Amaral, A., Schraa, O., Rieger, L., Gillot, S., Fayolle, Y., Bellandi, G., Amerlinck, Y., Mortier, S.T.F.C., Gori, R., Neves, R. and Nopens, I. (2016). Towards advanced aeration modelling: from blower to bubbles to bulk. Water Sci. Technol., 75(3), 552-560.

Abstract: Aeration is an essential component of aerobic biological wastewater treatment and is the largest energy consumer at most water resource recovery facilities. Most modelling studies neglect the inherent complexity of the aeration systems used. Typically, the blowers, air piping, and diffusers are not modelled in detail, completely mixed reactors in a series are used to represent plug-flow reactors, and empirical correlations are used to describe the impact of operating conditions on bubble formation and transport, and oxygen transfer from the bubbles to the bulk liquid. However, the mechanisms involved are very complex in nature and require significant research efforts. This contribution highlights why and where there is a need for more detail in the different aspects of the aeration system and compiles recent efforts to develop physical models of the entire aeration system (blower, valves, air piping and diffusers), as well as adding rigour to the oxygen transfer efficiency modelling (impact of viscosity, bubble size distribution, shear and hydrodynamics). As a result of these model extensions, more realistic predictions of dissolved oxygen profiles and energy consumption have been achieved. Finally, the current needs for further model development are highlighted.

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Schraa, O., Rieger, L. and Alex, J. (2016). Development of a model for activated sludge aeration systems: Linking air supply, distribution, and demand. Water Sci. Technol., 75(3), 507-517.

Abstract: During the design of a water resource recovery facility, it is becoming industry practice to use simulation software to assist with process design. Aeration is one of the key components of the activated sludge process, and is one of the most important aspects of modelling wastewater treatment systems. However, aeration systems are typically not modelled in detail in most wastewater treatment process modelling studies. A comprehensive dynamic aeration system model has been developed that captures both air supply and demand. The model includes sub-models for blowers, pipes, fittings, and valves. An extended diffuser model predicts both oxygen transfer efficiency within an aeration basin and pressure drop across the diffusers. The aeration system model allows engineers to analyse aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, controller design and tuning, and energy costs. This enables engineers to trouble-shoot the entire aeration system including process, equipment and controls. It also allows much more realistic design of these highly complex systems.

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Rosenthal, A., Roots, P., Yang, F., Kozak, J., Zhang, H. and Wells, G. (2017). Laboratory-scale investigation of integrated biological phosphorus removal and mainstream deammonification at the Terrence J. O’Brien Water Reclamation Plant. Proceedings WEFTEC.17, Chicago, Illinois, USA.

Abstract: In collaboration with the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), the environmental biotechnology research group at Northwestern University (NU) is conducting a laboratory-scale investigation of enhanced biological phosphorus removal (EBPR) integrated with mainstream deammonification using primary effluent as feed. The research presented here includes (1) A feasibility assessment of combining a high rate activated sludge (HRAS) process with EBPR (herein HRAS-P); (2) A feasibility assessment of mainstream deammonification without bioaugmentation in suspended growth and fixed film bioreactors; and (3) A demonstration of stable and efficient integrated HRAS-P + deammonification. After an extended period of variable phosphorus removal performance, HRAS-P discharged an average effluent ortho-P concentration of 0.5 mgP/L at an SRT of 2.5 days and an HRT of 4.8 hours. This was largely achieved through the implementation of sensor-based aeration control. Current results demonstrate the potential feasibility of mainstream deammonification, and also highlight the unique performance characteristics and challenges associated with biofilm and suspended growth deammonification reactors. Efficient retention of anammox biomass was a key advantage of the biofilm reactor, but nitrite oxidizing bacteria out-competition is an ongoing challenge for both. Suspended solids management techniques that achieve selective retention of anammox and ammonium oxidizing bacteria is a crucial ongoing focus for mainstream deammonification improvement.

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Schraa, O., Rieger, L., Alex, J. and Miletic, I. (2017). An innovative approach for modeling aerobic granular sludge processes. Proceedings WEFTEC.17, Chicago, Illinois, USA.

Abstract: An innovative biofilm model has been developed that allows modeling of a granular sludge sequencing batch reactor (GSBR) in a single tank without the need for a separate variable volume tank and a recycle stream. The model has a variable volume, supports separate or simultaneous feeding and effluent discharge, predicts the biofilm thickness without being constrained by a maximum thickness, supports non-uniform biofilm geometries, and models the impact of mixing intensity on biofilm detachment. The model has been calibrated and validated using data from lab-scale and pilot-scale reactors respectively and has demonstrated excellent predictive capabilities. Model calibration only required specification of the biofilm surface area within the reactor, the biofilm density, and the biofilm erosion velocity which were determined from the measured characteristics of the granules. This is significant because it enables engineers to use the model in the design of aerobic granular sludge processes which are becoming attractive due to their ability to intensify treatment.

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Juan-García, P., Kiser, M.A., Schraa, O., Rieger, L. and Corominas, Ll. (2017). Exploring the potential of dynamic air supply models to evaluate control strategies: the experience at the Girona WRRF. Proceedings ICA2017, Quebec, QC, Canada.

Abstract: A model-based treatment performance and energy audit of the Girona WRRF in Spain was conducted using advanced and fully dynamic air supply models. The focus of the study was on the aeration system, which represents 63% of the plant’s energy consumption. A dynamic model consisting of a process, a realistic controller, and a very detailed air supply model, was developed and calibrated to be used as the baseline for testing various scenarios. Results show that the implementation of an ammonia-based controller and a redistribution of the diffusers led to energy savings between 12 and 21%, depending on wastewater temperature. In addition, the model demonstrated that the current blower is too large, which causes an intermittent behaviour, endangering the equipment and shortening its life-time, plus limiting the minimum air-supply. The applied aeration system models enable engineers to identify bottle-necks by modelling equipment constraints (e.g. blower turn-down). Ignoring the air supply side in an assessment could result in an overestimation of energy savings or treatment performance and consequently in non-optimal control solutions or equipment selection.

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Miletic, I., Rieger, L., Schraa, O., Alex, J. (2017a). Predicting influent PO4 using a multivariate soft sensor. Proceedings ICA2017, Quebec, QC, Canada.

Abstract: A method for implementing a multivariate Partial Least Squares (PLS) soft sensor is discussed with a focus on sensor robustness. Practical aspects of implementation are examined including handling of missing or poor input data, sensor and data diagnostics, automated soft sensor calibration, and on-line sensor operation. Proof-of-concept examples are drawn using data from a simulated study, and a data set from a medium-sized European municipality. Results using the municipal data show that the PLS soft sensor can predict PO4-P values with COD, TSS, NHx pH, DO and temperature as model inputs.

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Miletic, I., Rieger, L., Schraa, O., Alex, J. (2017b). Improved on-line simulations of wastewater treatment plants using time series methods. Proceedings ICA2017, Quebec, QC, Canada.

Abstract: Time series analysis methods have been applied to water resource recovery facility (WRRF) data. Although time series methods are well understood, the practical aspects of their use and implementation are not often discussed. In the study presented in this paper, time series methods were used to (1) discuss and demonstrate a generic approach to selecting model order/complexity and to fit models to wastewater data, (2) predict future values of variables (i.e. create a forecast), and (3) show how these methods can be used to create input-output models that relate process variables to each other. In this paper, the focus is on the building of models for data on flow and TSS (Total Suspended Solids) that were measured at a Canadian municipal WRRF. The models accurately predict flow and TSS and are used for short-term replacement of missing or faulty measured data.

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Schraa, O., Rieger, L., Alex, J. and Miletic, I. (2017). Ammonia-based aeration control with optimal SRT control: improved performance and lower energy consumption. Proceedings ICA2017, Quebec, QC, Canada.

Abstract: Ammonia-based aeration control (ABAC) is a cascade control concept for controlling total ammonia nitrogen (NHx-N) in the activated sludge process. Its main goals are to tailor the aeration intensity to the NHx-N loading and to maintain consistent nitrification, which meets effluent limits but minimizes energy consumption and improves nutrient removal. One limitation to ABAC is that the solids retention time (SRT) control strategy used at a water resource recovery facility (WRRF) may not be consistent with the goals of ABAC. ABAC-SRT control is a strategy for aligning the goals of ammonia-based aeration control and SRT control. A supervisory controller is used to ensure that the SRT is always optimal for ammonia-based aeration control. The methodology has the potential to reduce aeration energy consumption by over 30% as compared to traditional DO control. Practical implementation aspects are highlighted for implementation at full-scale such as proper selection of the average DO set point for the supervisory controller, selection of the time interval used in calculating the rate of change in sludge inventory, using an MLSS controller to ensure that the SRT controller does not lead to excessively low or high MLSS concentrations, and proper tuning of the controllers. In conclusion, the ABAC-SRT concept is a promising approach for coordinated control of SRT, total ammonia nitrogen, and dissolved oxygen in the activated sludge process that balances both treatment performance and energy savings.

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Schraa, O. and Gray, M. (2017). Process Control Systems at Water Resource Recovery Facilities: Use of Process Simulation to Assist with Controller Design and Tuning. Proceedings ISA2017, Orlando, FL, USA.

Abstract: Process simulation is a useful tool to assist with control design and tuning at water resource recovery facilities (WRRFs). Simulation models serve as virtual WRRFs which can be used to design, tune, and test control systems. In addition, simulation models can be used as a basis for generating the models used in model-predictive control systems. Most WRRF simulation platforms provide the capability of implementing and testing feedback and other types of controllers. These controllers may be included as part of the unit process setup menus for common control loops such as DO (dissolved oxygen) and SRT (solids retention time) control or may be configurable using separate controller modules or blocks.

The paper provides a summary of the most common process disturbances and control loops found in WRRFs. This is followed by examples of using process simulation to design and test control systems for wet weather flow control, sequencing batch reactor (SBR) phase length control, and blower and air distribution control in aeration systems. In wet weather control, simulation can be used to determine the impact of bypasses, storm tanks, and step feed strategies on maintaining sludge inventory and minimizing the impact to the receiving water. For the case of SBRs, a Petri-Net discrete-event controller is simulated that shows the potential energy savings associated with ammonia-based control of the aeration phase length. A comparison of pressure-based and flow-based control of aeration blowers is presented that shows that flow-based control systems have a higher potential for energy savings and improved performance over pressure-based systems. Finally, an aeration system control example is presented that demonstrates the impact of temperature on control system performance and highlights that the optimal diffuser distribution in a plug-flow aeration basin depends on the temperature and loading conditions.

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Petrovich, M., Wu, C.-Y., Rosenthal, A., Chen, K.-F., Packman, A.I. and Wells, G.F. (2017). Nitrosomonas europaea biofilm formation is enhanced by Pseudomonas aeruginosa. FEMS Microbiology Ecology, 93(5), 1-9.

Abstract: Biofilms are useful in biotechnology applications such as wastewater treatment, where aggregation of cells on surfaces can increase retention of slow-growing organisms such as ammonia-oxidizing bacteria (AOB). The formation and morphological development of polymicrobial biofilms including AOB are not thoroughly understood. Here, we investigated the formation of Nitrosomonas europaea AOB biofilms in flow cell systems. Nitrosomonas europaea developed substantially greater biovolume in co-culture with heterotrophic Pseudomonas aeruginosa than when cultured alone. In single-species biofilms, N. europaea formed thin, dispersed layers of cells. Contrastingly, when N. europaea was added to flow cells containing pre-established P. aeruginosa biofilms, N. europaea associated closely with P. aeruginosa, resulting in dual-species clusters with greater quantities of N. europaea. These results indicate that P. aeruginosa enhances the formation of N. europaea in biofilms. This favorable association of N. europaea with heterotrophic biofilms is expected to facilitate development of improved strategies for retention of N. europaea and other slow-growing AOB in engineered bioreactors.

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Wells, G.F., Shi, Y., Laureni, M., Rosenthal, A., Szivák, I., Weissbrodt, D.G., Joss, A., Buergmann, H., Johnson, D.R. and E. Morgenroth, E. (2017). Comparing the resistance, resilience, and stability of replicate moving bed biofilm and suspended growth combined nitritation−anammox reactors. Environ. Sci. Technol., 51, 5108−5117.

Abstract: Combined partial nitritation−anammox (PN/A) systems are increasingly being employed for sustainable removal of nitrogen from wastewater, but process instabilities present ongoing challenges for practitioners. The goal of this study was to elucidate differences in process stability between PN/A process variations employing two distinct aggregate types: biofilm [in moving bed biofilm reactors (MBBRs)] and suspended growth biomass. Triplicate reactors for each process variation were studied under baseline conditions and in response to a series of transient perturbations. MBBRs displayed elevated NH4+ removal rates relative to those of suspended growth counterparts over six months of unperturbed baseline operation but also exhibited significantly greater variability in performance. Transient perturbations led to strikingly divergent yet reproducible behavior in biofilm versus suspended growth systems. A temperature perturbation prompted a sharp reduction in NH4+ removal rates with no accumulation of NO2− and rapid recovery in MBBRs, compared to a similar reduction in NH4+ removal rates but a high level of accumulation of NO2− in suspended growth reactors. Pulse additions of a nitrification inhibitor (allylthiourea) prompted only moderate declines in performance in suspended growth reactors compared to sharp decreases in NH4+ removal rates in MBBRs. Quantitative fluorescence in situ hybridization demonstrated a significant enrichment of anammox in MBBRs compared to suspended growth reactors, and conversely a proportionally higher AOB abundance in suspended growth reactors. Overall, MBBRs displayed significantly increased susceptibility to transient
perturbations employed in this study compared to that of suspended growth counterparts (stability parameter), including significantly longer recovery times (resilience). No significant difference in the maximal impact of perturbations (resistance) was apparent. Taken together, our results suggest that aggregate architecture (biofilm vs suspended growth) in PN/A processes exerts an unexpectedly strong influence on process stability.

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Rieger, L., Alex, J. and Schraa, O. (2016). Model-Supported Design, Testing, and Implementation of Process Control Strategies. in Ingildsen, P. & Olsson, G. (eds.). Smart Water Utilities: Complexity Made Simple. IWA Publishing, London, UK.

Abstract: Advanced process control has the ability to allow the full performance potential of your wastewater treatment plant to be realized by taking advantage of existing but unused capacities. In plants that are nearing their capacity limits this can lead to improved effluent quality and process stability. In plants that are under capacity, advanced control can allow you to operate your plant closer to effluent limits in order to achieve cost savings. While the benefits of a well-designed control solution are often clear, there are obstacles to implementation of these solutions in the field that can make it difficult to realize all the potential benefits.

Often the most significant obstacles are seemingly trivial issues such as poor data quality caused by sensor calibration and maintenance issues or lack of communication between plant designers, process engineers, control engineers, and plant operators. This case study will highlight the major obstacles in implementing advanced control solutions and present practical solutions tested in practice.

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Amaral, A., Schraa, O., Rieger, L., Gillot, S., Fayolle, Y., Bellandi, G., Amerlinck, Y., Mortier, S.T.F.C., Gori, R., Neves, R. and Nopens, I. (2016). Towards advanced aeration modelling: from blower to bubbles to bulk. Proceedings WWTmod2016, Annecy, France.

Abstract: The complexity of the whole aeration system (blowers to bubbles to bulk) has been neglected for far too long in the modelling of Water Resource Recovery Facilities (WRRF). We still use simplistic basis models extended with empirical relationships. The reason is that many of the mechanisms are very complex in nature and require significant research efforts. This contribution highlights the need for more detail in the different submodels of the aeration system and compiles recent efforts in aeration process modelling hereby covering physical models for the entire aeration system (blower, valves, piping, and diffusers), as well as adding rigour to the oxygen transfer efficiency modelling (impact of viscosity, bubble size distribution, shear, hydrodynamics). As a result of these model extensions more realistic predictions of dissolved oxygen profiles and energy consumption have been achieved or should become achievable. Moreover, a higher level of detail is required for systems that are operated at low oxygen dissolved (DO) concentrations. Finally, also the current needs in terms of further model development is discussed.

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Schraa, O., Rieger, L. and Alex, J. (2016). Development of a model for activated sludge aeration systems: Linking air supply, distribution, and demand. Proceedings WWTmod2016, Annecy, France.

Abstract: During the design of a wastewater treatment plant (WWTP), it is becoming industry practice for engineers to use simulation software to assist with process design. Aeration is one of the key components of the activated sludge process, and is one of the most important aspects of modelling wastewater treatment systems. A comprehensive dynamic aeration system model has been developed that captures both air supply and demand.  The model includes sub-models for blowers, pipes and fittings, valves, and diffusers. Oxygen transfer within aeration tanks is also included as part of the overall model. The aeration system model allows engineers to analyze aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, and controller design and tuning.

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Rosenthal, A.F., Balogun, O. and Wells, G.F. (2016). Morphological examination of nitrifying biofilms in a moving bed biofilm reactor using optical coherence tomography and fluorescence in situ hybridization. Proceedings WEFTEC.16, New Orleans, LA, USA.

Abstract: We present an imaging and image analysis approach to quantify both the overall morphology and the spatial distribution of nitrifying guilds in mixed-culture, nitrifying biofilms enriched on K1 carriers. For image acquisition we used optical coherence tomography to non-destructively visualize the three dimensional structure of entire internal cavities and fluorescent in situ hybridization of carrier cryosections to visualize the spatial distribution of ammonia oxidizing bacteria and nitrite oxidizing relative to the overall biofilm structure. Determination of morphological parameters relies on the adaptation of current image analysis methods to accomodate the complex, curved geometry of MBBR carriers.

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Schraa, O., Rieger, L. and Alex, J. (2016). Coupling SRT Control with Aeration Control Strategies. Proceedings WEFTEC.16, New Orleans, LA, USA.

Abstract: A new control strategy has been developed that integrates dynamic SRT control with ammonia-based aeration control. In this strategy, feedback control of dynamic SRT is employed which is shown to provide the most appropriate means of adjusting the waste flow rate in response to loading and operational changes. A new SRT set point controller is developed which determines the optimum set point for the dynamic SRT controller, given the output from the ammonia controller. Simulation experiments are shown to demonstrate the benefits of the new control strategy which include variation of the SRT throughout the year to operate at the lowest SRT that allows for the desired level of nitrification, less sludge washout during storms and faster recovery after storms, and potential reductions in energy consumption of almost 10% as compared to ammonia-based aeration control on its own and 25 to 30% compared to traditional DO control.

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Alex, J., Rieger, L. and Schraa, O. (2016). Comparison of Fine-Bubble Aeration Control Concepts with Respect to Energy Efficiency and Robustness. Proceedings WEFTEC.16, New Orleans, LA, USA.

Abstract: The presented study analyzes and compares four standard aeration control concepts for air supply and air distribution in terms of energy savings, controller performance and robustness. Realistic results were achieved by the use of state-of-the-art models for the treatment process complemented by detailed and fully dynamic models for the aeration equipment and the piping network. The study showed the significance of proper controller tuning. Simplified tuning rules have been derived for all tested control concepts.

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Rieger, L., Aymerich, I., Sobhani, R., Rosso, D., Schraa, O. and Corominas, Ll. (2015). Lower energy consumption but higher energy bills? The impact of energy tariffs on WRRF operating costs. Influents, Magazine of the Water Environment Association of Ontario, Winter 2015.

Abstract: The paper demonstrates the impact of equipment efficiencies and energy tariffs on the operating costs of a Water Resource Recovery Facility (WRRF). An example is given using a simulation benchmark environment using models for pumps and blowers with reduced efficiencies in the lower flow ranges. In the example, two ammonia-based aeration controllers (on-off and PID) are compared against a base case with a classic dissolved oxygen controller. An energy tariff from Southern California Edison is used to highlight the significant impact of time-of-use and peak power demand charges. The results show a reduction potential for energy of over 20% for both ammonia controllers with a slightly better performance of the on-off ammonia controller in terms of effluent total nitrogen and energy consumption. However, after applying the energy tariff from Southern California, the cost savings are reduced to 3% for the on-off controller and 10% for the PID controller.

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Rosenthal, A., Yi, J., Zhang, H.F., Balogun, O. and Wells, G. (2015). Quantitative image analysis of mesoscale biofilm structure with optical coherence tomography. Proceedings WEFTEC.15, Chicago, Illinois, USA.

Abstract: The metabolic processes supported in a biofilm are often determined by a biofilm’s mesoscale structure (10-4-10-2 meter scale), and mesoscale structure is in turn shaped by the interplay between microbial metabolism and detachment. Despite the widespread adoption of biofilm reactors for water pollution control, much is unknown regarding the relationship between mesoscale biofilm structure and process performance. Optical coherence tomography (OCT) has recently emerged as a promising technique to non-destructively capture mesoscale three-dimensional images of environmentally relevant biofilms. For this study, OCT was used to image the three dimensional structure of mixed culture biofilms enriched in a synthetic wastewater. Distinct morphological features such as pores and microcolonies were detectable as deep as 1mm within the biofilm matrix. Quantitative structural parameter values (e.g. porosity, thickness, diffusion distance) were consistent across multiple fields of view and compared favorably with images acquired by confocal laser scanning microscopy.

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Schraa, O., Rieger, L. and Alex, J. (2015). A Comprehensive Aeration System Model for WRRF Design and Control. Proceedings WEFTEC.15, Chicago, Illinois, USA.

Abstract: During the design of a water resource recovery facility (WRRF), it is becoming industry practice for process engineers to use simulation software to assist with the design of the plant and its aeration system. The aeration process is one of the key components of the activated sludge process, and as such, is one of the most important aspects of modeling wastewater treatment systems. A comprehensive aeration system model has been developed in SIMBA# that can model both the air supply and demand. The model includes sub-models for centrifugal and positive displacement blowers, pipes and fittings, valves, and diffusers. Both compressible and incompressible flow can be modelled. Oxygen transfer within aeration tanks is also included as part of the overall model. The aeration system model allows engineers to analyze aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, and controller design and tuning. This will allow more detailed system-wide testing before commissioning.

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Ogurek, M., Alex, J., Rieger, L., Schraa, O. and Schütze, M. (2015). A novel integrated approach for designing, testing and implementing WRRF process control solutions. Proceedings WEFTEC.15, Chicago, Illinois, USA.

Abstract: This paper describes an integrated approach for designing, testing and implementing process control solutions for Water Resource Recovery Facilities (WRRFs) using a novel simulation platform. The user-friendly modeling and simulation environment encapsulates expert knowledge and standardizes typical tasks and goes far beyond classic simulators in terms of the level of detail of modeling control solutions. The suite of available models ranges from various controller types, sensor and actuator models up to pipe networks. In this way it enables even inexperienced users to design state-of-the-art process control solutions. Proper designs are supported by the ability to test realistic solutions in simulation before they are implemented at a real plant. The implementation process is facilitated by an option to automatically translate control solutions (in form of graphical function blocks) into PLC code. A special solution is provided for sequential control (e.g. SBRs) using a graphical Petri-net approach.

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Rieger, L., Alex, J., Schraa, O. and Bott, C.B. (2015). A Highly Efficient Methanol Dosage Controller for Multi-stage WRRFs. Proceedings WEFTEC.15, Chicago, Illinois, USA.

Abstract: A new model-based methanol dosage controller for multi-stage Water Resource Recovery Facilities (WRRF) is presented and compared with classic control approaches. The new controller uses self-tuning observer models to predict the concentration of methanol consuming organisms (methylotrophs) and subsequently the remaining methanol concentration in the denitrification zone effluent. This information is used in methanol dosage controllers to reduce methanol over-dosage if the mass of methylotrophs is insufficient to make use of the dosed methanol. This bleeding of methanol into the downstream aeration zones causes additional chemical costs, increases oxygen demand, and produces additional sludge. It is shown that classic methanol dosage controllers cannot prevent over-dosage and that the presented control solution (i) reduces the likelihood of over-dosage, (ii) leads to cost savings in comparison with classic controllers, and (iii) increases the control authority of the dosage controllers by minimizing the grow-in time of methylotrophs.

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Aymerich, I., Rieger, L., Sobhani, R., Rosso, D. and Corominas Ll. (2015). The difference between energy consumption and energy cost: Modelling energy tariff structures for water resource recovery facilities. Water Research 81, 113-123

Abstract: The objective of this paper is to demonstrate the importance of incorporating more realistic energy cost models (based on current energy tariff structures) into existing water resource recovery facilities (WRRFs) process models when evaluating technologies and cost-saving control strategies. In this paper, we first introduce a systematic framework to model energy usage at WRRFs and a generalized structure to describe energy tariffs including the most common billing terms. Secondly, this paper introduces a detailed en- ergy cost model based on a Spanish energy tariff structure coupled with a WRRF process model to evaluate several control strategies and provide insights into the selection of the contracted power structure. The results for a 1-year evaluation on a 115,000 population- equivalent WRRF showed monthly cost differences ranging from 7 to 30% when comparing the detailed energy cost model to an average energy price. The evaluation of different aeration control strategies also showed that using average energy prices and neglecting energy tariff structures may lead to biased conclusions when selecting oper- ating strategies or comparing technologies or equipment. The proposed framework demonstrated that for cost minimization, control strategies should be paired with a spe- cific optimal contracted power. Hence, the design of operational and control strategies must take into account the local energy tariff.

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Rieger, L., Aymerich, I., Sobhani, R., Rosso, D., Schraa, O. and Corominas, Ll. (2015). Lower energy consumption but higher energy bills? The impact of energy tariffs on WRRF operating costs. Water Environment Association of Ontario Annual Conference 2015, Toronto ON, Canada; April 2015.

Abstract: The paper demonstrates the impact of equipment efficiencies and energy tariffs on the operating costs of a Water Resource Recovery Facility (WRRF). An example is given using a simulation benchmark environment using models for pumps and blowers with reduced efficiencies in the lower flow ranges. In the example, two ammonia-based aeration controllers (on-off and PID) are compared against a base case with a classic dissolved oxygen controller. An energy tariff from Southern California Edison is used to highlight the significant impact of time-of-use and peak power demand charges. The results show a reduction potential for energy of over 20% for both ammonia controllers with a slightly better performance of the on-off ammonia controller in terms of effluent total nitrogen and energy consumption. However, after applying the energy tariff from Southern California, the cost savings are reduced to 3% for the on-off controller and 10% for the PID controller.

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Rieger, L., Jones, R.M., Dold, P.L. and Bott, C.B. (2014). Ammonia-based feedforward and feedback aeration control in activated sludge processes. Water Environ Res., 86(1), 63-73.

Abstract: Aeration control at wastewater treatment plants based on ammonia as the controlled variable is applied for one of two reasons: (1) to reduce aeration costs, or (2) to reduce peaks in effluent ammonia. Aeration limitation has proven to result in significant energy savings, may reduce external carbon addition, and can improve denitrification and biological phosphorus (bio-P) performance. Ammonia control for limiting aeration has been based mainly on feedback control to constrain complete nitrification by maintaining approximately one to two milligrams of nitrogen per liter of ammonia in the effluent. Increased attention has been given to feedforward ammonia control, where aeration control is based on monitoring influent ammonia load. Typically, the intent is to anticipate the impact of sudden load changes, and thereby reduce effluent ammonia peaks. This paper evaluates the fundamentals of ammonia control with a primary focus on feedforward control concepts. A case study discussion is presented that reviews different ammonia-based control approaches. In most instances, feedback control meets the objectives for both aeration limitation and containment of effluent ammonia peaks. Feedforward control, applied specifically for switching aeration on or off in swing zones, can be beneficial when the plant encounters particularly unusual influent disturbances.

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Olsson, G., Carlsson, B., Comas, J., Copp, J., Gernaey, K.V., Ingildsen, P., Jeppsson, U., Kim, C., Rieger, L., Rodríguez-Roda, I., Steyer, J.-P., Takács, I., Vanrolleghem, P.A., Vargas Casillas, A., Yuan, Z. and Åmand, L. (2014). Instrumentation, Control and Automation in wastewater – what happened from London 1973 to Narbonne. Water Sci. Technol., 69(7), 1373–1385.

Abstract: Key developments of instrumentation, control and automation (ICA) applications in wastewater systems during the past 40 years are highlighted in this paper. From the first ICA conference in 1973 through to today there has been a tremendous increase in the understanding of the processes, instrumentation, computer systems and control theory. However, many developments have not been addressed here, such as sewer control, drinking water treatment and water distribution control. It is hoped that this review can stimulate new attempts to more effectively apply control and automation in water systems in the coming years.

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Nopens, I., Arnaldos, M., Belia, E., Jeppsson, U., Kinnear, D., Lessard, P., Murthy, S., O’Shaughnessy, M., Rieger, L., Vanrolleghem, P. and Weijers, S. (2014). Maximising the benefits of activated sludge modelling. Water 21, 16(10), 31-33.

Abstract: Activated sludge (AS) has been around for 100 years now, but the dynamic modelling of this process has been undertaken for only the last four decades, starting probably with the pioneering work of John Andrews (1974).The recent WWTmod2014 event (see box) and a brainstorm among the authors – as seasoned AS modellers – in Wendake, Québec, Canada, resulted in the development of some key points of direction with regards to the short-term needs in AS modelling. These range from further
model development over development of modelling tools as well as the way modelling projects are (or should be) better conducted.

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Hauduc, H., Rieger, L., Oehmen, A., Comeau, Y., van Loosdrecht, M.C.M., Héduit, A., Vanrolleghem, P.A. and Gillot S. (2012). Critical review of activated sludge modelling: State of process knowledge, modelling concepts and limitations. Biotechnol. Bioeng., 110(1), 24-46.

Abstract: This work critically reviews modeling concepts for standard activated sludge wastewater treatment processes (e.g., hydrolysis, growth and decay of organisms, etc.) for some of the most commonly used models. Based on a short overview on the theoretical biochemistry knowledge this review should help model users to better understand (i) the model concepts used; (ii) the differences between models, and (iii) the limits of the models. The seven analyzed models are: (1) ASM1; (2) ASM2d; (3) ASM3; (4) ASM3+BioP; (5) ASM2d+TUD; (6) Barker&Dold model; and (7) UCTPHO+. Nine standard processes are distinguished and discussed in the present work: hydrolysis; fermentation; ordinary heterotrophic organisms (OHO) growth; autotrophic nitrifying organisms (ANO) growth; OHO & ANO decay; poly-hydroxyalkanoates (PHA) storage; polyphosphate (polyP) storage; phosphorus accumulating organisms PAO) growth; and PAO decay. For a structured comparison, a new schematic representation of these processes is proposed. Each process is represented as a reaction with consumed components on the left of the figure and produced components on the right. Standardized icons, based on shapes and color codes, enable the representation of the stoichiometric modeling concepts and kinetics. This representation allows highlighting the conceptual differences of the models, and the level of simplification between the concepts and the theoretical knowledge. The model selection depending on their theoretical limitations and the main research needs to increase the model quality are finally discussed.

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Choubert, J.-M., Rieger, L., Shaw, A., Copp, J., Spérandio, M., Sørensen, K., Rönner-Holm, S., Morgenroth, E., Melcer, H. and Gillot, S. (2013). Rethinking wastewater characterisation methods – A position paper. Water Sci. Technol., 67(11), 2363-2373.

Abstract: Increasingly stringent effluent limits and an expanding scope of model system boundaries beyond activated sludge has led to new modelling objectives and consequently to new and often more detailed modelling concepts. Nearly three decades after the publication of ASM1, the authors believe it is time to re-evaluate wastewater characterization procedures and targets. The present position paper gives a brief overview of state-of-the-art methods and discusses newly developed measurement techniques on a conceptual level. Potential future paths are presented including on-line instrumentation, promising measuring techniques, and mathematical solutions to fractionation problems. This is accompanied by a discussion on standardization needs to increase modelling efficiency in our industry.
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Rosenthal, A.,Ramalingam, K., Beckmann, K., Deur, A. and Fillos, J. (2013). Experimental evaluation of the nitrite sensitivity coefficient in granular anammox biomass. Water Sci. Technol., 68(9), 2103-2110.

Abstract: Nitrite is widely reported to inhibit anammox activity and growth. One modeling approach for nitrite impairment of anammox growth is the use of a nitrite sensitivity coefficient which increases the endogenous decay coefficient of anammox bacteria proportionally to nitrite concentration. The objective of this study was to measure nitrite concentration profiles within active anammox granules incubated at fixed bulk nitrite concentrations and to compare these with nitrite concentration profiles predicted by a biofilm model that incorporates the nitrite sensitivity coefficient. We developed an apparatus for the repeated measurement of nitrite concentration profiles along the radius of granular anammox biomass over a period of 6 days at fixed bulk nitrite concentrations. Granular anammox biomass was obtained from a two-stage bench-scale partial nitritation/anammox reactor system. There was no apparent effect of nitrite concentration on nitrite utilization kinetics after 6 days at exposures up to 90 mg NO2-N/L. These findings suggest that anammox bacteria tolerate extended exposures to elevated nitrite concentrations, and in its present form, the nitrite sensitivity coefficient is not applicable for anammox growth modeling.

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Rieger, L., Takács, I. and Siegrist, H. (2012). Improving nutrient removal while reducing energy use at three Swiss WWTPs using advanced control. Water Environ Res., 84(2), 171-189.

Abstract: Aeration consumes about 60% of the total energy use of a wastewater treatment plant (WWTP) and therefore is a major contributor to its carbon footprint. Introducing advanced process control can help plants to reduce their carbon footprint and at the same time improve effluent quality through making available unused capacity for denitrification, if the ammonia concentration is below a certain set-point. Monitoring and control concepts are cost-saving alternatives to the extension of reactor volume. However, they also involve the risk of violation of the effluent limits due to measuring errors, unsuitable control concepts or inadequate implementation of the monitoring and control system. Dynamic simulation is a suitable tool to analyze the plant and to design tailored measuring and control systems. During this work, extensive data collection, modeling and fullscale implementation of aeration control algorithms were carried out at three conventional activated sludge plants with fixed pre-denitrification and nitrification reactor zones. Full-scale energy savings in the range of 16–20% could be achieved together with an increase of total nitrogen removal of 40%.
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Rieger, L. and Olsson, G. (2012). Why many control systems fail. Water Environment and Technology, 24(6), 42-45.

Abstract: When implementing real-time process control systems at wastewater treatment plants (WWTPs), several technical problems may have to be addressed. Sensors have to be reliable. Controllers should be properly designed and tuned. Fault detection should be implemented and safety nets designed. Actuators must have adequate control authority to deal with changing influent quality and quantity. But in their 1998 Water Science Technology (Vol. 37, No. 12, pp. 397–401) article “Reviewing, assessing, and speculating,” G. Olsson and B. Newell argue that it is the “human factor” that is often neglected even though it creates more problems than the technology.

Lack of incentives or misleading incentives for plant stakeholders create poor WWTP efficiency and suboptimal effluent quality. Greatly varying interests and demands of the stakeholders involved at all levels of urban water management have to be taken into consideration in a successful incentive structure. Without effective incentives, the sustained application of a control concept or any other optimization measure might be jeopardized.

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Xu, X., Fillos, J., Ramalingam, K. and Rosenthal, A. (2012). Quantitative analysis of methanol in wastewater by GC-MS with direct injection or headspace SPME sample introduction. Anal. Methods, 4, 3688-3694.

Abstract: Monitoring methanol (MeOH) concentration in wastewater treatment processes is important because methanol is a widely used carbon source for denitrification. Headspace solid-phase microextraction (HS-SPME) and direct liquid injection coupled with gas chromatography and mass spectrometry (GC/MS) were studied to analyze methanol in municipal wastewater samples in the mg mL-1 range. The conditions applied for both methods are presented. The extraction parameters for the HS-SPME method, such as SPME fiber selection, extraction temperature and time, agitation, pH value of the solution, and salt effect, were investigated. For the liquid injection method, the critical parameter was to select a polar column, the DB-WAX capillary column. Linear calibration curves were obtained over 3 orders of magnitude using methanol-d3 (MeOH-d3) or acetonitrile (ACN) as an internal standard. Both methods were validated with repeatability and recovery experiments. The liquid injection method requires a smaller injection volume and allows for faster analysis and better precision. The HS-SPME method also has reliable performance with less than 10% error. In addition, the comparison of the two methods on practical applications and maintenance is included.

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Villez K., Rieger L., Keser B. and Venkatasubramanian V. (2012). Probabilistic qualitative analysis for fault detection and identification of an on-line phosphate analyzer. Int. J. Adv. Eng. Sci. Appl. Math., 4(1-2), 67-77.

Abstract: On-line, real-time collection of measurements remains a key challenge in water quality monitoring and control due to unknown and varying quality of on-line sensor data. Today’s data quality assessment is typically based on a comparison of sensor-based measurements and grab samples of the sampled solution taken next to the on-line analyzer and analyzed in a laboratory. In this work, internal data is used for fault detection and identification of a phosphate analyzer to inspect the measuring process itself. These internal data is shown to be information-rich with respect to the analyzer’s status. Furthermore, this information is captured well by means of a newly developed method for qualitative analysis of time series. This method was developed with global optimality in mind and therefore lends itself to a probabilistic assessment of the qualitative representation of time series.
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Corominas, Ll., Villez, K., Aguado, D., Rieger, L., Rosén, C. and Vanrolleghem, P.A. (2011). Performance evaluation of fault detection methods for wastewater treatment processes. Biotechnol. Bioeng., 108(2), 333-344.

Abstract: Several methods to detect faults have been developed in various fields, mainly in chemical and process engineering. However, minimal practical guidelines exist for their selection and application. This work presents an index that allows for evaluating monitoring and diagnosis performance of fault detection methods, which takes into account several characteristics, such as false alarms, false acceptance, and undesirable switching from correct detection to nondetection during a fault event. The usefulness of the index to process engineering is demonstrated first by application to a simple example. Then, it is used to compare five univariate fault detection methods (Shewhart, EWMA, and residuals of EWMA) applied to the simulated results of the Benchmark Simulation Model No. 1 long-term (BSM1_LT). The BSM1_LT, provided by the IWA Task Group on Benchmarking of Control Strategies, is a simulation platform that allows for creating sensor and actuator faults and process disturbances in a wastewater treatment plant. The results from the method comparison using BSM1_LT show better performance to detect a sensor measurement shift for adaptive methods (residuals of EWMA) and when monitoring the actuator signals in a control loop (e.g., airflow). Overall, the proposed index is able to screen fault detection methods.
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Hauduc, H., Rieger, L., Ohtsuki, T., Shaw, A., Takács, I., Winkler, S., Héduit, A., Vanrolleghem, P.A. and Gillot, S. (2011). Activated sludge modelling: development and potential use of a practical applications database. Water Sci. Technol., 63(10), 2164-2182.

Abstract: This study aims at synthesizing experiences in the practical application of ASM type models. The information is made easily accessible to model users by creating a database of modelling projects. This database includes answers to a questionnaire that was sent out to model users in 2008 to provide inputs for a Scientific and Technical Report of the IWA Task Group on Good Modelling Practice – Guidelines for use of activated sludge models, and a literature review on published modelling projects. The database is analysed to determine which biokinetic model parameters are usually changed by modellers, in which ranges, and what values are typically used for seven selected activated sludge models. These results should help model users in the calibration step, by providing typical parameter values as a starting point and ranges as a guide. However, the proposed values should be used with great care since they are the result of averaging practical experience and not taking into account specific parameter correlations.
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Miletic, I., Bruwer, M.J., Pieterse, D., and MacGregor, J.F. (2011). Advanced control of final product quality for batch processes. Proceedings of the ChemInnovations 2011 Conference, Houston TX, USA, September 2011.

Abstract: xyz.

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Sutton, P.M., Rittmann, B.E., Schraa, O.J., Banaszak, J.E., and Togna, A.P. (2011). Wastewater as a resource: a unique approach to achieving energy sustainability. Water Sci. Technol., 63(9), 2004-2009.

Abstract: A wastewater-treatment flowsheet was developed to integrate uniquely designed biological processes with physical-chemical unit processes, allowing conversion of the organic carbon in the wastewater to methane, the removal and recovery of phosphorus and nitrogen from the wastewater, and the production of water suitable for reuse. In the flowsheet, energy is derived from the wastewater by first shunting a large fraction of the organic carbon in the wastewater to a solids slurry which is treated via anaerobic digestion. The anaerobic digestion system consists of focused pulsed (FP) pretreatment coupled to anaerobic membrane bioreactors (MBRs). Computer modelling and simulation results are used to optimize design of the system. Energy generation from the system is maximized and costs are reduced by using modest levels of recycle flow from the anaerobic MBRs to the FP pretreatment step.
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Sutton, P.M., Melcer, H., Schraa, O.J., and Togna, A.P. (2011). Treating Municipal Wastewater with the Goal of Resource Recovery. Water Sci. Technol., 63(1), 25-31.

Abstract: A new municipal wastewater treatment flowsheet was developed with the objectives of energy sustainability, and water and nutrient recovery. Energy is derived by shunting a large fraction of the organic carbon in the wastewater to an anaerobic digestion system. Aerobic and anaerobic membrane bioreactors play a key role in energy recovery. Phosphorus and nitrogen are removed from the wastewater and recovered through physical-chemical processes. Computer modeling and simulation results together with energy balance calculations, imply the new flowsheet will result in a dramatic reduction in energy usage at lower treatment plant capital costs in comparison to conventional methods.
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Corominas, L., Rieger, L., Takács, I., Ekama, G., Hauduc, H., Vanrolleghem, P.A., Oehmen, A., Gernaey, K.V., van Loosdrecht, M.C.M. and Comeau, Y. (2010). New framework for standardized notation in wastewater treatment modelling. Water Sci. Technol., 61(4), 841–857.

Abstract: Many unit process models are available in the field of wastewater treatment. All of these models use their own notation, causing problems for documentation, implementation and connection of different models (using different sets of state variables). The main goal of this paper is to propose a new notational framework which allows unique and systematic naming of state variables and parameters of biokinetic models in the wastewater treatment field. The symbols are based on one main letter that gives a general description of the state variable or parameter and several subscript levels that provide greater specification. Only those levels that make the name unique within the model context are needed in creating the symbol. The paper describes specific problems encountered with the currently used notation, presents the proposed framework and provides additional practical examples. The overall result is a framework that can be used in whole plant modelling, which consists of different fields such as activated sludge, anaerobic digestion, sidestream treatment, membrane bioreactors, metabolic approaches, fate of  micropollutants and biofilm processes. The main objective of this consensus building paper is to establish a consistent set of rules that can be applied to existing and most importantly, future models. Applying the proposed notation should make it easier for everyone active in the wastewater treatment field to read, write and review documents describing modelling projects.
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Hauduc, H., Rieger, L., Takács, I., Héduit, A., Vanrolleghem, P.A. and Gillot, S. (2010). Systematic approach for model verification – Application on seven published Activated Sludge Models. Water Sci. Technol., 61(4), 825–839.

Abstract: The quality of simulation results can be significantly affected by errors in the published model (typing, inconsistencies, gaps or conceptual errors) and/or in the underlying numerical model description. Seven of the most commonly used activated sludge models have been investigated to point out the typing errors, inconsistencies and gaps in the model publications: ASM1; ASM2d; ASM3; ASM3+ Bio-P; ASM2d+ TUD; New General; UCTPHO+. A systematic approach to verify models by tracking typing errors and inconsistencies in model development and software implementation is proposed. Then, stoichiometry and kinetic rate expressions are checked for each model and the errors found are reported in detail. An attached spreadsheet (see http://www.iwaponline.com/wst/06104/0898.pdf) provides corrected matrices with the calculations of all stoichiometric coefficients for the discussed biokinetic models and gives an example of proper continuity checks.
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Park, H., Rosenthal, A., Ramalingam, K., Fillos, J. and Chandran, K. (2010). Linking community profiles, gene expression and N-removal in anammox bioreactors treating municipal anaerobic digestion reject water. Environ. Sci. Technol., 44, 6110–6116.

Abstract: Anaerobic ammonium oxidation (anammox) requires 60% less oxygen and no external organic carbon compared to conventional biological nitrogen removal (BNR). Nevertheless, fullscale installations of anammox are uncommon, primarily owing to the lack of well-established process monitoring and control strategies that result in stable anammox reactor performance. The overarching goal of this study was to develop and apply molecular biomarkers that link microbial community structure and activity to anammox process performance in a bioreactor fed with actual anaerobic digestion centrate from a full-scale operational wastewater treatment facility. Over longterm operation, Candidatus “Brocadia sp. 40” emerged as the dominant anammox population present in the reactor. There was good correspondence between reactor nitrogen removal performance and anammox bacterial concentrations. During the period of reactor operation, there was also a marked shift in biomass morphology from discrete cells to granular aggregates, which was paralleled by a shift also to more stable nitrogen removal and the succession and establishment of bacteria related to the Chlorobi/Bacteroidetes superfamily. Basedonbatchassays,hydrazineoxidoreductase(hzo)expression and concentrations of the 16S-23S rRNA intergenic spacer region (ISR) were good quantitative biomarkers of oxygen- and nitrite-mediated inhibition. When applied to a continuous anammox reactor, both molecular biomarkers show promise as monitoring tools for “predicting” reactor performance.

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Park, H., Rosenthal, A., Jezek, R., Ramalingam, K., Fillos, J. and Chandran, K. (2010). Impact of inocula and growth mode on the molecular microbial ecology of anaerobic ammonia oxidation (anammox) bioreactor communities. Water Research, 44, 5005-5013.

Abstract: The composition of distinctly inoculated granular anammox and biofilm-based completely autotrophic nitrogen removal over nitrite (CANON) bioreactors was investigated from startup through continuous long-term operation via denaturing gradient gel electrophoresis (DGGE) and sequencing. The granular anammox reactor was seeded with sludge from an operational anammox reactor in Strass, Austria. The CANON reactor was seeded with activated sludge from a local wastewater treatment plant in New York City. The principal anammox bacteria (AMX) shifted from members related to Kuenenia stuttgartiensis present in the initial inoculum to members related to Candidatus Brocadia fulgida during pre-enrichment (before this study) and to members related to Candidatus Brocadia sp. 40 (during this study) in the granular reactor. AMX related to C. Brocadia sp. 40 were also enriched from activated sludge in the CANON reactor. The estimated doubling times of AMX in the granular and CANON reactors were 5.3 and 8.9 days, respectively, which are lower than the value of 11 days, reported previously. Both the granular anammox and CANON reactors also fostered significant amounts of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). The fractions of AMX and two groups of NOB were generally similar in the granular anammox and CANON reactors. However, the diversity and fractions of AOB in the two reactors was markedly different. Therefore, it is suggested that the composition of the feed and extant substrate concentrations in the reactor likely select for the microbial community composition more than the inocula and reactor configuration. Further, such selection is not equivalent for all resident communities.

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Rieger, L., Takacs, I., Shaw, A., Winkler, S., Ohtsuki, T., Langergraber, G. and Gillot, S. (2010). Editorial: Status and future of wastewater treatment modelling. Water Sci. Technol., 61(4), 821–823.

Abstract: Modelling of wastewater treatment systems, originally deve- loped by academics primarily motivated by their research interest, has become a standard engineering tool. Initially focussed on the activated sludge process, modelling is now routinely applied for the whole wastewater treatment plant, including sludge and sidestream treatment. The interest in process modelling, as measured by the number of publications (Figure 1), seemed to have reached a plateau in the early years of the millennium (Gujer 2006), but since then it has gained speed again. The number of citations has been exponentially growing for the last 25 years (Figure 2). IWA’s Good Modelling Practice Task Group (GMP TG) was fortunate to have the opportunity to review trends in this field in the past through literature and dialogs with academics, practitioners and industry leaders. The following is a personal perspective of the GMP TG on the future of modelling, and current challenges, drawn from the information gathered. The purpose of this editorial is to promote further discussion of the topics it raises.
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Rieger, L., Takács, I., Villez, K., Siegrist, H., Lessard, P., Vanrolleghem, P.A. and Comeau, Y. (2010). Data reconciliation for WWTP simulation studies – Planning for high quality data and typical sources of errors. Water Environ. Res., 82(5), 426-433.

Abstract: Model results are only as good as the data fed as input or used for calibration. Data reconciliation for wastewater treatment modeling is a demanding task, and standardized approaches are lacking. This paper suggests a procedure to obtain high-quality data sets for model- based studies. The proposed approach starts with the collection of existing historical data, followed by the planning of additional measurements for reliability checks, a data reconciliation step, and it ends with an intensive measuring campaign. With the suggested method, it should be possible to detect, isolate, and finally identify systematic measurement errors leading to verified and qualitative data sets. To allow mass balances to be calculated or other reliability checks to be applied, few additional measurements must be introduced in addition to routine measurements. The intensive measurement campaign should be started only after all mass balances applied to the historical data are closed or the faults have been detected, isolated, and identified. In addition to the procedure itself, an overview of typical sources of errors is given.
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Kaelin, D., Manser, R., Rieger, L., Eugster, J., Rottermann, K. and Siegrist, H. (2009). Extension of ASM3 for two-step nitrification and denitrification and its calibration and validation with batch tests and pilot scale data. Water Res. 43(6), 1680-1692.

Abstract: Although traditionally not taken into account by most of activated sludge models the production of nitrite as an intermediate of the nitrification–denitrification processes becomes of interest in some specific plant operational situations or in case of high sensi- tivity of the receiving ecosystems. The Activated Sludge Model No.3 (ASM3) was therefore extended for two-step nitrification and two-step denitrification in order to better describe nitrite dynamics especially during the treatment of communal wastewater. Nitrite was included as a new model compound and as an intermediate product of biological processes, both for heterotrophic and autotrophic bacteria. Two new model compounds replace XA, the original autotrophic biomass: Ammonium Oxidizing Bacteria, XAOB and Nitrite Oxidizing Bacteria, XNOB. Growth and decay processes of nitrifiers were split into AOB and NOB processes (3 additional processes) and heterotrophic anoxic processes were also doubled in order to account for two-step denitrification (4 additional processes). Default values from literature as well as laboratory measurements were considered for the choice of kinetic and stoichiometric parameters. The model was calibrated and validated with laboratory scale tests in batch reactors and with data from an Eawag activated sludge pilot plant configured conventionally with nitrification and pre-denitrification for the treatment of communal wastewater.
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Hug, T., Benedetti, L., Hall, E.R., Johnson, B.R., Morgenroth, E.F., Nopens, I., Rieger, L., Shaw, A.R. and Vanrolleghem, P.A. (2009). Mathematical models in teaching and training: mismatch between education and requirements for jobs. Water Sci. Technol., 59(4), 745-753.

Abstract: As mathematical modeling of wastewater treatment plants has become more common in research and consultancy, a mismatch between education and requirements for model-related jobs has developed. There seems to be a shortage of skilled people, both in terms of quantity and in quality. In order to address this problem, this paper provides a framework to outline different types of model-related jobs, assess the required skills for these jobs and characterize different types of education that modelers obtain “in school” as well as “on the job”. It is important to consider that education of modelers does not mainly happen in university courses and that the variety of model related jobs goes far beyond use for process design by consulting companies. To resolve the mismatch, the current connection between requirements for different jobs and the various types of education has to be assessed for different geographical regions and professional environments. This allows the evaluation and improvement of important educational paths, considering quality assurance and future developments. Moreover, conclusions from a workshop involving practitioners and academics from North America and Europe are presented. The participants stressed the importance of non-technical skills and recommended strengthening the role of realistic modeling experience in university training. However, this paper suggests that all providers of modeling education and support, not only universities, but also software suppliers, professional associations and companies performing modeling tasks are called to assess and strengthen their role in training and support of professional modelers.

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Park, H., Rosenthal, A., Deur, A., Beckmann, K., Ramalingam, K., Fillos, J. and Chandran, K. (2009). Molecular based characterization of the microbial ecology and activity of anammox bioreactors. Proceedings WEFTEC.09, Orlando, FL, USA.

Abstract: Anaerobic ammonium oxidation (anammox) is the biological oxidation of ammonia using nitrite as an electron acceptor to produce dinitrogen gas. Anammox requires 60% less oxygen and no external organic carbon compared to conventional biological nitrogen removal (BNR). Despite these significant advantages, full-scale installations of anammox are uncommon primarily owing to the lack of well established process monitoring and control strategies that result in stable anammox reactor performance. The primary aim of this study was to determine the impact of long term operation with actual anaerobic digestion centrate from a full-scale operational wastewater treatment facility on the community diversity of an anammox bioreactor. A secondary aim was to develop and apply molecular biomarkers based on structural and functional genes to permit inference of anammox population activity. Over long-term operation, bacteria related to C. Brocadia sp. 40 were consistently the dominant anammox bacteria present in the anammox reactor. There was also a marked shift in biomass morphology from discrete cells to granular clusters. This morphological shift was paralleled by a shift also to more stable nitrogen removal performance in general and the succession and establishment of bacteria related to the Chlorobi-Bacteroidetes superfamily. There was good correspondence between anammox reactor nitrogen removal performance and anammox bacterial concentrations. Biomarkers for anammox activity based on structural (16S-23S rRNA intergenic spacer region, ISR) and functional (hydrazine oxygenase, hzo) genes were developed and compared. Based on responses to exposure to oxygen and nitrite mediated inhibition and correlation with nitrogen removal performance, hzo expression was a more sensitive indicator of anammox activity than ISR.

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Vallet, B., Labelle, M.-A., Rieger, L., Bigras, S., Parent, S., Juteau, P., Villemur, R. and Comeau, Y. (2009). Inhibition of biological phosphorus removal in a sequencing moving bed biofilm reactor in seawater. Water Sci. Technol., 59(6). 1101-1110.

Abstract: A new process was developed to achieve denitrifying biological phosphorus removal in wastewaters containing high levels of nitrate and phosphate with a low level of organic matter. This could particularly be useful in recirculating systems such as aquariums or fish farms to prevent accumulation of nitrate and phosphates and to avoid regular cost extensive and polluting water replacement. Phosphorus (P) was removed from the influent in a sequencing moving bed biofilm reactor, stored in the attached biomass and then cyclically removed from the biomass by filling the reactor with anaerobic water from a stock tank. Phosphate was accumulated in the stock tank which allowed for use as fertilizer. The feasibility of the experimental design was demonstrated by using the activated sludge model No. 3 (ASM3) complemented by the EAWAG Bio-P module implemented in the WEST simulation software. A pilot scale experiment was conducted in two identical reactors in two runs: one to treat water from a marine mesocosm, the other to treat a synthetic freshwater influent. No biological phosphorus removal was achieved during the seawater run. During the freshwater run, average P removal efficiency was 20%, of which 80% was attributed to biological removal and 20% to chemical precipitation. The absence of efficiency in seawater was attributed to the high concentration of calcium.
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Hauduc, H., Gillot, S., Rieger, L., Ohtsuki, T., Shaw, A., Takács, I. and Winkler, S. (2009). Activated sludge modelling in practice – An international survey. Water Sci. Technol., 60(8), 1943-1951.

Abstract: The Good Modelling Practice Task Group (GMP-TG) of the International Water Association (IWA) is developing guidelines for the use of Activated Sludge Models (ASM). As part of this work the group created and sent out a questionnaire to current and potential activated sludge model users in 2007. The objectives of the questionnaire were (i) to better define the profile of ASM users, (ii) to identify the tools and procedures that are actually used and (iii) to highlight the main limitations while building and using ASM-type models. Ninety-six answers were received from all over the world, from several types of organisation. The results were analysed to identify the modellers’ perceptions of models depending on their profile. The results also highlighted the main topics of interest for improving modelling procedures which are standardisation of the available modelling guidelines and better experience and knowledge transfer.
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Belia, E., Amerlinck, Y., Benedetti, L., Johnson, B., Sin G., Vanrolleghem, P.A., Gernaey, K.V., Gillot, S., Neumann, M.B., Rieger, L., Shaw, A. and Villez, K. (2009). Wastewater treatment modelling: dealing with uncertainties. Water Sci. Technol., 60(8), 1929-1941.

Abstract: This paper serves as a problem statement of the issues surrounding uncertainty in wastewater treatment modelling. The paper proposes a structure for identifying the sources of uncertainty introduced during each step of an engineering project concerned with model-based design or optimisation of a wastewater treatment system. It briefly references the methods currently used to evaluate prediction accuracy and uncertainty and discusses the relevance of uncertainty evaluations in model applications. The paper aims to raise awareness and initiate a comprehensive discussion among professionals on model prediction accuracy and uncertainty issues. It also aims to identify future research needs. Ultimately the goal of such a discussion would be to generate transparent and objective methods of explicitly evaluating the reliability of model results, before they are implemented in an engineering decision-making context.
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Sin, G., Kaelin, D., Kampschreur, M.J., Takács, I., Wett, B., Gernaey, K.V., Rieger, L., Siegrist, H. and van Loosdrecht, M.C.M. (2008). Modelling nitrite in wastewater treatment systems: A discussion of different modelling concepts. Water Sci. Technol., 58(6), 1155-1171.

Abstract: Originally presented at the 1st IWA/WEF Wastewater Treatment Modelling Seminar (WWTmod 2008), this contribution has been updated to also include the valuable feedback that was received during the Modelling Seminar. This paper addresses a number of basic issues concerning the modelling of nitrite in key processes involved in biological wastewater water treatment. To this end, we review different model concepts (together with model structures and corresponding parameter sets) proposed for processes such as two-step nitrification/denitrification, anaerobic ammonium oxidation and phosphorus uptake processes. After critically discussing these models with respect to their assumptions and parameter sets, common points of agreement as well as disagreement were elucidated. From this discussion a general picture of the state-of-the-art in the modelling of nitrite is provided. Taking this into account, a number of recommendations are provided to focus further research and development on nitrite modelling in biological wastewater treatment.
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Kaelin, D., Rieger, L., Eugster, J., Rottermann, K., Bänninger, C. and Siegrist H. (2008). Potential of in-situ sensors with ion-selective electrodes for aeration control at wastewater treatment plants. Water Sci. Technol., 58(3), 629-637.

Abstract: A pilot-scale activated sludge wastewater treatment plant (WWTP) operated with nitrification and pre-denitrification was monitored with a set of on-line sensors for over 3 years. Wet-chemistry ex-situ analyzers, UV and UV-Visible in-situ sensors and in-situ sensors based on ion-selective electrodes (ISE) were used. New ISE sensors for ammonium, nitrate and nitrite, adapted to water and wastewater matrices, have been released in recent years, With adequate quality control they proved to be highly accurate and reliable in WWTP influents and activated sludge (AS) reactors even at the end of the biological treatment zone, working at low ammonium concentrations (1–2 mgN/l). The ammonium measurement was used to test several feed-forward and feed-back aeration control strategies. The first aim was to keep inorganic nitrogen compounds, i.e. ammonium, nitrate and particularly nitrite, as low as possible in the effluent, and within Swiss national standards (<2.0mgNH4-N/l, <0.3mgNO2-N/l, 24h average). All the strategies were successful at keeping ammonium low and subsequently at gaining denitrification capacity to significantly reduce the total nitrogen discharge. Some control strategies however generated temporary peaks of ammonium or even accumulation of nitrite.
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Miletic I., Boudreau, F., Dudzic, M., Kotuza, G., Ronholm, L., Vaculik, V. and Zhang, Y. (2008). Experiences in applying data-driven modeling technology to steelmaking processes. Canadian Journal of Chemical Engineering, 86, 937-946.

Abstract: Experience has shown that data-driven modelling methods are useful for improving steelmaking processes. In particular, principal components analysis and partial least squares are well-suited for industrial implementation because they address practical issues such as colinearity and missing data. In the course of applying these multivariate methods on-line, a need for a flexible computer infrastructure to better support data handling and model implementation was identified and met with an internally developed software calculation platform. Multivariate methods have been found useful for monitoring and for prediction and can also be applied as a foundation for other methods such as optimization.

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Rieger, L., Langergraber, G., Kaelin, D., Siegrist, H. and Vanrolleghem, P.A. (2008). Long-term evaluation of a spectral sensor for nitrite and nitrate. Water Sci. Technol., 57(10), 1563–1569.

Abstract: A spectral in-situ UV sensor was investigated to measure nitrite and nitrate concentrations in the effluent of the EAWAG pilot-scale plant. The sensor was used with a calibration that was based on data from another WWTP and was operated over a period of 1.5 years. The results showed constant accuracy although the sensor was operated with minimal maintenance (manual cleaning once a month). It could be shown that the sensor was able to accurately predict the nitrite and nitrate concentration with a precision of 0.32mgN/l (95%prediction interval at mean lab value of 1.15mgN/l) and 1.08mg N/l (at 5.55mgN/l) for nitrite and nitrate, respectively. The UV sensor showed good results for nitrite in the low concentration range and very accurate results for higher concentrations (up to 10mgN/l). This allows using the sensor for alarm systems as well as for control concepts at WWTPs.
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Rieger, L. and Vanrolleghem, P.A. (2008). monEAU: A platform for water quality monitoring networks. Water Sci. Technol., 57(7), 1079-1086.

Abstract: Continuous monitoring of water quality creates huge amounts of data and therefore requires new concepts to guarantee high data quality and to prevent data graveyards. Monitoring stations commonly used in practice today suffer from insufficient flexibility and a lack of standardization. That is, although a lot of monitoring tasks are comparable and should lead to robust and powerful platforms, most monitoring stations are case specific developments. In this paper the underlying ideas of a new generation of monitoring networks is described.
First a problem analysis of monitoring stations typically seen in current river monitoring practice is outlined, then the monEAU vision on monitoring networks will be discussed together with an overview of a planned system set-up with innovative data evaluation concept.
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Rosen, C., Rieger L., Jeppsson, U. and Vanrolleghem, P.A. (2008). Adding realism to simulated sensors and actuators. Water Sci. Technol., 57(3), 337-344.

Abstract: In this paper, we propose a statistical theoretical framework for incorporation of sensor and actuator faults in dynamic simulations of wastewater treatment operation. Sensor and actuator faults and failures are often neglected in simulations for control strategy development and testing, although it is well known that they represent a significant obstacle for realising control at full-scale facilities. The framework for incorporating faults and failures is based on Markov chains and displays the appealing property of easy transition of sensor and actuator history into a model for fault generation. The paper briefly describes Markov theory and how this is used together with models for sensor and actuator dynamics to achieve a realistic simulation of measurements and actuators.
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Miletic, I., Garbaty, R., Waterfall, S. and Mathewson, M. (2007). Model based optimization of scrap steel purchasing. Proceedings of the International Federation of Automation and Control (IFAC), Mining, Metals, and Metallurgy Symposium, August 2007, Quebec City, Canada.

Abstract: Scrap steel and iron ore are important feed stocks in the production of steel. Scrap steel is available on the open market as a commodity from various suppliers. The price and quality of scrap steel fluctuate and therefore require the periodic determination of the relative usage rate for each batch of steel made in a particular period of time. The determination of the usage rate results in an optimization programming problem that seeks to minimize scrap purchase costs. The solution of the programming problem should indicate which scrap supplier to purchase from, what scrap type to use, and in what lot quantities, in order to fill customer orders and to maintain desired inventory levels for the steel producer.

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Miletic, I., Garbaty, R., Waterfall, S. and Mathewson, M. (2007). Optimizing scrap blending with uncertainty. McMaster Advanced Control Consortium 2007 Annual Meeting, Hamilton, Ontario, Canada, May 2007.

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Rieger, L., Langergraber, G. and Siegrist, H. (2006). Uncertainties of spectral in-situ measurements in wastewater using different calibration approaches. Water Sci. Technol., 53(12), 187-197.

Abstract: Three calibration methods were applied to UV/VIS spectra recorded in the influent of six wastewater treatment plants (WWTPs) to measure total COD (CODtot), filtered COD (CODfil), nitrate and nitrite nitrogen (NOx-N) and total suspended solids (TSS). It could be shown that a calibration of the sensor using data sets from four Swiss WWTPs leads to an improvement of the precision in comparison to the global calibration provided by the manufacturer. A calibration to the specific wastewater matrix always improves the results and gives the highest accuracy. For CODtot a mean coefficient of variation CVx of 12.5% could be reached, whereas for NOx-N only weak results were achieved (average CVx = 36%).
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Rieger, L., Alex, J., Gujer, W. and Siegrist, H. (2006). Modelling of Aeration Systems at Wastewater Treatment Plants. Water Sci. Technol., 53(4-5), 439-447.

Abstract: A model for the response time of aeration systems at WWTPs is proposed. It includes the delays caused by the air supply system (consisting of blowers, throttles and pipes), the rise time of the air bubbles and all control loops except the master DO controller. Beside a description of the required step-change experiments, different approaches for model calibration are given depending on the available data. Moreover, the parameters for the oxygen transfer and the response time of the aeration system model are not clearly identifiable. The model can be used for simulation studies which compare different types of controllers under changing loading and process conditions. The results from full-scale experiments at three different plants show that the response times of the aeration systems are in the range of 4–5min. Taking all processes and time constants into account, some 30min are needed to reach a new steady state after a step change of the airflow rate.
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Copp, J.B., Belia, E., Snowling, S. and Schraa, O. (2005). Anaerobic digestion: a new model for plant-wide wastewater treatment process modeling. Water Sci. Technol., 52(10-11), 1-11.

Abstract: This paper describes a new anaerobic digestion model for wastewater treatment systems (MantisAD). The model has been developed specifically for plant-wide wastewater treatment modelling. That is, rather than modelling nitrogen as a series of fractions of other carbonaceous state variables, this new model includes six dedicated nitrogen state variables. This structure makes this new anaerobic model easier to incorporate into plant-wide models by simplifying the aerobic/anaerobic model interfaces. The model is complete and initial success with the model has been achieved. A comprehensive description of the model including a Petersen Matrix representation of the model is included as is a case study in which the model is applied to full-scale anaerobic digesters.
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Rieger, L., Thomann, M., Gujer, W. and Siegrist, H. (2005). Quantifying the uncertainty of on-line sensors at WWTPs during field operation. Water Res. 39(20), 5162-5174.

Abstract: It remains an ongoing task to quantify the uncertainty of continuous measuring systems at WWTPs during field operation. The commonly used methods are based on lab experiments under standardized conditions and are only suitable for characterizing the measuring device itself. For measuring devices under field conditions, a knowledge of the response time, trueness and precision is equally important. A method is proposed which can be used to characterize newly installed on-line sensors or to evaluate monitoring
data which may contain systematic errors. The concept is based on comparative measurements between the sensor and a reference. A linear regression is used to differentiate between trueness and precision. Various statistical tests are conducted to validate the preconditions of linear regression. The information about the trueness and precision of the measuring system under field conditions helps to adapt control strategies more effectively to the relevant processes and permits sophisticated control concepts. Moreover, the concept can help to define guidelines for evaluating the uncertainties of effluent quality monitoring to overcome the concerns about on-line sensors, improve the trust in these systems and to allow the use of continuously measuring systems for legislative purposes. The approach is discussed in detail in this paper and all statistical tests and formulas are listed in the Appendix.
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Schraa, O., Tole, B., and Copp, J.B. (2005). Fault Detection for Control of Wastewater Treatment Plants. Water Sci. Technol., 53(4-5), 375-382.

Abstract: Interest in real-time model-based control is increasing as more and more facilities are being asked to meet stricter effluent requirements while at the same time minimizing costs. It has been identified that biological process models and automated process control technologies are being used at wastewater treatments plants throughout the world and that great potential for optimising biotreatment may exist with the integration of these two technology areas. According to our experience, wastewater treatment plants are indeed looking for ways to successfully integrate their modelling knowledge into their process control structure; however, there are practical aspects of this integration that must be addressed if the benefits of this integration are to be realised. This paper discusses the practical aspects of monitoring, filtering and analysing real sensor data with an aim to produce a reliable real-time data stream that might be used within a model-based control structure. Several real case study examples are briefly discussed in this paper.

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Schraa, O., Belia, E. and Churn, C.C. (2005). Simulating ‘What Ifs’ at an Industrial Facility. Industrial Wastewater, 4(3).

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Langergraber, G., Rieger, L., Winkler, S., Alex, J., Wiese, J., Owerdieck, C., Ahnert, M., Simon, J. and Maurer, M. (2004). A guideline for simulation studies of wastewater treatment plants. Water Sci. Technol., 50(7), 131-138.

Abstract: A guideline for simulation studies of wastewater treatment plants is proposed. The aim of the
HSG-guideline is to define a reference quality level that helps to make the results of simulation studies
comprehensible and comparable and therefore increases the quality and reliability of mathematical modelling in wastewater treatment. The paper gives a summary of the HSG-guideline, written by a group of university members from Germany, Austria and Switzerland.

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Miletic I., Quinn, S., Dudzic, M., Vaculik, V. and Champagne, M. (2004). An industrial perspective on implementing on-line applications of multivariate statistics. Journal of Process Control, 14(8), 821-836.

Abstract: Multivariate statistics (MVS) has enjoyed popularity in the applied science literature over the last decade. It has also been well received by industrial practitioners; however, industrial applications have had mixed results. In this paper, we focus on our experiences in developing multivariate statistical systems for industrial use. From these experiences, we identify a methodology for developing useful, long-standing industrial applications. We highlight features we feel are important in the successful development of on-line MVS applications, both technical and non-technical. Specifically, we focus on on-line systems for manufacturing environments. Many of the applications discussed grew out of industry-university collaborations. To end the paper, we recommend topics open for further academic research with an industrial focus.

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Rieger, L., Langergraber, G., Thomann, M., Fleischmann, N. and Siegrist, H. (2004). Spectral in-situ analysis of NO2, NO3, COD, DOC and TSS in the effluent of a WWTP. Water Sci. Technol., 50(11), 143-152.

Abstract: An in-situ UV spectrometer was applied to the effluent of a WWTP in Switzerland and calibrated using a multivariate calibration algorithm based on PLS regression. Except for nitrite, the calibration was based on comparative measurements of the effluent in the plant laboratory. Samples made of stock solution added to three different matrices prepared in the EAWAG laboratory were used for the nitrite calibration because the effluent concentrations were always in the range of 0.06–0.26 mg/l. The results show very good precision for nitrite and nitrate. The measuring range for COD and DOC was not completely covered by the measurements, so the meaningfulness of the results is limited. Nevertheless the precision obtained for soluble COD is high enough for most applications at WWTPs. The accuracy of the TSS measurement is unsatisfactory as regards effluent limits since the spectrometer used does not cover the wavelength region up to 700 nm, which gives better signals for TSS calibration due to its strong correlation with turbidity.

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Rieger, L., Thomann, M., Joss, A., Gujer, W. and Siegrist, H. (2004). Computer aided monitoring and operation of continuously measuring devices. Water Sci. Technol., 50(11), 31-39.

Abstract: Extended studies of measuring and control systems in activated sludge plants at EAWAG revealed that the measuring devices remain the weakest point in control applications. To overcome this problem, a software package was developed which analyses and evaluates the residuals between a reference measurement and the sensor and collects the information in a database. The underlying monitoring concept is based on a two-step evaluation of the residuals by means of statistical evaluations using control charts with two different sets of criteria. The first step is a warning phase in which hints on probable errors trigger an increase in the monitoring frequency. In the second step, the alarm phase, the error hypothesis has to be validated and should allow immediate and targeted reactions from the operator. This procedure enables an optimized and flexible monitoring effort combined with an increased probability of early detection of systematic measuring errors. Beside the monitoring concept, information about the measuring device, the performed servicing actions and the responsibilities is stored. Statistical values for the quantitative characterization of the measuring system during operation will be given. They are needed to parameterise controllers or to guarantee the accuracy of the instrument in order to allow reliable calculations of effluent tax. In contrast to other concepts, not only is the measuring device examined under standard conditions, but so is the entire measuring chain from the liquid to be analysed to the value stored in the database of the supervisory system. The knowledge of the response time of the measuring system is then required in order to allow a comparison of the corresponding values.

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Siegrist, H., Rieger, L., Fux, C. and Wehrli, M. (2004). Improvement of nitrogen removal at WWTP Zürich-Werdhoelzli after connection of WWTP Zürich-Glatt. Water Sci. Technol., 50(7), 35-43.

Abstract: Optimisation of nitrifying activated sludge plants towards nutrient removal (denitrification and enhanced P-removal) leads to a substantial reduction of operating costs and improves effluent and operating conditions. At WWTP Zürich-Werdhöelzli, initially designed for nitrification only, an anoxic zone of 28% of total activated sludge volume was installed and allowed 60% nitrogen elimination besides several other optimisations. In 2001 the operation of WWTP Zürich-Glatt was stopped and the wastewater was connected to WWTP Werdhöelzli. To improve nitrogen removal, WWTP Werdhöelzli co-financed two research projects; one for separate digester supernatant treatment with the anammox process operating two SBRs in series and the other applying NH4 sensors for aeration control in order to decrease energy consumption and raise effluent quality. The results of both projects and the consequences for WWTP Werdhöelzli are discussed in this paper.

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Winkler, S., Rieger, L., Pressl, A., Gruber, G. and Saracevic, E. (2004). Application of ion-sensitive sensors in water quality monitoring. Water Sci. Technol., 50(11), 105-114.

Abstract: Within the last years a trend towards in-situ monitoring can be observed, i.e. most new sensors for water quality monitoring are designed for direct installation in the medium, compact in size and use measurement principles which minimise maintenance demand. Ion-sensitive sensors (Ion-Sensitive- Electrode – ISE) are based on a well known measurement principle and recently some manufacturers have released probe types which are specially adapted for application in water quality monitoring. The function principle of ISE-sensors, their advantages, limitations and the different methods for sensor calibration are described. Experiences with ISE-sensors from applications in sewer networks, at different sampling points within wastewater treatment plants and for surface water monitoring are reported. An estimation of investment and operation costs in comparison to other sensor types is given.

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Rieger, L., Alex, J., Winkler, S., Boehler, M., Thomann, M. and Siegrist, H. (2003). Progress in sensor technology – progress in process control? Part I: Sensor property investigation and classification. Water Sci. Technol., 47(2), 103-112.

Abstract: To ensure correctly operating control systems, the measurement and control equipment in WWTPs must be mutually consistent. The dynamic simulation of activated sludge systems could offer a suitable tool for designing and optimising control strategies. Ideal or simplified sensor models represent a limiting factor for comparability with field applications. More realistic sensor models are therefore required. Two groups of sensor models are proposed on the basis of field and laboratory tests: one for specific sensors and another for a classification of sensor types to be used with the COST simulation benchmark environment. This should lead to a more realistic test environment and allow control engineers to define the requirements of the measuring equipment as a function of the selected control strategy.

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Alex, J., Rieger, L., Winkler, S. and Siegrist, H. (2003). Progress in sensor technology – progress in process control? Part II: results from a simulation benchmark study. Water Sci. Technol., 47(2), 113-120.

Abstract: To show the impact of sensor behaviour on the control result, four strategies for aeration control are tested using different sensor characteristics. It is demonstrated, on the one hand, how an increasing response time will limit the achievable control quality and, on the other hand, how a given sensor characteristic can be taken into account for the controller design. The presented tests show that an improvement potential by control for WWTPs is available but this potential is limited compared to proper DO control with fixed set-points. To activate this control potential, sufficient control authority must be available and a careful control design is required. It can be shown that using feedback control, sensors with a small response time have significant advantages compared to conventional sensors. Using feed forward control, the improvement potential by control is considerably higher and additionally, the sensor delay can be integrated into the controller design. The presented discussion is based on simulation studies performed on a standardised benchmark case. For these tests it was necessary to include sensor models into the simulation model. It can be stated that the usage of sensor models is necessary for the application of dynamic simulation for the design and evaluation of WWTP control and in general to achieve realistic results.

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Dudzic, M. and Miletic, I. (2002). Multivariate statistical monitoring of a continuous slab caster. Proceedings of the 2002 American Control Conference, Anchorage, Alaska, USA, May 2002.

Abstract: Principal components analysis (PCA) is a multivariate statistical (MVS) technology that can be applied in process monitoring and fault diagnosis. Dofasco has developed on-line multivariate statistical process control (SPC) monitoring systems for both its #1 and #2 continuous casters (#1CC & #2CC) based on this technology. The paper introduces the reader to the basic process of steel casting and the motivation for MVS modeling. As well, the paper highlights various experiences involved in implementing on-line multivariate monitoring strategies and resulting outcome.

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Miletic, I., Hough, M.J., Quinn, S., Vaculik, V. and Dudzic, M. (2002). Industrial experience with maintenance of on-line multivariate statistical applications. Presented at the I.E.E.E. Industry Applications Society: Advanced Process Control Applications for Industry Workshop, Vancouver British Columbia, Canada.

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Rieger, L., Siegrist, H., Winkler, S., Saracevic, E., Votava, R. and Nadler, J. (2002). In-situ measurement of ammonium and nitrate in the activated sludge process. Water Sci. Technol., 45(4-5), 93-100.

Abstract: A new in-situ probe is presented for the continuous measurement of ammonium and nitrate in wastewater. It requires no sample preparation and is installed directly in the process liquid. This new low- cost probe significantly reduces investment and operating costs and requires minimum maintenance. The paper describes the sensor principle and test results from three different probe locations: the primary clarifier effluent, the activated sludge tank and the nitrifying biofilter influent. Reference measurements were carried out by means of conventional analyzers with ultrafiltration, an in-situ UV spectrometer for the nitrate and laboratory analysis of spot and 2h-composite samples. The aim of the study was to investigate the operational reliability and accuracy of the new probe and the expenditure required for its maintenance and calibration. The tests showed that the new probe performed very well overall and required minimum maintenance. Some problems were observed during the biofilter plant test. They are assumed to be related to substantial changes in the wastewater composition.

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Siegrist, H., Rieger, L., Koch, G., Kühni, M. and Gujer, W. (2002). The EAWAG Bio-P module for activated sludge model No. 3. Water Sci. Technol., 45(6), 61-76.

Abstract: An additional module for the prediction of enhanced biological phosphorus removal is presented on the basis of a calibrated version of ASM3. The module uses modified processes from ASM2d but neglects the fermentation of readily degradable substrate. Biomass decay is modeled in the form of endogenous respiration as in ASM3. The glycogen pool and biologically induced P-precipitation is not taken into account. The module was systematically calibrated with experimental data from various batch experiments, a full-scale WWTP and a pilot plant treating Swiss municipal wastewater. A standard parameter set allowed all data to be simulated.

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Thomann, M., Rieger, L., Frommhold, S., Siegrist, H. and Gujer, W. (2002). An efficient monitoring concept with control charts for on-line sensors. Water Sci. Technol., 46(4-5), 107-116.

Abstract: A monitoring concept for on-line sensors will be discussed which helps the WWTP staff to detect drift-, shift- and outlier effects as well as unsatisfactory calibration curves. The approach is based on the analysis of comparative measurements between the sensor and a reference method. It combines statistical analysis such as control charts and regression analysis with decision support rules. The combination of two different detection levels in the selected Shewhart control charts with additional criteria allows one to detect “out-of-control” situations early with an optimized measurement effort. Beside the statistical analysis the concept supports the operator with a graphical analysis to monitor the accuracy of on-line measurements efficiently. The widely applicable monitoring concept will be illustrated with examples for an ion-sensitive NH4+- and a MLSS-sensor.

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Koch, G., Kühni, M., Rieger, L. and Siegrist, H. (2001). Calibration and validation of an ASM3-based steady-state model for activated sludge systems – part II: prediction of phosphorus removal. Water Res., 35(9), 2246-2255.

Abstract: An ASM3-based steady-state model which can be used for estimating the average nitrogen- removal, sludge-production and phosphorus-removal rates of different biological phosphorus-removing systems (AAO, UCT, intermittent processes) is developed. It considers the wastewater composition, the oxygen and nitrate input in the anaerobic compartment and the interaction between biological phosphorus removal and denitrification for different operating conditions. The model is calibrated and validated with data from a number of long-term pilot and full-scale experiments for Swiss municipal wastewater. The steady-state model is adequate for a comparison of different BPR process configurations or for a first estimation of the nutrient-removal efficiency. It allows the plant performance and key parameters to be determined very quickly. Excel spreadsheets of the model for different flow schemes are available from the corresponding author.

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Vaculik, V., MacCuish, R.B., Dudzic, M.S., Miletic, I., Hough, M.J. and Smith, K.B. (2001). Innovative monitoring technology improves caster operation at Dofasco. Iron and Steel Maker, September 2001

Abstract: The Process Automation group at Dofasco successfully developed an improved system for online fault detection. This system provides better protection from non-sticker-type breakouts, has contributed to a lower number of breakouts per year and has raised pulpit operator confidence in operating the caster.

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Rieger, L., Koch, G., Kühni, M., Gujer, W. and Siegrist, H. (2001). The EAWAG Bio-P module for activated sludge model No. 3. Water Res., 35(16), 3887-3903.

Abstract: An additional module for the prediction of enhanced biological phosphorus removal is presented on the basis of a calibrated version of ASM3. The module uses modified processes from ASM2d but neglects the fermentation of readily degradable substrate. Biomass decay is modeled in the form of endogenous respiration as in ASM3. Moreover, an additional glycogen pool and biologically induced P-precipitation were not taken into account. The module was systematically calibrated with experimental data from various batch experiments, a full-scale WWTP and a pilot plant treating Swiss municipal waste water. A standard parameter set allowed all data to be simulated.

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Miletic, I.P. and Marlin, T.E. (1998). On-line statistical results analysis in real-time operations optimization. Industrial & Engineering Chemistry Research, 37(9), 3670-3684.

Abstract: This paper presents methods for the statistical analysis of plant operations optimization results with special consideration for real-time optimization (RTO) applications. The key challenge is to determine whether the results of an optimization calculation should be implemented in the plant. Since feedback data used to correct the model include noise and the effects of high-frequency disturbances, the results of the model-based optimization calculations are corrupted by a stochastic component. The methods developed in this paper apply multivariable statistical hypothesis tests based on control charts in order to distinguish between high-frequency disturbances propagated through the calculations and significant changes in the plant optimization variables with the goals of reducing the frequency of unnecessary changes in the implemented independent optimization variables and increasing plant profits. Only the statistically significant results are implemented in the plant. Case studies indicate that increased profit can be obtained by implementing fewer changes to the process because the preponderance of changes due to noise are rejected whereas most meaningful changes are implemented.

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Miletic, I.P. and Marlin, T.E. (1998). Results diagnosis for real-time process operations optimization. Computers & Chemical Engineering, 22, 475-482.

Abstract: Past work has shown that results analysis based on knowledge of the common cause measurement variability and the transmission of this variability in the real-time optimization (RTO) loop to the recommended operating conditions can be applied to an RTO system to increase operating profit and to decrease unnecessary operations changes. An extension to results analysis which detects and diagnoses the cause of ill-conditioning in parameter updating, via singular value decomposition, is developed here and is applied to a case study. The case study has shown that it is possible to evaluate conditioning on-line. With an appropriate selection of an updater control limit, a hypothesis test can prevent ill-conditioned parameter estimates from being used for plant optimization.

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Schraa, O. and Crowe, C.M. (1998). The numerical solution of bilinear data reconciliation problems using unconstrained optimization methods. Comput. & Chem. Eng., 22(9), 1215-1228.

Abstract: A new approach to solving steady-state data reconciliation problems with bilinear constraints is proposed. Previously it was shown by Crowe that these problems can be solved using the method of matrix projection. In this work the objective function and its constraints are put into unconstrained form using Lagrange multipliers. Unconstrained optimization methods based on analytical derivatives are then used to solve the unconstrained formulation. The unconstrained approach is tested on two different reconciliation problems from the literature using number of Newton-type unconstrained optimization methods. The unconstrained methods are compared in terms of robustness and efficiency in solving the example problems. The performance of the unconstrained approach is compared to that of the method of matrix projection and the projected Lagrangian algorithm implemented in GAMS/MINOS.

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Miletic, I.P. and Marlin, T.E. (1996). Results analysis for real-time optimization: Deciding when to change plant operation. Computers & Chemical Engineering, 20, 1077-1082.

Abstract: After the automation goals of safety, product quality and production rate have been achieved, opportunity exists in many plants to further increase profit by adjusting selected operating variables. When external conditions change frequently, incentive exists for the optimization to be performed in real time. However, real-time optimization might change conditions too frequently when responding to measurement noise and high frequency stationary disturbances. A results analysis method is proposed to decide when a significant change in plant operation has been proposed and should be implemented. The method is based on a knowledge of the common cause measurement variability and the transmission of this variability in the RTO loop to the recommended operating conditions. The basic expressions for the results analysis method are developed, and sample results from a case study are presented. Extensions are also briefly discussed.

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