Innovative Solution

WWTP, Performance evaluation, Control, Optimization, Simulation

technical

Wastewater treatment plants (WWTPs) in Europe, in industrialized countries and increasingly in developing countries are under a high pressure regarding treatment efficiency, operational costs and environmental impact (e.g. CO₂-footprint). The developed method combines state- of-the-art methods for simplified simulation studies and for using simulation to evaluate control concepts regarding functional, economic and environmental criteria.

Starting point of the analysis is a cost-effective set up of a reliable simulation model of the existing plant with sufficient accuracy and using essentially only operational data and routine lab samples. This step is based on the results of the HSG group (www.hsgsim.org, Alex et al. 2007).

The second step consists of the workshop-based development of a catalog of potentially beneficial measures to improve treatment plant performance. The measures are ranked regarding the effort necessary to apply the respective measure. Typically, operational measures are easier to adopt then structural changes in the treatment plant layout. Starting point of the workshop discussions are a portfolio of default measures (defined e.g. in state-of-the-art documents such as DWA-M 268 etc.).

The third step is logically the implementation of the measures into variants of the WWTP simulation model. Models realistically describing sensors and actuators require special attention.

In a following step (four), the measures are simulated and evaluated regarding treatment performance and cost effectiveness. The set-up of measures and performance evaluation approach is inspired by the results of the IWA Task Group on “Benchmarking of Control Strategies for WWTPs”.

After selection of feasible operational measures, these have to be implemented into the real-world plant automation system. In order to support this step, the used simulation system allows to describe simulated controllers using an industry standard programming language for controllers (PLCs: IEC 61131 ST).

The proposed solution was successfully applied to several WWTPs in Germany in close cooperation with different consultancies and plant operators. The method can be applied to activated sludge plants with nitrification/denitrification. Vast experience exists for European conditions (climate and legislative), but application to other climates is also possible.

The methodology was developed by the group “Environmental Software & Control” at ifak e.V. Magdeburg, Germany (www.ifak.eu, simba.ifak.eu). But each successful application requires a close cooperation with consultancies and operators of the respective WWTPs. Projects will be initiated by plant operating organizations via specialized consultancies. Ifak offers the service described, but the methodology will be also be published and can be utilized by all experienced users of dynamic simulation systems. Users of the simulation system developed by ifak (SIMBA) usually benefit first from the latest results of this methodology.

Parts of the method were applied in the past to different WWTP analysis and improvement projects (e.g. Alex et al 2009, Baumann, Alex 2003). Many important components of the methodology were developed in the framework of international working groups (HSG (www.hsgsim.org), IWA Task Group (www.benchmarkwwtp.org)). Final results of these working groups (reports) are expected to be published in the near future. But as the main components are already available, the proposed method can be considered as ready to use. More applications will hopefully be published in the future by different actors (simulation experts, consultants, WWTP operators).

The proposed method will allow giving answers to questions such as:

• Is a plant achieving the maximum pollutant removal efficiency utilising the given tank capacities?
• Is it possible to reduce the effluent load of pollutants for certain load situations like during storm events?
• Is it possible by better control to improve the plant performance significantly without extending the plant? Can certain effluent requirements met by operational measures only?
• Can be a comparable plant performance achieved with a reduced energy consumption or using less chemicals?
• Is it possible to improve removal efficiency and operational costs at the same time and which are the Pareto-optimal operation states?
• What are suitable and cost effective combinations of operational and constructive measures to meet increasingly stricter requirements?
• Will a proposed control approach improve the performance of this plant?

Using the proposed methodology (HSG approach) to set up simulation models based on available online data and routine lab samples will reduce the effort for this step by one order of magnitude. The simulation model will allow not only to answer design questions but is very well suited to answer questions regarding the set-up and parameterization of the plant control system as well (integrated process design). Thus the model can be used for different purposes and during different phased during the life time of a WWTP. After the design and construction phase, it will serve as a test bed for permanent optimization of the performance and adaptation to changing load, changing requirements, climatic and economic conditions. Thus the significantly reduced effort to set up the model is combined with a multitude of benefits.

In order to monitor the effectiveness of the proposed method, it will be necessary to monitor WWTP plant performance using suitable performance indicators (removal efficiency, energy efficiency, economical efficiency etc.) and to keep track of measures applied at the plant during its life time (extensions, changing control structures, changing set-points). For a reliable evaluation of the effectiveness of certain measures, the proposed simulation method itself can be utilized. A comparison of measures under identical boundary conditions is possible using the proposed simulation models. This task has to be performed by the plant operators and can be periodically supported by consultancies.

The application of the method will help all plant operators of activated sludge plants with strict removal requirements and economical and environmental pressures to improve the plant performance. The method can be applied by potent operation organizations (large water companies or associations) themselves, which can afford to train simulation experts, or has to be applied by specialized consultants or experts. The required qualification level of the simulation experts can be reduced relying on the proposed simplified approach.

It is possible to set up simulation models of existing or planned treatment plants with a minimum level on required data providing a reasonable reliable picture of the plant performance. This set-up is based on standard assumption regarding the model parameters (activated sludge model, clarifier model) based on a broad commitment of experts (here the DWA A131 design guidelines). For the description of typical dynamic load patterns, on cen refer to size-dependent standard influent patterns (see e.g. Langergraber et al. 2008), thus avoiding measurement campaigns with a high temporal resolution – 1 day mixed samples instead of 2h mixed samples. Using the improved quality and cost effectiveness of modern online measurements, the data situation to validate the simulation model for existing plants can be further improved.

In order to describe potential control concepts planned for the new plant or planned to improve an existing plant, it is necessary to describe important aspects of the control system as well on an appropriate level. The proposals made by the IWA Task Group on “Benchmarking of Control Strategies for WWTPs” give indications for simple sensor and actuator models. If necessary, it is possible to describe the automation system and related aeration systems even in more detail, as e.g. presented in recent publications. Using these extensions of the WWTP model, a realistic test of the control concepts will be possible. The methodology can be used to analyze proposed control and operation concepts for specific plants under investigations but also for a generic analysis of options for classes of WWTPs. For example, in a recent initiative (DWA working group KA6.9) in Germany, potential measures to treat an increased wet weather load from combined sewers in treatment plants will be analyzed for a standard pre-denitrification plant. Also standard controllers to improve nitrogen removal can be analyzed (Alex, Jumar 2009) using a standardized model (BSM2, Jeppson et al. 2006).

A general experience regarding the setup of advanced control concepts for WWTPs is, that the control design requires some effort to fit the local situations (sensor locations, control structure, typical load situations, actuator dimensioning etc.)), and require safety concepts to detect equipment or process failures and to switch to alternative safe control schemes und requires a careful start-up and tuning phase. A significant part of these requirements can be supported using an existing simulation model. A very useful option to test a planned controller in depth before the actual installation at the plant is the implementation of the controller with as much as possible implementation details in a simulation environment. This allows testing und debug the control algorithm in any operational situation in a very short time. This allows finding implementation failures or not well functioning components before the implementation in the real world. A significant shortcutting of the plant start-up phase and improving the plant operation reliability can be achieved.

The proposed method is the compilation of components developed by different national and international expert groups and by ifak. Thus parts of the method were applied by ifak and also by other experts with success. The holistic approach described heree will be applied in probably all future simulation studies. Some examples have been reported by Hetschel, Alex 2010 and Baumann, Alex 2003.

Dr. Jens Alex

ifak – Institut fuer Automation und Kommunikation e.V. Magdeburg Werner-Heisenberg-Str. 1 D-39106 Magdeburg, Germany

Tel.: +49 391 9901469

Fax: +49 391 9901590 

mail: jens.alex@ifak.eu

Literature:

• DWA A131:  ATV-DVWK Arbeitsblatt A131: Bemessungen von einstufigen Belebungsanlagen ab 5000 EW. DWA, Hennef, Germany (2000).
• Alex, J. : A simple three-layer clarifier model, Watermatex 2011 in San Sebastian, Proceedings, pp 264-271 (2011)
• Alex, J., Hetschel, M. and M. Ogurek : Simulation study with minimised additional data requirements to analyse control and operation of WWTP Dorsten, Germany., Water Science & Technology Vol. 60 No.6, pp. 1371-1377 (2009)
• Alex, J., Jumar, U. : Benchmark-Modell für anlagenweite Kläranlagenregelungen., at-Automatisierungstechnik, Jahrgang 57 (2009) Heft 12, S. 625-637
• Alex, J., Wichern, M., Halft, N., Spering, V., Ahnert, M., Frehmann, T., Hobus, I., Langergraber, G., Plattes, M., Winkler, S. Woerner, D. 2007 A method to use dynamic simulation in compliance to stationary design rules to refine WWTP planning. Poster presentation at the 10th IWA Specialised Conference on Design, Operation and Economics of Large Wastewater Treatment Plants, 10–13 September 2007, Vienna. Austria
• Baumann, P., Alex, J. (2003): Ammoniumspitzen und deren Verminderung durch regelungstechnische Maßnahmen. ATV-Landestagung, Heilbronn
• Hetschel, M., Alex, J. : Analyse und Verbesserung des Kläranlagenbetriebs mit vereinfachter dynamischer Simulation, KA – Korrespondenz Abwasser, Abfall 6/2010 (2010)
• Jeppsson, U., Rosen, C., Alex, J., Copp, J., Gernaey, K.V., Pons, M.-N. and Vanrolleghem, P.A. (2006). Towards a benchmark simulation model for plant-wide control strategy performance evaluation of WWTPs. Water Sci. Technol., 53(1), 287–295.
• Langergraber, G., Alex, J., Weissenbacher, N., Woerner, D., Ahnert, M., Frehmann, T., Halft, N., Hobus, I., Plattes, M., Spering, V. & Winkler, S. 2008 Generation of diurnal variation for influent data for dynamic simulation. Water Sci. Technol. 57(9), 1483–1486.
• Merkblatt DWA-M 268 Steuerung und Regelung der Stickstoffelimination beim Belebungs­verfahren, Ausgabe 2006.
• Ogurek, M., Alex, J., Schütze, M. : Simulation as tool for development, test and failure-minimising implementation of wastewater systems control., ICA 2009, 10th IWA Conference on Instrumentation, Control and Automation, 2009, Cairns, Australia, Poster Proceedings on CD (2009)
• Rieger, J. Alex, S. Winkler, M. Boehler, M. Thomann and H. Siegrist: Progress in sensor technology – progress in process control? Part I: Sensor property investigation and classification, Wat. Sci. Tech. 47, 2, 103-112 (2003).