Modelling of Wastewater Treatment Plants

As a result of urbanization, stricter environmental requirements and an aging infrastructure, Sweden's wastewater system is facing significant investments for expansion, renewal and efficiency improvements over the coming years. According to Svenskt Vattnens’ report Investeringsbehov och framtida kostnader för kommunalt vatten och avlopp (in Swedish), the annual investment requirement at Swedish WWTPs is currently expected to be about SEK 2.0 - 2.1 billion / year and then increase to about SEK 2.5 - 2.9 billion / years for the next 30 years. The requirements for modern wastewater treatment plants apply not only to effluent water quality but also to energy efficiency, greenhouse gas emissions and resource recovery. Wastewater treatment plants are dynamic complex systems with large variations and feedback flows. This requires an integrated, often dynamic, analysis.

Process Modelling

Dynamic process models describe hydraulics as well as physical, chemical and biological processes at the treatment plants by mathematical equations that include the best understanding of the real systems at the molecular and organism level. The equations are implemented in a computer software and a specific model is built for the treatment plant in question with its plant-specific processes, flows and data. Once calibrated and validated against historical data, the model can be used to evaluate various scenarios of process changes, load variations or stricter emission requirements.

The use of dynamic models allows:

• scenario analyzes and evaluation for major investments;
• optimization possibilities, by simulating different control and control strategies;
• evaluation of capacity and performance;
• simulation of new deployment and start-up attempts before actual implementation;
• increased knowledge of the purification processes themselves.

The use of modelling in design improves accuracy in sizing, which allows costly over- or under-investment to be avoided. In optimization projects, performance enhancements can be identified which means that investments for expanded capacity can be postponed or avoided altogether.

RISE Expertise

RISE has been involved in research, development and application of process models for wastewater treatment plants for over 8 years. The research includes further development of the models for e.g. greenhouse gas emissions, anaerobic co-digestion and heat recovery. The models have been applied to Swedish sewage treatment plants for evaluation of, among other things: expansions, increased influent load and stricter effluent requirements. We provide the basis for decisions with insights that would have been difficult or impossible to get based on classical design and calculation.

In our projects we have the opportunity to use all of the market's commercial software for modelling wastewater treatment plants (e.g. BioWin, SUMO or WEST). For research projects, we often use the general calculation platform Matlab / Simulink.