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Content archived on 2024-06-18

Reduced GREENhouse gas emissions and sustainable wastewater treatment by integrated Control and Operational STrategies

Final Report Summary - GREENCOST (Reduced GREENhouse gas emissions and sustainable wastewater treatment by integrated Control and Operational STrategies.)

Ensuring the supply of quality water for human consumption, irrigation and industrial operations is commonly regarded as one of the major challenges of this century. As stated by EU water policy “Waters [...] are under increasing pressure from the continuous growth in demand for sufficient quantities of good quality water for all purposes”. Therefore, the number of wastewater treatment plants (WWTP) is expected to keep increasing and to be intentionally designed to match up- and downstream requirements, i.e. to match the characteristics of the influent with the requirements for the effluent. As the number of WWTP increases, so does their environmental impact. The main identified negative impacts of WWTP include sludge disposal, electricity and chemicals consumption for operation and direct greenhouse gas (GHG) emissions. The reduction of GHG emissions has only recently been tackled in the last years due to the difficulty in quantifying and modelling those emissions. It has been estimated that CH4 produced from sewage treatment represented about 5% of the global methane sources, and together with the emissions of N2O this accounts for the 1.6% of GHG emissions in CO2 eq. Besides, N2O has been identified as the most important agent of ozone depletion for the 21st century.

Running a WWTP at low GHG emissions is admittedly not an easy task, especially while respecting the effluent limits and keeping the operating costs controlled. This task becomes even more complex given the common structure of incentives and objectives in a WWTP: operators are evaluated for keeping the process running and respecting the effluent limits while, it is the plant manager and/or chief operator who must focus on minimising the operating costs GREENCOST addresses these challenges by using mathematical modelling in order to combine the available information and provide a clear guide for WWTP management. To increase the credibility of this research, we have used as a case-study an innovative process patented at the University of Santiago de Compostela (Integrated system of methanogenic anaerobic reactor and membrane bioreactor for COD and nitrogen removal in wastewater).

In order to properly account for the environmental impacts of WWTP operation, a life-cycle assessment of the prototype was carried out. As a result thereof, the global warming potential and eutrophication potential were identified as the main impacts associated with the plant operation. Several objectives were then formulated for the plant operation, i.e. i) minimise the release of nutrients; ii) minimise the carbon footprint and iii) minimise the carbon footprint used to remove nutrients. Through simulation and mathematical programming it was established which operational strategies would lead to optimal performance according to each of the objectives. These operational strategies would involve changes design variables such as the hydraulic residence time (HRT), the anoxic volume of the plant, the solids retention time (SRT), the aeration flowrate and the recirculation rate. Furthermore, the model allowed identifying which microbial groups thrived in each of the operational strategies, thereby providing major insight about the process to the managing operators.

As a result, it was demonstrated that mathematical modelling is a well-adapted and useful tool to reduce the environmental impact and especially the global warming potential of complex WWTPs, while maintaining the main function of the plant, i.e. to produce a clean effluent from a wastewater influent. The data obtained from the mathematical analysis were also used to design a fully operational control system which will be eventually installed in the plant prototypes. A major benefit of this controller is that it regulates the plant carbon footprint as well as the effluent quality.

Additionally, data analysis tools were prepared in this project to assist the evaluation and benchmarking of energy consumption in real WWTPs. This task was carried out in collaboration with H2020 collaboration and support action ENERWATER. It was seen that the main methods used for benchmarking WWTPs energy consumption could be classified into three classes: normalization, statistical techniques and programming techniques. Advantages and disadvantages were identified for each one including the simplicity of use, clarity of results and robustness to measurement errors. These findings were instrumental in developing the ENERWATER methodology for measurement and reduction of energy expenditure in WWTPs.