Optimal WWTP process selection for treatment of domestic wastewater - A realistic full-scale retrofitting study

Retrofitting existing wastewater treatment plants has become a major challenge in the wastewater engineering area, in particular due to the increasing number of competing technologies and concepts (treatment versus resource problem). In order to support the expert-based approach and provide a rational design for WWTPs, we proposed the use of a mathematical programming based framework to manage the complexity and solve the multi-criteria retrofitting problem. The treatment alternatives were represented in a superstructure - coupled with a database containing information on performance and economics of treatment units. The optimization problem was solved for different objective functions and constraints corresponding to different retrofitting scenarios. The framework was evaluated on two full-scale domestic wastewater treatment plants (265,000 PE and 750,000 PE respectively). To this end, the design space included 9 side-stream treatment technologies and 9 main line technologies resulting in more than 200 retrofitting alternatives. The retrofitting problem resulted in the selection of a high-rate oxic reactor coupled with anammox technology in the main line as the optimal solution, due to the low utility consumption and high biogas production associated with this network. On the other hand, most of the side stream treatment alternatives have similar technical-economic benefits in terms of the whole plant retrofitting cost function. Therefore the selection of the optimal alternative is more based on local conditions and preferences in that case.