The most used process for biological nitrogen removal from municipal and industrial wastewaters is the
activated sludge process. Because of the importance of this process, as well as the large number of existing
facilities, a lot of research effort has been focused on optimizing the operation strategies or improving the
individual plant design. However, the systematic optimization of the process structure (process synthesis)
and operation conditions based on rigorous process models has not been presented in the literature. The
objective of this work is to address the simultaneous optimization of the process configuration and equipment
dimensionsi.e., process synthesis and designand the operation conditions of activated sludge wastewater
treatment plants for nitrogen removal based on a superstructure model. The model embeds up to five reactors
and a secondary settler, and allows flow distribution of the main process streams, i.e., nitrate and sludge
recycle streams and fresh feed, along the reaction zone. The objective function is to minimize the net present
value formed by investment and operating costs, while verifying compliance with the effluent permitted limits.
The investment cost computes the reaction tanks, aeration systems, secondary settler, influent pumping station,
and sludge pump costs. The operation cost computes the cost for pumping, aeration, dosage of an external
carbon source, excess sludge treatment for disposal, and fines according to pollution units discharged. Influent
wastewater flowrate and composition are assumed to be known. The activated sludge model no. 3 and the
Takács model are selected to describe the biochemical processes and the secondary settler, respectively. This
results in a highly nonlinear system with nonsmooth functions. Because of the problem complexity, in this
first approach, a nonlinear programming (NLP) problem (specifically a nonlinear programming with
discontinuous derivatives (DNLP) problem) is proposed and solved to obtain some insights for future models.
It was implemented and solved using general algebraic modeling system (GAMS). Results for case studies
are presented and discussed.