Wastewater treatment plant synthesis and design: Combined biological nitrogen and phosphorus removal

Abstract

In the present work, a previous superstructure model developed for simultaneous optimization of the process configuration and equipment dimensions, i.e., optimal process synthesis and design and the operation conditions of activated sludge wastewater treatment plants, is extended to account for phosphorus as well as nitrogen removal. Along the activated sludge treatment process, the wastewater stream is exposed to different environmental conditions (anaerobic, anoxic, and aerated zones) in order to facilitate the different microbiological processes such as the release and uptake of phosphorus and the nitrification/denitrification processes. The Activated Sludge Model No. 3 extended with the Bio-P module for computing biological phosphorus removal is used to model the reaction compartments and the Taka`cs model for representing the secondary settler. The performance criterion selected is the minimization of the net present value that includes investment and operating costs while verifying compliance with the effluent permitted limits. The problem is posed as a NLP problem, specifically a nonlinear programming problem with discontinuous derivatives DNLP. The optimization model is implemented and solved using a General Algebraic Modeling System, GAMS. Optimal configurations and designs obtained for several case studies are reported and discussed. The model itself and the resolution methodology prove to be robust and flexible enough to solve efficiently scenarios with a wide range of operation conditions, embedding conventional and nonconventional process configurations.