Optimal Synthesis of Activated Sludge Wastewater Treatment Plants For Nitrogen Removal

MIGUEL MUSSATI; SERGIO MUSSATI; ING. NOELIA ALASINO; AGUIRRE, A. P.; SCENNA, NICOLAS JOSE

Rio de Janeiro

Congreso; 2nd Mercosur Congress on Chemical Engineering. 4th Mercosur Congress on Process Systems Engineering; 2005

UNRJ

The predenitrification, postdenitrification and combined pre-postdentrification are the most used basic activated sludge wastewater treatment plant configurations for biological nitrogen removal from municipal and industrial wastewaters. In general, the denitrification schemes may require an external carbon source dosage since the influent easily biodegradable COD is consumed by aerobic microorganisms, resulting in a shortage of easily biodegradable COD for denitrification. This is even worse when the fresh wastewater has low COD/nitrogen ratios. However, the excess of external carbon added may result in a deteriorated effluent quality causing increased fines and/or operation costs for the excess carbon that is not used for denitrification, the higher cost for treating an increased amount of sludge for disposal and energy demand for aeration to remove the excess COD. The problem addressed in this paper is the simultaneous optimization of the system structure (process configuration) and the operation conditions (process stream flow rates and component concentrations) of activated sludge wastewater treatment plants for nitrogen removal, aiming at minimizing the total annual operation cost while predicting compliance with the effluent permitted limits. Defined influent wastewater specifications and process unit sizes are assumed. As the plant equipment is supposed given, investments are not included. The Activated Sludge Model No. 3 and the Tákacs model are used to model the reaction compartments and the secondary settler, respectively, resulting a highly non-linear system. The proposed superstructure, which embeds the pre-, post- and pre-post-denitrification configurations, is modeled as a Mixer Integer Non Linear Programming (MINLP) model with constraints, where binary variables (0/1) are used to select the optimal process configuration, and continuous variables to represent the real process variables (feed, clean effluent, waste sludge for disposal, recycles and aeration flow rates, external carbon source dosage rate and component concentrations). The resulting model is implemented and solved using General Algebraic Modeling System (GAMS). For the study case and conditions investigated, the postdenitrification system resulted to be the optimal process configuration that meets the effluent threshold values. The alternative plant configurations are evaluated and compared to the optimal solution.