Optimal wastewater stabilization ponds system design

OCHOA, M. P.; ESTRADA, V.; HOCH, P.

Salt Lake City

Congreso; 2015 AIChE Annual Meeting; 2015

The design of water networks is a major topic of process system engineering and has been receiving increasing attention during the last years due to public concern on scarce water resources, together with stringent regulations on wastewater discharge. Wastewater treatment networks (WWTN) consist on a series of regeneration and treatment processes, one major inlet stream along with mixers and splitters for all the interconnections between treatment units. In general, to define treatment units outlet concentration or removal ratio for contaminant is fixed (1). The use of these simplified models prevents the application of already developed synthesis tools for industrial processes. Only a few papers have considered more detailed models of treatment processes. For example, Yang et al. (2) proposed the implementation of short cut models in the WWTN for modeling different treatment units, thus yielding to some improvement on the whole process design.In this work, we present the design of a system of stabilization ponds using rigorous models for the three different types of lagoons: aerobic, facultative and anaerobic, based on first principles of mass conservation. A superstructure consisting on the different types of stabilization ponds is constructed for optimization of the wastewater treatment system network.The ponds are modeled taking into account mass balances of biomass of algae, the main groups of bacteria: heterotrophic, autotrophic, fermenting, acetotrophic sulphate reducing and acetotrophic methanogenic bacteria. In addition, mass balances for organic load are considered, such as slowly biodegradable particulate COD, inert particulate COD, fermentation products, inert soluble COD, and fermentable readily biodegradable soluble COD. For nutrients, ammonium and ammonia nitrogen, nitrate and nitrite nitrogen, sulphate sulphur and dissolved oxygen. Finally, molecular nitrogen and methane emissions are considered in the model following Sah et al. (3).The model is formulated as a series of nonlinear programming problem (NPL) implemented in GAMS (4). The objective function is to minimize the organic load and nutrients concentration of the effluent in order to conform to environmental regulations. The optimization is performed under different scenarios of organic load, ambient temperature and light intensity, which are the most influential variables of the process. Numerical results provide useful information about the complex relationships between microorganisms, nutrients and organic load in the WWTN design optimization.