System dynamics modeling approach for biological nitrogen removal process simulation, uncertainty analysis, and operation optimization

Nitrogen removal from wastewater is challenging due to the complex microbial interactions and dynamic environmental conditions. Such interplay between process parameters necessitate advanced mathematical modeling to optimize the nitrogen removal process. However, simulation of nitrogen removal involves complex process rate equations, requiring extensive model development and calibration. In addition, parameter uncertainty and model optimization limit the applicability of existing models to simulate complex nitrogen removal process. Therefore, in the current study, the system dynamics (SD) modeling approach was adopted for the first time to interconnect the influent wastewater characteristics, operational parameters, and microbial kinetics as input variables. The outputs of the resulting SD model included effluent characteristics such as concentrations of chemical oxygen demand (COD) and nitrogen compounds. The SD modeling approach was employed to reproduce previously reported experimental observations of effluent characteristics within a single-stage partial nitrification/Anammox (PN/A) system with a coefficient of determination (R2) of higher than 0.88 under continuous and intermittent aeration. The developed SD model was subsequently deployed to quantify the influence of uncertainties in microbial kinetics on effluent characteristics. Furthermore, the model was employed to determine the optimal aeration scheme required to maintain an ammonia removal efficiency of 90 % while minimizing the required energy demand, where optimization using the SD model reduced the energy requirements for biological ammonia removal by 19 %. In general, the developed SD model can be used to guide effective operation and control schemes considering the variabilities in microbial kinetics in the nitrogen removal process. In addition, as the SD modeling approach is generic by design, its application can be extended to other complex wastewater and sludge treatment processes.