Assessing operational issues in a modular sewage treatment plant and providing remedies via comprehensive modeling and simulation through GPS-X

Abstract

In this study, an existing but faulty modular sewage treatment plant (STP) with a capacity of 100 m3/day has been modeled and assessed using GPS-X software, combined with sensitivity analysis to confirm an underlying but critical operational issue of an STP. Currently, the STP shows <5 % ammonia removal at an average influent ammonia concentration of 50 mg/L, along with the problem of partial BOD removal. The treatment units comprise an equalization tank, two moving bed biofilm reactors (MBBRs) in series, a tube settler and a sand filtration unit. Sensitivity analysis helped decipher the most influential kinetics/stoichiometric parameters, which assisted in achieving a precise model calibration. Post calibration, the modified model was re-simulated for each constituent treatment unit based on the mass balance of four major output parameters viz. NH4-N, biochemical oxygen demand, dissolved oxygen and alkalinity. Real experimental/measured values spanning over 8 months were analyzed during the calibration phases (including both static and dynamic calibration phases). The calibrated GPS-X model was simulated using various input factors to find a possible remedial solution and understand the critical limitation of the existing plant. As a remedial measure, a pre-anoxic tank with an operational volume of 15 m3 and a recirculation ratio of 3 was proposed for the existing STP. A proper experimental design and statistical tools further helped achieve the most optimum and techno-economically feasible solution for an MBBR-based biological treatment unit. This helped effectively manage wastewater pollution in a modular treatment system.