Retrofitting low-performance units to abate sewer overflow pollution based on mathematical model and Sobol algorithm

Abstract

Abstract Optimal retrofit of low-performance units (LPUs) is promising to abate overflow pollutant mass loading of sewer systems during wet-weathers. This study presents a combination of mathematical model and Sobol algorithm to help identify LPUs of sewer systems and design retrofitting strategies. Therefore, the solution to minimize the overflow pollutant mass loading from sewers systems can be efficiently obtained. The developed method was demonstrated at a catchment served by one wastewater treatment plant in Chaohu City, Anhui Province of China, with five pumping stations and a total sewer length of 58.3 km. Within the catchment, there are three rivers and a pool, which are the main waterbodies for receiving the overflow pollutant mass loading from the sewer system. Among them, Donghuancheng river was the most polluted and thus selected as the object for water quality improvement by LPU retrofits in the paper. After identifying the LPUs in the sewer system along Donghuancheng river, and developing the associated retrofitting strategies using Sobol sequence, the mitigations for overflow pollutant mass loading were analyzed. Results show that by applying the optimal retrofit strategy, i.e., increasing the conveyance capacities of two pumping stations by 2.5~3.2 times and augmenting the diameters of 12 sewers by 1.25~1.29 times along Donghuancheng river, the mass loading of chemical oxygen demand (COD) discharged into the river could be reduced by 40.6%. To further coordinate the abatement of overflow pollution and retrofit investment, Sobol sensitivity analysis was conducted to screen the dominant LPUs to update the optimal retrofit strategy. Using the updated strategy, the overflow COD mass loading (1100 kg) per overflow event could be close to that of non-updated strategy (1039 kg) while reducing the retrofitting length of sewers by 40%. Therefore, on the basis of the presented method, decision-makers can flexibly develop an optimal retrofitting strategy for enhancing conveyance and interception efficiencies of sewage and respond to the abatement of overflow pollutant mass loading in a cost-effective way.