Impact of rainwater intrusion on wastewater characterization and performance of nutrient removal activated sludge process

BACKGROUND: The paper investigated the negative impact of rainwater intrusion and mixing with sewage, on the operation of an existing nutrient removal activated sludge plant. It evaluated the magnitude of observed changes, both in terms of flow increase and modification of major parameters, together with the resulting impact on system performance. The study was carried out at a selected plant. The influent characteristics of the plant were reviewed for 2021 and 2022, both for the dry and rainy periods. Collected data were evaluated together with the experimental support derived from the selected plant. RESULTS: Statistical assessment of the collected data revealed significant changes likely to affect the performance of the existing plant: Sewage flow was increased from 580 000 to 670 000 m(3) day(-1). Although chemical oxygen demand and total nitrogen were slightly reduced, a higher level of inorganic solids was observed. Assessment of nitrificationdenitrification processes yielded an aerobic sludge age, theta(XA) of 9 days and a total sludge age, theta(X) of 15 days, with a V-D:V-T ratio of 0.4. Solids mass balance indicated a biomass level of 6100 mg L-1 with 47% of fixed solids, which was increased to 7300 mg L-1 with 52% fixed solids with the wet weather flow. These values denoted reactor volume limitation for sustainable/safe operation. CONCLUSION: Aside from the impact of substantial treatment cost increase resulting from the incremental flow of 13.7 million m(3) year(-1) during wet weather periods, control and decrease of inorganic solids emerged as the key issue to safeguard system operation. The study also indicated clues for diverting runoff mixing as a tangible remedial action. (c) 2023 Society of Chemical Industry.