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

Abstract

BackgroundThe 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 on a selected plant. The influent characteristics of the plant have been 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.ResultsStatistical assessment of the collected data revealed significant changes likely to affect the performance of the existing plant: Sewage flow was increased from 580,000 m3/d to 670,000 m3/d. While COD and total N were slightly reduced, a higher level of inorganic solids was observed. Assessment of nitrification‐denitrification processes yielded an aerobic sludge age, θXA of 9 d and a total sludge age, θX of 15 d, with a VD/VT ratio of 0.4. Solids mass balance indicated a biomass level of 6,100 mg/L with 47% of fixed solids, which was increased to 7,300 mg/L with 52% fixed solids with the wet weather flow. These values denoted reactor volume limitation for sustainable/safe operation.ConclusionAside from the impact of substantial treatment cost increase due to the incremental flow of 13.7 million m3/year at 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.This article is protected by copyright. All rights reserved.