Modeling the effect of WWTP Bypass Events on Water Quality in Sebou river estuary, Morocco

Abstract

Historically, urban wastewater from Kenitra was directly discharged into the Sebou estuary (68 km) through six collectors, producing environmental degradation and prompting the Kenitra wastewater treatment plant (WWTP) construction in 2020. Although designed to treat all urban wastewater before discharge, WWTP (19.4 km) experiences bypass events during Eid al-Adha when organic loads surge due to widespread animal slaughter. This paper aims to model the Bypass impact on Sebou water quality. Given the strong dependence of water quality on hydrodynamics, a one-dimensional hydraulic model (HEC-RAS5.0.6) was used, calibrated, and validated using morphological datasets. The hydraulic simulation outputs (water levels and flow velocities) were then used in water quality module to model the biochemical oxygen demand (BOD₅). Three scenarios were examined: untreated discharge, discharge after treatment at WWTP, and Eid al-Adha bypass event. Results indicated a 90% BOD₅ reduction post-treatment, confirming WWTP efficiency. However, during bypass, BOD₅ surged to 4.3 mg/L, significantly deteriorating Sebou water quality, with varied residence-time, 3 days under high freshwater flow and 9 days under tidal dominance. These findings highlight the critical need for bypass prevention and adaptive management during peak-load events