Modeling the Effect of WWTP Bypass Events on Water Quality in Sebou River Estuary, Morocco

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) in 2020. Although designed to treat all urban wastewater before discharge, WWTP (19.4 km from the mouth) experiences bypass events during Eid al-Adha when organic loads surge due to widespread animal slaughter. This study aims to model the Bypass impact on Sebou estuary water quality, focusing on biochemical oxygen demand (BOD5) as a key indicator. 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 the water quality module to simulate BOD5 dynamics. Three scenarios were examined: untreated discharge, discharge after treatment at WWTP, and Eid al-Adha bypass event. The results indicated a 90% BOD5 reduction posttreatment, confirming WWTP efficiency. However, during the bypass event, BOD5 surged to 4.3 mg.L-1, significantly deteriorating Sebou estuary water quality. The pollution residence time varied from 3 days under high freshwater flow (300 m3 .s-1) to 9 days under tidal dominance (0 m3 .s-1). These findings highlight the urgent need for adaptive wastewater management (pre-treatment during peak periods and public awareness campaigns) during peak-load events to mitigate ecological risks and safeguard downstream communities relying on the estuary for water and livelihoods.