Water Quality Modelling of the Sebou River Estuary (Morocco) Before and After the Installation of the Kenitra City’s Wastewater Treatment Plant

Abstract

Urban wastewaters from Kenitra had been discharged without prior treatment through six collectors into the Sebou estuary (60 km) causing in the process health and environmental problems up until WWTP was installed in 2020. The waste water treatment plant (WWTP) had to gather all urban wastewaters in order to go through treatment. Thus, the WWTP, situated 17 km from the mouth of the estuary, has become the only discharge point for treated domestic wastewater into the Sebou estuary. This study aims to model the Sebou estuary water quality and assessment of the impact of WWTP. Our study started with hydraulic modelling of the river using a 1D model (HECRAS 5.0.6), since water quality is strongly depending on hydraulic regime. HEC RAS has been calibrated and validated using hydraulic and morphological database of the years 2019 and 2021. The spatiotemporal evolution of hydraulic variables (water velocity, water level, etc.) was calculated by the hydraulic model and used in the water quality module to simulate the biochemical oxygen demand (BOD 5 ). Two scenarios were put under examination, one is a simulation of discharges of untreated wastewaters by six collectors (mean BOD 5 of 300 mg/L) and the second is a simulation of discharge after treatment at the WWTP (mean BOD5 of 24 mg/L). The simulation showed an impact of the tidal cycle and fresh water flows (coming from the upstream) on the urban wastewater discharges fate. Calculations of the BOD 5 provided by the model are in good agreement with field measurements. The first simulation results show that water quality of the Sebou River does not meet WHO standards (average quality about 7 mg/l of BOD 5 ). The second simulation shows that the treatment plant reduces the concentration of BOD 5 in the river to about 3 mg/l compared to the case before their installation, the quality of the water in the estuary is changing to become good after having been just average. In addition, the BOD 5 concentration downstream of the WWTP changes according to the tidal cycle. Finally, the results show the very positive impact of Kenitra's WWTP on the water quality of the Sebou River estuary.