Abstract
The Gambia River is a tidal and tropical river that covers 12% of the area of The Gambia and is to date not harnessed for drinking water supply purposes. While the freshwater region of the river is mainly used for irrigation, groundwater wells are predominantly used for drinking water. Salinity intrusion and wastewater treatment requirements are challenging current practices. Previous investigation on water quality indicated that the upper regions, with permanent and pristine freshwater, would be well suited for drinking water supply after ultrafiltration (UF) treatment. At 120 km from the river mouth to the coast, where the water becomes saline, desalination is required. With sea level rise, this saline region is expanding further inland, requiring increasingly energy-intensive treatments such as nanofiltration/reverse osmosis (NF/RO), while also affecting groundwater quality. In the urban area near the river mouth, there is potential for water reuse to increase water availability, while addressing the increasing load of micropollutants from the discharged wastewater effluents.
UF could decrease the turbidity to ≤0.5 NTU with partial removal of dissolved organic carbon (DOC). During UF operation at controlled flux, an increase in transmembrane pressure was observed, attributed to fouling caused by the high turbidity (up to 140 NTU) of the water samples from the freshwater region. Dense NF/RO membranes removed salinity and achieved the WHO recommended guideline for drinking water (< 1 g/L as TDS) in the tidal regions with salinities up to 4 g/L during the beginning of the dry season between Tendaba and Jasobo at 100-120 km from the river mouth. The more open NF membrane could treat salinities in the tidal regions below 2 g/L at 100-110 km from the river mouth.
Micropollutants (MP), with a total concentration of up to 26.3 µg/L for pesticides, 38.4 µg/L for pharmaceuticals, and 2.1 µg/L for hormones, were found in a wastewater treatment plant (WWTP) effluent discharged 500 m near the ocean. This led to water quality degradation of the nearby stream, connected to The Gambia River, in which most of the MP concentrations were surpassing surface water quality standards. NF/RO membranes were able to remove most of the MP to concentrations below the surface water guidelines. With an appropriate pretreatment to mitigate fouling, and thus reduce energy consumption and process overall cost, NF/RO technology is well suited for water supply from desalination of the tidal region of The Gambia River and municipal water reuse for non-potable purposes, such as irrigation. The quality requirements will depend on membrane selection and water salinity. The findings of this research on desalination and water reuse can be extended beyond the local context and can contribute to addressing similar challenges in other countries worldwide