Water treatment plants abstracting from rivers face a unique challenge during seasonal shifts. Many areas in Africa are characterised by a distinctive rainy season, which brings with it the need to adjust the operational philosophy of water treatment plant companies. Conventional water treatment plants function to remove suspended particles in the source water through clarification and filtration. The main workhorse in this process is the clarifier. It is responsible for the majority of suspended material removal and it achieves this objective with the aid of flocculent dosing.
Flocculent dosing rates are dependent on suspended solids concentration. As the sediment load of the source water increases during the rainy season, more flocculent is required. If the relationship between sediment load and flocculent dose rates is maintained, the clarifier will continue to produce the same quality water as seen in the dry season. However, if the flocculent dosing is not adjusted in accordance with the sediment load, the turbidity and suspended solids concentration will increase in the water exiting the clarifier.
The effects of decreased water quality entering the filters are notable. If the system allows, an increase in backwash cycles will be observed. This results in power and water wastage as the filters are cleaned more frequently. If the backwash duration is not long enough, over time the filters will foul despite the increase in backwash cycles. The effect of this is the production of a lower volumes and poorer quality water. In extreme cases, filter structural failure and possible pump damage may result.
Consumption of disinfectant is related to the water quality. The poorer the quality, the more disinfectant is required to render the water safe for drinking. If the filters do not produce water of an adequate quality, either chemical consumption increases with a corresponding increase in operational costs, or unsafe drinking water is delivered into the distribution network.
Other possible impacts of the rainy season are the challenges faced by submersible pumps positioned directly in the river. High water volumes and increased flow may result in pumps being washed away. Often debris is carried by the water and, if large enough, could damage the pumps and pipework or even dislodge the pump from its mounting completely. Further complications may result if the pumps are able to transfer smaller items into the pipeline that become stuck at valves and bends. If submersible pumps are required, they should be positioned in a way as to protect them from the main water stream.
Operators of water treatment plants should develop a sound understanding of the relationship between flocculent dosing and sediment load in the source water. In this way they will be able to adjust the system parameters set by water treatment plant companies to ensure ongoing production of high quality drinking water.