The water treatment process at Blantyre Water Board is a typical conventional system. It involves chemical and physical processes of coagulation, sedimentation, rapid sand filtration and disinfection. The water treatment process is as follows:-
Sedimentation: Raw water from the source is pumped into a raw water (pre-settling) tank which ensures continuous and steady flow. At this stage, the heavier particles settle at the bottom of the tank. Then the water moves to the reaction tank.
Coagulation: A coagulant is introduced at the outlet of the raw water tank and it is allowed to mix rapidly with the water, as the water flows quickly into the reaction tank. As the coagulated water flows in the channels leading to clarifiers, particles which form as a result of the chemical reaction between coagulant and water grow into large sizes called ‘flocs’.
Sedimentation: The heavy flocs settle out in the clarifiers/reaction tanks while the light ones move with the water to the sand filters.
Rapid sand filtration: At the sand filters, water is filtered through all the solids which still remain in suspension after sedimentation in the clarifiers/ reaction tanks are removed.
Disinfection: After filtration, chlorine is added to the water to kill disease – causing organisms (germs) to render the water ‘safe’ to drink.