The Water Cycle
Water in a Pool
Flocculants or coagulants enhance the removal of dissolved, colliodal or suspended matter by bringing it out of solution or suspension as solids (coagulation) then bringing the solids together, (flocculation) producing a floc which can be trapped in a filter.
PAC, polyaluminium chloride or hydroxychloride is commonly used. It forms a gelatinous precipitate by hydrolysis. Other ones used are Alum, (aluminium sulphate, kibbed alum) sodium aluminate, iron chlorides and iron sulphates. Iron leaves a residual and must be kept low to prevent staining.
Dosing must be by pumps, to adjust to small quantities depending on the bather load. Coagulants mustn't reach the pool in appreciable quantities. The traditional dosage was 20 mg/l for a period of one pool turnover after backwashing, but this is bad for water and for swimmers. Improved filtration can be achieved by continuous dosing, the minimum is 0.05mg/l as aluminium, 0.1mh/l for iron.
In the case of PAC, it is delivered at 0.1ml/m3. Flocculation happens during contact time between the coagulant and pool water process, so it must be ensured that the coagulant mixes before the filters.