Field trials of appropriate hydraulic flocculation processes

Hydraulic flocculation has been used for water treatment throughout the world for many years and is particularly well suited for developing countries. Two types of flocculator are described, a horizontal-flow channel system, for which aspects such as retention time, channel slope, rapid mixing methods and baffle insertion were assessed and a contact flocculation-filter (CFF). Pilot plant tests were carried out over two years at Thyolo Water Treatment Works in Malawi using river water with crushed seed kernels from the indigenous tree Moringa oleifera Lam. (M. oleifera) or aluminium sulphate as coagulant. Both flocculation systems were able to reduce turbidity to below the World Health Organisationʹs guideline value of 5 NTU for drinking water from raw water with average turbidities during test runs ranging from 15–5600 NTU. The CFF method was particularly effective at low turbidities where <1 NTU was attained at low dosages (<25 mg/l) of M. oleifera. “Flash” mixing of coagulant using a simple equivalent of an in-line vortex mixer was shown to give more efficient treatment than mixing by alternative methods. M. oleifera as coagulant is shown to be feasible for treating river water for communities up to small town size and its production close to treatment plants would obviate importation of chemicals thus saving on foreign exchange, provide economic development of rural communities and reduce potential health hazards from chemical overdosing.