DIRECT HORIZONTAL-FLOW ROUGHING FILTRATION .2. PERFORMANCE, AND OPERATIONAL GUIDELINE

Direct horizontal-flow roughing filtration (DHRF) was found to be vers atile and capable of handling wide fluctuations in raw water with high turbidity (100-400 NTU) while operating conditions such as coagulant dose (I mg Al(III)/L), mixing intensity and time (e.g. 200 s(-1) and 1 min) and filtration rate (up to 5 m/h) remained unchanged (see also P art I, Aqua Vol. 45, no. 5, pp. 262-271). In the presence of humic sub stances the turbidity removal was reduced substantially. However, sati sfactory turbidity and colour removal could be achieved in DHRF by inc reasing coagulant dose or decreasing pH, or both. A design and operati onal guideline for DHRF is proposed. A comparative analysis between ho rizontal-flow roughing filtration, DHRF, and the conventional floccula tion-sedimentation process indicates that DHRF is an appropriate pre-t reatment technology for towns and small cities in many developing coun tries.