Direct filtration of Virginia surface waters : feasibility and costs

This research investigated the technical and economic feasibility of the direct filtration technique for treating high quality Virginia surface waters prior to domestic use. One of the flow schemes that was investigated eliminates the sedimentation basin while employing a full, conventional flocculation period. Another eliminates separate sedimentation and flocculation units altogether.

Major variables that were assessed in terms of their effect on direct filtration performances included raw water characteristics such as turbidity, color# total dissolved solids, total hardness, algae count, and total coliform count; filter design variables such as type and size of media (two dual media filters and one trimedia were evaluated); and operating variables such as hydraulic loading rate and nature and dosage of applied coagulant.

The most effective filtration scheme consisted of a three-minute rapid mix with alum and a cationic polymer as primary coagulants, followed by filtration at a 5 gallons per minute/sq ft loading rate through 20 inches of 1.3 mm effective size anthracite coal and 10 inches of 0.45 mm effective size silica sand. Color and algae concentrations had definite effects upon the direct filtration process. Results from the study show that raw waters with turbidities less than 10 NTU, color less than 15 APHA units, and algae concentrations less than about 1,000 colonies/ml should be excellent candidates for economical treatment by direct filtration.