Tracking filter performance with particle counting

Minor changes (1-2 mg/L) in the primary coagulant dosage can significantly affect particle counts in filtered water. Jar tests were inconclusive in se lecting a coagulant dosage for source water of low turbidity (< 5 ntu). Mea surements of turbidity in filtered water were also not definitively altered by small changes in coagulant dosage. On-line particle counting was succes sfully used in a 0.5 x 10(6)-m(3)/d (135-mgd) conventional water treatment plant to distinguish the effects on water quality of small changes in coagu lant dosage and filter flow. Measurable changes in particle counts in filte red water were detected when the aluminum sulfate dosage ranged from 5 to 1 0 mg/L at filter loading rates of 4-9.5 m/h (1.7-4.0 gpm/sq ft.). Particle count percentile plots and statistics (10th, 50th, 90th, 95th, and 98th per centiles) were valuable indicators of the performance of both individual fi lters and the overall plant.