Simple Polyacrylamide Dosing Systems for Turbidity Reduction in Stilling Basins

Abstract

Stilling basins are often ineffective in reducing turbidity and total suspended solids (TSS) in water containing fine sediments. This study determined the effectiveness of two polyacrylamide (PAM) dosing systems to flocculate suspended materials and the influence that porous baffles have on the two systems. Turbid water was generated by injecting soil into pond water flowing into a mixing basin for 30 min. Turbid water containing mostly fine suspended sediments was pumped from the surface of the mixing basin to a test stilling basin with physical and chemical treatments. The physical treatments were either an open basin or one with three porous baffles of 900 g m-3 coir matting. The chemical treatment was either passive using a PAM block or active using a PAM solution injected into the water pump intake at 4 mg L-1 in the pumped water. The passive treatment involved pumping turbid water over the PAM block at the basin entrance, dissolving the PAM as the water flowed over the block. Sampling for turbidity and total suspended solids (TSS) was done at 5 min intervals at the inlet, outlet weir, and four surface and bottom points inside the basin. In laboratory screening for PAM formulation and dose, tests conducted with whole soil resulted in greatly reduced turbidity, while those with only the suspended fraction had a much lower turbidity reduction. In the stilling basin, detention times of 1.5 or 24 h both had no effect on turbidity or TSS at the outlet. The turbidity of untreated discharges ranged from 220 to 260 nephelometric turbidity units (NTU), while both active and passive dosing significantly decreased the turbidity by 66% to 88%. Porous baffles had little effect compared to the PAM treatment. The active PAM treatment significantly reduced TSS at the outlet by up to 80%, but the 45% to 65% reduction by the passive system was not significantly different from the untreated tests. Patterns within the basins indicated that suspended flocs in PAM-treated water may have been intercepted and removed by the sloped dam wall, a phenomenon not observed in the untreated water.