Polyelectrolyte conditioning of ferric sulfate sludge

Tests were run on samples of ferric sulfate sludge collected from the sedimentation basins of a water treatment plant during basin cleaning operations. The purpose of the testing was to determine if cationic, anionic and nonionic polyelectrolytes could successfully condition the sludge to improve its dewaterability. In addition the parameters which affected the conditioning mechanism. were investigated. Tests that were run during the investigation included specific resistance, COD, total and volatile solids, zeta potential and pH. Polyelectrolyte conditioned and unconditioned sludge samples were applied to bench scale sand beds to determine the effect of conditioning on the dewatering rate of the sludge.

Results indicated that all types of polyelectrolytes used reduced the specific resistance of the sludge. However, specific resistance was only a qualitative measure of the effect of polyelectrolytes on gravity dewatering of the sludge. Anionic and nonionic polyelectrolytes exhibited chemical bridging as the mechanism of conditioning while cationic polyelectrolytes conditioned by both chemical bridging and charge neutralization. Both pH and sludge solids content were found to affect the conditioning process. The sand bed studies indicated that sludge conditioned with anionic polyelectrolytes produced a more porous floc structure that dewatered to a cake that was easily removed from the bed. Cost data for polyelectrolytes indicated that conditioning of the sludge by this method was very economical.