Effect of dissolved organic material and cations on freeze-thaw conditioning of activated and alum sludges

Freeze-thaw conditioning effectively dewaters alum and activated sludges, but it works better on alum sludge than it does on activated sludge. The main difference between alum sludge and activated sludge is that activated sludge has high concentrations of both dissolved organic material and ions. Dissolved organic material and ions may possibly alter the freezing process and decrease the effectiveness of freeze-thaw conditioning on activated sludge. The objective of this study is to investigate the effect of dissolved organic material and cations on freeze-thaw conditioning of sludges, and to improve the effectiveness of freeze-thaw conditioning on activated sludge. The results of this study show that although protein, carbohydrate and cation concentrations in activated sludge supernatant are initially high, they dramatically increase after freeze-thaw conditioning. The increase is likely to come from the release of extracellular and intracellular material to sludge supernatant. The observed increase in the DNA concentration in activated sludge supernatant after freeze-thaw conditioning indicates that freeze-thaw causes cell disruption. Alum sludge supernatant, on the other hand, initially contains low concentrations of proteins, carbohydrates and cations which do not noticeably change after freeze-thaw conditioning. When ECPs (extracellular polymers) and cations are extracted from activated sludge before freeze-thaw conditioning, the sludge settles and dewaters better after the freeze-thaw. The resulting aggregates are smaller and denser resembling the “coffee ground” aggregates of alum sludge.