Effect of cations on simultaneous enhancement of dewaterability and settleability of activated sludge under mild thermal treatment

Dewatering and settling are of great importance during activated sludge treatment. The activated sludge fetched from a thickening tank of the wastewater treatment plant was dosed with 0.01–0.10M cations under mild thermal treatment below 100°C. Specific resistance to filtration and sludge volume index were measured to evaluate the dewaterability and settleability, respectively. Results show that dewaterability and settleability were deteriorated under mild thermal treatment, but improved markedly with the addition of cations, and advanced further together with mild thermal treatment, indicating that the cations transformed the effect of mild thermal treatment from deterioration to enhancement. Calcium and Fe3+ were found to be the most effective cations in strengthening the sludge dewatering and settling under mild thermal treatment, and could act immediately after the addition. It is speculated that the cations interact with the binding sites of the polymer substances and cells released by mild thermal treatment, forming polymer–cation–cell complexes, intensifying the flocs inner connection and inducing a superb aggregation. The dewaterability and settleability were thus enhanced by cations under mild thermal treatment.