Fate and function of flocculant polymers in waste activated sludge pre-thickening: Effects on digestion, rheology and dewaterability

Flocculant polymers are commonly applied prior to or during anaerobic digestion, particularly for pre-thickening or recuperative thickening, making their fate during digestion important. This study assessed the impact of dosing synthetic powder, synthetic liquid polymer, and powder biopolymer prior to digestion, on sludge rheology, methane yield, and volatile solids (VS) destruction during digestion, and post-digestion dewaterability. Optimal polymer dose reduced yield stress by 20-40 %, improved methane yield by 10-15 mL CH₄ g^-1 VS (15-25 % increase compared to the base case), and enhanced digestate dewaterability. While polymers contributed to methane yield (median 180 mL CH₄ g^-1 VS_poly), they retained a variable 10-36 % residual effect after digestion, in terms of reduced dosing requirements for final dewatering. Overdosing, especially with biopolymers, inhibited VS destruction, likely due to excessive biomass aggregation. Strategic polymer dosing before digestion can enhance biogas production and downstream dewatering whilst minimising polymer use. This supports more cost-effective and sustainable sludge treatment.