Low-dose flocculant strategies for sewage sludge dewatering: mechanistic and techno-economic assessment of cationic polyacrylamide and aluminum sulfate

Effective dewatering of sewage sludge is essential to reduce transport, disposal and energy costs in wastewater treatment plants. Conventional conditioning uses high doses of polymeric flocculants or inorganic coagulants, increasing chemical consumption and sometimes limiting beneficial reuse. This study evaluates low-dose flocculant strategies for sewage sludge dewatering using cationic polyacrylamide (CPAM), anionic polyacrylamide (APAM) and aluminum sulfate [Al₂(SO₄)₃] under standardized jar-test conditions. Dewatering performance was assessed by final cake moisture content and linked to changes in zeta potential, particle size distribution, chemical oxygen demand (COD) and microstructure. In the absence of any chemical addition, the filter-press cake (raw-sludge control) had a moisture content of 82.7%. Low-dose CPAM at 0.01% (w/v) reduced this to 22.7%, comparable to CPAM 1.0% (24.8%) and alum at 15 mg/L (25.8%), whereas all APAM doses yielded significantly higher moisture (≈31-33%). One-way ANOVA confirmed highly significant treatment effects (p < 0.001), and individual correlations showed only weak dependence on zeta potential (R2 ≈ 0.26) and particle size (R2 ≈ 0.15), but a moderate relationship between COD reduction and moisture content (R2 ≈ 0.55). A three-parameter empirical regression model combining COD, absolute zeta potential and Dv90 explained 72.3% of the variance in moisture content, with COD as the dominant predictor. Scenario-based techno-economic analysis, using representative hauling and electricity tariffs, indicates that implementing low-dose CPAM can yield net savings on the order of 105 US$ per 1,000 t dry solids treated, primarily via reduced transport and thermal drying energy, while aluminum sulfate offers a low-cost alternative with slightly lower dewatering performance. These results demonstrate that carefully designed low-dose conditioning can substantially improve sludge dewaterability with reduced chemical demand and provide mechanistic guidance for flocculant selection and optimization.