Optimizing coagulant dosing for enhanced sludge settling and subsequent anaerobic digestion

Effective sludge management is critical for wastewater treatment plants. This study evaluated five coagulants for sludge settling and selected three for detailed testing: polyacrylamide (PAM), polyaluminum chloride (PAC), and polyferric sulfate (PFS). Response surface methodology was applied to optimize dosage and performance. For primary sludge, PAC at 100 mg/L removed 96 % turbidity and 91 % total suspended solids but increased sludge volume index (SVI) by 56.3 %. PAM (50-100 mg/L) mitigated this, limiting the SVI increase to 5.4-9.9 %. In waste activated sludge, PAM (100 mg/L) reduced SVI by 48 % (69.0 mL/g), while PAC improved supernatant quality. Combined PAC-PAM addition (100 mg/L each) caused only minor methane yield losses (3.4-7.9 %), whereas PFS inhibited anaerobic digestion, reducing methane by 15-23 % via sulfate-reducing bacteria. Economic analysis identified PAC-PAM as optimal, yielding 10.35 US$/m³. These findings establish a practical framework for optimizing coagulant use in integrated sludge treatment.