Coagulation in combination with anaerobic digestion for enhancement of resource recovery from faecal sludge

Poorly managed faecal sludge (FS) poses significant challenges to public health and the environment. Anaerobic digestion (AD) of FS provides an effective method for energy recovery while reducing FS associated threats. Recognizing the critical role of the dewatering process before AD, this study investigates the synergistic application of chemical coagulation and mesophilic AD for synthetic FS treatment. FeCl₃, AlCl₃, Fe₂(SO₄)_3, poly ferric sulfate (PFS) and poly aluminium ferric chloride (PAFC) were utilized at varying dosages to examine their impact on FS properties and subsequent biogas production from the dewatered FS. It was found that coagulation enhances sedimentation efficiencies and dewaterability through mechanisms such as charge neutralization, charge patching and bridging, thereby improving the FS feasibility for AD. Notably, polymer coagulant PFS showed good performance in balancing pollutant removal and methane recovery, contributing to facilitating the hydrolysis and acidogenesis microorganisms involved in the AD process. Optimal dosage was identified at 150 mg/g TS (1.7 g/L FS), achieving prominent removal efficiencies for total COD (67%), turbidity (85%), and total phosphorus (60%), while simultaneously enhancing AD performance with specific CH₄ production reaching 517 ml CH₄/g VS or 24.8 ml CH₄/g AD wet feedstock compared to 309 ml CH₄/g VS or 2.7 ml CH₄/g AD wet feedstock in untreated FS.