Sludge dewatering: A review of conventional methods and microbial assisted approaches

Sludge dewatering remains a major challenge in wastewater treatment due to strong water binding within extracellular polymeric substances and the complex structure of microbial flocs. Conventional mechanical, chemical, and physicochemical conditioning methods primarily remove free and interstitial water but are less effective in disrupting extracellular polymeric substance associated bound water, and are often linked to high chemical consumption, elevated energy demand, and increased downstream environmental pressures. This review synthesizes recent advances in sludge dewatering, focusing on extracellular polymeric substance architecture, water state distribution, and microbial-assisted mechanisms. By integrating key performance indicators, including capillary suction time, specific resistance to filtration, dry solids content, nuclear magnetic resonance relaxation characteristics, and extracellular polymeric substance fractionation, a unified framework is established linking sludge microstructure, water binding modes, and dewatering performance. Microbial-assisted strategies, including bioaugmentation, in situ activation of indigenous consortia, and enzyme-assisted conditioning, are evaluated in terms of direct extracellular polymeric substance degradation and indirect physicochemical modulation, such as changes in pH, ionic strength, redox conditions, and electrical conductivity. Reported improvements vary with sludge type and operating conditions, and many strategies showing strong laboratory performance demonstrate inconsistent results during scale up. Emerging hybrid microbial chemical and microbial physical processes are therefore examined to clarify how microbial pretreatment enhances bound water release while reducing chemical dosage and energy demand. Sustainability is assessed in terms of resource use, carbon emissions, and impacts on downstream sludge utilization. This review provides a mechanism-oriented framework to support process selection and optimization of sustainable sludge dewatering technologies.