PROCESS STABILITY AND KINETIC LIMITATIONS OF SINGLE PHASE ANAEROBIC DIGESTION OF SEWAGE....
ABSTRACT
Anaerobic digestion (AD) of municipal sewage sludge is an essential technology for volume reduction, pathogen destruction, and renewable bioenergy production. However, operating digestion systems in temperate or tropical winter environments without external heating remains a major challenge due to the temperature sensitivity of methanogenic microorganisms. This study evaluates the operational performance, kinetic limitations, and process stability of an unheated, single-phase, bench-scale anaerobic digester (26-liter capacity; 23-liter active volume) treating thickened primary and waste activated sewage sludge under natural psychrophilic-mesophilic boundary conditions (21°C–24°C). The reactor was operated across nine distinct steady-state organic loading rates (OLRs of 0.60 to 2.66 kg VS/m³/day) corresponding to solid retention times (SRTs of 67 to 15 days). Experimental results demonstrate that maximum operational efficiency was achieved at an OLR of 1.33 kg VS/m³/day and an SRT of 30 days at a core temperature of 23°C. Under these optimum parameters, the system achieved a volatile solids (VS) reduction of 35.73% and a maximum specific biogas yield of 0.362 m³/kg VS/day with a peak methane content of 39.42%. Progressive reduction of SRT to 25, 20, and 15 days (OLRs of 1.60, 2.00, and 2.66 kg VS/m³/day) at lower ambient temperatures (21.5°C–21.0°C) triggered severe volatile fatty acid (VFA) accumulation, reaching 1,400 mg/L as acetic acid. This acid accumulation exhausted the system's bicarbonate buffering capacity, causing the VFA/alkalinity ratio to spike from 0.243 to a critical value of 0.56, while the methane content collapsed to just 8.0%. These findings outline the precise biochemical and kinetic boundaries of unheated, low-temperature digestion, providing a design framework for cost-effective, decentralized wastewater treatment infrastructures in developing nations.
KEYWORDS: Anaerobic Digestion; Low-Temperature Digester; Process Stability; Volatile Fatty Acids; Psychrophilic Methanogenesis; Sewage Sludge
