Effect of Moisture Content on the Composting Process In a Biotoilet System

Compost Science & Utilization, (2005), Vol. 13, No. 3, 208-216

Miguel Angel Lopez Zavala and Naoyuki Funamizu

Department of Environmental Engineering, Graduate School of Engineering,
Hokkaido University, Sapporo, Japan

Biotoilet is a composting toilet that uses sawdust as a matrix for bioconversion of human excreta into compost and is managed with the aim of accelerating decomposition, optimizing efficiency, and minimizing any potential environmental problems. Understanding how the moisture content affects the biodegradation rates of feces is a key factor for setting criteria for the proper design and operation of the biotoilet. A research project in this respect was conducted in laboratory-scale composting reactors. Results showed that composting is characterized by different biological responses of microorganisms depending on the moisture content under which the process is conducted. Low moisture contents (< 64%) ensure aerobic degradation of feces, whereas high moisture levels (> 64%) cause both aerobic and anaerobic decomposition. Higher reductions in parameters such as total solids (TS), volatile solids (VS), and chemical oxygen demand (COD), and higher oxygen utilization rates were obtained at moisture contents near 65%. This moisture level is the critical moisture estimated during drying tests on sawdust. At high moisture contents, odors, anaerobic emissions, nitrite formation, and increase of sulphate concentrations were detected. Keeping moisture content near 60%, or little higher, but avoiding levels near or higher than 65% ensures an optimum performance of the biotoilet system.

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