Estimation of optimum moisture levels for biodegradation of compost bulking materials

Abstract

Moisture affects the physical and biological properties of compost and other solid statefermentation matrices. Aerobic microbial systems experience different respiration rates (oxygenuptake and CO2 evolution) as a function of moisture content and material type. In this study themicrobial respiration rates of 13 compost-bulking materials were measured by a pressure sensormethod at 6 different moisture levels. The experimentally determined respiration quotient (RQ)values were used to calculate CO2 respiration rates from O2 consumption. The RQ values of allmaterials were around 1.0 except for silage, oat straw and leaves which were about 1.5. A widerange of respiration and heat production rates were observed for different materials, with alfalfa hay,silage, oat straw, and turkey litter having the highest values. These four compost-bulking agents maybe particularly suitable for improving internal temperature and pathogen destruction rates fordisease-related mortality composting. Optimum moisture content was determined based onmeasurements across a range that spans the maximum respiration rate. A mechanistic model ofmoisture kinetics was also used to predict the optimum moisture levels. There was good agreementbetween experimental observations and modeled optimum moisture content. The optimum moisturecontent of each material was observed near WHC, which ranged from near 65 to over 85% on a wetbasis for all materials except a highly stabilized yard waste compost (optimum around 30% w.b.).This study demonstrates the importance of moisture content on the biodegradability of organicmaterials and specific respiration rates of each material. The results can be used to develop moisturemanagement and process control strategies to maintain compost and cover materials in anacceptable range.