The respiration rate of composting pig manure

The rate at which oxygen is consumed during composting is a measure of aerobic microbial activity and is linked to the rate of organic material decomposition. The rate of loss in mass is a function of the mass of the degradable organic fraction and is related to oxygen uptake rate by the reaction rate coefficient, k. The decomposition of a pig manure and straw mix was investigated at temperatures between 10°C and 70°C using respirometric techniques. The oxygen concentrations in the reactor were measured continuously for about 4 days and then converted to hourly oxygen uptake rates for each incubation temperature, T. The specific oxygen uptake rate was used to calculate the reaction rate coefficient at T, kT, for the observed fast and slow stages of decomposition. The effect of the environmental factors was taken into account using a multiplicative approach and a relationship, which expressed kT for each stage as a function of T, was formulated. The maximum measured rate of activity occurred during the fast stage at 60°C where kTfast = 0.31 day-1. Activity increased exponentially with the temperature in both stages up to about 60°C. At higher temperatures, the activity slowed but most noticeably in the fast stage. The dependence of kT on T during each stage was described by a double power expression, which predicted that activity would cease around 73°C. The relationships may be used to improve a compost model that is based on a first order reaction rate kinetics for the decomposition of organic material.