A simple respirometric technique for assessing compost stability

Stability is an important compost quality characteristic, but also one that is difficult to measure. A simple respirometric technique for the assessment of compost stability (SOUR test) was developed, that utilises a dissolved oxygen probe to measure changes in the oxygen concentration in an aqueous compost extract, under conditions ensuring optimum microbial activity and maximum reaction rates. A fully automated procedure is described and possible problems are discussed. The method was used to follow changes in stability of sewage sludge composts from a commercial plant using the windrow composting system. Two different batches of material were monitored. Results were compared with three other stability tests, a respirometric method utilising solid compost samples and the COD and optical density of compost water extracts. Both respiration tests indicated increasing compost stability with processing time and gave significant correlation both with compost age and with each other. The SOUR of the raw mixture was different for the two batches (9.3 and 18.4 mg O 2/g VS/h), but after composting and maturation both dropped to about 1 mg O 2/g VS/h. Measuring the respiration rate of an aqueous compost suspension rather than a solid compost sample offers certain advantages: the test is not affected by variations in the matric water potential of the samples; there is immediate contact between substrate, microbes and oxygen leading to maximum reaction rates; and the gas–liquid barrier for oxygen diffusion at the surface of the compost particles is omitted. The COD and the optical density of the water extracts did not show clear trends and did not allow any conclusions about compost stability to be drawn.