Assessing the Stability and Maturity of Compost at Large-scale Plants

Four different tests to assess compost stability were compared during green waste composting in windrows at a largescale composting plant. The tests used were: germination index, (based on seed germination); SOUR (specific oxygen uptake rate, respirometric method based on O2 consumption by the microorganisms); self-heating test, (reflecting the energy output from the aerobic process); and Solvita, (a commercial maturity test based on CO2 and NH3 production). These methods have been identified previously as suitable on-site parameters to establish the end-point of the composting process and were compared to the typical process parameters which included temperature, pH, electrical conductivity, moisture content and volatile solids. SOUR was found to give a useful indication of the degree of stabilisation of the organic matter during the first stages of the composting process. Solvita, was found more to reflect the degree of maturity of the compost, and was more useful at the later stages of the composting process. This study showed that SOUR and Solvita provided fast, simple and relatively unsophisticated methods to monitor compost stability and maturity at large-scale composting plants processing green wastes.