Windrow composting of wastewater biosolids: process performance and product stability assessment

The composting process at the Thames Water composting facility at Little Marlow, Buckinghamshire, UK, was monitored in order to access the value of both process parameters (temperature, drying tendency) and compost stability indicators (VS, COD, respiration rate) for process performance evaluation. A simple model was developed to describe the process rate on the basis of volatile solids and respirometric results. Pile core temperature exceeded 55°C in the first 2-3 days and reached ambient levels after 8-10 weeks. Compost stability, as determined by both wet (SOUR) and dry (DSOUR) respirometry increased with age, the two parameters correlating well with each other and composting time. The SOUR dropped from about 20 mg/g VS/hr for the fresh compost to 5 mg/g VS/hr at the end of the 4 weeks of turning and to 1.5 mg/g VS/hr after five months of maturation. The COD, optical density at 665, 465 and 280 nm, and the E4/E6 ratio of water extracts were also analysed as potential stability parameters, but the information revealed was limited.