The effects of sludge particle size on oxygen transfer and dynamics in the composting of digested sludge

Oxygen transfer limitation inside the sludge particles presents a significant technical challenge in the aerobic composting process. The objective of this study is to evaluate the effect of sludge particle size (SPS) on oxygen transfer, microbial activity and biological stabilization of organic matters in the composting of anaerobic digested sludge. Oxygen uptake rate (OUR), CO₂ production rate and respiratory quotient (RQ) were measured and evaluated in the tests carried out with 0.25 g, 1 g and 4 g particles of dewatered sludge. The effect of SPS on the moisture, volatile solid (VS) and water extract organic carbon (WEOC) was investigated in a lab-scale composting system. The results showed that OUR, CO₂ production rate, the biodegradation rate of WEOC and VS increased significantly with the decline of SPS within the early phase of composting. The moisture of smaller particles was considerably lower in the latter stage. There were also spatial and temporal differences in aerobic conditions inside of different size particles. Oxygen transfer inside solid particles was demonstrated to be the rate-limiting step in the early stage of composting.