Biodegradation potential of bulking agents used in sludge bio-drying and their contribution to bio-generated heat

Straw and sawdust are commonly used bulking agents in sludge composting or bio-drying. It is important to determine if they contribute to the biodegradable volatile solids pool. A sludge bio-drying process was performed in this study using straw, sawdust and their combination as the bulking agents. The results revealed that straw has substantial biodegradation potential in the aerobic process and sawdust has poor capacity to be degraded. The temperature profile and bio-drying efficiency were highest in the trial that straw was added, as indicated by a moisture removal ratio and VS loss ratio of 62.3 and 31.0%, respectively. In separate aerobic incubation tests, straw obtained the highest oxygen uptake rate (OUR) of 2.14 and 4.75 mg O2 g−1VS h−1 at 35 °C and 50 °C, respectively, while the highest OUR values of sludge were 12.1 and 5.68 mg O2 g−1VS h−1 at 35 °C and 50 °C and those of sawdust were 0.286 and 0.332 mg O2 g−1VS h−1, respectively. The distribution of biochemical fractions revealed that soluble fractions in hot water and hot neutral detergent were the main substrates directly attacked by microorganisms, which accounted for the initial OUR peak. The cellulose-like fraction in straw was transformed to soluble fractions, resulting in an increased duration of aerobic respiration. Based on the potential VS degradation rate, no bio-generated heat was contributed by sawdust, while that contribution by straw was about 41.7% and the ratio of sludge/straw was 5:1 (w/w, wet basis).

Research highlights
► Sludge bio-drying is an approach of biomass energy utility.
► Focuses on the biodegradation extent of bulking agents during composting or bio-drying.
► The sludge, straw and sawdust were incubated separately under an aerobic condition.
► Straw presented strong degradability while sawdust showed poor biodegradation.
► Contribution of bio-generated heat by straw was ca 41.7% for improving bio-drying.