Effects of lime addition on sewage sludge composting process

Previous study showed that co-composting sewage sludge with lime would effectively reduce the availability of heavy metals in the sludge compost. In the present study, effects of addition of lime on the microbial activities of sewage sludge composting were evaluated in a bench-scale in-vessel system through monitoring biological parameters including microbial respiration, microbial populations (thermophile and mesophile), and activities of related enzyme (dehydrogenase, alkaline phosphatase, β-glucosidase, and urease). Lime raised the pH of the sludge compost effectively at the initial period, and this effect became less obvious with an increase in composting time. Adverse effects of lime amendment on all biological parameters increased with an increase in lime application rates, but these effects were generally restricted to the early stage of the thermophilic phase. Addition of 0.63% lime prior to sludge composting slightly improved the microbial activity as indicated by the higher temperature and CO2 evolution and did not exert a significant inhibition on bacterial population, and the activities of β-glucosidase, alkaline phosphatase, and dehydrogenase after 100 days of composting. This together with our previous results on heavy metal availability supports the use of lime at a rate of <1.0% (w/w) to co-compost with sewage sludge.