Performance Evaluation in Composting of Sewage Sludge with Different Bulking Agents

Increasing sewage sludge production has become a new environmental problem in China because of its huge amounts of sludge, heavy metal concentrations, and potential pathogen risk. Composting with appropriate bulking agents is an effective method to treat sewage sludge. In this research project, wood chips, ceramsite, and plant ash were used as bulking agents during sewage sludge composting over a period of 44 days (in winter) at a pilot scale. The results indicated that wood chips were an efficient bulking agent to enhance composting performance while ceramsite failed to initialize sewage sludge composting. In addition, a lower weight ratio between sewage sludge and wood chips significantly improved the composting process. A ratio of 3 : 1 (wet basis) resulted in an increase of composting temperature in two days from 13 degrees C up to 67 degrees C and maintained high temperatures of more than 55 degrees C over some time. In addition, moisture content decreased from 78% to 50%, and a decline of volatile solids from 61% to 47% was observed. The addition of plant ash facilitated a further decrease in moisture content and the germination index of the compost rose from 85.3% to 106.3%. Furthermore, in the compost derived from the 3 : 1 ratio of sludge to wood chips, all monitored heavy metal concentrations were far below the limit levels.