Composting is one of the cost-saving ways for sewage sludge treatment to produce a final product that is stable, and free of pathogens and plant seeds. However, the loss of nitrogen through ammonia emission not only reduces the agronomic value of the composting product, but also leads to air pollution and is potentially health threatening. Five mixtures of sewage sludge and acidified sawdust were co-composted for 22 days with different initial pH values (3.51, 4.45, 5.51, 6.48 and 7.56). Acidified sawdust was used as a pH regulator and also bulking agent during composting. Changes in physicochemical properties were characterised by the temperature, organic matter degradation, carbon dioxide emission and pH value. The results showed that regulating the initial pH of composting materials to 5.51~6.48 was the most effective way in reducing ammonia emissions. Compared with the control group, the cumulative ammonia emission was reduced by 52.1% and the nitrogen loss decreased from 44.7% to 24.8% with no adverse effects on organic matter degradation and microbial activity.
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