Reducing ammonia emission via reactor composting technology

Composting is an important way to recycle manure into soil as an organic fertilizer or conditioner. However, approximately 50% of the total nitrogen in manure is lost in the form of ammonia during composting, and this loss has an environmental impact on human health. Therefore, to reduce ammonia emission during the composting process to improve the quality of organic fertilizer and reduce its impact on the environment, this study intends to explore a composting method with low ammonia emissions, economy, and high efficiency. In this study, we used quantitative data from full-scale composting systems (windrow composting and reactor composting) in an industrial sheep farm to compare the ammonia emissions from traditional windrow composting and reactor composting, and quantitatively analyzed the ammonia recovery efficiency and influencing factors of gaseous scrubbers, as well as the economic benefits of windrow composting, reactor composting, and reactor composting combined with scrubbers. The results showed that windrow composting and reactor composting emitted 193 g NH₃ and 75 g NH₃ for 1 ton dry compost materials, respectively. Compared with traditional windrow composting, reactor composting reduced ammonia emissions by 61.1% (P<0.01), which was further increased to 82.3% (P<0.01) when the reactor was composted with a gaseous scrubber. The ammonia recovery rate decreased with an increase in the absorption time. The result showed that ammonia recovery rate of the scrubber was 82.0% (P<0.05) when the solution was fresh and gradually decreased to 39.8% (P<0.05) after 5 h. In addition, the ammonia recovery rate was significantly influenced by the temperature and NH₄⁺-N concentration of the absorption solution. For instance, the ammonia recovery rate decreased from approximately 80% to 20% when the temperature of the solution increased from 30 ℃ to 40 ℃ to 50 ℃. In terms of the cost of composting, reactor composting needed 116.9 Yuan per ton of manure, which was higher than that of traditional windrow composting (87.4 Yuan per ton of manure). The ammonia reduction costs of reactor composting and the scrubber were 22.0 Yuan per kg NH₃ and 16.5 Yuan per kg NH₃, respectively, when compared with windrow composting, which was lower than the value of the European Union. In addition, reactor composting had a higher efficiency than windrow composting, with composting cycles of 8 and 45 days for reactor composting and windrow composting, respectively. In conclusion, reactor composting can significantly reduce ammonia emissions, and most of the ammonia can be recovered when combined with a gaseous scrubber. The pH, NH₄⁺-N concentration, and temperature of the scrubber solution significantly affected the ammonia recovery rate. Improving the ammonia recovery rate and operational stability of the gas washing tower is a direction for future research to develop ammonia emission reduction technology. Consequently, reactor composting combined with a gaseous scrubber is recommended, which is an efficient and feasible composting technology with great potential for reducing ammonia emission.