The progress of composting technologies from static heap to intelligent reactor: Benefits and limitations

Composting technology is widely used and different technologies have been applied from static heap to windrow composting, and to intelligent reactor composting since the rapid development of landless intensive livestock farms and improved awareness of environmental impacts. However, the lack of a systematic analysis has limited the objective evaluation of these different technologies from the perspective of treatment efficiency, and environmental and economic benefit. This study aimed to use quantitative data from full-scale composting systems (static heap, windrow composting and reactor composting with 8 t treatment capacity per day) installed on a commercial pig farm (6000 head) to compare their treatment efficiency, nutrient losses, product quality and investment and operational costs. The results showed that fresh feedstock entered the thermophilic phase in reactor composting much more quickly (within a few hours) than the other composting methods, and maintained a relatively stable high temperature (55–65 °C). This improved the biodegradation process and shortened the composting period. Within the first week, the organic matter degradation in the reactor composting treatment reached 35.7%, significantly greater than for the other treatments. Rapid heating and sustained high temperature promoted the removal rates of the antibiotics tetracycline, doxycycline and sulfamethoxazole. More than 90% of these antibiotics were degraded in the reactor composting treatment, and antibiotic resistance gene abundance was significantly reduced by 79% after composting. Nitrogen loss from the reactor composting was 34% less compared with windrow composting. Although the reactor composting has a higher equipment cost and greater depreciation than the other composting technologies, the total investment and operational costs are comparable to windrow composting. Moreover, with low nitrogen loss, high antibiotic and resistance gene removal rates, reactor composting has benefits regarding nutrient use efficiency and environmental impact.