Production of nitrate-rich compost from the solid fraction of dairy manure by a lab-scale composting system

•Nitrate-rich compost was efficiently produced from solid fraction of dairy manure.•Shorter thermophilic phase duration and lower aeration rate led to less N loss.•Shortened thermophilic phase duration had negligible effect on VTS degradation.•Nitrification was enhanced by an improved reactor with removal of condensate.•NO3− content in the end product was 15.5 × 103 mg kg−1-TS and GI reached 112.1%.

In the present study, we developed an efficient composting process for the solid fraction of dairy manure (SFDM) using lab-scale systems. We first evaluated the factors affecting the SFDM composting process using different thermophilic phase durations (TPD, 6 or 3 days) and aeration rates (AR, 0.4 or 0.2 l min−1 kg−1-total solid (TS)). Results indicated that a similar volatile total solid (VTS) degradation efficiency (approximately 60%) was achieved with a TPD of 6 or 3 days and an AR of 0.4 l min−1 kg−1-TS (hereafter called higher AR), and a TPD of 3 days resulted in less N loss caused by ammonia stripping. N loss was least when AR was decreased to 0.2 l min−1 kg−1-TS (hereafter called lower AR) during the SFDM composting process. However, moisture content (MC) in the composting pile increased at the lower AR because of water production by VTS degradation and less water volatilization. Reduced oxygen availability caused by excess water led to lower VTS degradation efficiency and inhibition of nitrification. Adding sawdust to adjust the C/N ratio and decrease the MC improved nitrification during the composing processes; however, the addition of increasing amounts of sawdust decreased NO3− concentration in matured compost. When an improved composting reactor with a condensate removal and collection system was used for the SFDM composting process, the MC of the composting pile was significantly reduced, and nitrification was detected 10-14 days earlier. This was attributed to the activity of ammonia-oxidizing bacteria (AOB). Highly matured compost could be generated within 40-50 days. The VTS degradation efficiency reached 62.0% and the final N content, NO3− concentration, and germination index (GI) at the end of the composting process were 3.3%, 15.5 × 103 mg kg−1-TS, and 112.1%, respectively.