Contribution of Acid Additive to Co-Composting of Chicken Manure: Gas Emission Reduction and Economic Assessment

Acidic additives have garnered significant attention due to their ability to reduce ammonia (NH3) emissions, enhance nitrogen retention, and balance function with cost-effectiveness. This study aimed to investigate the potential of oxalic acid (OA) in reducing gas emissions, promoting compost humification, and enhancing nutrient retention during the co-composting of chicken manure. Moreover, the comparative analysis of the effects and the economic benefits was conducted among OA groups with varying concentrations (OA1: 0.03 mol·kg−1, OA2: 0.10 mol·kg−1, and OA3: 0.15 mol·kg−1) and a sulfuric acid (SA) group (SA1: 0.03 mol·kg−1). The results indicated that the addition of OA can extend the thermophilic phase to some extent and achieve the required composting maturity. As the amount of OA increased, the NH3 emissions (p < 0.05) and the total nitrogen loss rate was decreased; however, this also increased the cost. No significant difference in the total nitrogen loss rate was observed between SA1 and OA1 (p < 0.05). It is worth noting that the addition of SA increased the SO42− content, leading to an increase of 113.52% in H2S emissions. In contrast, the addition of OA resulted in a reduction in H2S emissions by 29.92–45.90%. In terms of economic analysis, OA1 was the most effective (OA1 > OA2 > OA3 > SA1). Thereby, OA was proved to be a good alternative for SA in the co-composting of chicken manure, and 0.03 mol·kg−1 OA is recommended.