Straw filtering wastewater and aerobic composting of filter residues and manure on a pig farm

Pig-farm wastewater is characterized by high suspended solids, organic matter, and ammonia nitrogen content. It is very difficult to treat, due mainly to the complex composition of the pig manure and feed residues. Fortunately, the straw with a large specific surface area can be expected to serve as the better retention and adsorption for the suspended solids and nutrients. For example, nitrogen content decrease can be greatly contributed to the subsequent resource utilization of pig-farm wastewater. However, it is still a new challenge for the efficient reutilization of straw filter residues. Alternatively, aerobic composting can be used to produce the organic fertilizer for better soil fertility and structure, due to the easy operation, short reaction period, better reduction, cost saving, and environmentally friendly. It is a promising way to simultaneously dispose of straw filter residues and pig manure in an intensive pig farm. Compared with the dry straw, the filtered straw is easily decomposed to participate in the composting process, where the high porosity can facilitate to absorb the water and swells. Meanwhile, the composting process is accompanied by the emissions of greenhouse gases and stenches, such as methane (CH4), carbon dioxide (CO2), nitrous oxide (N2O), and ammonia (NH3), due to the complex composition of pig manure. Moreover, the composting microorganisms can quickly utilize the ammonium nitrogen that is adsorbed by the straw in the process of filtration. But the ammonium nitrogen is also easily volatilized to produce the NH3 for secondary pollution, leading to the reduced quality of compost due to nitrogen loss. However, it is still unclear on the relevant composting characteristics of the straw filter residues mixed with the pig manure. This study aims to filter the pig farm wastewater through maize straw and then systematically investigate the aerobic composting of straw filter residues and pig manure, the conversion of carbon and nitrogen during composting, as well as the emission of harmful gases. The results showed that the optimal conditions for the maize straw filtering the pig farm wastewater were as follows. The bulk density and height of the maize straw filter layer were 0.15 g/cm3 and 40 cm, respectively, whereas, the inner diameter of the filter column was 9 cm. The removal rates of Total Nitrogen (TN), Total Suspended Solids (TSS), and Chemical Oxygen Demand (COD) in the pig farm wastewater were 22.80%, 51.60%, and 76.81%, respectively, under the optimum conditions. Furthermore, the higher the initial ratio of the carbon to nitrogen (C/N) was, the better the composting was, the more the Total Organic Carbon (TOC) loss was, and the less the TN loss was, when the initial C/N, ambient temperature, moisture content, and ventilation rate were 20-35, 22.32-32.05 ℃, 65%, and 0.2 m3/h, respectively. The harmful gas emissions mainly occurred in the early stage of composting. At the initial C/N of 35, the highest composting temperature reached 65.96 ℃ and the high-temperature period (>50℃) was maintained for 21 days. Among them, the maintenance time above 60 ℃ was up to 12 days. The seed germination index (GI), the loss ratio of TOC and TN after composting of 36 d reached 81.03%, 57.73%, and 10.08%, respectively. Although the emissions of CH4 and CO2 increased, there was a significant decrease in the emissions of NH3 and N2O, as well as the nitrogen loss during composting. The potential greenhouse effect was 137.53 kg/t (in CO2 equivalent) for the three greenhouse gases of CH4, CO2, and N2O. Therefore, the local anaerobic reaction can be avoided to reduce the generation of CH4 under high C/N conditions. The findings can provide the favorable support for the resource utilization of the straw filter residues and the emission reduction of harmful gases in the aerobic composting process of straw filter residues and pig manure.