Research on comprehensive integration process and recycling technologies of biogas slurry

Abstract: As a new direction for treating and utilizing the biogas slurry, the filtration and membrane concentration has received widespread attention in China. The corn stalk is a biodegradable recycled material that is good for composting. In this study, the corn stalk, combined with volcanics and quartz sand, were used as the pretreatment material for the filtration of biogas slurry. The filter materials were filled in different filters (10 cm in diameter and 1 m in height) connected in series through putting the corn stalk in the first filter, volcanics in the second one and the quartz sand in the last one, respectively. Using the ultrafiltration membrane and nanofiltration membrane for biogas slurry concentration, the effect of the filtration on membrane concentration was studied and then the possibility of composting the corn stalk was investigated. The results showed that, after filtrating through the corn stalk, volcanic rock and quartz sand successively, the content of suspended solids in the biogas slurry was reduced from 3830 (in initial) to 142 mg/L, with more than 95% of the removal rate of the suspended solids. In addition, the COD was decreased, while other parameters did not changed significantly. Since a great amount of suspended solids in the biogas slurry was intercepted after filtration, resulting an appropriate C/N ratio for composting, the corn stalk can be composted successfully even under the condition that the initial moisture content was 75%. During the composting process, the temperature increased normally and maintained approximately 10 d of high temperature. After 28 d of composting, the corn stalk could meet the requirements of maturity, with a 5% total nutrient (total nitrogen, phosphorus and potassium) and slightly changed moisture. After ultrafiltration and nanofiltration, in the concentrated solution the concentrations of the total nitrogen, phosphorus and potassium were increased by 2.6, 1.9 and 4.5 times, respectively, when compared with untreated biogas slurry. Additionally, the Mg/Fe ratio and the amino acid concentration were increased by more than 10 and 8.8 times, respectively. After membrane concentration, the fertilizer potential of the biogas slurry was improved greatly, and the permeate liquid of the nanofiltration could achieve basic requirement of the farmland irrigation water quality standards and livestock and poultry breeding industry pollutant discharge standards. Furthermore, although the ammonium nitrogen was exceed, the BOD5, COD, suspended solids, total phosphorus, fecal coliform, and the ova of roundworm could all meet the requirement of the both standards. After some special treatment for the ammonium nitrogen, the permeate liquid may be used as the irrigation water or drained directly. This study can provide a good reference for the treatment and utilization of biogas slurry.