Influence of low-temperature anaerobic fermentation on sedimentation characteristics of dairy wastewater

Large volume of dairy wastewater has caused a great threat to the ecological environment, due to the difficulty to treat it. Traditional treatment requires more heating energy at the least temperature of 36℃. Alternatively, non-hazardous fertilizers are widely expected to treat wastewater, particularly easy soluble in water to transfer through dripping or spraying irrigation. However, the chemical fertilizers derived from wastewater contain many organic matters and nutrients, resulting in the blockage of spray nozzles. In this study, a feasible bio-treatment of dairy wastewater was proposed using the low-temperature anaerobic fermentation, thereby lowering energy consumption, while increasing the fluid permeability through dripping or spraying irrigation. The treated dairy wastewater was observed in the anaerobic fermentation at low temperatures (10-25 ℃) through an array of time points by no inoculants, as well as at a normal temperature of 36 ℃ by inoculants of granular anaerobic sludges. A full-length 16S rRNA gene sequencing was used to detect microbiological compositions. Response surface analysis was performed on fermentation parameters. The results showed that there was a significant influence of anaerobic fermentation under low temperature on the sediment characteristics of dairy wastewater. The sedimentation and layering performance of wastewater were significant under the condition of 9 days at 10 ℃, where the turbidity of supernatant was about 5. The most energy-efficient and economical condition was at 15 ℃ in the 4 d for the low-temperature anaerobic treatment. There was an average energy saving of 2 865 kW•h/d by l5 ℃ anaerobic fermentation with ambient temperature around 7.8 ℃, compared with fermentation at 36 ℃. Low-temperature anaerobic fermentation changed the sedimentation characteristics of wastewater over a longer time but with equal quality, where supernatant CODCr was reduced to (2 627.8±548.1) mg/L, nitrogen to (565.2±79.5) mg/L, fecal coliforms to around 360 g/L, the residue ratio of the dry filter by mesh diameter 75 μm filters approximately to 0 %. The dry matter ratio of supernatant dropped from 18.9% to 2.3% after treatment, while a part of of wastewater was recycled as irrigation water, where the total nutrients (N+P2O5+K2O) content of supernatants were 1.744 9 g/L. The number of fecal coliforms reduced down to 360 g/L, while, the mortality ratio of ascarid egg was above 95%. Both granular and low-temperature sludges showed the treating functions of wastewater (methanogenesis and wastewater turbidity change), but with different efficiency, where 199 otu were detected in the anaerobic sludges via the full-length 16S rRNA gene sequencing, while 207 otu in the acclimatized granular anaerobic sludges, and 96 otu were common in both sludges. Candidatus Cloacimonas was a dominant species in the long-term (over 6 mon) low-temperature conditioned anaerobic sludges, while genus Sulfurovum dominated in inoculated granular anaerobic sludges. In the study site with the mean annual temperature of 4.1 ℃, low-temperature (15 ℃) anaerobic fermentation saved much more energy consumption than the thermal anaerobic fermentation (36 ℃). The supernatant could transmit through to the nuzzle irrigation, the environmental non-hazardous proxies met with the discharge requirements of the national standard. The finding can provide a potential application for future recycling in agriculture and sustainable development.