UASB anaerobic fermentation and microbial community analysis of pig wastewater after straw filtration

Pig wastewater is often characterized by the complex composition of the protein, oil, cellulose, suspended and dissolved particulate solids. Among them, the suspended solids have posed a serious impact on biological fermentation. The granular sludge can clog to reduce the flocculation activity in the particle disintegration and inactivation. The reactor of sludge production can lead to shorten the age of sludge, thus affecting the reactor processing efficiency. The efficiency of the reactor can depend mainly on the shortening of sludge age, due to the increase of sludge production. Alternatively, the waste resources of corn stover are very rich in the annual output in China. Furthermore, the corn stover with a rich natural porous structure can be expected to adsorb and then maintain the suspended and dissolved particulate solids in the pig wastewater. Taking the corn stover after crushing and compaction as the filtration medium, negative-pressure filtration was carried out to remove the suspended and insoluble particulate solids in the pig wastewater, together with the parts of organic matter. The upper-flow anaerobic sludge bed/blanket (UASB) reactor was used for the anaerobic fermentation of filtered pig wastewater. The chemical oxygen demand (COD) removal rate, pH value and gas production were also explored with the increase of organic load in the fermentation. High-throughput sequencing was used to analyze the composition, bacterial and archaeal communities in the anaerobically digested sludge under the optimal load at the beginning of the experiment. The test results showed that the best performance of pig wastewater filtration was achieved, when the pressure was 40kpa, the thickness of the filter layer was 15cm, and the compaction degree of filter media was 1.6 times the density (148.8kg/m³). The removal rates of the total solid, volatile solid, and COD were 33.08%, 28.05%, and 23.01%, respectively. The anaerobic fermentation test showed that the maximum load of reactor was 11 kg/(m³·d) for the stable operation at the temperature of (35±1)℃, at which the influent COD concentration was 6 093 mg/L, the COD removal rate was 76.46%, and the gas production rate per unit volume was 1.21 m³/(m³·d); The optimal treatment load was 10.00 kg/(m3·d), at which the influent COD concentration was 6 068 mg/L, the COD removal rate was 60.26%, and the gas production rate per unit volume was 1.51 m³/(m³·d). The reactor was operated stably in the anaerobic fermentation without extreme acidification. The microbial diversity showed that the anaerobic fermentation was synergistic by a variety of microbial flora, where the main bacterial groups were Firmicutes and Bacteroidota in the hydrolysis and acidification stage; Specifically, the main archaeal group was Halobacterota at the phylum level. There was an abundant distribution of efficient methanogens in the UASB reactor at the methanogenic stage of anaerobic fermentation; The rich distribution of high-efficiency methanogens was also found with three methanogenic pathways, including the methyl-, acetate- and hydrogen-nutrient type. The findings can provide the technical application of crop straws to filter the pig wastewater in the waste treatment.