Process integrating phosphorus crystallization with A/O on treating biogas slurry of swine digestate

A large amount of livestock wastewater is ever-increasing in China. Especially, swine wastewater has drawn great concern as an important agricultural pollution source. Digestion can usually be used to produce a substantial amount of digestate for the pre-treatment of swine wastewater. A considerable proportion of swine digestate has to be discharged, in view of the limited capacity of land utilization. However, it is still lacking in the efficient treatment for the swine digestate containing great concentrations of contaminants in many swine farms at present. In this study, an engineering implementable approach was designed to integrate phosphorus crystallization with the A/O (anoxic/aerobic) process. The swine digestate was also treated to achieve the standard discharge, in order to efficiently recover the phosphorus resources. Both the anoxic and aerobic units include two series-wound reactors. Among them, the anoxic reactors were in columnar closed-to-plug flow reactors. The aerobic reactors were approximately fully mixed. Prior to the experiments, the device was running for two months. A mature biofilm was also attached to the carriers. The sludge reflux was avoided to consume the energy and biomass in the reactors. Ceramsite and fibers were added into the anoxic and aerobic reactor, respectively, as the biofilm carriers, respectively; The filling rate of the material was 50%. The efficiency and mechanism of pollutant removal were investigated using a self-designed experimental device. The real swine digestate was taken from dry manure in the swine farm as test water. The hydraulic residence times of anoxic and aerobic reactors were 3.4 and 8.6 h, respectively, in the A/O system; The nitrification liquid reflux ratio was 200%; the hydraulic residence time of the crystallization reactor was 15 min. Results showed that the process effectively removed those pollutants, where the removal rates of TP, NH₄⁺-N, TN, and COD were 91.20%, 94.67%, 83.28%, and 96.18%, respectively. The anoxic unit was the primary contributor to the COD removal with a contribution of 75.24%; The organics were mainly consumed by heterotrophic denitrification in the anoxic unit. In addition to heterotrophic denitrification, anammox was the possible mechanism to result in the TN removal. NH₄⁺-N was removed with the contribution rate of 17.30%. The anoxic unit contributed 47.66% to the TN removal in total. Microbial community structure analysis showed the presence of bacteria for anammox, in addition to heterotrophic denitrification/bacteria for partial denitrification in biofilm from anoxic unit. The aerobic unit contributed 62.75%, and 24.76% to the removal of NH₄⁺-N, and COD, respectively. Remarkably, the aerobic unit also contributed to the TN removal with a contribution of 30.92%. In addition to the assimilation of microorganisms, the TN removal was caused by simultaneous nitrification and denitrification, as well as anammox. Bacteria were detected to nitrify and denitrify anammox in the aerobic unit. A novel reactor was employed for phosphorus crystallization. The reactor shared the structure, including internal and external concentric cylinders. The mixed liquid was circulated in the reactor. There was a density difference between the internal and external cylinders during aeration in the internal cylinder. Moreover, a three-phase separator was added at the top of the reactor to effectively intercept the fine crystal core, in order to realize the efficient recovery of phosphorus. The phosphorus crystallization unit contributed 93.44% to the TP removal, while contributed 19.92% and 21.39% to NH₄⁺-N and TN, respectively. The product of phosphorus crystallization was MgNH₄PO₄.