Pilot-scale study on the treatment of swine wastewater by constructed wetland consisted with exoelectrogens-microalgae biofilm

Abstract: Constructed wetland (CW) is widely used in the treatment of swine wastewater, due to the operation cost-saving and simple maintenance. However, the pollutant removal efficiency is greatly affected by temperature in CW technology. Fortunately, the exoelectrogens-microalgae biofilm is composed of extracellular respiratory bacteria and microalgae, particularly with the high efficiency, energy saving, and low cost in wastewater treatment. Therefore, the stability of CW can be enhanced by the exoelectrogens-microalgae biofilm at low temperatures. It is a high demand to evaluate the technical reliability of long-term outdoor operations. This study aims to improve the treatment efficiency and microbial community of swine wastewater under natural temperatures, in order to explore the effect of temperature on exoelectrogens-microalgae biofilm in the enhanced CW system. Extracellular respiratory bacteria were achieved in the direct interspecific electron transfer process through direct contact or conductive substance mediation. As such, the hydrogen partial pressure and the concentration of metabolite greatly improved the electron transport efficiency and metabolic rate. Extracellular respiratory bacteria mainly included electroactive bacteria, such as Shewanella. The extracellular respiratory bacteria produced carbon dioxide by metabolizing organic matter, which was used as a carbon source for the microalgae. The oxygen produced by microalgae was used as an electron acceptor for the extracellular respiratory bacteria, thus realizing the material and energy mutual camping between them. Therefore, the construction of exoelectrogens-microalgae biofilm was a feasible way to improve the efficiency of CW wastewater treatment. There were high concentrations of carbon, nitrogen, and phosphorus in the swine wastewater. The treatment efficiency of swine wastewater and microbial community of the system under natural temperature was determined in the coupling exoelectrogens-microalgae biofilms (EMB) with the CW system. The outdoor experiment then lasted for 18 months. The results showed that the removal rates of chemical oxygen demand (COD), ammonia nitrogen, nitrate nitrogen, total nitrogen, total phosphorus, and nitrite nitrogen in the EMB-CW group in summer and winter reached 98.26/96.10%, 97.96/91.56%, 85.45/65.29%, 95.07/89.94%, 91.44/92.12%, and 85.45%/83.15%, respectively, with the strong adaptability at low temperature. The effluent concentrations of COD, ammonia nitrogen, nitrate nitrogen, total nitrogen, total phosphorus, and nitrous nitrogen were 97.50, 12.70, 1.24, 24.44, 1.97, and 0.17 mg/L, respectively, which fully met the Emission Standard for Livestock and Poultry Industry (GB18596-2001) in China. Compared with the CW group, the removal rates of the above pollutants in the EMB-CW group increased by 5.75, 6.95, 8.00, 13.75, 2.38, and 20.93 percentage points, respectively. Cyanobium, Shewanella and Azoarcus were the dominant microorganisms of the EMB-CW group in both winter and summer, and the stability of community structure was high, which was conducive to the removal rate of pollutants in winter for the reliability of the EMB-CW system. Consequently, the exoelectrogens-microalgae biofilm significantly improved the pollutant treatment efficiency and stability of the CW system under low-temperature conditions. The finding can provide an idea for the treatment of rural livestock and poultry farming wastewater. Therefore, better social benefits and technically feasible can be expected to gain for the application of exoelectrogens-microalgae biofilm in practice.