Effect of myriophyllum elatinoides wetland on nitrogen removal from swine wastewater under different pollution loads

Pollutions caused by livestock industry have posed substantial concerns on ecological environment. The random discharge of swine wastewater has resulted in excessive nutrients transport to rivers and lakes, in turn inducing eutrophication of surface water. Given that China is one of the largest pork producers in world, the treatment of swine wastewater is becoming a very urgent issue. In this study, 9 pilot-scale surface flow constructed wetlands vegetated with M. elatinoides were constructed to treat swine wastewater, in order to investigate nitrogen removal effect and influence factors of myriophyllum elatinoides wetlands. The pilot-scale M. elatinoides wetlands were exposed to swine wastewater under three strengths: low loading rates (swine wastewater diluted with fresh water at a 1:2 ratio), medium (swine wastewater diluted with fresh water at a 2:1 ratio), and high (pristine swine wastewater without dilution). The M. elatinoides wetlands were operated using an intermittent flow regime with a total volume of 180 L/d, and the theoretical hydraulic retention time was 33 d in the surface flow constructed wetlands. Water samples (inflow and outflow) were collected once a month from July 2014 to May 2015 for measuring ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), total nitrogen (TN), and chemical oxygen demand, and concurrently the water temperature, pH, and dissolved oxygen were determined in the field. Monthly sediment samples were collected to determine NH4+-N, NO3--N, TN, and dissolved organic carbon. The linear mixed-effect model was used to explore the key factors affecting NH4+-N removal in the M. elatinoides wetlands. The results showed that during the whole test period (July 2014 -May 2015), the removal rates of NH4+-N and TN in low, medium, and high load wastewater were relatively high, where the removal rates of NH4+-N and TN were 85.0%-98.7% and 83.6%-97.1%, respectively. Nitrogen removal was different under different pollution loads, whereas, the removal rates of NH4+-N and TN decreased with the increase of pollution loads. The results of linear mixed model analysis showed that the key environmental factors affecting NH4+-N removal in the wetland were the dissolved oxygen and NO3--N in wastewater, and the content of NH4+-N in sediment, where the dissolved oxygen in wastewater presented the greatest impact on NH4+-N removal in the wetland. In every unit increased in the concentration of dissolved oxygen, the effluent concentration of NH4+-N decreased by 1.33% on average. With the accumulation of NH4+-N in sediment, the effluent NH4+-N concentrations increased significantly (P<0.001). As the dissolved oxygen in wastewater decreased as the pollution loads increased, the sediment NH4+-N increased as well, indicating that the variation in nitrogen removal of different pollution loads in the M. elatinoides wetlands. The removal rates of NH4+-N and TN in wetlands under different pollution loads were above 80.0%, although gradually decreased with the increase of pollution loads. Therefore, the M. elatinoides can be used as an ammonia-tolerant plant to treat high-load swine wastewater. The finding can provide a promising potential application of M. elatinoides wetlands in large-scale farms.