Swine wastewater is characterized by high levels of ammonia nitrogen (NH₄⁺-N), suspended solids (SS), and chemical oxygen demand (COD). The discharge of untreated swine wastewater results in excessive nutrient transport to streams, rivers, and lakes, in turn inducing eutrophication of surface water. Furthermore, groundwater pollution has also been shown to occur as a result of swine wastewater discharges. 

In recent years, environmental pollution problems caused by swine production have created substantial concern in China. The swine industry is located in large part in Chinese rural areas, where it is not feasible to build sewage treatment plants in these locations given the high construction and operation costs involved. Low-cost constructed wetlands (CWs) have been utilized to control pollution from animal wastewater in many sites all over the world. Surface-flow CWs with macrophytes have shown good performance in treating swine wastewater. 

Using a N mass balance method, researchers from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) investigated nitrogen removal processes in Myriophyllum aquaticum mesocosms with swine wastewater (SW), 50% diluted swine wastewater (50% SW), and two strengths of synthetic wastewater, 200 mg NH₄⁺-N L^-1 (200 NH₄⁺-N) and 400 mg NH₄⁺-N L^-1 (400 NH₄⁺-N). 

The researchers found that, after a 28-day incubation, the average NH₄⁺-N and TN removal rates were 99.8% and 94.2% for 50% SW and 99.8% and 93.8% for SW, which were greater than 86.5% and 83.7% for 200 NH₄⁺-N, and 73.7% and 74.1% for 400 NH₄⁺-N, respectively. "This result reflects a maximum areal TN removal rate was found in M. aquaticum mesocosms with 50% SW and SW"said Dr LIU Feng, a lead researcher at ISA. "During the incubation period, the observed dynamics of NO₃^--N concentrations in water and gene copy numbers of ammonia-oxidizing archaea (AOA), ammonia-oxidizing (AOB), nirK and nirS in soil unraveled strong nitrification and denitrification processes occurring in M. aquaticum mesocosms with swine wastewater". 

The researchers also found that plant uptake and soil N accumulation accounted for 17.9-42.2% and 18.0-43.8% of the initial TN load, respectively. The coupled nitrification and denitrification process was calculated to account for, on average, 36.8% and 62.8% of TN removal for 50% SW and SW, respectively. These findings demonstrated that the N uptake by M.aquaticum contributed to a considerable proportion of N removal. In particular, the activities of ammonia-oxidizing and denitrification microbes responsible for nitrification and denitrification processes in M.aquaticum mesocosm accelerated NH₄⁺-N and TN removal from swine wastewater. 

The research was financially supported by the key CAS Programs (KZZD-EW-11-3), the CAS STS program (KFJ-EW-ZY-006), and the National Science and Technology Supporting Project (2014BAD14B01, 2014BAD14B05). 

The study entitled "Nitrogen removal and mass balance in newly-formed Myriophyllum aquaticum mesocosm during a single 28-day incubation with swine wastewater treatment" has been published in Volume 166, January 2016 of Journal of Environmental Management, details could be found at http://www.sciencedirect.com/science/article/pii/S0301479715303741.