Nitrogen removal from wastewater plant secondary effluent in a compound natural treatment system

A method for removal of low concentrations of nitrogen in the secondary effluent from a wastewater plant was evaluated. The method can be summarized as follows: Primary Subsurface Vertical Flow Wetland (PSVFW) → Submerged Macrophyte Oxidation Ponds (SMOPs) → Secondary Subsurface Vertical Flow Wetland (SSVFW). Pilot experiments were conducted to assess the efficiency of total nitrogen (TN) removal from secondary effluent from the Chengjiang Wastewater Treatment Plant, China. At a total hydraulic retention time (HRT) of 21.38 days and a hydraulic loading (HL) of 0.024 m3 m−2 d−1, the average TN concentration in the influent decreased from 6.60 mg L−1 in the influent to 0.87 mg L−1 in the effluent, thus meeting the Class-III surface water standards. PSVFW mediated the ammonification of organic nitrogen (ORG-N) and the nitrification of NH3-N (ammonia), resulting in a TN reduction of 7.58%. In the SMOPs, the concentration of NH3-N was further reduced, and most NO3−-N (nitrate nitrogen) was denitrified under aerobic conditions, resulting in a 63.18% TN reduction. In the SSVFW, the concentrations of the remaining NO3−-N and ORG-N declined even further so that the whole process accounted for an 86.82% TN removal efficiency relative to the influent TN concentrations.