Nitrogen removal in integrated anaerobic-aerobic sequencing batch reactors and constructed wetland system: a field experimental study

A study was conducted to assess the performance of an integrated system consisting of an anaerobic digester (AD), an aerobic sequence batch reactor (ASBR), and three horizontal subsurface flow constructed wetlands (HSSFCWs) vegetated with the perennial grass Phragmites karka for removal of nitrogen in tannery wastewater. Nine grab wastewater samples were collected weekly between September and December 2016 from the influent and effluent at each unit of operation of the plant and analyzed for physicochemical parameters. The AD removed 67% of oxidized nitrogen (NOX-N); the removal was improved to 76% in the ASBR with a further aeration period in the HSSFCWs, and the NOX-N removal efficiency improved progressively to 96%. The removal efficiency for nitrate (NO3-N) was 82% and for nitrite (NO2-N) 88%; for ammonium (NH4-N) removal efficiency was 77% higher in the HSSFCWs compared to the AD and the ASBR. Despite the considerable percentage of NO3-N removed, in the effluent was above the country's permissible limit (39.3mg/l). The high concentration might be due to a weaker denitrification process in HSSFCWs caused by lack of carbon and external organic sources in the wastewater sufficient to carry out the process. The results suggest that the addition of supplementing carbon sources (methanol, sugars, or volatile fatty acids) to the effluent would achieve better performance. Based on these findings, the application of combined anaerobic/aerobic system connected with constructed wetlands process allows the achievement of higher efficiency in removing nitrogen compared to the use of an anaerobic, aerobic, or constructed wetland system along.