Influence of septic tank attached growth media on total nitrogen removal in a recirculating vertical flow constructed wetland for treatment of domestic wastewater

Among different decentralized wastewater treatment systems, constructed wetlands are particularly robust, reliable and cost-effective technologies. However, traditional treatment wetland designs have a limited capacity to remove total nitrogen. The recirculating vertical-flow constructed wetland (RVFCW) system is a novel modification of the vertical flow wetland (VFW), allowing for increased denitrification by circulating the nitrified effluent back into a recirculation tank, where it is mixed with primary treated wastewater. Microcosm experiments were conducted to investigate the effects on nitrogen removal of mixing recirculated VFW effluent with raw wastewater after different degrees of primary treatment, with and without attached growth media. The results show that the inclusion of attached growth media in the first chamber of the recirculation tank resulted in enhanced total nitrogen removal. The microcosms that contained a mixture of raw wastewater and VFW effluent showed denitrification efficiency of 83% after 48 h of contact time. An increase in the denitrification efficiency (up to 99.5%) was observed in microcosms that also contained attached growth media. The majority of nitrate-N (NO3-N) removal was achieved in the first 24 h. Inclusion of media increased the denitrification efficiency after 48 h contact time from 36 to 93% and from 31 to 88% in microcosms containing VFW effluent mixed at a ratio of 3:1 with wastewater after 1 and 2 days residence time in a septic tank respectively. It was inferred that the lower the degree of pre-treatment of wastewater into which the recirculated VFW effluent was mixed, the greater is the denitrification rate and thereby the lower TN concentration in the effluent.