Effect of pre-treatment of wastewater in HRAP on nitrogen removal in subsurface flow gravel bed constructed wetland

Abstract

This study discusses the effect of application of High Rate Algal Pond (HRAP) as a pre-treatment unit on nitrogen transformation and removal in Sub-surface flow Gravel bed Constructed Wetland (SGCW). Experiments were carried out at the University of Dar es Salaam main campus wastewater treatment facility in two different settings. In Phase I, effluent from primary facultative pond was fed directly to unplanted gravel bed SGCW, with a discharge of 0.69 m3/d. During Phase II, the HRAP with 0.6 m depth, received effluent from primary facultative pond at a flow rate of 1.2 m3/d and discharged it to unplanted gravel bed SGCW with 0.8 m deep, filled to 10 cm above water level with 20 mm diameter aggregates, at a flow rate of 0.69 m3/d. Both HRAP and SGCW were operated at a retention time of 5 days. The results showed that improved nitrogen removal occurred with inclusion of HRAP unit. In Phase I, an average nitrogen removal of 41.3% was achieved, while in Phase II total nitrogen reduction in SGCW improved to 57.3%. These results show the significance of HRAP for better performance of nitrogen removal in gravel bed SGCW. It was further observed that the removal efficiency of nitrate-nitrogen was superior in SGCW phase II (68.2%) compared to SGCW phase I (20.7%). Hence this research proved that provided the influent is fully nitrified in the pre-treatment system such as HRAP, denitrification becomes the major contributor to the nitrogen removal process in SGCW.