Nitrogen removal in saturated zone with vermicompost as organic carbon source

Abstract

Bioretention basin or commonly known as rain garden is one of the stormwater management best practices for reducing runoff pollutants while providing aesthetic appeal. Rain gardens typically comprise three layers, namely, ponding, retention and drainage. In this study, a fourth layer, the saturated zone, above the drainage layer was studied at the laboratory scale to promote denitrification activity. The effects of vermicompost composition (used as organic carbon source for denitrification process) and empty bed contact time (EBCT) were evaluated, in laboratory column tests, on nitrogen removal performance achieved in this zone. Vermicompost of 2 and 5 vol%, with the remaining portion comprised of sand in the media mixture, was used as organic carbon source in the saturated zone, while media with 100% sand was used as a control. Runoff was simulated using diluted mem- brane bioreactor effluent, having total nitrogen (TN) concentrations of 7.0-12.8 mg L-1, was used as influent. Vermicompost composition of 5% at 3-h EBCT provided significantly higher average TN removal efficiency of 52 ± 20%. This was 3 and 2 times more than that of 100% sand and 2% vermicompost, respectively (p < 0.05, based on 2-sample t-test). This study also demonstrated that the EBCT and the amount of organic carbon source used in the saturated zone are important design criteria for optimum nitrogen removal.