Enhanced wastewater nutrients removal in vertical subsurface flow constructed wetland: Effect of biochar addition and tidal flow operation.

Abstract

Dissolved oxygen (DO) and carbon stock in substrate medium play a vital role in the nutrient removal mechanism in a constructed wetland (CW). This study compiles the results of dynamics of DO, ammonium N (NH4+-N), nitrate (NO3-N), sulfate (SO4-2), phosphate (PO4-3), chemical oxygen demand (COD), in three setups of vertical-flow constructed wetlands (TFCWs) (SB: substrate\xa0+\xa0biochar; SBP: substrate\xa0+\xa0biochar\xa0+\xa0Colocasia esculenta plantation; SP: substrate\xa0+\xa0Colocasia esculenta (SP), operated with tidal flow cycles. Experimental analyses illustrated the continuous high DO level (2.743-5.66\xa0mg\xa0L-1) in SB and SBP after the I and II cycle of tidal flow (72\xa0h flooding and 24\xa0h dry phase). COD reduction efficiencies increased from 15.75 - 61.86% to 48.55-96.80% after tidal operation among operating TFCWs. N (NH4+-N) and N (NO3-N) removal were found to be 88.16%, and 76.02%; 49.32, and 57.85%; and 40.23%, and 48.94\xa0% in SBP, SP and SB, respectively. The theory of improved nitrification and adsorption through biochar amended substratum was proposed for TFCW systems. PO4-3 and SO4-2 removal improved from 22.63 to 80.50%, and 19.69 to 75.20%, respectively after first tidal operation in all TFCWs. The microbial inhabitation on porous biochar could promote the transformation of available P into microbial biomass and also helped by the plant uptake process while SO4-2 reduction in TFCWs could be mainly due to sulfate-reducing bacterial activity and nitrate reduction process, mainly facilitated by high DO and biochar addition in such setups. The study suggests that effluent re-circulation through tidal operation and biochar supplementation in the substratum could be an effective mechanism for the improvement of the working efficiencies of CWs operated with low energy input systems.