Simultaneous Removal of Nitrate and Sulfate from Greenhouse Wastewater by Constructed Wetlands

This study evaluated the effectiveness of C-enriched subsurfaceflow constructed wetlands in reducing high concentrations of nitrate (NO3 -) and sulfate (SO4 2-) in greenhouse wastewaters. Constructed wetlands were filled with pozzolana, planted with common cattail (Typha latifolia), and supplemented as follows: (i) constructed wetland with sucrose (CW+S), wetland units with 2 g L-1 of sucrose solution from week 1 to 28; (ii) constructed wetland with compost (CW+C), wetland units supplemented with a reactive mixture of compost and sawdust; (iii) constructed wetland with compost and no sucrose (CW+CNS) from week 1 to 18, and constructed wetland with compost and sucrose (CW+CS) at 2 g L-1 from week 19 to 28; and (iv) constructed wetland (CW). During 28 wk, the wetlands received a typical reconstituted greenhouse wastewater containing 500 mg L-1 SO4 2and 300 mg L-1 NO3 -. In CW+S, CW+C, and CW+CS, appropriate C:N ratio (7:3.4) and redox potential (-53 to 39 mV) for denitrification resulted in 95 to 99% NO3 removal. Carbon source was not a limiting factor for denitrification in C-enriched constructed wetlands. In CW+S and CW+CS, the dissolved organic carbon (DOC)/SO4 2ratios of 0.36 and 0.28 resulted in high sulfate-reducing bacteria (SRB) counts and high SO4 2removal (98%), whereas low activities were observed at DOC/SO4 2ratios of 0.02 (CW) to 0.11 (CW+C, CW+CNS). On week 19, when organic C content was increased by sucrose addition in CW+CS, SRB counts increased from 2.80 to 5.11 log[CFU+1] mL-1, resulting in a level similar to the one measured in CW+S (4.69 log[CFU+1] mL-1). Consequently, high sulfate reduction occurred after denitrification, suggesting that low DOC (38–54 mg L-1) was the limiting factor. In CW, DOC concentration (9–10 mg L-1) was too low to sustain efficient denitrification and, therefore, sulfate reduction. Furthermore, the high concentration of dissolved sulfides observed in CW+S and CW+CS treated waters were eliminated by adding FeCl3. Simultaneous Removal of Nitrate and Sulfate from Greenhouse Wastewater by Constructed Wetlands