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 (NO 3 -) and sulfate (SO 4 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 SO(4)(2-)and 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)/SO(4)(2-)ratios of 0.36 and 0.28 resulted in high sulfate-reducing bacteria (SRB) counts and high SO 4 2-removal (98%), whereas low activities were observed at DOC/SO(4)(2-)ratios 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