Enhancement of nitrate removal in constructed wetlands utilizing a combined autotrophic and heterotrophic denitrification technology for treating hydroponic wastewater containing high nitrate and low organic carbon concentrations

• Heterotrophic denitrification (HD) process is commonly used in constructed wetlands.
• Constructed wetlands are not sufficient for treating hydroponic wastewater.
• Hydroponic wastewater contain high nitrate and low organic carbon.
• Autotrophic denitrification (AD) process is efficient for hydroponic wastewater removal.
• The use of combined HD and AD techniques improves nitrate removal in CWs.

To enhance the nitrate removal in constructed wetlands (CWs) for treating hydroponic wastewater discharged from greenhouses, the effectiveness of HF (horizontal flow)-HF hybrid CWs utilizing a combined sulfur-based autotrophic (based on the optimum conditions from batch experiment) and heterotrophic denitrification was evaluated for treating hydroponic wastewater containing high nitrate and low organic carbon concentrations. The optimum ratio of sulfur: limestone:immobilized bead with Thiobacillus denitrificans (T. denitrificans) was found to be 3:1:4; the optimum initial cell density was above 1 × 106 cells; the optimum temperature was 25–35 °C; and the optimum sulfur sources were thiosulfate and elemental sulfur to effectively treat hydroponic wastewater utilizing autotrophic denitrification with T. denitrificans in batch experiments. In the HF–HF CWs utilizing the combined autotrophic and heterotrophic denitrification, the average removal efficiencies of nitrate were higher in the order of T2 (71.5%, thiosulfate treatment—combination of heterotrophic and autotrophic denitrification) >T3 (66.6%, element sulfur treatment—combination of heterotrophic and autotrophic denitrification) ≫T1 (43.0%, control—heterotrophic denitrification only). In the HF–HF CWs, the maximum nitrate removal efficiency by the thiosulfate treatment was slightly greater than that by the treatment with elemental sulfur, whereas the sulfate production influence on autotrophic denitrification by elemental sulfur (SO42−: 89.1 mg L−1) was lower as compared to thiosulfate (SO42−: 38.3 mg L−1). Because the sulfate production is an important factor to meet acceptable drinking water quality discharge standard (Sulfate concentration in the effluent was below 250 in US EPA, and 200 mg L−1 in South Korea), elemental sulfur was a more suitable sulfur source in HF–HF hybrid CWs. Overall, a combined process of using E/L/B (element sulfur/limestone/immobilized bead with T. denitrificans) column in HF–HF hybrid CWs would promote autotrophic and heterotrophic denitrification. Therefore, a combined autotrophic and heterotrophic denitrification process in HF–HF CWs would be more suitable than the heterotrophic denitrification alone (conventional technology in CWs) for treating nitrate in hydroponic wastewater since hydroponic wastewater contains little organic carbon.