Enhancing denitrification in constructed wetland with algae addition

Abstract

Constructed wetlands (CWs) can be used for tertiary treatment of wastewater; however, carbon source shortages limit denitrification. We studied the effect of algae addition as an external carbon source in CWs and found that the nitrogen removal efficiency of CWs is highly dependent on the algae dosage. Optimal nitrogen removal percentage (80.5%) can be achieved by adding 81.1 mg·L−1 dry weight algae to the influent when the chemical oxygen demand/nitrogen (COD/N) ratio reaches 5.3. Longitudinal changes in the nitrogen concentrations, organic matter concentrations, and nitrogen functional genes were also analyzed. The algae addition strengthened the anoxic environment, boosted the volatile fatty acid concentrations, and improved the ratio of nitrite reductase gene (nirS) and copper-containing nitrite reductase (nirK)/16S rRNA, as well as the ratio of nitrate reductase gene (narG)/16S rRNA, thereby expanding the active space for denitrification. The addition of algae could potentially provide enough carbon to enhance denitrification during treatment of wastewater with a low COD/N ratio.