Pretreatment methods for aquatic plant biomass as carbon sources for potential use in treating eutrophic water in subsurface-flow constructed wetlands

Plant biomass is usually added to constructed wetlands (CW) to enhance denitrification. In this study, we investigated effects of different pretreatments on two common external plant carbon sources, cattail and reed litter. We determined the average ratio of chemical oxygen demand (COD) to total nitrogen (TN), designated as C/N, in water samples after addition of litter subjected to various pretreatments. The C/N in the water samples ranged from 4.8 to 6.4 after addition of NaOH-pretreated cattail litter, which was four to six times greater than that of water from the Yapu River and 3.84–39.15% higher than that of systems that received untreated cattail litter. The C/N of systems that received H₂SO₄-pretreated carbon sources varied from 1.7 to 3.6. These two methods resulted in TN and total phosphorus (TP) levels lower than those in river water. The C/N was 1.4–1.7 after addition of CH₃COOH-pretreated reed litter, which was 34.87–53.83% higher than that of river water. The C/N was 2.5 in systems that received mild alkali/oxidation-pretreated reeds, which was 30.59% higher than that of systems that received non-pretreated reeds. The residue rates of cattail and reed litter subjected to various pretreatments were greater than 60%. Our results showed that NaOH, H₂SO₄, and mild alkali/oxidation pretreatments were useful to rapidly improve the C/N of river water and enhance denitrification.