Comparison of nutrient retention efficiency between vertical-flow and floating treatment wetland mesocosms with and without biodegradable plastic

Abstract

Treatment wetlands are ecological systems that are engineered to improve polluted water quality through macrophyte, soil, and microbial remediation and are used commonly for urban and agricultural runoff treatment. However, constructed wetlands used for marine aquaculture effluent treatments are understudied when compared to their freshwater counterpart. We compared the nutrient retention and the microbial communities of two types of constructed wetland mesocosms, a vertical-flow treatment wetland (VFTW) and floating treatment wetland (FTW) in subtropical south Florida. To enhance nutrient retention efficiency, we implemented biodegradable plastic (polycaprolactone), as an external carbon source and monitored the performance of VFTW and FTW for the treatment of marine aquaculture effluent. Polycaprolactone surface were covered by various cyanobacterial genera including Oscillatoria, Leptolyngbya, Brasilonema, and Trichormus and some plastic-degrading bacteria such as Pseudomonas. The presence of a biodegradable plastic in FTW improved the overall performance of nitrogen removal (nitrite plus nitrate) by 14% through denitrification. The pattern of nutrient removal between two treatment wetland mesocosms were significantly different (p < 0.01), with over 87–91% retention of total nitrogen in VFTW and no retention in FTW, the latter due to poor retention of nitrite plus nitrate and production of organic nitrogen from the system not present in inflow waters. Total phosphorus was retained in both mesocosm types, with higher retention (74–81%) in the VFTW than in the FTW (17–40%). The nutrient retention in VFTW was higher overall compared with FTW mesocosms regardless of biodegradable plastic presence.