Effects of salinity on purification efficiency of constructed wetland with mangrove plant Rhizophora stylosa for eutrophic wastewater treatment

There is obvious effect for eutrophic wastewater treatment by constructing mangrove wetland, but mangrove plants grow under circumstances of a certain salinity, so this may influence the wastewater purification efficiency. Rhizophora stylosa, one of dominant mangrove species in South China, was used to construct simulated wetlands for eutrophic wastewater treatment. After 9 months irrigation to 8 groups of wetlands with wastewater at salinities of 0⁰/₀₀, 5⁰/₀₀, 10⁰/₀₀, 15⁰/₀₀, 20⁰/₀₀, 30⁰/₀₀, 40⁰/₀₀ and 50⁰/₀₀, respectively, the growth, photosynthesis, amounts of nitrogen (N) and phosphorus (P) assimilated by plant, and the treatment efficiency were surveyed. The optimum salinity for Rhizophora stylosa growth was 15~20⁰/₀₀. The net photosynthesis rate, stomata conductance and transpiration rate of leaves increased with the elevation of salinity, reached the highest between 10⁰/₀₀ and 20⁰/₀₀, and then decreased. At salinities of 15⁰/₀₀ and 20⁰/₀₀, the content of N in leaves and stems was the highest, but in roots it decreased with the salinity increase. The content change of P in leaves and roots was similar with N, while at low salinity the content of P in stems was higher than that at high salinity. The removal rate of N in wastewater was above 80% for all treatments of different salinities, but at salinities of 10~30⁰/₀₀ there was the highest treatment efficiency with removal rate of 84.1~85.2%; the removal rate of P in wastewater was 98.11~98.82% for all treatments, and at salinities of 10⁰/₀₀ and 20⁰/₀₀ it was a little higher than at other treatments. The amounts of N and P removed by plant assimilation accounted for- - 5.9~11.2% of total N and 1.3~3.3% of total P removed from wastewater, respectively, and most of them transferred into sediment. At the optimum salinity for the plant growth, there was the highest purification efficiency for eutrophic wastewater treatment by constructed wetland with Rhizophora stylosa.