Study On Multiple Constructed Wetland System For Treatment Wastewater

Based on literature review and practical investigations, achievements in researches and progresses in development and application of constructed wetland techniques for wastewater treatments were summarized. Re-modification of constructed wetland treatment system was proposed and tests based on new ideas were conducted with objectives to seeking alternative solutions to the existing problems for use of constructed wetlands in municipal sewage treatments.Using the constructed multiple wetlands system (in the campus of Central South University of Forestry and Technology) as experimental site, this study was conducted to evaluate the pollutant removal efficiency of the system in relation to seasonal and annual changes with focuses on contribution of individual treatment cells and impact of seasonal changes on their treatment efficiencies. Removal of TN and TP by wetland plants was further investigated and discussed. The general goal of this study was to establish a better theoretical foundation as well as to provide useful information and experiences for design, operation and management of the constructed wetland project in practice.Results obtained from the present study showed that the constructed multiple wetland system was effective in reduction of the pollutants in wastewaters from the campus. The removal rate of major pollutants remained higher than 60% even in the cold winter when most plant species stopped to grow and the bioactivity of the system was low. The treatment capacity of different cells varied to some degrees with seasonal changes while the treatment efficiency of the whole system was not significantly affected by temperature.The aerobic biochemical pool and subsurface flow.wetland played most important roles in removal of CODcr, TN and TP. The removal rate of the biological regulating pool was found to be low. The ecological function of this pre-treatment unit in the multiple wetland ecosystem, however, is essential with respect to primary decomposition and transformation of organic pollutants.Plant analysis indicated that the uptake of nitrogen (N) and phosphorus (P) by plants was positively correlated with plant biomass and the correlation coefficient (R2) was 0.8934 and 0.7887 for these two elements respectively. The concentration of nitrogen (N) and phosphorus (P) in plant tissues was also positively correlated with tissues biomass but the correlation level was low, with R2 values 0.4996 for N and 0.4850 for P. The removal of N and P due to plants uptake was reversely related to wastewater loads. The uptake of these two nutrient elements was higher when wastewater load was lower. Plants in SFW (surface flow constructed wetlands) removed more N and P than those in the SSFW (subsurface flow constructed wetlands) and biological regulating pool. In general plant uptake was not found to be the main factor in N and P removal as its contribution efficiency was low, varying within a range between 2% and 10%.