Studies On The Function Of Substrates And Pollutants Removal Mechanisms In Subsurface Flow Constructed Wetlands

In view of the importance of substrates for wastewater purification in constructed wetlands,based on the idea of local conditions and ecological low consumption,the mechanisms of pollutants removal in subsurface flow constructed wetlands?SSF CWs?with slag and gravel as substrates have been studied.The phosphous adsorption mechanism and influence factors of slag and gravel were analyzed by batch adsorption experiments.This research revealed the degradation and removal of main pollutants according to the purification effect of two different substrate subsurface flow constructed wetlands for treating polluted river water.The ecological mechanism among substrates,wetland plants and micirobial communities was identified based on the analysis of the effects of substrate on the plant and microbial communities in SSF CWs.The mechanism of pollutants removal in SSF CWs was clarified by the correlation analysis between the seasonal variation of the micobial diversity and the removal rate of the pollutants in two substrates SSF CWs.The main conclusions are listed as follow:?1?The experiment result of batch phosphorus adsorption showed that the Freundlich isotherm equation provided better fitting coefficients for slag,whereas the Langmuir equation was better fitted for gravel.The Qm of slag and gravelwere 3.15 mg/g and 0.81 mg/g,respectively.The experimental data of both substrates fitted well to the pseudo-second-order model with high correlation coefficients?R2?of over 0.99.This finding indicated that physical absorption,chemisorption and ligand exchange were the main P removal mechanisms.The P adsorption of slag was not affected by the initial p H,the type and concentration of organic matter along with concentration of ammonia nitrogen,but the P adsorption capacity of gravel decreased with the increase of p H and ammonia concentration.?2?During the four years operation,the slag and gravel SSF CWs performed high removal efficiencies for suspended solids?SS?,chemical oxygen demand?COD?,5-day biochemical oxygen demand?BOD5?.In the first two years,the removal rate of SS,COD and BOD5 in SSF-slag reached 90%,72% and 84% respectively,which is higher than the removal efficiency of SSF-gravel.In the third and fouth year,the performance of removal of SS,COD and BOD5 also tended to be consistent because of the similar internal environment.In addition,the removal rates of SS,COD and BOD5 were approximate linear positive correlation with the pollutant loading rate.?3?During the first two years,the removal efficiencies of of TN and NH₃-N were similar in both two SSF CWs.With the extension of operation time,the removal efficiencies of TN and NH₃-N were better in SSF-gravel than that of SSF-slag.Further more,the SSF-slag acheived more than 10% removal efficiency of total phosphorus?TP?and phosphate?PO₄-P?than that of SSF-gravel.In addition,the removal rate of TP was approximate linear positive correlation with the influent loading.?4?Because the both two SSF CWs had no spatial limitation,the wetland plants grew rapidly in the first year.Compared to the SSF-slag,the SSF-gravel was more conducive to the continuous growth of the plants.At the fourth harvest season,the biomass and density of plant in SSF-gravel were 25 roots/m2 and 0.3 kg/m2 higher than that in SSF-slag.And in SSF-gravel,plants played more important roles for nutrient removal.In both two SSF CWs,the diversities of microbial community in the plants rhizosphere were all higher than that in the substrates area.Further more,the aerobic environment around the rhizosphere stimulated the survival of paticular microbial Achenobacter calcoaceticus??-proteobacteria?.?5?The microbial diversities in both two SSF CWs were higher in the winter than that in the summer.In the winter,the microbial diversity in the influent area was higher than that in the effluent area,while in the summer,the microbial diversity of in the effluent area was higher than that in the influent area.The results of cluster analysis showed that the microbial similarity in the two SSF CWs was from 75.15 to 88.06%,which indicated that the two units had similar oxygen environment.In winter,the denitrifying bacteria has been detected which indicated that the SSF CWs have better insulation effect.The dissolved oxygen?DO?concentration of influent was higher in winter and lower in summer,but in summer,the DO concentration was increased along the flow.The results of two-way ANOVA showed that DO concentration was the main influencing factor for microbial community.?6?The removal efficiencies of SS and TP were not affected by seasonal variation,while the removal rates of COD and BOD5 were higher in summer and autumn,lower in winter and spring.The removal rate of TN and NH₃-N was affected by seasonal variation significantly.And the removal of TN was significantly correlated with the microbial diversity index.The DO concentration was the most important condition for nitrogen removal.In winter,the average DO concentration was 4.6±0.15mg/L which was better for nitrification,NH₃-N was effectively transformed to NO3-N,but the denitrification was limited.In summer,the average DO concentration was lower than 0.5 mg/L,and increase to 2 mg/L along the flow,but nitrification was limited,denitrification became the main removal way for nitrogen.