A Study On Remediation Of Rural Eutrophic Water And Mechanism By Using Constructed Wetlands

The study was based on constructed wetlands (CWs) in small-scale and pilot-scale reactors treating the rural eutrophic water. The treatment performance and mechanisms of CWs, growth character of plants, metabolic properties and functional diversity of the microbial communities in CWs were analyzed.Four types of CWs, Typha angustifolia/gravel bed horizontal subsurface flow constructed wetland(HFCW), Cyperus alternifolius/gravel bed HFCW, Typha angustifolia/gravel bed vertical subsurface flow constructed wetland(VFCW), Cyperus alternifolius/gravel bed VFCW, had good treatment performance. Under the condition that COD, TN and TP concentration of the influent were 45～110mg/l, 1.5～5.2mg/l and 0.22～1.54mg/l respectively on hydraulic rention time (HRT) of 3d, COD concentration of the effluent could gain the level of class IV of surface water quality according to GB3838-2002, TN concentration could meet the standard of classIII except class IV in August, TP concentration could stabilized at the level of class II except class III and IV in August and September. In the period of algal bloom(July, August and September), the removal rate of Chla in CWs was 82.5%～99.2% under the condition that the concentration of Chla was about 103.5mg/m³ï½ž252.4mg/m³. The area loading rates of COD, TN, TP were found linearly correlated with their removal rates. Compared to four types of CWs, removal rates of Cyperus alternifolius CWs were higher than that of Typha angustifolia CWs from October to January of the next year, but lower from May to September in both HFCWs and VFCWs. Between two flow patterns Typha angustifolia CWs, COD removal rate of HFCWs was higher than that of VFCWs in all times, but TN and NH₄⁺-N removal rates were lower than that of VFCWs in autumn and winter, TP removal rates was higher than HFCWs in winter and spring. The contamination removal rates of contamination in two flow patterns Cyperus alternifolius CWs had no remarkable difference.The growth characters of plants were different among four types of CWs. Along the beds in HFCWs, the plant height and tillered plant number both sloped down. But in VFCWs, there were no remarkable difference. The aboveground biomass of Cyperus alternifolius was much more than that of Typha angustifolia. The concentration of nitrogen in Cyperus alternifolius was much higher than that in Typha angustifolia, but the concentration of phosphorus had no much variance. Among the difference cells in CWs, the variance of nitrogen concentration was more remarkable than phosphorus. The concentration of nitrogen distribution could be summarized as follow: hf-I > vf > hf- II > hf-III> hf-IV. It could be explained that the plant in CWs treating the rural eutrophic water was easier to be menaced by nitrogen not by phosphorus. The total nitrogen and phosphorus absorbed by plant aboveground biomass in different cells decreased along the beds in HFCWs. This variance was positively correlated with the aboveground biomass, while showed no apparent relationship with the concentration of nitrogen and phosphorus. According to the contributing of nitrogen and phosphorus about removal method analysis, it was found that plant uptake played very important role.In this study, researcher selected effective determination methods like dissolved organic carbon (DOC), UV-Visible absorption and Three-Dimensional Excitation Emission Matrix (3DEEM) analysis, to conduct a research on distribution and transport of dissolved organic matter (DOM) by CWs. There were four types of fluorescence peak in the influent spectrum of 3DEEMs, which were attributed by four kinds of DOM: visible fulvic acid-like, UV-fulvic acid-like, tryptophan-like and tyrosine-like. The positions of four types of fluorescence peak in the difference sampling interstitial water and effluent were similar. The protein-like fluorescence peaks (tryptophan-like peak and tyrosine-like peak) in interstitial water and effluent samples of CWs were observed weaker than that in influent samples. The fulvic acid-like fluorescence peaks (visible fulvic acid-like peak and UV-fulvic acid-like peak) in interstitial water and effluent samples of CWs were stronger than that in influent samples. It showed that DOM was not only utilized by the microorganisms and plants in CWs, but also transformed to other forms. In this study, DOM in influent was likely to transformed to fulvic acid-like matter which was a sort of stodgy food to microorganisms. The microbial experiment results revealed that the abundance of bacteria in four types of CWs substrates in top layer was larger than in middle and bottom layer. In HFCWs, the abundance of bacteria was higher in front end than after end. There were no apparent relationship between the abundance of microorganism in substrate and the removal efficiency of COD, TN and TP.The metabolic properties and functional diversity of the microbial community in four types of CWs were analyzed by the sole-carbon-source utilization profiles using ECO Biolog microplates. There were four sampling points in HFCWs substrate bed( upper forepart layer, lower forepart layer, upper back-end layer and lower back-end layer) and two sampling points in VFCWs substrate bed(upper layer and lower layer). The results showed that there was apparent difference among the microbial communities in different substrates, as well as in different seasons. In autumn, the variation of average well color development explained that the metabolic activity of lower forepart layer substrate microorganisms was remarkable lower than that of other three substrate microorganisms in HFCWs, as well as the activity of upper layer was lower than that of lower layer in VFCWs. But in the next spring, the activity of lower forepart layer in HFCWs and upper layer in VFCWs increased a lot. Among the 31 carbon sources, polymers, carbohydrates, carboxylic acids and amino acids had greater rates of color development for all substrates. But the metabolic capability of amines and phenolic compounds was much different. The results of principal component analysis showed the different structures and metabolic properties of different substrate microorganisms. The results of functional diversity indices showed that there was greater variance in different substrate microorganisms. In HFCWs at autumn, the significance of richness indices difference among four substrates was smallness. The diversity and evenness indices of lower forepart layer substrate microbial community were greater than that of other three substrate microbial community, as well as the the diversity and evenness indices of upper layer was lower than that of upper layer in VFCWs. At the next spring, the diversity and evenness indices of lower forepart layer substrate microbial community in HFCWs increased much more, as well as the upper layer substrate microbial community in VFCWs. The spacial distribution analysis of COD, TN and TP in the substrate showed that there were different characters at different pollutants.In HFCWs, regardless of seasons and plants, the concentration of COD, TN, TP decreased along the beds and increased from upper layer to lower layer. However in VFCWs, the concentration variance of COD was small and uncertain in all year. The concentration of TN decreased from upper layer to lower layer in warm seasons, but the concentration stabilized from 1/2 section to effluent in cold seasons. The concentration of TP decreased from upper layer to lower layer in cold seasons, but the concentration stabilized from 1/2 section to effluent in warm seasons. The removal rate of COD, TN and TP in the beginning 1/4 section was in the highest flight to the total removal.The special distribution of NH₄⁺-N was not consistent with that of COD, TN and TP. The concentration of NH₄⁺-N increased from influent to 1/4 sections in CWs and then decreased along the beds. But the concentration of effluent was higher than influent in the most time of a year. The removal of NH₄⁺-N was weakness. The result showed that the concentration of NH₄⁺-N in effluent was below 0.57mg/l (it could meet the level of class II of surface water quality according to GB3838-2002).The pilot-scale treating system carried out the compound system engineering applying BF bed-constructed wetlands based on small-scale test result. Result indicated that BF bed was able to reduce the concentration of ammonia nitrogen, improve water-entering resolving of oxygen contents of CWs influent. It had a good effect to strengthen the follow-up CWs purification. The pilot-scale CWs had better treatment performance to the BF bed effluent. COD concentration of the CWs effluent could meet the level of class III-IVof surface water quality according to GB3838-2002, TN concentration could get the standard of classIV, TP concentration could stabilized at the level of class II. The removal rate of Chla of CWs increased from 75% to 90% in the last half year. The removal effect of NH₄⁺-N was not satisified. But the effluent concentration of NH₄⁺-N was below 1.0mg/l (it could meet the level of class III of surface water quality according to GB3838-2002). Integrated vertical constructed wetlands (IVCWs) had a comparative purifying effect to HFCWs under higher loading situation (the HRT of IVCWs was 1/3 less than HFCWs). In a word, the BF bed-CWs purifying system could improve the quality of supplemental water under the loading of 300t/d. With the CWs being mature (the growth of wetlands plants, the maturity of the wetlands substrate microorganism community), the effect of purifying is hopeful to improve in the future.