Study On Pretreatment Efficiency And Mechanism Of Micro-Polluted Drinking Water Resource By The Constructed Wetland

In recent years, as the increasing pollution of the water environment, the water quality of the water resource becomes worse, so that the conventional water treatment processes couldn't adapt to the present situation of water quality. Thus an efficient, simple and economic micro-polluted drinking water resource pretreatment process is badly wanted. This research was carried out in Yuqing Lake Reservoir in Jinan, and there were three types of the constructed wetland systems, including horizontal flow constructed wetland system, up and down fold flow constructed wetland system and surface flow constructed wetland system. All the wetland systems were used to pretreat the raw water which came from Yellow river after sedimentation. The objective of this study was to investigate the pretreatment efficiency and mechanism of the constructed wetland systems.The results of pretreatment efficiency experiment showed that the average removal efficiencies of COD, TP, TN and NH4+-N were 49.55%, 49.36%, 52.81% and 48.36% in the horizontal flow constructed wetland system respectively. The average removal efficiencies of COD, TP, TN and NH4+-N were 55.87%, 50.42%, 58.74% and 55.74% in the up and down fold flow constructed wetland system respectively. The average removal efficiencies of COD, TP, TN and NH₄⁺-N were 38.27%, 41.69%, 45.80% and 39.86% in the surface flow constructed wetland system respectively. Among the three types of the constructed wetland systems, the up and down fold flow constructed wetland system had the best performance, followed by the horizontal flow constructed wetland system and the surface flow constructed wetland system performed the worst.From the analysis of the impact factors in the constructed wetland systems, it can be seen that a key factor was to control the influent flux, and proper influent flux can obtain the optimum treatment effect. The disposal performance was closely correlated with the temperature of different seasons. And the trend of treatment effects in summer had the best treatment performance. They performed better in autumn than in spring, and in winter they performed the worst. During the experiment, the removal efficiencies of the pollutants increased with an increase of the pollutants concentration in the raw water, and there was no obvious deviation. This indicated that the concentration of the pollutants in YuQing lake reservoir influent can't threaten the steady operation of the constructed wetland system. It was also observed that the systems had the ability to bear higher load of pollution. Meanwhile, according to the degration situation of different contaminants, first order kinetics removal model for these three constructed wetland systems was established.According to the longitudinal variation of pollutants concentration in constructed wetland and different height, it can be clearly observed that the pollutants removal efficiencies in the constructed wetland systems declined along the water flow direction, and most of the pollutants were removed in the front of the water flow direction. Moreover, the upside of the subsurface constructed wetland system had a better treatment effect.From the analysis of the plants and substrates in the constructed wetland systems, it can be clearly observed that the plants removed the nitrogen and phosphorus directly by absorption, and this process performed better in the growing season of plants. On the other hand, the pollutants in the raw water were filtrated and absorbed by the plants'roots. The roots also provided a habitat for the microorganism and then, the microorganism played a degradation function on the pollutant, which had a significant effect in the constructed wetland systems. The plants on the foreside of the bed grew better than those grew on the rearward, and they can also absorb more nitrogen and phosphorus. This regulation was accorded with the one that the pollutants removal efficiencies declined along the water flow direction. The substrate in the constructed wetland systems can absorb a lot of pollutants, and there were many microorganisms on the mature substrate surface. Experiment showed that the substrate had a strong ability of biodegradation, and played an important role in the process of pollutants removal.The microbial community characteristic was investigated by PCR-DGGE, and the results showed that different types of constructed wetlands exerted great influence on the structure and distribution of microbial community. At the same time, it revealed the regularity that the structure of microbial community in the constructed wetland system varied with the orientations of the water flow. In the up and down fold flow constructed wetland system, the predominant microbial community was Vogesella sp. and Microvirgula sp. in the foreside of the system, and the predominant microbial community was Rhodoferax sp. and Fulvimonas sp. in the rearward. In the horizontal flow constructed wetland system, the predominant microbial community was Cetobacterium sp. and Thermofilum sp. in the foreside of the system, and the predominant microbial community was Sulfuricurvum sp. and Xylophilus sp. in the rearward. In the surface constructed wetland system, the predominant microbial community was Fulvimonas sp. in the foreside of the system, and the predominant microbial community was Salinisphaera sp. in the rearward. This also revealed the variational law of the microbial community along water flow direction in the constructed wetlands.Five strains of phosphorus-accumulating organisms, six strains of nitrobacteria and six strains of denitrifying bacteria were isolated from the three constructed wetland systems. From the 16SrDNA sequences measurement the full sequences of the seventeen strains that screened out the constructed wetland systems were obtained, and similarity retrieval was carried out in GenBank and the most similar strains were obtained. It was found that the isolated strains belonged to Bacillus sp. and Delftia sp., which indicated that there were a lot of bacteria belonging to those two genera in the constructed wetland systems. It played an important role in removing the pollutants.Economic analysis of the up and down fold flow constructed wetland system and the surface flow constructed wetland system were carried out when influent quantity was 2m3·d-1and the circulate cycle was 20 years. The results showed that the construction cost of the former system was 2076.73 yuan and the operation cost was 0.05 yuan/t. the construction cost of the latter system was 2365.01 yuan, and the operation cost was 0.05 yuan/t. It can be concluded that the constructed wetland pretreatment system was an economical pretreatment system.