| Constructed wetland sewage treatment system has many advantages such as low construction investment, simple operation and management and ecological landscape effect. Thus constructed wetland has present outstanding technical superiority and broad application prospects in the comprehensive pollution treatment in developing regions. However, constructed wetland is sensitive to season variation, and displays a significant reduction in treatment efficiency in winter, which becomes the drawbacks of constructed wetland limiting its application promotion.In this thesis, based on the important role of plants played in constructed wetland, different wetland plants with adaptability of coldness or heat were investigated. Then plants with different seasonal adaptability were scientifically arranged, and an enhancement measurement was proposed that hardy submerged plants (Potamogeton crispus) collocated with thermophilic emergent plants (reed) in constructed wetland. The ability of pollutants removal by P. crispus in winter was examined. And I studied the stress tolerance of P. crispus and reeds to sewage environmental, respectively. Then constructed wetland with seasonal plants collocation was build and operated in a continuous flow mode for a year. Mechanism was traversed in the seasonal variation on the aspects of feature of plants succession, alternately playing leading roles in pollutants removal, feasibility of seasonal plants collocation, effluents quality variation and microscopic changes. This supplied a theoretical basis for the constructed wetland perennial stable operation. The main conclusions are as follows:(1) In the cold conditions in winter, hardy plants P. crispus grew well, improved the DO concentration in water by 52.13% compared with the control group, and displayed a better pollutants removal efficiency than the control group, reeds wetland and Vallisneria spirals wetland, with COD, NH4+-N, TN, TP in 86.59%,87.32%, 46.86%,76.27%, respectively. Constructed wetland with P. crispus had the ability of quickly reducing the concentration of COD, NH4+-N, TP to a level within the national limit of class III surface water. This supplied firm and direct evidence of the feasibility to apply P. crispus in constructed wetland in winter.(2) The threshold value of ammonia tolerance by P. crispus is 4mg/L. Ammonia of higher concentrations (≥4 mg/L) resulted in active oxygen metabolism imbalance, continuous strengthen of lipid peroxidation, and impact of membrane permeability and photosynthesis to a certain extent. However, there was no apparent change on plants, which kept grow. In this thesis, the ammonia concentration in the influent of seasonal plants collocation wetland was 4-5mg/L, which is the standard of Wastewater Treatment Plants Effluent Level A. Thus it was within the tolerance range of P. crispus and supported the feasibility. According to plant traits of P. crispus, the ammonia concentration in the influent should not be too high, if not, measures should be taken to avoid damage to P. crispus in accordance with the actual situation,.(3) Strong photosynthesis of submerged plant can increase water pH level, causing stress on the reed. With water pH value increasing from 7.5 to 10.5, the relative growth rate of reed reduced, the assimilation effects was restricted, resulted in limited TN removal efficiency. At pH of no more than 8.5, system displayed good removal of pollutants. Therefore, in an engineering practice, pH level of influent in reed wetland should be controlled within a reasonable range. The collocation by reeds and P. crispus is feasible, only sometimes in summer when the photosynthesis is particularly strong, precaution should be taken to apply appropriate measures to adjust pH.(4) Results of the dynamic flow constructed wetlands with seasonal plants collocation showed that P. crispus and reeds alternately played the role of advantage, in summer, the primary wetland in the experimental group namely P. crispus wetland contributed less than the primary wetland in the control group. In winter, P. crispus wetland displayed priority and the contribution rate of contaminant removal reached the highest level of the year, at which time, the control group was with the annual minimum contribution rate. This was consistent with the feature of plant growth and development, and also consistent with the amount variation of microorganism in the wetland. In addition, in winter, the total removal efficiency in the experimental group of COD, NH4+-N and TP were significantly increased compared with the control group, the removal rates were 42.08%,70.14% and 67.95% respectively in the experimental group. P. crispus wetlands highlighted its advantages in winter and made up for shortcomings of reed wetland of treatment efficiency reduction. Looking at the concentration of contaminants in the effluents in two groups throughout the year, it was found that the experimental group achieved a long-term stability for ammonia removal, the annual average ammonia removal efficiency was 70.15%, higher than that of the control group which was 61.40%. The collocation of seasonal plants in constructed wetland succeeded in maintaining a stable sewage treatment performance throughout the year.(5) Results showed that the number of total bacteria in the primary wetland reached highest in winter in the experimental group namely P. crispus wetland and in summer in the control group, which coincide with results of contaminants concentration. It indicated that with season changes, the variation of plant growth status affected microbial activity. The oxygen transfer system and secretion of root exudates by P. crispus increased the number of microorganisms in the winter, thereby affecting contaminants removal. The oxygen enrichment effect of P. crispus led to significant amount increase of ammonia oxidizing bacteria (AOB) in wetland, resulting in high removal efficiency of ammonia nitrogen in spring and winter. Effect of plants on denitrifying bacteria was not obvious, this was consistent with the little difference in TN removal between different groups. |
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