Effect And Mechanism Of Botanic Purifying-tank During Treatment For Heavily Polluted River Water And Engineering Demonstration

In the restoration and treatment of polluted city rivers, the biological and ecological treatment became more and more important. Particularly, the technique with aquatic plants was gradually used widely, owing to its good purifying efficiencies, special economic benefits, low energy consumption and simple operation. Further, the technique was helpful to restruct and restore the aquatic ecosystem. In this paper, purifying-tanks were constructed (with various planting densities, two aquatic plants of Pontederia cordata L. and Nymphaea tetragona were cultivated). Engineering demonstration and the investigation on aquatic plants' physiological characteristics during treatment for compound heavily polluted water were performed. Plants' purifying efficiencies for polluted water, the synergy mechanism between plants and microbes in the treatment, the effect of the engineering demonstration of the treatment for polluted water with the situ combined biological technique, all those were analyzed. The mechanism of the biological purifying efficiencies of polluted water was studied. All above aimed to provide scientific basis for the technical design of the practical engineering, system integration and operation regulation.N. tetragona and P. cordata were hydroponic cultivated in 8 purifying-tanks with various planting densities (densities: k1/4=5, k1/2=10, k1/1=20; aeration and no aeration) to investigate diurnal variation of physiological characteristics of plants including peroxidase (POD) and catalase (CAT) activities of roots tissues, soluble proteins (SP), chlorophyll (Chl) contents of leaves and main physical and chemical characteristics of polluted water. Morphology, tillers, roots densities, biomass and nitrogen (N) and phosphorus (P) accumulations of plants were measured. Correlation between diurnal variation of physiological characteristics of plants and N and P removals of polluted water was analyzed, and effects of planting densities on physiological characteristics of the plants and purifying effects of polluted water were studied. Effects of long-term aeration on physiological characteristics of the plants and purifying efficiencies of the purifying-tanks during treatment for heavily polluted water were analyzed, physiological characteristics and N and P removals between two plants were comparatively analyzed. ERIC-PCR was used to analyze correlation between microbial communities on the plants roots, DO secretion from the plants and purifying efficiencies of the heavily polluted water. A tank without plant was for control (CK). With engineering demonstration, purifying effects of the compound technique of artificial aeration, floating-beds and biological films were analyzed. Results showed POD and CAT activities of the roots tissues, Chla, Chb and SP contents of the leaves had positive correlations with NH₄⁺-N and soluble P removals of polluted water. In purifying-tanks with higher planting densities, the plants had longer roots, stems and leaves, more tillers, roots densities, biomass and N and P accumulation, higher Chla, Chb and SP contents of the leaves and NH₄⁺-N and soluble P removals of polluted water, while POD and CAT activities of the roots tissues declined. With stronger sunlight, POD and CAT activities of the roots tissues, Chla, Chb and SP contents of the leaves, NH₄⁺-N and soluble P removals of polluted water all increased. In summer, with effects of photosynthesis midday depression, SP contents of the leaves and DO concentrations of polluted water showed double-peak styles at about 12:00 pm and 14:00 pm, and the bottom between two peaks appeared about 13:00 pm. In 10:00 am-14:00 pm, CAT and POD activities of the roots tissues, Chla, Chlb and SP contents of the leaves, NH₄⁺-N and soluble P removals of polluted water all exceeded the other times at the day.Long-term aeration affected physiological characteristics of the plants and the purifying efficiencies of polluted water. Effects of artificial aeration on N. tetragona were heavier than P. cordata. Roots, stems and leaves lengths of the plants at the aeration were shorter than the no aeration, respectively. Chla, Chlb and SP contents of the leaves were lower than the no aeration. While POD and CAT activities of roots tissues at the aeration exceeded the no aeration. N and P accumulations of plants tissues were higher at the no aeration than the aeration. While plants biomass at the aeration were lower than the no aeration. For P. cordata, tillers and roots densities in aeration purifying-tanks were more than the no aeration, while they changed contrarily for N. tetragona For P. cordata, BOD₅, COD and NH₄⁺-N removals of polluted water at the aeration exceeded the no aeration by 5.97%,7.12%,6.29% in summer, whereas TP and soluble P removals were significantly lower by declining 38.18% and 43.79%. For N. tetragona, BOD₅, COD and NH₄⁺-N removals of polluted water at the aeration exceeded the no aeration in spring and summer, while TP and soluble P removals were significantly lower. But in autumn, BOD₅, COD, NH₄⁺-N, TP and soluble P removals of the effluents at the aeration were lower than the no aeration by decreasing 5.71%, 5.99%, 6.06%, 12.36%, 9.95%.P removals by P. cordata were better than N. tetragona, while N removals between two plants did not differ significantly. Effects of heavily polluted river water on N. tetragona were heavier than P. cordata. Living-term of N. tetragona was longer than P. cordata under the stress of heavily polluted water. POD activities of roots tissues, Chla and SP contents of leaves between two plants all differed significantly, while Chlb contents of leaves and CAT activities of roots tissues did not. POD and CAT activities of roots tissues of N. tetragona exceeded P. cordata by 0.0961 U/(g·min) and 1.4206 mg/(g·min) in spring, while Chla, Chlb and SP contents of leaves of P. cordata exceeded N. tetragona by 0.0010 mg/g,0.0231 mg/g and 24.91 mg/g. Roots lengths of N. tetragona were longer than P. cordata, while stems and leaves lengths were shorter, and tillers, roots densities, biomass and N and P accumulations were lower than P. cordata, respectively. TP and soluble P removals of the purifying-tanks were lower than tanks with P. cordata, while NH₄⁺-N removals did not differ significantly.Microbial communities on the plants roots, DO secretion from the plants and purifying effects of polluted river water were correlated. 10:00am-14:00pm, with stronger sunlight, the more O₂ secretion from the plants and the more microbial diversities on the plants roots resulted in the better purifying efficiencies. In the purifying-tanks with higher planting densities, the more sum O₂ secretion from the plants resulted in the higher DO concentrations of heavily polluted water, the more microbial diversities on the plants roots, which resulted in the better purifying efficiencies. With seasonal change of the plants' growing (spring, initial growing stage; summer, fast growing stage, autumn, last growing stage), the corresponding change of the O₂ secretion from the plants directed to the corresponding change of the microbial diversities on the plants roots and the purifying efficiencies of polluted water.Water qualities analysis of engineering demonstration showed, the compound technique of artificial aeration, floating-beds and biological films effectively treated polluted water. Artificial aeration affected the effects of TP and soluble P removals of polluted water.Innovation productions of this paper were as follow. (1) Physiological analysis of the mechanism of the treatment for polluted water with aquatic plants was conducted. Quantized analysis of the relationship between purifying efficiencies and physiological characteristics of plants was performed. (2) Microbial communities variation on the plants roots were analyzed. Variation of the environment for microbes resulted from the change of physiological characteristics of plants, which resulted in the variation of microbial communities on plants roots. (3) It was found that N. tetragona had good growth in heavily polluted water, and its living-term was longer than P. cordata, and its N removals were higher than P. cordata. So, N. tetragona had a good application prospect in the ecological restoration engineering of polluted city rivers. (4) Effects of long-term aeration on physiological characteristics of plants during treatment for heavily polluted water were analyzed, which indicated aeration tolerance varied with plant species and plant's seasonal growth. This work provided theoretical evidence for optimizing plants allocation and regulating aeration technique in the ecological restoration engineering of polluted rivers. (5) Relationships between purifying efficiencies of polluted water, physiological characteristics of plants and microbial communities variation on plants roots were analyzed. It provided theoretical evidence for purifying-tanks' future application for the restoration of polluted water. This work was of profound theoretical and practical significance.