Since the 1960s, scholars at home and abroad have been using plants for sewage treatment research. 1974 West Germany built the first artificial wetland for wastewater treatment, our country was also in the 1980s to implement the implementation of the water purification ponds to crocus ponds based on sewage treatment and the use of ecological projects to achieve the double benefits of pollution control and increase production. In recent years, the use of mangrove wetlands in coastal areas for wastewater treatment has also achieved remarkable results. It can be seen that, whether it is artificial wetlands, purification ponds, or land treatment system, the use of aquatic plants to purify sewage has a wide range of prospects for development. In order to achieve better purification effect and higher purification efficiency, it is necessary to carry out experimental research to explore the mechanism of wastewater purification by aquatic higher plants. 1 aquatic higher plants to explore the mechanism of wastewater purification 1.1 Champange plant purification of lead / zinc mine wastewater example study Broad-leaved Champange belongs to monocotyledonous perennial aquatic plants, with rhizomes, with other stems continue to extend the rapid development of the group, can form aquatic plants purification of ponds in the absolute dominance of the population. The Institute of Environmental Sciences of Sun Yat-sen University and the Environmental Protection Monitoring Station of Shaoguan Fankou Lead/Zinc Mine, in response to the pollution situation of large discharge of Fankou tailings wastewater and large heavy metal content, designed and built an Aphanizomenon Purification Pond by using the local waste ores and sand gravels. According to the experimental results, the untreated lead/zinc mine wastewater containing Pb, Zn, Cd and total suspended solids exceeded the standard, but after the purification pond, the removal rate of SS reached 99%, and the removal rate of Pb, Zn and Cd reached 84%-90%, and all the indexes reached the industrial discharge standard. The black and gray wastewater was turned into clear water by the fragrant bushes community, and the fragrant bushes plants could grow luxuriantly, and many kinds of algae, fishes and Cyperus rotundus plants also appeared in the pond. 1.2 Example study of mangrove plants purifying oily wastewater and urban sewage Mangrove forests belong to the unique wetland ecosystems of tropical coasts, including terrestrial ecosystems and aquatic ecosystems, which have the ecological functions of preventing wind and waves and protecting farmland, and have high economic and ornamental values due to the richness of their biological resources and their beautiful sceneries. For sewage treatment, mangroves also have a unique role. Li Mei and other artificial simulated wetland with autumn eggplant for a period of one year of oily wastewater purification test, found that with the increase in concentration of oily wastewater treatment, the relative purification rate of the plant on the oil is: 50mg / L group for 75.76%, 100mg / L group for 67.55%, and 800mg / L group for 42.94%, can be seen with the concentration of the purification efficiency increases. The size of oil content was root > leaf > stem > branch. The experiment also showed that the suitable concentration of autumn eggplant to purify oily wastewater should not be more than 200mg/L. White bone loam is also a perennial mangrove plant. The same will be normal, 5 times, 10 times the concentration of synthetic sewage discharged into the white bone soil artificial simulation of wetlands, a year of experiments proved that: the white bone soil simulation of wetlands on sewage heavy metal purification rate of 88% or more, of which the Pb purification rate of 97.91%, Zn purification rate of 89.47%; N purification rate of 88.04%. Because N, Pb and Zn were absorbed by the white bone soil as the architectural elements of the plant body, the amount of absorption was large, so the purification rate of the same kind of artificial sewage was the largest compared with that of the tung-flower tree (purification rate of N: 60.58%, Pb: 93.62%) and the autumn eggplant (purification rate of N: 60.58%, Pb: 93.44%). It can be seen that white bone loam has a strong adaptability and tolerance to sewage containing heavy metals. 1.3 Herbaceous plants purification of paper wastewater example study Hao Dengfeng et al. chose seven aquatic plants: water hyacinth, water peanut, big drift, floating pimples, tumbleweed, broad-leaved incense and wild rice, the establishment of a plant treatment system to deal with paper wastewater, the waste mixture made of three concentration levels of wastewater injected into the plant system. Through the experiment, the removal rate of suspended solids in the wastewater of 7 kinds of plants are more than 70%, of which water hyacinth, water peanut, tumbleweed is more than 84%; the removal ability of TN, TP size: water hyacinth > big drift > water peanut > floating duckweed, water hyacinth > wide-leafed parsnip > wild rice. But CODCr and BOD5 removal rate are less than 50%. Wastewater chromaticity is only water peanuts, water hyacinth removal effect is obvious, water peanuts 9 days after the removal rate of up to 73.33%, water hyacinth up to 54.67%, so that the black and stinking water treatment is relatively clear. 1.4 Purification mechanism to explore 1.4.1 The plant's own traits and resistance Aquatic plants due to long-term life in an oxygen-poor, low-light environment, their own morphology and anatomy to form special traits. Roots, stems and leaves form a complete aeration organization to ensure the need of organs and tissues for O2; leaves are fleshy, such as the epidermis of cattail has a thick cuticle, the fence organization is developed, and the epidermal cells of roots, stems and leaves at the point of pollution are arranged closely, etc. The structure of the plant can resist the decline of assimilation function and excessive transpiration of water due to the damage of pollution, which enhances the pollution resistance and resistance of the plants of cattail. 1.4.2 Absorption and enrichment of plants Aquatic plants have developed root system, which is good for absorbing substances in water. Such as water hyacinth need a lot of N, P nutrients, it absorbs the rapid growth, for purification of eutrophic water effect is obvious; balsam fern plant absorption of heavy metals in wastewater, the size of the absorption capacity in order of roots & gt; underground stems & gt; leaves, and in accordance with a certain proportion of the habitat from the absorption of a variety of elements, the formation of a new dynamic equilibrium, to prevent an element of absorption of too much caused by toxicity. Plant absorption of pollutants, especially heavy metal ions, pesticides and other synthetic organic matter, etc., will be enriched, fixed in the body or soil, reduce the amount of pollutants in the water body. Studies have shown that Pb, Zn into the body of the balsam pear, mainly accumulated in the cortical cells in the cell wall, only a small amount of entry into the protoplasm, it can be seen that the cell wall of the heavy metals have a high affinity. 1.4.3 Sedimentation, adsorption and filtration in the purification pond The aquatic plants in the purification pond grow vigorously, have developed root system, have large contact area with the water body, and form a dense filtration layer. For example, the fragrant bushes, its underground stems and roots form a crisscrossed underground stem network, when the water flow is slow, heavy metals and suspended particles are blocked and settled, preventing them from being lost with the water. At the same time and on its surface for ion exchange, chelation, adsorption, precipitation, etc., insoluble colloids for the root system adhesion and adsorption, agglutination of the bacterial colloid to the suspended organic matter and metabolism products settled down. Author: Unknown 1.4.4 Biochemical effect The biochemical effect also plays a big role in the process of plant purification of sewage, and a lot of research has been done in this regard. Photosynthesis produces O2 and atmospheric O2 directly transported to all parts of the plant, and diffuse into the water, on the one hand, the root system through the release of O2, oxidative decomposition of sediment around the root system; on the other hand, the bottom of the body of water and the substrate soil to form a number of anaerobic and aerobic cells, to create conditions for microbial activity, and then the formation of the "inter-root zone". In this way, plant metabolites and residues and dissolved organic carbon provide food source for the colonies in the wetland; at the same time, a large number of microorganisms form gray biofilm on the surface of the substrate, which increases the number of microorganisms and decomposition and metabolism area, so that pollutants (enriched or settled) in the roots of the plants can be decomposed by microorganisms and utilized, or be removed by the process of biological metabolism and degradation. In eutrophic water bodies, we can also rely on the microorganisms on the roots of aquatic plants to make denitrifying bacteria and ammonia bacteria accelerate the conversion process of NH3-N to NO2-N and NO3-N, which facilitates the uptake and utilization by aquatic plants and reduces the nutrient salts released from the bottom mud into the water body. 1.4.5 Inhibition of competition for planktonic algae In water bodies with serious eutrophication, algae grow wildly and water quality deteriorates. After planting aquatic plants, they compete with planktonic algae for nutrients as well as the required light and heat conditions, and at the same time secrete algal inhibitory substances to destroy the normal physiological and metabolic functions of algae, forcing them to die in order to prevent the toxins they bring. This can increase the transparency of the water body, improve the DO content in the water, and promote the growth of submerged plants and *** biotic bacteria to further purify the water quality. 1.5 Factors affecting the purification effect of plants 1.5.1 Selection of purification plants There are many higher plants for purifying sewage, such as water hyacinth, water peanut and cattail, etc. However, the ideal purification species needs to be selected considering the factors such as high purification rate, low cost and shock load resistance. The South China Institute of Environmental Sciences carried out experiments for 2 years and evaluated 11 species of higher aquatic plants in South China in terms of purification capacity, resilience, ease of management, comprehensive utilization value and landscaping, and screened out black algae and false amaranth as the better purifying species. Therefore, it can be seen that the purification ability of plants is related to the purification efficiency. 1.5.2 The size of wastewater pH value pH value is different, the wastewater can affect the growth of plants differently, which in turn affects its purification efficiency. The results of purifying acidic paper wastewater with 7 kinds of herbaceous plants such as water hyacinth and water peanut show that the pH value of wastewater can not be lower than 5.84, otherwise the physiological mechanism of plants will be damaged and the purification function will be decreased, which will lead to the plants can not absorb the heavy metals well. 1.5.3 Nature of wastewater The concentration of organic pollutants in wastewater, the size of N and P content, and whether the pollutants are easily degraded and other properties are very important for the purification efficiency of plants. For example, Fenglian treatment of oil refinery wastewater to implement the best conditions for operation: 65mg / L & lt; COD & lt; 130mg / L, the critical effective point for COD = 262.6mg / L13. Once the critical point is exceeded, the greater the degree of injury to the plant, the smaller the purification effect. Similarly, N, P nutrients are necessary for plant growth, but high concentrations are harmful. For water hyacinth, paper wastewater N, P concentration of 15mg / L ~ 20mg / L or so the best purification. 1.5.4 Purification time The length of the purification time and seasonal changes in the plant purification efficiency should not be ignored. When the aquatic plant Fenglian purifies the eutrophic lake water for ≤2d, the purification result is not obvious; when it is extended to 7d, the purification efficiency will be increased by 50%～80%. At the same time, the plant in the temperature changes in the climate under the normal play its function, but the cold weather will make some plants freeze, physiological metabolism is blocked, can not be very good purification of sewage. For example, when the temperature drops to 4 degrees Celsius in South China, the static cultivation of the bromeliad will be frozen, and it is difficult to overwinter. Of course, plant resistance to pests and diseases, wastewater flow and flow rate, the size of the dissolved oxygen in the wastewater and other factors also constrain the purification ability of aquatic plants. 2 Utilization and development of plant purification 2.1 Advantages of aquatic higher plants purification technology Aquatic higher plants treatment of sewage is an emerging bioengineering technology, with the following four advantages: ① low cost, little disturbance to the environment; ② is conducive to the protection and improvement of the original environment, there is a high value of beautification of the environment; ③ governance of the pollution can be harvested plants and bio-energy, to obtain economic benefits, such as water hyacinth purification ponds, every year, can produce biogas per hectare, and can produce biogas per hectare. For example, water hyacinth purification pond, each hectare can produce biogas 58400m3, equivalent to save standard coal 46.72t; ④ operation is simple, less investment, its capital investment, operating costs and energy consumption are 1/3 ~ 1/5 of the conventional secondary treatment methods. 2.2 Current Problems At present, aquatic plants purify the sewage, but also there are some problems. First of all, improper control of management, failure to salvage excess or dried plant residues in a timely manner will lead to the generation of secondary pollution (such as eutrophication, the release of toxic substances). Second, a plant can generally only absorb and degrade one or a limited number of environmental pollutants, and may be toxic to other pollutants with high concentrations, so there are limitations to the role of promotion. Again, aquatic plants themselves grow in the sewage, very easy to screen in the water surface to produce a self-screening effect, will oppress the environment; at the same time, the density is too large will breed mosquitoes and bacteria. Fourthly, the plants with large resistance cannot be scientifically screened, and the system process design of purification does not consider the optimal configuration and post-treatment problems, resulting in the purification effect is not obvious and inefficient. 2.3 The future direction of development (1) can continue to use aquatic plants a variety of combinations of built configurations or multi-level aquatic plant tandem ponds, the formation of a certain level of purification, which is conducive to the growth period and purification function of the seasonal alternation of complementary. (2) For the treatment of sewage during the low temperature period in winter, it is necessary to take the overwintering measures of mulching or changing the ecological position of the plants that are not cold-tolerant among them. (3) Can be combined with other engineering technologies to build a composite sewage treatment process. If scholars use coal ash adsorption and plant oxidation pond composite treatment of wastewater, divided into three systems: mixed adsorption → rapid settlement → water hyacinth oxidation pond self-purification system to remove COD for more than 80%, the water can also be recycled for production. (4) Molecular biology and genetic engineering technology can be applied to the higher plants for pollution control, promote hyperaccumulation of plants, through the improvement and change to make their growth cycle shorter, faster growth rate, and improve the purification capacity. (5) The aquatic plant purification system should have a post-treatment cleaning process, so that it can be turned into a treasure, providing abundant biological resources and energy resources.