At present, the main methods to remove SO2-4 from water are neutralization method, reverse osmosis membrane method, biochemical treatment method and wetland method. The former ones have high operating costs and different effects, and some of them still have problems such as secondary pollution or imperfect technology, so a cheaper and cleaner treatment method is adopted, that is, using wetlands to remove sulfur.
Generally speaking, coal mining, especially underground mining, needs to drain groundwater, forming streams or rivers on the surface into depressions and forming wetlands. Wetlands have remarkable ecological functions, which can purify water quality, adjust air humidity and temperature, cultivate all kinds of wet aquatic plants and improve the living environment. According to the investigation, the ecological function of coal mine wetland is often ignored, either abandoned or seriously damaged. The purpose of this study is to try to solve the desulfurization problem of the drainage outside the terminal by using the wetland formed by the drainage in the mining area, which can be said to be the last link of the above comprehensive integrated treatment scheme, and also a special topic to solve the ecological restoration and ecological utilization of the coal mine wetland.
The research on sulfate removal from water by constructed wetland is still in the exploratory stage. Constructed wetlands belong to the category of artificial structures. The usual practice is to build several treatment tanks, cover them with sediment, plant plants, and rely on the effects of plants, sediment and other factors to achieve desulfurization effect. Coal mine wetland obviously does not belong to the above-mentioned artificial wetland, and the research on ecological function and pollution removal ability of coal mine wetland is still relatively small at present. According to the related literature at home and abroad, the desulfurization effect of constructed wetlands varies greatly, some can reach 9 1.9%, some are 53%, and some even have almost zero removal rate. The main reason is the difference of wetland scale, water quality, climate, sediment and aquatic vegetation. Therefore, when studying coal mine wetlands, we must understand the basic conditions of ecological geology.
Constructed wetland is a simulation of natural wetland system, which uses ecological methods to remove pollutants and achieve the purpose of purifying sewage. It utilizes the synergistic effect of physics, chemistry and biology in natural ecosystem, and realizes efficient purification of sewage through filtration, adsorption, precipitation, ion exchange, plant absorption and microbial decomposition (Peng et al., 2000). Practice shows that compared with other sewage treatment methods, constructed wetland system has the characteristics of high efficiency, low investment, low operating cost, low maintenance technology and basically no electricity consumption (Ding Jianghua et al., 2000). Since 1974, the first constructed wetland system for sewage treatment was built in West Germany, and it has developed rapidly due to its superior performance (Liu Zilian et al., 2005). From Europe in the 1980s to the United States, Australia and other regions and countries, the research work in this field has been widely carried out. At present, there are more than 600 constructed wetland projects in the United States to treat municipal, industrial and agricultural wastewater; In Denmark, Germany, Britain and other countries, at least 200 constructed wetland systems (mainly subsurface flow wetlands) are in operation, and more than 80 constructed wetland systems are put into use in New Zealand (Li Li et al., 2007). A lot of monitoring shows that the effect of wetland purification of sewage is obvious. For example, Knight(2000) analyzed more than 65,438+0,300 reported data, and the average purification efficiency of constructed wetlands for livestock effluent was: BOD5, 65%; TSS，53%； NH4—N，48%； Total nitrogen 42%, total phosphorus 42%. The database data of American environmental protection agency shows higher processing efficiency, with BOD5, TSS, TN, NH4-N, NO3-N and TP as high as 95%, 88%, 67%, 6 1%, 72% and 76% respectively (Braskerud et al., 2002).
Wetland research in China started late. Since the Seventh Five-Year Plan period, experiments have been carried out, and the research results on the technological characteristics, technical points and engineering parameters of constructed wetlands have been obtained (Hu Kangping et al., 199 1). Since the 1990s, the research on constructed wetlands in China has found that plants such as rushes and cattails can meet the national second and third class surface water standards in purifying sewage in constructed wetlands, and constructed wetlands can be widely used in industrial wastewater treatment, agricultural water treatment and rainwater treatment. The study on the removal of algae from water by constructed wetland ecosystem shows that constructed wetland system also has its own characteristics in advanced treatment of sewage or reduction of eutrophication and inhibition of algae growth. Dozens of cities across the country have carried out research on constructed wetlands, many of which have been put into production; Many cities have established reed constructed wetland sewage treatment systems. Since the operation of these systems, they have produced good economic and social benefits and contributed to the environmental protection in China. The study on the treatment of lead-zinc mine wastewater in Shaoguan City, Guangdong Province and the planting of cattail in constructed wetland showed that cattail had a good effect on purifying industrial wastewater containing lead and zinc, and the removal rates of COD, SS, Pb, Zn, Cu and Cd were 92. 19%, 99.62%, 93.98% and 97.02% respectively. In addition, the experimental results of treating iron ore acidic wastewater by constructed wetland show that (Tang Shuyu, 1996), the pH value of acidic water is increased from 2.6 to 6.1; The removal rates of copper ions, iron ions and manganese ions are 99.7%, 99.8% and 70.9% respectively. In the aspect of removing sulfate ions from wastewater by wetland, through consulting domestic and foreign literature, it is found that the previous research is not sufficient, and the desulfurization effect in a few literature reports is quite different. The research data show that after biochemical pretreatment, the SO _ 42-of textile wastewater changes from 1235mg/L to 1244mg/L before and after passing through the wetland, and the removal rate is almost zero (Yin Jun et al., 2004). The SO2-4 removal rate of the rainwater wetland treatment system in Hidden River, Florida, USA reaches 53% (Wang Shihe et al., 2007); Another study shows that the sulfide degradation rate of livestock house sewage can reach 88.3% after passing through the wetland (Wang Zhisan et al.,1995); In the study of purifying pig manure water by wetland, it was found that the removal rate of SO2-4 reached 965,438+0.9% (Liu Kairong et al., 65,438+0.997). Foreign scholars believe that the removal rate of inorganic sulfur in domestic sewage by constructed wetlands can reach 95%(Buisma et al., 1990).
In terms of wetland design, foreign scholars found through tracing experiments that under the same wetland area, the removal effect of BOD by the wetland system with 0.45m packing depth is slightly better than that by the wetland system with 0.3m depth (George, 2000). In the Handbook of Constructing Wetlands to Treat Municipal Sewage, the US Environmental Protection Agency thinks that the water depth of the inlet area of subsurface flow wetland is generally 0.4m, and the depth of substrate should be 0. 1m, that is, the overall depth of the system is 0.5m(USEPA, 2000). Domestic scholars have studied the COD removal rate at three water depths of 20cm, 40cm and 60cm, and found that when the water depth is 60cm, the COD removal rate can still reach 84.9% even if the hydraulic load is high (433.3cm/d) (Wang Shihe et al., 2003). Another study found that the increase of influent load caused the decrease of hydraulic retention time and effluent rate, which was not conducive to sewage purification. On the other hand, the influent load is too small to give full play to the purification potential of the wetland, so there is a good influent load in the wetland system (Wu Zhenbin et al., 200 1). The results show that low flow rate and high hydraulic retention time (HRT) have good removal effects on organic matter and TSS (total suspended solids), and too high HRT will increase the transpiration of water in the constructed wetland. In view of the important role of wetland plants in treating organic matter and heavy metals in wastewater, at present, the research on plant selection in constructed wetlands abroad is deepening. Generally speaking, there are three commonly used plants, namely, Cyperus alterniflora, Phragmites communis and Typha Typha (Ciria et al., 2005; Karathanasis et al., 2003). Some foreign scholars have studied the removal efficiency of eight kinds of plants in the constructed wetland treatment system, and found that cattail has the strongest removal ability (Klomjek, 2005). The application of plants in constructed wetland system in China is basically the same as that in foreign countries. When studying the treatment effects of cattail, canna, Juncus Juncus, Phragmites australis, Yingpu, Zizania latifolia and Huanghuayingwei, it was found that the treatment effects of cattail, Canna, Huanghuayingwei, Zizania latifolia and Yingpu were relatively good (Lu Min et al., 2004). Graptopetalum alterniflora, Vetiveria zizanioides, Typha Typha, Phragmites australis and Juncus Juncus are widely used plants in constructed wetlands in China (Jing Yuan Xiao et al., 2002; Liao Xinti, 2002; Level,1997; Wang et al., 2004).
From the above summary, we can find that the research and application of wetland sewage treatment is a hot issue at home and abroad, and some theoretical and practical achievements have been made. However, as a special ecosystem, wetland has its own complexity, the types of wastewater are complex and diverse, and the specific situation is also very different. Therefore, there are still many problems to be solved urgently in purifying wastewater, especially coal mine wastewater, which can be said to be "crossing the river by feeling the stones". At present, the evaluation of wetland's ability to purify pollutants at home and abroad is mostly based on the principle of solute balance, which is considered to be the result of wetland's purification ability by subtracting the dissolved mass at the inlet and outlet of wetland. This evaluation method has many shortcomings. First of all, it must rely on long-term and massive monitoring data. Second, it is impossible to give more accurate purification efficiency data per unit area. Third, the wetland can only be evaluated after it is completed. In order to design wetlands more scientifically, it is necessary to make a reasonable prediction on the purification capacity of wetlands before construction. At present, the design of wetlands at home and abroad often focuses on hydraulic parameters and chemical indicators, but pays little attention to the key factors that affect the purification effect, such as plants and sediments, especially the lack of comprehensive analysis of the research results of various elements of wetlands. In fact, many existing studies either regard wetland as a "reaction kettle" filled with plants and sediment, or only study the multidisciplinary problems of wetland purification from the perspective of plants, chemistry and other disciplines.
In addition, although the research at home and abroad has proved the effectiveness and practicability of wetland wastewater treatment, most of the research focuses on the removal of nitrogen, phosphorus, pH value and metal ions in wastewater, and there is little research on the removal of sulfate ions with high content in acidic wastewater. High sulfur wastewater is a kind of pollution produced in industrial production, especially in coal mining. There are few studies on the removal of sulfate ions from water by wetlands at home and abroad, and the conclusions are quite different. The reason for this phenomenon is that the environment of each wetland studied by predecessors, including climate, sediment, area, plant species and quantity, and the nature of wastewater discharged, including water quantity, pH value, sulfate concentration, COD, BOD5, etc., are very different. Therefore, in the study of specific wetlands, field investigation and sampling should be conducted to evaluate the removal of SO2-4 by wetlands. Fundamentally speaking, it is the lack of ecological geology research on the structure of wetland ecosystem, which leads to insufficient research on wetland sewage purification and insufficient function.