Wastewater and coal gangue seepage discharged from coal mines have high sulfur content. According to the actual situation of Dayugou mine, even if the comprehensive integrated treatment method is adopted, the sulfur removal effect of the effluent water is not obvious, and the SO2-4 in the water is still as high as 1994.21~2144.06mg/L. Although the existing Coal Industry Pollutant Emission Standard (GB20426-2006) has no clear limitation on the SO2 -4emission concentration is not explicitly limited, the high sulfate water still has a serious impact on the groundwater of Dayugou and the water quality of Liangshuiquan Reservoir.
Currently, the main methods for removing SO2-4 from water are neutralization, reverse osmosis membrane method, biochemical treatment and wetland method. The former kinds of high operating costs, varying results, some of which are also secondary pollution or technology is not perfect, etc., more cheap and clean treatment methods, namely, the use of wetlands to remove sulfur.
Generally speaking, coal mining, especially well mining are required to drain groundwater, the formation of small streams or rivers on the surface into the depression, the formation of wetlands. Wetlands have significant ecological functions, and can play a role in purifying water, regulating air humidity and temperature, reproducing a variety of wet - aquatic plants, and improving the human environment. According to the research, at present, the ecological functions of coal mine wetlands are often neglected and either abandoned or severely damaged. The purpose of this study is to try to use the wetland formed by mine drainage to solve the problem of de-sulphurization of terminal external drainage and to make it resourceful, which can be said to be the final part of the aforementioned comprehensive and integrated treatment scheme, and at the same time, it is also to solve the specialized topic of ecological restoration of wetlands and ecological utilization of wetlands in coal mine mountains.
The study of removing sulfate in water by artificial wetland is still in the exploratory stage, artificial wetland belongs to the category of artificial structures, the usual practice is to build several treatment pools, the pools are covered with substrate and planted with plants, relying on plants, substrate, and other elements of the role to achieve the effect of sulfur removal; Coal mine wetland obviously does not belong to the abovementioned artificial wetland, and the research on the ecological function of the wetlands of the coal mine mountain, and the decontamination ability is still relatively rare. Research on the ecological function and desulphurization capacity of coal mine wetlands is still relatively rare. According to the relevant literature at home and abroad, the desulfurization effect of artificial wetlands varies greatly, some can reach 91.9%, some 53%, and even some removal rate is almost zero. The reason for this is mainly the differences in wetland scale, water quality, climate, substrate and aquatic vegetation. Therefore, the basic conditions of ecological geology must be identified when conducting research on coal mine wetlands.
Artificial wetland is a human simulation of natural wetland system, using ecological methods to remove pollutants in order to purify sewage, which utilizes the synergistic effect of physical, chemical and biological in natural ecosystems to achieve efficient purification of sewage through filtration, adsorption, ****sedimentation, ion exchange, plant absorption and microbial decomposition (Peng Chaoying et al., 2000). Practice shows that, compared with other methods of sewage treatment, artificial wetland systems are characterized by high efficiency, low investment, low operating costs, low maintenance technology, and basically no power consumption, i.e., "one high, three lows, and one no" (Ding Jianghua et al., 2000). Since the first artificial wetland system for sewage treatment was built in West Germany in 1974, it has gained rapid development due to its superior performance (Liu Zilian et al., 2005).In the 1980s, research in this area has been widely carried out in Europe, America, Australia and other regions and countries. At present, there are more than 600 artificial wetland projects in the United States for the treatment of municipal, industrial and agricultural wastewater; at least 200 artificial wetland (mainly submerged wetland) systems are in operation in Denmark, Germany, the United Kingdom and other countries, and more than 80 artificial wetland systems are put into use in New Zealand (Li Li et al., 2007). And a large number of monitoring shows that the effect of wetlands in purifying sewage is obvious. For example, Knight (2000) et al. analyzed more than 1,300 reported data, and the purification efficiencies of artificial wetlands on water discharged from livestock rearing were, on average, 65% for BOD5, 53% for TSS, 48% for NH4-N, 42% for TN and 42% for TP. Database information from the U.S. environmental protection agency showed higher treatment efficiencies of up to 95%, 88%, 67%, 61%, 72% and 76% for BOD5, TSS, TN, NH4-N, NO3-N and TP, respectively (Braskerud et al., 2002).
China's wetland research started late. From the "Seventh Five-Year Plan" period, we began to test and obtain the research results on the process characteristics, technical points and engineering parameters of artificial wetlands (Hu Kangping et al., 1991).Since the 1990s, China's research on artificial wetlands has found that the purification of wastewater by lampsbane, cattail and other plants in artificial wetlands can reach the national second- and third-grade surface water standards, and that artificial wetlands can be widely applied to industrial and commercial wastewater treatment, and can be used for the treatment of industrial wastewater. Artificial wetlands can be widely used in industrial wastewater treatment, agricultural water treatment, rainwater treatment and so on. In the research of using artificial wetland ecosystem to remove algae in water body, it shows that artificial wetland system also has characteristics in deep treatment of sewage or reducing eutrophication of water body and inhibiting algae growth. Dozens of cities across the country have carried out research on artificial wetlands, and many of them have been put into production; a number of cities have already established reed artificial wetland sewage treatment systems. Since the operation of these systems, they have produced good economic and social benefits, and contributed to the environmental protection of China. The study of lead and zinc mine wastewater treatment in Shaoguan City, Guangdong Province, planted in the artificial wetland with fragrant bushes showed that (Yang Chengsheng et al., 2000), the use of fragrant bushes to purify lead and zinc-containing industrial wastewater is very effective, and the removal rates of COD, SS, Pb, Zn, Cu and Cd are 92.19%, 99.62%, 93.98%, 97.02%, 96.87% and 96.39%, respectively, and the water quality The water quality was significantly improved, and the main pollutants TSS, Pb, Zn, Cu and Cd reached the discharge standard. In addition, the experimental results of artificial wetland in treating acidic wastewater from iron ore mine showed that (Tang Shuyu, 1996), the pH value of acidic water increased from 2.6 to 6.1; the removal rates of copper ion, iron ion and manganese ion were 99.7%, 99.8% and 70.9% respectively. In terms of using wetlands to remove common sulfate ions from wastewater, a review of domestic and international literature reveals that previous studies are not yet sufficient and that the desulfurization effect varies greatly among the few literature reports. Research data showed that SO2-4 of textile wastewater after biochemical pretreatment changed from 1235 mg/L to 1244 mg/L before and after passing through the wetland, and the removal rate was almost zero (Yin Jun et al., 2004); the SO2-4 removal rate of the Hidden River stormwater wetland treatment system in Florida, U.S.A., reached 53% (Wang Shihe et al., 2007); another study showed that the degradation rate of sulfide could reach 88.3% after the livestock and poultry house wastewater passed through the wetland (Wang Zhisan et al., 1995); in the study of wetland purification of swine manure water from pig farms, it was found that the removal rate of SO2-4 reached 91.9% (Liu Kairong et al., 1997); foreign scholars have concluded that artificial wetland The removal rate of inorganic sulfur in domestic sewage can reach 95% (Buisma et al., 1990).
In terms of wetland design, foreign scholars have found through tracer experiments that under the same wetland area, the BOD removal effect of wetland system with a filler depth of 0.45m is slightly better than that of wetland system with a depth of 0.3m (George, 2000). The USEPA, in its manual on constructed wetlands for municipal wastewater treatment, considers that the water depth in the intake area of a submerged wetland is generally 0.4m, and the depth of the substrate should be 0.1m deeper than that of the water, i.e., the overall depth of the system is 0.5m (USEPA, 2000). Some scholars in China have studied the removal rate of COD under three water depth conditions of 20cm, 40cm, and 60cm, and found that with a water depth of 60cm, the removal rate of COD can still reach 84.9%, even if the hydraulic load of operation is high (433.3cm/d) (Wang Shihwa et al., 2003). It was also found that the increase in influent load caused a decrease in hydraulic retention time and effluent rate, which is not favorable to the purification and treatment of wastewater. However, on the other hand, too small an influent load cannot fully utilize the purification potential of the wetland, so a better influent load exists for all wetland systems (Wu Zhenbin et al., 2001). Studies have shown that low flow rate and high hydraulic retention time (HRT) have better removal effects on organic matter and TSS (total suspended solids), and too high HRT will increase the transpiration of water in artificial wetlands. In view of the important role of wetland plants in the treatment of organic matter and heavy metals in wastewater, at present, the plant selection of artificial wetlands in foreign countries is constantly deepening the research, in general there are generally three kinds of plants are more commonly used, for the windmill grass, reed and balsam (Ciria et al., 2005; Karathanasis et al., 2003). Some foreign scholars have studied the removal effect of eight kinds of plants on pollutants in artificial wetland treatment system and found that the removal ability of balsam fern is the strongest (Klomjek, 2005). The application of plants in the domestic artificial wetland system is basically the same as that of foreign countries. When studying the treatment effect of seven common wetland plants in Wuhan area, namely, balsam fern, plantain, cordgrass, reed, campanula, wild rice and yellow-flowering warbler's tail on the domestic wastewater, it was found that among them, balsam fern, plantain, yellow-flowering warbler's tail, wild rice and campanula had a relatively better effect on the treatment (Lumine et al., 2004). Tumbleweed, vetiver, balsam fern, reed and lampsbane are the plants with more applications in artificial wetlands in China (Jing Yuanxiao et al, 2002; Liao Xinti, 2002; Cheng Pingli et al, 1997; Wang Quanjin et al, 2004).
Through the above summary, it can be found that at present, the research and application of wetland treatment of wastewater is a hot issue at home and abroad, and certain theoretical and practical results have been achieved, however, due to the fact that wetland as a special ecosystem has its own complexity, coupled with the complexity of the type of wastewater, the specific situation varies greatly, so there are still a lot of problems in the use of wetland purification of wastewater, especially in coal mine wastewater, there are still a lot of problems in the use of wetland purification of wastewater, especially coal mine wastewater. Therefore, there are still many problems to be solved in utilizing wetland to purify wastewater, especially coal mine wastewater, and it can be said that it is still "crossing the river by groping the stones". At present, the evaluation of the ability of wetland to purify pollutants at home and abroad is mostly based on the principle of solute balance, and the solute mass of the wetland inlet and outlet is subtracted, and the result is considered to be the purification capacity of the wetland. This evaluation method has many drawbacks, firstly, it must rely on long-term and large amount of monitoring data as the basis, secondly, it cannot give more accurate data of purification efficiency per unit area, thirdly, it can only be evaluated after the wetland is built, and if you want to carry out the wetland design in a more scientific way, it is necessary to make reasonable prediction of wetland purification capacity before the construction. At present, the wetland design at home and abroad tends to focus on the hydraulics parameters and chemical indexes, and involves less on the key factors affecting the purification effect, such as plants, substrate, etc. Especially, there is a lack of comprehensive analysis on the research results of the wetland elements, and a lot of existing studies, in fact, either regard the wetland as a "reactor" with plants and substrate, or only from the plants and substrate.
Many existing researches, in fact, either regard wetland as a "reactor" with plants and mud, or study the multidisciplinary problem of wetland purification only from the perspective of a single discipline, such as plants and chemistry.
In addition, although domestic and foreign studies have proved the effectiveness and practicality of wetland treatment of wastewater, however, most of the studies focus on the wetland on the wastewater of nitrogen, phosphorus, pH and metal ions to remove the study, very few studies on the removal of sulfate ions for the acidic wastewater, the content of which is quite high. High-sulfur wastewater is a type of pollution produced in large quantities in industrial production, especially in coal mining, and there have been few studies at home and abroad on the use of wetlands for removing sulfate ions from water, and the conclusions obtained have varied widely. The reason for this phenomenon is that the environments of each wetland studied by the previous researchers, including climate, substrate, area, plant species, number, etc., as well as the nature of the discharged wastewater, including water volume, pH, sulfate concentration, COD, BOD5, etc., are all very different. Therefore, when studying a specific wetland, a survey sampling should be carried out in the field to evaluate the SO2-4 removal effect of the wetland. Fundamentally, it is the lack of ecological and geological studies on the structure of wetland ecosystems that has led to the lack of studies on the purification of wastewater by wetlands, so that their functions have not been fully utilized.