[Step-by-step composite subsurface flow constructed wetland for rural sewage treatment] What is constructed wetland?

Environmental and ecological restoration action

Name of graduate student

Discipline (field)

Specialized courses

Student number

Research topic Cai Zhengquan, Master of Environmental Engineering Class 4 [1* * * * * *] New stepped composite subsurface constructed wetland for rural governance.

domestic sewage

Instructor Chen Yonghua

A new type of stepped composite subsurface constructed wetland for agricultural treatment

Rural domestic sewage

Cai Zhengquan

Abstract: the content of nitrogen and phosphorus in rural sewage is very high, and it contains a lot of viruses and bacteria. Removing nitrogen and phosphorus from sewage is the key to purification. Although there are many sewage treatment methods at home and abroad, there are still some shortcomings, such as poor effect, high cost, difficult operation and maintenance, and large area. The new stepped composite constructed wetland is a process of connecting the upward subsurface constructed wetland and the horizontal subsurface constructed wetland in series. Adding a sedimentation tank in front of the wetland can effectively prevent the substrate from blocking and improve the effluent quality. In the first-class wetland, water falls into the substrate from bottom to top, which increases the dissolved oxygen content in the wetland, thus enhancing the nitrification reaction. Denitrification reaction in two-stage horizontal subsurface flow wetland greatly improves the ability of removing N from sewage and has great anti-pollution influence. In this paper, the optimal parameters of equipment will be designed according to different substrates, plant configurations and water distribution methods, and at the same time, the requirements of small floor space, high treatment efficiency and convenient management and operation will be met to achieve the optimal effect of sewage treatment.

[Keywords:] parameter design of rural sewage constructed wetland

This paper mainly aims at the shortcomings and defects of the current constructed wetland technology in treating rural domestic sewage. For example, Wu Shubiao and others used the improved family constructed wetland combined system to treat the domestic sewage of rural restaurants around Beijing, Ye and others used TICW to treat the domestic sewage of a rural area in Ningbo, and SSFCWS successfully treated the rural domestic sewage of Pingxiliang Village in Urumqi. Although these cases have satisfied the reasonable treatment of sewage to a certain extent, there are still some shortcomings, such as too large area, poor landscape effect of single plant configuration, high cost and difficult operation. Therefore, the author improved and designed a new stepped composite constructed wetland on the original basis, which effectively solved various problems.

1. Discharge and treatment of rural domestic sewage

1. 1 Characteristics of domestic rural domestic sewage discharge

China's rural area accounts for 57.09% of the national area. According to the representative rural survey report, 96% of China's rural areas have no drainage channels and sewage treatment systems, and 25 million tons of sewage are directly discharged every year. The main pollution characteristics of its emissions are as follows: ① The sources are scattered. Affected by geographical conditions, the layout of villages in China is scattered, and the discharged sewage is also scattered, so the amount of sewage that can be collected centrally is less.

② Wide sources. The pollutant concentration is low. The living standard of villages and towns in China is low. Influenced by traditional living habits, the domestic sewage in villages and towns not only includes kitchen sewage, washing water and fecal wastewater, but also includes percolating wastewater generated by domestic garbage stacking, and the concentration of sewage pollutants is relatively low. ③ The water quality and quantity fluctuate greatly. With the changes of farmers' living standards and lifestyles, the water quality and discharge of township domestic sewage fluctuate greatly. At present, many villages and towns still use masonry channels, and there are a lot of rainwater and groundwater in rainy season or when the groundwater level is high.

Flowing into sewage collection channels, causing large fluctuations in water quality and quantity.

④ It is difficult to collect and the treatment rate is low. Due to the scattered layout, small scale and complex terrain conditions of villages and towns in China, centralized collection of sewage is not easy. The discharge of township sewage is relatively small, and there are many and scattered discharge points. The main pollutants are nitrogen and phosphorus, and contain a lot of nutrients, bacteria and viruses, which brings certain difficulties to the treatment of village sewage.

⑤ High biodegradability. The main pollutants of domestic sewage in villages and towns are nitrogen and phosphorus, which basically contain no heavy metals and have high biodegradability.

1.2 Application of Constructed Wetland Technology in Rural Domestic Sewage Treatment in China

1.2. 1 Concept and characteristics of constructed wetland

Constructed wetland consists of gravel, gravel and other substrates and aquatic plants growing on them. Sewage flows on or below the surface of wetland substrate, and degrades nutrients in water through a series of processes such as substrate adsorption, plant absorption and microbial transformation. It is a unique soil plant diffusion system different from natural wetlands. Constructed wetland has the advantages of stable effluent quality, strong ability to remove nutrients such as nitrogen and phosphorus, low capital construction and operation costs, convenient maintenance and management, etc., and is suitable for the treatment of intermittent small-discharge sewage. Because of its good environmental and ecological benefits, it is highly praised by many countries. Its types include surface flow constructed wetland, horizontal flow subsurface flow constructed wetland and vertical flow subsurface flow constructed wetland. They have their own advantages and disadvantages: the advantages of surface flow constructed wetland are low investment and operation cost, but the disadvantages are poor treatment effect, easy breeding of mosquitoes and flies and odor, and large area. The advantage of horizontal subsurface constructed wetland is good treatment effect, and it is not easy to produce mosquitoes and flies, but the disadvantage is that it is easy to cause blockage. Compared with the other two kinds of wetlands, the vertical flow subsurface constructed wetland has better treatment effect and smaller area, but the disadvantage is that the operation cost and infrastructure cost are higher, and mosquitoes and flies are easy to breed in summer.

1.2.2 Application of Constructed Wetland Technology in Rural Domestic Sewage Treatment in China

At present, due to the popularization and application of constructed wetlands, there have been several successful cases of constructed wetlands applied to rural sewage treatment in China. For example, Wu Shubiao and others used the improved family constructed wetland combined system to treat the domestic sewage of small restaurants in rural areas around Beijing. The results show that the system has a good removal effect on COD, NH3-N and TP, and the removal rates are 92.4%-97.9% and 84.5%-96.0% respectively. 79.5%-90.7%, the average effluent concentration is 33.6 mg/L, 2.88 mg/L and 0.66 mg/L, but its plant configuration is single, its anti-pollution load is small, and its landscape value and economic value are low. Ye et al. used tower constructed wetland to treat domestic sewage in a rural area of Ningbo. Waterfall flow is beneficial to improve the oxygenation effect of sewage. The number of dominant bacteria such as ammonifying bacteria and nitrifying bacteria in the soil in the system increased, and the nitrogen removal effect was remarkable. The removal rates of COD, NH3-N, TN and TP are 72%, 82%, 83% and 63% respectively, but the occupied area is too large. Yan Yi and others successfully treated the rural domestic sewage in Pingxiliang village of Urumqi with SSFC coal water slurry. The results show that the constructed wetland can still operate normally in arid areas with harsh climatic conditions, large temperature difference between day and night and large seasonal differences. The average removal rates of COD, BOD, NH3-N and TP are 78.3%, 6 1. 1% and 46.4% respectively.

2. Design of stepped composite subsurface constructed wetland.

2. 1 structural arrangement

As shown in the figure, the whole wetland is an inverted trapezoidal composite subsurface constructed wetland, and the outer layer is a concrete pool, with a height of 1600cm. The whole device is buried underground, and an impermeable layer is set in the pool, with the burial depth of 1400cm, the upper length of 3000cm, the lower length of 1800cm and the width of 1200cm. Inlet plate height 1400cm, thickness 100cm, inlet plate height 1200cm, thickness 100cm, center plate height 1200cm, and outlet plate thickness 100cm. The left side is an upward vertical subsurface constructed wetland, and the right side is a horizontal subsurface constructed wetland. Sewage first enters the sedimentation tank from the left inlet pipe, then overflows from the inlet tank partition into the inlet channel, enters the substrate from below the inlet sedimentation tank partition, flows into the right horizontal subsurface constructed wetland from the direction of the middle partition plate, and then comes out from the lower end of the outlet tank partition and is discharged from the outlet pipe. The substrates of the upward vertical subsurface flow constructed wetland are 200 cm thick gravel (particle size 1.2-3.6 cm), 400 cm thick small gravel (particle size 0.7- 1.8 cm), 400 cm thick waste brick (particle size 3-5 cm) and 200 cm thick soil layer respectively.

. Horizontal undercurrent artificial

The wetland substrate consists of 400cm large gravel, 400cm small gravel, 400cm fly ash and 200cm soil layer from bottom to top. Choose plants with high economic and aesthetic value such as Zizania latifolia or Canna, and the planting density is 5-6 plants /M 2.

2. 1 Summary of device design ideas and advantages

2. 1. 1 Preventing blocking

Because the main components of rural domestic sewage are N and P, there are a lot of organic matter and bacteria, and even sediment. A sedimentation tank is designed at the front end to precipitate large insoluble substances in sewage, including sediment, macromolecular particles, insoluble particles and inactive phosphorus. This effectively prevents them from blocking the substrate.

The ammonia nitrogen removal rate is 2. 1.2.

The sedimentation tank is equivalent to a pretreatment unit in the whole system. It is buried underground, with a relatively constant temperature and is not easily affected by the fluctuation of external temperature, providing a good breeding ground for bacteria, nitrifying bacteria and ammonifying bacteria. Some colloidal impurities or impurity particles adsorbed with ammonia nitrogen in the water are precipitated in the sedimentation tank and then oxidized and removed by nitrifying bacteria, which leads to the decrease of the volume quality of ammonia nitrogen in the effluent of the sedimentation tank. First, sewage falls into the substrate from the reservoir, which increases the dissolved oxygen content in the water and is beneficial to nitrification. The micropores and large attachment area of waste bricks provide good breeding conditions for nitrifying and denitrifying bacteria. At the same time, the water body is rich in carbon source, which can ensure the growth and reproduction of nitrifying bacteria in the lower part from the wetland matrix, and provide carbon source and NO -3 for denitrifying bacteria when they continue to ascend to the central region. On the surface, the roots of plants provide oxygen. Generally speaking, the level of dissolved oxygen in water increases, which is beneficial to nitrification. There are many nitrifying bacteria and insufficient denitrifying bacteria. Then the sewage enters the horizontal subsurface constructed wetland, which is rich in denitrifying bacteria, which is beneficial to denitrification.

The phosphorus removal rate is 2. 1.3.

Phosphorus in sewage exists in dissolved state and granular state, which can be divided into organic state, inorganic state and polyphosphate state. Among them, inorganic phosphates mainly come from detergents, and organophosphorus compounds mainly come from biological processes, food residues in domestic sewage and active or inactive organisms (such as bacteria in treatment tanks) in wastewater. When sewage enters the sedimentation tank, some granular phosphorus will settle down. The removal of part of phosphorus by vertical flow wetland bed includes the biochemical action of wetland plants absorbing wetland microorganisms and the adsorption, complexation and precipitation of substrates. Studies have proved that the absorption of plants and the activities of microorganisms have little contribution to the removal of phosphorus in sewage, and the adsorption and precipitation of substrates account for 70%-87% of phosphorus removal. The main materials of horizontal subsurface flow constructed wetland bed are fly ash and gravel.

Al 3+, Ca2+ and other metal ions and stones. Soluble phosphate in sewage is easy to react with Fe 3+

Metal oxides, hydroxides and clay minerals undergo adsorption and precipitation reaction through ligand exchange to generate insoluble phosphate and fix it.

2. 1.4 Other advantages and features

This kind of equipment is buried underground and is less affected by temperature and weather. At the same time, the landscape value is high, the cost of matrix is low, and it is building waste bricks, fly ash, etc., and it occupies a small area.

3. Conclusion and prospect

In view of some shortcomings and shortcomings of some previous constructed wetland technologies in treating rural domestic sewage, the device has changed the water distribution mode, wetland structure layout, substrate and plant selection, etc., which can prevent substrate blockage, resist the influence of temperature and weather, and effectively remove organic matter, TN and TP. However, there are still many shortcomings in this design, and efforts need to be made in the selection of matrix. This equipment can't move, and its ability to cope with changes is poor.

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