Technology of treatment of urban wastewater by constructed wetland

1. Definition of constructed wetland

The constructed wetland is a kind of technology which is similar to the marshland by artificial construction and control operation. The sewage and sludge are distributed to the artificially constructed wetland in a controlled way. In the process of the sewage and sludge flowing in a certain direction, the physical, chemical and biological synergies of soil, artificial medium, plants and microorganisms are mainly used to treat the sewage and sludge. Its mechanism of action includes adsorption, retention, filtration, REDOX, precipitation, microbial decomposition, transformation, plant shelter, residue accumulation, transpiration of water and nutrient absorption and the role of various animals.

2. Defects and solutions in the design of constructed wetland at present

2.1 Blockage of waterway channels.

Because the constructed wetland is immersed in water for a long time, the sediment and soil (or fine sand) will form soil colloids, which will firmly adsorb the inner wall of the pipeline or the surface of the filler, and the blockage will be formed over time. Before the sewage enters the constructed wetland, the grating, filtration and precipitation are essential facilities, which can block a large number of solid sediments and colloidal substances from the constructed wetland. Water channel blockage can be divided into inlet and outlet water pipe blockage and filling blockage, the solution is as follows.

1) In view of the blockage of the inlet and outlet water pipes, the design should consider reserving the dredging position and dredging regularly once a year. There is no need for special configuration of dredging equipment, there are professional sewer dredging companies and equipment on the market, such as pipeline dredging equipment, high pressure water equipment, air cleaning equipment, etc., depending on the size of the caliber of the pipeline and the degree of blockage, generally can receive good dredging effect.

2) For the problem of filling blockage, two schemes can be considered in the design. Firstly, filter tanks are installed at the inlet and outlet respectively. The tanks are filled with filter material and a reasonable mix of matrix particles is selected. No aquatic plants are planted in the tanks to facilitate regular cleaning or replacement of the filter tanks. Second, the use of combined constructed wetlands. Firstly, surface flow constructed wetland (or ecological retention pond) is adopted. Because the surface flow constructed wetland has larger water capacity, longer residence time of sewage, and digestion of aquatic plants, the removal effect of solid particles and suspended solids in sewage is better.

2.2 Seasonal differences in sewage treatment effect

Seasonal differences and north-south regional differences in the treatment of urban sewage by constructed wetlands are objective and cannot be completely eliminated, but the differences can be minimized through optimal design.

1) The freeze-tolerant aquatic plants can be replaced in winter, and the surface flow constructed wetland can be planted with melanoderma, Siberian iris, mushroom grass, cress, etc.; Kale, calendula, beet with red leaf and privet with small leaf can be planted in the constructed wetland.

2) Adopt combined constructed wetlands. The sewage treatment effect of a single constructed wetland in winter is not as good as that in other seasons, and the combination of constructed wetlands can make up for the mutual defects. In order to prevent mosquito breeding in summer, surface flow constructed wetlands can be solved by breeding frogs and silver carp. The practice shows that the surface flow constructed wetland can remove most of the solid particles and suspended matter by flowing under the ice when the surface of most parts of North China is frozen in winter.

2.3 Combined constructed wetland system

Each constructed wetland has its own advantages and disadvantages. After years of operation of constructed wetlands in our country, it is found that a single constructed wetland can not meet the requirements of the development of sewage treatment. In recent years, the combined constructed wetlands have developed rapidly. The combined constructed wetland system described in this paper is as follows: grid filtration precipitation (anaerobic) - emergent plant surface flow constructed wetland (strong anaerobic reoxygenation is poor, good removal of solid particles) - drop water (reoxygenation) - subsurface flow constructed wetland (good reoxygenation is poor) - drop water (reoxygenation) - surface flow constructed wetland (strong anaerobic reoxygenation is poor).

2.4 Specific Examples

Heze Qilihe Wetland Park, from Linshang Road in the east to Xingmin Road in the west, planning a river course of 5 kilometers, the project is divided into three phases, the first phase of the construction of the three-level ecological retention pond, subsurface flow constructed wetlands and surface flow constructed wetlands and landscape projects on both sides of the river, has become a good place for the masses to play. The constructed wetland in this case has four units.

Unit 1: Pretreatment of three-level ecological retention pond. There are a large number of residents on both sides of the river, three public toilets and a pig farm, and all the sewage that used to go straight to the Qili River is now intercepted into the third-level ecological detention pond. The first stage: anaerobic fermentation; The second stage: artificial aeration reoxygenation digestion; The third stage: flocculation precipitation, grid filtration. Then it enters the surface flow constructed wetland after falling water. The effluent through this unit can remove most solid particles and some suspended solids.

Unit 2: Surface flow constructed wetland, designed water depth of 40cm. Vertically divided into three blocks for easy maintenance and seedling replacement. Aquatic plants planted ornamental effect and strong cold resistance of thousands of bracken and German iris. In late September, cress was planted in the row spacing to increase the winter landscape effect and sewage purification effect. The results showed that the removal rates of total nitrogen, ammonia nitrogen, total phosphorus and chemical oxygen demand were 14.07%, 13.82%, 8.48% and 23.04% higher in the constructed wetland planted with water celery than in the blank control group, and the growth rate of water celery was stable in the low temperature season. There is no disease phenomenon, and it can be extended to constructed wetlands as a choice of hardy overwintering plants.

Unit 3: Horizontal subsurface flow constructed wetland. For easy maintenance and replacement of plants, it is vertically divided into three blocks. The water inlet is equipped with a 30cm drop water reoxygenation device, the water inlet and the water outlet are respectively equipped with coarse sand filter devices, and no plants are planted on them. In order to facilitate maintenance and seedling replacement, it is vertically divided into three blocks. Emergent water plants spring, summer and autumn configuration of asparagus, resplendent flowers and canna. When temperatures drop in late September, the ground portion is harvested in blocks, leaving the roots of the underground portion intact and replanting winter hardy plants such as kale, red beet and calendula. In winter, the temperature drops and the activity of microorganisms decreases. In this case, the method of covering pig manure on the surface of subsurface flow constructed wetland was adopted to keep warm. The results showed that the average removal rates of total nitrogen and ammonia nitrogen reached 57.8%-68.5%, respectively.

Unit 4: Surface flow constructed wetland, designed water depth of 40-100cm. Through the treatment of the above three units, the water body entering this unit basically meets the secondary discharge standard, and the water quality is basically close to the natural water body. The purpose of increasing the water depth is to facilitate three-dimensional planting of aquatic plants: planting willow, metasequoia, privet leaf on the shore; In shallow water area, flowers such as reed, calamus, irises and iris are planted. The submerged plants planted foxtail algae and bitter grass with strong cold resistance; Floating leaf plants planting lotus, water lilies; Floating plant planting water hyacinth, winter planting mushroom grass, these two plants due to strong growth, control in a certain range of growth.

The maintenance period of this case is two years after the completion of construction, and the water bodies in summer and winter meet the second-level discharge standards.

In addition, the use of combined constructed wetlands reflects the good effect of combined constructed wetlands in the treatment of sewage interception.

3. Conclusion

Constructed wetland can be composed of one or more constructed wetland units, but each constructed wetland has its own advantages and disadvantages. In order to construct a constructed wetland which can meet the standard of sewage treatment, the appropriate constructed wetland unit should be selected on the basis of reasonable design standards in both design and application stage. The constructed wetland discussed in this paper has formed a good internal circulation after years of operation, established a stable ecosystem, and has better economic and ecological benefits. It is a new technology for the treatment of sewage in constructed wetlands that is constantly being studied, applied and developed, and is worthy of reference and promotion in places with conditions to construct constructed wetlands.