Constructed Wetland Technology
With the rapid development of the country's economy, environmental problems have become increasingly prominent, and the contradiction between the shortage of fresh water resources and the improvement of people's living standards has become increasingly serious. The task of water resources protection is also becoming increasingly difficult. In recent years, environmental science research has been developing rapidly, and various sewage treatment methods have emerged. Among them, ecological technology-artificial wetland technology has low investment, good effluent water quality, strong impact resistance, simple operation, low construction and operating costs, and convenient maintenance The ammonia nitrogen removal rate is high, and at the same time, the sewage treatment can be organically combined with environmental ecological construction, and the characteristics of creating urban ecological landscape while treating sewage are gradually accepted by the country, and widely promoted and applied.
Constructed wetland sewage treatment system
I. Explanation of Constructed Wetland Technology
1.1 Definition of constructed wetland
Constructed wetland (Constructed Wetland) is a artificially constructed packed bed composed of various fillers (such as gravel) for the treatment of sewage. The surface of the bed is planted with aquatic plants (high survival rate, long growth cycle, beautiful and economic value) (Such as reeds, pampas grass, etc.), there are a large number of anaerobic and aerobic microorganisms in the bed. Plants and microorganisms in the bed form a unique flora and fauna ecosystem. Sewage flows through the gaps in the packing of the bed, and is intercepted and removed by the natural degradation of the ecosystem.
Constructed wetlands are based on the ordinary constructed wetlands, and through the improvement of fillers, plant types and structures, the bottleneck problem of artificial wetlands with low treatment efficiency during wintering in northern China is solved.
1.2 Constructed wetland structure
Constructed wetlands are composed of six parts:
① Simple civil construction, cofferdam and other infrastructure;
② Up and down water pipes
③ Plant reeds suitable for growth in water;
④ Special substrate with various water permeability, such as sand, gravel, residue, special materials, etc .;
⑤ Body of water (water flowing below or above the surface of the substrate);
⑥ Aerobic and anaerobic microbial communities;
For constructed wetlands, the length-to-width ratio of a single pond is between 1: 1 to 1:10, the slope of the pond bottom is about 1% to 8%, and the depth is between 0.5m to 2.5m. The operation mode can be connected in series and parallel.
1.3 Characteristics of constructed wetlands
① Constructed wetland structure is relatively simple and construction cost is low;
② Constructed wetlands are easy to operate and have low operating costs;
③ The selected plants have strong purification ability and developed root systems, and have good adaptability to environmental changes;
④ Constructed wetland runs stably and is resistant to shock loads;
⑤ It has anti-freezing and heat insulation facilities in winter, which can ensure a good treatment effect during winter operation, and its technology can be applied to northern wastewater treatment projects;
⑥ The constructed wetland operation mode is efficient, the effluent water quality is stable, and there is no secondary pollution;
⑦ Plant recovery and water reuse can directly and indirectly generate economic benefits;
⑧ Constructed wetlands can increase the area of green land, which can beautify and improve the ecological environment.
2. Mechanism and influencing factors of constructed wetlands
2.1 The role of the matrix
The matrix in artificial wetland is also called filler and filter material. While the substrate provides growth medium for plants and microorganisms, it can also directly remove pollutants through precipitation, filtration, adsorption, ion exchange, plant absorption and microbial decomposition.
2.2 The role of wetland plants
For plants in constructed wetlands, our company has long-term research, repeated screening experiments, and decided to adopt the advantages of strong adaptability to the surrounding environment, developed root systems, and strong purification capabilities.
Plants grown in constructed wetlands have four functions:
① Plants can significantly increase the attachment of microorganisms (roots of plants);
② Plants can transmit atmospheric oxygen to the roots, allowing microorganisms to grow in a micro-oxygen environment;
③ The growth of plant roots and stems can loosen, enhance or stabilize the water permeability of the substrate;
④ Plant growth absorbs nutrients in sewage (such as N, P, etc.);
2.3 The role of microbial agents
Microecological preparations are a bacterial group mainly composed of microorganisms such as Pseudomonas, Alcaligenes, Actinomycetes, fungi, yeasts, etc., through their own metabolism, absorption, oxidative decomposition to achieve the purpose of removing organic pollutants.
At the beginning of the construction of the constructed wetland, the types and number of microorganisms were not sufficient. As the wastewater continued to enter the constructed wetland, microecological preparations were added regularly and quantitatively to form a dominant flora in the constructed wetland.
Micro-ecological preparations combined with artificial wetland biochemical treatment form a complete wastewater treatment technology system. Its removal mechanism is: the micro-ecological system forms a huge biofilm reactor in the entire wetland system, and the organic pollutants dissolved in the wetland wastewater are transferred to the wetland. The biofilm covered by the filler surface is transferred and degraded by biochemical reactions.
2.4 Mechanism of removing pollutants from constructed wetlands
The types of pollutants removed by constructed wetlands include: N, P, SS, chroma, organic pollutants, heavy metals and pathogens. The wetland operation shows that under the condition that the small ecosystem's food chain is not complete, the artificial wetland has a concentration of less than 200mg / L, and after 24 hours of hydraulic retention, the COD removal rate of the artificial wetland can reach more than 70%, and BOD 5 removal The rate can reach more than 85%, the concentration of effluent COD is below 90mg / L, the concentration of BOD 5 is about 10mg / L-30mg / L, and the SS is less than 20mg / L. After long-term operation, the ecosystem composed of plants and microorganisms in the wetland is further improved, the biomass of the population is gradually increased, and the quality of the effluent water will be better.
2.4.1 Removal mechanism of suspended matter
Suspended solids are retained and removed by microbial growth and wetland substrate surface adsorption mechanism. Wetlands are very effective in removing suspended solids. Suspended water values are generally around 10 mg / L. To prevent blockage caused by higher suspended solids load, Pretreatment is usually set before water to avoid blockage caused by excessively high suspended matter load.
2.4.2 Removal mechanism of organic matter
Constructed wetlands have a strong ability to remove organic pollutants. Organic matter in wastewater contains particulate organic matter and soluble organic matter. Insoluble organic matter can be quickly trapped and then decomposed or used by filtering in the wetland matrix. Soluble organic matter is absorbed, absorbed, and anaerobic by the plant root biofilm Degradation and removal of aerobic biological metabolism. Due to the unique microbial environment of the wetland, a good living condition of aerobic, facultative and anaerobic bacteria in the system is formed. Especially the surface layer of the medium has high microbial activity and strong ability to remove organic matter.
The reaction mechanism is expressed by the following equation:
The anaerobic process controlled by facultative and obligate anaerobic bacteria proceeds in steps. The main products of the first step are fatty acids such as acetic acid, lactic acid, ethanol, CO 2 and H 2 .
Sulfate-reducing bacteria and methanogens use these fatty acids for metabolic activities.
2.4.3 Nitrogen removal mechanism
Nitrogen mainly exists in the form of organic nitrogen and inorganic nitrogen in sewage. Organic nitrogen includes urea, amino acids, purines and pyrimidines, and inorganic nitrogen includes ammonia, nitrous acid (NO 2- ) and nitrate (NO 3- ). The main forms of inorganic nitrogen in constructed wetlands are NH 4 + , as well as NO 2- , NO 3- , N 2 O, and dissolved N 2 . The mechanism of nitrogen removal mainly includes volatilization, microbial transformation (amination, nitrification / denitrification), plant absorption, medium adsorption and precipitation filtration, etc., but it is mainly accomplished through microbial nitrification / denitrification.
2.4.4 Removal mechanism of phosphorus
Phosphorus removal in constructed wetlands is accomplished through the cooperation of substrate adsorption, plant absorption, and adsorption sedimentation storage. Phosphorus removal in the constructed wetland system has three main effects: ① normal assimilation of microorganisms or absorption by plants; ② phosphorus uptake of polyphosphate bacteria; ③ physical and chemical effects of substrates. Among them, the inorganic phosphorus is mainly absorbed by plants, which is mainly due to the long-term growth of large-scale plants such as reeds on the demand for inorganic phosphorus.
2.5 Influencing factors and countermeasures of constructed wetlands
The physical and chemical environment of the wetland affects the biological processes, which in turn affects the physical and chemical environment of the wetlands. Important factors are temperature and dissolved oxygen.
2.5.1 Effect of temperature
Temperature varies greatly with day, night, and season. Temperature changes can cause natural changes in some chemical and biological processes, and the relative removal rate in effluent water quality will decrease, or even zero.
After our company's long-term scientific research and repeated experiments, we have concluded a set of effective winter insulation operation methods, including:
① Artificial wetland padding is 1.0-1.5 meters (0.8-1.2 meters in the south);
② The artificial wetland water inlet pipe is covered with fillers of more than 20 cm, which can play the role of antifreeze and heat preservation;
③ In winter, the reeds harvested at the end of autumn are laid on the wetland surface, and a layer of plastic film can also be laid on it. Especially in snowy weather, snow also plays a role in heat preservation. The temperature of the waste water treated in the packed bed can be maintained at 150C— Around 200C. In order to solve the problem that artificial wetlands cannot be used in winter in the north over the years;
2.5.2 Dissolved Oxygen Effect
After the sewage enters the underflow wetland, there are two main aerobic factors: carbonized oxygen demand (CBOD) and nitrated oxygen demand (NOD). It mainly shows the oxygen demand of deposited debris, dissolved organic oxygen demand and dissolved nitric oxygen demand. If the dissolved oxygen in the water is too low or too high, it will affect the microbial activity and thus affect the treatment effect. The recovery of oxygen in water is mainly through atmospheric reoxygenation, direct mass transfer to the water surface, conduction transmission by plant stems and leaves, and oxygenation in water when entering wetlands.
Although natural reoxygenation and plant aeration have been able to meet most of the needs of microorganisms, in order to improve the efficiency of microbial treatment, artificial wetlands are added with aeration during the pretreatment before entering the wetland. Artificially increasing the dissolved oxygen in the water improves the treatment efficiency .
3. Prospects of artificial wetland technology
Since the first constructed wetlands in West Germany in 1974, various wetlands have been used to treat a large number of different wastewaters around the world. The Fourth International Symposium, held in Vienna, Austria, in September 1996, marked that artificial wetlands, as a unique new wastewater treatment technology, have officially entered the field of water pollution control. In recent years, the developed wetland technologies in the western developed countries have flourished. North America has mostly adopted free surface flow constructed wetlands, and Europe has tended to subsurface flow systems. Most artificial wetland systems are planted with reeds and other wetland plants. It is believed that root growth and reed root area will increase water conductivity. Constructed wetland systems in Europe tend to do secondary treatment of rural communities, while North America tends to advanced treatment of larger populations. In Australia and South Africa, it is used to treat various types of wastewater.
Although there have been newspapers attached to the research results of constructed wetlands since the Seventh Five-Year Plan period, it is still in its infancy as a whole. On the one hand, the relevant departments at the national level have not formed enough attention, and the local provinces, municipal governments and related departments are also mostly on the sidelines. On the other hand, although the research of many scientific research institutes and experts has a certain scale, it is still on the "point" of research. It is not integrated and systematic, and has not formed long-term, systematic wetland operation experience and design. Data accumulation. At present, the practical application of artificial wetlands is only concentrated in a few areas in the southern (mainly Shenzhen) tropical and subtropical regions. Due to the artificial wetland process itself, the low temperature and low-efficiency operation in the northern region in winter have always been the defects in the operation of the artificial wetland. There is no mature and complete experience in the design and operation of artificial wetland in the northern region.
In general, the constructed wetland sewage treatment system is a better way of wastewater treatment, especially it fully utilizes the production potential of resources, prevents re-pollution of the environment, and obtains the best benefits of sewage treatment and recycling. It is suitable for the treatment of urban sewage and industrial wastewater with little change in water quality and low management level, such as domestic sewage treatment in small and medium towns in rural China.
Therefore, it can be said that artificial wetland is a relatively advanced wastewater treatment process with low investment, low operating cost, and good economic, environmental, and social benefits, and should be further promoted and applied.