What Is a Constructed Wetland? A Detailed Explanation of This Wastewater-Treatment Term
A constructed wetland is an integrated ecosystem that applies the principles of species symbiosis and material-cycle regeneration in ecosystems and the coordination of structure and function. On the premise of promoting the benign cycle of pollutants in wastewater, it gives full play to the production potential of resources, prevents re-pollution of the environment, and achieves the optimal benefit of wastewater treatment and resource recovery.
Microorganisms in the wetland system are the main force in degrading pollutants in the water body. Aerobic microorganisms decompose most organic matter in wastewater into carbon dioxide and water through respiration; anaerobic bacteria decompose organic matter into carbon dioxide and methane; nitrifying bacteria nitrify ammonium salts; denitrifying bacteria reduce nitrate nitrogen to nitrogen gas, and so on. Through this series of actions, the main organic pollutants in wastewater are degraded and assimilated into part of the microbial cells, while the rest become harmless inorganic substances that return to nature.
It is worth noting that the technologies and standards in this field continue to evolve and improve.
Compared with conventional wastewater treatment plants, constructed wetlands have obvious advantages such as lower investment and lower operating costs. In rural areas, because of the relatively low population density, the investment in constructed wetlands is generally 1/3 to 1/2 less than that of conventional wastewater treatment plants. In the treatment process, constructed wetlands basically adopt gravity flow, with almost no energy consumption during treatment and low operating costs. A wastewater treatment plant charges about 1 yuan per tonne of wastewater, while a constructed wetland averages less than 0.2 yuan.
A courtyard constructed wetland built in Yuantian Village, Pixian County, Sichuan. [1]
Constructed wetlands also perform better than conventional wastewater treatment plants in rural areas. First, constructed wetlands use purely biological technology for water purification, whereas treatment plants use chemical methods, so treatment plants generate large amounts of sludge and residue rich in harmful chemicals that affect the environment during treatment, while constructed wetlands produce no secondary pollution. Second, constructed wetlands use aquatic plants and flowers as the main treatment plants, which provide a good landscape effect while treating sewage and help improve the rural environment. In addition, constructed wetlands offer sustainable economic benefits: higher-value aquatic plants with purification effects can be selected and planted on the wetland, generating economic returns while treating wastewater.
The operation and management of constructed wetlands is also simpler and more convenient than that of treatment plants. Because constructed wetlands operate entirely by biological means, they basically need no dedicated personnel-only periodic cleaning of the grille tank and grease trap, and an annual harvest of aquatic plants. The main treatment role in a constructed wetland is played by microorganisms, not by soil filtration, so wetland design should address clogging of the media, plant death and overwintering. The service life of a constructed wetland is generally calculated as 10-15 years, meaning a well-designed wetland system needs its filter bed cleaned only after 15 years; once it reaches its service life, the system can be put back into use after the filter bed is cleaned. In addition, constructed wetlands have a short construction period; building a conventional treatment plant and laying the associated pipes often takes more than a year, while the average construction period of a constructed wetland is within three months, so a constructed wetland yields results faster.
Conclusion: In rural areas with low population density and low pollution emissions, the 'constructed wetland' domestic sewage treatment facility has many advantages. It makes full use of the terrain around farmers' houses, is adapted to local conditions and simple to implement-it can be built on vacant land beside a house, or renovated from a pond or a park's landscape pool; its scale can be large or small, shared by twenty or thirty households or built for a single household; it requires low investment, is easy to maintain and occupies a small area, and with aquatic plants it also achieves a landscape beautification effect. [2]
The removal of SS mainly relies on physical sedimentation and filtration, while the removal of BOD mainly relies on microbial adsorption and metabolism; the metabolites are all harmless, stable substances, so the residual BOD concentration in the treated water can be very low. The principle of COD removal from wastewater is basically the same as that of BOD.
Mechanism of action: Removal and impact of pollutants on physical sedimentation-settled solids are removed by gravity settlement and filtration in the wetland; settleable and flocculable solids are intercepted and removed through inter-particle gravitational attraction and the interception by plant roots. Chemical/microbial metabolism: the metabolic action of suspended bottom mud and bacteria attached to plants decomposes suspended solids, colloids and soluble solids into inorganic matter; nitrogen is removed through biological nitrification-denitrification; some trace elements are utilized and oxidized by microorganisms and plants and removed by interception or binding. Natural die-off: bacteria and viruses in unsuitable environments undergo natural decay and death; plant metabolism removes pollutants through plant uptake of organic matter; plant root exudates inactivate E. coli and pathogens; a considerable amount of nitrogen and phosphorus is absorbed and removed by plants; perennial marsh plants, harvested once a year, can absorb nitrogen and phosphorus, synthesize them and remove them from the constructed wetland system.
Furthermore, this technology also sees broad application and practice in related fields.
Plants are an important component of constructed wetlands. According to the different dominant plant species, constructed wetlands are divided into types such as free-floating plant wetlands, floating-leaved plant wetlands, emergent plant wetlands and submerged plant wetlands. The plants in a wetland can have a very important influence on its sewage-purification function.
It is worth noting that the technologies and standards in this field continue to evolve and improve.
Moreover, plants provide a larger surface area for the adsorption and growth of microorganisms. Plant roots are important habitats, attachment sites and breeding grounds for microorganisms. Relevant literature shows that the number of microorganisms in the plant rhizosphere is much greater than that in the non-rhizosphere, and microorganisms play an important role in degrading pollutants in the water.
Constructed wetland treatment systems feature large buffer capacity, good treatment effect, simple process, low investment and low operating cost, and are very suitable for wastewater treatment in small and medium-sized towns.
At the same time, the supporting processes and equipment are being continuously optimized and upgraded.
(3) The biological and hydraulic complexity increases the difficulty of understanding its treatment mechanism, process kinetics and influencing factors, and the design and operating parameters are imprecise; therefore, poor design often causes the effluent to fail to meet design requirements or discharge standards, and some constructed wetlands even become pollution sources.
In addition, according to existing data, when the plant density on the upper and lower surfaces increases, the treatment efficiency of the constructed wetland system improves; it takes 2-3 growth cycles to reach its optimal efficiency, so it takes several years after construction to reach fully stable operation. Therefore, the biggest problem with constructed wetland technology is the lack of detailed data from long-term operating systems.
In general, the constructed wetland wastewater treatment system is a good wastewater treatment method, especially because it gives full play to the production potential of resources, prevents re-pollution of the environment and achieves the optimal benefit of wastewater treatment and resource recovery; therefore it has high environmental, economic and social benefits, and is relatively suitable for treating municipal wastewater with small flow, little water-quality variation and not-very-high management level, such as in China's rural areas and small and medium-sized towns. As a new wastewater treatment technology, constructed wetlands need further improvement, and it is necessary to study the characteristics and operating data of different regions in more detail to provide more reasonable parameters for future construction.
The use of constructed wetlands to treat sewage can be traced back to 1903, with one built in Earby, Yorkshire, England, considered the world's first constructed wetland for sewage treatment, which ran continuously until 1992.
The gradual attention to and application of constructed wetland ecosystems around the world began after the German scholar Kickuth proposed the root-zone method theory in the 1970s. The root-zone method emphasizes the role of higher plants in wetland sewage treatment systems: first, they can supply oxygen to the heterotrophic microorganisms in their rhizosphere, creating an oxygen-rich micro-environment in the reducing substrate; microorganisms grow on the roots of aquatic plants and form a symbiotic cooperative relationship with the higher plants, increasing the degradation rate of pollutants in the wastewater; away from the root zone an facultative and anaerobic environment is created, favoring facultative and anaerobic purification; on the other hand, the growth of aquatic plant roots helps improve the hydraulic conductivity of the bed substrate layer. [3]
It is worth noting that the technologies and standards in this field continue to evolve and improve.
The core technology of constructed wetlands is the subsurface-flow wetland. It generally consists of two-stage wetlands in series with treatment units in parallel. The wetland is filled with different media and planted with different purifying plants according to the pollutants to be treated. Water is purified by the physical, chemical and biological pathways of the substrate, plants and microorganisms together, achieving significant removal efficiencies for BOD, COD, TSS, TP, TN, algae, petroleum, etc. In addition, the unique flow pattern and structure of this process form good nitrification and denitrification zones whose removal of TN, TP and petroleum is markedly better than other treatment methods. It mainly includes an internal structure system, an active-enzyme-body media system, a plant cultivation and combination system, a water distribution and collection system, anti-clogging technology and winter-operation technology.
In addition, many other factors must be taken into account in actual engineering applications.
Ditch-type wetland beds include a plant system, a media system and a collection system. They mainly collect and treat non-point-source pollution such as rainwater, achieving purification of rainwater and sewage through filtration, adsorption and biochemical action. They are an effective means of water-quality treatment and protection in small watersheds. [6]
With the rapid development of environmental protection, people have gained a broad understanding of wetland functions. As the 'kidney of the earth,' wetlands undertake the purification and treatment of the earth's natural water bodies. Because natural urban wetlands are gradually shrinking and disappearing, constructed wetlands have received increasing attention and development thanks to their unique advantages. As a new type of ecological sewage purification method, the constructed wetland system water-purification technology is based on planting specific wetland plants on the constructed wetland substrate to establish a constructed wetland ecosystem. When sewage passes through the wetland system, the pollutants and nutrients in it are absorbed or decomposed by the system, and the water quality is purified.
Furthermore, this technology also sees broad application and practice in related fields.
Aquatic plants remove BOD5, COD, TN and TP from sewage mainly through the microorganisms attached to and near the root zone, so aquatic plants with well-developed roots and strong tolerance to sewage should be selected.
In addition, many other factors must be taken into account in actual engineering applications.
If the treated sewage contains no toxic or harmful components, its comprehensive utilization can be considered from the following aspects: (1) as feed, generally select aquatic plants with crude protein content >20% (dry weight); (2) as fertilizer, consider plants with high fertilizer-effective components that are easily decomposed; (3) for biogas production, consider the carbon-nitrogen ratio of fermenting, gas-producing plants, generally selecting a carbon-nitrogen ratio of 25-30.5:1; (4) as industrial or handicraft raw materials, e.g. reeds can be used for papermaking, and bulrush, rush, cattail and sedge are all raw materials for weaving straw mats.
Furthermore, this technology also sees broad application and practice in related fields.
(4) Because these plants are mainly vegetative, their uptake and utilization rate of N is high; therefore, when configuring plants, attention should be paid to their N uptake and utilization effect, and they can be used as dominant plants for N removal, thereby improving the system's N removal effect.
Furthermore, this technology also sees broad application and practice in related fields.
(2) Using the characteristic that the growth of these plants (mainly the growth of tubers, bulbs and fruits) requires large amounts of P and K, they can be applied as dominant plants for P removal to improve the system's P removal effect.
Moreover, from an industry-development perspective, market demand is also driving technological progress.
Different plants have different adaptability to sewage. Generally, high-concentration sewage is concentrated in the front part of the wetland process. Therefore, when building a constructed wetland, the front-stage processes such as strongly-oxidizing ponds and subsurface-flow wetlands generally select plant varieties with strong pollution tolerance. In the terminal processes such as stabilization ponds and landscape ponds, because the sewage concentration is lower, more consideration can be given to the landscape effect of the plants.
In establishing a constructed wetland system, the selection and configuration of plants is an important consideration. When establishing the system and configuring the planting, the main function of the system and the botanical characteristics of the plants must be fully combined. Only in this way can the respective advantages of different plants be brought into full play to achieve a better treatment and purification effect.
The planting configuration of wetland plants should be determined according to the specific application environment and system process; for plant types with a wide application range, their advantages in that process should be fully considered so that they can give full play to their strengths and occupy a dominant position.
It is worth noting that the technologies and standards in this field continue to evolve and improve.
A constructed wetland consists of a buffer ditch and four wetlands of similar area (figure). Each wetland stage is a biological treatment system composed of a gravel bed, aquatic plants, microorganisms and substrate. Each wetland stage is paved with a 500 mm thick gravel bed. The gravel sizes in stages I-IV are 40-50 mm, 30-40 mm, 20-30 mm and 10-20 mm respectively; the gradual decrease in gravel size corresponds to the gradual reduction of organic matter and the progressive improvement of water quality as the sewage flows through stages I-IV. The gravel size within the same stage should be similar, and the gravel bed surface of each stage should be fairly flat. Aquatic plants tolerant of high-concentration organic matter are planted in the gravel beds.
The sewage preliminarily treated by the acidification tank percolates successively through wetland stages I-IV, forming a biofilm on the gravel bed; under the oxidative decomposition of microorganisms the organic matter in the sewage is degraded, while the aquatic plants directly absorb and utilize the organic matter, so that the sewage is purified. In addition, the gravel bed is also a highly efficient filter, making the water with high suspended solids become noticeably clearer after passing through the constructed wetland; this physical effect is especially important in the early stage of constructed wetland operation.
Furthermore, this technology also sees broad application and practice in related fields.
This paper discusses the mechanism and advantages of constructed wetland sewage treatment technology, expounds the construction of constructed wetlands and the research progress in sewage treatment, and shows that the sewage treatment efficiency of constructed wetlands is closely related to the type and degree of pollution, the type of constructed wetland, wetland plant species, substrate type, hydraulic retention time and hydraulic load. In China, there are three typical types of constructed wetlands-vertical-flow, subsurface-flow and surface-flow-mainly used to treat sewage from septic tanks, farms, paper mills, oilfields, coal mines, eutrophic lakes and urban life. Common plants for building constructed wetlands include reed (Phragmites australis), cattail (Typha orientalis), canna (Canna indica), pondweed (Potamogeton spp.) and hornwort (Ceratophyllum demersum); common substrate components are gravel, zeolite, sand, soil or furnace cinder. Through a systematic summary of China's research progress and application examples of constructed wetlands in sewage treatment, it is concluded that the establishment of constructed wetlands for pollutant removal has good application prospects, and that basic theoretical research on constructed wetlands should be further strengthened and their application further promoted in the future. [9]
On June 13, 2021, to further strengthen the protection and restoration of aquatic ecosystems, promote the recycling of regional reclaimed water and guide localities in the water-quality purification work of constructed wetlands, the Ministry of Ecology and Environment issued the Technical Guidelines for Constructed Wetland Water Quality Purification. [10]
The Guidelines consist of 4 chapters and 25 sections, systematically and comprehensively regulating constructed wetland water-quality purification projects from design, construction and acceptance to operation and maintenance. At the same time, they clarify four basic principles to be followed in implementation. First, accurate positioning: constructed wetland water-quality purification projects only undertake the water-quality improvement of low-pollution water such as the effluent of wastewater treatment plants that meets discharge standards, and do not directly undertake pollution-control tasks. Second, ecology first: prioritize natural or near-natural ecological means to improve the ecological quality of water, and insist on selecting native species. Third, adaptation to local conditions: carry out process design according to local actual conditions, and encourage the use of easily available land such as pits, depressions and wasteland, as well as urban green belts and corner plots, for constructed wetland construction. Fourth, clear performance: strengthen inflow and outflow supervision and clarify pollutant reduction requirements; insist on equal emphasis on construction and management, improve the operation and maintenance mechanism, guarantee operation and maintenance funds, and achieve long-term operation. [10]
The issuance and implementation of the Guidelines will play a positive role in standardizing and guiding localities in the construction, operation and maintenance of constructed wetland water-quality purification projects and in promoting the building of beautiful rivers and lakes. [10]
