Constructed Wetlands for Small Communities: Bed Grading, Planting and Honest Limits
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.
The economics only work in the right place
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. Running cost follows the same pattern: treatment basically adopts gravity flow, with almost no energy consumption. A wastewater treatment plant charges about 1 yuan per tonne of wastewater, while a constructed wetland averages less than 0.2 yuan.
Construction speed is the underrated part. 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. Maintenance is correspondingly light: wetlands basically need no dedicated personnel, only periodic cleaning of the grille tank and grease trap, and an annual harvest of aquatic plants.
What actually removes the pollutants
The main treatment role is played by microorganisms, not by soil filtration. 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.
Mechanistically, 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. Nitrogen is removed through biological nitrification-denitrification, and a considerable amount of nitrogen and phosphorus is absorbed and removed by plants, with perennial marsh plants harvested once a year.
Grading the bed is the whole design
The core technology is the subsurface-flow wetland. It generally consists of two-stage wetlands in series with treatment units in parallel, filled with different media and planted with different purifying plants according to the pollutants to be treated, achieving significant removal efficiencies for BOD, COD, TSS, TP, TN, algae, petroleum, etc.
A worked configuration: a constructed wetland consists of a buffer ditch and four wetlands of similar area. 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. The sewage percolates successively through stages I-IV, forming a biofilm on the gravel bed, while the aquatic plants directly absorb and utilize the organic matter. In the early stage of operation the gravel bed works mainly as an efficient filter, and this physical effect is especially important then.
Plants are structure, not decoration
Plants provide a larger surface area for the adsorption and growth of microorganisms, and plant roots are important habitats, attachment sites and breeding grounds for microorganisms. The number of microorganisms in the plant rhizosphere is much greater than that in the non-rhizosphere. According to the dominant species, wetlands are divided into free-floating plant wetlands, floating-leaved plant wetlands, emergent plant wetlands and submerged plant wetlands.
Selection follows the treatment job. 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. For nitrogen, because some plants are mainly vegetative, their uptake and utilization rate of N is high, so they can be used as dominant plants for N removal. For phosphorus, using the characteristic that the growth of tubers, bulbs and fruits requires large amounts of P and K, those plants can be applied as dominant plants for P removal. Placement follows concentration: high-concentration sewage is concentrated in the front part of the wetland process, so front-stage processes such as strongly-oxidizing ponds and subsurface-flow wetlands select plant varieties with strong pollution tolerance, while terminal processes such as stabilization ponds and landscape ponds can weight landscape effect. Common plants 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.
Where wetlands disappoint
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. Clogging is the operational counterpart, so wetland design should address clogging of the media, plant death and overwintering. The service life is generally calculated as 10-15 years, meaning a well-designed wetland system needs its filter bed cleaned only after 15 years, and can be put back into use afterwards. It also takes 2-3 growth cycles to reach optimal efficiency, so it takes several years after construction to reach fully stable operation.
The concept is not new. 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, which emphasized the role of higher plants in supplying oxygen to the heterotrophic microorganisms in their rhizosphere.
Regulation has caught up with the practice. On June 13, 2021, the Ministry of Ecology and Environment issued the Technical Guidelines for Constructed Wetland Water Quality Purification, consisting of 4 chapters and 25 sections. The Guidelines are explicit about positioning: such 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. If you need a wetland to do primary treatment, you have mis-specified it.