Constructed wetlands description
- Constructed wetlands are basically artificial wetlands, in which wastewater is purified by a combination of various physical, chemical and biological processes, such as sedimentation and filtration, precipitation, adsorption and chemical decomposition, microbial decomposition and plant metabolism.
- Wetlands have the ability to transform and retain substances and hence significantly improve the quality of treated water.
- Constructed wetlands (CWs) are cost-effective and technologically effective solution suitable especially for treatment of municipal wastewater in small villages and buildings that are not connected to municipal sewage system (houses, restaurants, hotels, resorts, etc.). Wastewater is discharged into an appropriately constructed tank, in which contaminants are removed by means of organisms naturally living on the roots of selected aquatic and marsh plants.
- A typical constructed wetland consists of a mechanical pre-treatment tank (septic or settling tank) and one or more filter beds. These can be classified as: horizontal flow CWs – the most common, mostly chemical processes occurring in the absence of oxygen are involved in purification of the effluent; vertical flow CWs – mainly chemical processes occurring in the presence of oxygen (e.g. removal of nitrogen by nitrification) are used for purification; and hybrid CWs (combination of both).
- Efficiency of constructed wetlands can be enhanced by, for example:
- Intermittent feeding and emptying of root filter beds – increase in concentration of oxygen in the filter filling → increase in efficiency of ammonia nitrogen.
- The use of special filtration material with sorption properties (slag, zeolite) – increase in efficiency of removal of phosphorus by sorption
- Sludge management –reed bed systems
Main advantages of the CW technology
- Efficient treatment of municipal as well as some types of industrial wastewater
- Low operational costs
- Effective also in intermittent operation (tolerant to fluctuations in flow and quality of wastewater)
- Low maintenance requirements
- Aesthetic aspect
- Odour elimination
Fields of use
Constructed wetlands are commonly used as a primary system for wastewater treatment in small towns or as a tertiary treatment in large wastewater treatment plants. The main fields of application are:
- Municipal sewage water (villages, settlements, establishments, houses);
- Agricultural wastewater (pig farms, wine, fruit, dairy farms, etc.);
- Leaks from solid waste landfills;
- Polluted rainwater (roads and highways, parking lots, airfields, storage facilities, greenhouses, etc.);
- Industrial wastewater contaminated with organic substances (food, chemical, paper, textile, leather and other operations;
- Stabilisation and mineralisation of sewage sludge.
- Similarly to constructed wetlands, Reed-bed systems are based on the natural ability of reed to transport atmospheric oxygen into the root zone. Root exudates at the same time enhance good prosperity of the necessary bacterial consortium and the feed material is biologically transformed by the action of several simultaneous processes. In specially designed shallow sludge tanks there is besides aerobic zone also an anoxic and anaerobic layer, which enables the sludge to be populated by a wide range of microorganisms, cooperating on the decay process of the organic matter.
- Therefore mineralisation takes place and at the same time the input material loses water (by drainage, evaporation, evapotranspiration).
- The end products are CO2, which is being removed by ventilation, H2O, which is being drained by a drainage system, and humus.
- Reed-bed systems are planted withPhragmites Plants are first let grow for 1 year so that they form a compact, dense cover. Reed-bed systems reach maximum capacity after two years.
- There are several processes taking place in the reed-bed systems. Namely it is dewatering by a series of drainage pipes and vegetal evapotranspiration, then mineralisation of organic substances and formation of humus, and finally liquidation of pathogenic microorganisms.
Advantages of using reed-bed systems
- Low energy demand (in fact it is, and only if necessary, the costs of pumping of treated water or sludge);
- Low operational and maintenance costs (the system is fully automated);
- Operation of the system does not create noise or smell;
- Green areas of vegetated tanks can be incorporated into a landscape design.
Reed-bed systems can be designed for dewatering especially sewage sludge, removal of contaminants such as organic waste, hydrocarbons and heavy metals. Particular set-up of a reed-bed system is always designed specifically for a particular type of wastewater and based on individual requirements of a client – namely for example for effluent containing deicing-defrosting fluids containing glycol, extinguishing water, fuel residues, herbicides, pesticides, etc.
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