| Short description |
Treatment systems that use natural processes involving wetland vegetation, soils, and their associated microbial assemblages to improve the quality of drainage water or other liquid fractions.
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| Best Available Technique: |
Not indicated |
| Objective |
Removal of the nutrients, or other pollutants, by means of biomass (plants and microorganisms) uptake and removal from the system through harvesting and denitrification.
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| Level of complexity |
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| Usual scale |
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| Innovation stage |
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| General diagram |
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| Applied to |
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| Typical technology combinations |
Manure separation + treatment of theseparation liquids
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| Pictures |
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Pilot plant for the study of the use of water hyacinth (Lu et al., 2008).
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| Theroetical fundamentals and process description |
Wetlands are typically constructed to reduce loss of nutrients and oxygen demanding fractions to the aquatic environment. The nitrogen removal in at constructed wetland takes place the same way as in traditional wetlands by denitrification. However, the construction of the wetland makes potential for a higher nitrogen removal per acreage than in a normal wetland.
Establishment of wetlands along watercourses can result in considerable nitrogen removal, because flooded areas will cause conditions that improve denitrification, which is the primary mechanism for nitrogen removal in wetlands. Furthermore, there may be a small effect from the growing plant biomass absorbing some of the nitrogen and from the discontinuance of cultivation of the area. The reestablishment of a wetland will most often take place by demolishing drains or ditches in the wetland, so that the water runs out naturally as groundwater or surface water. Water from the more elevated areas is led to the wetland through ditches or drains that are broken off at the border of the wetland, and the water is aimed to be spread across the wetland.
Organic matter content in manure is decreased by biological decomposition. Besides the solids and organic matter, the most important constituents in liquid manure are nitrogen and phosphorus and these can both be uptake by plants in constructed wetlands if conditions are appropriate. Wetlands have successfully been applied, also at industrial scale, for the treatment of dairy, cattle, swine and poultry manures, mainly with marsh vegetation.
Nevertheless, reported nitrogen removal efficiencies are usually very low, 20-60%. A more sound approach, for nutrients recovery, is the use of floating aquatic macrophytes characterized by higher nitrogen uptake efficiencies, over 90% in optimized conditions. Moreover, floating species offer simple harvesting systems, from hand collection to mechanical conveyors. There are relatively few studies focused on livestock manures treatment withfloating plants and they refer mainly to water hyacinth and duckweed. Ammonia (NH4) may be lost from the system through volatilization (NH3-NH4+), taken by plants or microbes, or oxidized to nitrite in nitrification process, while nitrate (NO3-) and nitrite (NO2-) are removed by plant uptake and denitrification process. Denitrification is the most important removal pathway for nitrogen in most wetlands, with a risk of N2O emissions, while adsorption in solids is the main responsible of phosphorous removal (Cronk., 1996).
Cronk (1996) reported some recommended plant species to be used in wetlands for the treatment of animal manure.
The NRCS (National Resources Conservation Service, USA) recommends that wetlands for animal wastewater treatment are designed with the following requirements:
- Allowable BOD5 loading rate of 73 kg ha-1 day-1
- A residence time at least of 12 days
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| Environmental effects |
Effects on air (emissions):
Risk for emission of N2O, since the nitrification and denitrification process are difficult to control.
Effects on water/soil (and management):
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The amount of nitrogen that a wetland can remove depends on local factors such as hydrology, pH-factors, redox-conditions, temperature, conservation and land use as well as the extent of the nitrogen load (Hoffmann et al. 2008). Generally, Danish research shows nitrogen removals in percentages varying between 48 and 99 per cent of the induced nitrogen, of which half of the investigations show a removal of more than 90 per cent of the load (Hoffmann et al. 2008). The extent of the nitrogen removal thus depends on how much nitrogen is led through the wetland. Over a length of time it must be presumed that the effect per hectare newly established wetland is reduced with an increasing area of wetlands, as the best locations can be expected to be selected first.
Other effects:
- Nutrient recovery (C, N, P and other nutrients). N and P removal rates dependent of environment temperature and pH (microbial growth), and consequently its efficiency present seasonal variations.
- Nutrient concentration that can enhance the capability of manure/slurry management. Recovered biomass presents a very wide range of application, going from biofertilizer to substrate for biogas or bioethanol production, as well as for animal feed.
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| Biosecurity aspects |
Not indicated |
| Technical indicators |
Conversion efficiency:
Possibility to obtain an outflow of BOD5<30 mg L-1, TSS<30 mg L-1 and NH3+NH4+-N <10 mg L-1 and a removal P rate of 23 mg P m-2 day-1 (Cronk., 1996; DeBusk et al., 1995)
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| Observations |
- When plants surpass a certain density in the wetland, its growth rate tends to decrease and, consequently, its biological capacity to remove the nutrient load also reduces. For this reason, it is necessary to continually control the density (Costa et al., 2003).
- It can be considered a simple and low energy intensive technology (biomass mediated), although the biomass harvesting is the most complex/costly step.
- High energy cost for drying if harvested material is converted to animal feed due to their high moisture content.
- Nutrients are lost from the agricultural production, unless they are captured in harvested vegetation.
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| Economic indicators (Economic figures are rough indications, which cannot be used for individual project planning) |
Investment cost: Constructed wetlands are relatively inexpensive and easy to construct; the main expense is soil works.
The construction costs for a wetland depend considerably on the conditions, but they are estimated to vary between 470 and 7,517 Euro per hectare (Hoffmann et al. 2008). If presumed that the interest rate is 5 per cent and the investment is written off over 30 years, and a contribution margin of 268 Euro per hectare per year is included at the same time, the annual cost (at establishment costs of 4,027 Euro) 530 Euro per hectare and the cost per kg removed N will be 3.5 Euro per hectare per year, if 150 kg N is removed per year per ha.
Operational costs - explanation: Constructed wetlands, engineered systems designed to simulate natural wetlands, are low cost, simple and low energy intensive technologies that require little maintenance after construction.
Non economically quantifiable benefits: Production of a valuable product that could be used in different industrial applications.
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| Literature references |
- Costa, R.H.R., Zanotelli, C.T., Hoffmann, D.M., Filho, P.B., Perdomo, C.C., Rafikov. M. (2003). Optimization of the treatment of piggery wastes in waterhyacinth ponds. Water Science and Technology. 48(2), 283–289
- Cronk, J.K. (1996). Constructed wetlands to treat wastewaters from dairy and swine operations: a review. Agriculture, Ecosystems and Environment. 58, 97-114.
- DeBusk, T.A:, Peterson, J.E., reddy, K.R. (1995). Use of aquatic and terrestrial plants for removing phosphorus from dairy wastewaters. Ecological Engineering. 5, 371-390.
- Hoffmann, C.C., Baatrup-Pedersen, A. og Jensen, P.L. (2008): Vådområder. Virkemidler til reduktion af N-udvaskningsrisiko, C1: Miljøforvaltning i risikoområder.
- Knudsen, Leif, and Camilla Lemming. Environmental measures in Denmark. Knowledge Centre for Agriculture. http://www.balticdeal.eu/measure/buffer-zones/?aid=3873&sa=1
- Lu, J.; Fu, Z.; Yin, Z. Performance of a water hyacinth (Eichhorniacrassipes) system in the treatment of wastewater from a duck farm and the effects of using water hyacinth as duck feed. Journal of Environmental Sciences, 2008, 20(5), 513-519.
- Shu, L., Schneider, P., Jegatheesan, V., Johnson, J. (2006). An economic evaluation of phosphorus recovery as struvite from digester supernatant. Bioresource Technology 97, 2211–2216. doi:10.1016/j.biortech.2005.11.005
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| Real scale installation references |
Wetlands are often large-scale projects laid out by the authorities. In many situations, however, smaller areas of flooded meadows can be established at low cost by the individual farmer or in a co-operation between a few landowners. The establishment can take place by interrupting and/or re-laying drains and allowing drain water flood the meadow.
Other examples of constructed wetlands: In the USA such wetlands are used for the disposal of the liquid fraction that are collected from feedlots (Foged, 2009), in Holland for reject water from a nitrification -denitrification plant (Foged, 2009), and in Denmark they are in connection to high-tech biogas plants. |
| Examples of suppliers |
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