Maussymbayeva D.K., Master of ecology

Innovative University of Eurasia, Kazakhstan

Constructed wetlands for wastewater treatment

Annotation. The constructed wetlands are used for wastewater treatment. There are surface flow constructed wetlands, subsurface flow with horizontal flow constructed wetlands and subsurface flow with vertical flow constructed wetlands. Wastewater is treated by adsorption, filtration, ion exchange, denitrification and other processes within aerobic, anaerobic and anoxic conditions found in the wetlands. The constructed wetlands are proved as effectual tool for wastewater purification of different influent quality.

Key words: constructed wetlands, wastewater treatment, surface flow, subsurface flow.

 

Wastewater is inevitable part of humans’ life in cities, rural area, industry and agriculture, which requires treatment. There are different methods for water purification and one of them is constructed wetland. The advantages of the constructed wetlands are that they can be constructed under various climatic conditions and for different quality of the influent. 

Constructed wetlands imitate natural wetlands frame for wastewater treatment. The main features of constructed wetlands are macrophytes, root media, soil and water. Wastewater is treated by ion exchange, filtration, adsorption, absorption, nitrification, denitrification, sedimentation, precipitation, oxidation and reduction processes in the wetland [1].

Macrophytes are able to promote the disintegration of wastewater, eliminate pathogenic microorganisms and pollutants. There are different plants that possess such property, however the ordinary reed (Phragmites australis) and the reedmace (Typha latifolia)  are specifically efficient. Both plant species have a large biomass: aboveground  and beneath the soil surface. Roots develop well vertically and horizontally creating a big surface area, thus, the spacious rhizome systems have a good ability to absorb nutrients and ions. Aerobic microorganisms exist around rhizosphere in a root zone and anaerobic microorganisms are located below those soil layers. Also, those pollutants are extracted by natural filtration in the substrate media [2].

There are floating, submerged and emergent plants used in the wetlands. Soil or gravel are utilized as a substrate for root growth of macrophytes in subsurface flow wetlands. Pretreated wastewater passes substrate media by gravity, horizontally or vertically, where it contacts with microorganisms living in conjunction with the substrate and macrophyte roots [3].

Constructed wetlands are effectual in treating different types of pollutants such as: nitrogen, phosphorus, organic matter, organic chemicals and pathogens, adsorption of heavy metals, through a complex interrelated system of vegetation, bulk water, media and microorganisms.

Design of constructed wetlands comprise surface flow constructed wetlands, subsurface flow constructed wetlands with horizontal flow and subsurface flow constructed wetlands with vertical flow, single or multi stage treatment plants.

Surface flow wetlands reside flat basins where water streams at low velocities within and atop the substrate, similar to natural marshes. The channels media usually include a mixture of peat or clay based soils, gravel, shredded rock  [2].

Wastewater run vertically or horizontally through the substrate media, consisting of sand, soil, gravel or artificial media in subsurface flow constructed wetlands (Figure 1).  D:\Docs\InEU\materials for creation\Untitled 1.png

Figure 1- Vertical subsurface flow constructed wetland chamber

According to the figure, wastewater contact with the substrate media and plants rhizoshere where treatment take place. Purification of water in subsurface flow wetlands is more efficient than in surface flow systems at large dose [2].

The constructed wetlands have a weakness of the system, which is a slow speed of operation compare to a general wastewater treatment plant [2].

The roles of emergent macrophytes in wastewater treatment in surface flow constructed wetlands are the physical effects of plants (decreasing of wind speed which act in precipitation of suspended sediments, averts re-suspension), filtration response, supply of the media for bacterial colonisation and plant metabolism (plant absorption and setting free oxygen from roots) [1].

Wastewater purification from nitrogen and organics in ecosystems is particularly significant due to the following reasons: guideless outlet of nitrogen into natural watercourse contributes to eutrophication of lakes and rivers; unpolished organic materials frequently attenuate dissolved oxygen concentration in surface water channels, resulting in the death of hydrobionts. Relevant literature demonstrates higher organics disposal fulfillment in subsurface flow wetlands by contrast with surface flow wetlands, nevertheless, such systems often demonstrate poor nitrogen removal rate [4].

The characteristics of wetland media are also significant features which assess the environmental condition (such as redox potential) within the porous media. To promote deprivation of nitrogen and organics in constructed wetlands, the media should hold the following characteristics:

-  Ensure co-existence of aerobic and anaerobic or anoxic pores within the matrix to enforce nitrification, denitrification and organics removal;

-  Ensure a carbon source for denitrification.

Organics can be demolished aerobically and anaerobically in subsurface flow wetlands. Aerobic process can use oxygen from atmosphere, supplied through convection as wind action, and macrophyte rhizoshere. Anaerobic process of organics degradation can proceed within pores of the media, missing oxygen [4].

Different constructed wetland design may be grouped for improvement of treatment process efficiency within them, particularly for nitrogen. These combined systems are often incorporate horizontal subsurface flow and vertical flow constructed wetlands established in various practicable ways. Whereas nitrification is not attained in horizontal flow wetlands due to its limited  oxygen  transfer  capacity, nitrification of  ammonium into nitrate can be proceed in vertical flow wetlands. Subsequently, denitrifying  bacteria can lower nitrate  into nitrogen gas amid anoxic conditions and in  the  presence  of an organic  substrate [5].

Constructed wetlands demonstrate their efficient treatment alternative for acid mine drainage, hazardous waste wastewaters, oil refinery wastes, compost and landfill leachates, agricultural wastes and pre-treated industrial wastewaters [6].

 

REFERENCES

1.       Vymazal J. Emergent  plants  used  in  free  water  surface  constructed  wetlands: A review. Ecological  Engineering (2013).  

2.       Shutes R.B.E. Artificial wetlands and water quality improvement. Environment International 26 (2001) pp.441-447. 

3.       Kadlec R.H. Comparison of free water and horizontal subsurface treatment wetlands. Ecological engineering 35 (2009) pp. 159–174.    

4.       Saeed T., Sun G. A review on nitrogen and organics removal mechanisms in subsurface flow constructed wetlands: Dependency on environmental parameters, operating

conditions and supporting media. Journal of Environmental Management 112 (2012) pp. 429-448. 

5.       Avila C., Garfi M., Garcia J. Three-stage  hybrid  constructed  wetland  system  for  wastewater treatment and reuse in warm climate regions. Ecological  Engineering  61 (2013) pp. 43-49. 

6.       Constructed wetlands to treat wastewater. Wastewater gardens  information sheet IS20120105. pp. 1-26.