Free-Water Surface CW

Constructed wetlands are secondary treatment facilities for household (blackwater or greywater, in some cases also brownwater) and/or biodegradable municipal or industrial wastewater. Constructed wetlands are a treatment step of DEWATS systems and they can even be used as a tertiary treatment system for polishing after activated sludge or trickling filter plants (HOFFMANN et al. 2010). The plants grown in the wetland may be used for composting or biogas production (see also composting small scale, composting large scale or anaerobic digestion). Effluents, if they correspond to the WHO guidelines (see also WHO 2006): Guidelines for the safe use of wastewater excreta and greywater Volume I, Volume II, Volume III and Volume IV) may be used for fertigation. Unlike the Horizontal Subsurface Flow Constructed Wetland, the free-water surface constructed wetland allows water to flow above ground exposed to the atmosphere and to direct sunlight. As the water slowly flows through the wetland, simultaneous physical, chemical and biological processes filter solids, degrade organics and remove nutrients from the wastewater.

Raw blackwater should be pre-treated to prevent the excess accumulation of solids and garbage. Once in the pond, the heavier sediment particles settle out, and this also removes the nutrients attached to them. Plants and the communities of microorganisms that they support (on the stems and roots), take up nutrients like nitrogen and phosphorus. Chemical reactions may cause other elements to precipitate out of the wastewater. Pathogens are removed from the water by natural decay, predation from higher organisms, sedimentation and UV irradiation.

Although the soil layer below the water is anaerobic, the plant roots exude (release) oxygen into the area immediately surrounding the root hairs, thus, creating an environment for complex biological and chemical activity.

Basically, there are three different types of constructed wetlands (CWs). They are classified according to the water flow regime as:

• Free-water surface constructed wetlands (FWS)
• Horizontal flow constructed wetlands (HF)
• Vertical flow constructed wetlands (VF)

These three types of CWs may be combined with each other in hybrid constructed wetlands in order to exploit the specific advantages of the different systems.

One of the main advantages of CWs is, that they are natural systems and thus not require chemicals, energy or high-tech infrastructure. Moreover, they are suited to be combined with aquaculture or sustainable agriculture (irrigation).

In a free-surface constructed wetland (also known as surface flow CW or free water surface CW), water flows above ground and plants are rooted in the sediment layer at the base of the basin or floating in the water. As the water slowly flows through the wetland, simultaneous physical, chemical and biological processes filter solids, degrade organics and remove nutrients from the wastewater. The channel or basin is lined with an impermeable barrier (clay or geo-textile) covered with rocks, gravel and soil and planted with native vegetation (e.g., cattails, reeds and/or rushes). The wetland is flooded with wastewater to a depth of 10 to 45 cm above ground level. The wetland is compartmentalized into at least two independent flow paths. The number of compartments in series depends on the treatment target. The efficiency of the free-water surface constructed wetland also depends on how well the water is distributed at the inlet. Wastewater can be fed into the wetland, using weirs or by drilling holes in a distribution pipe, to allow it to enter at evenly spaced intervals.

The basin is planted advantageously with native plants. Compared to subsurface wetlands (horizontal flow or vertical flow), free-surface CW’s can be vegetated with emergent, submerged and floating plants (SA’AT 2006; TILLEY et al. 2008).

To avoid clogging and the excess accumulation of solids and garbage, pre-treatment is necessary. Pre-treatment of wastewater separates solid materials (e.g. faeces or kitchen slop) as well as grease or oil from the liquid. Depending on the situation, there are several possibilities such as grease trap, septic tank, biogas settler, anaerobic baffled reactor, imhoff tank, UASB reactor, or compost filter (HOFFMANN et al. 2010).

Pre-treated wastewater enters the basin via a weir or a distribution pipe. It is important for the treatment effect that it is distributed over the whole width. Once in the pond, the wastewater flows slowly through the basin and the heavier sediment particles settle, also removing nutrients that are attached to particles. Plants, and the communities of microorganisms that they support (on the stems and roots), take up nutrients like nitrogen and phosphorus. Chemical reactions may cause other elements to precipitate out of the wastewater. Pathogens are removed from the water by natural decay, predation from higher organisms, sedimentation and UV irradiation. Although the soil layer below the water is anaerobic, the plant roots release oxygen into the area immediately surrounding the root hairs, thus creating an environment for complex biological and chemical activity (TILLEY et al. 2008).

Free-surface CW’s normally require more surface than a subsurface system (e.g. a horizontal flow or vertical flow wetland). This is because the porous subsurface filter medium in subsurface systems provides a greater contact area for treatment activities. Consequently, compared to a subsurface filter, free-surface wetlands are designed bigger for the same volume of wastewater (SA’AT 2006).

The open surface can act as a potential breeding ground for mosquitoes. However, good design and maintenance can prevent this (e.g. proper pre-treatment, read more about it in WALTON 2003) .

Free-water surface constructed wetlands are generally aesthetically pleasing, especially when they are integrated into pre-existing natural areas. Thus, they can provide wildlife habitat beside the treatment process.

Care should be taken to prevent people from coming in contact with the effluent because of the potential for disease transmission (TILLEY et al. 2008; SA’AT 2006) and the risk of drowning in deep water.

The biggest health risk arises from settled wastewater in the pre-treatment facility during inspections and emptying. A proper emptying process (invalid link or motorised) can decrease the health risks (TILLEY et al. 2008). After that, also sludge must be treated correctly, for example in drying beds or composting facilities.

The capital costs of constructed wetlands are highly dependent on the costs of sand since the bed has to be filled with sand and on the cost of land (HOFFMANN et al. 2010). Financial decisions on treatment processes should not primarily be made on capital costs, but on net present value or whole-of-life costs, which includes the annual costs for operation and maintenance (HOFFMANN et al. 2010).

Compared to other intensive (high-rate) aerobic treatment options (e.g. activated sludge), constructed wetlands are natural systems, which work extensively. That means treatment may require more land and time, but you can safe costs because of lower operation, which requires no or only little electrical energy and operators can be trained people from the community (low-skilled people). It also means that there is no need for sophisticated equipment, expensive spare parts or chemicals (GAUSS 2008). According to HOFFMANN et al. (2010) constructed wetlands are usually cheaper to build than high-rate aerobic plants but for larger plants, they are usually more expensive in terms of capital costs.

For large-scale treatment plants of more than 10 000 PE in areas where land is available cheaply, free-water surface and waste stabilisation ponds have lower capital costs than horizontal subsurface-flow constructed wetlands (horizontal and vertical). Surface-flow constructed wetlands have also often lower maintenance and repair costs than in subsurface systems. On the other hand, if land area is not available or ground prices high, the large surface can be a big disadvantage (SA’AT 2006).

In general the O&M requirements for constructed wetlands are relatively simple (no high-tech appliances or chemical additives), allowing community organisations or a private, small-scale entrepreneur to manage the system after adequate capacity building and with technical support (GAUSS 2008).

Regular maintenance should ensure that water is not short-circuiting, or backing up because of fallen branches, garbage, or beaver dams blocking the wetland outlet. Vegetation may have to be periodically cut back or thinned out. In general free-surface CW’s are easier to regulate then subsurface systems. On the other hand they have not a great cold temperature tolerance and odour and mosquito problems can occur if operated and maintained incorrectly. A number of systems have had problems with clogging and unintended surface flows (SA’AT 2006).

An important part of O&M is to empty the sludge of the pre-treatment facilities (e.g. septic tank). This should be done in a proper and safe way (see invalid link and motorised emptying and transport). The filter bed of the constructed wetland may also be changed sometimes. The old material, full of earth and organic matter may be directly used as soil amendment or composted first, similar to a planted drying bed.