Horizontal Subsurface Flow 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 filter media acts as a filter for removing remaining solids, a fixed surface upon which bacteria can attach, and a base for the vegetation. Although facultative and anaerobic bacteria degrade most organics, the vegetation transfers a small amount of oxygen to the root zone so that aerobic bacteria can colonize the area and degrade organics as well. The plant roots play an important role in maintaining the permeability of the filter.

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.

Besides horizontal subsurface flow constructed wetlands there are two other types of constructed wetlands (CWs). They are classified according to the water flow regime as:

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

The 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).

The design of a horizontal subsurface flow constructed wetland depends on the treatment target and the amount and quality of the influent. It includes decisions about the amount of parallel flow paths and compartmentation. The removal efficiency of the wetland is a function of the surface area (length multiplied by width), while the cross-sectional area (width multiplied by depth) determines the maximum possible flow. Generally, a surface area of about 5 to 10 m2 per person equivalent is required.

A horizontal subsurface flow constructed wetland is a large gravel and sand-filled channel that is planted with aquatic vegetation. As wastewater flows horizontally through the channel, the filter material filters out particles and microorganisms degrade organics.

The water level in a Horizontal Subsurface Flow Constructed Wetland is maintained at 5 to 15 cm below the surface to ensure subsurface flow (TILLEY et al 2008).

Pre- and primary treatment is essential to prevent clogging and ensure efficient treatment. This 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 tanks, anaerobic baffled reactors, imhoff tanks, biogas settlers, UASB reactors, or compost filters (HOFFMANN et al. 2010). The influent can be aerated by an inlet cascade to support oxygen-dependent processes, such as BOD reduction and nitrification.  

Pre-treated wastewater flows slowly through the porous medium under the surface of the bed in a horizontal path until it reaches the outlet zone. The bed should be lined with an impermeable liner (clay or geotextile) to prevent leaching. It should be wide and shallow so that the flow path of the water in contact with vegetation roots is maximized. A wide inlet zone should be used to evenly distribute the flow. A well-designed inlet that allows for even distribution is important to prevent short-circuiting. The outlet should be variable so that the water surface can be adjusted to optimize treatment performance.

At the outlet, the water level is controlled with an adjustable standpipe. A common design suggests a water level of about 60 cm which is maintained at ca. 5 to 15 cm below the surface of the CW to avoid anaerobic conditions in the bed. An important role of treatment efficiency is the oxygen supply. Horizontal filter beds have a very small external oxygen transfer and a smaller inlet compared to a vertical flow constructed wetland. Therefore they require a larger area.

Small, round, evenly sized gravel (3 to 32 mm in diameter) is most commonly used to fill the bed to a depth of 0.5 to 1 m. To limit clogging, the gravel should be clean and free of fines. Sand is also acceptable, but is more prone to clogging than gravel (EAWAG/SANDEC 2008) . The inlet and the outlet zone should be constructed with coarse gravel.

In recent years, alternative filter materials, such as PET, have been successfully used. The water level in the wetland is maintained at 5 to 15 cm below the surface to ensure subsurface flow (TILLEY et al 2008) .     

If topography allows for gravity flow, horizontal flow filters are not dependent on energy and can be operated by gravity. For designing the filter bed, expert knowledge is required. Parameters such as hydraulic load or organic loading (e.g. BOD) (HOFFMANN 2010) and experience of the designer specify the size of the filter as well as the filter substrate (see also vertical CW).

Plants

In contrast to a non-planted filter, plants in constructed wetlands are an important part of the design. Plants are aesthetically pleasant and serve as a habitat for wildlife. Dead plant material is a natural insulation layer and protects the filter bed during winter in cold climates. Furthermore, the vegetation transfers oxygen to the filter zone and plants and its roots provide an appropriate habitat for microbiological growth in the root zone. But the most essential function of the vegetation, i.e. the roots system is to maintain the permeability in the filter (HOFFMANN et al. 2010 and TILLEY et al. 2008).

Any native plant with deep, wide roots that can grow in the wet, nutrient-rich environment is appropriate. Phragmites australis (reed) is a common choice because it forms horizontal rhizomes that penetrate the entire filter depth. The plant should build a deep root zone to transfer at least a minimum of oxygen into the filter body (TILLEY et al. 2008).

Bamboo or papyrus should also be possible, but that has not been investigated yet (HOFFMANN et al. 2010). Other suitable plants and their characteristics can be found in the document HOFFMANN et al. 2010 (see further readings below).

Significant pathogen removal is accomplished by natural decay, predation by higher organisms, and filtration. In case of domestic wastewater disinfection (by tertiary treatment) might be necessary, depending on the intended reuse application (HOFFMANN et al. 2010). As the water flows below the surface, any contact of pathogenic organisms with humans and wildlife is minimized. The biggest health risk arises from settled wastewater in the pre-treatment facility. A proper emptying process (human powered 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 risk of mosquito breeding is reduced since there is no standing water compared to the risk associated with Free-Water Surface Constructed Wetlands. The wetland is aesthetically pleasing and can be integrated into wild areas or parklands.

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 costs can be safed because of lower operation, which requires no or only little electrical energy and operators can be trained people from the community (low-skilled people). Furthermore, 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-surface-flow constructed wetlands and waste stabilisation ponds have lower capital costs than subsurface-flow constructed wetlands (horizontal and vertical) due to the high amounts of sand and gravel fill required for the bed of the sub-surface flow constructed wetland. Plants and liners may substantially add to the costs if they are unavailable locally (EAWAG/SANDEC 2008). Moreover, design and construction of subsurface-flow constructed wetland requires skilled technical staff. However, the cost may be reduced if the material is acquired locally.

In general the O&M requirements for constructed wetlands are relatively simple (no high-tech appliances or chemical additives), which may allow a community organisation or a private, small-scale entrepreneur to manage the system after adequate capacity building and with technical support (GAUSS 2008). However, a CW will always require some maintenance for the duration of its life. This aspect is frequently overlooked in decision-making processes.

During the first growing season, it is important to remove weeds that can compete with the planted wetland vegetation. With time, the gravel will become clogged with accumulated solids and bacterial film. The filter material at the inlet zone will require replacement every 10 or more years. Maintenance activities should focus on ensuring that primary treatment is effective at reducing the concentration of solids in the wastewater before it enters the wetland (TILLEY et al. 2008) . Maintenance should also ensure that trees do not grow in the area as the roots can harm the liner.

A very critical situation occurs, when the filter smells like “foul eggs”. This is an indicator for anaerobic conditions. In this case the filter should be rested and the loads must be readjusted (HOFFMANN et al. 2010). Pre-treatment facilities need to be checked regularly if they work properly and they have to be emptied frequently and sludge must be discharged correctly (see human-powered emptying and transport and motorised emptying and transport).