Vertical Flow CW

Constructed wetlands are secondary treatment facilities for household (blackwater or greywater, in some cases also brownwater) and/or biodegredable 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). How the vertical flow CW works: By intermittently dosing the wetland (4 to 10 times a day), the filter goes through stages of being saturated and unsaturated, and, accordingly, different phases of aerobic and anaerobic conditions. During a flush phase, the wastewater percolates down through the unsaturated bed. As the bed drains, air is drawn into it and the oxygen has time to diffuse through the porous media.

The filter media acts as a filter for removing solids, a fixed surface upon which bacteria can attach and a base for the vegetation. The top layer is planted and the vegetation is allowed to develop deep, wide roots, which permeate the filter media. The vegetation transfers a small amount of oxygen to the root zone so that aerobic bacteria can colonize the area and degrade organics. However, the primary role of vegetation is to maintain permeability in the filter and provide habitat for microorganisms. Nutrients and organic material are absorbed and degraded by the dense microbial populations. By forcing the organisms into a starvation phase between dosing phases, excessive biomass growth can be decreased and porosity increased.

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.

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

• Free 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, there are suited to be combined with aquaculture or sustainable agriculture (irrigation).

The vertical flow constructed wetland can be designed as a shallow excavation or as an above ground construction. Clogging is a common problem. Therefore, the influent should be well settled in a primary treatment stage before flowing into the wetland. 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 tank, anaerobic baffled reactor, imhoff tank, biogas settler, UASB reactor, or compost filter (HOFFMANN et al. 2010). The design and size of the wetland is dependent on hydraulic and organic loads. Generally, a surface area of about 1 to 3 m2 per person equivalent is required. Each filter should have an impermeable liner and an effluent collection system. A ventilation pipe connected to the drainage system can contribute to aerobic conditions in the filter. In cold climates (annual average < 10°C), an area of 4 m2/p.e. is necessary. In warmer climates (annual average > 20°C), 1.2 m2/p.e. is enough, if the filter is designed correctly (HOFFMANN et al. 2010).

Operation

In vertical filter beds wastewater is intermittently applied (either by pump or self-acting syphon device) onto the surface and then drains vertically down through the filter layers towards a drainage system at the bottom. In some cases, the distribution pipes are covered with gravel to avoid open water puddles. The treatment process is characterised by intermittent short-term loading intervals (4 to 12 doses per day) and long resting periods during which the wastewater percolates through the unsaturated substrate, and the surface dries out. The intermittent batch loading enhances the oxygen transfer and leads to high aerobic degradation activities. Therefore, vertical filters always need pumps or at least siphon pulse loading, whereas horizontal flow constructed wetlands can be operated without pumps (if topography allows). The treatment process of constructed wetlands is based on a number of biological and physical processes (adsorption, precipitation, filtration, nitrification, predation, decomposition, etc.) (HOFFMANN et al. 2010).

Substrate (Adapted from HOFFMANN et al. 2010)

Structurally, there is a layer of gravel for drainage (a minimum of 20 cm), followed by layers of sand and gravel.

The provision of a suitably permeable substrate in relation to the hydraulic and organic loading is the most critical design parameter of subsurface flow constructed wetlands. Most treatment problem occur when the permeability is not adequately chosen for the applied load.The drainage pipes at the base are covered with gravel. On top of this gravel layer, there is a sand layer (40-80 cm thick) which contains the actual filter bed of the subsurface flow CW. On top of the sand layer there is another gravel layer (about 10 cm), in order to avoid water accumulating on the surface. The top gravel layer does not contribute to the filtering process.

Design recommendations regarding the substrate to be used in subsurface flow filters are:

• The sand should have a hydraulic conductivity (kf-value) of about 10-4 to 10-3 m/s.
• The filtration sand layer needs to have a thickness of 40 to 80 cm.

The recommended grain size distribution for the substrate is shown in the graphic below.

The substrate should not contain loam, silt or other fine material, nor should it consist of material with sharp edges (the figure below illustrates the properties of suitable sand).

What kind of filter material should be used depends on the local conditions and the experiences of the design engineer. HOFFMANN et al. (2010) recommend sand as a substrate, because in their point of view it is the most suitable substrate for the application of subsurface flow CWs for wastewater or greywater treatment in developing countries.

Plants

Depending on the climate, Phragmites australis (reed), Typha sp. (cattails) or Echinochloa pyramidalis are common plant options (TILLEY et al. 2008) . Bamboo or papyrus should also be possible, but have not been investigated yet (HOFFMANN et al. 2010). More possible plants and their characteristics can be found in HOFFMANN et al. 2010. Testing may be required to determine the suitability of locally available plants with the specific wastewater.

Plants are aesthetically pleasant and serve as a habitat for wildlife. Dead plant material is a natural insulations layer and protects the filter 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). Due to the high oxygen supply into the filter, the rates of nitrification are higher than in a horizontal flow filter (MOREL and DIENER 2006). Due to good oxygen transfer, vertical flow wetlands have the ability to nitrify, but denitrification is limited. In order to create a nitrification-denitrification treatment train, this technology can be combined with a Free-Water Surface or Horizontal Flow Wetland.

Pathogen removal is accomplished by natural decay, predation by higher organisms, and filtration. The risk of mosquito breeding is low since there is no standing water. The system is generally aesthetic and can be integrated into wild areas or parklands. Care should be taken to ensure that people do not come in contact with the influent because of the risk of infection.

Greywater, which has been treated in subsurface flow constructed wetlands generally meets the standards for pathogen levels for safe discharge to the environment without further treatment. In case of domestic wastewater, the situation could be different and for safety reasons, disinfection (by tertiary treatment) might be necessary, depending on the intended reuse application (HOFFMANN et al. 2010).

The biggest health risk arises from settled wastewater in the pre-treatment facility. This should be considered during inspections and emptying. 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 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 save 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 means also 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-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 require 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), allowing community organisations or a private, small-scale entrepreneur to manage the system after adequate capacity building and with technical support (GAUSS 2008). However, a CW will require 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. Distribution pipes should be cleaned once a year to remove sludge and biofilm that might block the holes. With time, the gravel will become clogged by accumulated solids and bacterial film. Resting intervals may restore the hydraulic conductivity of the bed. If this does not help, the accumulated material has to be removed and clogged parts of the filter material replaced. 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. 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). It needs to be controlled regularly whether pre-treatment facilities 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).

Vertical systems require more technical expertise than other wetland technologies such as waste stabilisation ponds, surface flow CWs or [8259-horizontal CWs (HOFFMANN et al. 2010).