Reed

“Sewage treatment reed-beds may be at least as biodiverse as naturally occurring reed-beds and will add to the overall biodiversity and ecohydrology of a catchment whilst saving energy.” - International Journal of Ecology

In a low-impact context, they’re wastewater treatment systems that use growing wetland plants as the active component in getting effluent clean enough to discharge back into the receiving environment. Alternately called treatment wetlands, constructed wetlands or reed bed treatment systems, they have the potential to be low-cost, zero energy input, low-tech, high-efficiency systems that can be used to help protect streams and rivers from almost any source of effluent or dirty water.

Despite appearances to the contrary, reed beds and constructed wetlands work in a very similar way to conventional treatment systems. Primary settlement takes place in a septic tank; secondary aeration is provided by the plants, which draw oxygen down to the roots via the leaves, where it becomes available for aerobic bacteria; tertiary polishing is carried out if the reed bed is built large enough, providing further removal of nitrogen and phosphorus.

The following physical, chemical and biological treatment mechanisms all come into play (see glossary for explanations):

• sedimentation
• bacterial action
• filtration
• nutrient uptake
• adsorption
• precipitation
• decomposition
• volatilisation

The terms reed bed and constructed wetland are often used interchangeably, but there are a number of distinct system types within the general category of treatment wetlands. Although these different system types can be used in series on any given project, the design protocol for each type should not be used interchangeably. The different reed bed types are usually categorised as follows:

Soil-based constructed wetlands or free water surface (FWS) wetlands: most closely resemble a natural marsh. They consist of a lined shallow basin, backfilled to c.150mm with loam soil and planted with a selection of tall wetland plants. The effluent is treated as it moves slowly through the plant stems and leaf litter that accumulates in the shallow water of the marsh. Integrated constructed wetlands (ICW) and wetland ecosystem treatment (WET) systems both fall into the constructed wetland category.

Horizontal flow gravel reed beds or horizontal sub-surface flow (HSSF) systems: generally smaller in footprint area, they consist of a 6-700mm deep bed of washed limestone gravel into which common reed (Phragmites australis) and other wetland plant species are planted. The effluent is treated as it flows through the gravel and plant roots.

Vertical flow (VF) reed beds: similar in layout to a raised sand polishing filter or stone trickling filter. Vertical flow reed beds have a smaller footprint area than horizontal flow gravel reed beds, so may be more suited to small sites. The VF reed bed consists of c.1m depth of gravel, of progressively smaller particle sizes towards the bed surface. Effluent is dosed over the bed surface either by pumped feed or gravity dosing mechanism and treatment occurs as it trickles down over the gravel media and plant roots. Bear in mind that VF reed beds filled or topped with sand can be prone to blocking unless the exact aggregate grades are used and the correct degree of pretreatment is consistently achieved.

Ponds may also be used to provide additional storage volume. This extends the residence time within the overall system while keeping the footprint area modest. Ponds also have the advantage of offering an additional habitat dimension – particularly towards the end of a system where the effluent is cleaner. They also offer greater penetration of UV light than marsh areas, for extra die-off of pathogens. They pose a potential safety hazard however, and are thus often excluded from domestic systems.

With our current global challenges of climate change and species extinction, reed beds and constructed wetlands can offer a zero energy input way to get our sewage clean, and also provide reliable sewage treatment at a time when continuation of reliable ongoing electricity supply to our municipal and domestic sewage systems is not guaranteed, thus helping to preserve the habitat integrity of our rivers, lakes and coastal waters for biodiversity.

Soil-based constructed wetlands are less suited to small sites (<1 acre), due to their open nature, but work well on larger sites where a natural habitat appearance is desired. They can be a low-cost, low-resource system on clayey soils, but can also be plastic lined where needed.

Advantages:

• Constructed wetlands are open systems; with water sitting on a soil base and movement of water through a dense filter of leaf litter and plant stems. This means that they are very resilient to sludge overloading and hydraulic shock loading (i.e. sudden overloads of effluent). The water simply rises up over the sludge, or where sludge accumulation is excessive, it can be removed by direct excavation with a mechanical digger.
• They can be very cost-effective systems where heavy clay is present, negating the need for plastic lining. In such sites, full secondary and tertiary effluent treatment can be achieved for not much more cost than excavation and planting, following the septic tank.
• Clay-lined constructed wetlands are one of the lowest embedded energy input sewage treatment systems available, (along with dry toilets, which don't even need the septic tank; and willow systems, which pay back carbon in firewood every year after construction).
• They are probably the best treatment wetland option for wildlife because they directly resemble marsh habitats, albeit nutrient-enriched ones.
• Where combined sewers are present, receiving both stormwater from roof surfaces and sewage effluent, constructed wetlands can be designed and sized to accommodate the rainfall-dependent flow patterns and still produce reliable effluent quality.

Limitations:

• Because they are open systems, they are more susceptible to odour generation. If odours are present in the septic tank, they can be channelled down the pipe to the wetland inlet. The solution is to ensure that eco-friendly detergents etc. are used in the home and that you position the wetland at a suitable separation distance from any houses.
• They contain open water to a depth of c. 200mm, which can be a safety hazard. The most common solution is to omit the pond (up to 1m deep) from the design, and to fence the wetland from children, animals and the general public.
• Due to their open nature they can be a potential source of pathogen contamination, transferred by pets, small animals or birds, and may thus be unsuitable for small sites or areas immediately adjacent to veg gardens or orchards.
• They need a bigger area than gravel reed beds: 100m² minimum wetland size vs. 25m² minimum reed bed size for a three-bedroom house.

Horizontal flow reed beds are more suitable for sites where space is limited, or where it’s desirable that the effluent is covered by a gravel surface. They need a tougher liner than soil-based wetlands, and tend to be more formal in final appearance, which can suit some garden layouts.

Advantages:

• Gravel reed beds usually have a smaller footprint area than soil-based constructed wetlands, so can be used on sites that don’t have space for a larger system.
• Because the water is entirely covered with gravel, they do not pose any potential drowning hazard, and are generally pathogen-free at ground surface level.
• Odours can also be less than from soil-based constructed wetlands because of the covered nature of the system.
• Since the water level in gravel reed beds is generally fixed at c.50mm below gravel surface, they may be used on sites where only small head losses are permitted, such as sites that are quite level, or where relatively high groundwater means that the final percolation trench level needs to remain as high as possible.
• Small modular reed bed units are relatively easy to install compared to using a plastic liner. That said, the author of Permaculture Guide to Reed Beds recommends that in all but the smallest of sites, you use sizing that is about double the UK guidelines design size, so these small units may be insufficient alone.

Limitations:

• Because gravel is used rather than soil, the liner needs to be stronger due to the risk of complete emptying and drying of the plants. Thus both liner costs and gravel costs can push the price higher than a soil-based wetland. However, where free-draining soils are present, requiring a durable liner anyway, gravel systems can be cheaper due to the smaller footprint area, so treat each site individually.
• The gravel media has the potential for clogging if the septic tank isn't properly maintained. One potential solution is to install two septic tanks before the reed bed, or to use a septic tank filter unit at the outlet pipe. Nonetheless, maintenance is a bigger factor for reed beds than for soil-based wetlands.
• If you are using a small modular unit then it is extra important that your mechanical treatment unit is functioning at top efficiency all of the time to ensure that the overall system performs as designed. (Modular units are usually used only for tertiary treatment after a mechanical aeration unit).
• At some stage, the gravel will clog up anyway. Bacteria mass, sediments and plant debris will all contribute to the eventual congestion of the gravel. While soil-based wetlands have an adjustable flow control unit that can simply be raised as sediment levels rise, gravel reed beds will need a complete overhaul every 15 to 30 years depending on influent quality, system size and throughput volumes.
• Although effluent exposure is minimised, bear in mind that some effluent may still be exposed for some or all of the time at the surface or at the reed bed inlet, depending on the final design. As such they cannot be treated as sterile. Like soil-based wetlands, they should ideally be fenced to keep out pets, livestock and small children.

Vertical flow reed beds are generally used to pretreat effluent entering a horizontal flow reed bed, where a higher quality of effluent is needed in a small space. They can also be effective where good soil percolation characteristics exist, but where a quick burst of treatment is needed prior to discharge to ground. Their main drawback is that a pump is usually needed to provide the required distribution of effluent over the reed bed surface. That said, a gravity splitter, dosing box or syphon may all be used where suitable falls are present.

Advantages:

• Vertical flow reed beds are good for stripping ammonia (the smelly component) from septic tank effluent. They’re also efficient for BOD and suspended solids reductions in tandem with horizontal flow reed beds.
• They can be very effective where space is limited, because they reduce the overall size needed for secondary treatment, which in turn reduces the required size of the follow-up tertiary treatment wetland and final infiltration area.

Limitations:

• A pumped feed is usually needed, which can add to ongoing energy needs and costs. However, where there is a fall on the site a gravity dosing box, siphon system or effective splitter unit may be used instead.
• VF reed beds are best used in conjunction with a horizontal flow bed or other treatment component, and the requirement for an extra system can add to the cost of a project.
• Incoming liquid needs to be sufficiently clean that it avoids clogging the pea gravel/sand surface layer. Thus ongoing septic tank maintenance is important (but unless you’re on a mains sewer, maintenance is important whatever sewage system you have).
• Greater inspection frequency is needed to ensure that the effluent spread is effective and the distribution network isn’t clogged.

There are a number of stages to any reed bed project. The more you can do yourself, the lower the overall cost. Reed beds and constructed wetlands generally require planning permission, so if you are good at preparing site drawings, system drawings and good at sourcing design information, this is something you can do yourself. Otherwise you may wish to hand over to a consultancy that specialises in constructed wetland and reed bed design. The excavation, lining, pipework and manhole construction all require input by somebody skilled in that kind of thing. You may hire a mini digger and do it yourself, or you may wish to hire a groundworks contractor, landscaper or sewage treatment system specialist. Planting with the right plants is the final stage (to be carried out before connecting the septic tank!). If you have a boggy field nearby with the right plants, that will be the lowest carbon footprint approach. You may also buy from specialist suppliers if you are stuck. Planting isn't rocket science, but be sure to put the green bit up and the rooty bit down (it happens…). The Permaculture Guide to Reed Beds gives guidance on all of these stages, so if you want to do it yourself, it may be a good book to start with.

See our further information section for summary guides to constructing different types of reed beds, and here’s a pictoral guide to installing a horizontal flow reed bed.

Below is a rough guide, based on sizing recommendations from the EPA (Ireland) and GBG-42 (UK).

Date on Lowimpact:2013-01-06