Willow

"I’m interested in how we can use natural biological systems and green technologies to meet environmental challenges. We're only at the start of developing such solutions but I think they are very much within reach." - Louis Hennequin, postgraduate researcher, Imperial College London

They're an increasingly popular method for managing domestic wastewater. Far from being simple plantings, they're engineered ecosystems that leverage the natural capabilities of specific willow varieties (Salix species), in conjunction with soil and microbial action, to purify effluent.

There are two basic willow system designs used for dealing with domestic sewage - willow filters and zero discharge willow facilities.

Typically unlined systems where willows are planted over a percolation pipe layout or drainage field (with modified piping to prevent blockage), through which effluent flows after primary treatment in a septic tank.

The fast-growing, specially selected willows possess extensive root systems that actively absorb water and the dissolved nutrients (like nitrates and phosphates) from the effluent, effectively using them as fertilizer. A significant volume of the absorbed water is returned to the atmosphere through transpiration from the willow leaves and evaporation from the soil surface. This process is particularly effective during the growing season.

As the effluent percolates through the soil matrix, physical filtration occurs. Simultaneously, a rich microbial community in the soil and within the rhizosphere (the area around the willow roots) breaks down organic pollutants and pathogens.

This system may be used on sites with poor percolation characteristics to encourage uptake of effluent, or may be used on sites with well drained soils to provide additional uptake of nitrites and phosphates from the effluent. Percolation through the soil will occur for some or all of the year, depending on the size of the system and the uptake rate through the trees vs. the volume of effluent from the septic tank.

Also known as willow evapotranspiration (ET) beds, these are designed for complete containment and on-site processing of effluent, with no liquid discharge to the surrounding environment.

Effluent is pumped into a lined, soil-filled, planted basin for 100% evapotranspiration to air in summer and full storage in winter. This is typically lined with plastic (or indigenous marl clay where suitable impermeable subsoil exists) and backfilled with loose subsoil or top-soil to _c_.1.5m deep. This approach is typically used in areas of impermeable subsoil where a surface discharge is prohibited or impractical and where a guaranteed zero discharge is required all year round. However it can also be an excellent system for sites where zero environmental impact is required.

Willows have long been employed for a wide variety of uses, including basketry, furniture, erosion control, fencing and hurdles. Although willow wood is slow to dry for burning as logs, willows have been increasingly grown in recent decades as a short rotation coppice crop to produce biomass fuel for municipal power plants and district heating systems.

In the early 1990s Danish engineers started researching willow planted systems for protecting waterways and groundwater from sewage pollution. Quick growing Scandinavian-bred willow cultivars were used in these trial systems and the uptake of effluent was found to be considerably greater than typical evapotranspiration rates from the equivalent area of grassland. So much so, in fact, that the research led to the development of zero discharge willow facilities which had such a high uptake rate that they could be used to evaporate 100% of the septic tank effluent pumped into them. Danish designers have been offering willow designs at home and abroad since then. Ongoing research throughout Europe and the rest of the world is helping to test the applications of willow systems for sewage treatment and disposal and to find new ways to convert sewage pollution into valuable nutrients and biomass.

Provides households with a tangible means of self-reliance and responsible resource management.

• Allows for on-site management of household wastewater, reducing dependence on municipal sewer systems or less effective, costly, conventional private systems like standard septic tanks with problematic soakaways. Particularly advantageous in rural areas where connection to mains sewerage may be impractical or prohibitively expensive.
• Directly converts a waste stream into useful resources. The vigorous growth of the willows results in a significant yield of biomass.
• Fosters a deeper connection with natural processes and responsible stewardship.
• However, zero discharge systems can be expensive compared to a septic tank and leachfield, and willow trees are not always suited to small sites.

• The adoption of on-site, nature-based solutions represents a shift away from centralized, often corporate-controlled, infrastructure, which has gone horribly wrong in recent years, discharging raw sewage into seas and rivers.
• Keeps money in communities, that would otherwise have gone to corporate shareholders.

While often designed for individual households, willow treatment can scale to the community level.

• The skills involved in designing, installing, and maintaining willow beds can become a shared community asset, with successful installations serving as educational models that inspire wider adoption of sustainable practices.
• On-site systems are not typically vulnerable to large-scale failures that can affect centralized plants. In situations where centralized services (power, water, sewage) might be disrupted, robust on-site systems like willow beds provide essential sanitation, a critical component of community health and stability, especially as many designs can operate with minimal or no external energy input once established.
• The local production of a renewable fuel source (willow biomass) and the overall improvement in local water quality contribute to the collective well-being and resilience of a community. Learning to work with natural systems for fundamental needs is a vital skill for navigating uncertain futures.

The environmental credentials of well-designed willow treatment systems are arguably their most significant benefit.

• They provide highly effective purification of domestic wastewater, significantly reducing the pollutant load (nitrates, phosphates, BOD, suspended solids, and pathogens) before it can impact groundwater or surface water bodies. This is crucial for protecting local aquatic ecosystems from eutrophication and contamination.
• Zero discharge willow facilities, by design, eliminate any effluent discharge to the surrounding environment, offering the highest level of protection, particularly in ecologically sensitive catchments or areas with vulnerable aquifers.
• The willow plantation itself becomes a vibrant habitat, supporting a variety of insects, birds, and other wildlife, thereby enhancing local biodiversity.
• The rapid growth of the willows means they are actively sequestering atmospheric carbon. If the harvested biomass is used as a substitute for fossil fuels, the system can be considered carbon-neutral over its lifecycle.
• Treatment relies on natural biological, chemical, and physical processes, minimizing or eliminating the need for electricity (willow filters) or the addition of chemicals associated with conventional wastewater treatment.

Willow filters and zero discharge willow facilities have two very different design protocols, construction input and budget. Zero discharge facilities are best designed by a specialist and then constructed with careful attention to detail to ensure that the system size and layout are correct. Willow filters are more straightforward and can be DIY or excavated by a local machine contractor.

Laid out as a drainage field (aka leachfield, infiltration area or percolation area), but with a modified pipe layout. Dig the drainage field trenches as set out in the standard guidance (having followed the correct procedures to determine the size needed to satisfy local legislation and to protect the groundwater and adjacent surface waters). Then, instead of using standard perforated piping, use a trench infiltrator unit to ensure that the willow roots won't clog the pipe perforations and block the system. This is essentially an upside down gutter, placed on the trench base and covered with soil. The higher up this can be in the ground, the easier it will be for the effluent to be accessed by tree roots. Too shallow will risk effluent exposure at surface level in times of heavy rain, and too deep will risk being beyond the easy reach of the roots of the trees. That said, due to the deep reach of willow roots, the bigger risk of having the trench base too low down is that the effluent will be introduced closer to bedrock or ground water.

Purpose-made trench infiltrator units are best for this work, but a lower-key alternative is to cut 300mm twin-walled piping (the corrugated outer/smooth inner piping used to pipe small drains in fields) down the middle and lay the two halves open-end down on the trench base. For distribution of effluent either split the effluent evenly between 4 trenches in the standard distribution box or use a ribbit splitter to get better-quality distribution (particularly useful for ground with good drainage where the effluent may otherwise drop too quickly into the ground and pollute the groundwater).

Ideally set out the area in a 6m wide rectangle to maximise the uptake efficiency of the willows. A more beautiful layout will also work if you prefer, but take care to allow plenty of air movement through the trees and exposure to sunshine. Plant with 6 rows of willows, spaced at _c_.80cm apart with willow cuttings at 30-50cm spacing within each row. A sheet of black silage pit cover plastic is recommended for covering the whole area in year 1. Plant the 30cm cuttings through the plastic (50% deep) and weight the plastic down well to avoid wind lift (which will kill your cuttings and lose you a full year of growth). Then in the February following planting, cut all cuttings back to 15cm above ground level and lift the plastic off. The ground will be weed free at that stage and a reduced hand-weeding input will be fine for year 2. Note that NO sprays should be used since these will quickly kill willows at any time of year.

First determine the effluent volumes likely to be produced, the evapotranspiration rate from local meteorological data, and soil type and depth. With this data, it's possible to determine the required facility size. A typical system size is _c_.6m x 35m for drier parts of the UK, but wetter locations can be as much as double this length (where careful design is employed to exclude a large proportion of the rainfall landing on the system). Keep the width relatively narrow to avoid a build-up of humidity in the centre of the basin. Here's the order of work:

• Excavate the full area, to _c_.1.5m deep, using a 45 degree bank slope.
• Lay a liner across the full basin, with protective geotextile membrane to protect the waterproof layer.
• Fit the inlet pipe (40mm pumped feed from a pump sump following the septic tank), using a 'top hat' fitting to assist with guaranteeing a waterproof seal. Remember that if any water leaks in or out it will seriously damage the capacity of the system to achieve a zero discharge.
• Lay a base drainage pipe along one side of the basin floor, running the full length and ending in a 300mm inspection well.
• Erect the 300mm inspection well in sand to hold it in place. Also, fit a 110mm inspection well at the opposite end, unconnected to the base drain.
• Backfill the basin with 1m of soil, taking care not to compact it in any way.
• Cover the soil with 10cm of sharp sand.
• Lay a central spreading system for effluent introduction - typically a Danish plastic bioblock medium with a 40mm spreading pipe attached to the top of the plastic media blocks. Wrap this with non-woven geotextile to allow effluent to flow into the underlying sand, but to avoid soil falling into the bioblocks.
• Add the top layer of soil, without compacting it.
• Build a perimeter embankment around the internal perimeter of the lined basin and fold the liner plastic up over this to reduce the area open to rainfall. Cover this with soil, and seed with grass to prevent erosion. Build a central bund to cover the spreading pipe from frosts and to contain any odours.
• Plant with willow cuttings. A plastic sheet may be used in year 1 to prevent weed growth. This is then removed after coppicing in the February following the first year of growth.
• Coppice on a 3-year rotation basis and keep the septic tank maintained as needed to prevent ingress of suspended solids.

Planning permission is generally needed for any new sewage treatment system, so if you're building a new house and new zero discharge willow facility then apply for planning for everything together. To carry out a major upgrade of a sewage treatment system usually requires planning, even for an existing house, but ask your local planning authority anyway since there may be exemptions.

If all you want to do is plant willows over an existing drainage field, then you can do that without planning permission. However, remember that willow roots are invasive and may well clog your system. Only plant willows near or over a drainage field if you're happy to get a digger in and redo your whole system in a few years time if clogging occurs. This may well be a worthwhile gamble if your system is already ponding slightly and you want to dry out the ground in the drainage field area or protect a local watercourse from pollution that's already occurring. In such a case, willows might be enough to soak up the excess liquid and keep the ground free of effluent, while also mopping up nitrates and phosphates and converting these nutrients (or pollutants depending on your perspective) into firewood.

There are c.526 species of willow in the world, mostly native to the Northern Hemisphere. Although there are many different willow species native to the UK and Europe, the osier, Salix viminalis is the main species used for breeding biomass willow cultivars. This is also the willow used for sewage treatment applications due to the high growth rates and consequent high uptake of liquid and nutrients. If you're planting a willow filter yourself, be sure to use only hybrid biomass willows and not basket willows or locally-growing willows. Find a farmer who grows them for biomass and ask for some if you like. The difference in growth rates is striking.

Danish willow systems are often put into first use immediately following planting in March or April (the end of the dormant season for willows). As the growing trees in a willow system become more mature, the volume of effluent evapotranspired into the air will increase, so some local authorities prefer to have 1 year of willow growth prior to first connection to the septic tank for new properties. Note that this may necessitate watering in the first year to ensure that adequate moisture and nutrients are available for the growing trees. Every 3 years the willows will reach their full height (for filter purposes anyway), and are ready to coppice. As the years go by, the willow roots will become more established and the number of willow poles per stem will increase, further increasing the uptake of effluent and generation of biomass.

• Ensure that the septic tank is desludged on an annual basis. This will avoid carry-over of solids to the willow facility.
• Keep the willow evapotranspiration system moist during dry conditions in the first season to ensure plant health.
• Control weeds around the willows in the first year, and after that if grass or other weed growth is excessive.
• Eliminate or minimise bleach and other toxic materials from the waste stream to maximise bacterial activity and the effectiveness of the whole system. No herbicides, paints, solvents or hydrocarbons should be allowed to enter the sewage system nor be used in the willow area.
• Cut back willows on a rotation basis to continually generate new growth. Since the systems work best for a 3-year rotation it would make sense to harvest 1/3 of the system each year. However in a typical 6-row basin, if coppicing is done in rows, the central row can become overgrown and fail to thrive. Consequently the coppicing programme often follows two years of coppicing (left side, then right side) then one year of no cutting. Row coppicing is more efficient than block coppicing (dividing the basin into 3 blocks of trees) since it maximises the “clothesline effect” whereby willows get the most air and sunshine to increase moisture uptake.

This topic introduction provides a very brief summary of willow filters and zero discharge willow facilities. If you'd like to know more, visit the websites listed below to contact a designer directly.

• Septic Tank Options and Alternatives
• Willow System Treatment Options

Date on Lowimpact:2025-06-04