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  • HPH373: Using a willow bed to treat sewage – with Laila Dudhill

    HPH373: Using a willow bed to treat sewage – with Laila Dudhill

    Laila Dudhill explains some of the challenges she faced when planning her new build home at the bottom of a sloping field. Not only did she have to deal with drainage across the site, diverting the natural flow of water away from the house, but also come up with a foul drainage strategy that would off-set phosphate discharge into the local watercourses.

    Interview with Laila Dudhill

    Laila Dudhill has a 40-acre farm in Herefordshire, in one of the river catchment areas in the county that is sensitive to phosphate pollution. Her land is partly situated on heavy clay and on a gradient, so a lot of thought has had to go into dealing with all aspects of water run off and foul water treatment. In the early stages of planning her project she developed rheumatoid arthritis, which increased her determination to step away from the responsibilities of the farm and influenced her designs for the new house.

    The original barn and field

    ‘Class Q’ permitted development allows for a change of use

    Class Q planning permission allows for a change of use from agricultural buildings to residential dwellings. Laila’s barn was a 1990s steel frame structure and the Class Q permitted development allowed her to build to the same footprint, same envelope and same size. The steel frame was a problem for Laila as the steels would have created thermal bridging and heat loss. This meant she had to go back to the Local Authority after receiving Class Q permission and gain consent to replace the steels (in other words, build new). She also needed to alter the angle of the roof slightly so that it would be optimised for solar PV. This resulted in the pitch being slightly higher than the original barn.

    The old barn is dismantled

    Any new home must address phosphate pollution

    Herefordshire has a large number of intensive poultry units, which has resulted in high phosphate levels in the River Wye and the River Lugg. The Council is concerned about more phosphates coming from the foul drainage of new properties, so every planning application in this area must demonstrate what measures will offset the phosphates.

    There are ways to off-set phosphate discharge

    Laila explains that her foul drainage strategy will double the offset requirement. Her main system is a willow bed and a septic tank, rather than the more common biodigesters. A biodigester requires a large drainage field, which was a problem for Laila being on heavy clay soil. It also cleans the effluent much better than a septic tank, so wouldn’t provide enough nutrients for a willow bed. 

    The willow bed satisfied her planning requirements, but Laila has been able to continue to add to her phosphate off-setting in various ways. Taking agriculture off the land gives you a phosphate credit along with planting trees, particularly fruit and walnut trees.

    The contours of the land and the flow of water

    The natural water flow was straight past the barn

    The site’s drainage has been of particular concern

    Topographical surveys and drainage studies were carried out, including percolation tests to determine the speed water drained through the ground. The field in front of the house was clay, with very poor drainage, whilst the woodland at the back had extremely fast drainage. Neither situation is good for a drainage field. Either the water sits on top of the soil with potential run-off to the watercourse or it goes through the soils so fast that the nutrients aren’t retained in the soil. The septic tank and willow bed will retain all nutrients from her foul water system, with periodic de-sludging of the tank required.

    Having lived on the site since 2006, Laila had a good knowledge of how water flowed through the site to a pre-existing pond (that’s shown on Ordnance Survey maps). She tells us about springs that issue from within the woods, but that she had not experienced any flooding around the barn until last winter, which was then repeated this year. She is not in a flood zone and believes this is because she is on a slope, so water doesn’t rise around the property but flows past in extreme weather. A new flood alleviation pond has been created to hold excess surface water and the recent severe weather tested the system and showed that some adjustments were needed.

    A flood alleviation pond was dug

    Water is piped from the flood alleviation pond under Laila’s driveway, to the head of a brook that emanates at the edge of the woodland and also picks up the woodland springs. Water from the brook eventually feeds into the River Arrow within the Lugg catchment which is why the foul drainage from the house needs to be managed separately to the rest of the surface water run-off.  The rain water which falls on the house can simply be discharged into a free draining area of the woodland.

    Digging the giant willow bed

    Lining the willow bed

    Back-filling the willow bed

    The willow bed is an effective way to treat the sewage

    From the house, all the sewage goes into a septic tank, which links into a willow bed. This is lined so nothing can escape into the surrounding soil. The bed is backfilled with soil and planted with willow, which uses the water from the foul drainage in transpiration and uses the nutrients to grow. Laila is researching which varieties to grow, how close to plant them and how often they should be coppiced so that people will want to use the willow.

    The willow bed requires a lot of space

    Laila’s drainage consultant Alan Corner looked at a Danish study and an Irish study to work out how big the willow bed needed to be. The specification is based on the number of bedrooms in a house, as that generally determines occupancy. Strictly speaking, Laila’s is a four bedroom house, although she will usually be living there on her own. The specification for the willow bed has come out at 47 metres by 8 metres, about the size of an Olympic swimming pool!

    “I’ve coined a phrase, I’m going to grow a crop from my crap”

    Financially, it’s been expensive and probably it would not be an option for most people, but Laila says it was a really interesting process and a good test case.

    Get recommendations when building up your team

    Laila tells us that she has found the whole process positive, with one thing organically leading to another and people always happy to explain things and answer questions. Initially she had a recommendation for a planning consultant, who then guided her through the planning application process and put her in touch with a groundworker. She interviewed a few architects and selected one.

    Building a house can be stressful

    Even when people are fantastic and things are exciting, Laila points out there is still stress, which is not ideal for her rheumatoid arthritis. But she has support from her rheumatology team, who try to keep her condition tolerable.

    Dividing the farm has helped finance the new home

    Laila feels fortunate that her farm can be divided quite nicely into two (by the main road), so she can sell the farmhouse, land and outbuildings on one side, whilst keeping the rest for her new home. She is currently still living in the farmhouse, but will need the sale to go through in order to complete the build.

    Embracing simplicity makes it easier to build and achieve the Passivhaus standard

    Laila’s father was an engineer and her mother was a mathematician, and so Laila understands the benefits of keeping things simple. Passivhaus Consultant Nick Grant has been advising and she has been happy to implement all his recommendations. This ultimately leads to doing more with less.

    The foundations for the new home are dug

    The binding layer of the foundations

    The foundations are nearly complete

    The groundworks and Isoquick base are now complete

    Laila’s structural engineer was happy that the stony ground the original barn was sitting on was sound for an Isoquick insulated raft foundation. The site was excavated down to around 750mm below the finished floor level and then built up with a sub-base material, which was compacted, and a final 50mm fine drainage stone to create a flat surface. The Isoquick raft sits on top of this followed by a layer of concrete, so they are ready for the superstructure.

    We will revisit Laila later in her build to hear the next steps in her journey.

    Find out more

    The Passive House Builder

    Corner Water Consulting

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  • HPH179 : How to deal with sewage for a self build home – with Cath Hassell

    HPH179 : How to deal with sewage for a self build home – with Cath Hassell

    Cath Hassell from ech2o consultants explains why connecting to mains drainage is generally the best option for a new build home. She also runs through some alternatives if this is not possible.

    Interview with Cath Hassell

    Cath Hassell from ech2o consultants talks us through the options available when planning drainage for your self-build.

    Practical experience informs drainage design

    Drainage is not something usually learnt by architects during their studies. Large architect firms typically employ drainage engineers. On a job with a smaller budget, the builders deal with drainage as they have the know-how.

    There’s a choice of materials for drainage pipes

    Cath explains that taking sewage away from the house is essentially a simple process, covered by Building Regulations. Lay a pipe in a trench (at a one in 40 drop) up to your site boundary, and backfill. The local water authority will then make the final connection.

    Builders tend to prefer using uPVC for the pipe. However uPVC pipe has to be laid in gravel, displacing earth that may need to be removed from site. So Cath always specifies clay pipe, which can simply be backfilled with site earth, and has plastic connectors for ease of use.

    If you’re near to mains drainage, use it but don’t overload it

    According to Cath, the Environment Agency are now saying if you’re near to mains drainage, you have to use it for your new build. There are few areas where the sewage systems are overloaded; issues only happen during rain events.

    That said, it’s important to minimise what has to be taken away as it takes energy to pump along the sewer system and treat. The first step is to contact your local council or Building Control department to find out whether you’ve got separate sewers that take 1) foul and waste water, and 2) rain water, or combined sewers where they’re mixed together.

    If possible, you want to keep rainwater out of combined sewers. And even if you have separate sewers, Cath recommends dealing with rainwater onsite. One way is to harvest rainwater, as Cath has explained in a previous podcast. She also mentions SuDS (sustainable drainage systems) for managing surface water.

    Cesspools aren’t an environmentally friendly option

    Where mains sewers aren’t available, cesspools have been used to capture waste to be processed elsewhere. While they ensure it doesn’t go into the ground or water, Cath says that environmentally it’s a really bad idea and it’s very costly anyway, explaining, “You’ve got to pay somebody to come in, pump it out and then take it to a sewage treatment plant and pump it into the plant.” Her advice is to deal with it onsite.

    Use a low-flush, dual-flush loo

    Unless you’re on a site in a dry part of the UK and solely reliant on rainwater, Cath wouldn’t bother with composting toilets. They take up space and many are in fact dehydrating toilets that require electricity to dry out the poo. She points out that some people fixate on composting toilets but don’t even compost their vegetable waste!

    However, if you’re building a workshop / cabin / sleepover place in the garden, there are some simple compost toilets available and preferable to a chemical toilet. Cath likes the look of this product.

    A septic tank is not just a hole in the ground

    Septic tanks have been a popular option for 800,000 properties in the UK not on mains drainage.

    What a septic tank does is settle out the solids to be broken down by anaerobic bacteria. Some of the solids sink to the bottom, and some float to the top forming a crust. Historically, the resultant effluent left the tank to be treated via a ditch, stream, soakaway or drainage field.

    A lot of those systems failed, and an EU Groundwater Directive was passed in 2006 to prevent further contamination. On 1st January 2015, the UK finally changed their regulations and Cath explains why you can no longer use a septic tank if you want to go into a stream or surface water. “The bacteria will go ‘wayhey, poo, brilliant’, they’ll start eating it, they’ll start to multiply.” This takes the oxygen from the water, which means that there’s less for fish and plants. And because you’re putting in a whole pile of nutrients, you’ll also get nitrification and algal blooming. So now you have to use what’s called a package sewage treatment plant.

    Package sewage treatment plants guarantee good quality effluent

    Package sewage treatment plants use aerobic bacteria to more effectively break down the waste. This requires introducing air into the system, normally using a pump. Or in the case of a Biodisc – a make of package sewage treatment plant – by use of a rotating disc inside.

    Without air the aerobic bacteria will die, so these systems have to be run continuously even if you don’t live onsite full time. The electrical cost may therefore be a consideration. But unlike septic tanks, these systems provide certainty that you will not be putting nutrients into the water or your drainage field.

    Drainage fields can clog up for various reasons

    A drainage field is a series of slotted pipes in a herringbone pattern laid across the land so that the effluent from a septic tank or package sewage treatment plant will percolate down through the ground.

    Septic tanks often clog drainage fields if:

    • The tanks are poorly maintained
    • The ground is fissured rock, where drainage is too fast and there are no aerobic bacteria left to deal with the bits of organic matter going into the field
    • The ground is clay, where eventually the clay gets clogged up and rain may wash off bits of matter into neighbouring streams

    There are alternatives to drainage fields

    Soil type may mean a standard drainage field cannot be installed for a septic tank or package sewage treatment plant. If this is the case, Cath has some suggestions:

    • Effluent from a package sewage treatment plant can be discharged into a nearby stream or ditch that leads to a stream
    • Reed beds, constructed wetlands and willow trenches can deal with foul water and waste water onsite

    You can get horizontal flow or vertical flow reed beds. Sometimes they’re called constructed wetlands, although this term can mean different things as explained in Septic Tank Options and Alternatives by Feidhlim Harty (written for Ireland, but relevant to the UK).

    Reed beds are sometimes seen as environmentally better options. However, Cath urges people not to put them in just to bring biodiversity on the site, as there are much better ways including bat and bird boxes, planting and hedges.

    Horizontal flow reed bed from Building Regulations Part H

     

    Vertical flow reed bed from Building Regulations Part H

    You now have to show you’re maintaining your sewerage system

    If you have an existing septic tank you will have to prove by 2020 that your system is working and being maintained properly. Right now, however, the Environment Agency is concentrating on new builds meeting regulations.

    Maintenance of septic tanks is relatively simple, as they contain no moving parts. Contractors take off the crust at the top, and remove sludge from the bottom. With a package sewage treatment plant, they still ‘de-sludge it’ but also check the pump is working properly.

     

    Find out more

    Visit the website of ech2o consultants

    Follow Cath on Twitter

    Download information from Cath

    Download the map of UK water companies

     

    Transcript

    Download a transcript of the interview with Cath Hassell

    Watch Cath’s stand-up gig: a plumber in a world of engineers

     

     

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