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