There is some outline information below, but bear in mind that you may need to get help (such as a report) from a sanitation consultant in order to have the system passed by the Environment Agency or other relevant body. CAT offers a consultancy service, through which our sanitation consultant can give technical advice on the type of sanitation system that will best meet your needs and satisfy regulations. For example, you could just book an hour or so with our consultant if you want to discuss the options for off mains sewerage.
A vertical flow reed bed for secondary treatment should allow 2 square metres per person (but decreasing as number of people increases - so for 50 people the reed bed area could fall to 1m2 per person). The bed area is calculated on the amount of sewage each person produces, which is assumed to be around 200 litres per day in domestic settings (but less where use is daytime only). If the bed is for grey water only you would calculate the area on the basis of water produced, so for small systems that would be 1m2 for every 100 litres of sewage. Grey water reed bed areas are likely to be 30% less than for complete sewage because approximately 30% of domestic waste water goes down the toilet.
A horizontal flow reed bed may be used for tertiary (third stage) treatment, but not for secondary treatment. This will need about 1 square metre per person, but with a minimum size of 6 square metres. This bed turns nitrates into nitrogen gas, and won't be needed if you're only treating greywater - unless the effluent is flowing into a pond where a better quality is needed.
The grade of pea gravel in a reed bed does not have to be too precise. 5-10mm is fine, as it's only a thin layer and is there to support the sand, and stop it washing down into the coarse gravel below (see our tipsheet for a diagram). The gravel used should ideally be washed, and graded to 20mm (but this is flexible). It will take 2-3 years for the reeds to establish, but the system can be used in that time as the reeds need the nutrients and the beds can work without reeds. Reeds aerate and provide frost protection.
For controlling the discharge of effluent to a vertical reed bed, we recommend a Floating Outlet or "FLOUT" which are less prone to blocking up. You can get them from Watercourse Systems.
In a vertical flow reed bed the ammonia turns into nitrates. There should be a settlement tank between vertical and horizontal flow reed beds, to take out solids that are created by the vertical flow bed (such as dead microbes). This tank will very rarely need emptying.
Aeration pipes on small domestic systems are not strictly necessary - the coarseness of the materials used should ensure enough air gets in. If they are to be included then the British standard for secondary filters is 100mm diameter, a minimum of four per unit, and maximum of 2m centres. That would be overkill on a small reedbed, where a couple would be sufficient. To make them, drill 10mm holes at four cardinal points every 20cm along the length of the 100mm tubing.
Construction can take from 2 days to 4 weeks depending on size, the materials used, if you hit bed-rock, etc. Approximate costings: £5,000 for DIY (hiring digging equipment, lining the tanks, plumbing, etc) or £10,000 plus if done professionally.