There are a number of different factors in deciding what sort of rainwater harvesting system one would like, one has space for or a system that one can afford.
Questions that need to be addressed are in the first instance budgetary.
This can not only be ones allocated figure to purchase the equipment, but in the longer term, the monetary savings that can be accrued by using this water for non-potable areas within the home or watering the garden, as opposed to buying it from the water authorities (your local Statutory Undertakers).
Scale of water retention needs to be addressed.
Do you opt for surface mounted water butts, primarily for use in the garden, or an underground tank, or tanks; the latter being possibly linked together. Either way you must ensure that there is an adequate system in place for run-off from these retention vessels when full.
Depending on the locality of the property, this discharge could either be to the mains sewerage/water system or to soak aways within the curtilage of the property, (carried out in accordance to BRE Digest 365). Building Control will require storm water run-off to be catered for in this way for new builds, where possible.
In the case of soakaways, if it is not known whether the soil has an adequate degree of permeability, or the roof area to be drained into the soakaway exceeds 100m2 you will need to:
- Carry out an on-site percolation test
- Decide on the construction type for the soakaway
- Calculate the required storage volume and
- Consider space requirements, site layout, topography, water table, subsoil type etc.
If you have the space, underground storage affords a greater capacity. This could be one tank or multiple tanks if a good volume of water is required for summer garden watering for example.
If linking tanks, for example from different sides of the property, don’t forget that you can link one to another just using gravity and the evening out of levels between the two. Just remember that the highest point anywhere along the linking pipe will limit the amount travelling from one tank to another! The advantage of doing this is that you’d only need one pump to transfer water for onward usage from the ‘primary’ tank. The linking pipe (near to the bottom of both tanks) need not be large in diameter, 32mm would suffice. This can transfer over a longer period of time if the primary tank is running low after a good watering of the garden in the ‘dry’ summer months.
Most domestic system tanks have a maximum capacity of 7500 ltrs. These are not small!
You’ll have to take into account access to the area where they are to be buried and the moving of a mini digger to dig out the hole. Then there’s the transport of sub-soil waste off-site either by skip or grab-lorry. Larger industrial tanks are available, but again have cost and space issues of their own. Some lower cost tanks come in two parts, to be slotted together on-site; the better systems come as one unit which is always better from a potential leakage point of view.
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