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Surface water run off is increasingly becoming a matter of concern as more and more land is paved over. Rainwater harvesting takes surface water and stores it for later use. Permeable paving allows water to pas through into the sub-soil. See also SUDS
RAINWATER HARVESTING Contribution by Alistair Tunnington, Managing Director, Combined Harvesters Ltd 
Why harvest rainwater ?
The new buzzword that will be heard over the next few years will be 'rainwater harvesting'. We live in an increasingly environmentally aware society and one that is ever conscious of frugality with regard to our necessary expenses. There is nothing new about collecting rainwater. Storage jars used for water collection some 2000 years ago have been found as far away as Thailand. In more recent times, we need to look closer to home to Spain and their Cisternas. All that has changed is an ever pressing need to help save our most precious resource and as a by product save money. It is possible to save a substantial amount of water by the simple fitting of a rainwater tank or an above/ below ground storage tank. This stored water can have a variety of uses from watering of your garden , washing of vehicles or to add to the mains water to your house (this however should be filtered). Rainwater is practically chemical free and a great source to irrigate arid gardens, wash outside surfaces, flush toilets, fire fighting applications, building cooling systems etc. And of course there is no burden of guilt during the ever longer periods of water restrictions.
Rainwater harvesting is a relatively new concept and is the storage of stormwater discharge locally for re-use in non-potable applications such as garden irrigation, toilet flushing etc.
It is surprising how much water can be collected from a standard roof , patio ,driveway or any hard-standing surface around the home. For every inch of water that falls on a surface of 1000 square feet it is possible to collect approximately 600 gallons of water . It is a simple task to work out the rainfall on average for your area-:
Available rainfall
= Annual rainfall for your area (mm)** x Roof area (m2) x Evaporation Factor* x Filter Loss Factor*
* assumed as 0.9
** example rainfall
E.G.,
Typical 3 bedroom house (4 people), the yield would be;
800** x 120m2 x 0.9*x 0.9* = 77,760 litres
Expected usage
45% of 150 litre per person per day
E.G.,
150 x 4 x 365 x 45% = 98,550 litres
If you calculate 5% of the lower value, then a tank size of 3,888 litres or one that is closest to that figure, would be the most suitable tank size. One of the benefits of a modular system is that additional tanks can be joined to the existing at a later date with ease.
If this water is put to use washing the car, watering the plants, washing the windows and even flushing the toilets , imagine the savings on water rates and the benefits to the environment. Apart from the above benefits there are others , less obvious-:
- Grants for some domestic rain harvesting systems are available in some counties.Commercial installations can be 100 % tax deductable.
- Eco rating points can be achieved (BREEAM)
- Local planners are acting frequently more favourably towards eco-friendly schemes.
- The infinite benefits of washing the old Merc during a hose-pipe ban quite legitimately, and the look on the neighbour's faces !
There are many methods of rainwater harvesting , barrels fitted to a downspout from a roof come in many shapes and sizes , some practical and some aesthetic. Thought should be given to sturdiness , child friendliness and safety, a cheap system will crack when the frosts come and would be a waste. Also run off of excess should be considered. Underground tanks can store anything from 1000 litres to as much as 10,000. Although a common size would be 2800 litres. A special pump will deliver the water to a hose pipe on demand or to a header tank in the loft. A flotation valve automatically shuts off the pump if the tank is emptied. A soakaway system will remove excess water when full naturally into the water table.
With the increasing demand for drinking water as a result of high density housing and commercial development, rainwater harvesting is seen as a potential solution for overcoming water shortages. It is widely considered that this may well be the subject of future legislation. What regulatory requirements are there for rainwater harvesting systems? There are currently no direct building regulations for rainwater harvesting. However, drainage is covered by the Building Regulations and can influence rainwater harvesting system design such as soakaway or city drain connections for tank overflow. Several other associated regulations apply; In England and Wales the Water Supply Regulations 1999 (Water by-laws 2000 in Scotland and Water Supply (Water Quality) Regulations (Northern Ireland) 2002) must be adhered. These regulations are enforced by the water supplier. The essential requirements of the regulations specific to rainwater harvesting are; (1) no cross bonding of rainwater piping with potable supply pipes (2) prevention of backflow to potable water supplies according to CEN EN1717 or air gaps type AA/AB/AC etc (3) the clear marking of rainwater pipes according to WRAS # 9-02-05 and (4) material specifications, where contact with wholesome water is permissible meet the prevailing approved list.
Did you know? - German households use rainharvesting systems up to 300 % more than households in the United Kingdom. However the use of these systems have increased by over 300 % in the last two years here.
- Your typical rainharvesting system can yield up to 50 % of your water needs, a typical period for payback on a family with a high water demand is three years.
- An average 4 bedroom family home roof can capture up to 100,000 litres of water each year, whereas a typical family's consumption for standard use such as toilet flushing, garden watering , washing etc is 70,000 litres.
For more details contact: Alistair Tunnington 0151-334-8655 al@combinedharvesters.co.uk 3 Mosedale Road,
Croft Business Park,
Bromborough,
Wirral,
Merseyside
CH62 3QZ
For more information visit their website: www.combinedharvesters.co.uk |
