Oasis Rainwater Harvesters
Rainwater Harvesting Systems
Sizing your Rainwater Harvester
Calculating The Size of your Rainwater Harvester Tank
1. Annual rainwater yield:
How much Rain do you get for your Rainwater Harvester?
The ideal rainwater harvester tank size is one that will provide a minimum of 21 days water storage and which holds between 5% and 10% of the annual rainfall amount to prevent stagnation. There are not many periods in the UK when there is no rain for more than 21 consecutive days!
See the map below for UK average rainfall in your area.
Not all the rain which fals on your roof will go into the rainwater harvester. Some is absorbed by the tiles, other water is trapped by the gravel of a flat roof, etc. In order to calculate the amount which will be collected, we use Roof Correction Factors
Roof Correction Factors
Clay tiles, glazed, fired = 0.9
Clay tiles, concrete tiles and slates = 0.8
Flat roof with gravel = 0.6
2. Water use /person/year
Annual rainfall (l/sq.mtr) x roof area x roof type corrector factor = Rainwater yield (l/yr)
| Toilets/person/year | 11800 ltr x | No. of occupants | = |
| Laundry/person/year | 3700 ltr x | No. of occupannts | = |
| Cleaning/person/year | 800 ltr x | No. of occupants | = |
| Garden per sq. metre | 60 litres x | cultivated garden area sq.mt. | = |
Total Demand
3. Tank Size - Below is the equation used
Rainwater yield/yr + Total demand x 21 days (safety reserve) =Min. litre tank size
2 365 days
Example
A building with a tiled pitched roof area of 100sq. metres in Ipswich (av. annual rainfall 640mm max.) 4 occupants, clay tiles and 60sq. mts. garden
100 x 640 x 0.8 = 51,200litres/year rainwater yield
Total Demand = 4 x (11800 + 3700 + 800 + 3600) = 79600 ltres/year
51,200 + 79,600 x 21 days = 3762.74 litre tank (min) size for 21 days m 2 365
This tank size is also OK for the stagnation factor as it is between 5% and 10% of the annual rainwater yield of 51,200 litres
The Government introduced sustainability rating system for houses in 2006. The system will become increasingly important when selling your house.
The Sustainability Rating System uses ‘stars’ to demonstrate the overall sustainability of a home. The range is from 1-6 stars. Six stars indicate exemplary development in sustainability terms.The rainwater harvester in the above example would reduce the water consuption of potable water to less than 80 litres/person/day, thus assuring the house a full 6 star rating.
The rainwater harvesting systems (RWH) we use are manufactured by GRAF in Germany and marketed under the Garantia brand. The underground systems range from 1600 to 6500 litre tanks.
I think the environment should be put in the category of our national security. Defense of our resources is just as important as defense abroad. Otherwise what is there to defend? ~Robert Redford, Yosemite National Park dedication, 1985
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Waste Tech Environmental Ltd
Tel. 01757 288022/289423
Email wastetech@eircom.net
