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 Post subject: Rain Water Collection
PostPosted: Wed Dec 01, 2010 10:41 pm 
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Location: Was Plouasne 22830, now Ploeuc sur Lie 22150, France
These notes relate to properties both on MAINS DRAINAGE and those with a SEPTIC TANK DRAINAGE

For properties on MAINS DRAINAGE, the law now requires that appliances that use NON-MAINS WATER should have a meter on the service that supplies the appliances, this is so that the correct amount for sewage treatment can be charged to the property

For properties on a septic tank drainage and also those who use the collected water for car washing or garden use only, there is no need for a meter on the NON-MAINS water supply

The collection of rainwater for either garden use, or garden and other NON-POTABLE use (I will deal with the collection of rainwater for potable use at the end of these notes) can be broken down into several simple stages.

1. How To Work Out Consumption, In Litres Per Item Of Equipment, And Average Use A Day Per Person:

1) W/C = 6 to 9 l per flush, average use 4 times a day per person
2) Wash hand basin = 5 l average use 5 times a day per person
3) Kitchen sink = 15 l average use 3 times a day
4) Shower = 80 l for 5 minutes average use 1 a day
5) Bath = 150 l
6) Dish Washer = 50 l per wash
7) Washing Machine = 150 l per wash
8) Swimming Pool 50000 to over 80000 l
9) Garden Watering less than 50 sq m = 150 to over 500 l
10) Garden Watering more than 100 sq m = 500 to over 1500 litres
This does not include car washing you can double the amount quoted in 9 & 10 if you include car washing

Using the above figures 1, 2, and 4; times the number of persons, then add fig 3 (15 l x 3 times), fig 6 once a day, times by seven, add fig 7 times by two, (2 washes a week), this should give you a pretty good idea of the amount of water used in one week, not including garden needs

2. How To Work Out The Amount Of Rain Fall Which Could Be Captured In One Year

Work out the plan area of the property, if the roof pitch is greater than 15%;=71/2° or more use the coefficient of 0.8, if the roof pitch is less than 15% less than 71/2° use the coefficient of 0.7
Taking the 6 areas of France for the first example of the amount of rain fall to be expected in litres a sq m/yr
West of France 772 ltr/mt²
North of France 634 ltr/mt²
East of France 771 ltr/mt²
South east of France 716 ltr/mt²
South west of France 858 ltr/mt²
The centre of France 787 ltr/mt²

[So a house with a plan area of 175 mt² in the south west of France with a pitched roof of more than 15% = 71/2°
Ex: 175 x 0.8 x 858 = 120,120 litres a year: note 1000 litres = 1 cubic metre, so 120,120 divided by 1000 = 120 cubic metres; not that I would suggest that you fit a 120 cubic metre tank, this is just an example]

To get a more accurate rain fall figure, go into the meteofrance web site, and look at the monthly precipitation for your department,(first page left hand side, click, Climat, Metropole, select department, then bulletins archives, or, particularities, mon department carastistiques climatiques, another widow will appear showing the max & min temp, also the monthly rain fall, in millimetres)

Two examples, taken over a 30 year period, 1971 to 2000 from Meteofrance records:

- Dept 57 Moselle; Oct, Nov, Dec, are the wettest months, followed by Jun & July, with the amount of rain in mm/month Oct = 70, Nov = 65, Dec = 80, Jun& Jul both months have on average 65mm

- Dept 19 Correze; Sept, Oct, Nov, are the wettest months, followed by April; on average they all have 90mm rainfall

Taking the same-sized house with a roof pitch of more than 15%, 71/2° and the average monthly amount of rain fall in Dept 57

This would be say, 70mm / sq m; 175 x 0.8 (coef) x 70 = 9800 litres divide by 1000 = 9.8 cubic metres of water

Dept 19, 175 x 0.8 x 90 = 12600 litres divide by 1000 = 12.6 cubic metres of water

Once you have worked out your greatest weekly/monthly demand, I would then look for the closest amount of monthly rainfall to match that, and then install a tank or tanks for a 1 or 3 month supply of rainwater depending on the amount of land available and costs involved

3. Now To Trap The Rainwater

1) Install leaf guards to the gutters
2) Either fit a filter in the pipe work leading to the collecting tanks or a filter in the tank where the pipe discharges into the tank, these need regular attention and cleaning out
3) The tanks: they can either be concrete (heavy but more robust) or plastic (lighter, can be manhandled but need more protection) in one piece or they can be of a two piece variety clipped together in the middle
The tanks need to be cleaned out every two years, so they need access to get inside them!!!!
4) Don't forget the overflow! I would fit a trap to the overflow, with the spill-over level above the soffit of the overflow pipe, to stop any animals getting inside the tank/s I would also fit a grill or better still a tidal flap valve (in French it is called a "clapet antiretour") which only opens when water wants to escape
5) The pump, I would fit an above ground, combined pump and pressure vessel (about 50 litres) with all controls mounted on the set (pressure switch, pump run dry cut off, pressure gauge) [the pressure vessel is a steel container with a rubber bladder (which holds the water) pre-charged with an inert gas normally nitrogen at around 2 bar, when the pump starts the water is pumped into the bladder compressing the gas, until a set pressure is reached, the pump automatically shuts off when the preset pressure is reached, when a tap is turned on the compressed gas forces the water held in the pressure vessel to travel down the pipe to the appliance which is opened, this continues until either the tap is turned off or the pressure drops sufficiently enough to start the pump, the pump continues to run until the pressure has built up again]; close to the tank/s, the pump could be either a single stage or a multi stage impeller pump mounted on top of the pressure vessel; a pump will only suck water 7mts (21 feet) vertical lift MAXIMUM, any horizontal pipe runs will reduce the vertical lift, owing to the frictional resistance of the pipe work, so it needs to be close to the tank/s and depending on the amount of water in cubic metres needed in one hour (litres / hour divided by 1000 = cubic metres) this calculation will give you the pump capacity in cubic meters water / hour
6) A multi-stage impeller pump similar to a bore hole pump could be installed in the tank, and a remote pressure vessel, (as in 5 above) could be installed with the controls close to the pressure vessel, this would avoid the need for any pump house or similar to be constructed, if the tanks were remote from the main building
7) The suction side of the pump and inside the tank from which the water is drawn (if there is more than one tank I would try to interconnect them in the bottom quarter of the tank, to ensure a through flow and mixing of the water, other wise the water in the first tank would tend to stagnate if the water flowed from one tank to another by a top connection of the tanks), should by a flexible armoured hose (to stop it collapsing when the pump starts) fitted with a combined foot valve / strainer, hanging vertically in the water, the foot valve helps to keep the pump primed, (so avoiding pump cut out and the need to re-prime the pump), fixed to a float, this is to allow the foot valve strainer to hang in the water below the water surface, so avoiding sucking up any surface debris

4. The Plumbing Side


If the collected rainwater is only going to be used for the garden or car washing, nothing else is needed, but if it is going to be used for flushing toilets and washing machine the water should be marked as NON-POTABLE WATER and a filter (10 to 20 microns) fitted to filter out any small particles of dirt which could interrupt the functioning of the appliances (it would be better to fit 2 filters one on line the other being washed out [every 2 to 3 weeks])
If the water was intended for POTABLE use, after the filter set, a bacteria filter and steriliser (UV lamp, silver iodide or similar) would need to be fitted, See section below called 'For those who want to be self sufficient'.


The rainwater could be connected to the water company's service pipe but with SPECIAL PRECAUTONS.

Items needed: Three number, quarter turn valves of the size of the pipes; one "Tee"; one disconnecter (similar to 'Danfoss Socla disconnecting valve BA2760) to be mounted on the water companies side of the "Tee” to the rainwater branch; two non-return valves (Danfoss-Socla EA221B or EA291NF), one on the rainwater branch and the other on the house side of the interconnection, a filter set as described above should be fitted, to the rainwater branch before the non-return valve on the "Tee" connection to the water companies service

Short description of the lay out, starting from the water company’s side. Fit the first quarter turn valve, then the disconnecting valve, then the "Tee" with the non-return valves fitted on the other 2 sides of the "Tee" after the quarter-turn valves.

To change from "mains water" to rainwater, first turn off the "mains" water valve, then turn off the house side valve, open a tap to drop the pressure, open the rainwater valve, then the house side valve, check that the pump runs, close the open tap, and check that the pump shuts off

This is my suggested way of collecting and using rainwater, there are other ways which do not involve an interconnection with the water company's water service, a completely stand-alone water supply for non- potable use, which involves a water supply pipe from the water mains to re-fill the rainwater tanks if they run dry, via an air break. The way this is done is a tap is placed over a 'tun' dish with about 20cm (8") air space between the nose of the tap and the top of the 'tun' dish, the out-let from the 'tun' dish leads to the tank via a 40mm PVC waste pipe, the only draw back in my eyes, is if you run dry and you re-fill the tank, and the next day it rains, all that water you have paid for, to re-fill the tank could have been saved

5. Costs

A very rough figure to go on is:

1) Tank 500€
2) Pump 300€
3) Leaf guards for the gutters and rainwater filter 500€
4) Interconnection fittings 400€

Total 1700€, NOT INCLUDING any digging out or sand for infilling or labour.

Depending on how you use water you could be up to 95% self sufficient if not 100% and only relying on mains water for drinking and potable use

6. For Those Who Want To Be Self Sufficient

Another form of Rain Water Filter
Australian Model

For a completely off the mains, water supply, or for a potable water cross connection, "mains water" rainwater.

The Australians use a "first flush" dirt trap (DT); this consists of about 2 m of 160mm PVC pipe, fitted vertically, at the bottom is a 90° bend with a "screw on" cap on it. This has a small hole (1/8" or 2 mm dia.) drilled in it as a drain hole, at the upper end is a tapered reducer (160mm x 80mm or the dia of the rain water pipe) this connects to the branch of a 90° sweep tee, a float which is larger than the reduced end of the reducer, is placed inside the 160mm pipe to close off the top end of the 160mm pipe as it fills with water (this is the first flush, which contains the dirt laying in the gutter, and laying on the roof, and is trapped in the dirt trap) the idea of the small hole in the cap is to allow the dirt trap to drain slowly and let the float to drop, the cap needs to be a screw on cap so that the dirt which collects in the DT can be removed. The float could be an English 41/2" plastic ball valve float with a brass bolt fitted in the threaded boss, or a French ball valve float provided it is large enough and the right shape to close off the top of the reducer of the DT.

If you can imagine it, this talk through might help you to understand the principle of a dirt trap.

The rain water pipe (RWP) descends vertically then turns through 90°, a 90° sweep tee with the 150mm pipe dirt trap is fitted to the branch, the RWP then turns through 90° to descend to the ground and the collection drain to the rain water tank; when it rains the first run off takes the dust and dirt with it, and as it runs down the RWP and turns through 90° it is slowed down and then it drops into the 150mm dirt trap, the float rises as the DT until it closes the top of the DT, the water flowing down the RWP is by now 'clean' of all dust and dirt, and as the DT is closed the rain water flows over the branch to the collecting tank, the short distance between the branch and the top of the DT, should cause a turbulence and cause any remaining dirt to drop out of suspension and be caught in that short section, so it should drop in to the DT, as the float drops with the lowering of the water level in the DT, as the water drains out of the hole in the cap of DT.

Three meat and one veg

An old wild fowling motto : little powder, plenty lead, kill 'em dead

English born, Living in France, Therefore European

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