Rainwater Harvesting.......what's it all about then?
Rainwater harvesting is the collection of rainwater from flat
or pent roofs, it’s filtering to varying degrees and storage in a vessel
either above or below ground, further filtering and delivery to the required
source via various methods.
Rainwater can be used for the following :
Toilet flushing
Clothes washing
Garden irrigation
Ponds
Car washing
Exterior cleaning in an around the home/business
Animal drinking ( subject to water quality )
Keeping this as plain English, let’s go through the various stages:
• Well, the rain is falling on the roof so let’s make every attempt to keep
the roof clean, I don’t mean get up there with a brush and bucket! Keep
the roof free from birds, especially pigeons. Bird faeces form a big problem
in the end use of rainwater for obvious reasons.
• Next is the gutter line, various preparatory products are available to
keep the gutter free from debris and leaf matter. Releaf gutter guard is
a bristle wrapped around a wire that sits neatly in the guttering and stops
leaves from settling and contaminating the rainwater run- off. Gardena gutter
cleaner and 5m extendable pole for pre-fitting of Releaf.
• Downspouts- a balloon filter is a good idea to stop debris from going
down the downspout, it is possible to take a length of gutter guard and
fit this in the pipe making the job a little more efficient
• Next is the downspout filter used in the stead of an in tank filter. Downspout
filters vary a lot in their quality, most have useless filtration properties
and are there to divert the rainwater in to the collection tank. Some feature
a good from of filtration as low as 100 microns or one tenth of a millimetre.
The smaller the ‘gaps’ in the filter though, the quicker the filter will
clog up so bear the maintenance side in mind. An excellent all round filter
diverter is the British made RainCatcher filter.
• As said before the rainwater can be stored in tanks either above or below
ground, so let’s look at this separately.
Above ground tanks.
• Come in a myriad of shapes and sizes, the biggest single off the shelf
tanks being around the 25,000 litre size, the smallest a 100 litre slim
line water butt. The principles are the same and the main thing to consider
is keeping the water clean and stopping the water from freezing in the winter.
The problem with cleanliness mainly centres on Legionnaires disease which
is mostly a problem with contaminated water being made airborne at a certain
temperature, mainly warm. Anti-microbial products such as Sprayguard are
available on the market to help control this; tanks can also be insulated
against the cold/warmth. In winter it is generally advised to empty all
above ground tanks to one third of capacity when temperatures drop below
zero degrees, if possible, rainwater delivery systems should also be switched
over to mains supply in the most extreme of circumstances. Wherever possible
it is recommended that below ground tanks are used , however in circumstances
where this is not possible, it is recommended to use a smaller tank that
sits against a wall and can be linked in a line whenever more capacity is
required, such as the award winning, British made, RainCatcher 700 litre
water storage tank.
Below ground tanks
Also come in a variety of shapes and sizes, some very important choices
need to be considered here:
1. Where is the level of my water table? A lot of tanks cannot be submerged
and have been found to pop out of the ground like corks out of a bottle,
so always seed advice from an expert such as our in house technical team..
2. Does my tank need to be back-filled with concrete? This is the case with
most GRP tanks and custom made models, however most of Combined Harvesters
Graf Garantia one piece tank do not have this requirement. The required
amount of concrete, apart from being incredibly un-environmentally friendly,
costs at least as much as the tank and sometimes a lot more! So you should
look at all options and this can be done by discussing said options with
one of our experts.
3. Can I actually get the tank to where I want it? Some tanks come in two
halves for easier transportation; however this in turn poses a problem due
to the fact that the tank must be fitted professionally and with great care
to ensure the seal does not leak! Somewhat defeating the object of the exercise.
4. Are cars or Lorries going to be driven over the tank? The turrets and
lids of the tanks will need to be ‘load bearing’ to suit the weight of the
vehicles passing over the tank.
5. Possibly the most important question: What capacity of tank do I need?
This will depend on what you are intending to use the rainwater for, a simple
calculation can be made to establish this, however this should only be used
as a guide. Take 5% of ( roof square metres x the annual rainfall for your
area x 0.9 filter loss factor x 0.9 evaporation) Generally speaking, a family
of 4 in a 3 bed house fully using rainwater will need a tank of between
3800 litres and 4200 litres. Never put in a bigger tank than you need, the
excess water may stagnate and cause all sorts of problems. Remember all
tanks that specialist companies produce can be linked together retrospectively,
possibly not the case however with a lot of GRP custom made tanks so beware
( measure twice and cut once ).
6. What is my ground like? Seems obvious but if your garden sits on granite
bedrock then you’re not getting through that with a shovel! Proprietary
‘flat tanks’ such as the Graf Garantia Lilo are great for low excavations
and come in three linkable sizes 1500,3000 and 5000 litres.
7. Filtration: If you haven’t fitted a downspout filter then an in-tank
option is available, this will feature a filter that generally is down to
300 microns, a calming inlet that stills the incoming rainwater and stops
sediment disturbance and an overflow siphon that allows the finer contaminent
particles to overflow to a soakaway.
Ok so we’ve sorted the tank. Now we need to sort out how we are going to
get the rainwater to our required source. Here we have two basic options:
Direct feed systems
Direct feed systems are when the rainwater is delivered to source either
via a pump which ’pushes’ the rainwater from inside the tank or via pumps
which draw from the tank via a floating intake.
In tank:
A pressure pump sits inside the tank, when rainwater is required at source
either by flushing a toilet or turning a tap, the rainwater is automatically
send down the MDPE pipe to source. The main disadvantage of these systems
is that when there is no rainwater available, the tank is topped up to a
certain level with fresh water.
Draw.
A management system sits either internally or externally and draws rainwater
via a floating intake in the tank further filtering the water on demand
and delivers to source, some of these systems have a back-up tank attached
which is topped up, some have the advantage of being insulated, allowing
them to sit outside of the property, negating the obvious problem of noise
pollution in the home.
OR
Gravity feed systems.
Gravity systems are basically a specially modified loft tank or a tank sitting
a minimum of 3 metres above required source. A cheaper or low wattage pump
sits in the main rainwater storage tank, when the header/loft tank demands
water, it is delivered to the tank and then by gravity to the required source.
If there is an absence of rainwater; only the loft tank is topped up. These
types of systems are definitely an advantage as they are by far the cheapest
to run.
OK, so that’s the basics…below are a few questions answered for you!
What is Rainwater Harvesting?
Rainwater harvesting is a way of saving the rainwater which would normally flow off a roof and down the drain, and using it as piped water to flush toilets and for the garden watering, yard washdown, vehicle and car washing, and even for your washing machine, instead of using expensive treated drinking (potable) water.
Are there planning regulations associated with rainwater Harvesting?
They are not required directly by Building Regulations, although they may be linked with the Planning Permission for the storm-water management of the site. Building Regulations do cover the installation itself, tank siting & pipe runs etc.
Will a system affect my homes eco rating?
Yes, rainwater harvesting is an important ER criterion. The EcoHomes rating system addresses all aspects of reducing potable water demand in a dwelling. Installing a rainwater harvesting system adds to the credit rating for water use.
What types of rainwater harvesting systems are there?
Un pressurised: rainwater is gravity fed from a header tank, usually in
the loft to the point of use.
Pressurised: The rainwater is pumped directly from the above or under-ground
tank to the required point of supply. i.e. toilet, outside tap etc.
Is there a danger of legionella?
No, the system does not provide the conditions necessary for the cultivation of Legionella. With the water stored underground it is dark cool and is kept well oxygenated. Legionella cannot cultivate in these conditions.
How is the system maintained?
As a general rule filters should be thoroughly cleaned once a year and we recommend a full 'flushing' of the system every three years. A treatment with an anti-algicide such as sprayguard is also recommended. A correctly designed harvesting system overflows on a regular basis to remove floating matter and the pumps are very reliable. Maintenance should only be carried out by qualified or experiences personnel. Combined harvesters have their own teams available year round to keep systems working efficiently.
What kind of pumps are used in the systems?
Pumps can be housed either internally in the tank or mounted within a control unit fitted in a garage, plant room etc. Pumps should have run dry protection and should have a pressure switch fitted to stop hunting. Internal tank pumps need to be constantly submersed in water to prevent damage from the air, from debris or sediment that may be sucked in. An external pump or control unit should have an audible alarm to identify faults in the system, when fresh water is being used etc.
How much does it cost to run a rainwater harvesting pump?
It typically takes 1.5- 2.0 kWh to pump 1 cubic meter of water (1000 litres). For a typical house using rainwater for WCs, washing machine and the garden, pumping costs are between 5-10p per week
Do I need to have a water meter?
This is not generally necessary; however the absence of one will reduce you seeing the benefits of harvesting rainwater immediately.
What does the British Standard BS8515:2009 cover?
BS8515 covers the design, installation, water quality, risk management and maintenance of rainwater harvesting systems. This applies to both new and retrofit properties.
What is the payback period?
This figure will depend upon the rainwater that you collect and the use. A typical domestic client will see a payback between 5-10 years. Commercial clients should be able to reduce this figure to 2-5 years.
Is it only for new builds?
No systems can easily be retro-fitted.
What design aspects are to be considered to comply with BS8515?
Designs should be provided by an industry expert, amount and intensity of rainfall, type of intended applications both now and in the future will be considered. Tank size will depend upon ground conditions and surface water. All these as well as filtration requirements, end use by clients, site restrictions and other factors not necessarily covered by BS8515 will be considered when designing a system.
What happens when there is no rain?
In the absence of rain, correctly designed systems will have a 'mains top up' facility ( not always required for garden systems ). This will fill the tank with the minimum required amount of water to keep it functioning until the rain returns or a small holding tank on the management unit..
Is rainwater harvesting suitable for work as well as at home?
Yes, in schools, hospitals, offices, commercial premises, rainwater can typically be used for toilets, vehicle washing, yard wash down and watering plant pots/gardens.
Can it be used when there is a hosepipe ban?
It is possible to use a hosepipe connected to a rainwater tank, provided
that the tank is not connected to a mains water supply. Many people are
doing this already by using a water butt.
Is it only for houses?
No, bungalows and commercial premises are also very suitable, the only limitation is the area of the roof to capture rain, in fact commercial and industrial buildings can make the largest savings.
Where is it installed?
The tank should be buried under a car or vehicle park, landscaped area,
garden, patio or drive, with space left for the round access cover. Most
systems are designed so that they can accept cars driving over them if suitably
installed. Or an above ground system can be installed next to the house
or property.
Are there any grants available?
Not for domestic installations yet, although several organisations are petitioning the government for assistance. For commercial installations, there is a tax relief scheme (ECA) for suitable approved equipment on the Water Technology List.
How clean is the water?
The rainwater is filtered as it enters the storage tank, to remove particles and other matter. It is kept in the dark and kept oxygenated to discourage algal growth, and properly designed systems are designed with calming inlets, which ensure that any sediment at the bottom of the tank does not get stirred up. The water is not drinking water fit for humans.
How do you stop debris from entering the system?
A filter is fitted along with a ‘calming inlet’. The filter has to meet strict criteria covering its weather resistancy, accessibility and efficiency.
How much rainwater does a system collect?
This depends on the area and angle of your roof, and your rainfall. Averages of 100,000 litres per household are commonly quoted, much more for large roofed commercial buildings.
Is the tank covered by BS8515:2009?
Yes, all tanks are covered. They need to be water tight, discourage microbial
growth, avoid stagnation and thereby legionella .Tanks need to be suitably
load bearing and need to resist floatation.
Pipes to and from the tank need to be clearly marked, rainwater pipes can
be green or black (not blue) .Special marker tape can be purchased to show
piping on a scan.
Does the tank need an overflow?
Yes, the overflow needs to be the same diameter or even larger than the inlet, it must also be fitted with back flow prevention. Overflows are important as a tank should be the correct size so as to overflow at least twice a year to ‘flush the system’.
How much would this save on water bills?
Depending on your normal usage, it can save 45 to 50% for the domestic user and 80% for the commercial user of the treated drinking water from the mains. Having metered water is the best way of appreciating the difference.
So, why should people buy Rainwater Harvesting systems?
To save on water bills and show an appreciation for this increasingly precious resource whilst making a difference to our environment.
How does rainwater harvesting work?
A storage tank is fitted to your storm water drain from your roof, and falling rain enters the tank through a filter which removes leaves and other matter. The storage tank is usually buried under car or vehicle parks, a garden or under the entrance access or drive, and contains a pump which pumps the rainwater to the building where it is piped to the toilets, and to the outside taps. Above ground tanks are also available.
How much water can a system save?
Depending on your normal usage, it can save 30 to 50% of the treated drinking
water from the mains in houses and up to and up to 80% of the treated drinking
water in a business or commercial building.
What can you use the water for?
Filtered, untreated rainwater should only be used for non- drinking or bathing purposes: toilet flushing, gardens and vehicle or yard wash downs.
How much does it cost?
Domestic systems can cost from about £2500 up to £4000 plus including installation costs, depending on size of tank. Commercial systems can cost a lot more depending on size and requirement, but usually have a much quicker 'pay back' period due to the size of roof and high usage.
Could rainwater get into my drinking supply?
Not in a properly designed system, the pipe work is entirely separate and should be identified as non-potable. BS8515:2009 stipulates that backflow prevention should be fitted upstream of or at a point where any two systems meet. This form of back flow prevention must be of type AA or AB Air gap conforming to BS13076 and BS EN13077.
Do I need a big roof area to make it worthwhile?
No, most domestic roofs are more than adequate, but the bigger and flatter the roof area, the more rain will be captured, and the more the rainwater will substitute for treated mains water.
Is Rainwater Harvesting a new idea?
No, collection re-use of rainwater from roofs can be traced back thousands of years in hot, dry countries around theMediterranean. In continental Europe, some 100,000 are installed annually; Germany has been using and refining the technology since the early 80s.
Can a roof affect the efficiency of a system?
Yes, all these need to be free-draining and suitable to stop debris from entering the system. Other roofs such as green roofs etc absorb water and results in less run off and more colouration.
This guide has been produced to assist professionals who are interested in the specifications of rainwater harvesting systems. Enabling them to make an informed choice of supplier based on the criteria set down in BS8515:2009. At Combined Harvesters Ltd we are committed to a long term approach to the supply of all rainwater harvesting and stormwater management products, ensuring quality throughout the chain.
Save your water…save your money…save your environment.
Can I use rainwater for my garden?
Absolutely, in fact the water is a better quality with less harmfull chemicals.