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Rain Catchment

There are four basic components to a well designed rainwater harvesting system.

  1. The collection area, most often the roof
  2. The conveyance group which consists of the gutters, downspouts, first flush diverter and piping to the tank or tanks.
  3. The storage facility or cistern
  4. The distribution group consisting of the piping and filtration (if used) 

In some cases it is advantageous to combine components of the conveyance and distribution groups since they can perform a portion of both tasks.  The quality of water collected and stored depends mostly on tanks and first diverters more than any other component.

Collection Area
Rainwater can be collected off of any commercially available roofing material for irrigation and secondary uses. The only material currently certified by the National Sanitation Foudation (NSF) to comply to Standard 61 for potable water, is Galv-alum sheet metal. It is possible however, to treat the water from virtually any surface, including composition shingles, to achieve drinking water quality. 

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Conveyance Group
Gutters, downspouts, overhead or underground piping, cistern filters and first diverters (sometimes called roofwashers) are components of the conveyance group.

The "roof washer" or "first flush" device should be as simple as possible so that little or no maintenance is required.  The purpose is to divert the first few gallons of water off the roof to a drain (or other catchment) so that the water in the cistern stays as clean as possible.  A length of dead end PVC pipe with a spill over point somewhere above the top of the tank is the simplest form presently in service.  There are no moving parts and a small hole drilled at the dead end will allow the captured water to drain after the rain stops so the diverter is ready for the next rain event.  Rainharvesting offers models with a floating ball and seat that prevents mixing the clean water with the diverted water which is recommended for potable applications. The drained water can be directed to a plant bed or swale so it is not wasted.

Cistern filters remove debris from the water before it enters the tank. The best type clears the debris automatically so the filter element does not need to be cleaned after every rain event. Rainkeeper has a complete line of cistern filters that are matched to the collection surface area and work with residential, commercial and municipal size rooftops. All are self-cleaning and the larger units are made of stainless steel.

Piping can be run overhead or underground depending on the location of the tank and eave height of the roof. Either way, the rain should enter the tank at the bottom through a calmed inlet to prevent stirring up sediment.

It is best to provide some kind of gutter protection as a first line of defense against leaves, twigs, small animals and other debris getting into the downspout and collection pipes. We recommend the Raintube because it is the only system with cradle-to-cradle certification as a green product and will reduce the fire hazard of combustible leaf clutter in the gutters.

Proper gutter drainage is also important to prevent tastes, odors, insects and microbes from building up between rains.

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Storage
The storage facility is the most expensive component and should be carefully fitted to the site and intended use of the water.  Most applications will benefit from an above-ground tank so that the water will flow out of the cistern by gravity.  In-ground tanks are more costly to install and require a pump to recover the water, but they are out of sight and the water stays cooler.  The most economical and easiest to install above-ground tanks are vertical polyethylene, flat-bottom tanks with an 18" manway on the top for maintenance.  These should be black or opaque green to eliminate algea growth inside the tank, but can be painted white for cooler storage.

Galv-alum tanks fitted with liners are the most desirable and durable of the affordable tank systems followed by lined corrigated steel. These tanks offer protetion from the sun, are attractive, and can be painted to blend in with the surroundings. If the liners are NSF61 certified, the water is potable.

Other tank materials include precast concrete, shotcrete, sealed concrete block, fiberglass, bolted or welded steel, wood and a variety of plastics, including polyethylene. Not all plastics, however, are suitable for collecting drinking water.

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Distribution Group
The distribution group is made up of differing components depending on the intended use of the water.  If the water is going to be used for domestic potable consumption, it is recommended that some form of filtration and disinfection be installed.  This can be as basic as a sand filter or as sophisticated as an ozone or reverse osmosis system depending on local conditions and personal preference.  If the water is for landscape or wildlife use, there is usually no need to filter or treat it. Water is extracted from the tank through a floating intake fitted with an inlet screen and check valve. This provides the cleanest water and leaves the sediment at the bottom undisturbed.

Pumps should be carefully selected according to the intended use of the water. Pumps with composite or stainless steel wetted parts are best for potable water. When irrigating, pumps must be matched to the pressure and delivery rate required by the spray heads or drip emitters.

Pex pipe is perferred since it will not leach chemicals or metals into the slightly acidic rainwater. Copper and galvanized pipe should be avoided in domestic water systems unless the water has been buffered to raise the pH.

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