Rain Tank FAQ
What can I use rainwater for?
The most common uses of rainwater harvesting include watering your lawn and garden, washing your car and windows; non potable uses such as toilet flushing, and potable uses from drinking and cooking to dishwashing and showering.
How much water do I need?
Information to calculate your rainwater storage needs:
Monthly water demand
Monthly rainfall statistics
There are a few straightforward calculations you can use to predict the amount of water you’ll need. These calculations will indicate averages only, please keep in mind your own habits and conservation ability.
Indoor Water Predictions
Type of Consumer Water use in metric Water use in imperial Average Consumer: 225 – 275 Litres/Person/Day 50 – 60 Gallons/Person/Day Rainwater Dependent: 115 – 180 Litres/Person/Day 25 – 40 Gallons/Person/Day
The simple calculation to determine your needs, therefore, is:
# of residents X (95 to 230) Litres = your water needs per day.
# of residents X (25 to 60) Imperial Gallons = your water needs per day.
To translate that information into your storage requirement you’ll need to figure out how much time you’ll be spending at your residence. Consider these points to help you:
- How many residents?
- Is it a full time residence or part-time?
- If part time, when are you mostly there? Are there plans to live there full time in the future?
- How many visitors? For how long? At what times of the year?
Outdoor Water Predictions
These guidelines will help you determine how much outdoor water you’ll need. These numbers apply typically during the summer as you’ll rarely have to water your lawn or garden during a west coast winter:
- 1 watering can = 3.3 gal. (15 L)
- 3 ft. shrub in hot weather (1 week) = 7 gal. (32 L)
- 18 in. pot in hot weather (1 week) = 1.8 gal. (8 L)
- 40 deck pots – drip water (1 week) = 50 gal (227 L)
- 1 sprinkler full flow (for 1 hour) = 240 gal. (1100L)
- Car washing (1/2 hour) 120 gal. = (550 L)
- Pressure washing (1/2 hour) = 40 gal. (180 L)
How much water can I collect?
The total amount of water you can collect from your roof depends on 3 things:
- The annual rainfall in your area – visit Climate Normals and Averages
- The area of your roof (the area measured as the horizontal plane under the roof including overhangs).
- The efficiency of your harvesting system. This is based on factors such as evaporation loss from winds, tree cover, roofing material, and the sizing of the pipes for storm events. Well designed systems capture 75-85% of the rainfall.
*Be aware that more precise information may be available for your location. Even across a small island or municipality, rainfall patterns may differ greatly. Check with neighbours, town leaders, and any other information sources you can find.
Calculate your harvest potential: Average rainfall l X efficiency X roof area
Measure your roof area: Length down one side of your house X width of your house.
- 1″ of rain on 1 square foot of roof area produces 0.52 imperial gallons of water
- 1 mm of rain on 1 square meter of roof area produces 1litre of water
Average rainfall and harvesting potential on BC’s Gulf islands
Precipitation averages are from Environment Canada. Harvest amounts are from a 1000 sq. ft. roof at 75% efficiency.
Location Annual Precipitation Summer Precipitation (June – Sept) Annual Harvest (gallons) Summer Harvest(gallons) Saturna Island (Capmon) 33.” (840 mm) 5.1″ (130 mm) 12,900 2,000 Salt Spring Island 38.1″ (975 mm) 4.9″ (125 mm) 15,000 1,900 Cortes Island (Tiber Bay) 49.4″ (1255 mm) 9.1″ (230 mm) 19,700 3,550 Bowen Island 67.8″ (1,700 mm) 9.8″ (250 mm) 26,400 3,800
How much filtering do I need?
The amount of filtering required in your system depends whether your usage is for outside use, toilet flushing or drinking.
There are two basic considerations:
1. SCREENING AND FILTERING THE RAINWATER BEFORE IT ENTERS YOUR STORAGE TANK.
Primary screening can be screens that cover the entire gutter system to take out twigs, leaves and needles from close-by trees before entering the downspouts. There are a number of products on the market that do this job. Keep in mind accessibility for periodic cleaning.
If debris from trees is less of a problem then filter screens (rain heads) can be installed at the top or bottom of the downspouts. Some devices are designed to shed debris (self-cleaning) and others have a collection pocket or canister that can be emptied periodically.
A third method is to install a filter basket that fits inside the 16” lid opening at the top of most storage tanks. The filter basket can be used in addition to gutter and downspout screens and has the added advantage of accommodating an optional fine cloth filter that will filter out pollen as well. Keep in mind all screens and filters must be easily accessed.
The more screening and filtering you can do before entering the tank means there will be less sediment buildup in the tank, and less future cleaning.
2. FILTERING THE STORED WATER AS IT LEAVES THE STORAGE TANK.
Further filtering on the outlet from the tank depends on the type of use.
a) If used for outside use only (garden, washing windows or cars) then no additional filtering would be necessary.
b) Water for toilet flushing and laundry needs to be filtered to a higher level to ensure valve and controls do not clog up or become damaged from particles in the water.
c) Water for drinking or human contact must be further filtered to one half a micron to ensure the proper function of an ultra-violet (UV) disinfection unit to ‘purify’ the water for human consumption or contact. As the water quantity needed for drinking or showering is relatively small, a separate tank or separate connection to the main storage tank would save the cost of unnecessarily disinfecting water that would be going to outside use or toilet flushes. A professional person familiar with UV disinfection must be consulted for this application.
There are a number of filtering devices on the market, and a rainwater professional can advise what is best suited for your application.
Can I store enough water for the whole summer?
Yes. Many west coast residents subsist strictly on harvested rain water. The most important factor is a large, high quality storage tank; storage size must be sufficient to get you through the dry summer months. The drier the summers and the higher the summer use – the larger the storage tank required.
How big does my storage tank need to be?
Different areas of the west coast have different rainfall amounts and patterns. This changes the important factors – roof area and storage tank size – required to meet your water needs. If you plan on making it through an entire summer you’ll need to know the following:
Number of residents X number of days residing X average daily usage – summer harvest
Example: 2 people spending their weekends at the cottage for June, July, and August, using a conservative 110 Litres per day:
2 residents X 24 days X 110 Litres/day = 5,280 Liters.
This couple would need over 5,000 Liters of water to get them through the summer to satisfy their indoor needs. If their average summer rainfall harvest is only 1900 Liters, as on Salt Spring Island, they would need a minimum 3,300 Liters of storage capacity.
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Water Tank FAQ
Can the tanks be buried?
- Tanks are manufactured specifically for either above ground or below ground use.
- Above ground tanks have smooth sides and are for ground level mounting only. They can however, be dropped into the ground to a depth of 25% of the tank height.
- Underground tanks are designated as ‚cisterns™ and can be buried completely with up to two feet of dirt cover unless otherwise noted. (Refer to burying instructions.) Underground tanks are heavily ribbed or are ball shaped to resist ground pressure.
Will a high water table be a problem?
- If the water table is likely to rise a foot or more above the base level of a buried tank, the tank must be anchored down against possible floating up when the tank is drained. (See suggested anchoring details) Dirt cover and water in the tank will of course help keep the tank in place.
- Consult dealer or factory for high water table installations.
What base is required?
- A firm, level base that is continuous under the entire tank is required.
- Base can be sand, fine gravel, clean dirt, cement or properly supported and closely placed planks.
- Pebbles larger that half inch should be removed.
Can a platform be used?
- Yes, however the tank bottom must be continuously supported, e.g. with two layers of 3/4 inch plywood nailed to a suitable support structure. In the interest of safety, an engineer should be consulted. NOTE – Weight of water is 8.33 pounds per US gallon or 10 pounds per Canadian gallon.
What about algae growth?
- Algae growth is promoted by light penetration into the tank. Growth can be reduced by locating the tank away from sunlight and choosing a darker tank colour and by disinfection.
How heavy are the tanks?
- For a rough guide, the weight of above ground tanks is obtained by taking the gallon size and dividing by five. E.g., a 1000 gallon tank will weight approximately 200 lb.
- For underground tanks, take the gallon size and divide by three.
Are they easy to handle?
- All tanks can be moved by pick-up truck.
- Polyethylene is a very resilient material and damage is unlikely to occur during careful handling and transportation.
- Extra care must be taken in extreme cold weather.
Can I put pressure on the tank?
- NO! Polyethylene tanks must be open to atmosphere or vented at all times. They can be used to provide a head of water by mounting at a higher elevation but cannot be subjected to any external pressure source.
What happens if my tank freezes?
- It is unlikely that a partially full tank will split. The walls will only swell under the pressure of ice. A tank will start to freeze only during extended periods of cold weather.
- Piping should be protected or drained during freezing weather because the smaller volume of water in the piping will freeze more easily and may split the pipe.
- If freezing is a concern, the tank can be insulated with a waterproof insulation or put in an insulated shed.
- An underground tank offers the best protection from freezing. It will also keep the water cool in summer.
What fittings come with the tank?
- Above ground vertical water tanks are fitted with a 2 inch female threaded pipe connection at the bottom and a 16 inch access cap at the top. Access caps have breather valves that let air in and out as the water level changes. Your local plumber or plumbing store can help you with making the proper connections.
- Additional connections for inlet pipes are added separately to customer requirements.
What about collecting rainwater?
- Collecting rainwater is an excellent way to bridge periods of short water supply.
- A filter basket should be used to screen out leaves, twigs and debris coming from the eaves troughs.
- Water for drinking, cooking and dish washing must of course be boiled or disinfected.
- Clean rainwater can be used for large volume uses such as toilet flushes, laundry, showers, washing cares and watering the garden. (When throwing water around during those dry spells be sure to let your neighbours know where it is coming from!)
Does sunlight affect the tanks?
- Over time, exposure to sunlight causes the plastic to harden which makes it more prone to cracking if the tank receives a heavy blow.
- All Premier Plastics tanks are impregnated with an ultraviolet inhibitor that slows this hardening effect.
How long is the warranty?
- All above ground tanks carry an eight year warranty, however, you can expect many more years of service.
- All underground water tanks and septic tanks carry a ten year warranty. Polyethylene will virtually last forever underground.
Are the tanks safe for drinking water?
- YES – tanks are manufactured from food grade polyethylene and meet specifications contained in FDA regulations 21 CFR 177.1520(c)3.1 and NSF Standard 51.
- Maintaining the quality of water in the tank is of course your responsibility. Information on water quality can be obtained from water supply companies, well drillers or local public health departments.
Will I notice a difference in taste?
- To keep the water fresh, your piping should be connected so that incoming water is always flushing through the tank.
- Water that has been sitting in any type of vented container for a period of time will develop a ‚flat™ taste. This is because air in the water has been lost. This can partly overcome by half filling a closed vessel and shaking it vigorously to put the air back in. (Refer to item on drinking water.)
How will a storage tank help a slow producing well?
- Many people are experiencing reduced flow rate from their wells. Well pumps are usually sized to provide water on demand, i.e. If your bathtub fills at 5 gallons a minute then the well must produce water at 5 gallons a minute for the time the bath is running.
- A storage tank allows short periods of high demand to come from the tank and not directly form the well.
- A family using 400 gallons per day would need their well to produce no more than 400 gallons over 24 hours if a storage tank is used. On average, a well needs only to produce half a gallon a minute every minute for 24 hours to supply the family.
- A common practice is to dump a truckload of water down a slow producing well. If water deliveries were put into a storage tank no loss from seepage would occur, resulting in cost savings.
- Sizing and system layout can be obtained from your local well driller.
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