by ERNE on OCTOBER 24, 2005 - 4 Comments in OFF-GRID 101, WATER
By Parker Abercrombie and Art Ludwig, author of Water Storage: Tanks, Cisterns, Aquifers, And Ponds for Domestic Supply, Fire And Emergency Use. Buy the book at the end of this story. Includes How to Make Ferrocement Water Tanks.
An ideal off-grid water storage system would draw water from an abundant, clean, year round spring or creek. There would always be enough water supply to meet water scarcity, and there would be no concerns about extra water for fire protection or emergencies. But since few of us are blessed in this way, nearly all water storage systems include some form of storage, most commonly a water tank. Even if you get your water from a municipal water system, you may want to store water at your home for fire protection or emergency preparedness. Water storage can be used to:
cover peaks in demand
- smooth out variations in supply
- provide water security in case of supply interruptions or disaster
- save your home from fire
- meet legal requirements
- improve water quality
- provide thermal storage and freeze protection
- enable a smaller pipe to serve for a distant source
We’re going to consider each of these reasons to store water, then take a detailed look at water tanks, and a brief look at other ways to store water.
Cover Peaks in Demand
The most common function of water storage is to cover short-term use flows that are greater than the flow of the water source. For example, a tiny, one gallon-per-minute spring supplies 1440 gallons a day. This is several times more than most homes use in a day. However, almost every fixture in the home consumes water at a faster rate than 1 gallon per minute (gpm) while it is turned on. Even a low-flow shower head uses about 1.5 gpm.
By using water stored in a tank, you can supply water to the shower faster than it is flowing from the spring. On completing the shower, the water will be coming in faster than it is going out, and the tank level will rise back up.
If you had a 10,000 gal tank, you could run a 100 gpm fire hose creating the kind of blast used to bowl over hostile crowds on the stored water from this tiny spring, for an hour and a half! Hopefully the fire would be out by then, as the tank would take several days to refill.
Smooth Out Variations in Supply
In some circumstances, your storage needs will be affected by variations in the water supply. For instance, if the supply is rainwater, you will need enough storage to make it through the intervals between rainfalls. A six-month, rainless dry season requires a heck of a lot more storage than the most common kind of variable supply a well pump that cycles on and off.
If you have a well that taps stored groundwater, a tank will save wear and tear on your pump, because the pump won’t have to switch on and off every time you open a tap.
Provide Water Security in Case of Supply Interruptions or Disaster
In many places, the water supply chain from source to tap is long and made of many delicate links. If a cow steps on the supply line, a pump breaks, a wire works loose, the electricity goes out, the city misplaces your check, or there is a natural disaster, your water flow could stop. By locating your storage as few chain links away as possible from your use point, a large measure of security is added.
Save Your Home from Fire
Designing a system to be effective for combating fire can change its specifications radically. To put out a fire, your stored water needs to be available at a flow rate many times greater than normal.
Meet Legal Requirements
Sometimes you may be required to install water storage simply to meet a legal requirement. On the other hand, you may be able to trade increased water storage for slack on a different legal requirement. For example, if you provide a large amount of water with good pressure that is reserved for fire emergency, a sprinkler system, and/or a hydrant, the fire department might allow you to build a narrower driveway with a smaller turnaround further from the house than they would otherwise�thereby saving you a fortune.
Improve Water Quality
The water coming out of a properly designed tank can be of significantly higher quality than the water that goes into it. This is mostly due to attrition and settling. Add an ozonator, and a tank becomes a substantial treatment step.
Provide Thermal Storage and Freeze Protection
Water has higher specific heat stores more thermal energy per unit of weight than any other common material. A large thermal mass of water stored within a solar greenhouse or home can help to keep it cooler in the day and warmer at night.
Also, as water changes to ice, it radiates a tremendous amount of stored energy. Imagine how much gas it would take to melt a water tank-sized ice cube; when water freezes, it releases this same amount of energy. This is why irrigating for frost protection is effective. The stored energy in water can prevent a water tank or nearby components from freezing (though in the coldest climates this may not be sufficient).
Evaporation consumes even more energy, which is why swamp coolers and cooling towers are effective.
Water is also an effective heat transfer medium.
Finally, in rare instances it can be economical to use elevated water storage as a battery, from which electricity is extracted by running it through a hydroelectric turbine.
Enable a Smaller Pipe to Serve for a Distant Source
If the flow of your source exceeds the peak demand, you can connect to it directly without storage. However, if the source is distant, it may be cheaper to run a small pipe to nearby storage, and a big pipe from there to the use point. The small pipe would be sized to the average use, the big pipe to the peak use. The savings in materials and labor from running a smaller pipe over most of the distance can often pay for the storage and then some.
Ways to Store Water
If you want to store water, here’s your options:
Source direct (no storage)a rarely applicable but desirable option if you have a clean source higher in elevation and flow than the water uses.
Store water in soil inexpensive supplemental irrigation storage in soil (not advisable in landslide areas).
Store water in aquifers free bulk storage safe from evaporative loss, but only accessible by pump and subject to contamination and extraction by other users.
Store water in ponds inexpensive bulk storage of water, most appropriate where rainfall exceeds evaporation and the majority of water need is for non-potable uses.
Store water in tanks most expensive but most flexibility in location and best protection and control of the stored water.
Water Tanks 101
Tanks are the most common way to store water. A well-designed tank offers nearly complete control of storage conditions, including:
- security against leakage
- protection from mosquitoes and vermin
- shade so algae will not grow
- minimal or no evaporation
- valve-controlled inlets and outlets
Water tanks come in all different shapes, sizes, and materials. Most tanks will have:
- an inlet
- an outlet
- service access
- a drain
- an overflow mechanism
- air venting
- provision for sunscreen
Water tanks also can have a host of optional features.
Before installing your water tank you will need to decide where to put it, whether it will be above or below ground, how big it will be, and what material it will be made of. You may also need to meet regulatory requirements.
Situating Water Tanks
The location of your water tank will largely determine:
- which parts of your land can be supplied with tank water by gravity
- the amount of pressure at every point in the system
- the length and cost of pipe runs, control wire runs, and line-of-sight for radio links
- how visually obtrusive your tank will be
- the vulnerability of the tank and pipes to hazards such as falling trees, rocks, and landslides
- the size of tank it is feasible to build
- ease of construction and service access
To situate a water tank, you need to consider:
If you have a hill, put the tank at an elevation on it that yields adequate pressure. (230 ft elevation produces 100 PSI, the maximum recommended pressure for home.)
In places where there is no hill handy, you can:
- make a water tower (to artificially increase the elevation)
- use a small pressure tank (to pressurize water as it is needed and have no water when the power goes out)
- use a huge pressure tank (to store pressurized water at low elevation)
- put a tank on your roof (and live with low pressure, like most people worldwide)
Stability of Soil and Slope
You don’t want your tank to sink into the ground, or slide down the hill. The load per unit of area from water tanks is actually quite low. A person walking can easily place much higher point loads on the soil. On the other hand, no one has feet as big as a water tank. It’s the aggregate load from a water tank all that area being pushed on at the same time that can push your building pad down into the gully.
However, undisturbed native soil is sufficiently strong to support even large tanks. In the case of a tank on a slope, where you don’t have a natural flat spot, put the tank on cut (newly exposed, undisturbed soil), rather than fill (freshly dumped, loose soil). For a really large tank or any tank on fill, it’s a good idea to consult with an engineer.
Aesthetics, Sacred Spots
Water tanks can be big, and although they can be beautiful, they are more often ugly. When locating a water tank, either:
- put it where it doesn’t matter
- conceal it well
- make it beautiful
Ideally, you want your tank downstream from whatever hazards and weak links lie between you and your water source. Rivers that flood, gullies that wash out, landslides, falling trees and rolling boulders it’s best if as few of these hazards as possible are between you and your tank.
Burying your water tank makes it less obtrusive, cooler, and more secure, as well as providing good protection from frost and sunlight. However, you probably won’t be able to install a gravity drain, so you’ll need a pump to get the water out and cleaning is more difficult. The design of buried storage is more structurally challenging, and inspection, repair, and replacement are more difficult. The buried tank will need to be protected from surface water leaking in, and protected from children and pets falling in and drowning.
Sizing Water Tanks
The size of your storage is one of the main factors that will determine under what circumstances you will find yourself short of water, and for how long. Will demand outstrip supply every morning? When there is a fire? A day after the well pump goes out? It will also do a lot to determine what your system costs.
Sizing your tank is a matter of figuring out what degree of water security you want, then finding the tank volume that makes the most of your water supply within your budget and other limiting factors. This is a good time to remember the reason(s) you want storage, as they will drive the calculation of tank size:
- You want more water security than a direct connection to the source can provide.
- The yield of the source cannot directly provide for peak demand.
- The yield of the source is less than that required for firefighting.
- The source is less secure than water stored in a tank (e.g., if the source requires pumping water, while water stored in a tank doesn’t).
- The pipeline distance to the source is so far that it is more economical to use a smaller size pipe and a tank, than a pipe large enough to carry the peak flow all the way from the source to the users.
The biggest variable by far is how much water security you’re aiming for. In general, the more storage you have, the better your water security. Without storage, the security the percentage of time you’ve got water is equal to the security of the source. The more storage you’ve got, the longer an interruption to the source supply you can cover with stored water.
Is it possible to have too much storage? Yes. Too much storage can lead to freezing or water age problems. More likely, it simply constitutes a waste of the Earth’s valuable resources. Because of the high up-front cost of storage, it is rare to see anyone except the super-wealthy install too much storage volume.
Sizing a Tank For Demand Peaks which Exceed Flow
Although water needs are usually expressed as a value-per-person for a 24-hour day, in actuality just about all of this water will be used during a period of 10 to 12 hours. Over half of the entire day’s water use may happen between dinner time and bedtime, or in the morning, depending on the culture. Water provided by the source during low-demand periods (e.g., overnight) can be stored for use during high-demand periods.
Sizing a Tank When You Have Limited Water Supply with Scheduled Use
This approach is the inverse of the approach above. It may be appropriate if the water supply is limited and there are known lengths of time without water use. Instead of sizing the tank to cover use, you size the tank to cover production of the source during the longest time without water use. If you store all the water that is produced during the longest time without usage, you’ll have maximized your limited supply.
Sizing a Tank to Cover Use During Interruptions in Supply
Most systems have at least a day’s worth of storage to cover supply interruptions due to servicing, a fault within the system, or a disaster such as an earthquake or power outage. To size your tank for supply interruption, consider what is likely to jeopardize your supply and for how long.
Sizing a Tank When Production Is Intermittent
If your water production is intermittent (for instance, from harvesting rainwater), your tank should cover the maximum cumulative deficit between production and consumption.
Sizing a Tank for Firefighting
If your system is part of a project that requires permits, there are likely specific legal requirements for the system’s firefighting capabilities which you will have to research and fulfill. These probably include much more storage, bigger pipes and higher pressure than you could otherwise imagine.
Size and Structural Integrity
As tanks get bigger, the structural engineering issues get much bigger. Tanks of a thousand gallons are no great challenge. A 10,000 gal (40 m3) tank requires serious consideration of the loads that will be operating on it. Any tank over 30,000 gal (110 m3) should be professionally engineered.
The tank shape determines how the material will resist the applied force and thus how easy it will be to resist a given load. This is something that you should consider carefully if you are making or modifying a tank.
There are plenty of choices for water tank materials, each with advantages and disadvantages. Common materials include galvanized steel, various plastics, fiberglass, concrete, ferrocement, brick, and rock and mortar. However, a few circumstances are unequivocally hazardous and to be avoided:
PVC exposed to sunlight PVC breaks down in sunlight, reacting to form carcinogens, which leach into the water. It is a plumbing code violation to have potable water in un-shaded PVC for this reason. You can see physical evidence of the change on the outside of the pipe; it darkens, becomes chalky and brittle. The reaction progresses from the outside in. To the extent PVC should be used at all, it should be buried or indoors. If you have PVC that has already degraded, you should replace it.
Pre-1997 PVC which was made with more toxic plasticizers.
Flexible PVC water bed bladders or trash cans which contain a high level of toxic plasticizers.
Pre-1980 tank coatings including coal tar and lead-based paint These were great for corrosion resistance but oops! they poison the water.
Lead pipe and pre-1987 lead-soldered copper pipe Solders and flux currently contain less than 2% lead. Before 1987 they typically were half lead. Lead pipe can be recognized by its softness.
Western red cedar The same stuff that smells good and keeps it from rotting is toxic when ingested.
Fly ash in concrete especially when exposed to acidic water.
Often the worst hazards are not the base material, but solvents, additives, mold-release agents, fungicides, etc.
Many, if not most, water tanks are installed with little or no regulator involvement, but rules and enforcement can vary quite a bit depending on where you are. You’ll need to inquire locally to find out what you’ll be subject to.
Water storage may be subject to zoning, building department, fire department, and health department rules. If you have a homeowners association, it may regulate water tanks, perhaps just because they are a structure. Your insurance company may have rules or incentives relating to water storage as a fire safety resource, a flooding hazard, or simply as another asset to insure.
Some of the rules you may encounter will be consistent with your own interests, and some will run counter to them. You may run into rules concerning:
You may or may not be allowed to build a water tank within the building setbacks from your property lines the zoning department can tell you.
In some neighborhoods, rules may prohibit above ground tanks. A beautiful ferrocement tank with an attractive shape and color, or one that looks just like a boulder, may be able to overcome anti-tank prejudice.
It will generally be the building department that enforces plumbing code requirements about pipe sizes, materials, placement, etc. Tanks over a certain size may require a permit 5000 gal/ 18.9 m3 in our county. Large, constructed tanks may require a permit with an engineer’s stamp on the structural plans. Some of the tank-specific requirements we’ve heard of:
Lockable lid to guard against malicious contamination and drowning hazard
Sealing lid to guard against entry of roof runoff and creatures
Overflow with mosquito trap
Soil report for the structural soundness of the soil that supports the tank
The fire department may require that a hydrant be attached to the tank, with a certain-sized connection (4 in our area). They may require a set-aside or reserve of a specific size (2000 gal/ 9.5 m3 in our area) that can only be accessed via their hydrant not a bad idea, if you want them to be able to save your house. This can be accomplished by putting the hydrant outlet at the bottom and the domestic supply outlet higher up.
The health department may have their own rules, or defer to the building department. The vector control department may want to ensure that your water storage does not breed mosquitoes.
Ideas for Really Cheap Storage
If economy is an overriding consideration, here are some suggestions for really cheap storage:
Salvage 30 or 55 gallon drums can often be scrounged for little or nothing. The bungs often have 3/4 pipe threads, facilitating attachment of inexpensive plumbing, for example small diameter polyethylene tubing. Drums can also be drilled, tapped, threaded, or fitted with bulkhead fittings to make inlets and outlets in any position.
Tote bins used for palletized, bulk transport of liquids. They can be made into passable small tanks, if they’ve contained something non-hazardous. They are usually 275 gal (1 m3) HDPE containers.
Above-ground swimming pools are the cheapest, funkiest storage going. Not a long-term solution, but you can’t beat the cost. The plastic walls are usually PVC.
Ponds can be relatively inexpensive for large volumes of water.
Aquifers usually don’t cost anything and can store vast amounts of water.
For More Information
For information on all types of water storage, worked examples of how to size water tanks, and extensive information on tank materials, see Water Storage, Oasis Design Press 2005. http://www.waterstorage.ws.
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