Here is I hope some handy info for consideration on calculating cistern size and water use.
I obtained this info from a number of sources and employed selectively it in developing such a system.
As a matter of oft neglected or unknown importance in the conversation pertaining to water collection and storage from roof rain water sources, it is advisable to employ a “washer” and filter system, as a roof naturally collects many types of organic substances and debris, from bird droppings and leaves, to soot and ash from a solid fuel house heating system.
A “washer” is basically a roof water sub system designed to divert a volume of roof water in the beginning of a period of precipitation, allowing debris to literally be washed from a roof surface, and not collected in a cistern/tank. (As untouched rain water is a great solvent, it is pure water being it has essentially been distilled through natural evaporation, it is it’s own built in scrubber, the idea is to have a system which takes advantage of this attribute). The traditionally accepted amount used in washing is open to interpretation due to a number of variables, frequency of rain, type and use of house heating, seasons, etc, but 1/3 of total annual rainfall should be considered “wash” and other lost water.
A filter is a traditionally a 2 stage device, which employs a series of course screens/perforated metal, set at an angle prior to the entrance of input flow into the filter system, this aspect diverts large debris, such as leaves, twigs, dead animals etc, out of the filter system. Below this debris diverter there is a compartment/tank section, with a valved outflow at the lowest point, this valve and section allows for draining of filter to avoid freeze damage and permit a portion of roof water to escape during the roof washing operation, then it is closed, where continued collected roof water can flow into an upwards flow filter bed. There are actually two filter beds, the first is upwards flow, the second stage being a downwards flow, which is fed by the upwards flow filter, allowing for both filter beds to drain dry avoiding freeze damage. Both filter beds use perforated metal plates, on each end of the filter mediums, which consist of large stone, from roofing pea stone to golf ball size, which hold layers of finer gravel, sand, and charcoal from washing out during filtering. It is a very simple design, can be built out of wood, metal or concrete. The use of 304 stainless is highly recommended for all or and metal components in the filter/washer system, because of it food safe and non corrosive alloy characteristics. Also it is recommended to employ diverter valves or gates in the conductor pipes coming from gutters and scuppers.
One method for determining the capacity of the cistern required is to multiply the square foot roof area used to collect water by inches of rainfall, and divide by 1.6
Then determine the daily gallons of water requirements per person per day times 365 days.
Using this equation, a family of 5, @ 5 gals per day, times 365, figures out to an annual requirement of 9,125 gallons of water to be collected and stored. Any figure works, as you might only want potable water for cooking, canning, other food processing, water for livestock, and intimate washing of body and cookery, and your needs could be met with smaller, or larger demands.
An important point to keep in mind, you don’t receive all this water normally in one shot, and you have to figure in for dry spells and low precipitation years, so a minimum storage capacity is a prerequisite in determining cistern capacity.
There are different schools of thought on how large a cistern, based on use verses potential precipitation, but 1/3rd of total yearly requirements is considered a bare minimum, usually half a years needs is considered prudent. You can not have too much clean safe water.
In my AO, we receive on average 47 inches a year, figure 1/3 as wash/waste, on our roof we have the potential for 53,000 gals of usable water. A 6’x6’x12′ inside dimension block and motor cistern, set on a concrete footer, =’s 2,537 gals capacity. We have more water in normal weather years than we require, @ a rate of use of 10 gals per day for 2 people. You must take into consideration your climactic conditions in all instances, as for example you may receive all your rain in only one season, so it is critical to have cistern capacity to carry through the dry seasons, or any of the myriad of different weathers seen across the continent.
Another aspect of cisterns, which Mike touched upon in his previous comment above involves gravity feed, or what is also known as water column. Water column is a function of height only, not volume, a 1 inch pipe 10 foot tall has the same pressure at the bottom of this column of water as a tank 10 foot tall by 100 foot wide.
Remember here we are talking about sustainable resources and all it entails to employ, use, and run them. Labor is valuable, so is time, in this sense, it makes common sense to have a tall as possible cistern, use the water column to benefit, either by placing your cistern strategically where gravity can provide direct flow to your plumping, or assist what type of pump system you have incorporated, saving muscle power or off grid power. It is that whole holistic thing again, where you try to incorporate as many sustainable and beneficial aspects of your labor and resources into as many functions and dual uses/multi tasks as can be figured for. When you have to hand pump your water, or use precious fuel or off grid power to run an electric pump, you end up using less water by rational standards. So that can end up being a safety margin in usage. Things to think about.
We have what is called a rotary vane pump, or a SIGMA Double Acting Semi Rotary Hand Wing Pump. It is plumbed into our domestic water system, in parallel to a 12 volt service pump which runs off an off grid battery bank. With the use of a couple of ball valves and check valves, a pressure bladder tank, we have the option of running either pump, and have a pressurized domestic water system. The rotary hand pump can produce a max 37psi, 20 ft of lift, and 25ft discharge. We have to run down to the basement and pump our system to full pressure a couple times or three a day if our battery bank is getting low like on a run of cloudy days or little wind, as we have solar panels and wind turbines to charge our battery set. (It is a big honking 850lb 12 VDC 1200 amp hr fork truck battery).
The block cistern as built incorporates a troweled on mortar liner and concrete filled blocks to fully reinforce and waterproof it. It has a sump formed in the concrete footer to collect any sediment, and to fully drain the cistern for annual clean-out when precipitation is adequate to permit such maintenance. A conductor pipe from the outlet of the roof washer/filter, terminates about 6 inches from the footer, enclosed in a “tinkers damn”, or baffle, made of block sitting on the footer so when flow is present, turbulence is minimized to limit stirring up any accumulated sediment. Before we filled it for the first time, we washed the interior surfaces with a strong solution of water and baking soda, let it set for a day, gave it a good rinsing with a hose, and let her fill up with rain. The baking soda being highly alkaline, neutralized the high mineral content of the mortar and concrete, improving the quality of the collected water. I hear some folks let theirs fill up, leave it to set for awhile, drain it and refill to use. Probably either method works equally well as it basically fully cures the concrete if I understand the science correctly.
Notes:
To determine possible cistern volume, gallon capacity can be determined within a high degree of accuracy using the two following trusty pipe fitters formula’s *:
Round tank capacity in gallons, measurements in inches:
Capacity = diameter x diameter x .7854 x length divided by 231
Rectangular tank capacity in gallons, measurements in feet:
Capacity = length x width x height x 7.48
*(Regardless of type of cistern design, be sure to calculate using inside measurements of your type of tank to obtain accurate capacity)
Source for rotary hand pumps:
http://www.handpumps.com Rintoul’s above now carries a wider range of hand pumps, the Sigma pump is been renamed “Excelsior E2”
There are a variety of online papers, from university and state/county agricultural extension offices on the theory and construction of cistern systems. Though like most agrarian and rural related self determining/self sufficiency resources which in the past where promoted and sanctioned by state entities, they are disappearing rapidly.
Just to reiterate some important aspects which can get lost in the forest for the trees, the cleaner your water which is first collected and stored in your cistern, the longer it will remain viable and sanitary. After all the idea is to have a source of safe water to begin with. I know that sounds simple, but just collecting as much water as possible is but one object of the scope of such a system. All sorts of organic contaminants can possibly be collected and build up, to pollute your water source without suitable washing and filtering of roof water, from dead mice and birds, frogs, toads, insects, and animal droppings, vegetable matter and tannins from tree leaves, pollen, etc. Then there is soot, ash, creosote and other byproducts of combustion to consider that can deposit on your roof if you heat your building with a solid fuel stove or furnace.
The imperatives of a roof washer/filter bed are something which only you can determine, but if potable water storage is the prerequisite, their importance can not be overstated.
