Slice of Heaven in a Can (SOHIAC) is my project to develop an inexpensive, ecologically sound, minimalist solution to the issue of homelessness due to disaster or societal conditions, by reusing surplus ISO shipping containers as a basic construction module.  

Providing safe storage of water is an important part of the habitability equation. Even in areas where rainfall is very abundant overall, as it is in my target area in Hawai’i, there are wet seasons and dry seasons, and wet years and dry years, so it is important to make sure the storage capacity is at least equal to the need over the driest period of the driest year. And the type of storage cistern or tank needs to be considered carefully.  

The photo above is a pair of custom steel rainwater tanks at a house across my back fence in Austin. In this application the caught rainwater is used for landscape watering during periods when city water restrictions are in effect.  In Hawai’i this type is also used in more expensive houses.

A less expensive and more common type of steel tank is made of corrugated steel. This type of tank is a common sight in rural Texas, where it is used for both rain and well water. These are a very popular type in Hawai'i (23% of total in use, but 85% of recent installations), generally with a mesh cover instead of a solid top, but they are generally considered to be ugly.  I concur.

The traditional rainwater tanks in Hawai’i were large redwood or cypress barrels, some of which are still in use after over 100 years. Today they are considered deluxe, because of their natural beauty and longevity, but they are expensive to install ($7-10,000) so their use today is limited (11% of total in use, 1% of new installations).  

Cement or masonry cisterns are prized by their owners for the natural buffering action they provide to the natural acidity of much of Hawai’i’s rain. Rainwater absorbs sulfur emissions from volcanic venting, which converts into weak sulfuric acid. For other kinds of tanks it is normal to add a box of bicarbonate of soda every few weeks to neutralize the acid, but with cement tanks this step is often not required. This type is subject to cracking and typically requires periodic maintenance to repair leaks. The latest variation on this type is ferro-cement construction (2%), in which a rebar cage is sprayed with concrete, much like a swimming pool. These are at the high end of the price range and are typically constructed in round shapes that eliminate corner cracking.  (Pictured is a traditional Texas style stone cistern at the Ladybird Wildflower Center)  (Maybe 1% of total in use due to cost) 

In the middle price range ($2,000 for 1,000 gallon tank) are blow molded polyethylene tanks, which are available in various sizes of 100s and 1,000s of gallons. They are molded in dark color to suppress the algae growth that can occur with translucent tanks. The larger ones need to be partially sunk in the ground or strapped to the building to prevent tipping during seismic events. One budget trick is to shop for B grade tanks, which are structurally sound but which need to be cleaned inside because the plastic beads used in the molding did not completely melt. These are usually snapped up quickly at the Hilo factory because of the cost savings. (1% of total in use, but rising quickly) 

 A fairly new variation on this kind of tank are the surplus 330 gallon food grade poly tanks that come on a plastic pallet, in a steel cage. Used for bulk transfer of sauces, fruit purees and the like, they are widely available in agricultural areas at fairly low cost for clean used ones. Currently I can buy these for $120 each in the Austin area, and if I can’t find them in Hawai’i I’ll have a couple shipped over from Texas, because I think this is an ideal size for this project, and is a lower price than anything else I’ve found yet. And, again, it is reusing a container that just piles up in certain places. And due to the modular nature, additional capacity can be added 330 gallons at a time. I'll paint them with opaque paint to suppress algae growth. 

 At the low end of the price range are Doughboy type swimming pools (under $1,000 for 10,000 gallons), and their use in Hawai’i is very widespread because of the low investment. Usually covered with mesh that allows direct capture of rainfall as well as runoff from the roof, this type is the most easily contaminated from small animals, bird droppings and insects getting into the water. The other big issue is that off the shelf vinyl swimming pool liners normally include a fungicide which is not healthy to drink. So if this type is chosen, it’s important to purchase a “food grade” liner, which adds to the expense. Also this type is very prone to algae growth and often requires “shocking” with chlorine bleach to suppress algae and micro-organism growth. Their service life is also short, 5 years being typical. And finally, in the event of seismic activity, this type is the most likely to collapse. Lots of drawbacks, but quite popular due to relatively low cost. (57% of total in use, but 6% rate of new installs)

 One new type to consider is the fabric bladder tank created by Monolithic. It is a bit like the Doughboy type, but uses a ferrocement ring for containment, and adds a top to the liner, creating a kind of sealed pillow shape that expands and contracts as the water level changes. They guarantee the liner for 10 years, and say the life expectancy is 20, so this could be a good investment for a small village, say. For one SOHIAC home it would be overkill, but for a group of a dozen? Something to keep in mind if the SOHIAC concept catches on. 

 One consideration on tank size we do not have to deal with in this project is the requirement most Hawai’ian fire insurance policies have for a 10,000 gallon water tank on premises for emergency use by the fire department. Because this is a steel building fire insurance is not a necessity.

 Calculations: 

Water needs:

 Mainland water requirements are often calculated at 110 gallons per person per day. Hawai'ians use a lot less water, particularly during dry periods, so 30 - 5o gallons a day are a more typical figure. For this application there will be no dishwasher (heavy user of water) and no toilet flushing (usually about 1/3 of the total - details to follow) so I figure it this way, per person:

Use                                               Typical Hawai'ian                 SOHIAC           

Cook/Drink/Dishwash           5                                                    3

Laundry                                         7                                                    -   (laundromat)

Bathing                                           16                                                  8  (low flow shower)

Toilet Flushing                      20                                                  -  (Ecosan) 

          TOTAL                      48                                      11

So the 330 gallon tank works out to a 30 day supply for 1 person. Nifty!

 For a round tank, tank diameter (feet) (squared) x depth of water (feet) x 5.9 gallons/foot = volume (gallons).

For example, if your tank measures 8 feet across and the depth of water you are calculating for is 6 feet:

82 = 8 x 8 = 64

64 x 6 x 5.9 = 2265.6 gallons.

For a rectangular tank, tank length (feet) x tank width (feet) x depth of water (feet) x 7.5 gallons/foot = volume (gallons). For example, if your tank is 8 feet square and the water depth is 6 feet, then

8 x 8 x 6 x 7.5 = 2880 gallons.

For a complete free online manual on rainwater catchment see http://www.ctahr.hawaii.edu/oc/freepubs/pdf/rm-12.pdf 

Love, David

Steel tank photo #1  and text © 2010 David Kinne

Search on SOHIAC for other posts in this series. 

– Other photos courtesy US Plastics, Texas Commission on Environmental Quality, Cottage Chronicles, BH Tanks and Monolithic.com