Sea Glass

Sea Glass

Friday, February 5, 2010

Should there be a cistern in your future?

Cisterns on Sanibel

Cisterns have been used in many areas for centuries, predominantly in places where ground water is scarce, such as the Virgin Islands, Bermuda, etc. Even though Sanibel’s Island Water Association (IWA) appears to have access to abundant aquifer water, does it not make sense anyway to conserve rain water as much as possible? On Sanibel, wells into the aquifers are not allowed and many island homes use more water for landscape irrigation than they do for potable uses. So we’ve done a brief study of cisterns that could be used locally.

Since many homes on barrier islands, such as Sanibel and Captiva, are elevated on pilings or raised stem walls, they provide ideal areas for water storage. Cisterns come in many forms, so estimating the cost is difficult. Most literature I’ve come across refers to the cost of a cistern as being about $1.00/gallon. Cost for cisterns in the USVI, built as part of the home’s foundation, appear to run about $1.25-$1.50/gallon.

I’ve looked at a product called “Rainpillow”, which is essentially a plastic bag. These come in a variety of sizes (1,000-40,000 gal) and are lightweight (a 3000 gallon kit weighs a little over 100 lbs.). The costs appear to run from $2.50/gal (1,000 gallon kit) to $.60/gal (10,000 gallon kit).

This is based on the use of a 10000-gallon “Rainwater Pillow” (size is 23’ x 23’x 3’) as a storage device, as a model for our Sea Glass project, for example, since information is readily available for all required equipment, i.e. tank, filters, pumps, etc. Also, this size tank would most likely fit and could be set up under a raised stem wall foundation. I’ve been told from the company that it can be accessed through a 4’ x 4’ access hole. This projection is also based on the use of stored water for landscape purposes only. Accurate information on the cost of equipment necessary to make cistern water potable was not readily available.

While stored water can be easily treated to be used for “greywater” devices, it would appear that landscaping might be the most directly beneficial since there is light treatment involved and irrigation water, on Sanibel, has to come from the IWA. The IWA has stated that a typical Sanibel home with a “conservation” landscape package would use approximately 9,000 to 14,000 gal/month to support the landscaping. A key problem with water storage for landscaping is that during the “dry” half of the year, when irrigation is most important, the average monthly (November through April) rainfall is only 1.9 inches/month, according to UF extension service tables. One inch of water is collected at the rate of .6 gallons per square foot of roof area. So a 3,000 sq. ft. roof area will collect 1800 gallons of water at the rate of 1”/month.

Since the landscape area for a typical home in Sea Glass would most likely be smaller than a typical Sanibel lot, I’ve taken the liberty to estimate the landscape water demand at 6,000 gal/month. Under this scenario, it appears that the 10000-gallon storage tank would need replenishment water from the IWA for about 5 months of the six-month “dry” season. This replenishment would not be required during the months of the average “wet” season since the s month average is approximately 7.0 inches/month.

This is the 20 year projection for this system:

Total gallons of water collected from cistern: 1498400 gallons
Total supplemental irrigation water purchased: 188000 gallons for $752.00 (present cost)
Total system cost: $13,759.00
Cost of system per gallon/stored: $1.37
Cost of water per 1000/gallon: $9.18

The cost of water is approximately twice what the cost of the IWA would be as it is now. This could be easily offset if any type of rebate or tax credits were to be involved if forward thinking law makers were act as they should.

Jeff Good
Benchmark General Contractors, Inc.

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