On increasing the water crop by desalination of salty water - November 27th, 1972


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On increasing the water crop by desalination of salty water - November 27th, 1972

Material Information

Title:
On increasing the water crop by desalination of salty water - November 27th, 1972
Creator:
Parker, Garald G. (Garald Gordon), 1905-2000
Publication Date:
Language:
English
Physical Location:
Box 2

Subjects

Subjects / Keywords:
Aquifers -- Hydrogeology -- Florida ( lcsh )
Hydrology -- Florida -- Biscayne Aquifer (Fla.) ( lcsh )

Record Information

Source Institution:
University of South Florida
Holding Location:
University of South Florida
Rights Management:
The University of South Florida Libraries believes that the Item is in the Public Domain under the laws of the United States, but a determination was not made as to its copyright status under the copyright laws of other countries. The Item may not be in the Public Domain under the laws of other countries.
Resource Identifier:
032968560 ( ALEPH )
891343127 ( OCLC )
G16-00688 ( USFLDC DOI )
g16.688 ( USFLDC Handle )

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Book

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PAGE 1

. ' . ) FROM THE DESK OF THE CHIEF HYDROLOGIST: On Increasing the Water Crop by Desalination of Salty Water. Florida is surrounded and underlain everywhere by salty water. To the east our shores are washed by the Atlantic Ocean; to the west.is the Gulf of Mexico; and underneath us at some depth everywhere the ground water is salty. The ocean waters generally average about 35,000 ppm {parts per million) of dissolved solids, of which about 19,500 ppm are sodium chloride (connnon table salt). The ground waters, however, range widely in their dissolved solids and sodium chloride content. Some Florida ground water has a chloride (Cl) content of less than 10 ppm whereas. other ground waters, such as those in the deeply buried formations,-contain brines with Cl content several times that of the oceans. '!be U.S. Public Health Service standards call for drinking waters to have no more than 500 ppm of Cl and most people can definitely taste salt in water at 500 ppm. Others do not taste it until the Cl content is 1,000 ppm or more. Most public water supplies in the state furnish fresh water to consmners ranging from about 10 ppm to less than 100, at costs ranging from about 10 cents to more than a dollar per thousand gallons. Such water not only tastes better but it requires less soap, is not so corrosive of metal fixtures and, especially for persons requiring a low-sodium diet, is far more healthful than waters of higher sa~inity {chloride content). According to accounts of early settlers in our District, shallow wells up to a hundred or more feet deep produced fresh water right down to the very shore line of the Gulf. Early supplies fqr all our coastal .cities Mere from such shallow wells and,_ in a few instances, some are still in use, though on a marginal basis, at cities such as Dunedin and Clearwater in Pinellas County. Tampa and St. Petersburg, until the late 1920's also obtained municipal supplies from downtown well fields but encroaching salt water from Tampa Bay and . the Gulf of Mexico at that time ruined the well fields and forced the development of new fresh-water supplies inland. Latest of our cities to lose its well field to salt-water encroachment is New Port Richey in Pasco County; and nearby Port Richey, plus the entire urbanizing strip along the coast from St. Peter.sburg-north to Citrus County, is facing the same prospect. Recent hydrogeologic studies by our staff and the staff of the U.S. Geological -Survey have shown that in a strip paralleling the shore from Charlotte Harbor on the south to the Withlacoochee on the north, a wedge of salt water is slowly moving inland from the Gulf of Mexico. Its movement has been initiated by the lowering of fresh-water levels in this shore zone as developers have dug drainage ditches to drain the swamps and marshes for housing sites, and have dredged deep channels into the underlying limestones (top of the permeable Floridan Aquifer) to provide boat or ship ch~nnels, and lastly, have provided large-scale well pumping facilities for municipal, agricultural or commercial purposes. Of these, the well pumping is by far the least damaging. The ditchers and dredgers have caused the most harm. Lowering the fresh-water level on land upsets Nature's long-established equilibrium between salt and fresh water in the coastal zone and, in attempting to reach a new equilibrium based on the new, lower fresh-water levels inland, salt water inexorably moves inland. So now we find salty coastal ground water where once there was only fresh water, and the only answer previously_available for residents of this salty-water invaded zone was to .purchase inland well-field sites, develop supplies there and at great cost pipe the water to the coastal areas. And even today this is still a practical

PAGE 2

Page -2-way of .supplyipg the needed fresh water. But it can't always be so. Our freshwater supplies are limited by Nature. We can take out only so much and then we exceed Nature's annual replenishment. When we do this we begin "mining" water, or to think of it another way, we no longer live on the interest of the principal, we begin to use up the principal itself. Eventually, living on the financial principal we go broke; so also we can hydrologically go broke. Our forecasts of fresh-water availability indicate that, foronce-only uses as we are generally now doing in the District, the time of water mining will begin sometime between 1985 and 1990. But here in Florida we will never be out of fresh water. We can always obtain all we need by desalination of the limitless ocean waters or, better yet, by desalination of the brackish ground water of the encroaching wedge previously described. There are several methods --practical ones --for doing this. These include: (1) the distillation processes, including MSF (multi-stage flash)-and VTE (vertical tube evaporator); (2) the membrane-separation processes, such as the RO (reverse osmosis), ED (electrodialysis); and (3) crystal~ization (freezing) processes. Nationally, research in these various processes has been carried out under the direction of the Office of Saline Water in the U. s. Department of the Interior. Additional research is being done by a number of universities and by several private corporations. As a result,a number of technological breakthroughs have occurred recently and successful desalination plants, both operational and pilot (experimental) are now in operation. Costs of producing fresh water with chlorides of less than 250 ppm, are reported ranging from about 35 cents a thousand gallons to $2.25 a thousand gallons. Here in Florida we are in the forefront of desalination activity. '!he Navy's Key West plant distills sea water using the MSF process. Their costs to produce 2.6 mgd of fresh water have ranged from 85 cents a thousand to $2.29. Why the Navy chose to use sea water instead of brackish water from the Floridan Aquifer . by way of a deep.well is not clear. Farther north in the Florida Keys, at the Ocean Reef Club on Key Largo, a Floridan Aquifer well 1,000 feet deep supplies feed water with a chloride content of 5,000 ppm. This brackish water is desalinated by the RO process to 200 ppm at a reported cost of less than the cost of using Floridan Aqueduct water piped from the Florida City Well Field. Nearer to home the Siesta Key ED plant has been operating successfully since 1969 producing 1.5 mgd from brackish Floridan Aquifer well water. Production costs are reported to be about 31 cents per thousand gallons but overall costs (including lease-purchase payments, maintenance, distribution, servicing, etc.) run up to about 81 cents, figures reported by Mr. Martin Murphy, Plant Superintendent to the Office of Saline Water. Incidentally, this is now the largest ED plant in the U.S. and is soon to be enlarged to 3 or 4 times its present capacity in order to meet expected demands of the next 5 years. The newest, and probably the most efficient, desalination plant in the U. s. is the .5 mg~ RO plant at Rotunda West, on Charlotte County's Placita Peninsula,

PAGE 3

Page -3-about 20 miles west of Punta Gorda. This. new plant was .installed at a cost of $385,000, including wells, pumps and treatment plant. Rotllllda plans to expand this plant to 2 ~gd in the near future. Cost of treating the 7,000 ppm Floridan Aquifer water to reduce it to less than 250 -ppm.are expected to be about 50 cents a thousand gallons and the company figures that it can make a pr_ofit at 85 cents a thousand. I So, as I say, we need never run out of good, clean fresh water. It will only cost us more than it now does, but perhaps not much more. Costs:quoted above indicate that desalinated water produced by the revers~ osmosis process is competitive right now with water developed from distant fresh-water well fields. The time is upon us now, particularly for the coastal communities, to begin planning for augmenting their present sources with desalinated water produced within their own use areas. Certainly the Southwest Florida Water Management Dis ct will ook favorably upon such developments. Garald G. Parker, Chief Hydrologist Senior Scientist November 27, 1972 t t


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