MIAMI -- The crisis in drinking water supplies has never been the lack of availability of water -- it has been the lack of potable water that could be economically treated to meet drinking water standards. Up until recent history, drinkable water sources have been plentiful enough to meet the demands that the public has placed on them. Now, however, with population trends having shifted to the South, Southwest and West, the demand for drinking water has begun to exceed the potable water supplies.
By analogy, the existing drinking water sources have been the fruit at the bottom of the tree - easy pickings. That bottom hanging fruit has now been picked clean, and communities are going to have to work harder to pick the fruit from the top of the tree, i.e. turning non-potable water into alternative drinking water sources.
To illustrate the problem, one can look to the water woes of Martin County, Florida, on the southeast coast of Florida. Historically, the water utilities within Martin County have drawn their fresh water supply from an aquifer just below ground level. This water was fresh enough to require mere chlorination before consumption by the public. As the population of Martin County has grown, however, this surficial aquifer has reached its withdrawal capacity. Without an economically viable alternative water supply, growth in Martin County would grind to a halt.
To solve their problem of finding an alternative water supply, utilities in Martin County and communities across the nation have turned to membrane technologies -- filtering out impurities in water by forcing water under pressure through fine membranes which act as filters to produce fresh drinking water. In Martin County alone, one membrane plant has already been built and one is in the permitting stage with completion due in less than three years.
Membrane technology for drinking water production is not new. It has been around since the 1960's. What is new today is a drastic reduction in the costs to construct and operate membrane water treatment facilities. This revolution in cost reduction for membrane systems has opened alternative sources of drinking water to communities throughout the nation that until now had no practical role to play in the drinking water mix.
A quick primer for the membrane uninitiated. Membranes are constructed so that water molecules readily pass through the membrane while larger impure molecules will not. These larger molecules are what have kept alternative sources of drinking water inaccessible. Prime among these larger molecules which make water undrinkable are salt, calcium, phosphorus, nitrate, and radium. Membranes can now be constructed to remove virtually all dissolved soids and impurities from water or to remove specific impurities that render a water source undrinkable. This versatility allows almost any source of water to be used as an alternative drinking water source.
In Martin County, the main impurity at issue was salt. Right below the freshwater surface aquifer in Martin County is a deep aquifer with high concentrations of salt. This salty aquifer would provide enough water for expansion of Martin County, if it could be cost-effectively mined.
The present level of membrane technology effectively removed the technical feasibility barrier from using alternative water sources for drinking water. All that was left were the economic considerations -- initial capital costs to construct the facility, and continuing operating and maintenance costs for the facility once constructed.
As to the first consideration, membrane capital costs have been almost halved in the last ten years. Ten years ago, membranes may have lasted only about 3 years before requiring replacement. Now, their life expectancy is at least 5 years, and probably will last 7 to 8 years. As experience in operations has been gained and advances in technology have been introduced, the scope and cost of the equipment needed to efficiently treat water has dropped fast.
Recent membrane facilities constructed in the United States have been built at a price of $3.00 per gallon for plant sizes ranging from 2 to 10 million gallons per day (mgd). This cost varies depending on the location of the source of water, the level of effort to bring the water to the treatment facilities, and the cost of disposing of the brine water created as a by- product of the membrane process. This provides a basic benchmark to compare membrane facilities with conventional treatment facilities which cost roughly $2.25 per gallon in the same locales.
On the operating side, the largest component of cost in operating membrane facilities has been and remains the cost of energy necessary to drive the water through the membranes. Here again, significant reduction in overall energy requirements have been achieved in the last ten years. As an example, the town of Jupiter, Florida operates a membrane treatment facility using brackish water withdrawn from an underground aquifer. When its plant was designed in 1986, it operated at a pressure of 305 psi. The town recently completed an expansion of this facility with the latest membrane technology and operates now at a reduced pressure of only 230 psi to produce the same volume of water. This represents a 24 percent reduction in energy consumption. The facility will also utilize other energy recovery technologies to reduce energy consumption another 12 percent. The operating costs for this new facility equates to $0.85 per thousand gallons of treated water.
A second significant operating cost component is associated with personnel required to staff these facilities. Membrane processes have been demonstrated to be well suited to automation. It is now possible to operate very large facilities with personnel present only on a part-time basis. Advances in automation have made membrane personnel costs lower on a relative scale than other traditional treatment technologies.
With these reductions in capital and operating costs, membrane technology has brought previously unattainable potable water sources into economic play. In essence, every water source can now be analyzed as economical options in the effort to meet the rising demands for drinking water, as membrane technologies have at least brought cost levels low enough for consideration.
The costs of membrane treatment facilities still rise above the existing cost of providing drinking water for most communities, which will likely necessitate a certain amount of education of utility customers as to the benefits to existing customers for increased costs to be incurred for the benefit of new customer growth. Of help in this education process is the generally superior quality of water produced by membrane facilities over conventional treatment. Blending new membrane-treated water with existing conventionally-treated water will raise the overall quality of drinking water for existing customers. Adding membrane facilities to existing conventional facilities may also forestall even more expensive replacements of existing facilities as new federal drinking water standards make existing treatment facilities non-compliant.
At the cost levels presently experienced with membrane treatment technologies and the potential benefits to drinking water quality that spring from such technology advances, alternative water sources can be utilized while maintaining reasonable water rates.
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