Six of these were within 100 miles of the Gulf and 

 the remaining five were scattered far inland. In 

 each of the 1 1 cities the unit cost for desalting 

 water was found to be less or about the same as 

 the unit cost for water from conventional sources. 



Twenty-six cities were eliminated from further 

 consideration because it was found that the 

 calculated unit costs for desalted water from 

 individual plants was significantly higher than the 

 unit costs for conventional water. Nine of these 26 

 cities are in close geographic proximity in the 

 lower Rio Grande Valley and have similar water 

 problems. They provide a case for further study 

 because of the possibility of obtaining economical 

 municipal water supplies from one or more large 

 desalting plants instead of many smaller ones. 



A comparison of unit costs for the 1 1 candidate 

 cities is summarized in Table 3. The costs given are 

 total costs. They include water production, trans- 

 portation, treatment, and processing costs. In 

 addition, for desalting, the cost of brine disposal is 

 included. Although the cost data given are prelim- 

 inary, pending detailed engineering studies for 

 each city, the methods and technical data used in 

 the computation are sufficient to establish a 

 predictable range of accuracy plus or minus 10 per 

 cent. 



A comparison of capital costs of desalting 

 plants versus conventional plants in the eleven 

 candidate cities is given in Table 4. In favor of 

 desalting in all 11 cities is the lower capital 

 investment required for desalting as compared to 

 development of conventional water supplies. 



Southwest Research Institute points out that 

 the cost of desalted water calculated for any of the 

 Texas cities would prohibit its use as agricultural 

 water. The least expensive desalted water cal- 

 culated in the study was 30 cents per thousand 

 gallons. Translated to a dollars per acre foot basis 

 this water would cost about $98. In the lower Rio 

 Grande Valley where the demand for irrigation 

 water is great, the least expensive desalted water 

 calculated for any city was 32.1 cents per thou- 

 sand gallons which is the equivalent of $105 per 

 acre foot. The current cost of irrigation water in 

 the valley is $10 to $15 per acre-foot. 



It seems reasonably clear from the Southwest 

 Research Institute study that in locaUties where 

 special circumstances prevail and for uses where 

 the value of the marginal product is high, desalted 

 water can compete on an economic basis with 



Table 3 



COMPARISON OF UNIT COSTS OF DESALTING 



VERSUS CONVENTIONAL WATER SUPPLY SYSTEMS 



IN THE ELEVEN CANDIDATE CITIES 



(Unit costs include capital amortization) 



Costs are expressed in dollars of 1966 value. 

 Plant capacities based on projected 1985 municipal 

 water requirements. 



Calculated at 750 parts per million total dissolved solids 

 {ppm TDS) product water. All other cities are calculated 

 at 500 ppm TDS. 



Incremental supplies for additional water requirements. 

 All other cities are calculated for total water supply 

 requirements for 1985 for both supply and quality 

 improvement. 



Several alternate conventional sources of water supply 

 for El Paso are under active consideration. The study of 

 the feasibility of development of these sources has not 

 progressed to the point where costs can be estimated. 

 No proved alternate source of water supply with less 

 than 1,000 ppm TDS is available to Port Mansfield, 

 therefore no cost calculations were made for compari- 

 son. 



For plants in this size range, OSW has under develop- 

 ment new types of distillation processes which may be 

 expected to compete favorably with the electrodialysis 

 plant concept for which this desalting cost was calcu- 

 lated. Preliminary estimates indicate unit costs as low as 

 48cf/1 ,000 gallons for the El Paso site. 



water from conventional sources. But for agri- 

 culture, which has low marginal product value, the 

 cost of desalted water, at least from plants in the 

 10 mgd size range, is not remotely competitive 

 with conventional water. In calculating relative 

 costs reaUstically it is important to prepare and 

 analyse the schedule of the value of marginal 

 product of water for various quantities that might 

 be available to a region. There is also the need to 

 compare marginal costs of desalting water and 

 water from conventional sources. 



An important item in connection with the 

 comparison of water costs from desalination and 

 costs from conventional sources relates to the 



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