Gil Morrill, the city's consulting engineer studied alternative solutions for the problem including alternative water supplies. The most cost effective and environmentally sound was the reverse osmosis system. While funding was being secured for the project, Las Animas was planning for the design of the plant with a model system. A pilot filter run over several weeks in early 1994 indicated that the dissolved solids (ranging from the high end from 3,600 to 4,700 Mg/L) would be reduced to below 750 Mg/L; hardness, ranging from 1,600 to 1,988 Mg/L would be reduced to below 150 Mg/L; and sulfate, ranging from 2,100 to 2,800 Mg/L would also be reduced to below 150 Mg/L. The somewhat high iron and manganese content of the water was anticipated to cause scaling of the RO membranes if it was allowed to oxidize. Consequently, the city's wells were modified with foot valves so a full water column could be maintained.
To keep sulfates in solution and prevent membrane fouling from this source, the system will add a scale inhibitor. To recondition the permeate, the city will blend 80-100 percent raw water back into the system. Corrosion control will be further enhanced with the addition of sodium hydroxide and zinc orthophosphate. Fluoride levels will be restored through the addition of hydrofluorosilicic acid. Disinfection will be accomplished through chlorination. Concentrate from the RO units will be discharged back into the river through an industrially permitted discharge point. The ratio of finished water to wastewater produced in the process is expected to be 50:50.
Construction bids were opened August 26, 1995. The low bid was $3.7 million. Average monthly water bills will be about $35, roughly the same as what the average customer pays now, given existing monthly bills and home treatment system expense. This compares with the Colorado average monthly municipal water bill of $26.19. As of the writing of this report, the residents of Las Animas are extremely happy with the water produced by the RO plant. One lady reports how nice it is to be able to "boil beans" again.
This report was extracted from an article written by Barry Cress, a specialist with the Colorado Division of Local Governments.