EVALUATING THE ECONOMICS OF A UNIQUE HYBRID RO DESIGN AFTER THREE YEARS OF TREATING BRACKISH GROUNDWATER

摘要

In November 2008, the City of Oxnard, located sixty miles northwest of Los Angeles, commissioned its first, large scale, brackish water reverse osmosis (BWRO) desalination water treatment facility as part of its comprehensive regional water resources development program. The uniquely designed RO uses energy saving technologies in a hybrid configuration to minimize power consumption. The 7.5 MGD ( 28,400 m3/day) RO has been operating stably for three years and affords the opportunity to compare the tradeoff between its original capital cost and its operating cost based on actual performance data. Downstream cartridge filtration, the two stage BWRO system treats ground water with an average salinity expressed in TDS of 1,500 mg/1 and a temperature ranging between 18 °C and 25 °C. The plant employs a hybrid design by using two different RO membranes with differing permeabilities and rejections in each of the two stages. Contrary to a typical hybrid design, the Oxnard inverts the membrane installation by placing the higher permeability membranes in the first stage while higher rejecting membranes improve the permeate quality coming from the second stage. The flux imbalance that would normally occur in such an reversed hybrid is offset by the presence of an Energy Recovery Device (ERD) between the two stages. This leads to a more equitable flux throughout the system. The selection of an ERD increased the capital cost of the plant. However, the use of a hybrid design in combination with the ERD, offers operational cost savings relative to a more conventional, lower capital, system. Placing the higher rejecting membranes in the second stage where they are needed most means that the overall permeability of the plant is lower than if the higher rejecting membranes had been located in the first stage.. The operational savings associated with this unique membrane combination offsets the higher capital cost of the ERD which makes the design preferable both economically and technically. This paper evaluates the capital and operating cost associated with the reversed hybrid design and compares the Oxnard design to other Brackish RO designs including a similar design that takes advantages of the latest RO membrane chemistry as well as manufacturing developments that were not available when Oxnard was commissioned in 2008. The evaluation is based on actual operating data spanning the past three years, including actual feed and permeates water ion analysis. The difference in operating cost will be compared to the differences in capital cost, and a 2 year return on investment will be shown. The technical and economic evaluation of the system, based on three years of operating data, illustrates the use of an innovative design employing the latest energy saving technologies to reduce energy consumption and operating costs. This evaluation can be used to design an RO system and evaluate the tradeoff between capital cost and operating cost, while seeking to tailor the element selection to meet specific permeate quality targets.