"Brackish water supplies are dwindling now, and soon these supplies will be fully depleted due to such factors as drought, pollution and overusage," said Dr. Irving Moch, president of I. Moch & Associates, a water consulting firm located in Wilmington, Delaware. "Very simply, we are having a more difficult time preserving our nation's aquifers."
Moch believes that sea water will become a viable replacement for our shrinking brackish water supplies. In particular, he sees reverse osmosis as the purification method of choice for newly constructed industrial sea water plants in the United States. (Dr. Moch will be presenting a paper on this subject, entitled "Pricing a Reverse Osmosis Sea Water Plant for Industrial Use," at the 1998 International Water Conference, to be held in Pittsburgh, Pa., October 19p;21.)
"Reverse osmosis for the purification of sea water is a relatively new technology in the United States," Moch said. "Industrial engineers and planners need to understand not only the individual issues involved with RO sea water systems, but also they need to know what these new systems will cost, both initially and in the long run."
RO's Sea Water Applications
Recognizing RO's capital cost difficulties, membrane manufacturers have undertaken extensive R&D programs for more than 20 years to improve membrane performance. They looked to increase membrane salt rejection, stability and life while also enhancing flow per module. Today, reverse osmosis has gained popularity in sea water treatment applications worldwide, mostly in areas where brackish water is scarce (such as the Middle East, South East Asia and parts of the Mediterranean).Based on case studies of plants in these regions along with individual research, a methodology was developed for pricing an industrial sea water facility utilizing RO. For purposes of this study, Moch priced out capital and long-term costs for a hypothetical two mgd plant. This plant took in sea water with a salinity of roughly 35,000 ppm and desalinating that water down to an ultrapure standard of 10 ppm using a double-pass reverse osmosis system. The first pass, at 1000 psig or less, would have high recovery rates, resulting in a water quality of 250 to 350 ppm.
RO System Stages
Capital costs remain a prime disadvantage of reverse osmosis plants compared to brackish facilities. Extensive cost-saving methods must be undertaken in the planning stages of any new sea water facility. However, at the same time, a well thought-out design must emerge. The design must permit the facility to take advantage of all operating efficiencies.Proper pretreatment filtration devices and cutting-edge membrane technologies are two such items that will enable a new sea water plant to operate efficiently. Long-term costs will then be minimized and in many cases remain lower than most alternative systems. When studying the capital cost of an RO plant for sea water use, engineers and planners must review pre-treatment membrane and post-treatment considerations.
In Dr. Moch's hypothetical plant, during the first stage of treatment raw water is filtered to prevent fouling of the membranes and then mixed with sulfuric acid to lower the pH and prevent any plating of carbonate compounds and salts on the membranes. Although some pre-treatment technologies are rather expensive (particularly some chemical processing), research has found that proper pretreatment is critical to reducing a long-term cost. Bacteria also must be removed, most likely through a separate RO membrane. Current treatment methods eliminate well over 99 percent of bacteria at a relatively low cost. At this point most bacteria and large particulate matter has been eliminated and the feed water is ready for further processing.
Passing feed water through the actual RO membrane is the second stage of treatment. This process is by far the most expensive operation in an RO sea water plant, both in capital and long-term costs. The feed water would be introduced under pressure into pressure vessels containing the membranes. These membranes provide separation of selected monovalent and divalent ions. Factoring in an average life-span of three to seven years (based on the amount of usage and the quality of the feed water), the only long-term cost will be changeout of the RO element. With the proper operation, replacement of the pressure vessels in which the element is contained will be unnecessary. Reducing or eliminating the need to replace these expensive vessels decreases cost considerably.
Post-treatment, the third and final stage, is almost as important as pre-treatment from a quality standpoint, but fewer opportunities exist to trim costs. In most instances, and in Moch's example, the pH of the water will need to be restored, and depending on end use, the water may need to be chlorinated. Dissolved gases (CO2 and H2S) would be removed from the permeate water by a degassifier. Off-gas can be vented or introduced into a scrubber for removal of regulated constituents, such as hydrogen sulfide. The cost of a degassifying system depends on the extent to which it is needed. For industrial applications using sea water, degassifying should not be a very involved procedure and therefore should not present major expense.
In order to test the end water's final salt content, some treatment facilities use conductivity as a test. This process remains the most inexpensive way to test the quality of the end product. In general, post-treatment operations are inexpensive compared to the first two operations in the purification series.
Transition to Sea Water
Due to high costs for sea water plants compared to brackish water plants, the market for sea water facilities in the United States has been small, limited to areas where high-quality water was in extremely short supply. Soon however, Moch believes that many areas in the United States will begin to face brackish water shortages. U.S. water engineers will need to learn how to minimize the capital and operational costs associated with RO sea water treatment so that Americans can continue to enjoy a steady supply of inexpensive water."The bottom line is this," Moch said. "Soon sea water will be used as the raw material for a significant portion of the industrial water produced in this country. It's a question of when, not if."
