The movement of seawater into fresh water aquifers is severely affecting the quality of industrial and domestic water supplies and overtaxing treatment facilities. Companies and municipalities have proven that reverse osmosis (RO) membrane systems offer one of the most effective methods to battle salt water intrusion.
The rapid growth of seaboard cities and industries has altered the normal flow of fresh water to the sea. Their draw of water from coastal fresh water wells sometimes exceeds the ability of underlying aquifers to replenish the fresh water supply, allowing salt water to seep into fresh water sources (Figure 1). This "salt water intrusion" is contaminating the water supply of existing populations and industries, threatening their future growth.
Figure 1
RO technology offers immediate relief from and a long-term preventative solution for this type of groundwater contamination. Ingalls Shipbuilding, Pascagoula, Mississippi, U.S.A., uses RO to purify brackish well water, while the Orange County Water District in California, U.S.A., uses RO to halt intrusion of seawater into its fresh groundwater supply.
SALT WATER INTRUSION OVERBURDENS DI SYSTEM
Ingalls Shipbuilding, a long-time contractor for the U.S. Navy, services nuclear and conventional fleet ships. The company uses high purity water to maintain high-pressure boilers and electronic cooling systems aboard the naval vessels.
For decades, Ingalls had obtained high quality (low TDS) well water for their treatment processes prior to use in these critical applications. However, water quality shifted significantly during the mid-1970's as increased demand on shallow aquifers in the area created ideal conditions for saltwater intrusion.
Before 1977, Ingalls used weak acid, weak base ion exchange (IX) and carbon filtration to treat process feedwater. This treatment process worked satisfactorily until the quality of the water dramatically decreased. The chloride content in the feedwater rose from 200 mg/l (as ion) to more than 1200 mg/l (as ion) over a three-year period. This salinity increase is directly attributed to salt water intrusion as a result of excessive draw on the local aquifer.
High costs associated with the regeneration and maintenance of the weak anion/cation IX systems made treatment of this source infeasible, according to Louis Janus, manager of the chemistry lab at Ingalls' pure water facility. As shallow well water quality continued to deteriorate, the company began using water from deeper wells of 122 meters (400 ft.) and 144 meters (800 ft.). Although these wells were less affected by salt water intrusion, their water contained 800 mg/l TDS (as CaCO3) and a high total organic carbon (TOC) content. Activated carbon was replaced on a nearly continuous basis due to the high levels of tannins and other organic contaminants in the feedwater. Three operators worked full-time to maintain the water treatment system.
RO: A COST- EFFECTIVE SOLUTION
In late 1977, the company installed a water purification system designed and manufactured by Osmonics, Inc., Minnetonka, Minnesota, U.S.A. (Figure 2). An Osmo RO unit provides the foundation for the system, which produces 133 1pm (35 gpm) of 18 megohm-cm quality water. Instead of the three operators previously required, only one full-time operator is needed to run the system. The RO/DI system has produced more than 34,000 m3 per year (9 million gallons per year) of ultrapure water for nine years with no significant downtime.
Figure 2
The system has consistently net the U.S. Navy's high pressure boiler feedwater standards of less than 0.2 mg/l silica (from a feed of more than 30 mg/l silica) and 18 megohm-resistivity. The RO unit removes more than 95% of ionic elements and all organics of 200 molecular weight and greater.
The filtration efficiency of the RO machine has allowed Ingalls to maintain product quality with the original IX resin delivered with the system in 1977. It also has eliminated the need for activated carbon treatment. The Osmo-411 HR (CA) sepralators (membrane elements) run for over 18 months without cleaning. The ability of the Sepa cellulosic membrane to withstand continuous exposure to chlorine in the feedwater is essential to control the growth of bacteria during daily shutdowns of 8 to 16 hours.
Groundwater quality in coastal areas continues to deteriorate. By the year 2000, all groundwater in the Mississippi Gulf area is expected to be unsuitable for use without extensive treatment to reduce the high dissolved solids level induced by salt water intrusion. The Osmo RO system will continue to play an important role in the success of Ingalls' high purity operations.
A PREVENTATIVE APPROACH TO THE INTRUSION PROBLEM
Another example of RO employed to fight salt water intrusion can be found on the West Coast of the United States. The Orange County Water District in California has taken steps to stop inland advancement of subterrain salt water. The district operates Water Factory 21, a 19,000 m3d (5 mgd) RO facility that recovers purified water from secondary waste water effluent. RO permeate is deep-well injected near the Pacific seaboard and forms a pure water barrier between intruding salt water and fresh water supplies. This well-known municipal effort to protect existing groundwater from seawater contamination is one of several efforts of such scale in the world.
FUTURE OUTLOOK
The use of RO is expected to dramatically increase as coastal fresh groundwater supplies continue to be overdrawn. Municipalities and manufacturing facilities will increasingly rely on crossflow membrane technology to prevent salt-water intrusion and purify brackish water contaminated wells.
