Water softeners are commonly used to produce treated water for use as makeup to boilers and cooling towers. Softeners are also commonly employed in residential and commercial applications to improve water quality by removing calcium and magnesium hardness. Regardless of the application, the salt brine used to regenerate water softeners ultimately is discharged into the environment. This places increased pressure on fresh water quality, which is a concern for environmentalists and sustainability advocates.
Salt is the most abundant, non-metallic natural mineral. US salt reserves are estimated at 55 trillion metric tons. Plus the oceans contain 2.7% salt by weight. At the current world consumption rate of 240 million tons per year, the existing US dry salt reserves are enough to supply our needs for over 100,000 years. Unlike precious metals or oil, the world has an inexhaustible supply of salt.
In 2009, the U.S. salt production equaled 27,329,000 tons with a market value of just under $2 billion. Of this, 66% of the salt was used for highway deicing, 13% for water conditioning (softener regeneration), 8% for chemical processing and 6% for human consumption.
Notwithstanding the natural prevalence of huge salt deposits located throughout the US, China and the world, concerns have been raised by conservationists over the impact that the discharge of salt brine has on the environment. Some areas have detected a gradual increase in dissolved solids in fresh water supplies, aka: total dissolved solids (TDS) “creep,” which has been traced back to a corresponding increase in sodium chloride levels. Since TDS values greater than 1000 ppm compromise the water quality used in agriculture, aquaculture and potable water systems, regulators have been spurred to limit salt discharge into fresh water supplies and soils. In an effort to meet these restrictions, softener manufacturers and owner/operators have been forced to improve the brine regeneration efficiency in residential and industrial water softeners.
Three primary methods for improving softener regeneration efficiency are commonly adopted.
1. Optimize the salt dosage
2. Recover and reuse the brine
3. Zero discharge of salt brine
OPTIMIZE SALT DOSAGE
Water softeners operate in the service cycle until the exchange capacity of the resin has been depleted. At that time, the softener is removed from service for regeneration. This involves passing an 8% to 15% brine (sodium chloride) solution through the resin bed at a controlled rate. The strong salt solution displaces the calcium and magnesium hardness from the exchange sites and restores the resin back to the sodium form. Following a slow and fast rinse cycle, the softener is ready to be returned to service.
The salt dosage, also known as the regeneration level, determines the softening capacity of the resin expressed as Kilograins (1 Kilograin = 1000 grains) per cubic foot. The higher the salt dosage, the greater the exchange capacity. But this relationship is not linear. A 5 pound per cubic foot salt dosage produces a softening capacity of 17,800 grains per cubic foot. A 15 pound regeneration level establishes an exchange capacity of 29,300 grains per cubic foot. A 300% increase in salt dosage only produces a 65% increase in exchange capacity. In this way, higher salt dosages are less efficient than lower salt dosages.
A practical way to express the brine efficiency level is grains of exchange capacity per pound of salt. The theoretical maximum efficiency is 6,000 grains per pound of salt. A practical peak efficiency is 5,100 grains/lb. In general, a good target is 3,350 gr/lb. California, however, has established a more ambitious efficiency target of 4,000 gr/lb. In our example, the 5 pound regeneration level produces an efficiency of 3,560 grains per pound, which fails to meet the California target, but is acceptable overall. At 15 pounds regeneration level, however, the efficiency is reduced to 1,953 grains per pound, which is unacceptable by any of these standards.
Optimizing brine efficiency reduces salt costs and minimizes brine discharges to the environment. Using an example of an 80 cubic foot softener producing 120,000 gallons of soft water per day from a raw feedwater containing 18 grains per gallon hardness, we are able to compare the difference in salt consumption when the softener is operated in the low efficiency(15 lbs salt/cu. ft.) mode versus the high efficiency (5.3 lbs salt/cu. ft.) mode.
Because the high efficiency (lower salt dosage) regeneration produces a lower exchange capacity, the softener must be regenerated more frequently. In our example, the softener is regenerated 3 times every two days (540 regenerations per year) in the high efficiency mode versus 1 time per day (360 regenerations per year) in the low efficiency mode.
More frequent regeneration consumes more rinse water and produces more waste water notwithstanding that the amount of salt consumed is less. The following table indicates the difference in annual salt consumption.
|
Efficiency Mode |
Pounds of Salt per Year |
|
Low (15 lbs/cu.ft.) |
402,960 |
|
High (5.3 lbs/cu.ft.) |
232,140 |
|
Reduction in Salt |
170,820 |
Salt prices vary depending on volume purchased, location and market conditions. Assuming a price of $0.033 per pound when purchased in bulk (25 ton) quantities, the annual cost savings is $5,637.
This may seem like a significant savings, but because the softener must be regenerated more frequently when operated in the high efficiency mode, we must factor in the added expense of labor, waste disposal, and opportunity costs. In many cases, these other costs tend to offset the savings in salt.
Because salt costs are so low as compared to other operating and opportunity costs, reducing salt consumption does not necessarily reduce the overall cost for producing soft water. Although better for the environment, operating at higher salt efficiencies does not guarantee a reduction in overall soft water cost.
BRINE RECOVERY AND REUSE
Various control systems and retrofits are commercially available for reclaiming a portion of the salt brine for recycle back to the softener. These systems are effective only on high salt dose softeners where the sodium chloride level at the drain is high enough for reuse.
During the typical regeneration cycle, a 10% brine solution (30% salometer) passes through the softener and is discharged to drain. This process displaces calcium and magnesium from the resin and thereby restores the exchange sites to the sodium (Na) form. The brine strength is about 3 times the stoichiometric amount required to elute the hardness from the resin. This is done to insure a complete regeneration with minimal hardness leakage.
Collecting samples at the drain during the regeneration cycle and testing for percent salt saturation yields an informative elution curve. Here we see that the waste water in the “middle” of the regeneration cycle has a percent saturation greater than 30%, which, in theory, contains enough salt for the start of the next softener regeneration. However, the brine solution also contains a high concentration of calcium and magnesium ions that have been displaced from the resin. Therefore, the waste brine must be analyzed to determine the portion that has a high enough ratio of sodium to calcium. The curve also indicates that the slow rinse cycle does not contain enough sodium to be effective as a regenerate.
Tests performed to determine the efficacy of brine reclaim systems indicate that the presence of calcium and magnesium in the used brine discounts the benefit of using reclaimed brine. The overall impact is that the capacity and efficiency of using reclaimed brine is the same or slightly less than the results obtained with a standard brine system.
Some operators claim good success with brine reclaim systems, but there are conflicting opinions regarding the overall benefits of this approach. In any case, low salt costs make the retrofit of existing brine systems unattractive for those seeking a favorable return on investment through brine reclaim.
ZERO DISCHARGE BRINE SYSTEMS
In cases where salt discharge is restricted, it is technically possible to design a zero discharge brine recovery system. The process involves removing impurities from the waste brine making it acceptable for recycle and reuse.
At the completion of the service cycle, the softener is placed into the regeneration mode. After passing through the resin bed, the brine solution is collected for purification. As mentioned previously, used brine consists of a solution of sodium chloride along with calcium and magnesium salts that have been displaced from the resin. The hardness salts are sparingly soluble at high pH and can be precipitated from solution by the addition of caustic soda (NaOH). The hardness salts are then removed by settling and filtration. The excess caustic soda in the clarified brine is next neutralized with hydrochloric acid (HCl) to produce the neutral salt NaCl. The reclaimed salt solution is now of acceptable purity for recycle back to the softeners. The precipitated impurities are hauled off as a solid waste.
Because of the low salt costs, this design approach offers little to no dollar and cents savings and is generally applicable to zero discharge plants as mandated by law.
Another option for zero discharge plants is to contract with a service provider to furnish fully regenerated portable exchange softener beds. These vessels are placed into service until they exhaust at which time they are exchanged for another set of freshly regenerated units. The softener regenerations take place off-site at the service provider’s location. Technically, this approach does not reduce or eliminate salt discharge into the environment, since the regeneration still takes place, but at a remote location. However, for site-specific reasons, portable exchange softeners can satisfy a short or long-term soft water demand.
SUMMARY AND CONCLUSIONS
In general, most softeners are operated at a high regeneration level for three practical reasons: (1) softener requires fewer regenerations (2) effluent has less hardness leakage and (3) salt is cheap. This practice, however, results in poor regeneration efficiency and maximum salt discharge into the environment.
The softener regeneration efficiency can be improved by making retrofits to the brine system. This includes the following strategies:
1. Reduce the salt dosage to achieve at least 3,350 grains per pound of salt and regenerate the softener more frequently.
2. Reclaim a portion of the used salt brine for recycle back to the softener during the next regeneration. This is only applicable to softeners that must be operated in the high salt dosage mode.
3. Capture and purify the used salt brine by chemical precipitation, settling, filtration and neutralization to achieve zero discharge. Alternatively, contract with a service provider for portable exchange softener units.
In all cases, because of the inherent low cost of salt, adopting these conservation strategies will not necessarily lower the overall cost for producing soft water. But you will be doing your part to reduce salt discharge into the environment and thereby enhance and maintain the quality of our freshwater reservoirs.
Water Technology Report 