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Traditional Salt-Based Water Softening and Its Impact on the Environment
Why Use Water Softeners
A water-softening is designed to reduce dissolved minerals (primarily calcium, magnesium, some manganese, and metals such as ferrous iron ions) in hard water. These "hardness ions" can cause several types of problems, as follows:
- The hardness ions impede the cleaning effect of detergent and soap. In particular, the metal ions react with soaps and calcium-sensitive detergents, hindering their ability to foam and form a precipitate. With softened water, less soaps and detergents are needed for cleaning and laundry. Softened water also increases the lifespan of your fabric and textile through hundreds of wash cycles.
- The minerals of minerals and magnesium carbonates tend to attach and stick to the surfaces of pipes and water heats, resulting in scale buildup, which restricts water flow in pipes. In boilers, the mineral deposits act as an insulation that impairs heat transfer to water. The net result of such thick scale buildup is that more energy is required (resulting in energy wastage and inefficiency) to heat the water in a boiler. With softened water, one saves energy in water heating.
- The ions in the electrolyte can also lead to metal corrosion, causing corrosion in the plumbing system within the house or the commercial building.
The Chemical Process of Water Softening: Ion Exchange
Salt-based water softening works on the principle of ion exchange or ion replacement, a reversible process. (Ions are atoms or groups of atoms that can lose or gain electrons in water and therefore have an electrical charge.) Calcium (Ca2+) and magnesium (Mg2+) ions in the hard water are exchanged or replaced with sodium ions (Na+) (or infrequently, potassium K+ ions) during the reversible ion-replacement reactions, as sodium ions leave the ion-exchange polymeric resins (or beads, or a class of minerals called zeolites) and go into the water, while calcium and magnesium ions in the hard water are bound to the resins or beads. Essentially, the ions switch places during water softening: sodium from the resin matrix migrates into the water, and calcium and magnesium move from water to attach to the resin matrix. The resin or bead matrix is itself inert and does not participate in the chemical reaction, and its physical structure is not changed during water softening.
The entire ion exchange takes place in a chamber or tank. The displaced sodium (or potassium) ions pass downward through the resin "bed" and out the softener drain; thus, the softener delivers "soft" water which now has sodium instead of calcium or magnesium ions. In other words, as the water to be treated passes through a resin bed, negatively charged resins absorb and bind with hardness and/or metal ions, which are positively charged. Chemically, the resins initially contain univalent hydrogen (H+), sodium (Na+) or potassium (K+) ions, which then exchange with divalent calcium (Ca2+) and magnesium (Mg2+) ions in the water. As this is a replacement reaction, this ion exchange eliminates precipation and soap-scum formation; thus, when the raw water passes through the resins, the "hardness ions" replace the sodium (or potassium) ions, which are released into the water. The "harder" the raw water, the more sodium (or potassium) ions are released from the resins and into the "softened" water.
When all the sodium in the resins or beads matrix have been replaced and saturated with calcium and magnesium ions, then the resin in the exchange chamber is "full" and cannot perform any more water-softening reactions. It is now time for another process called regeneration (also called back-flushing). During regeneration, salt (sodium chloride) is added to the exchanger chamber; the high level of salt displaces the calcium and magnesium ions and replaces them with sodium. After regeneration, the resins or beads can soften more hard water.
However, water-softening polymeric resins do not last forever: Frequency of regeneration depends on the hardness of water, the amount of hard water treated or softened, the size of the water-softening system, and the capacity of resin to absorb hardness minerals. Over time, the resins' capacity to soften water by ion exchange decreases between regeneration, especially if the water also contains iron or other metals. Why? It is because the negatively charged anionic functional groups of polymeric resins bind with positively charged hardness and metal ions (such as positively charged sodium ion and positively charged iron and metal ions). The metal ions that bind to resins tend not to get flushed out during regeneration, unlike calcium and magnesium ions that do get flushed out during back-flushing. As a result, over time more positively charged metal ions irreversibly bind with negatively charged resins, and the resins' capacity to perform ion exchange decrease. In general, the polymeric resins in traditional salt-based water-softening systems have to be replaced every five to 10 years (depending on the hardness of water being treated and the other factors discussed previously) because the resins have been "spent" and cannot be regenerated any further. It is clear then that salt-based water softeners are not an ideal, permanent solution to the hard-water problem.
There is also another problem associated with scale buildup when using a traditional salt-based water-softener system: During regeneration, the calcium and magnesium ions which are discharged with the rest of the household sewage may precipiate out as hardness scale on the inside of the sewage and discharge pipe. As a result, the part of the household plumbing that connects with the municipal sewers may have scale buildup as well.
Lead and Other Metals in Your Softened Water
Softened water is not healthy to drink—not only for sodium-sensitive, hypertensive patients, but also for healthy people as well. For hypertensive people on a low-sodium diet, softened water contains sodium. But softened water may also contain metals (such as lead and copper), which have been leached from the metal pipes, faucets, and soldered joints of the pipes.
Why would softened water contain metals? Softened water may contain metals because water is a universal solvent. When water is heated or softened, it is more prone to leach metals from water pipes and other parts of the indoor plumbing system. Copper in pipes, lead in soldered joints, and metal faucets are especially vulnerable to heated or softened water. Thus, in addition to containing sodium, softened water may have lead, copper, and other metals.
The Environmental Impact of Sodium and Chloride in Water Softeners: What Happens When Backwash Water Reaches Sewage Treatment?
It is widely known that traditional salt-based water-softening systems are not environmentally friendly. The salt contributes to the salinity problem when discharged with other wastewater into the municipal sewage-treatment plant. In general, higher salinity in the wastewater increases the treatment costs and reduces the potential for reuse of treated wastewater for irrigation and industrial purposes.
There are several problems associated with brine and minerals discharge into the sewage-treatment system, as follows:
- Higher wastewater-treatment costs—It has been estimated by one municipal sewage-treatment plant that it costs about U.S.$0.20 to add a pound of salt (NaCl) to the water-softening system, but it would cost 25 times that amount, at U.S.$5.00, to remove that much chloride at the treatment plant. (A typical household uses up to 100 pounds of salt per month for water softening. Most chloride in the sewage treatment comes from residential water softeners, while a small amount comes from soaps, detergents, and other cleaning products—particularly laundry products.) One California sanitation district in Los Angeles County estimated that it would cost at least U.S.$300 million just to build an additional treatment facility to perform microfiltration and reverse osmosis to remove chloride from the treated wastewater, with approximately 50% of this infrastructural cost going to install a 46-mile pipe (called "brine line") on land to connect this city to the Pacific Ocean so as to transport that extra salt waste to the Pacific Ocean, and then to build another three-mile underwater pipe at the ocean. It is important to transport the extra salt not just to the beach of the Pacific Ocean, but to the outer ocean (at least three miles away from land), so that this additional salt will not affect the local marine life by increasing the salinity of seawater.
- High levels of dissolved sodium and other minerals in the treated wastewater—highly saline treated wastewater containing high levels of dissolved minerals (including the concentrations of calcium, magnesium, potassium, and anions of chloride and sulfate) will add to the total dissolved solids (TDS) of a treatment plant's discharged water. A high TDS will affect a sewage-treatment agency's ability to comply with state and federal discharge standards. Generally, wastewater-treatment plants remove very little of the dissolved salts and minerals because they are primarily designed to remove grit and detritus, grease/oil, and organic solids (e.g., food and human waste)—and not designed for chloride removal. For these other types of wastes, they can be removed by settling (in sedimentation basins) and biological degradation; but chloride cannot be removed using these treatment methods. Again, to remove the additional chloride at sewage-treatment plant via microfiltration and reverse osmosis, it would be prohibitively expensive for many towns and cities.
- Salt is a major pollutant—in many states (including California), salt is considered a pollutant when discharged into the environment. As we know, when hardness or salinity rises, soaps and detergents become less effective, home appliances and plumbing wear out faster, and water-heating systems become less energy efficient due to scaling. When discharged with treated wastewater into rivers and lakes, chloride (Cl-) can harm aquatic life and damage agricultural crops by causing leaf burn of drying of leaf tissue, thus reducing crop yields.
When excess chloride and sodium ions are discharged with treated wastewater, the reusability of this treated wastewater is reduced. As discussed previously, when treated effluent is discharged into surface waters (e.g., rivers), downstream farmers may fear the use of high-chloride river water for irrigation. Crops such as strawberries and avocados are salt-sensitive and chloride-sensitive.
Be Good to the Environment: Avoid Traditional Salt-Based Water Softeners!
Salt—both sodium ions and chloride ions—pose many serious environmental threats, from harming aquatic and marine life in the discharge areas (whether rivers, lakes, or oceans), to damaging the delicate biochemical balance in the soil (if treated effluent is reused as irrigation water). Removing salt in typical municipal wastewater-treatment plants is extremely expensive, as additional microfiltration and reverse osmosis facilities must be built to remove chloride from the treated wastewater. The best way to reduce the cost of municipal sewage treatment associated with excess salt discharge into the sewers is to minimize salt discharge into the sewers in the first place.
The best way to avoid discharging tons of salt into the sewers is to replace existing traditional salt-based water softeners with newer salt-free water softeners. APEC's FUTURA system uses catalytic-conversion media to neutralize calcium and magnesium and reduce scale buildup.
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