Water is a universal solvent. Soft rainwater picks up naturally occurring dissolved minerals, including calcium and magnesium carbonates, as it passes over rocks and through soil, which begins to make it hard. Most source water contains some amount of hardness. Water softening is often required to make source water suitable for use in manufacturing processes, boilers, cooling towers, and rinse water applications.
The primary purpose of hard water softening is to prevent the precipitation and buildup of hard water minerals in equipment and piping. Reduction or elimination of hard water scaling can be performed using physical treatment equipment, or, in limited circumstances, through the use of chemical additives.
Definition and Source of Hardness
Limestone dissolved in water is termed "hardness." Ground water dissolves limestone from deposits formed eons ago through the following steps: a) carbon dioxide reacts with water to produce carbonic acid which in the environment exists primarily as the bicarbonate ion (HCO3-1); b) microscopic marine organisms consume this as carbonate and form calcite skeletons which were deposited over millions of years to form limestone deposits which are found extensively in many parts of the world; c) ground water is often slightly to moderately acidic due to anaerobic decomposition and action of bacteria in the soil. This acidic characteristic causes limestone to be dissolved, in the form of calcium and bicarbonate ions, thus becoming hard.
Problems Associated with Hard Water
Calcium carbonate is moderately soluble in water and will come out of solution (i.e., form a precipitate) in the form of a hard scale when its concentration in water exceeds its solubility constant. This tendency may cause build-up in hot and cold water pipes, water heaters, boiler tubes, cooling towers and any other surfaces it contacts. It also reacts with soap and detergent forming a precipitate in the form of a "scum" which is evident as spotting on glasses and silverware and as "bathtub ring." The buildup in boilers can interfere with the transfer of heat and can even lead to boiler tube failure.
Types of Water Hardness
This refers to hardness whose effects can be removed by boiling the water in an open container. Such waters have usually percolated though limestone formations and contain bicarbonate HCO3– along with small amounts of carbonate CO32– as the principal negative ions. Boiling the water promotes the reaction by driving off the carbon dioxide gas.
2 HCO3– → CO32– + CO2
The CO32– reacts with Ca+2 or Mg+2 ions, to form insoluble calcium and magnesium carbonates which precipitate out. By tying up the metal ions in this way, the amounts available to form soap scum are greatly reduced. While boiling is not an economical way to soften water, the chemistry demonstrated by boiling defines the difference in temporary and permanent hardness.
Waters that contain other anions such as chloride or sulfate cannot be softened by boiling, and are said to be "permanently" hard. These must be softened by other methods, including lime soda or, more frequently, ion exchange softening described below.
Conventional Water Softening
Conventional water softening is most often based a process known as ion exchange, utilizing a synthetic polymeric (plastic) material in the form of very small beads called ion exchange resin. The resin is porous so that each bead has tremendous surface area and the surface area is chemically constructed to contain billions of active or "exchange sites." These sites have considerable affinity for metals in the water with valences (i.e., charges of +2 and +3). Thus, when water containing calcium, magnesium (the two major constituents of hard water), dissolved iron, copper or aluminum, the active sites attract and "hold" these ions.
However, in order to do so, the sites must have a less tightly held ion to "exchange" for the metal hardness ions. While the resin does not prefer ions with a single charge, fortunately under conditions of high concentration and extended time of exposure, sodium ions can be "forced" onto the active sites by slowly passing a concentrated solution of sodium chloride (table salt) over the resin. Thus, a relatively inexpensive, readily available, safe chemical can be used to "regenerate" the resin. Ion exchange resins are also used to produce high purity Demineralized water but the resin and regeneration chemicals are different (see ion exchange).
Disadvantages include the need to dispose of waste water high in salinity and the fact that water treated with an ion exchange water softener has sodium added. According to the Water Quality Association (WQA), the ion exchange softening process adds sodium at the rate of about 8 mg/liter for each grain of hardness removed per gallon of water. A "grain" of hardness is equal to 17.1 mg/l or ppm of calcium carbonate.
A water softener typically consists of two tanks, a larger one into which rock or pellet salt is added and a smaller tank containing the ion exchange resin through which the hard water passes. A control valve fixed atop the resin tank of the industrial water softener causes the system to recharge or regenerate based on passage of a pre-set time or it meters the water treated and initiates regeneration based on a pre-set number of gallons treated.
When regeneration is initiated, the first step is to backwash the resin bed with raw water to fluff the resin and remove entrained dirt and sediment. Then brine is slowly deducted from the salt tank at a set flowrate for a specific time (set as a function of the resin volume and the influent water hardness), followed by a slow draw of raw water to slowly displace the brine solution. This is followed by a faster flow of raw water to thoroughly flush any remaining salt from the resin bed. During this entire process, the control valve either prevents raw water from flowing to service or allows raw water to by-pass the softener during regeneration. Often two parallel industrial water softeners are operated together, in order to avoid any interruption to soft water.
Disadvantages to conventional softening are consumption of copious volumes of salt and the discharge of highly saline waste water from the regeneration step. The WaterProfessionals® routinely work with clients to include partial brine reclaim systems to minimize this waste. They also evaluate the use of nanofiltration as an alternative approach, depending on the hardness and volume requirements for a specific application.