Water may be generally classified as "soft" or "hard" water. Hard water is characterized by the elevated concentration of polyvalent metal ions (cations). Soft water is characterized by a low concentration of these metal ions. The most common hard water metal ions are those of calcium and magnesium, which are divalent metal ions expressed as: Ca+2 and Mg+2 respectively.
While there are no hard and fast definitions of what soft water and hard water entail, we generally define soft water as having less than 75 mg/L calcium carbonate (CaCO3) and hard water as having greater than 150 mg/L calcium carbonate (CaCO3)(Sawyer and McCarty, 1967).
Typically, groundwaters are harder than surface waters, due to water dissolving and then carrying calcium and magnesium from the surrounding rocks.
Hard water, when it dries, creates water spots (white scale) on vehicle windows and finishes, household windows, shower doors, tile, etc. It also creates scale on the insides of water distribution pipes, boilers, and water heaters. Elevated water temperatures create the scale much faster than cold water temperatures, thus creating a problem for water heaters or boilers. The scale creates a barrier to the efficient transfer of heat, which requires a greater quantity of fuel to heat to the same temperature. Premature failure of boilers and hot water heaters is most often contributed to this scaling.
There are three basic treatment processes to treat hard water:
1) Ion Exchange: this process involves the use of a high ion exchange resin. An example of this resin is a polystyrene type. The resin is specifically designed to hold sodium ions on its ion exchange sites. As the hard water passes through the beads of resin, the calcium and magnesium ions replace the sodium ions that are attached to the resin. This process removes the calcium and magnesium from water being treated and releases the sodium to the treated water. The sodium does not form this scale, nor cause water spots. At some point in time, all of the sodium has been replaced with calcium magnesium and the resin can therefore no longer remove it from the raw water. The resin is regenerated by stopping the flow water through the resin bed, and then backwashing the resin bed with a highly concentrated solution of rock salt (a sodium chloride brine). This concentrated brine solution removes the calcium and magnesium ions from resin and replaces it with sodium ions. The resin bed is then rinsed gently with soft water, and made available for use. In many communities utilizing groundwater sources, the well sites possess ion exchange treatment units to soften groundwater prior to the addition of a disinfectant or fluoridation. The ion exchange process is more cost-effective when treating groundwaters, as they are typically a non-carbonate form of water hardness. That is, the hardness is specifically a predominance of the calcium and magnesium ions by themselves, not attached to their anionic components of carbonates, etc. Surface waters are typically the carbonate form of water hardness, which is more cost effectively treated by utilizing a lime or lime soda ash treatment of process.
Advantages & Disadvantages
a) An advantage of the ion exchange treatment process is that it does not change the pH or alkalinity of the water. Other advantages include excellent process reliability, process stability, and chemical safety.
b) Disadvantages of the ion exchange process include:
1) First is the increase of total dissolved solids (TDS) in the treated water due to the release of sodium.
2) Second is the cost of disposing of the regeneration cycles salt brine. This regeneration backwash water contains sodium chloride, calcium chloride, and magnesium chloride in a concentrated solution that ranges from 30,000 to 50,000 mg/L TDS. The quantity water may vary from 1.7 % to 7.5% of the softened water total depending upon the raw water source, the type of hardness being removed, and the quantity of water being treated. Proper disposal of this regeneration backwash water is obviously difficult due to these parameters.
3) Depending on the pH of the treated water, most waters treated by the ion exchange process, are corrosive waters due to loss of the calcium and magnesium. Blended water or the addition of stabilizing chemicals will correct this.
4) Resin problems:
a) Iron in the ferrous state must not be allowed to enter the ion exchange process, as it will oxidize to the iron oxide state on the resin and become a permanent resident on the resin. If the iron oxide state is achieved prior to entering the ion exchange process, it will be removed from the process water during treatment and is able to be removed from the resin during the normal backwash cycle. Best practice is to remove all iron prior to the ion exchange process.
b) Modern ion exchange resins are very resilient, with the life expectancy in excess of 15 years, when the process is properly operated. Excessive chlorine residuals will break down the resins, and must therefore not be applied to the resin beds. Surface waters, with accompanying biological growths, higher turbidities, and color values must be treated prior to the ion exchange process in order to prevent these materials coating the resin beads and interfering with the softening process.
2) Membrane Filtration: this is a physical process whereby either reverse osmosis or nanofiltration are utilized to physically remove the calcium and magnesium ions from the raw water source. The type of membrane will determine the degree of treatment.
Advantages and Disadvantages
c) Advantages include: hardness removal without large quantities of chemicals involved such as lime and sodium chloride (rock salt), simplicity of operation, increased operator safety due to lack of potentially hazardous chemicals.
d) Disadvantages include: high cost of membranes, (which are experiencing a trend of decreasing cost over the past several years), proper disposal of concentrated rejection water, and a potential requirement for pre-treatment of surface waters prior to membrane filtration.
3) Chemical Precipitation: this process is characteristically both chemical and physical in nature. This type of softening process does not completely remove all hardness from the treated water (such as an ion exchange process), and therefore requires less of a requirement for a downstream water stabilization process. Approximately 45 to 90 mg/L of hardness, expressed as calcium carbonate (CaCO3 ), will exist in the treated water which will provide an adequate corrosion protection value for the water distribution system and consumer plumbing fixtures. This may be accomplished by either of two manners:
a) Sodium hydroxide (NaOH)(caustic soda) may be utilized for chemical precipitation. Advantages: produces less sludge than lime, or lime-soda ash processes. Disadvantage: higher total chemical cost.
b) Utilizing lime, or lime and soda-ash. This is the preferred method of most facilities, and therefore the one we will detail.
Lime-soda ash Treatment Process
The raw water is brought to a rapid mixer where calcium hydroxide Ca(OH)2 is added to it. In the majority of cases soda ash is also added to it, for what is termed a "lime-soda ash" softening process. (On a side stream, dry lime (CaO) is "slaked" by adding water to it to create the Ca(OH)2.) This slurry is added to the water being treated to increase the pH to 10 for calcium removal or to a pH of 11 for magnesium removal. The addition of the lime and soda ash to the hard water creates a precipitate consisting of calcium carbonate and magnesium hydroxide. The water then flows into flocculation basins for a detention time for approximately 15 to 20 minutes. The lime utilized in the slaking process generates approximately 8 to 10% silica grit by weight. Much of this grit is removed in the lime slaker, but a fair quantity usually finds its way into the rapid mix and flocculation basins. The grit must be removed periodically from the flocculation basins to minimize damage to the flocculation equipment. The water then flows into sedimentation basins specifically designed for capture of the calcium and magnesium precipitates. Up-flow or solids contact clarifiers are usually utilized for combining the flocculation and sedimentation process into one unit. We discussed these clarifiers in the sedimentation chapter previously. Solids contact clarifiers are especially suited for this type of treatment. Rectangular sedimentation basins are usually avoided due to the excessive wear that can occur on the chains and flights due to the abrasive qualities of the grit and precipitates.
After the sedimentation process, the pH of the water must be properly adjusted prior to further treatment. This is usually accomplished by the addition of an acid or carbon dioxide (CO2 ). "Recarbonation" is the term utilized to describe the addition of carbon dioxide to the water being treated. For smaller facilities, carbon dioxide is usually purchased and delivered in liquid or gas cylinders, or in dry ice containers. Larger facilities have found it more cost-effective to produce carbon dioxide onsite in submerged combustion burners where natural gas is burned to create the carbon dioxide or by utilizing cleaned and scrubbed exhaust gases from furnaces, lime regeneration or carbon regeneration equipment process units. Generally speaking re-carbonation by itself can only reduce the pH to approximately 8.3. Additional reductions in the pH value will require the addition of an acid.
Advantages and Disadvantages
Advantages of this type of process include: the ability to soften water yet maintain an adequate water stability for corrosion protection; and its cost-effectiveness in treating large quantities of surface water.
Disadvantages include: proper disposal of the large quantity of high pH sludge; constant removal of the calcium carbonate scale on slaking equipment, rapid mixers, and flocculation basin equipment; safety issues regarding dosing sodium hydroxide or soda ash, and the slaking and feeding of lime. Extreme care must be taken when working around dry lime (CaO). The addition of water generates a lot of heat. One must never apply water from a hose directly to a dry lime spill or accumulation in a slaker, as there exists the potential of explosion resulting from escaping gases in heat from the interior of the lime pile.
Operational & Design
Impacts of calcium carbonate scale and particles on downstream piping and filtration processes are as follows:
a) Slaked lime creates calcium carbonate scale on anything it comes in contact with. This includes pump impeller's, pipes, pipe fittings, and all assorted equipment and containment vessels. The best installations utilize rubber hoses, with a minimal number of fittings, to convey the lime slurry. This is to reduce the number of man hours required to remove calcium carbonate from quick couplings, etc. The constant flexing of the rubber hose reduces scale buildup. It is highly recommended that the lime slaker be located within the shortest distance possible to the rapid mixer to minimize the length of rubber hose/couplings used in conveying the slaked lime. Sample lines and the sludge piping must be cleaned on a routine basis. Scale must be removed periodically to maintain pipe flows. This may be accomplished by the use of high-pressure water blaster's (up to 10,000 psi, although experience has shown that a cost effective range may be in the order of 4,000 psi), high concentrations of chlorine, and alum solutions as appropriate.
b) Provisions must be made for the removal of grit from the flocculation basins on a routine basis. The frequency will be determined by the quantity of grit in the dry lime, the lime slaker's grit capture rate, and the number of pounds of lime fed him each day.
In summary, some facilities utilize both lime and soda ash in combination to better control the process and chemical costs. Generally speaking, chemical precipitation is utilized only when large quantities of water need to be softened. Obviously, chemical precipitation is followed by a stabilization process be designed and operated to control the pH and alkalinity of the processed water.