|Figure 1: Typical lime-soda softening process.|
Precipitative softening uses chemical precipitation to reduce hardness in source waters and improve clarification prior to filtration. A typical precipitative softening process includes rapid mix, flocculation, sedimentation, recarbonation, and filtration. In some cases, pretreatment by aeration and presedimentation may also be practiced. Aeration is used to remove carbon dioxide from groundwater with high carbon dioxide concentrations. Presedimentation is used to treat source water of high turbidity to provide a more uniform water quality.
The primary cause of hardness is the presence of multivalent ions, such as calcium (Ca2+) and magnesium (Mg2+). These ions or minerals can cause scaling of pipes and equipment in drinking water and process water systems. Precipitation is achieved by raising the pH of water and provoking the precipitation of calcium carbonate (CaCO3) and magnesium hydroxide Mg(OH)2. Precipitates are removed by means of conventional processes such as coagulation-flocculation, sedimentation, and filtration. After precipitation, the water is recarbonated to lower the pH in order to reduce scale formation, typically near pH 8.4. In addition to the removal of hardness, precipitative softening can be used for the removal of arsenic, radionuclides, dissolved organics (including disinfection byproduct precursors), color, and microbial contaminants.
The most important parameters controlling the removal of precipitative softening is pH. For calcium carbonate precipitation the pH is raised to approximately 10 and for magnesium hydroxide precipitation the pH is raised above 11. The removal of other substances is also dependent of pH. Arsenic removal is greatly increased at pH greater than 10.5. As pH increases, more total organic carbon (TOC) and color are removed. Removal of radionuclides also improves as pH increases.
The three most common precipitative softening alternatives include lime softening, lime-soda ash softening, and caustic softening. The selection of one of these alternatives is based on cost, water quality and owner and operator preferences. Lime softening is typically used for water containing low concentrations of non-carbonate hardness. Lime-soda softening may be required with high concentrations of non-carbonate hardness. Caustic soda softening is typically used when the treated water has inadequate carbonate hardness to react with lime. Softening by lime and lime-soda ash is generally less expensive then by caustic soda. However, caustic softening produces less sludge and unlike lime does not deteriorate during storage. Figure 1 shows lime-soda softening.
Softening may be two stage wherein excess lime is added to the first stage to pH 11 for magnesium control, followed by recarbonation to near pH 10 in the second stage for calcium control where soda ash may or may not be added, followed by final recarbonation.
Softening may be split wherein one stream is softened and another is conventionally treated. These waters are then blended to achieve target final hardness.
Because powdered activated carbon may be added, or chlorine or other oxidants may be present, softening is oftentimes concurrent with other processes.
Two parameters frequently used to describe the clarification process are the overflow rate and the detention time. The overflow rate is the process loading rate and is usually expressed in gpm/sf or gpd/sf. Overflow rates for conventional sedimentation generally range from 0.3 to 1 gpm/sf (500 to 1500 gpd/sf). Overflow rates for other processes can vary significantly. There are proprietary sand-ballasted clarification systems that have been demonstrated to operate effectively at overflow rates as high as 20 gpm/sf. Typical detention times range from 1 to 2 hours, although many states require up to 4 hours for full-scale surface water treatment.
The most commonly used filter type in softening process is a dual-media filter comprised of anthracite and sand; however, mono-media (sand), multi-media (garnet, anthracite, and sand), and other media configurations - including the use of granular activated carbon - are also used in drinking water treatment. During filtration, the majority of suspended particles are removed in the top portion of the filter media. Filters are backwashed to dislodge and remove particles trapped within the filter bed, to reduce head loss, and to keep the filter media clean.
The filter loading rate is a measure of the filter production per unit area and is typically expressed in gpm/sf. Typical filter loading rates range from 2 to 4 gpm/sf; however, higher filter loading rates, 4 to 6 gpm/sf, are becoming more common at full-scale. This can be a critical parameter because it determines the water velocity through the filter bed and can impact the depth to which particles pass through the media. The filter run time describes the length of time between filter backwashes during which a filter is in production mode. The filter run time is not only an indicator of the effectiveness of prior treatment (i.e., the ability of the coagulation and clarification steps to remove suspended solids), but also plays a role in the effectiveness of the filter itself. Filter performance, particularly with regard to particulate contaminants, is often poorest immediately following a backwash. As the filter run time increases and the concentration of solids in the media increases, the filtration process often performs better with regard to particulate contaminant removal.
Softening large quantities of sludge, and disposal can be very expensive. The amount of sludge produced depends on the water's hardness. Depending on the hardness of water, the average water treatment plant produces 1,000 to 8,000 pounds of solids per million gallons of water treated. Lime sludge is frequently recycled to the clarification process to improve precipitation, reduce chemical usage, and improve process performance. Sludge generated by softening can also be disposed by discharge to a sanitary sewer, drying lagoons, and land application. If the sludge contains high concentrations of metals or toxic substances it may be required to be disposed in a hazardous waste landfill.