7.3. Source Dependant Treatment Process Selection

The basis for selecting treatment process alternatives is established by the characteristics of the raw water and finished water quality goals. Consideration must be given to future implementation of more stringent drinking water quality standards and to possible changes and variability in the raw water quality. Thus the goals and objectives, as well as the restrictions and constraints defined in the preceding section, all bear upon the selection of alternative processes. Furthermore, the availability of major equipment, postinstallation services, and the capability of the operators and maintanence personnel, as well as the waste handling requirements, and the availability and cost of water treatment chemicals, all greatly affect the selection of the water treatment process, especially in remote regions and developing countries (Kawamura, 2000). The selection and design of the water treatment processes to be used at a particular facility are dictated by practicability, reliability, flexibility, and overall economics. Engineers experienced in water treatment plant design are needed to determine the best treatment system for any particular situation, and their advice should be obtained in early stages of project planning (Technical Manual, 1985). The interface between ground water and surface water is an areally restricted, but particularly sensitive and critical niche in the total environment. At this interface, ground water that has been affected by environmental conditions on the terrestrial landscape interacts with surface water that has been affected by environmental conditions upstream. Furthermore, the chemical reactions that take place where chemically distinct surface water meets chemically distinct ground water in the hyporheic zone may result in a biogeochemical environment that in some cases could be used as an indicator of changes in either terrestrial or aquatic ecosystems. The ability to understand this interface is challenging because it requires the focusing of many different scientific and technical disciplines at the same, areally restricted locality. The benefit of this approach to studying the interface of ground water and surface water could be the identification of useful biological or chemical indicators of adverse or positive changes in larger terrestrial and aquatic ecosystems (Winter et al., 1998).Data on the surface water quality, taken over a sufficient period of time (5 to 10 years), should be both reviewed and evaluated to assess the physical, chemical, microbiological and radiological characteristics of raw water. A risk assessment must also be made in regard to possible contamination of the water supply by chemical spills or radioactive wastes. Moreover, the degree of present and future land development in the watershed must be studied (Kawamura, 2000). Raw water characteristics vary widely, the major differences being between surface and groundwater, hard and soft water, and river water compared to reservoir water. Therefore, groundwater systems are more prevalent than surface water, but more people drink water from surface water systems (Flynn, 2009). If groundwater is selected as the source of the process water, the same considerations associated with surface water apply. Groundwater as a raw water source necessitates additional studies, such as the geological conditions, water table, the drawdown of the water table as the result of pumping, problems associated with seawater intrusion, and the potential leaching of industrial wastes, domestic wastes, agricultural chemicals, and fertilizers into the groundwater (Kawamura, 2000).For the reasons just mentioned, data analysis is a very important aspect of evaluating the quality of a water source. Components in the raw water whose maximum concentration levels are limited by the Drinking Water Quality Standards must be analyzed and evaluated carefully. The treatability of the raw water may be evaluated through the use of bench-scale tests and a pilot study (Kawamura, 2000).If there is a an existing water treatment plant in the vicinity of the proposed plant site, the design engineers should consult the operational data of the existing plant because it will provide valuable information on the treatability of the raw water (Kawamura, 2000).

7.3.1. Surface Water Treatment

Surface water treatment can be accomplished by a variety of process trains, depending on source water quality. Some examples are given below, beginning with conventional treatment. All surface waters require disinfection, so regardless of the treatment train chosen to treat a surface water, that process train must include disinfection (Logsdon et al., 1999).Disinfection Only with No Filtration: The number of water systems for which treatment of surface water consists only of disinfection is a small fraction of the total systems using surface water and is likely to decrease as a result of population growth and increasing difficulty associated with watershed ownership or control. Nevertheless, some systems, including some very large ones, now use this approach to water treatment. Conventional Treatment: Disinfection is included in conventional treatment, with the point or points of addition of disinfectant varying at different treatment plants. A conventional treatment train is appropriate for source waters that are sometimes or always turbid, with turbidity exceeding 20 to 50 ntu for extended periods of time. A modern hypothetical conventional filtration plant (Figure 7.1) for treatment of the Ohio River (depending upon its location on the river) would need to treat water having turbidity ranging from as low as about 10 ntu to a high of over 1000 ntu during floods. Coagulant dosages might be as low as 10 mg/L to over 100 mg/L during floods. Depending on the coagulant of choice, addition of alkalinity might be needed at some times. Rapid mixing would be followed by flocculation. Sedimentation might be accomplished in conventional long rectangular basins, or in basins aided by tube or plate settlers. Filtration would probably involve use of dual media (anthracite over sand). With the present emphasis on lowering disinfection by-product formation, chlorination would probably take place after sedimentation or after filtration. Total organic carbon concentrations on the Ohio generally are not so high as to require extraordinary measures for control of TOC (Logsdon et al., 1999).

Figure 7.1. Conventional treatment, surface water.

Conventional Treatment with Pretreatment: Some surface waters carry loads of sediment so high that water treatment plants employ a presedimentation step prior to the conventional treatment train. Earlier in the twentieth century, plain sedimentation with no chemical addition was practiced to remove a portion of the suspended solids before conventional treatment. Now, it is common to add some polymer or coagulant to enhance the first sedimentation step and reduce the load on the remainder of the plant. Thus, while the conventional treatment train can treat a wide range of source waters, some may be so challenging that even conventional treatment requires a form of pretreatment. Predisinfection using chloramines or chlorine dioxide may be used at some plants to decrease the concentrations of bacteria in the source water (Logsdon et al., 1999). Processes for Source Waters of Very High Quality: For source waters having very low turbidities, low concentrations of TOC, and low concentrations of true color, some of the treatment steps employed in a conventional treatment plant may not be needed, or other filtration processes may be suitable. Treatment of very highquality source waters can be accomplished by filtration without prior clarification using diatomaceous earth filtration, slow sand filtration, or by direct filtration, which deletes the sedimentation step from the conventional treatment train. Figure 7.2 is a process schematic diagram for direct filtration with an alternative for in-line filtration, in which flocculation is omitted. For waters not likely to form high concentrations of DBPs upon chlorination, free chlorine is a probable disinfectant (Logsdon et al., 1999).

Figure 7.2. Direct and in-line filtration treatment, surface water.

Dissolved Air Flotation: For reservoirs and other surface waters with significant algal blooms, filtration processes lacking clarification can be quickly overwhelmed by filter-clogging algae. The processes suitable for low-turbidity source waters are not very successful when treatment of algal-laden water is necessary. The sedimentation basins employed in conventional treatment are not very successful for algae removal, though, because algae tend to float rather than to sink. The density of algae is close to that of water and when they produce oxygen, algae can create their own flotation devices. Therefore, a process that is better suited for algae removal is dissolved air flotation (DAF), in which the coagulated particulate matter, including algae if they are present, is floated to the top of a clarification tank. In DAF, the clarification process and the algae are working in the same direction. Like conventional treatment, DAF employs chemical feed, rapid mix, and flocculation, but then the DAF clarifier is substituted for the sedimentation basin. A DAF process scheme is shown in Figure 7.3.

Figure 7.3 Dissolved air flotation/filtration treatment, surface water.

Waters having high concentrations of algae may also have high concentrations of disinfection by-products (DBP) precursors, so predisinfection with free chlorine could lead to DBP compliance problems. Chlorination just before or after filtration and use of alternative disinfectants, such as chloramines, may need to be considered (Logsdon et al., 1999). Membrane Filtration: Membrane filtration covers a wide range of processes and can be used for various source water qualities, depending on the membrane process being used. Microfiltration, used for treatment of surface waters, can remove a wide range of particulate matter, including bacteria, protozoan cysts and oocysts, and particles that cause turbidity. Viruses, however, are so small that some tend to pass through the microfiltration membranes. Microfiltration is practical for application to a wider range of source water turbidities than slow sand filtration or diatomaceous earth (DE) filtration, but microfiltration can not handle the high turbidities that are encountered in many conventional treatment plants. Microfiltration does not remove dissolved substances, so the disinfection process appropriate for water treated by this process will depend on the dissolved organic carbon (DOC) and precursor content of the source water.Advantages for membrane filtration include very high removal of Giardia cysts and Cryptosporidium oocysts, ease of automation, small footprint for a membrane plant, and the feasibility of installing capacity in small increments in a modular fashion rather than all at once in a major expansion, so that capital expenditures can be spread out over time. A microfiltration process train is shown in Figure 7.4 (Logsdon et al., 1999).

Figure 7.4. Microfiltration treatment, surface water.

7.3.2. Groundwater Treatment

Many groundwaters obtained from deep wells have very high quality with respect to turbidity and microbiological contaminants. If they do not have mineral constituents requiring treatment, they may be suitable for consumption with disinfection as the only treatment. The minerals in groundwater in many cases result in the need or the desire for additional treatment (Logsdon et al., 1999). Disinfection Only, or No Treatment: Some groundwaters meet microbiological qualitystandards and have a mineral content such that disinfection may be the only required treatment, and in some states disinfection may not be required. This may change when the Groundwater Rule is promulgated by USEPA. Circumstances favoring this situation are that the aquifer has no direct connection to surface water and the well has been properly constructed so the aquifer cannot be contaminated at the well site. For groundwaters of high quality, the most commonly used disinfectant is free chlorine (Logsdon et al., 1999). Removal of Iron or Manganese, or Both, Plus Disinfection: If the minerals in the aquifer include iron or manganese, these inorganic constituents may be found in groundwater. For removal of iron and manganese, oxidation, precipitation, and filtration are commonly employed. Figure 7.5 shows processes for iron and manganese removal. Presence of organics in the source water can impair removal of iron and manganese by oxidation and filtration. Iron can be oxidized in many instances by aeration. Treatment at a pH of 8 or higher promotes a more rapid oxidation of iron by aeration, if natural organic matter (NOM) is not present in significant concentrations. Chlorine, potassium permanganate, chlorine dioxide, or ozone can be used to oxidize iron and manganese. Potassium permanganate is commonly used for manganese, which is more difficult to oxidize than iron. Greensand has been used in conjunction with potassium permanganate for iron and manganese removal in numerous treatment plants, especially for small- or medium-sized systems. Greensand can adsorb excess permanganate when it is overfed and later remove iron and manganese when permanganate is underfed, allowing operators to attain effective treatment without continuously matching the permanganate dosage to the iron and manganese content of the raw water. When chemical oxidants are used rather than aeration, pressure filters are sometimes used to accomplish iron or manganese removal without the need for repumping following treatment (Logsdon et al., 1999).

Figure 7.5. Iron and manganese treatment, groundwater.

Precipitative Lime Softening: Hard water contains excessive concentrations of calcium and magnesium. Both groundwater and surface water can be treated by precipitative lime softening to remove hardness. Treatment involves adding slaked lime or hydrated lime to water to raise the pH sufficiently to precipitate calcium or stil higher to remove magnesium. If noncarbonate hardness is present, addition of soda ash may also be required for precipitation of calcium and magnesium. In precipitative lime softening the calcium carbonate and magnesium hydroxide precipitates are removed in a settling basin before the water is filtered. At softening plants that employ separate rapid mix, flocculation, and sedimentation processes, recirculating some of the lime sludge to the rapid mix step improves CaCO₃ precipitation and agglomeration of precipitated particles. Solids contact clarifiers combine the rapid mix, flocculation, and sedimentation steps in a single-process basin and generally are designed for higher rates of treatment than the long, rectangular settling basins. A two-stage softening process is shown in Figure 7.6. Solids contact clarifiers are an attractive alternative, especially for groundwater, because of the possibilities of lower capital cost and smaller space requirements, and are used more often than separate flocculation and sedimentation units (Logsdon et al., 1999).Use of solids contact clarifiers may reduce problems related to deposition of precipitates and scaling in channels and pipes connecting unit processes. When magnesium is removed, settled water has a high pH (10.6 to 11.0) and the pH must be reduced. Typically, this is accomplished by recarbonation (i.e., addition of carbon dioxide). Solids formed as a result of recarbonation can be removed by secondary mixing, flocculation, and sedimentation facilities. At some softening plants, carbon dioxide is added after the secondary settling to bring about further pH reduction and to stabilize the water.Although two-stage recarbonation is more effective in optimizing hardness removal and controlling the stability of the softened water, a less expensive single stage recarbonation process is sometimes used in excess lime treatment. Aeration sometimes is used before lime softening to remove carbon dioxide from groundwater, because lime reacts with carbon dioxide. The decision of whether to use aeration or simply to use more lime for carbon dioxide treatment can be aided by conducting an economic analysis of the cost of aeration versus the costs of the extra lime and the extra sludge produced (Logsdon et al., 1999).

Figure 7.6. Two-stage excess lime softening treatment, groundwater.

Ion Exchange Processes: The most common ion exchange softening resin is a sodium cation exchange (zeolite) resin that exchanges sodium for divalent ions, including calcium, magnesium, and radium. When radium is present along with calcium or magnesium or both Ca and Mn, the hardness removal capacity of the resin is exhausted before the capacity for radium removal is reached, so hardness breaks through first. After the resin has reached its capacity for hardness removal, it is backwashed, regenerated with a sodium chloride solution, and rinsed with finished water. The regeneration step returns the resin to its sodium form so it can be used again for softening. A portion of the source water is typically bypassed around the softening vessel and blended with the softened water. This provides calcium ions to help stabilize the finished water (Logsdon et al., 1999).Anion exchange resins are used in water treatment with equipment similar to that used for water softening with cation exchange resins. Anions such as nitrates and sulfates, along with other compounds, are removed with this process. Ion exchange processes can be used for water softening and, in some instances, are used for removal of regulated contaminants such as nitrate or radium. Ion Exchange is appropriate for water low in particulate matter, organics, iron, and manganese (Logsdon et al., 1999).Pretreatment to remove iron and manganese should precede ion exchange if those inorganics are present. High concentrations of NOM can foul some ion exchange resins. Ion exchange, which is generally used in smaller plants, offers advantages over lime softening for water with varying hardness concentration and high noncarbonate hardness. Figure 7.7 is an ion exchange plant process diagram (Logsdon et al., 1999).

Figure 7.7. Ion exchange softening, groundwater.