The design of treatment facilities will be determined by feasibility studies, considering all engineering, economic, energy and environmental factors. All legitimate alternatives will be identified and evaluated by life cycle cost analyses. Additionally, energy use between candidate processes will be considered. For the purpose of energy consumption, only the energy purchased or procurred will be included in the usage evaluation. All treatment process systems will be compared with a basic treatment process system, which is that treatment process system accomplishing the required treatment at the lowest first cost (Technical Manual, 1985).
Contaminant removal is the principal purpose of treatment for many source waters, particularly surface waters. The quality of treated water must meet all current drinking water regulations (Pontius, 1998). Furthermore, to the extent that future regulations can be predicted by careful analysis of proposed drinking water regulations, water treatment processes should be selected to enable the water utility to be in compliance with those future regulations when they become effective (Logsdon et al., 1999).The U.S. Environmental Protection Agency (EPA) sets two types of standards (URL 2): Primary standards are set to provide the maximum feasible protection to public health. They regulate contaminant levels based on toxicity and adverse health effects. The goal of standard setting is to identify maximum contaminant levels (MCLs) which prevent adverse health effects. Secondary standards regulate contaminant levels based on aesthetics such as color and odor, which do not pose a risk to health. These secondary maximum contaminant levels (SMCLs) are guidelines, not enforceable limits. They identify acceptable concentrations of contaminants which cause unpleasant tastes, odors, or colors in the water. SMCLs are for contaminants that will not cause adverse health effects. When water utility customers and water utility management place a strong emphasis on excellent water quality, the maximum contaminant levels (MCLs) of drinking water regulations may be viewed as an upper level of water contaminants that should be seldom or never approached, rather than as a guideline for finished water quality. Many water utilities choose to produce water that is much better in quality than water that would simply comply with the regulations. Such utilities may employ the same treatment processes that would be needed to provide the quality that complies with regulations, but operate those processes more effectively. Other utilities may employ additional treatment processes to attain the high finished water quality they seek (Logsdon et al., 1999).Both surface waters and groundwaters may have aesthetic characteristics that are not acceptable to customers, even though MCLs are not violated. Utilities in some states may be required to provide treatment to improve the quality of water that has problems of taste, odor, color, hardness, high mineral content, iron, manganese, or other aesthetic problems resulting in noncompliance with secondary MCLs. Improvement of aesthetic quality is very important, however, because customer perceptions of water quality often are formed based on observable water quality factors, most of which are aesthetic. Water that has bad taste or odor or other aesthetic problems may be perceived as unsafe by customers. This can cause a loss of confidence in the utility by its customers, and might cause some persons to turn to an unsafe source of water in lieu of using a safe but aesthetically objectionable public water supply (Logsdon et al., 1999).Much is known in general about the capabilities of various water treatment processes for removing both regulated contaminants and contaminants causing aesthetic problems. A comprehensive review of drinking water treatment processes appropriate for removal of regulated contaminants was undertaken by the National Research Council (NRC) in the context of providing safe drinking water for small water systems (National Research Council, 1997), but many of the NRC’s findings regarding treatment processes are applicable regardless of plant size. For removal of particulate contaminants, filtration and clarification (sedimentation or dissolved air flotation) processes are used. Site specific information on process capabilities may be needed, however, before engineers select a process train for a plant, particularly when no previous treatment experience exists for the source water in question. Pilot plant studies maybe an appropriate means of developing information on treatment processes and the water quality that can be attained by one or more process trains under evaluation. As soon as candidate treatment processes and treatment trains are identified, the potential need for a pilot plant study should be reviewed and the issue resolved. Carrying out a pilot study prior to process selection could take from 1 to 12 months for testing onsite and an additional 2 to 6 months for report preparation, but sometimes such a study holds the key to a cost-effective design and to ensuring that the quality goals will be met by the process train selected (Logsdon et al., 1999).The interaction of various processes on treated water quality must be considered in the regulatory context and in the broader context of water quality. Drinking water regulations generally have been written in a narrow context that focus on the contaminant or contaminants being regulated. Sometimes an approach to treatment for meeting a given MCL can cause problems of compliance with other regulations. For example, use of increased free chlorine residual might be an approach to meeting the CT requirement of the Surface Water Treatment Rule, but this could cause trihalomethanes (THMs) in the distribution system to exceed the MCL and possibly taste and odor problems. Maintaining a high pH in the distribution system might be helpful for meeting the requirements of the Lead and Copper Rule, but high pH increases the possibility of THM formation and decreases the efficiency of disinfection by free chlorine (Logsdon et al., 1999).Some interactions between treatment processes are beneficial. Ozone can be used for a variety of purposes, including control of tastes and odors, disinfection, and oxidation of iron and manganese. Improved filter performance in terms of longer runs or improved particle removal or both can be an additional benefit of using ozone; however, ozonation by-products must be controlled to prevent biological regrowth problems from developing in the distribution system (Logsdon et al., 1999).
A comparison of source water quality and the desired finished water quality is essential for treatment process selection. With the knowledge of the changes in water quality that must be attained, the engineer can identify one or more treatment processes that would be capable of attaining the quality improvement. Depending on a water utility’s past experience with a water source, the amount of data available on source water quality may range from almost nonexistent to fairly extensive (Logsdon et al., 1999). Source water quality is an issue that can be used to eliminate a process from consideration, if the process has not been proven to be capable of successfully treating the range of source water quality that would be encountered at the site in question (Logsdon et al., 1999). For nearly every type of water use, whether municipal, industrial, or agricultural, water has increased concentrations of dissolved constituents or increased temperature following its use. Therefore, the water quality of the water bodies that receive the discharge or return flow are affected by that use. In addition, as the water moves downstream, additional water use can further degrade the water quality. If irrigation return flow, or discharge from a municipal or industrial plant, moves downstream and is drawn back into an aquifer because of groundwater withdrawals, the groundwater system also will be affected by the quality of that surface water (Winter et al., 1998).The source of raw water can be an attactive target for an adversary. Whether it is a lake, river, or well field, many sources are remote can offer an attacker numerous opportunities to attempt a contamination or physical attack. It is believed that large quantities of contaminant are required to successfully poison a water source such as a lake or river (Winslow, 2005). Learning about the source or origin of the raw water can be helpful for estimating the nature of possible quality problems and developing a monitoring program to define water quality. For surface waters, information about the watershed may reveal sources of contamination, either manmade or natural. Furthermore, an upstream or downstream user may possess data on source water quality. For groundwaters, knowledge of the specific aquifer from which the water is withdrawn could be very useful, especially if other nearby water utilities are using the same aquifer (Logsdon et al., 1999). The capability of a water treatment plant to consistently deliver treated water quality meeting regulatory and water utility goals is strongly enhanced when the range of source water quality is always within the range of quality that the plant can successfully treat. Frequently, the source water database is limited. Water quality characteristics that may vary over a wide range, such as turbidity, can be studied by using probability plots. With such plots, estimates can be made of the source water turbidity that would be expected 90 or 99 percent of the time. When treatment processes such as slow sand, diatomaceous earth, or direct filtration are considered, careful study of the source water quality is needed to ensure that the high-quality source water required for successful operation of these processes will be available on a consistent basis. Source water quality problems can sometimes signal the need for a particular process, such as use of dissolved air flotation to treat algae-laden waters. When surface waters are treated, the multiple barrier concept for public health protection should be kept in mind. Sources subject to heavy fecal contamination from humans or from livestock (cattle, hogs, sheep, horses, or other animals capable of transmitting Cryptosporidium) will probably require multiple physical removal barriers [sedimentation or dissolved air flotation (DAF) followed by filtration] (Logsdon et al., 1999).
Process reliability is an important consideration and in some cases could be a key aspect in deciding which process to select. Disinfection of surface water is mandatory, so this is an example of a treatment process that should be essentially fail-safe (Logsdon et al., 1999).Unless the treatment plant can be taken out of service for a period of time for maintenance and repair work, two or more of all essential items, such as pumps, settling basins, flocculators, filters, and chemical feeders must be provided. The degree of importance of each item must be evaluated on a case-by-case basis, considering that safe water has to be supplied at all times (Technical Manual, 1985). The only acceptable action to take for a failure of disinfection in a plant treating surface water is to stop distributing water from the treatment works until the problem is corrected and proper disinfection is provided or until a “boil water” order can be put in place so the public will not drink undisinfected surface water. To avoid disinfection failures and to minimize downtimein the event of an equipment failure, backup disinfection systems or spare parts must be kept on hand for dealing with emergencies. Process reliability would be a very important factor in evaluating alternative disinfection systems, as well as other processes whose failure could have immediate public health consequences (Logsdon et al., 1999).Process reliability needs to be evaluated on a case-by-case basis, because factors that influence reliability in one situation may not apply at another situation. Factors that can influence reliability include (Logsdon et al., 1999):- Range of source water quality versus the range of quality the process can successfully treat- Rate of change of source water quality-slow and gradual or very rapid and severe- Level of operator training and experience- Staffing pattern-24 hours per day or intermittent, such as one shift per day- Mode of operation
The choice of processes to incorporate into a treatment train may be influenced strongly by the existing processes when a treatment plant is evaluated forupgrading or expanding. Site constraints may be crucial in process selection, especially in pre treatment when alternative clarification processes are available,some of which require only a small fraction of the space needed for a conventional settling basin. Hydraulic constraints can be important when retrofitting plantswith ozone or granular activated carbon (GAC) adsorption. The extra head needed for some treatment processes could result in the necessity for boosterpumping on-site to accommodate the hydraulic requirements of the process. This adds to the overall cost of the plant improvements and, in some cases, mightresult in a different process being selected. The availability of high head can influence process selection in some instances. Pressure filtration might be selectedfor treatment of groundwater after oxidation, for iron or manganese removal.In this situation, use of gravity filtration would involve breaking head and pumping after filtration, whereas with pressure filters it might be possible to pumpdirectly from the well through the filters to storage (Logsdon et al., 1999).
The ability of a water treatment plant to accommodate changes in future regulations or changes in source water quality is quite important. In the present regulatory environment, water utilities must realize that more regulations are likely in the future. For some utilities, these future regulations may require additional treatment or more effective treatment, such as when a previously unregulated contaminant is present in the source water or a maximum contaminant level is lowered for a contaminant in the utility’s source water. Some water treatment processes target a narrow range of contaminants and may not be readily adaptable to controlling other contaminants. On the other hand, a surface water treatment plant employing coagulation and filtration might be able to attain sufficient arsenic removal to comply with a future lower MCL, depending on the arsenic concentration in the source water, the coagulant chemical and its dosage, and the pH of treatment. The coagulation and filtration treatment train in this example has more flexibility for dealing with a changing regulatory requirement (Logsdon et al., 1999). Source water quality should be well established when a treatment plant is planned, so that good decisions on treatment processes can be made. Most treatment plants are built to last for several decades, and changes can occur in the quality of source waters with the passage of time. Long-term eutrophication of lakes can lead to increased algae blooms and to taste and odor problems. Some treatment plant operators believe that the water at present is more difficult to treat than it used to be. The defense against such problems is to incorporate process flexibility in a treatment plant, so that both present and unforeseen future quality problems can be addressed and finished water quality meeting the expectations of the utility and its customers can be produced for the long term (Logsdon et al., 1999).
After treatment processes are selected, designed, and on-line, the water utility must be able to operate them successfully to attain the desired water quality. The issue of system size versus treatment complexity becomes important with smaller systems. If successful treatment plant operation requires more labor than a small system can afford, or if the level of technical skills exceeds that readily attainable in a community, treatment failure may occur. Availability and access to service and repair of equipment involves considerations of time and distance from service representatives, and this may be problematic for some small, very remote water utilities. Selected treatment processes need to be operable in the context for which they will be employed. System size is not the only determining factor in successful operation. Sometimes, management is not sufficiently progressive or does not realize the necessity of providing well trained staff with modern tools and techniques to facilitate successful treatment plant operation. In this situation, utility management needs to be informed of the complexities and requirements for treatment processes before plans for treatment are adopted. Introduction of relatively complex treatment processes at a water utility whose management is not supportive of actions that will be needed for successful operation is a recipe for trouble (Logsdon et al., 1999).
Cost considerations usually are a key factor in process selection. Evaluation of costs for alternative process trains using principles of engineering economics might at first seem to be straightforward, but this may not be the case. When different treatment trains are evaluated, their capabilities are not likely to be identical, so the resulting treated-water quality from different trains likewise may not be identical (Logsdon et al., 1999). All cost estimates will contain a certain amount of experienced judgment or educated guesswork concerning the various cost elements, which comprise the estimate (Cilensek, 2005). The cost of water treatment is dependent on three factors: (1) quality of the raw water, with costs increasing as raw water quality deteriorates, (2) the degree of treatment required, so that the purer the finished water required, the more it will cost to produce it, and finally (3) the volume of water required and hence the size of the treatment plant, with the cost of water per unit volume decreasing as the capacity of the treatment facilities (Gray, 2008).The basis for process comparison has to be decided upon in such situations. If a certain aspect of water quality improvement is beneficial but not really necessary, perhaps it is not sufficiently valuable to enter into cost considerations. For example, both diatomaceous earth filters and granular media filters with coagulation pretreatment can remove particulate matter, but the process train employing coagulation, flocculation, and sedimentation can remove more color and total organic carbon (TOC) from source water. For treatment of a water with low color and low TOC concentrations, the treatment for particulate contaminant removal may be sufficient, and the use of a lower-cost filtration process, such as diatomaceous earth filtration, might be favored. On the other hand, if additional water quality improvement is needed, then any process train under consideration must be able to attain that improvement (Logsdon et al., 1999).The various types of cost estimates used are based on the level of Project design, which defines the known scope of work. The scope of work can range from only a Daily treatment flow rate and major process facilities, to a complete set of plans and specifications for contruction (Cilensek, 2005). Cost estimates should be made taking into consideration the entire life cycle cost of a process train. Both capital and operating and maintenance (O&M) costs must be included in the estimate. Estimating O&M costs can be difficult, and sometimes unforeseen major changes in the economy occur and invalidate earlier estimates. The need for repairs, for maintaining an inventory of spare parts or extra equipment, for operator staffing, and for routine maintenance activities must be included in cost determinations. Some water utilities have encountered high expenses for equipment upkeep and frequent replacement, negating the initial savings on the capital investment. Smaller utilities in particular must consider not only the amount of labor associated with the various treatment processes being considered, but also the skills required of that labor. For small utilities located in predominantly rural settings, far from large communities and far from sources of technical assistance on which to draw during times of crisis, the possibility of being able to attract and keep workers who can operate complex treatment equipment may become an important consideration. For some utilities, contract O&M arrangements or a circuit rider may be necessary for successful long-term operation (Logsdon et al., 1999).
Environmental compatibility issues cover a broad spectrum of concerns that include residual waste management, the fraction of source water wasted in treatment processes, and energy requirements for treatment. The effect of water treatment extends beyond the treatment plant. The benefits of providing safe drinking water are very great, but caution must be taken that the treatment processes selected to provide that safe water do not create serious environmental problems. Making quantitative calculations about public health benefits and environmental damages attributed to alternative treatment processes is likely to involve much guesswork and only a limited amount of solid data, but the difficulty in making firm estimates about overall environmental effects should not discourage engineers and owners from considering these issues (Logsdon et al., 1999). Residuals, or sludge and other by-products of water treatment, are commonly thought of when environmental compatibility is considered. Disposal of large volumes of water works sludge to surface waters is no longer permitted in most locations. Therefore, the residuals produced by coagulation, enhanced coagulation, and lime softening need to be dealt with in an environmentally acceptable manner. Disposal of brines from ion exchange or some membrane processes can present difficult issues in locations where brackish water or salt water is not nearby. Treatment of residuals can account for a significant portion of the total cost of water treatment; in some instances, concerns about residuals could influence process selection (Logsdon et al., 1999).Water wastage is an issue that may be important in areas where water supplies are limited. Treatment employing membrane processes has some advantages over other approaches to filtration, but if the fraction of water rejected by a membrane process is excessive, then less water is available to satisfy the demand for treated water. Recycling of high-volume process waste streams, with or without additional treatment, is also practiced in many areas (Logsdon et al., 1999).Energy usage by water utilities could become an environmental concern in the future. Developing estimates of future costs is very difficult. Those who consider the possible effect of future energy cost increases might look to the mid- to late 1970s, when the energy crisis and sharp increases in fuel prices occurred in the United States. A before-and-after comparison of the delivered prices of coagulant chemicals, sludge disposal costs, and electricity could be useful in an assessment of the vulnerability of a treatment plant employing coagulation versus vulnerability of a microfiltration plant to future energy price hikes (Logsdon et al., 1999).
The influence of treatment processes on desired water quality in the distribution system is a factor to be considered in process evaluation, and includes (Logsdon et al., 1999):
Regulatory requirements related to water distribution system monitoring are such that even if finished drinking water at the treatment plant meets MCLs, water quality deterioration in the distribution system could result in regulatory compliance problems. Treatment processes should be selected to enhance water stability. For example, ozone’s ability to break the molecular bonds of large organic molecules and form smaller organic molecules or molecular fragments can result in the formation of a more suitable food source for bacteria found in water, so use of ozone can promote growth of bacteria in water. If this growth takes place within a filter bed in the treatment plant, water with greater biological stability can be produced. On the other hand, if little or none of the organic matter were metabolized by bacteria in the filter bed, the organics would pass into the distribution system and could promote the growth of biofilms there. Distribution system biofilms can cause a variety of problems, including microbiological compliance violations, tastes and odors, excessive chlorine demand and free chlorine depletion, and corrosion of water mains. If the pH and alkalinity of finished water are such that the water will not be stable over time, water quality in the distribution system may change sufficiently to cause corrosion problems, even though the water did not seem to be problematic at the treatment plant (Logsdon et al., 1999). When multiple water sources are used by a single water utility, problems of water incompatibility can arise. These might be caused by the nature of the source waters, such as a water having high mineral content being mixed in a distribution system with a water of low mineral content. In addition, this situation could arise when a conventionally treated surface water and water treated by reverse osmosis are put into a common distribution system. Alternatively, water from different sources might be treated by different disinfection techniques. In general, it is considered inadvisable to mix chloraminated water and water disinfected with free chlorine in a distribution system. At the zone where the two different waters interact, the free chlorine can chemically react with the monochloramine, reducing the available free chlorine residual and forming dichloramine or nitrogen trichloride. Taste and odor complaints may also result from this practice (Logsdon et al., 1999).
Feasibility to scale processes up to very large sizes or to scale them down to very small sizes can be important in some cases. Complex treatment processes, such as coagulation and filtration of surface water or precipitative lime softening, can be scaled down physically, but the costs of equipment and the need for a highly trained operator may make the scaled-down process impractical. Processes that are practical and manageable at 10 mgd (38,000 m3/day) or even 1 mgd (3,800 m3/day) may be too complex at 0.01 mgd (38 m3/day). On the other hand, processes that work very well for small water systems may not be practical for large systems. Membrane filtration has worked very well for small systems, but microfiltration plants in the size range of 100 to 500 mgd (3.8 x 105 to 1.9 x 106 m3/day) would at this time entail a very large amount of piping and valving to interconnect large numbers of small modules. Processes that employ treatment modules (e.g., microfiltration) are expanded to larger sizes by joining together more modules. This can become problematic for a 100-fold size expansion. On the other hand, granular media filters can be expanded by designing the filter to have a large or small surface area (Logsdon et al., 1999).
