When the objective of water treatment is to provide drinking water, then we need to select technologies that are not only the best available, but those that will meet local and national quality standards. The primary goals of a water treatment plant for over a century have remained practically the same: namely to produce water that is biologically and chemically safe, is appealing to the consumer, and is noncorrosive and nonscaling. Today, plant design has become very complex from discovery of seemingly innumerable chemical substances, the multiplying of regulations, and trying to satisfy more discriminating palates. In addition to the basics, designers must now keep in mind all manner of legal mandates, as well as public concerns and en-vironmental considerations, to provide an initial prospective of water works engineering planning, design, and operation.
The growth of community water supply systems in the United States started in the early 1800s. By 1860, over 400, and by the turn of the century over 3000 major water systems had been built to serve major cities and towns. Many older plants were equipped with slow sand filters. In the mid 1890s, the Louisville Water Company introduced the technologies of coagulation with rapid sand filtration.
The first application of chlorine in potable water was introduced in the 1830s for taste and odor control, at that time diseases were thought to be spread by odors. It was not until the 1890s and the advent of the germ theory of disease that the importance of disinfection in potable water was understood. Chlorination was first introduced on a practical scale in 1908 and then became a common practice.
Federal authority to establish standards for drinking water systems originated with the enactment by Congress in 1883 of the Interstate Quarantine Act, which authorized the Director of the United States Public Health Services (USPHS) to establish and enforce regulations to prevent the introduction, transmission, or spread of communicable diseases.
Today resource limitations have caused the United States Environmental Protection Agency (USEPA) to reassess schedules for new rules. A 1987 USEPA survey indicated there were approximately 202,000 public water systems in the United States. About 29 percent of these were community water systems, which serve approximately 90 percent of the population. Of the 58,908 community systems that serve about 226 million people, 51,552 were classified as "small" or "very small. " Each of these systems at an average serves a population of fewer than 3300 people. The total population served by these systems is approximately 25 million people. These figures provide us with a magnitude of scale in meeting drinking water demands in the United States. Compliance with drinking water standards is not
uniform. Small systems are the most frequent violators of federal regulations. Microbiological violations account for the vast majority of cases, with failure to monitor and report. Among others, violations exceeding SDWA maximum contaminant levels (MCLs) are quite common. Bringing small water systems into compliance requires applicable technologies, operator ability, financial resources, and institutional arrangements. The 1986 SDWA amendments authorized USEPA to set the best available technology (BAT) that can be incorporated in the design for the purposes of complying with the National Primary Drinking Water Regulations (NPDWR). Current BAT to maintain standards are as follows:
For turbidity, color and microbiological control in surface water treatment: filtration. Common variations of filtration are conventional, direct, slow sand, diatomaceous earth, and membranes.
What BATs Are
Chlorine Carbon Dioxide
Packed Tower Aeration
Diffused Aeration Oxidation Processes RO
For inactivation of microorganisms: disinfection. Typical disinfectants are chlorine, chlorine dioxide, chloramines, and ozone.
For organic contaminant removal from surface water: packed-tower aeration, granular activated carbon (GAC), powdered activated carbon (PAC), diffused aeration, advanced oxidation processes, and reverse osmosis (RO).
For inorganic contaminants removal, membranes, ion exchange, activated alumina, and GAC.
For corrosion control: typically, pH adjustment or corrosion inhibitors. The implications of the 1986 amendments to the SDWA and new regulations have resulted in rapid development and introduction of new technologies and equipment for water treatment and monitoring over the last two decades. Biological processes in particular have proven effective in removing biodegradable organic carbon that may sustain the regrowth of potentially harmful microorganisms in the distribution system, effective taste and odor control, and reduction in chlorine demand and DBP formation potential. Both biologically-active sand or carbon filters provide cost effective treatment of micro-contaminants than do physicochernical processes in many cases. Pertinent to the subject matter cover in this volume, membrane technology has been applied in drinking water treatment, partly because of affordable membranes and demand to removal of many contaminants. Microfiltration, ultrafiltration, nanofiltration and others have become common names in the water industry. Membrane technology is experimented with for the removal of microbes, such as Giardia and Cryptosporidium and for selective removal of nitrate. In other instances, membrane technology is applied for removal of DBP precursors, VOCs, and others.
Other treatment technologies that have potential for full-scale adoption are photochemical oxidation using ozone and UV radiation or hydrogen peroxide for destruction of refractory organic compounds. One example of a technology that was developed outside North America and later emerged in the U.S. is the Haberer process. This process combines contact flocculation, filtration, and powdered activated carbon adsorption to meet a wide range of requirements for surface water and groundwater purification.
Utilities are seeking not only to improve treatment, but also to monitor their supplies for microbiological contaminants more effectively. Electro-optical sensors are used to allow early detection of algal blooms in a reservoir and allow for diagnosis of problems and guidance in operational changes. Gene probe technology was first developed in response to the need for improved identification of microbes in the field of clinical microbiology. Attempts are now being made by radiolabeled and nonradioactive gene-probe assays with traditional detection methods for enteric viruses and protozoan parasites, such as Giardia and Cryptosporidium. This technique has the potential for monitoring water supplies for increasingly complex groups of microbes.
In spite of the multitudinous regulations and standards that an existing public water system must comply with, the principles of conventional water treatment process have not changed significantly over half a century. Whether a filter contains sand, anthracite, or both, slow or rapid rate, constant or declining rate, filtration is still filtration, sedimentation is still sedimentation, and disinfection is still disinfection. What has changed, however, are many tools that we now have in our engineering arsenal. For example,, a supervisory control and data acquisition (SCADA) system can provide operators and managers with accurate process control variables and operation and maintenance records. In addition to being able to look at the various options on the computer screen, engineers can conduct pilot plant studies of the multiple variables inherent in water treatment plant design. Likewise, operators and managers can utilize an ongoing pilot plant facility to optimize chemical feed and develop important information needed for future expansion and upgrading.
Technology and ultimately equipment selection depends on the standards set by the regulations. Drinking water standards are regulations that EPA sets to control the level of contaminants in the nation's drinking water. These standards are part of the Safe Drinking Water Act's "multiple barrier" approach to drinking water protection, which includes assessing and protecting drinking water sources; protecting wells and collection systems; making sure water is treated by qualified operators; ensuring the integrity of distribution systems; and making information available to the public on the quality of their drinking water. With the involvement of EPA, states, tribes, drinking water utilities, communities and citizens, these multiple barriers ensure that tap water in the U.S. and territories is safe to drink. In most cases, EPA delegates responsibility for implementing drinking water standards to states and tribes. There are two categories of drinking water standards:
• A National Primary Drinking Water Regulation (NPDWR or primary standard) is a legally-enforceable standard that applies to public water systems. Primary standards protect drinking water quality by limiting the levels of specific contaminants that can adversely affect public health and are known or anticipated to occur in water. They take the form of Maximum Contaminant Levels (MCL) or Treatment Techniques (TT).
• A National Secondary Drinking Water Regulation (NSDWR or secondary standard) is a non-enforceable guideline regarding contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. EPA recommends secondary standards to water systems but does not require systems to comply. However, states may choose to adopt them as enforceable standards. This information focuses on national primary standards.
Drinking water standards apply to public water systems (PWSs), which provide water for human consumption through at least 15 service connections, or regularly serve at least 25 individuals. Public water systems include municipal water companies, homeowner associations, schools, businesses, campgrounds and shopping malls. EPA considers input from many individuals and groups throughout the rule-making process. One of the formal means by which EPA solicits the assistance of its stakeholders is the National Drinking Water Advisory Council (NDWAC). The 15-member committee was created by the Safe Drinking Water Act. It is comprised of five members of the general public, five representatives of state and local agencies concerned with water hygiene and public water supply, and five representations of private organizations and groups demonstrating an active interest in water hygiene and public water supply, including two members who are
NDWAC advises EPA's Administrator on all of the agency's activities relating to drinking water. In addition to the NDWAC, representatives from water utilities, environmental groups, public interest groups, states, tribes and the general public are encouraged to take an active role in shaping the regulations, by participating in public meetings and commenting on proposed rules. Special meetings are also held to obtain input from minority and low-income communities, as well as representatives of small businesses.
The 1996 Amendments to Safe Drinking Water Act require EPA to go through several steps to determine, first, whether setting a standard is appropriate for a particular contaminant, and if so, what the standard should be. Peer-reviewed science and data support an intensive technological evaluation, which includes many factors: occurrence in the environment; human exposure and risks of adverse health effects in the general population and sensitive subpopulations; analytical methods of detection; technical feasibility; and impacts of regulation on water systems, the economy and public health. Considering public input throughout the process, EPA must (1) identify drinking water problems; (2) establish priorities; and (3) set standards.
EPA must first make determinations about which contaminants to regulate. These determinations are based on health risks and the likelihood that the contaminant occurs in public water systems at levels of concern. The National Drinking Water Contaminant Candidate List (CCL), published March 2, 1998, lists contaminants that (1) are not already regulated under SDWA; (2) may have adverse health effects; (3) are known or anticipated to occur in public water systems; and (4) may require regulations under SDWA. Contaminants on the CCL are divided into priorities for regulation, health research and occurrence data collection.
In August 2001, EPA selected five contaminants from the regulatory priorities on the CCL and determined whether to regulate them. To support these decisions, the Agency determined that regulating the contaminants presents a meaningful opportunity to reduce health risk. If the EPA determines regulations are necessary, the Agency must propose them by August 2003, and finalize them by February 2005. In addition, the Agency will also select up to 30 unregulated contaminants from the CCL for monitoring by public water systems serving at least 100,000 people. Currently, most of the unregulated contaminants with potential of occurring in drinking water are pesticides and microbes. Every five years, EPA will repeat the cycle of revising the CCL, making regulatory determinations for five contaminants and identifying up to 30 contaminants for unregulated monitoring. In addition, every six years, EPA will re-evaluate existing regulations to determine if modifications are necessary. Beginning in August 1999, a new National Contaminant Occurrence Database was developed to store data on regulated and unregulated chemical, radiological, microbial and physical contaminants, and other such contaminants likely to occur in finished, raw and source waters of public water systems.
After reviewing health effects studies, EPA sets a Maximum Contaminant Level Goal (MCLG), the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, and which allows an adequate margin of safety. MCLGs are non-enforceable public health goals. Since MCLGs consider only public health and not the limits of detection and treatment technology, sometimes they are set at a level which water systems cannot meet. When determining an MCLG, EPA considers the risk to sensitive subpopulations (infants, children, the elderly, and those with compromised immune systems) of experiencing a variety of adverse health effects.
Was this article helpful?