Choosing the Right Disinfection Technology for a Municipal Drinking Water Plant - Part 1
Managers of water treatment plants in the United States are very aware of the challenges they face to meet today’s standards for potable drinking water. They must deal with increasingly stringent regulations from the federal government on down to perhaps even tougher rules at state and local levels. One only has to go on the U.S. Environmental Protection Agency (USEPA) web site (www.epa.gov) to see the incredible scope of what concerns this regulatory body today.
This article does not claim to be a cure-all for the entire water industry’s problems, but it addresses one key aspect: the basic factors that influence the selection of a disinfection technology.
The obvious place to start is by looking at the nature of the raw water source that the utility uses. The next important consideration is the rules and regulations that are being promulgated with regard to drinking water. The final step is to take a look at the common technologies existing today for disinfection as well as oxidation, and the supplementary role of filtration. Hybrid solutions combining two different techniques may prove to be the best answer for difficult treatment problems.
Sources of Drinking Water
The raw water source or sources a given municipality has to process into drinking water is the first consideration in the selection of a disinfection process. These sources usually are classified into two categories: Ground Water (e.g., from wells and underground aquifers) and Surface Water (e.g., from rivers, lakes and reservoirs). Seawater also is a raw water source, but it has not been a significant source in North America to date.
The USEPA classifies Drinking Water Systems into two categories for regulatory purposes: Large Systems (water treatment plants [WTPs] serving more than 10,000 customers) and Small Systems (WTPs serving 10,000 customers or fewer). For practical reasons, it does not yet regulate drinking water systems serving fewer than 25 consumers.
In conjunction with the implementation of the 1996 amendment to the Safe Drinking Water Act (see regulations), USEPA conducted a detailed study of the U.S. WTPs for drinking water. Excluding plants or systems serving fewer than 25 people, close to 200,000 plants were identified. Many of these plants were WTPs serving populations of 25 to 10,000 and primarily using groundwater.
However, some groundwaters are contaminated by surface water (referred to as groundwater under the influence of surface water) and therefore need surface water treatment techniques. More importantly, most source waters tend to be unique as to needed treatment because of the wide range of impurities they may contain. These contaminants include
• A variety of undesirable inorganic dissolved solids,
• Dissolved or suspended organic matter (plants and algae) that create undesirable color, taste or odor, turbidity or by-product precursors (i.e., substances that can react with a given disinfectant to result in harmful DBPs), and
• Harmful microorganisms including bacteria, viruses and cysts.
In a single water source these characteristics can vary with changes in climatic conditions at different times of the year.
The obvious conclusion to this problem of multifaceted water sources is that no one disinfection technology is ideal or even adequate for all drinking water disinfection applications. Furthermore, with the ever-increasing complications of regulations, the utility may have to address new situations that previously did not exist (e.g., the presence or absence of protozoa such as Cryptosporidium).
Regulations: A Strong Influence on Disinfection Practice
In the U.S. municipal drinking water market, rules and regulations issued by the USEPA are the dominant influence in the selection of the proper disinfection process. These regulations lead directly to enforcement steps that individual states take to meet local treatment needs. Over the years, drinking water regulations have addressed all kinds of concerns as to what constituents could adversely affect the health of those ingesting the water. They also have explored problems such as possible hazards from the transportation of bulk chlorine or disinfection by-products that might originate in the treatment process itself.
Possible contaminants being examined include microbial pathogens, bacteria, viruses and a host of organic and inorganic chemicals. Any substance is suspect, even in trace amounts, if it might cause problems for human beings. Over the past three decades, USEPA rules and regulations have been evolving and changing at an ever-increasing pace.
These regulations typically start when the federal government reacts to public concerns or political pressures and enacts legislation that addresses specific environmental issues. USEPA then develops regulations or rules to implement the legislation, and States follow with their own specific requirements to ensure compliance at the state level. It is at the state level that the disinfection process can be influenced the most, since at this level there is the threat of fines and even imprisonment for non-compliance or deliberate violations.
Serious attention to water quality standards began in the United States as early as 1948 with passage of the Water Pollution Control Act (later known as the Clean Water Act [CWA]). While dealing more broadly with contaminants in the nation’s water (e.g., those due to discharges from industrial plants and municipal sewer systems), CWA had a series of amendments that began identifying specific contaminants and even specifying the levels of pollutants in water for selected categories of usage including public water supplies.
In 1974, Congress zeroed in on the issue of contaminants in U.S. drinking waters by passing the Safe Drinking Water Act (SDWA). This act is the basis for the present-day regulatory approach to drinking water standards. The resultant EPA regulatory approach was to move toward identifying contaminants of concern, developing information on health effects and establishing National Interim Drinking Water Regulations (NIDWRs) to limit human exposure to certain constituents in drinking water.
In 1986, Congress made major amendments in the SDWA, reflecting dissatisfaction with USEPA progress in establishing NIDWRs. The amendments also reflected a belief that oversights of U.S. water utilities should be enforceable. EPA thus began to name specific substances for regulation, with quantitative limits in terms of Maximum Contamination Level (MCL) and Maximum Contamination Level Goal (MCLG). These levels are expressed in milligrams per liter (mg/L) and the MCLs are enforceable at specified levels until they are revised.
Reference 1 includes the table "National Primary Drinking Water Standards" taken from EPA 810-F-94-001A (1994) that lists close to 100 contaminants grouped by type. For each category, in addition to columns for MCL and MCLG, the table has columns for Potential Health Effects from Ingestion in Water (such as cancer) and Sources of Contaminant in Drinking Water (such as chlorobenzene originating from waste solvent from metal degreasing processes).
It may not be economically or technically feasible to monitor certain contaminants such as viruses. For these, in place of the MCL, the table has "TT" that indicates there is a Treatment Technique Requirement. For example, the requirement might specify fine filtration in combination with the use of a disinfectant.
in Distribution Systems
As a part of amendments to the SDWA in 1989, EPA addressed a problem that had been plaguing the water industry for years, the occurrence of coliform bacteria in otherwise high-quality drinking water. It placed an emphasis on water quality in distribution systems, requiring maintenance of chlorine residuals of a concentration to inhibit microbiological regrowth.
White traces the history of the problem of bacterial regrowth, starting with past tendencies to place more emphasis on THMs than on pathogenic organisms.2 He devotes more than 15 pages to various aspects of the problem and its solution.
Present New Threat
Over the past 15 years, there has been a growing recognition in the water industry of the potential health hazards from two tiny protozoan parasites in raw water: Cryptosporidium parvum and Giardia lamblia. They have many similarities and both can cause serious illness when ingested by humans. They exist in the form of cysts (protective sacs) that make them more resistant to destruction by disinfection. Other physical separation means seem necessary.
Many outbreaks of infection from such protozoa have been attributed to the Cryptosporidium organism, which has a form referred to as an oocyst. It is smaller than the Giardia cyst, ranging in size from 4 to 6 microns (as opposed to the Giardia’s 10 to 16 microns), thus is somewhat more difficult to remove. Newer regulations that require reducing the number of oocysts to a very low level present a real problem for adequate treatment processes.
At the typical water treatment plant, a dose level of chlorine at 1 to 3 ppm is ineffective against Cryptosporidium. Therefore, the emphasis has been on separation processes such as settlement, flocculation, clarification and fine or ultrafine filtration. These processes have been supported in some cases by ozone or chlorine dioxide treatment.
Multitude of USEPA ‘Rules’
The designation "rule" has evolved in place of "regulation" for specific guidelines issued by USEPA. It is vitally important to be aware of these rules when choosing or recommending disinfection processes or offering advice about them. The Water and Wastewater Equipment Manufacturers’ Association (WWEMA), whose members are suppliers of equipment, disinfectants or related services to water utilities, have a Drinking Water Regulatory Committee that, among other activities, tracks pertinent rules and periodically issues a summary report to its members.
The USEPA rules are not irrefutable edicts but rather are subject to scrutiny by various groups and open to change if future study or tests warrant it. The normal procedure by USEPA is to issue a preliminary draft with proposed dates for publication of the rule and a target date for finalizing the rule up to a year or more later. Comments from interested parties are encouraged and seriously considered.
A brief description of several of these rules that affect disinfection methods will best explain their nature and why all persons involved in treatment of drinking water need to keep informed about them. They all have acronyms for ease of reference but these sometimes look quite cryptic. The accompanying sidebar (see page 35) lists some of these alphabetically.
Ground Water Rule (GWR): This rule applies solely to the more than 150,000 groundwater systems and states that such systems must comply with the Total Coliform Rule (TCR). Total coliforms are a group of closely related bacteria that are used to determine the effectiveness of water treatment, evaluate the integrity of the distribution system and serve as a rough screen for fecal contamination. The GWR, proposed in March 2000, is to specify appropriate use of disinfection and encourage the use of alternative approaches, including control of contamination at its source.
Enhanced Surface Water Treatment Rule (ESWTR): According to White, a 1989 rule, called the Safe Water Treatment Rule (SWTR), mandated specific filtration and disinfection requirements aimed at microbiological contaminants. It specifically names Giardia, viruses, Legionella, heterotrophic plate count (HPC) and coliform bacteria. Conspicuous by its absence was the feared Cryptosporidium. Under the SWTR all public water systems using surface water sources were required to achieve 3- to 4-log removals. These levels were far below the California rule of 5-log minimum.
The SWTR was then amended by what was called the Interim Enhanced Surface Water Treatment Rule (ISWTR) to strengthen microbial protection, including Cryptosporidium, and to address risk trade-offs with DBPs. The rule was to be applied to public water systems that use surface water or groundwater under the direct influence (GWUDI) of surface water and serve at least 10,000 people. In addition, States were required to conduct sanitary surveys for all surface water and GWUDI systems, including those that serve less than 10,000.
Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR): With a proposed date of March 2000 and a final rule date of January 2001, this rule applies the already existing Interim ESWTR to small systems (those serving 10,000 or fewer). One of its goals is to significantly reduce the level of Cryptosporidium in finished drinking water supplies through improvements in filtration. It spells out many details with regard to filtration systems.
USEPA is now carrying this rule even further with a Long Term 2 Enhanced Surface Water Treatment Rule (LT2ENSWTR). With a final rule target date of May 2002, the purpose of this rule is to provide protection against microbial contamination that goes beyond even that of IESWTR and LT1ESWTR. USEPA will do this by using data obtained from the Information Collection Rule (ICR).
Disinfection/Disinfection By-products Rule: In 1992, USEPA released what would later be known as the Disinfection/Disinfection By-Products rule (D/DBP). Its objective was to strike a balance between microbiological and chemical risks. DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs) were to be regulated by specified MCLs for users of chlorine, with aldehydes and bromates regulated by MCLs for users of ozone.
By 1994, USEPA proposed the Stage 1 D/DBP Rule that was promulgated in 1999. It updated previous regulations for total trihalomethanes and reduced exposure to three chemical disinfectants (chlorine, chloramine and chlorine dioxide) giving specific figures for maximum residual level goals and levels for these chemicals. It also stated new MCL goals and MCLs for total THMs, HAAs, chlorite and bromate.
Now, USEPA is moving on to Stage 2 D/DBP rule with the proposed dates as February 2001 and final rule, May 2002. Its purpose is to provide more protection against DBPs beyond that provided by the Stage 1 D/DBP. Commenters have expressed concern over the practicality of this new proposed rule. Based on new data to come from further research, USEPA will reevaluate these regulations and reintroduce them as appropriate. Together with the LT2ESWTR, the agency wants to finalize the Stage 2 D/DBP rule to ensure a proper balance between microbial and DBP risks.
Part 2 of this article will appear in the January issue and will discuss disinfection processes for drinking water systems.