Disinfection

What is water disinfection?

This section examines the chemical and microbiological principles of water and applies them to drinking water disinfection. This section also focuses on major water disinfection regulations and how and why they are set at their respective values.

The EPA defines disinfection as it pertains to public water systems as the process designed to kill most microorganisms in water. This includes essentially all pathogenic (disease-causing) bacteria. There are several ways to disinfect, with chlorine being most frequently used in water treatment.

Disinfection does not sterilize the water. Sterilization is the destruction of all microorganisms, which is unnecessary. Not all microorganisms are pathogens, and some are even helpful (such as those present in stomachs and intestines to help digest food).

EPA and the SDWA regulations do not apply to bottled water. Bottled water is regulated by the Food and Drug Administration (FDA), under the Federal Food, Drug, and Cosmetic Act.

1. The primary purpose of disinfection is to kill or inactivate pathogens in the water.

Disinfection is the selective destruction or inactivation of pathogenic organisms. Water provides a favorable environment for the growth of many biological organisms, some of which are capable of causing disease.

An effective disinfection process will produce water free of disease-causing microorganisms (pathogens).

Pathogens are disease-causing organisms. Many of the largest disease outbreaks in history can be attributed to waterborne pathogens. Because of their propensity to kill large numbers of the population, these and all other pathogens should be controlled in the treatment process.

Pathogens must be deactivated or destroyed to prevent waterborne diseases.

In fact, the processes of coagulation, flocculation, sedimentation and filtration were developed to control pathogenic contamination in drinking water. Pathogens, which are found in either the forms of bacteria, viruses, or protozoa, are not “removed” by disinfection. Some are killed, some are temporarily inactivated, and some are not affected at all by chemical disinfectants or biocides. Because of this fact, it is mandatory to optimize our disinfection processes.

Biological Contamination

One of the most common contaminants water utilities collect samples and analyze are part of the biological class of contaminants. While only a few bacteriological organisms are pathogenic, this entire class of organisms can wreak havoc on a water system.

From a public health standpoint, we are mainly concerned with the bacteriological and viral disease-causing (pathogenic) organisms. Since it can be difficult and costly to try and analyze all the different strands of viruses and bacterial organisms, which might find their way into a water system, an “indicator” organism is preferred. In drinking water supplies and systems, the indicator of choice is the total coliform group.

The total coliform group of bacteria are the indicator microorganisms.

Pathogenic organisms include those derived from fecal contamination and viruses, which typically use bacteria as a host for replication. These disease-causing organisms that can be found in drinking water include, but are not limited to Escherichia coli (E. coli) and various strains of Vibrio and Enterococcus, various enteroviruses, and parasites such as Cryptosporidium and Giardia.

Most of these organisms find their way into a drinking water system through some type of contamination with fecal matter. Perhaps an animal feed lot is upstream from a water supply, or an underground sewer collection system is leaking next to a drinking water underground well. Therefore, the main source for these contaminants is human or animal feces.

The presence of E. coli in drinking water indicates that potentially harmful bacteria may be present.

Below is a list of waterborne diseases and illnesses.

Bacteria	Internal Parasite from Protozoa	Virus caused
Anthrax	Dysentery	Gastroenteritis
Dysentery	Ascariasis (round worm)	Heart anomalies
Cholera	Cryptosporidiosis from Cryptosporidium	Hepatitis A
Gastroenteritis (Stomach Flu)	Giardiasis from Giardia	Meningitis
Leptospirosis		Poliomyelitis
Paratyphoid
Salmonella
Shigellosis (Shigella)
Typhoid fever

2. The secondary purpose of disinfection is to provide a residual safeguard.

After regulatory standards are met at the treatment process, water quality degrades as it moves through the distribution system. The degree of water quality degradation is affected by its chemical and biological composition, as well as the physical condition of the distribution system it travels through.

Improving the quality of the water after it enters the distribution system is usually not a viable endeavor. However, proper distribution system management, which includes flushing, cleaning, pressure maintenance, backflow prevention, monitoring and line replacement programs, collectively, will help prevent substantial water quality degradation in the distribution system.

The water delivered to the consumer will almost always be of a lesser quality than what it was when it entered the system. How much the water quality degrades is largely the responsibility of the distribution operator.

Although, only about 1% of water is used for drinking and cooking, the quality standard for all public water supplies is based on the use of water for those purposes.

Disinfectant residuals and monitoring.

As part of a multi-barrier regulatory framework to control microbial contamination, public water systems (PWSs) must maintain a disinfectant residual for water that is delivered to customers through a distribution system.

Disinfectant residuals in the distribution systems serve three main purposes:

• to protect against microbial contaminants,

• to act as an indicator of distribution system upset,

• and to limit growth of heterotrophic bacteria and Legionella within the distribution system.

A disinfectant residual must be continuously maintained during the treatment process and throughout the distribution system.

Disinfection equipment shall be operated and monitored in a manner that will assure compliance with disinfectant residual regulatory standards.

The disinfection equipment shall be operated to maintain the following minimum disinfectant residuals in each finished water storage tank and throughout the distribution system at all times:

• a free chlorine residual of 0.2 mg/L (milligrams per liter); or

• a chloramine residual of 0.5 mg/L (measured as total chlorine) for those systems that distribute chloraminated water.

• The maximum residual disinfectant level (MRDL) allowable in a distribution system is 4.0 mg/L for chlorine and chloramine and 0.8 mg/L for chlorine dioxide.

Distribution Sampling

Sampling Plan

The Total Coliform Rule (TCR) requires each water supply system to develop and follow a written sampling plan. Each plan must be representative of the system and specifically identify sampling points throughout the distribution system. Sampling plans must be approved by the state regulatory agency and it is necessary to check with the state agency to determine the details of their review process and what documents need to be submitted.

Monitoring Frequency

The routine monitoring frequency for community water systems is based on the population served. Routine monitoring frequencies for non-community systems are based on the source of supply and, in some cases, the population served. The distribution system sampling frequency is required to be included in the written sampling plan.

Sampling Frequency of Routine Samples Based on Population Served

Population	Minimum Samples/Month	Population	Minimum Samples/Month
25-1,000	1	59,001 to 70,000	70
1,001 to 2,500	2	70,001 to 83,000	80
2,501 to 3,300	3	83,001 to 96,000	90
3,301 to 4,100	4	96,001 to 130,000	100
4,101 to 4,900	5	130,001 to 220,000	120
4,901 to 5,800	6	220,001 to 320,000	150
5,801 to 6,700	7	320,001 to 450,000	180
6,701 to 7,600	8	450,001 to 600,000	210
7,601 to 8,500	9	600,001 to 780,000	240
8,501 to 12,900	10	780,001 to 970,000	270
12,901 to 17,200	15	970,001 to 1,230,000	300
17,201 to 21,500	20	1,230,001 to 1,520,000	330
21,501 to 25,000	25	1,520,001 to 1,850,000	360
25,001 to 33,000	30	1,850,001 to 2,270,000	390
33,001 to 41,000	40	2,270,001 to 3,020,000	420
41,001 to 50,000	50	3,020,001 to 3,960,000	450
50,001 to 59,000	60	3,960,001 or more	480

The samples must be collected in sterile containers and not contaminated during the sampling process.

Obtain routine samples from designated sites. A common sampling point is an outside house faucet that is on the water main side of the street.

The point must be sanitary

Do not use fire hydrants for sample collection

Do not sample from faucets with overhanging plants

Do not sample from faucets with an insect nest

Do not sample from leaky faucets

Do not take samples on rainy or windy days

Do not sample where mains have been recently flushed

Do not sample from vacant buildings, use only active service connections

Do not sample from locations with a water softener

A manufactured sampling station installed directly over the water main is preferred. These provide a locked sampling station that remains secure and clean, with easy accessibility, and instant flushing. 

Test results are reported as:

• positive (coliform found)
• negative (not found)
• or unsuitable for analysis

Water Quality Violations

Whenever a routine sample tests positive for total coliform, two provisions of the Total Coliform Rule take effect:

• repeat samples must be taken
• and the original coliform-positive sample must be tested for the presence of fecal coliforms or E. coli to determine whether an actual or potential violation of the coliform MCL exists.

The reason repeat samples are required is to investigate whether the original coliform-positive sample was caused by a contamination problem that exists throughout the distribution system or if it is a localized problem that exists only at that one sampling point. With this information, appropriate corrective action can be taken to eliminate the problem as quickly as possible.

The Total Coliform Rule specifies how many repeat samples must be taken, when, and from what locations. It also directs the water utility to collect a specific number of samples the following month, based on the number of routine samples ordinarily collected.

Non-community systems serving more than 1,000 people have the same requirements as community water systems. Non-community systems serving fewer than 1,000 people are required to collect one routine sample per quarter. When the routine sample tests positive, four repeat samples are required in the same quarter and five samples are required the following quarter.

Whenever a routine or repeat sample tests positive for total coliform, the water agency must collect a set of three or four repeat samples within 24 hours of receiving the laboratory results.

• at least one of the repeat samples must be taken from the same tap as the original coliform-positive sample

• the remaining repeat samples in the set must be collected at nearby taps (within five service connections of the original sampling point), upstream and downstream of the original.

Repeat samples must be taken until no coliforms are detected or until the MCL is exceeded and the state is notified.

Determining Compliance

The MCLG for total coliforms (including fecal coliforms and E. coli) is zero. The MCL, based on the presence-absence concept, is as follows:

1. For water systems analyzing at least 40 samples per month, no more than 5.0 percent of the samples (including routine and repeat samples) may be positive for total coliforms.

2. For water systems analyzing fewer than 40 samples per month, no more than one sample per month may be positive for total coliforms.

The Revised Total Coliform Rule makes another significant change in the way compliance is calculated. All valid coliform-positive samples, routine and repeat samples, must be counted when calculating compliance with the monthly MCL. Under previous regulations, check or repeat samples were not included in the monthly MCL calculation. On a case-by-case basis, the state may declare a sample invalid for one of several reasons, including interference by heterotrophic bacteria during laboratory analysis, as previously discussed.

Total-coliform-positive samples may be invalidated by the state under any of the following conditions:

1. The analytical laboratory acknowledges that improper sample analysis caused the positive result

2. The state determines that the contamination is a local plumbing problem

3. The state has substantial grounds to believe that the positive result was unrelated to the quality of drinking water in the distribution system

Reporting and Notification Requirements

Reporting frequencies for coliform test results increase in step with the urgency of the problem. A water agency must report the results of monthly coliform testing to the state regulatory agency within the first 10 days of the following month.

Any time a water agency fails to collect a sample as required, the state must be notified within 10 days after the system learns of the violation. An invalid sample result is considered a failure to monitor and must be reported.

If the MCL is exceeded, the state must be notified no later than the end of the next business day and the public within 14 days. This requirement could occur when the water agency exceeds its monthly coliform-positive limit or when test results show the presence of fecal coliforms or E. coli in any sample.

The most critical situation exists when either of two situations occurs:

1. A routine sample tests positive for total coliforms and for fecal coliforms or E. coli, and any repeat

    sample tests positive for total coliforms.

2. A routine sample tests positive for total coliforms and negative for fecal coliforms or E. coli, and any

    repeat sample tests positive for fecal coliforms or E. coli.

These situations are considered an acute risk to health. This occurrence is a Tier 1 violation, which requires that the state and the public be notified within 24 hours.

Distribution System Maintenance

Maintaining an efficient distribution system can help maintain good water quality. Water entering a distribution system from a source is routinely disinfected with a chemical such as chlorine. While the water travels through the distribution system the disinfectant will do its job by inactivating pathogenic organisms. As this occurs, the amount of chlorine in the system (residual) will reduce. The further the water travels, the lower the residual level. The water also becomes older as it travels through the distribution system and the water can become stagnant, resulting in discoloration, odors, and low chlorine residuals.

One way to help keep a disinfectant level at acceptable levels is to help move the water through the system by flushing dead ends and areas furthest away from sources. By flushing and helping to move water through the distribution system, the water and with it the disinfectant residual travels faster through the distribution system and the water does not become stagnant.

Sometimes, residuals drop very rapidly or cannot be maintained within a distribution system. This typically occurs when the initial dose of the disinfectant is not high enough at the source water, the water stays in the distribution system too long because of low use, or there is some other problem within the distribution system.

When this occurs, distribution operators may choose to add a disinfectant in water storage tanks. Since disinfectants are added at the sources of supply they are typically found at higher levels in the distribution system around these sources. Storage tanks are commonly placed on the outer edges of distribution systems and if water demands (usage) are low, disinfectant residuals can drop below acceptable levels.

This is when distribution operators can add disinfectants, such as calcium hypochlorite granules or liquid sodium hypochlorite to storage tanks. This will help improve disinfectant residuals within the tank and then in the distribution system as water is taken out of the tanks during times of usage.

Consumer Confidence Report (CCR)

The CCR is an annual report sent to all customers receiving water from a utility. This report provides information on the sources of supply, updates on new or emerging water quality regulations, health effects from contaminants found in their drinking water, levels for all contaminants found in the drinking water supply, and any violations which may have occurred.

This CCR is very helpful in communicating to the public the safety of their water supply. The information within the report is from the prior calendar year and must be sent to all customers by July 1 of the following year. The report also must be provided in each language spoken within the utilities service area if the population speaking that language is greater than ten percent of the total population.

Types of Disinfectant Chemicals

There are several chemical disinfectants available in drinking water applications. The most used in the United States is chlorine. The basics will be covered below as well as a background on the other chemicals available. The chemicals will be broken down into sub-categories based on their practical usage in the United States.

MOST COMMON

• Chlorine – The most widely used disinfectant in the United States is free chlorine.

Chlorine can be added:

• as a gas in the form of chlorine gas
• as a solid in the form of calcium hypochlorite
• or as a liquid in the form of sodium hypochlorite.

Most likely, you have a bottle of sodium hypochlorite in your house. We call it bleach. The use of free available chlorine has declined over the years because of the discovery of disinfectant by-products.

• Chloramines – The use of chloramines has become more common in recent years to reduce DBPs, mainly trihalomethane (THM). Chloramines are also referred to as combined chlorine as it is the combination of chlorine and ammonia.

Chloramines are also effective at eliminating taste and odor problems and the residual lasts longer in the distribution system. However, chloramine disinfection is not as strong as chlorine and the improper addition of ammonia can lead to excessive amounts of ammonia in the treated water which results in nitrification.

MORE COMMON DISINFECTANTS

• Chlorine Dioxide – used as a water treatment disinfectant and oxidizer. It does not react with ammonia which is an issue with chlorine. Chlorine dioxide is used as a disinfectant but is also very effective at removing iron, manganese, taste, odor, and color from treated water. Chlorine dioxide reacts with sulfide compounds helping to remove them and eliminate their characteristic odors. Phenolic tastes and odors can be controlled with chlorine dioxide.

Cryptosporidium is resistant to chlorine, but is not resistant to chlorine dioxide. Up to 70 percent of chlorine dioxide is converted to chlorite, which is a regulated disinfectant by-product so the dosage rates when using it as a disinfectant must be lower than 1.4 mg/L. Chlorine dioxide must also be made on site which necessitates higher operational and maintenance costs.

• Ozone – Ozone was first used in Europe in the early 1900’s. It is a strong disinfectant that also reduces taste and odor issues. The drawback of ozone is that it is very expensive to produce, has high electrical costs, has limited solubility, and does not leave a residual in the treated water because it is so reactive.

Ozone is one of the most powerful water treatment compounds available to system managers today. It is a technology that has been in continual commercial use for over 100 years and has distinct properties that allow disinfection of even heavily compromised water streams. With the 1996 reauthorization of the Safe Drinking Water Act, ozone was named as among the best available technologies for water system compliance with National Primary Drinking Water Regulations as overseen by the US Environmental Protection Agency.

If bromide is present in the water, ozone can react with it to form bromate, an undesirable DBP. Ozone is very efficient at disinfecting Cryptosporidium, so it is generally used as a secondary disinfectant along with chlorine or chloramines.

At ambient temperatures, ozone is an unstable gas, partially soluble in water; generally, more soluble than oxygen. Due to its instability, ozone quickly reverts to oxygen. Ozone cannot be produced at a central manufacturing site, bottled, shipped, and stored prior to use. It must be generated and applied on-site.

• Ultraviolet (UV) – Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in frequency than visible light and lower in frequency compared to x-rays. The UV spectrum has a larger wavelength than x-rays and a smaller wavelength than visible light and the order of energy, from low to high, is visible light, UV, and x-rays.

UV is known to be an effective disinfectant due to its strong germicidal (inactivating) ability. UV disinfects water containing bacteria and viruses and can be effective against protozoans, such as Giardia lamblia cysts or Cryptosporidium oocysts. UV is used in the pharmaceutical, cosmetic, beverage, and electronics industries. In the United States, it is used for drinking water disinfection; however, high operating costs compared to disinfection by chlorination has limited its usage.

Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very effective disinfectant, although the disinfection can only occur inside the unit. No residual disinfection in the water exists to inactivate bacteria that may survive or may be introduced after the water passes by the light source. The percentage of microorganisms destroyed depends on the intensity of the UV light, the contact time, raw water quality, and proper maintenance of the equipment.

Water should be sampled and tested for bacteria counts regularly. Sample before and after the UV unit to test its performance. Water should also be sampled in the distribution since bacterial regrowth can occur downstream of the UV unit.

RARE DISINFECTANTS

• Iodine – It is commonly used for emergency treatment in the form of droplets or tablets. It is not used by the water treatment industry because of its cost and the potential health hazards to pregnant women and possible thyroid issues, which can develop with frequent use.

• Bromine – It is not used by water treatment facilities as it is very corrosive and can cause severe skin burns. It is used more commonly as a disinfectant in swimming pools. When it reacts with choline (a common nutrient from plants and animals) in water, it can create disinfectant by-products. It was used by the United States Navy for a time, but most systems have been removed because of bromine’s corrosiveness. Because bromine is a very reactive chemical residual are hard to obtain.

• Sodium Hydroxide and Lime – more frequently used to sterilize pipes. They are not used as an everyday disinfectant because of the bitter taste that is left behind after application. Sodium hydroxide and lime are more often used to increase the pH of the water in the distribution system after treatment with gas chlorine.

Chlorine and Chlorinated Disinfectants

Disinfection with chlorine and chlorinated compounds result in a disinfectant residual of free chlorine or total chlorine. Free chlorine is the amount of “free” and available chlorine to perform the disinfection process.

As chlorine combines with nitrogen related compounds (as we will describe in more detail with chloramination) a combined chlorine residual occurs. This combined chlorine, along with any available free chlorine makes up a total chlorine residual.

Chlorine Gas

Chlorine is the most common disinfectant used in drinking water. It is in the form of a gas, provides a relatively long lasting residual, and tends to lower the pH of the water. Chlorine gas is greenish yellow in color and has a high coefficient of expansion. Therefore, chlorine cylinders should not be filled more than eighty-five percent. Chlorine cylinders come in one hundred, one hundred fifty, and one ton sizes. Only forty pounds per day (ppd) can be withdrawn from the smaller sized cylinders unless they are equipped with an evaporator.

Cylinders are equipped with a fusible plug, which is designed to melt at temperatures between 158F and 165F to prevent the cylinder from combusting. One-ton cylinders have six fusible plugs and two valves to withdraw the chlorine. Chlorine gas is 2.5 times heavier than air and ventilation in chlorine rooms needs to close to the floor. Commonly vents are twelve inches above the floor. Leaks can be detected by waving an ammonia soaked rag, which creates a white cloud.

Chlorine gas has an immediately dangerous to life and health (IDLH) level of 10 ppm. Chlorine can combine with organics in drinking water created halogenated disinfection by-products.

Pictured above is a standard chlorine scale and trunnion system. The trunnion acts as a form of storage while also keeping an accurate measure of the amount of chemical left in the tanks. In California, the tanks must also be secured with straps due to earthquakes.

Hypochlorites of Sodium and Calcium

Sodium hypochlorite comes in the form of a liquid and the chlorine concentration is commonly twelve and one half percent. Calcium hypochlorite comes in the form of a solid as granules or tablets and is typically sixty-five percent. Calcium hypochlorite is also known as high test hypochlorite (HTH). When dissolved in water hypochlorites tend to raise the pH. Hypochlorites can combine with organics in drinking water created halogenated disinfection by-products.

Chlorine Dioxide

Chlorine dioxide is highly effective in controlling waterborne pathogens while minimizing disinfection by-products. It is an effective means of controlling taste and odor issues. It can be expensive to use, especially in smaller quantities and can be difficult to handle.

Chloramine

Chloramine is a disinfection process using chlorine and ammonia together. This disinfection process is referred to as chloramination and it results in a combined or total chlorine residual. There are several reasons chloramination is used instead of chlorine alone. Chloramines are produced under three different processes;

• pre-ammoniation with post-chlorination
• pre-chlorination with post-ammoniation
• or concurrent addition of chlorine and ammonia

Concurrent addition produces the lowest disinfection by-products and pre-chlorination produces the highest disinfection by-product levels.

Chloramination vs. Chlorination

Both chemicals are widely used to disinfect drinking water. Each has their benefits and drawbacks. As previously described, Chloramination results in a total chlorine residual and chlorine creates a free chlorine residual.

Both free and total are efficient with inactivating/killing microorganisms, including heterotrophic plate count bacteria and pathogenic organisms. They both can penetrate biofilm and reduce coliform regrowth. While free chlorine is a stronger oxidizer, chloramines provide a longer lasting residual.

At the correct ratio between chlorine and ammonia, taste and odor problems can be controlled. If water contains organic compounds, free chlorine can combine with these compounds creating disinfection by-products, such as trihalomethane and halo acetic acid compounds. Chloramines reduce this disinfection by-product formation.

Breakpoint Chlorination

As chlorine is initially added to water, reducing compounds are destroyed. Both organic and inorganic reducing agents contribute to this first stage of disinfection. As a result, no chlorine residual is present. Understanding this is critical, but it is also counter intuitive. As you add chlorine no chlorine residual is detected. More chlorine must be continually added.

The next stage of breakpoint chlorination is the formation of chloroganics and chloramines. At this point, a residual begins to be detected. As the chloroganics and chloramines start to be destroyed, the residual starts to decrease. Once all the chlororganics and chloramines are destroyed, breakpoint is hit, and all the chlorine demand is satisfied. At this point, any chlorine added is directly proportional to the chlorine residual measured.

If disinfecting with chloramines, an ideal chlorine to ammonia ratio is 5:1. This means for every part of ammonia added, there should be five parts of chlorine. At this point, the highest total chlorine residual is realized, and taste and odor issues are minimized.

Lower chlorine to ammonia ratios result in free available ammonia. This creates a potential food source for microorganisms and results in a decreased disinfectant residual. This results in a condition referred to as nitrification.

Therefore, it is important to monitor for nitrogen related components to control this condition. If the chlorine to ammonia ratio increases, the disinfectant residual also decreases, and unwanted taste and odor compounds increase. If a free chlorine residual is desired and ammonia is not added along with chlorine, then chlorine needs to be continually added until breakpoint is reached. 

Nitrification

Nitrification is an aerobic process in which bacteria reduce ammonia and organic nitrogen into nitrite and then nitrate. Nitrite rapidly reduces free chlorine and can also interfere with the measurement of a free chlorine residual. This results in a loss of total chlorine and ammonia and an increase in heterotrophic plate count bacteria.

Higher temperatures and longer detention times in storage facilities increase the potential for nitrification. Water utilities using chloramination as a disinfection practice usually have a nitrification monitoring plan. This plan would specify and describe steps the utility will take to monitor, prevent, and reduce the effects associated with nitrification.

For example, the plan would specify when increased monitoring would be required. It would specify the constituents, which would need to be monitored. It would also describe maintenance activities within the distribution system such as a proactive flushing program to help distribute the chlorine residual and remove stagnant water.

Another example would be to properly cycle the water within storage tanks to prevent or reduce stratification, higher temperatures, and stagnant water. Constituents routinely monitored in systems using chloramines include:

• ammonia

• total and free chlorine

• nitrite

• and heterotrophic plate count bacteria

A reactive approach, which is commonly used by water utilities, is a process referred to as “batch” chlorination. Batch chlorination is the process of adding chlorine to water storage facilities when residuals become too low and/or when nitrification compounds are present.

There are a variety of methods and chemicals that can be used to disinfect drinking water with chlorine and chlorine related compounds being the most common. The disinfection process is critical in making sure drinking water is safe to drink by eliminating pathogenic microorganisms from the water supply.

It is important to disinfect source water supplies and it is important to maintain detectable chlorine residuals within the distribution system. There are side effects related to the disinfection process including taste and odor issues and the potential formation of unwanted disinfection by-products. Therefore, it is important to have adequate monitoring programs and to make sure the appropriate disinfectant is used.

Factors Influencing Disinfection

Depending on the disinfection process, whether physical or chemical, there are things affecting the effectiveness. For example, UV light must come in direct contact with the organisms to work. We will look at six variables influencing the disinfection process: pH, temperature, time, turbidity, organic compounds, and non-organic reducing agents.

The pH of the water plays a pivotal role in the effectiveness of disinfection especially when using chlorine or chlorine related compounds. When using free chlorine with water, hypochlorus and hydrochloric acids are formed. In dilute solutions with a pH above 4, the formation of hypochlorus acid is most complete and leaves little chlorine in the solution.

However, hypochlorus acid is a weak acid and poorly dissociated at pH levels below 6. The higher the pH, the greater percent of hypochlorite ion exists. Hypochlorus acid has a greater disinfection potential than hypochlorite ion. Therefore, pH plays an important role with disinfection. At a pH of approximately 7.2, 60% of dissolved chlorine exists as hypocholrus acid.

At a pH of 8.5, approximately 90% of the dissolved chlorine exists as hypochlorite ions. Therefore, chlorine as a disinfectant is more efficient at pH levels around 7.

Temperature also influences disinfection. The higher the temperature the more efficiently water can be disinfected. At lower temperatures, longer contact times are required. Adding larger quantities of chlorine can speed up the disinfection process. One major disadvantage to warmer waters exposed to the atmosphere is the increased dissipation rate of chlorine into the atmosphere.

As with temperature, time plays a role in disinfection. The more time a disinfectant is in the water, the greater the disinfecting abilities. This also plays a part in the formation of disinfectant by-products, especially when using chlorine or chlorine related compounds. Water in the distribution system having long residence times, like in the far reaches of the distribution system, often have elevated disinfection by-product levels.

Excessive turbidity in water supplies will greatly reduce the efficiency of the disinfection process. Any suspended solids present in the water supply can shield microorganisms from the disinfectant. In addition, some types of suspended solids can create an increase in chlorine demand, this results in less available chlorine to react with pathogens.

If some of the turbidity or if other substances in the water are in the form of organic compounds, chlorine disinfectants are greatly reduced. In addition, unwanted by-products can be formed, including trihalomethanes and halo acetic acids. The overall affect is a reduction in the overall chemical available for disinfection.

Various other non-organic reducing agents can also impact the disinfection process. The demand for chlorine for all reducing agents must be satisfied before chlorine becomes available for disinfection. Inorganic reducing agents impacting chlorine disinfection include, but are not limited to hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions.

The challenge of a public water system is to maintain, as much as possible, the quality of the water from the treatment facility to the consumer’s tap.

The information in this site is intended solely for the personal non-commercial use of the user who accepts full responsibility for its use. While we have taken every precaution to ensure that the content of this site is both current and accurate, errors can occur.