This article originally appeared in the March 2007 issue of Water Quality Products as "The ABCs of Bacteria Removal"
Removing or inactivating bacteria in the water we drink is essential to protecting the health of any person drinking the water. Removing or inactivating bacteria in water can be done in a variety of ways, depending on the water source, level of bacteria and inorganic matter, and resources available. Many bacteria treatment systems involve the addition of a chemical to kill or inactivate, but bacteria can be removed physically by filtration or through the use of ultraviolet (UV) radiation. Each system has its pros and cons, so it is important to take everything into account when deciding on a method for how to remove bacteria from water. Once you have a system in place it is important to continually monitor the system to ensure ongoing removal of bacteria. Frequency of monitoring will depend on applicable regulations, type of system and resources available.
How to Remove Bacteria From Water: Chemical Oxidation/Disinfection
The most common method for how to remove bacteria from water is through the use of chlorine; in fact, about 98% of public water systems use some form of chlorine for disinfection. Chlorine is frequently used because it is inexpensive and effective. There are several forms of chlorine including chlorine gas, sodium hypochlorite and calcium hypochlorite. Chlorine gas is widely used in large municipal applications, while sodium hypochlorite is commonly used in residential and smaller system applictions. Calcium hypochlorite is a granular form of chlorine used frequently in swimming pools and spas.
When chlorine is added to water two species are formed: hypochlorous acid, which is electrically neutral, and hypo-chlorite ion, which is electrically negative. Hypochlorous acid is a much stronger disinfectant and oxidizer than hypochlorite. The pH level determines how much hypochlorous acid and hypochlorite is formed. The lower the pH, the more hypochlorous acid is formed, while hypochlorite is formed at higher pH levels. This explains why measuring oxidation reduction potential (ORP) gives a better idea of disinfection than just measuring free chlorine. ORP actually measures the potential for oxidation by taking into account the difference in oxidizing power of the two chlorine species as well as the pH levels. Lowering the pH level will increase the ORP because the hypochlorous acid is a stronger oxidizer and much more prevalent at low pH levels. The Stage 1 Disinfectant and Disinfection Byproduct Rule, however, establishes maximum residual disinfectant levels for chlorine, chloramine and chlorine dioxide. This means systems are monitoring directly for these disinfectants to ensure they comply with the regulations.
Chloramine is a newer disinfectant that has emerged due to the formation of disinfection byproducts with traditional chlorine disinfection. Chloramine is the combination of chlorine and ammonia. Systems add the ammonia downstream of the chlorine injection site in order to allow the chloramine to form naturally. Chloramines can occur naturally if water already contains ammonia. Chloramines became an alternative to chlorine because they have a tendency to form fewer disinfection byproducts. The residual chloramine is more stable and longer lasting so it provides better protection against bacteria re-growth in systems with large tanks or dead ends. Chloramine does pose problems for fish aquariums as it is toxic to aquatic life and it is dangerous for dialysis applications. Chloramine is also more difficult to remove from water than chlorine.
Chlorine dioxide is also gaining in popularity due to changes in regulations regarding disinfection byproducts. Chlorine dioxide has been used in water treatment for several decades as an oxidizer to aid in removal of iron, manganese and other odor-causing organic substances. Chlorine dioxide is effective over a broad pH and temperature range, and kills viruses and cysts like Cryptosporidium and Giardia that chlorine alone cannot kill effectively. It is commonly used as a pre-oxidant because it does not react with organic materials to form the byproducts such as trihalomethanes and haloacetic acids that chlorine does. It does form chlorite ions, however, which are regulated by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act (SDWA) with an maximum contaminant level (MCL) of 1 ppm. Chlorine dioxide can be generated in either liquid or gaseous form. In the U.S., chlorine dioxide gas cannot be transported in any concentration, so it must be generated on site. There have been some recent advances in delivery of chlorine dioxide as a disinfectant, including generators, stabilized solutions, tablets, powders and sanitary wipes.
Ozone & UV Treatment
Ozone is a disinfectant that is commonly used in bottled water applications, industrial water treatment and food processing. It is becoming widely accepted in public water supplies as well as residential treatment due to its advantages over chlorine. One of the most important advantages is that like chlorine dioxide, ozone is extremely effective at killing parasites and other waterborne viruses that are resistant to chlorine. Ozone removes color and odors from water, while chlorine adds distinctive odor and taste. Ozone also does not form common disinfection byproducts like trihalomethanes or haloacetic acids; however, it can potentially form bromate if bromide is present in the water being treated. Bromate is considered a carcinogen and is regulated by the EPA under the SDWA with a MCL of 0.010 mg/L and a MCL goal of zero. Ozone is a gas and cannot be transported so it must be generated on site, either by corona discharge or UV radiation. Each system has its advantages, so consult with the manufacturers to determine your best option.
UV light is also gaining in popularity due to studies indicating its efficiency, safety and cost-effectiveness. The American Water Works Association conducted a study that indicates use of UV light as a disinfectant does not produce any carcinogenic or mutagenic chloro-organic byproducts. Water travels through a disinfection chamber where it is exposed to UV radiation, which kills or inactivates bacteria, viruses and cysts without adding anything to the water. Water may require pretreatment to remove minerals because bacteria can hide behind mineral particulates and enter the finished water. Pretreatment should be considered for water with a high turbidity level.
Bacteria also can be removed by microfiltration. Filters are typically rated absolute or nominal. An absolute rating is the size of a particle in micrometers (µm) that will pass through the filter. It is intended to represent the pore size opening in the filter media. The term “absolute” specifies that no particle larger than that rating will be able to pass through the filter. This limits the absolute rating to types of media with consistent pore size, which will show 100% retention of particles. A nominal rating refers to the pore size of the particulate; the filter media retains the majority of particulates (60 to 98%) at the specified pore size. While filters with low absolute ratings may aid in reducing bacteria, smaller viruses are more difficult to remove with filtration alone. Many times these filters include silver to inhibit further growth of bacteria or iodine to kill or inactivate any present bacteria or viruses.
Removing bacteria from drinking water is imperative to protecting public health. Given better testing methods and further research, more bacteria are coming to light, such as Helicobacter pylori, Adenoviruses, Aeromonas hydrophila and Cyanobacteria. These are on the EPA’s Contaminant Candidate List for possible future regulation. Lack of clean drinking water is still a leading cause of death throughout the world. Proper disinfection and treatment are essential to protecting public health. Regular testing of drinking water is a good way to ensure that the treatment being used is effective and the water is free from microbial contamination.