rss

WaterOperator.org Blog

Articles in support of small community water and wastewater operators.

Controlling Legionella in Drinking Water Systems

Controlling Legionella in Drinking Water Systems

Photo Credit: CDC Public Health Image Library ID #11148 by Janice Haney 2009; Edited with cropping.

The prevalence of Legionella bacteria in drinking water and distributions systems has gained notice over the past several years due to its increasing rate of infection in the United States. Inhalation or aspiration of small aerosolized Legionella bacteria from water can cause Pontiac fever and Legionnaires’ disease most frequently in sensitive or immunocompromised populations. Between 2000 and 2015, the National Notifiable Diseases Surveillance System (NNDSS) reports that the incident rate of Legionnaires’ disease in the U.S. increased from approximately 0.42 cases per 100,000 persons to 1.89 cases per 100,000 persons. According to the Ohio Department of Health, potential reasons for this change in rate might include increased monitoring and awareness, higher population susceptibility, climate change, water-saving fixtures, and/or aging infrastructure. As of 2019 Legionnaires’ disease is reported to afflict and kill more people in the U.S. than any other waterborne disease.

Existing research indicates that, though Legionella bacteria can be found in all parts of the water treatment system, they amplify best inside protozoan hosts and near the biofilm typically found within premise plumbing or drinking water systems. The resiliency of biofilm to disinfection acts as a protective barrier for Legionella while creating an environment abundant in nutrients. Protozoan hosts also offer defense against extreme temperatures and treatment technologies. A 1994 study by Kramer and Ford found that hundreds of Legionella bacteria can be contained within a single amoeba vesicle. L. pneumophila, the species responsible for most human infections, can also differentiate into various life cycle forms that alter susceptibility to water treatment. This symbiotic relationship with other microorganisms complicates Legionella disinfection.

Hot spots for growth include showerheads, faucets, plumbing systems, cooling towers, hot tubs, fountains, and distribution systems where water stagnation, insufficient disinfectant residual, warm temperatures (77-124°F), or excess nutrients foster biofilm formation. As a result, the most frequent outbreaks from Legionella have been documented in hotels and healthcare facilities. Management of outbreaks can start at the site of these impacted buildings as well as the treatment plant. Drinking water utilities can participate in prevention by understanding the conditions that favor propagation and the methods to control growth.

The U.S. EPA established a Maximum Contaminant Level Goal (MCLG) for Legionella at zero microorganisms. While this is not an enforceable limit, the Agency believes that if Giardia and other viruses are removed or inactivated as required under the Surface Water Treatment Rule, Legionella will also be controlled. Requirements to manage bacterial contamination under the Revised Total Coliform Rule and Ground Water Rule also contribute to Legionella management. Though some systems may routinely monitor for Legionella bacteria, testing methods can often yield both false positives and false negatives. Given the complications of environmental monitoring as well as the cost, management generally starts in response to outbreaks or sporadic cases.

Ongoing research has identified that potential drinking water treatment methods for Legionella include chlorination, copper-silver ionization, ultraviolet (UV) light, ozonation, and thermal disinfection. Among these technologies, chlorine, chlorine dioxide, chloramine, and ozone are the most widely used disinfectants. A combination of these techniques offers the most effective defense against recolonization and biofilm formation. To inactivate individual bacteria as well as those contained within biofilm, operators should also pay attention to the contact time and concentration of disinfectant used during treatment. Equally important to contact time is the maintenance of disinfectant residuals throughout distribution. The National Academy of Sciences’ Management of Legionella in Water Systems details the recommendations for proper disinfection using free chlorine, chlorine dioxide, monochloramine, and technologies more commonly used by building water systems.

To effectively manage Legionella in drinking water, utilities must also collaborate with impacted buildings. Facilities that have experienced outbreaks can develop their own management plan using the Center for Disease Control’s (CDC) Developing a Water Management Program to Reduce Legionella Growth & Spread in Buildings and the World Health Organization’s Legionella and the Prevention of Legionellosis. This literature, along with the CDC training on Legionella Water Management Programs and the other resources linked within this guide will ensure that your community members, especially those at greater risk to illness, are protected from Legionella.

Peracetic Acid (PAA) in Wastewater Disinfection

Peracetic Acid (PAA) in Wastewater Disinfection

Peracetic acid (PAA) has grown in popularity over the last several years for its use in the disinfection of wastewater and stormwater. Utilities use disinfectants as the primary mechanism to inactivate and destroy pathogenic organisms that spread waterborne disease. An appropriate disinfectant will sufficiently treat any disease-causing microbes including bacteria, spores, helminthes, and protozoa. While PAA technology has been employed in Canada and Europe for the last 30 to 40 years, this disinfectant has only become noticed in U.S. municipal wastewater treatment within the last 10 years. Competing with chlorine, an already well-established disinfectant, its use is still slow growing, however systems are discovering that PAA offers several benefits to wastewater treatment that chlorination does not.

What is peracetic acid? The alternative disinfectant is a clear, organic peroxide compound that readily hydrolyzes to acetic acid and hydrogen peroxide in water. It’s characterized as a strong oxidant and fast reacting disinfectant. Commercially available peracetic (CH3CO3H) is purchased in an equilibrium mixture of acetic acid (H3CO2H), hydrogen peroxide (H2O2), and water (H2O). Manufacturers typically add a stabilizer as well. The following formula represents the equilibrium equation: CH3CO2H + H2O2 ←→ CH3CO3H + H2O.

PAA can generally be purchased in concentrations of 5% to 22%. When PAA decomposes in water, free hydrogen peroxyl (HO2) and hydroxyl (OH) radicals are formed. These radicals have significant oxidizing capacity that play an active role in microbial disinfection. According to the EPA, bacteria are destroyed through cell wall lysis and leakage of any cellular constituents.

Wastewater systems consider moving to peracetic acid for several reasons. Unlike chlorine, PPA decomposes into biodegradable residuals of vinegar (acetic acid) and hydrogen peroxide that can pass fish toxicity tests without removal. These residuals are not toxic, mutagenic, or carcinogenic. Bioaccumulation in aquatic organisms is also highly unlikely. Neither chlorinated compounds nor harmful disinfection by-products (DBPs) are produced with its use. As such, PAA has been considered the potential answer to tough DBP regulations. Peracetic acid can also disinfect over a wide range of pH and is unaffected by nitrate and ammonia concentrations.

Chemical handling of PPA is toted for being easier and safer than chlorination. The disinfectant can be stored for long periods of time exhibiting less than 1% decrease in activity per year when properly stored. Its use does not require any special risk management plans (RMPs) required by the EPA when handling certain toxic chemicals. For systems that operate under cooler conditions to prevent contamination or elevated temperatures, PAA has a low freezing point. Switching to PAA requires minimal retrofitting with the chemical itself being offered at prices competitive to other disinfectants.

There can be some disadvantages to peracetic acid. Depending on the formula purchased, PAA introduces varying amounts of acetic acid into the wastewater effluent. This can contribute to biological oxygen demand (BOD) and may not be appropriate for systems that are struggling to meet these limits. The biggest challenge wastewater systems face is regulatory approval. While PAA has been approved by the EPA as a primary disinfectant, each state regulatory agency must also approve its use. A WaterOnline guest column includes an infographic of states that have approved PPA as of 2017. The guest column discusses how systems can approach local regulatory agencies to seek approval on a case-by-case basis.

The overall effectivity of PPA will depend on wastewater characteristics, the PAA concentration, contact time, and the reactor configuration. Dosage will depend on the target organisms, wastewater quality, and level of inactivation required. When monitoring PAA residuals, operators can use the same analyzer and method as for chlorine residuals. A standard EPA sampling method does not yet exist. The lack of established methods and protocols for PAA makes approval difficult for local regulatory agencies. To help investigate the use and implications of PAA in wastewater, the Water Research Foundation (WRF) completed a study to evaluate effluent toxicity as well as dosage and contact times required to meet compliance. Metro Vancouver’s Northwest Langley WWTP in Canada has also published findings from a multi-year pilot program that used PAA as a disinfectant. More studies will have to expand on existing research until peracetic acid can become easily and widely adopted.