Cooling Towers Can Harbor Legionella

Plan to keep Legionella bacteria out of ethanol plants rather than respond to a crisis. This article is published in the June issue of Ethanol Producer Magazine.
By Randy McDaniel | May 13, 2016

Legionella bacteria have been making a lot of headlines.

In August 2015, for example, 12 people died and 120 became seriously ill after Legionella bacteria contaminated a Bronx hotel cooling tower in New York City. Later in the year, a major pharmaceutical company in North Carolina stopped production and furloughed employees for about 72 hours to clean its cooling towers after finding that they contained high levels of Legionella. In February 2016, a $100 million lawsuit was filed against a Michigan hospital after a Legionellosis outbreak.

Ethanol producers need to better understand the Legionella threat and its potential implications to prevent something similar from occurring in their plants.

Legionella bacteria are present in water environments. Cooling towers are a known Legionella reservoir that can lead to the amplification of these bacteria. Cooling towers associated with the ethanol manufacturing typically are operated with ideal growth conditions for Legionella. These include water temperatures of 77 to 108 degrees Fahrenheit, low water velocities and the presence of scale, sediment, iron, biofilms and amoebae.

Legionella bacteria can cause Legionnaires’ disease and Pontiac fever, collectively known as Legionellosis. People must inhale an aerosol of Legionella originating from an environmental reservoir to contract the disease.

Legionnaires’ disease is an especially virulent form of pneumonia with a fatality rate of 15 to 80 percent, generally requiring hospitalization. Patients with Pontiac fever demonstrate flu-like symptoms, but are not at risk of death. 

Healthcare professionals report cases of Legionellosis to the Centers for Disease Control and Prevention through the National Notifiable Disease Surveillance and Supplemental Legionnaires Disease Surveillance systems. According to the CDC, from 2000-’09, 22,418 cases of Legionellosis were reported, a nearly threefold increase during that period. The development and use of urine antigen tests are the most likely cause for the increased incident reporting.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers and American National Standards Institute published Standard 188-2015 last year to establish minimum risk management requirements for building water systems. The standard has no enforcement capability, but sections were adopted in New York City after the outbreak, where all owners with cooling towers now must comply. Regardless, the standard provides a process to reduce the risk of Legionellosis at ethanol plants.

Hazard Assessment
Disease prevention begins with a site survey to determine if a facility has any specific systems listed in Section 5.1 of the standard that can be reservoirs of Legionella—cooling towers being one of many. Facility mangers also should determine if their sites have any characteristics listed in Section 5.2 that mostly are related to healthcare and could potentially involve potable water. I do not anticipate ethanol plants needing to evaluate their potable water systems. 

At-risk facilities should form a team and develop a water management program (WMP) following the broad outline shown in the accompanying diagram. Specific elements are detailed in the standard. The team must have authority to make decisions, have knowledge of the water systems and know about Legionella and risk mitigation solutions. 

During the hazard assessment, the cooling tower system should be inspected for three principle hazards. First, a hazard exists if any Legionella bacteria are present in a biofilm or dispersed in the water. A second hazard is the presence of favorable Legionella growth conditions such as suspended solids, iron, ideal temperatures or stagnation. A third hazard is Legionella dissemination. For example, if drift eliminators are not working properly, cooling water aerosols could spread Legionella beyond the ethanol plant property. 

The WMP documents the control strategies for each identified risk. It includes procedures for system startup, shutdown and maintenance. It establishes test frequency for all mitigation procedures and establishes remediation procedures when, or if, the plan does not yield the desired results. Procedures include emergency protocols should a Legionellosis case be reported.

Risk Mitigation Strategy
It is common for cooling system water treatment to include some form of biocide application to control bacteria that affects scaling, corrosion and fouling. Biocides are applied in accordance with their labels and their application is federally regulated by the U.S. EPA.  Most labels permit intermittent and continuous feed strategies.

A common procedure at ethanol plants is to apply the oxidizing biocide, sodium hypochlorite (bleach), in the cooling tower. Continuous feed strategies maintain a free residual chlorine in the tower water which can be monitored.  Intermittent feed strategies shock the system with a brief, high concentration of bleach. Care is taken with both strategies because bleach usage can adversely impact system corrosion. Either feed strategy can be automated using ORP sensors that track oxidation reduction potential. Biocide effectiveness is affected by contact time and concentration, although pH also can affect performance.

Plants that discharge water into the environment may not find some commercially available biocides to be an option. If bleach is used, the water must be dechlorinated before discharge to meet the requirements of a plant’s National Pollutant Discharge Elimination System permit. Total organic loading also impacts bleach effectiveness. Bleach consumption increases with an increase in outside organic matter such as bird droppings, insects, cottonwood seed or even DDG dust. The cooling tower is an effective air washer.

After deciding which biocide strategy to implement, this decision must be documented in the WMP along with a method for verifying the proper dosing of the biocide. This can be as easy as regularly measuring actual chemical consumption against the design consumption. Action plans also are needed describing what steps will be taken if the biocide has not been properly dosed.

One could argue that if no person has been diagnosed with Legionellosis linked to the cooling tower, the risk mitigation strategies are working. Though this may satisfy a strict interpretation of a plan validation, it is fraught with legal risk and could lead to the potential death of an individual who contracts Legionellosis. 

Directly testing for Legionella through an accredited lab is an option. While it once was believed that controlling total bacteria levels is sufficient, Legionella is found in water with either high or low levels of total bacteria present and in clean or dirty systems. Direct testing is the only way to confirm the presence or absence of these bacteria. Testing typically is conducted monthly or quarterly.

Finally, the WMP serves as a playbook for actions to be taken for various concentrations of Legionella, or if someone is diagnosed with Legionellosis. OSHA has provided guidelines for both situations. If direct test results are positive, but below 100 colony forming units per milliliter (cfu/ml) of Legionella, a review of the program and additional biocide treatment of the system is warranted. If direct test results exceed 100 cfu/ml of Legionella, prompt cleaning or biocide treatment of the system is recommended. If direct test results exceed 1,000 cfu/ml of Legionella, immediate cleaning or biocide treatment is recommended and prompt action must be taken to prevent employee exposure.

It may be necessary to turn off the cooling tower fans to help prevent employee exposure. Without the fans, the cooling system temperature will increase and have a negative impact on fermentation temperature control. Each plant develops its own response protocol, but it can be as easy as increasing the concentration of bleach to a higher level of free residual chlorine for a period of time, which will impact discharge water treatment. Testing should be repeated after any remediation to ensure Legionella are destroyed. 

If someone is diagnosed with Legionellosis, the CDC procedures call for shutting off the heat source and cooling tower fans. The system is treated to maintain at least 10 ppm of free residual chlorine for at least 24 hours, tested at two-hour intervals. The use of a dispersant also is part of the procedure, then the system is drained and refilled. 

Local or state authorities should be included in the implementation of this procedure because the disposal of this water could have a problematic effect on the environment. Dechlorinating is required at a minimum. After this procedure, the plant could be left with several hundred thousand gallons of partially fermented mash to further process and several hundred thousand gallons of cooling tower water that requires proper disposal.

Even the most thorough WMP will not guarantee the absence of Legionella, but controlling it is the key to preventing Legionellosis. Understanding the bacteria, the disease and establishing effective risk management can prevent Legionella from adversely impacting ethanol plants. Writing a detailed water management program helps a management team make decisions should an emergency occur.

Author:  Randy McDaniel
Strategic Accounts Manager,
Weas Engineering Strategic