Heat-killing of Legionella in biological sludge from a paper and pulp mill water treatment plant

Paper and pulp mills use biological water treatment plants to reduce Chemical Oxygen Demand (COD) release to the environment. In the end of the process, microorganisms are concentrated into a biological sludge. Among the microbes thriving in these plants are Legionella, causing the Legionnaires disease. Combustion of the biological sludge produced at a plant results in unwanted downstream effects on the production and probably increased formation of nitrogen oxides (NOx) in the recovery boiler. Due to the disadvantages of combustion, the possibility to sterilize biological sludge has been investigated as a part of the continuously ongoing work at Metsa to improve occupational safety and reduce impact on the environment in a proactive way. A method to eradicate Legionellae in biological sludge would improve safety and ecological sustainability if the sludge instead is safely composted and used as e.g. soil fertilizer. Here we have assessed the time to death upon sludge heat treatment of a pathogenic L. pneumophila serogroup 1 strain, a L. longbeachae strain and the bacteria naturally occurring in biological sludge at the Metsa Board, Husum mill, Sweden. Time to death decreased with increasing temperatures up to 65 degrees C, where higher temperatures resulted in neglectable gain in time to death.