Abstract
Efficient control of pathogenic bacteria, specifically Legionella pneumophila, is one of the main concerns when operating industrial cooling towers. Common practices to limit proliferation involves use of disinfectants, leading to formation of disinfection by-product and increase in water corrosiveness. A disinfectant-free Legionella control method would make the industry more environmentally friendly. A pilot-scale cooling tower (1 m3/h) operated with demineralized water was used to investigate the potential of high-pH conditioning as a disinfectant-free alternative for control of L. pneumophila and other pathogens. One control experiment was performed under standard full-scale operation involving sodium hypochlorite dosage. Thereafter 3 alkaline pHs of the cooling water were tested: 9.0, 9.4 and 9.6. The tests lasted between 25 and 35 days. The cooling water from the basins were analysed for total cell count by flow cytometry, L. pneumophila concentration by plate count and occasional qPCR analyses targeting the mip-gene, bacterial and eukaryotic community analyses with 16S and 18S rRNA gene amplicon sequencing, relative abundance of eukaryotic to prokaryotic DNA by qPCR of the 16S and 18S rRNA gene. The L. pneumophila analyses showed considerable growth at pH 9.0 and pH 9.4 but was maintained below detection limit (< 100 CFU/L) at pH 9.6 without disinfection. Interestingly, the results correlated with the overall abundance of protozoa in the water samples but not directly with the relative abundance of specific reported protozoan hosts of Legionella. The pathogenicity based on 16S rRNA gene amplicon sequencing of the cooling water DNA decreased with increasing pH with a strong decline between pH 9.0 and pH 9.4, from 7.1% to 1.6% of relative abundance of pathogenic genera respectively. A strong shift in microbiome was observed between each tested pH and reproducibility of the experiment at pH 9.6 was confirmed with a duplicate test lasting 80 days. High-pH conditioning ≥ 9.6 is therefore considered as an efficient disinfectant-free cooling tower operation for control of pathogenicity, including L. pneumophila.
Highlights
Evaporative cooling towers are prone to the growth of Legionella pneumophila, a bacterial pathogen which induces a severe form of pneumonia, named Legionellosis, when inhaled (Prussin II et al 2017; Ricketts et al, 2012)
The results demonstrate that a pH of or above 9.6 constitutes an efficient conditioning for the control of L. pneumophila in open recirculating cooling tower, alternatively to biocide dosage
L. pneumophila was unable to grow at pH 9.6
Summary
Evaporative cooling towers are prone to the growth of Legionella pneumophila, a bacterial pathogen which induces a severe form of pneumonia, named Legionellosis, when inhaled (Prussin II et al 2017; Ricketts et al, 2012). Together with other artificial water systems such as shower plumbing, swimming pools or wastewater treatment plants, they represent important sources of outbreaks due to the formation of aerosols spreading to the surround-. Evaporative cooling towers are the most often confirmed sources of Legionellosis outbreaks and can contaminate large numbers of people as their aerosols can spread over several kilometres (Paschke et al, 2019). Complex chemical conditioning and delayed response caused by sparse and indirect plate count monitoring (Bentham, 2000). In the Netherlands, L. pneumophila plate count is the standard monitoring method, with a detection limit of 100 CFU/L. While the Drinking Water Act requires response actions to be taken when the concentration exceeds 100 CFU/L, the Environmental Protection Act that applies to evaporative cooling towers does not specify any threshold (National Academies of Sciences, 2020). The industrial areas can implement their own risk management plan, often using a 104 CFU/L threshold according to technical guidelines (ESCMID, 2017)
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