22 ENVIRONMENT and ENERGY
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Risk management approaches to control legionella
This document describes risk management approaches and technologies for controlling legionella growth in premise plumbing systems. The information presented is based on the references reviewed during the preparation of this document.
By the Environmental Protection Agency
In general, technologies for controlling legionella have been shown to offer some degree of effectiveness against legionella. However, the long-term eradication of legionella from a premise plumbing system has not been demonstrated consistently with any of these technologies. Complex plumbing systems, such as those found in a multi-storey building, might have areas where there is less exposure to disinfectants and heat, which could provide opportunities for bacteria to grow. Legionella bacteria may be found in biofilms or in sections of the plumbing system with long water residence times, depending on the pipe materials, water temperature and other system-specific factors. The effectiveness of a technology against legionella growth in biofilm or legionella ingested by amoebae is often cited as a concern. Other studies suggest that disinfectants, disinfection byproducts and other environmental pollutants may induce an increase in antimicrobial resistance of bacteria, including pathogens such as L. pneumophila( Ashbolt et al., 2013).
The retention of viable legionella in amoebae cysts is an important factor for risk management of water distribution and premise plumbing systems. Springthorpe et al.( 2014) discusses the importance of the association between opportunistic pathogens, such as legionella, and free-living protozoa( which include amoebae), and how the protozoa might lead to long-term persistence of the pathogens by allowing them to relocate and / or avoid interventions, such as disinfection. Wang et al.( 2013) suggest that natural systems may provide conditions, such as an abundance of beneficial microbial diversity, which may help prevent and potentially control the growth of opportunistic pathogens that can be found in engineered environments.
Establishing and maintaining a disinfectant residual throughout the system is critical for the effectiveness of chlorine, monochloramine, chlorine dioxide and CSI treatments. Maintaining a disinfectant residual provides increased protection in the event legionella is released into the premise plumbing system( for example, sloughing off of biofilm material containing legionella) or enters a premise plumbing system through the PWS distribution system. Ozone and UV disinfection do not produce a disinfectant residual( USEPA, 2007). Therefore, water treated with only these methods, in some cases, may be susceptible to subsequent contamination unless treatment is at the point of use or supplemental treatment is provided. For these reasons, more than one type of treatment or control measure may be necessary to inhibit legionella growth in a premise plumbing system( VHA, 2014).
The effectiveness of a particular technology is dependent upon building-specific characteristics, such as pipe material, age and condition; water usage rates and water age; and water quality parameters( for example, pH, hardness, organic contaminants, inorganic contaminants, and types of waterborne pathogens). Therefore, decisionmakers may want to consider the specific conditions of each premise plumbing system before making a decision and ensure that the conditions are adequate for the selected approach.
The physical and chemical characteristics of the finished water from water treatment plants
March 2017 Volume 23 I Number 1 www. plumbingafrica. co. za