Water Treatment for Closed Heating and Cooling Systems (BG 50/2021)

Common customer questions & answers

QQuestion BG 29/2021 gives guidelines for pseudomonas levels, but BG 50/2021 doesn’t. Why is this?

AAnswer Pseudomonads are environmental slime-forming bacteria that can enter a system via debris and/or mains water supplies. Depending on location, the levels of pseudomonads within mains water will vary and can be as much as 1000 cfu/ml. Pseudomonads that enter systems may promote the formation of biofilms and will contribute to the overall TVC within a system, but should not normally exceed TVC levels.

Pseudomonads are facultative anaerobes which means they can act as either aerobic or anaerobic species depending on conditions within the system. When anaerobic, they act as nitrate/nitrite reducing bacteria (NRB) and can utilise nitrite corrosion inhibitors as a food source.

BG 29/2021 states that microbiological limits for TVCs have been retained at 10,000cfu/ml in line with recommendations for the avoidance of biofilms in cooling towers (HSG 274 Part 1). Evidence has shown that this overall TVC limit can achieve an acceptable level of control over both free-flowing and surface-adhering bacteria. Microbiological limits for pseudomonads have been revised to 1000 cfu/ml as an additional indicator of the potential for biofilm growth. Pseudomonads should be determined using a pseudomonas selective agar so that results are directly comparable with overall TVC counts. It is important that individuals submitting samples for analysis should request the appropriate closed system analysis and ensure results are expressed as cfu/ml.

If a system has been properly cleaned and dosed with a biocide, it is anticipated that pseudomonad levels should be minimal. However, in older systems, there are many factors which may influence the establishment of pseudomonads. These include leaks and subsequent water top-up, poor circulation, functioning temperature and presence of biocide. It is therefore important that regular monitoring is maintained throughout the life of a system to ensure that the system does not develop significant biofilms which may encourage sulfate-reducing bacteria (SRB) colonisation and subsequent microbiologically influenced corrosion (MIC). The build-up of biofilm within a system will also prevent the inhibitor from contacting the metal surfaces thus preventing the corrosion inhibitor from working effectively and potentially leading to under deposit corrosion.

Opinions vary about the significance of high pseudomonad counts in older systems, with some practitioners arguing that the limits should be the same as for TVC and that 100 cfu/ml would certainly be no cause for concern. Various data from actual sites suggest that pseudomonad counts are typically around 10% of TVC counts in a “dirty” system (one operating around the TVC limit) but only 1% of TVC in a clean system.

In BG 50/2021, the analysis of TVC (and SRB) throughout the life of a system is considered sufficient as an indicator of system water quality and the limits for TVCs are set at 10,000 cfu/ml (i.e. in line with BG 29/2021 and other industry standards). As pseudomonads form part of the TVC count, if TVC counts are low, it is considered that pseudomonad counts will also be low. If TVC counts are detected at elevated levels, then the water treatment specialist should decide whether pseudomonads should be analysed.

If elevated levels of pseudomonads are detected, the water treatment specialist should decide on a suitable control limit for pseudomonads based on their knowledge of the system and operational requirements. In particular, they should investigate to establish whether the system is still protected by biocides and whether circulation of biocide-treated water is able to reach all extremities, dead legs and stagnant areas.

QQuestion Which edition of BSRIA’s Water Treatment for Closed Heating and Cooling Systems guide should I be using?

AAnswer The current edition of BSRIA’s Water Treatment for Closed Heating and Cooling Systems guide is BG 50/2021. It was revised in partnership with industry bodies with input from a steering group, members of which are listed in the guide. It supersedes all previous editions of the guide. It is BSRIA’s intention that, in normal circumstances, BG 50/2021 should be used in preference to the superseded guide, BG 50/2013. The superseded edition is still available as it provides useful reference information for older projects. Users of BG 50/2021 should be aware that it is a guide, not a regulation. It is for individual project teams to decide whether to follow the guidance, and this should be made clear in project documentation.