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Case Studies

Learn from a real-life case study about the dangers of ineffective biocide dosing in chilled water systems. Monitoring and maintaining microbiological control.

Maintaining Water Purity: Biocide Dosage in Chilled Systems

We were called in to a building in central London with problems in the chilled water system (CHW). The building was maintained by a large national FM company with a water treatment subsidiary. Lack of control of the pseudomonads within the system was concerning the client who had spent tens of thousands of pounds on biocide dosing. Our brief was to give an independent view of previous actions and compare to the British Standard and the BSRIA guides.

What we found was interesting:

  1. All actions were being recommended based on the evidence of a single sample point – BS8552 details the sample numbers and locations needed for a meaningful understanding of the system. There was no rationale behind the treatment programme and no desired outcomes.
  2. Action was recommended quoting BSRIA BG29 – despite this being a ten-year-old building! Advice was given on a case by basis without consideration of geometric mean or trends.
  3. There was no understanding of the system volume.
  4. A biocide was put forward with no efficacy testing against the system bacteria.
  5. Biocide was dosed with no idea of system volume and potentially under-dosed leading to resistant strains of bacteria developing.
  6. When a dose of biocide showed to be ineffective, the recommended action was to dose the same biocide, in the same way, at the same concentration.
  7. The system was not regularly circulated and contained numerous bypasses and dead-legs. Circulation was not achieved after dosing biocide.

We reported to our client the following:

  1. Treatment needs an objective. Use the correct guidance and knowledge of their system to develop a rationale for treatment with desired outcomes. Monitor those outcomes, judge performance and take any remedial action on the basis of informed evidence.
  2. If a biocide needs to be dosed make sure it’s going to be effective before dosing it:
    • Test its efficacy against the target organism from your system at the concentration you plan to use
    • Determine the real system volume – tracer testing is generally the best way if done well
    • Have an understanding of all bypasses, dead-legs and dirt pockets in the system.
    • Dose the biocide
    • Circulate the system
    • Open all the bypasses, flush the dead-legs and dirt pockets to ensure biocide reaches every area
  3. Independently monitor regularly to ensure control is being maintained.

Dosing biocides ineffectively will have several undesirable effects:

  1. Biocides will break down into compounds that provide nutrient for bacteria including, for some biocides, SRBs.
  2. Sub lethal dosing will lead to resistant strains of bacteria. Evolution is very quick when generation times are measured in minutes!
  3. There are limited biocides available and due to the BPR it is unlikely that there will any more in the future. Misuse in systems can easily lead to a situation where no biocide is available!


We highlighted some serious failings and overspend on unnecessary and counterproductive actions. As a result of our work our client was able to regain microbiological control of their system. Money was saved on unnecessary treatment and the outcome was a well maintained system with a long potential life.

Where to find us?

Royal House
3 Kingdom Close
Segensworth East
PO15 5TJ
Phone Number:
01489 55 77 77