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  Persistent high biological counts in a closed-loop cooling system

a.     despite dosing isothiazolone, glutaraldehyde, and a biodispersant.

2.     Answer First

a.     You’re likely dealing with an entrenched biofilm that’s resisting standard treatments. A combined shock program—high-dose biodispersant + rotated biocide + mechanical filtration—should be implemented immediately.

3.     Likely Reasons for Persistent Biological Contamination

a.     Chemical Ineffectiveness

                                               i.     Biocide                                    Suboptimal

Biocide dose – low

Contact time – insufficient

                                                                                                pH – High or low

                                                                                                Temperature

                                                                                                Dilutions from leaks

                                             ii.     Bio dispersant                         Suboptimal

Product selection in combination with biocide

                                                                                                Poor penetration

Low Dose

b.     BioFilm                                                Existing biofilm shedding microbes

Low flow low sheer areas

c.     Mechanical system                             Dead legs

Poorly maintained filters – recolonization

Sand filters

4.     Chemical and pH

pH                   Glute ½ life     Iso ½ life         Difference

            8.0                   30 – 45 days    20 – 30 days    Both stable – Glute is more durable

9.0                      3 – 5             2 – 3                Both start to degrade – Iso faster

10                       8 – 12 Hrs     1 – 3 Hrs

10.5                    1 – 3             < 1                                                      

a.     Glutaraldehyde degrades via aldol condensation and polymerization, forming non-biocidal polymers that may contribute to fouling, especially at pH >9.5.

b.     Isothiazolone degrades via alkaline hydrolysis, breaking the active ring structure and yielding non-biocidal fragments—rapid and irreversible above pH 9.

5. Dosage Review:
   
   1. Isothiazolone: 10–25 ppm active (higher if slug dosing weekly)
   2. Glutaraldehyde: 50–200 ppm active (often 100 ppm slug for control)
   3. Biodispersant: follow manufacturer dose; if it’s <100 ppm, consider increasing.

6.     Conduct a Biofilm Penetration Strategy

6. 1. Apply a shock dose of dispersant (e.g., 200 ppm non-ionic or amphoteric dispersant) with high-dose oxidizing or non-oxidizing biocide.

                                               i.     Let circulate for 6–12 hours.

                                             ii.     Filter heavily.

7. Consider temporary installation of a side stream filter (1–5 micron) to capture biomass slough-off.

8.     Inspect and Eliminate Stagnation Points

9. Audit the loop for dead legs, idle branches, or bypasses.
   
   1. If present, retrofit with continuous flow or flush valves.
10. Confirm minimum velocity (~3 fps) is maintained in all parts of the loop.

11.  Switch or Rotate Biocides

11. 1. Microbial resistance can build to repeated treatments. Try this:

                                               i.     Replace glutaraldehyde with DBNPA or THPS, both effective in closed loops.

12.  Confirm Test Accuracy

12. 1. Ensure microbial monitoring uses both culture-based (dip slides, HPC) and ATP-based testing for more insight.
    2. Take samples from multiple representative points—not just the supply header.

13.  Mechanical Cleaning if Needed

13. 1. If counts remain high after chemical cleaning, consider mechanical cleaning (e.g., pigging or flushing loop segments).
    2. Review loop filters—clean or replace them frequently during biocide treatment cycles.

14.  Longer-Term Improvements