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Control strategies for the presence of heterotrophic micro-organisms

This strategy is drawn from AS/NZS 3666.3 Air-handling and water systems of buildings - Microbial control - Performance-based maintenance of cooling water systems and provides an approach to dealing with the presence of heterotrophic micro-organisms in the recirculating waters of cooling towers.

Control strategies for the presence of other heterotrophic micro organisms

Test result (cfu/mL)

Required control strategy

< 100 000

(1) Maintain monthly monitoring
Maintain water treatment program.

> 100 000

< 5 000 000

(2) Investigate problem
Review water treatment program.
Take necessary remedial action (including immediate online disinfection) and undertake control strategy (3).

(3) Retest water within 3 to 7 days of plant operation

  • If the test result is < 100 000 cfu/mL, repeat control strategy (1).
  • If the test result is > 100 000 cful/mL but < 5 000 000 cfu/mL, undertake control strategy (2).
  • If the test result is > 5 000 000 cfu/mL, undertake control strategy (4).

> 5 000 000

(4) Investigate problem
Review water treatment program.Take necessary remedial action (including immediate online disinfection) and undertake control strategy.

(5) Retest water within 3 to 7 days of plant operation

  • If the test result is < 100 000 cfu/mL, repeat control strategy (1).
  • If the test result is > 100 000 cfu/mL but < 5 000 000 cfu/mL, repeat control strategy (4).
  • If the test result is > 5 000 000 cfu/mL, investigate the problem, review the water treatment program, and carry out immediate online decontamination.