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Control strategies for the presence of Legionella

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 Legionella in the recirculating waters of cooling towers.

Control strategies for the presence of legionellae

Test result (cfu/mL)

Required control strategy

Not detected (< 10)

(1) Maintain monthly or at least three monthly monitoring
Maintain water treatment program.

Detected as < 1000

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

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

  1. If not detected, continue to retest water every 3 to 7 days until 2 consecutive samples return readings of 'not detected', then repeat control strategy (1).
  2. If detected at < 100 cfu/mL. repeat control strategy (2).
  3. If detected at > 100 cfu/mL, investigate the problem and review water treatment program, and immediately carry out online decontamination.
  4. If detected at > 1000 cfu/mL, undertake control strategy (4).

Detected as > 1000

(4) Investigate problem

Review water treatment program.

Take necessary remedial action (including immediate online decontamination) and undertake control strategy (5).

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

  1. If not detected, continue to retest water every 3 to 7 days until 2 consecutive samples return readings of 'not detected', then repeat control strategy (1).
  2. If detected at < 100 cfu/mL, repeat control strategy (2).
  3. If detected at > 100 and < 1000 cfu/mL, investigate the problem and review water treatment program, immediately carry out online decontamination), and repeat control strategy (5).
  4. If detected at > 1000 cfu/mL, investigate and review the water treatment program, immediately carry out system decontamination, and repeat control strategy (5).