Engineering for

Legionella Control at

Cooling Towers

Clive Broadbent AM, FIE Aust.

www.broadbent.com.au

Legionella in the environment

Chain of Causation

Total Microbial Control

Temperature

Biofilm

Protozoa, bacteria

Nutrients

Heat Rejection by Evaporation

Towers on Roof

Towers Indoors

Industrial Applications - small

Larger

Very

Large

Inside

Warning

At Mines

Refining metals

Co-located

Single Flow – for cleaning

Single Flow

Power Stations

Variation of Legionella Counts with

Basin Water Temperature

kW v. Cooling Tower Water Temperature

Total Microbial Control

Temperature

Biofilm

Protozoa, bacteria

Nutrients

Poor Practices

Stagnant water - deadleg

Duty/Standby changeovers

Nutrients - algae on the top deck

Overflow – chemical loss

Uncontrolled Water Losses

Windage

Air intakes

Location

Location

Location

Outbreak Lessons

Outbreak

- chillers

Pumps

Tower 1

Tower 2

Biocide dosing

Two Independent Systems

C1

P1

CT1

C2

P2

CT2

Two Interconnected Systems – Scheme 1

Auto valves

interlocked with

pumps

Allows P1 to

serve C2 or P2 to

serve C1 in event

of failure

Design water flow

maintained

through each

cooling tower

C1

P1

C2

P2

CT1 CT2Auto

Valves

Auto

Valves

C1

P1

C2

P2

CT1 CT2Manual

Valves

Auto

Valves

Auto valves at

towers replaced

with manual

isolating valves

On auto operation,

uncontrolled water

losses occur when

one pump/chiller/

tower is sufficient

for load

Two Interconnected Systems – Scheme 2

Piping (no balance line)

Chemical dosing point (return lines)

Details

Site dust load high

Piping/pumping irregularities

No balance line between towers

Return piping at high level

No anti-syphon trap

Overflow on pump shutdown

Plant, incl. chillers, on manual control

Contd

Details

Chemical injected at return line

Componentry (dosing pump) unreliable

Only one biocide in use

non-oxidising

low concentration

Contractual difficulties

Major L.D. outbreak

Contd

Examples of

Developments

Safety

Example

Example

Snow making

Air Compressors

Towers in the Snow

Dry Basin

Water Saving

Understand and improve plant operation

(best practice)

Uncontrolled water losses (identify, remove)

Fix basin leaks

Increase cycles of concentration if relevant

Clean mud out of fill

Strategies

Splashguards

Eliminators

Sheets rolled back

Cooling Tower Makeup Water

with 5.5 C Range

Water Savings

For a cooling tower rejecting 2,000 kW, return water 35OC, supply water 29.5

OC, recirculation is

s/L87183.4x5.5

2000

Evaporation is s/L82.01000x430,2

996x2000 (where 996 is water density, 2430 is enthalpy of vaporisation).

Drift is 0.002% of flow = 0.002 L/s

Take TDS as 30 ppm (Melbourne)

For 2 cycles, bleed required = s/L82.0)3060(

30x82.0

s/L87

183.4x5.5

2000 s/L87

183.4x5.5

2000 and water

used is 0.82 + 0.82 + 0.002 = 1.64 L/s

For 6 cycles, bleed = s/L16.0)30180(

30x82.0

and water used is 0.16 + 0.82 + 0.002 = 0.98 L/s

Cycles 1.25 1.5 2 2.5 3 4 5 6 7 8 9 10 12 14 16 18 20

Water Use L/s 4.1 2.5 1.6 1.4 1.2 1.1 1 1 1 1 .9 .9 .9 .9 .9 .9 .9

Cooling Tower Makeup Water

with 5.5 C Range

Cooling Tower Makeup Water

with 5.5 C Range

0.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.61.71.81.92.02.12.22.32.42.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Cycles of Concentration

Makeu

p L

/s

Reclaim System at Office

Cost - $140,000

Payback – 2 years

Water saving – 1,135,520 litres reclaimed in 2

months

Reuses tower blowdown

Reclaim tank/flusher tank

Reclaim System at Office

Cooling water system primary disinfection by

bromine generated at site.

Reclaim system secondary disinfection by

electrochlorination – converts high chlorides

in blowdown water into chlorine

Riverside Centre

Reclaim Tank

Standards

AS/NZS 3666 - Microbial Control

Part 1 - Design, installation, commissioning

Part 2 - Operation, maintenance

Part 3 - Performance-based maintenance of

cooling water systems

Design features for towers (prescriptive – Part 1)

Convenient accessible openings

Components that can be removed

Sumps that can be readily drained

Materials compatible with use of disinfectant and hosing

with water jets

Use of components that minimise corrosion

Efficient drift eliminators (0.002% loss)

Minimal internal components such as structural brackets

which can collect sediment

Surfaces which can be readily cleaned

Protection of wetted surfaces from direct sunlight

Operation & maintenance features

(prescriptive – Part 2) Layout of system

Correct and safe operating procedures

Maintenance, cleaning and disinfection procedures and their frequency

Regular water treatment regimes

Bleed rate

Testing requirements pH

total dissolved solids or conductivity

bacterial counts

Disinfectant levels

Safety precautions

Person or contracting agency responsible for: overseeing and recording the work

ensuring that the plant operates normally

Performance-based maintenance (Part 3)

Assessment

the problem

Preventive measures

known controls

Operational procedures

continuing controls

Verification

monitoring the controls

Incident reporting

evidence

Companion Standard

AS 5059 – 2006

Power station cooling tower

water systems – management

of Legionnaires’ disease

health risk

Other Hazards

Artesian Bores

Mine

Are we there yet?

While there is an environmental pathogen and

there are people susceptible to infection, there

may always be cases of disease.

Legionella control is based on a partnership of

disciplines including engineering. Scientific

understanding and wisdom have already come

a long way. But we are still on a journey of

discovery. May all travel well.

Thank you

for

your attention