Legionella Risk in Cooling Towers: ASHRAE 188 and What Indian Industries Must Do

Cooling towers are among the highest-risk sources of Legionella because they combine warm water in the range where Legionella multiplies (roughly 20–50°C), biofilm and aerosol drift. ASHRAE Standard 188 manages that risk through a documented water management programme: a responsible team, a hazard analysis, control limits, monitoring, corrective action and verification that the controls work.

This guide covers why Indian towers are vulnerable, what ASHRAE 188 asks for, and the practical controls and testing regime that keep a tower in check.

Why are cooling towers a Legionella risk?

Legionella bacteria multiply in warm water, and Indian cooling towers operate in the 25–45°C band year-round. Towers also produce drift, fine aerosols that can carry bacteria beyond the tower. Legionnaires' disease is contracted by inhaling contaminated aerosols; it is not caught by drinking water and is not normally spread from person to person.

Temperature alone does not create an outbreak. Legionella amplifies where several conditions coincide:

• Biofilm on fill, basins and pipework, which shelters bacteria from biocide.
• Sediment, scale and corrosion products that provide surface area and nutrients.
• Stagnation in dead legs, standby circuits and towers left idle.
• Inconsistent biocide residual, often after make-up water events that disturb biofilm.
• Warm, humid ambient conditions that keep the water in the growth range for most of the year.

Which facilities in India face the highest risk?

Risk is highest where cooling towers sit near vulnerable people or critical operations: hospitals, data centres, hotels and large commercial buildings. A data centre tower running at 6–8 cycles of concentration, 24 hours a day, cannot simply be switched off for cleaning, so control has to be continuous rather than periodic.

What does ASHRAE Standard 188 require?

ASHRAE Standard 188 (current edition 188-2021) sets the minimum requirements for a Legionella water management programme covering each at-risk building water system, including cooling towers. It does not prescribe a single biocide or test frequency; it requires the owner to define, document and verify controls suited to each system.

• A programme team with named responsibilities.
• A description of each water system, with flow diagrams.
• A hazard analysis identifying where Legionella could grow and spread.
• Control locations, with control measures and limits for each.
• Monitoring procedures and frequencies.
• Corrective actions for when limits are breached.
• Confirmation that the programme is implemented as designed and is effective.
• Documentation and communication of all of the above.

ASHRAE Guideline 12 complements the standard with practical guidance on minimising Legionellosis risk in building water systems, and BSRIA BG 50 covers treatment of closed heating and cooling circuits. ASHRAE 188 is a US consensus standard, not an Indian statutory requirement, but it is increasingly used as the benchmark in due diligence by multinational tenants, operators and investors.

Which control measures keep Legionella in check?

Effective control combines water chemistry, biocide and physical cleanliness. Navbharat Water's ASHRAE 188 plans hold oxidising biocide at 0.2–0.5 mg/L free residual, add UV or ozone as supplemental disinfection, and use side-stream filtration to 10 microns to remove the sediment that shelters bacteria.

Chemistry and hygiene reinforce each other. Scale and corrosion deposits protect biofilm from biocide, so a well-controlled tower is also easier to disinfect. At a 2 MLD hyperscale data centre WTP in Pune, RO and softening of make-up water raised cycles of concentration from 3 to 6.5, cutting make-up consumption by 50%. Physical measures matter too: effective drift eliminators, removal of dead legs, and a cleaning and disinfection routine for basins and fill.

How often should cooling tower water be tested for Legionella?

Navbharat Water's plans include quarterly Legionella testing by an NABL-accredited laboratory, backed by continuous online tracking of conductivity, pH and biocide dose. Culture results take several days, so the daily defence is keeping control parameters within limits; laboratory tests then verify that those controls are working.

• Continuous: conductivity, pH, biocide dose and residual, tracked on the Smart Water platform with automated alerts.
• Routine: visual inspection of basins, fill and drift eliminators, and general bacterial counts as a trend indicator.
• Quarterly: Legionella culture testing by an accredited laboratory.
• Additional: sampling after a shutdown, restart, major repair or any positive result.

Quarterly testing on its own is not a programme. A tower can test clean in one quarter and amplify rapidly weeks later if biocide control lapses, which is why continuous monitoring carries the day-to-day load.

What should happen when Legionella is detected?

The plan should set response levels in advance, agreed with the testing laboratory, so a positive result triggers action rather than debate. Responses typically escalate in three steps: verify controls and resample, increase biocide and investigate the cause, then carry out online disinfection and a full clean. Every step and result should be documented.

Investigation matters as much as disinfection. A positive result usually means a control failed somewhere: a dosing pump stopped, a dead leg was brought back into service, or the residual dropped during a load change. Fixing the cause, and recording it, is what prevents the same result next quarter.

How should an Indian facility build its water management plan?

Start with an inventory and hazard analysis of every tower and connected system, then set control limits, monitoring frequencies and response steps for each. Most sites already have treatment data; the gaps usually lie in documentation, defined limits and verification. The ASHRAE 188 programme elements listed above provide the structure.

• List every cooling tower, evaporative condenser and connected circuit, with flow diagrams.
• Identify hazards: stagnant sections, standby units, nutrient sources, drift paths near air intakes.
• Set control limits for residual, pH, conductivity and cycles, and name who checks them.
• Define laboratory testing frequency and response levels.
• Write corrective action procedures, including shutdown and restart.
• Review the plan annually and after any change to the system.

Sites that use treated sewage as make-up need extra care. An on-site MBR STP with tertiary polishing can supply make-up water: at a 150 KLD IT park STP in Hyderabad, 120 KLD is recycled daily for cooling towers and landscaping. The plan must then cover the reuse water's quality and disinfection as a control point.

Navbharat Water implements ASHRAE 188 water management plans for data centre and IT park cooling towers through its IT Parks and Data Centers offering (/industries/data-centers), with continuous biocide and conductivity tracking on Smart Water Monitoring (/solutions/smart-water). Hospital programmes are covered under Healthcare and Pharma (/industries/healthcare-pharma).

Frequently asked questions

Is ASHRAE 188 mandatory in India?

No. ASHRAE 188 is a US consensus standard, not an Indian statutory requirement. It is, however, the most widely recognised framework for Legionella risk management, and multinational tenants, operators and investors increasingly expect an ASHRAE 188 water management programme during due diligence, particularly for data centres, hospitals and hotels.

At what temperature does Legionella grow?

Legionella multiplies most readily between about 25°C and 45°C, which is the normal operating range of Indian cooling towers throughout the year. Below about 20°C it survives but multiplies slowly, and it is killed at temperatures of 60°C and above. Cooling towers cannot use temperature as a control, so they rely on biocide and hygiene.

Is quarterly Legionella testing enough?

Not on its own. Quarterly culture testing by an NABL-accredited laboratory verifies that controls are working, but results take several days and represent a single moment. Day-to-day protection comes from keeping biocide residual at 0.2–0.5 mg/L and tracking conductivity, pH and dosing continuously, with alerts when any parameter drifts.

Can treated sewage be used as cooling tower make-up water?

Yes, if it is treated and controlled properly. An MBR STP with tertiary polishing produces outlet BOD below 10 mg/L, suitable for cooling tower make-up. The water management plan must treat the reuse supply as a control point, with disinfection and monitoring, because higher nutrient levels can increase biocide demand.

Which biocides are used for Legionella control?

Oxidising biocides such as chlorine or bromine are the primary control, held at 0.2–0.5 mg/L free residual in Navbharat Water's plans. Non-oxidising biocides such as DBNPA can be used alongside them, and UV or ozone may be added as supplemental disinfection. The choice depends on water chemistry, system materials and discharge limits.