Chapter 3: Cooling towers and evaporative condensers

3.1 General

• 3.1.1 For the purposes of this standard, the term “cooling tower” shall also include “evaporative condenser.”
• 3.1.2 Cooling towers present a significant risk for exposure to Legionella as they allow favorable conditions for the growth of Legionella bacteria.
• 3.1.3 Plumes arising from cooling towers also pose a significant risk for transmission of Legionella bacteria.
• 3.1.4 The risks should be minimized with proper design, start-up and commissioning, maintenance, and testing, as indicated in the following sections.
• 3.1.5 Cooling towers may be subject to local regulations.

3.2 Design

• 3.2.1 This section applies to new facilities and to major upgrades to existing facilities.
• 3.2.2 The minimum separation distance shall be 10 metres between the cooling tower and any air intake or other critical receptor.
• 3.2.3 A separation distance exceeding 10 meters shall be used if there is a greater risk of contamination.
• 3.2.4 Do not locate cooling towers near kitchen exhaust fans, truck bays, other sources of organic materials, or outdoor areas that are densely populated.
• 3.2.5 Consider factors such as the height of adjacent structures, direction and velocity of prevailing winds, and the presence of enclosures and screenings around adjacent cooling towers while siting cooling towers.
• 3.2.6 Cooling towers shall be equipped with high-performance drift eliminators.
• 3.2.7 The design air velocity through the fill and the drift eliminators shall not exceed 3 m/s.
• 3.2.8 There shall be no bypass of air around the drift eliminators.
• 3.2.9 The drift eliminators shall be easily removable for cleaning, inspection, and/or replacement.
• 3.2.10 Tower construction material in contact with the water shall be corrosion resistant and compatible with disinfectants, biocides, and other cleaning agents.
• 3.2.11 Provide bolted and/or gasketed removable access openings to facilitate inspection and cleaning.
• 3.2.12 Design cooling tower basins for easy accessibility for cleaning and disinfection.
• 3.2.13 Protect wetted surfaces from direct sunlight to minimize bacterial growth under warm conditions.
• 3.2.14 Eliminate dead legs and reservoirs where water stagnation can occur.
• 3.2.15 Eliminate short-circuiting of cooling tower air with the discharge back into the cooling tower inlet.
• 3.2.16 For water chemical treatment purposes, include provisions for automatic continuous bleed, manual cleaning at regular intervals, application of scale and rust inhibitors on a continuous basis, and application of biocides, including shock chlorination, at timed intervals using automatic equipment.
• 3.2.17 Connect the discharge from cooling tower drain and/or overflow into the sanitary sewer such that back pressure, surcharge, cross-contamination, and/or reverse flow cannot occur.

3.3 Start-up, shut-down, and commissioning

• 3.3.1 No cooling tower shall be handed over, accepted, or put into service until commissioning is successfully completed.
• 3.3.2 Commissioning shall include, but may not be limited to, the following:
  • 3.3.2.1 Completion of a written maintenance program, including the manufacturer's required maintenance and treatment program.
  • 3.3.2.2 Thorough clean-out of all mechanical equipment prior to service.
  • 3.3.2.3 Disinfection of the entire system following the disinfection procedure in section 3.4 Operation & Maintenance.
• 3.3.3 When a system is to be shut down for more than three days, it shall be drained. When this is not practical, stagnant water shall be pre-treated with an appropriate biocide regimen before start-up, allowing for proper contact time according to the supplier's recommendations.
• 3.3.4 For any start-up at any time, follow the cleaning and disinfection procedure indicated in Section 3.4 Operation & Maintenance below.

3.4 Operation & maintenance

Inspection

• 3.4.1 Weekly inspection of cooling towers shall include as a minimum:
  • 3.4.1.1 Visual inspection under normal operating conditions, for signs of microbial growth, algae, water leaks, splashing, blockages, and restrictions at air inlets.
  • 3.4.1.2 Inspection of water treatment equipment for correct operation and adequate stock of chemicals.
• 3.4.2 Monthly inspection of cooling towers shall include as a minimum:
  • 3.4.2.1 The requirements of the cooling tower weekly inspection.
  • 3.4.2.2 Examination of water flow through the tower for normal unrestricted flow.
  • 3.4.2.3 Examination of drift eliminators internally and externally for damage and for excessive drift.
  • 3.4.2.4 With system power off, examination of the internal structure of the tower for the condition of the plant and equipment. Report any deterioration of materials, particularly the fill, drift eliminators, basin, and water distribution system.
  • 3.4.2.5 Test for free chlorine or other biocide levels.
• 3.4.3 Yearly inspection of cooling towers shall include as a minimum:
  • 3.4.3.1 The requirements of the cooling tower monthly inspection.
  • 3.4.3.2 A more detailed inspection of all system components and a detailed assessment by a water treatment specialist for indications of corrosion, biofilms, or deposits.

Cleaning

• 3.4.4 Cooling towers shall be maintained in a clean working condition whenever the equipment is in use.
• 3.4.5 Start-up and annual cleaning of cooling towers shall include as a minimum:
  • 3.4.5.1 Use additives to aid in cleaning, including detergents and anti-foaming agents.
  • 3.4.5.2 Circulate the water for at least one hour throughout the system to provide coarse cleaning of the wetted surfaces.
  • 3.4.5.3 Switch off equipment and drain to waste in a manner approved by the local water authority.
  • 3.4.5.4 Thoroughly clean the internal shell, fill and sump off the cooling tower, moving or flushing away all debris.
  • 3.4.5.5 Refill with clean water.
  • 3.4.5.6 Dose with free chlorine or other biocide at recommended levels and circulate for one hour.
  • 3.4.5.7 Clean all filters, strainers, water nozzles, and fittings.
  • 3.4.5.8 Refill with clean water and treat again as required.

Disinfection

• 3.4.6 Disinfection shall be carried out at system start-up and when required by bacterial testing results.
• 3.4.7 Shock chlorination is one acceptable method for disinfection of cooling towers.
• 3.4.8 The disinfection procedure using shock chlorination for cooling towers shall be as follows:
  • 3.4.8.1 Ensure that the tower has been cleaned according to the cleaning procedure.
  • 3.4.8.2 Turn OFF the cooling tower fan during shock chlorination.
  • 3.4.8.3 Shock chlorinate the whole system including the cooling-tower distribution basin and fill with the circulating pump(s) in operation.
  • 3.4.8.4 During shock chlorination, maintain pH at less than 7.
  • 3.4.8.5 Maintain free chlorine residual of at least 5 ppm for at least 6 hours, or alternatively, a residual of 15 ppm for at least 2 hours.
  • 3.4.8.6 This shall be followed by continuous, automatically controlled feed of suitable water treatment chemicals with scale and rust inhibitors.
  • 3.4.8.7 Use an effective biocide program for microbiological control, complying with all federal, provincial/territorial, or municipal requirements.

Water treatment plan

• 3.4.9 The written water treatment plan for control of microbiological activity including Legionella, scale, and corrosion shall be site specific, prepared by a qualified person as part of the Legionella Bacteria Control Management Program (LBCMP), and shall include at minimum where chemical treatment is used:
  • 3.4.9.1 Specification of all equipment and chemicals used for the purpose of treating the open recirculation loop.
  • 3.4.9.2 Use of corrosion inhibitors or other manufacturer-approved product.
  • 3.4.9.3 Use of surfactants and other chemicals to control fouling due to formation of biofilms.
  • 3.4.9.4 Application of biocides or the use of other measures to control the growth of bacteria.
• 3.4.10 Water treatments shall be used according to supplier recommendations, or as recommended by local specialist water treatment contractors.
• 3.4.11 Interactions between the chemicals must be considered when planning the water treatment program. For example, the effectiveness of chlorine-based biocides may be reduced by interactions with surfactants.
• 3.4.12 Chemical dosing application points shall be in the turbulent zones of the distribution system to ensure rapid dilution and mixing. Acceptable methods of dosing application may include:
  • 3.4.12.1 Continuous drip feed
  • 3.4.12.2 Shock dosing
  • 3.4.12.3 Metered dosing
• 3.4.13 Consider the use of alternating biocides, as Legionella bacteria often develop a resistance to a single type of biocide.

3.5 Minimum bacterial testing requirements

Water sampling locations

• 3.5.1 Water-sampling locations for bacterial testing shall be in accordance with the facility LBCMP.
• 3.5.2 The facility LBCMP should consider, at a minimum, the following locations for water sampling:
  • 3.5.2.1 Storage tanks or reservoirs in the cooling tower system
  • 3.5.2.2 Cooling tower basins

Bacterial testing results

• 3.5.3 Record the bacterial testing results on the facility LBCMP testing form (LBCMP-4).
• 3.5.4 Report the bacterial testing results in accordance with the regional and national reporting protocols.

Minimum bacterial testing requirements and frequency (normal operation)

• 3.5.5 While the system is in normal operation, perform weekly dipslide tests to determine the total bacteria count (TBC) at each water sampling location. Refer to Figure 1 located in Appendix D.
• 3.5.6 While the system is in normal operation, perform monthly Legionella bacteria culture tests to identify total Legionella pneumophila including all serogroups (LPTOT) bacteria levels. Refer to Figure 1 located in Appendix D.
• 3.5.7 Perform a qPCR test 24 hours after completion of the system start-up cleaning and disinfection. Refer to Figure 1 located in Appendix D.

Minimum bacterial testing requirements (emergency mode)

• 3.5.8 Use the emergency-mode bacterial test procedure if:
  • 3.5.8.1 There have been multiple reported cases of Legionnaires' disease in the past 30 days in the region within a 10 km radius of the building location and/or,
  • 3.5.8.2 The building has had recurring Legionella bacteria levels detected by the Legionella bacteria culture test that required disinfection of the system within the past 90 days and/or,
  • 3.5.8.3 Regional health authorities direct more stringent bacterial testing
• 3.5.9 Unless directed otherwise by the regional health authority, or due to regional restrictions, a weekly Quantitative Polymerase Chain Reaction (qPCR) test should be performed to obtain rapid, Legionella specific results, to determine the Legionella Pneumophila bacteria count at each water sampling location.

Dipslide testing limits and actions

• 3.5.10 (Level D1—Green)—When the TBC is less than 10,000 Colony forming unit (cfu):
  • 3.5.10.1 Continue with normal Operation and Maintenance (O&M).
• 3.5.11 (Level D2—Yellow)—When TBC is greater than or equal to 10,000 and less than or equal to 100,000 cfu:
  • 3.5.11.1 Review and adjust the O&M procedures and the water treatment program as required to ensure acceptable bacterial levels in the system.
  • 3.5.11.2 Continue operation with adjusted O&M and water treatment program.
• 3.5.12 (Level D3—Orange)—When TBC exceeds 100,000 cfu:
  • 3.5.12.1 Shut down the cooling tower system within 7 days and perform cleaning and disinfection procedures in accordance with one of the following two options.
    • Option 1—Shut down within 48 hours to perform cleaning and disinfection procedure.
    • Option 2—Perform a running disinfection within 48 hours. Shut down within 7 days to perform cleaning and disinfection procedure.
  • 3.5.12.2 Review and adjust the O&M procedures and the water treatment program as required to ensure acceptable bacterial levels in the system.
  • 3.5.12.3 Continue operation with adjusted O&M and water treatment program.
  • 3.5.12.4 If the condenser water temperature is less than or equal to 18 °C, continue operation with the revised O&M procedures.
  • 3.5.12.5 If the condenser water temperature is above 18 °C, wait 2 to 7 days then perform a Legionella bacteria culture test in accordance with clauses 3.5.13 through 3.5.17 below.

Legionella bacteria culture testing limits and actions

• 3.5.13 Perform Legionella bacteria culture test in accordance with ISO 11731-1998 and ISO/TS 11731-2-2004.
• 3.5.14 Where the analysis result indicates interfering flora are present that may have affected the results:
  • 3.5.14.1 Perform a running disinfection of the cooling tower system within 48 hours.
  • 3.5.14.2 Review and adjust the O&M procedures and the water treatment program as required to ensure acceptable bacterial levels in the system.
  • 3.5.14.3 Wait 2 to 7 days then perform a Legionella bacteria culture test in accordance with clauses 3.5.13 through 3.5.17.
• 3.5.15 (Level C1—Green)—When the LPTOT count is less than 10 cfu/mL:
  • 3.5.15.1 Continue with normal O&M.
• 3.5.16 (Level C2—Orange)—When the LPTOT count is greater than or equal to 10 and less than or equal to 1,000 cfu/mL:
  • 3.5.16.1 Shut down the cooling tower system within 7 days and perform cleaning and disinfection procedures in accordance with one of the following two options.
    • 3.5.16.1.1 Option 1—Shut down within 48 hours to perform cleaning and disinfection procedure.
    • 3.5.16.1.2 Option 2—Perform a running disinfection within 48 hours. Shut down within 7 days to perform cleaning and disinfection procedure.
  • 3.5.16.2 Review and adjust the O&M procedures and the water treatment program as required to ensure acceptable bacterial levels in the system.
  • 3.5.16.3 Continue operation with adjusted O&M and water treatment program.
  • 3.5.16.4 Wait 24 hours then perform a qPCR test in accordance with clauses 3.5.18 through 3.5.21.
• 3.5.17 (Level C3—Red)—When the LPTOT count exceeds 1,000 cfu/mL:
  • 3.5.17.1 Immediately implement measures that will eliminate water dispersion by aerosol from the affected cooling tower system then clean and disinfect the system before putting the system back into service.
  • 3.5.17.2 Review and adjust the O&M procedures and water treatment program to ensure acceptable bacterial levels in the system.
  • 3.5.17.3 Continue operation with adjusted O&M and water treatment program.
  • 3.5.17.4 Wait 24 hours then perform a qPCR test in accordance with clauses 3.5.18 through 3.5.21.

Quantitative polymerase chain reaction testing limits and actions

• 3.5.18 Perform qPCR Test in accordance with ISO/TS 12869
• 3.5.19 (Level Q1—Green)—When the Legionella pneumophila bacteria count is less than 10 GE/mL:
  • 3.5.19.1 Continue with normal O&M.
  • 3.5.19.2 Wait 2 to 7 days then perform a Legionella bacteria culture test in accordance with clauses 3.5.13 through 3.5.17.
• 3.5.20 (Level Q2—Yellow)—When the Legionella pneumophila bacteria count is greater than or equal to 10 and less than or equal to 100 GE/mL:
  • 3.5.20.1 Review and adjust the O&M procedures and adjust the water treatment plan as required to ensure acceptable bacterial levels in the system.
  • 3.5.20.2 Continue operation with adjusted O&M and water treatment program.
  • 3.5.20.3 Wait 2 to 7 days then perform a Legionella bacteria culture test in accordance with clauses 3.5.13 through 3.5.17.
• 3.5.21 (Level Q3—Orange)—When the Legionella pneumophila bacteria count exceeds 100 GE/mL:
  • 3.5.21.1 Shut down the cooling tower system within 7 days and perform cleaning and disinfection procedures in accordance with one of the following two options.
    • 3.5.21.1.1 Option 1—Shut down within 48 hours to perform cleaning and disinfection procedure.
    • 3.5.21.1.2 Option 2—Perform a running disinfection within 48 hours. Shut down within 7 days to perform cleaning and disinfection procedure.
  • 3.5.21.2 Review and adjust the O&M procedures and the water treatment program as required to ensure acceptable bacterial levels in the system;
  • 3.5.21.3 Continue operation with adjusted O&M and water treatment program.
  • 3.5.21.4 Wait 24 hours then perform a qPCR test in accordance with clauses 3.5.18 through 3.5.21.

3.6 Operation, maintenance and testing schedule

• 3.6.1 Table 3.1 below indicates a summary of the minimum frequency for operation, maintenance and testing requirements of cooling towers.