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Hong Kong's Code of Practice for Fresh Water Cooling Towers (2023 edition) comes in three parts: design and installation, operation and maintenance, and water treatment methods. For a building owner already running towers, Part 2 is the one that bites. It sets indicative water quality criteria — total Legionella below 10 cfu/mL and heterotrophic colony count below 100,000 cfu/mL — requires cleaning, desludging and disinfection at least every 6 months, defines exactly what to do at each Legionella count, and requires operation and maintenance records to be kept for at least 2 years and produced on request. The 2023 edition was published on 21 November 2023 and fully implemented on 21 May 2024.
The Code of Practice sits underneath the Fresh Water Cooling Towers Scheme — the EMSD scheme that lets buildings in designated areas use fresh water for evaporative cooling instead of being restricted to air-cooled plant. Joining the scheme is what makes your towers lawful. The Code is the rulebook you agree to operate by.
It helps to be precise about its status. The Code itself sets out an order of priority: legislation and subsidiary legislation first, then relevant Codes of Practice and technical standards, then this Code. So it is not, by itself, a statute. But it is the benchmark against which your towers are inspected, and departing from it is something you would have to justify — to EMSD, and to anyone asking questions after an incident.
The single most useful thing to understand: the Code is written for the O&M contractor and the water treatment service provider to carry out. It is not written to relieve the owner. Engaging a contractor delegates the work, not the responsibility.
| Part | Covers | Who needs it |
|---|---|---|
| Part 1 | Design, installation and commissioning | New towers, replacements, major alterations |
| Part 2 | Operation and maintenance | Every building already running towers |
| Part 3 | Water treatment methods | You and your treatment provider, together |
If you own or manage an existing building, Part 2 is where your obligations live day to day. Part 3 is where the chemistry you are paying for is described. Part 1 matters when something is being built or swapped out.
Part 2 gives a table of indicative cooling water quality criteria. The Code is explicit that this table is for reference — it is guidance on what healthy water looks like, not a pass/fail certificate. That said, these are the numbers an inspector knows, and the numbers your treatment programme should be holding.
| Parameter | Criterion |
|---|---|
| Total Legionella count | Less than 10 cfu/mL |
| Heterotrophic colony count (HCC) | Less than 100,000 cfu/mL |
| pH | 7 – 10 |
| Conductivity | Less than 1,500 µS/cm |
| Total dissolved solids | Less than 1,000 ppm |
| Suspended solids | Less than 150 ppm |
| Calcium hardness | Less than 500 ppm CaCO₃ |
| Total alkalinity | 80 – 500 ppm CaCO₃ |
| Chloride | Less than 200 mg/L |
| Sulphate | Less than 200 mg/L |
| Total iron | Less than 1.0 mg/L |
| Free residual chlorine | 0.5 – 1.0 mg/L* |
| Biocide / inhibitor level | Per manufacturer's specification |
* The Code notes operators may decide a suitable concentration with due consideration of existing pipe conditions. Source: Table 2.1, Code of Practice Part 2, 2023 edition.
Two of these deserve comment. pH 7–10 is a very wide band, and the top of it is a place where chlorine barely functions — above pH 8 most of your free chlorine has dissociated into the weak hypochlorite ion. A tower can sit inside the Code's pH range and still have effectively no disinfection. We explain why in effects of pH on various biocides. And total iron below 1.0 mg/L is not really a health number — it is a corrosion tell-tale. Iron in the water is pipe that used to be in the wall (see metal corrosion in water systems).
This is the part of the Code that people most often get wrong, usually by assuming there is one threshold. There are several, and each carries a different required response. Part 2 sets them out as control strategies:
| Total Legionella count | Indication | What the Code requires |
|---|---|---|
| Not detected (<10 cfu/mL) | System under control | Maintain quarterly monitoring and maintain the water treatment programme |
| ≥10 and <1,000 cfu/mL | Review programme | Investigate, review the treatment programme, take remedial action including immediate on-line disinfection, then resample within 3–7 days |
| ≥1,000 cfu/mL | Implement corrective action | Investigate, review the programme, and carry out immediate emergency decontamination, then resample within 3 days |
After disinfection you do not get to stop at one clean result. The Code requires two consecutive samples with no Legionella detected before you may return to the routine quarterly regime. If the resample still shows ≥100 and <1,000 cfu/mL, you clean and disinfect again and repeat the cycle. If it comes back at ≥1,000 cfu/mL, you are back to emergency decontamination.
Read that ladder again as a business risk rather than a chemistry one. A single result at 1,000 cfu/mL does not just trigger a decontamination — it starts a resample loop you cannot exit until two clean samples in a row. That is weeks of disruption and cost that a functioning treatment programme simply avoids.
Legionella gets the attention. But the Code also sets an action ladder on heterotrophic colony count — a general measure of how much bacterial life is in the water:
| HCC result | What the Code requires |
|---|---|
| <100,000 cfu/mL | Repeat routine monitoring |
| ≥100,000 and <5,000,000 cfu/mL | Immediately carry out cleaning and disinfection |
| ≥5,000,000 cfu/mL | Investigate — this is also an emergency decontamination trigger |
HCC is worth watching precisely because it moves before Legionella does. A rising heterotrophic count means the water is becoming hospitable — nutrients, biofilm, somewhere for amoebae to live. Legionella is a passenger on that ecosystem. By the time your Legionella count is climbing, the HCC has usually been telling you for a while.
The Code names three triggers. Any one of them is enough:
That third trigger is the interesting one, and it is the one that catches badly-run systems. You do not need a catastrophic count to end up in emergency decontamination. You only need a programme that keeps failing to fix an ordinary one.
Three operational rules from Part 2 that are routinely missed:
We cover the practical side of this in how often a cooling tower should be cleaned and restarting a cooling tower after shutdown or typhoon.
Everything above is the Code — the rulebook you operate by. It is worth separating that from the statutory layer, because they are two different things and people merge them constantly.
EMSD has regulated the water quality of fresh water cooling towers under the Public Health and Municipal Services Ordinance, Cap. 132 (PHMSO) since 2011. With powers delegated from the Director of Food and Environmental Hygiene with effect from 24 January 2011, EMSD can enter premises and take water samples. It has been carrying out inspections and sampling since April 2011. These are random surveillance inspections — you do not get notice.
What happens then depends on the number:
| EMSD's sample result | What EMSD issues | Consequence |
|---|---|---|
| 10 – 1,000 cfu/mL | An advisory letter | You are asked to restore water quality through on-line disinfection per the Code. Not an offence — but it is now on record that you were told. |
| ≥ 1,000 cfu/mL | A nuisance notice under the PHMSO | You are required to carry out emergency decontamination with re-testing, for abatement within a prescribed period. Failing to comply with the notice is an offence. |
And there is a consequence that has nothing to do with fines. EMSD publishes the building locations of fresh water cooling towers found at or above 1,000 cfu/mL with nuisance notices issued, in a list covering the past three months. It is a public PDF on the EMSD website, updated as results come in. If you manage a hotel, a mall or a Grade A office, that is the part of this article to reread. The chemistry is recoverable. Being on the list is a matter of public record, and your tenants, your guests and your competitors can all read it.
It is reasonable to ask whether this is a theoretical risk. EMSD publishes its own sampling statistics, so we do not have to guess — we track every published round in EMSD cooling tower sampling results.
In the first half of 2026 (January to June), EMSD's routine surveillance took 348 water samples from fresh water cooling towers across 219 buildings. Of those samples, 1 was at or above 1,000 cfu/mL and drew a nuisance notice. But 35 came back between 10 and 1,000 cfu/mL — the advisory-letter band.
That is the number worth sitting with. Roughly one sample in ten from Hong Kong's routine surveillance showed detectable Legionella above the Code's threshold. Not a catastrophe; not rare either. EMSD's own characterisation of that band is blunt — it says such cases “more often reflect intermittent inadequate vigilance” in routine maintenance and water treatment.
Prosecution is not the realistic risk for most buildings. Being the one in ten is. And the difference between the buildings in that group and the ones outside it is not luck — it is whether someone is actually running the programme between inspections.
Figures are EMSD's published routine-surveillance statistics for January to June 2026, retrieved 15 July 2026. EMSD updates them periodically; check the source for current numbers.
Part 2 requires that operation and maintenance manuals and records are kept by authorised personnel, are readily available for inspection on request, and are kept for at least 2 years. Defects identified in any report must be notified to the owner, occupier, manager or their nominated representative immediately.
Inspection also looks for updated installation documents and completed inspection and maintenance checklists, records and logbook for the past 1 year.
This is why we send a dated written report after every visit, and why you should expect that from any provider. Treatment that works but is not documented gives you no defence. Two years is not an archiving preference — it is the window in which someone can ask you to prove what you did. The printable compliance checklist puts the recurring duties and a monthly record grid on one sheet for the plant room.
If you strip the Code down to what an owner must actually ensure happens:
None of that is exotic. It is ordinary competent maintenance, done on a schedule, with records. The reason it goes wrong is almost never that someone could not understand the chemistry. It is that nobody owned the calendar.
The Code sets out its own order of priority: legislation and subsidiary legislation first, then relevant Codes of Practice and technical standards, then the Code itself. It is the operating benchmark for the Fresh Water Cooling Towers Scheme and the basis on which towers are inspected, so departing from it is something an owner would need to justify.
The Code of Practice Part 2 requires cooling tower systems to be regularly cleaned, desludged and disinfected at least every 6 months. That is a minimum. Towers with heavy fouling, high dirt loading or elevated risk need cleaning more often.
Below 10 cfu/mL is treated as not detected and you maintain quarterly monitoring. At 10 cfu/mL or above, but under 1,000, you must investigate, review the treatment programme and carry out immediate on-line disinfection, then resample within 3 to 7 days. At 1,000 cfu/mL or above, immediate emergency decontamination is required, with a resample within 3 days.
At least 2 years. Operation and maintenance manuals and records must be kept by authorised personnel and be readily available for inspection on request. Inspections also check for completed checklists, records and logbook covering the past 1 year.
It depends on the count. Between 10 and 1,000 cfu/mL, EMSD issues an advisory letter asking the owner to restore water quality through on-line disinfection per the Code of Practice. At 1,000 cfu/mL or above, EMSD issues a nuisance notice under the Public Health and Municipal Services Ordinance requiring emergency decontamination with re-testing within a prescribed period, and failing to comply with that notice is an offence. EMSD also publishes a public list of buildings issued with nuisance notices.
Yes. With powers delegated from the Director of Food and Environmental Hygiene under the Public Health and Municipal Services Ordinance with effect from 24 January 2011, EMSD can enter premises and take water samples from fresh water cooling towers. It has carried out random surveillance inspections and sampling since April 2011.
No. The Code assigns the work to the O&M contractor and water treatment service provider, but responsibility for the installation stays with the owner. What protects an owner is treatment that works plus records that prove it.
We will survey the system, test the water against the Code's criteria, and tell you plainly where you stand.