45 terms you will meet in reports, quotations and EMSD paperwork — each with its Traditional Chinese equivalent and a plain-English definition.
A
Advisory letter 勸諭信
A letter EMSD sends a building owner when its own sample from a cooling tower shows Legionella between 10 and 1,000 cfu/mL. It asks the owner to restore water quality through on-line disinfection under the Code of Practice. It is not an offence — but it puts the problem on record.
Alkalinity 鹼度
A measure of water's ability to neutralise acid, usually reported as ppm of calcium carbonate. It buffers pH against sudden swings and affects whether water tends to form scale or corrode metal.
Approach temperature 逼近溫度
The gap between two temperatures a cooling system is trying to close: for a cooling tower, how near the water leaving it gets to the outside wet-bulb temperature; for a condenser, how near the cooling water gets to the refrigerant. Engineers watch it because it doubles as a free fouling gauge — as deposit builds on the heat-transfer surfaces, the approach widens and the plant burns more energy for the same duty. Just 1/32 of an inch of scale can cut heat-exchange effectiveness by over 10%, so a slow upward drift is worth chasing — and if there is no scale to explain it, biofilm is the likely cause.
B
Biocide 殺菌劑
A chemical that kills bacteria and other microorganisms in water. Chlorine-based biocides are the most common in Hong Kong buildings. How well a biocide works often depends on the water's pH, which is why dosing without pH control can fail.
Biofilm 生物膜
A slimy layer of bacteria attached to wet surfaces inside pipes, tanks and cooling towers. It shelters Legionella from biocides, so a system can test badly even while chemicals are being dosed. Removing it usually needs physical cleaning, not just more chemical.
Blowdown 排污
Deliberately draining part of the cooling water to the sewer so dissolved minerals do not keep concentrating. Blowdown is the main control on cycles of concentration. Too little blowdown causes scale; too much wastes water and chemicals.
Breakpoint chlorination 折點加氯
Dosing enough chlorine that it first reacts with ammonia and organic matter in the water, and then leaves free chlorine available to disinfect. Below the breakpoint, most of the chlorine is tied up as chloramines and disinfects poorly.
C
Calcium hardness 鈣硬度
The amount of dissolved calcium in water, reported as ppm of calcium carbonate. High calcium hardness is the main driver of scale on hot surfaces. It is one of the inputs to the Langelier Saturation Index.
cfu/mL 菌落形成單位
Colony-forming units per millilitre — the unit laboratories use to count live bacteria in a water sample. Hong Kong's cooling tower thresholds are written in cfu/mL: below 10 counts as "not detected", and 1,000 or above triggers emergency action.
Chloramine 氯胺
A compound formed when chlorine reacts with ammonia or other nitrogen compounds in the water. Chloramines disinfect much more slowly than free chlorine and cause the strong "chlorine smell" people complain about. Heavy chloramine formation means the chlorine dosed is not doing the work.
Code of Practice 實務守則
In Hong Kong water treatment, usually the EMSD's Code of Practice for Fresh Water Cooling Towers (2023 edition), published in three parts. It sets the water quality criteria, the Legionella action levels, the cleaning frequency and the record rules. We explain it in plain English here.
Conductivity 電導率
A measure of how much dissolved mineral is in the water, in microsiemens per centimetre (µS/cm). More dissolved solids means higher conductivity. In cooling systems it is the everyday signal used to control blowdown.
Cooling tower 冷卻塔
Equipment that cools a building's air-conditioning water by evaporating part of it into the air. A tower is warm, wet and open to the sky — the classic environment for Legionella if it is not treated and maintained.
Corrosion 腐蝕
The gradual loss of metal from pipes and equipment into the water. It shows up as rusty water, rising iron in test results, and eventually leaks. Corrosion is controlled with inhibitors and by keeping the water chemistry balanced — see metal corrosion in water systems.
Cyanuric acid 氰尿酸
A stabiliser that protects chlorine from being destroyed by sunlight, present in trichlor tablets. Useful for outdoor pools — but as it accumulates it slows chlorine's killing action, so the level needs to be monitored and managed.
Cycles of concentration 濃縮倍數
How many times the dissolved minerals in cooling water have been concentrated compared with the fresh make-up water. It is set by the balance between evaporation and blowdown, and it decides both your water bill and your scale risk. Full explanation and calculator here.
D
Desludging 清淤
Removing the settled sludge from a cooling tower basin or tank. Sludge is food and shelter for bacteria, so it must be physically taken out. The Code of Practice requires cooling towers to be cleaned and desludged at least every 6 months.
Disinfection 消毒
Killing the microorganisms in a water system, usually with chlorine-based chemicals. On-line disinfection is done with the system still running; emergency decontamination is the stronger, stop-everything procedure required after a high Legionella result.
Dosing 加藥
Adding treatment chemicals to a water system, by hand or by automatic pump. Automatic dosing keeps chemical levels steady between site visits, and can be tied to real-time monitoring so the system corrects itself.
Drift 飄水
The fine mist of water droplets that escapes a cooling tower with the air stream. Drift is how bacteria in tower water can reach people on the street or in nearby buildings — which is why tower water quality is treated as a public health matter, not just maintenance.
Duty holder 責任人
The person or company responsible for a water system's safety — for a cooling tower, ultimately the building owner. Hiring a contractor delegates the work, not the responsibility. See who is responsible.
E
Emergency decontamination 緊急消毒
The stop-everything procedure required by the EMSD Code of Practice when a cooling tower is badly out of control: cleaning, desludging and disinfection, then a resample within 3 days. Any one of three findings triggers it — a total Legionella count of 1,000 cfu/mL or above, a heterotrophic colony count of 5,000,000 cfu/mL or above, or on-line disinfection, and cleaning and disinfection, having failed to control Legionella and HCC. Two consecutive samples with no Legionella detected are needed before routine quarterly monitoring resumes. When EMSD's own sample is the one at 1,000 cfu/mL or above, the work is carried out under a statutory nuisance notice — see what to do in the first 24 hours.
EMSD 機電工程署
The Electrical and Mechanical Services Department of the Hong Kong Government. It runs the Fresh Water Cooling Towers Scheme, publishes the Code of Practice, and has been able to enter premises and sample cooling towers at random since 2011.
F
Free chlorine 餘氯
The chlorine in water that is still available to kill microorganisms, as opposed to chlorine already combined with other substances. It is the working number a technician checks on site — and its killing power drops sharply as pH rises.
FWCT Scheme (Fresh Water Cooling Towers Scheme) 淡水冷卻塔計劃
The EMSD scheme that lets buildings in designated areas use fresh water for evaporative cooling instead of being restricted to air-cooled plant — joining it is what makes your towers lawful. The Code of Practice is the rulebook you agree to operate by, and the benchmark against which towers are inspected. Towers registered under the Scheme are also required to complete an annual audit.
H
HCC (heterotrophic colony count) 異養菌落總數
A general count of the live bacteria in a water sample, in cfu/mL — a measure of how hospitable the water has become. The EMSD Code of Practice's indicative criterion is below 100,000 cfu/mL; at 100,000 or more the Code requires immediate cleaning and disinfection, and 5,000,000 or more is an emergency decontamination trigger. HCC usually rises before Legionella does — see the Code explained.
I
Inhibitor 緩蝕劑／阻垢劑
A chemical dosed to slow corrosion (a corrosion inhibitor) or to stop minerals depositing as scale (a scale inhibitor). Inhibitors must be matched to the metals in the system and the local water chemistry — the reason a treatment programme is designed, not poured.
L
Legionella 退伍軍人桿菌
Bacteria that cause Legionnaires' disease, a serious lung infection, when inhaled in fine water droplets. Legionella multiplies in warm, stagnant water and shelters in biofilm — the reason cooling towers and tanks are regulated. Full guide: Legionella in water systems.
LSI (Langelier Saturation Index) 飽和指數
A calculated number that shows whether water tends to deposit scale (positive LSI) or dissolve metal (negative LSI). It combines pH, temperature, hardness, alkalinity and dissolved solids. Explanation and calculator: the Langelier Saturation Index.
M
Make-up water 補充水
The fresh water added to a cooling system to replace what is lost to evaporation, drift and blowdown. Its quality sets the starting point for the tower's chemistry — everything in the make-up water gets concentrated as the water evaporates.
N
Nuisance notice 妨擾事故通知
A statutory notice under the Public Health and Municipal Services Ordinance. EMSD issues one when its sample from a cooling tower shows Legionella at 1,000 cfu/mL or above. It requires emergency decontamination and re-testing — and failing to comply is an offence. See the enforcement layer explained.
O
On-line disinfection 在線消毒
Disinfection carried out with the cooling tower still running, rather than drained down for a physical clean. The Code of Practice requires immediate on-line disinfection — together with investigating and reviewing the treatment programme — when a total Legionella count comes back at 10 cfu/mL or above but below 1,000, followed by a resample within 3 to 7 days; at 1,000 cfu/mL or above the requirement is emergency decontamination instead. It is also what an EMSD advisory letter asks an owner to do to restore water quality. Because it leaves the sludge and biofilm that shelter bacteria in place, it does not replace a full clean and desludge, which means draining the basin with the tower offline.
ORP 氧化還原電位
Oxidation-reduction potential, measured in millivolts. It shows how strongly the water can oxidise — in practice, how actively the disinfectant is working right now. Monitoring systems use ORP as a live health signal between laboratory tests.
P
pH 酸鹼值
How acidic or alkaline water is, on a scale of 0 to 14, with 7 as neutral. Chlorine disinfects well near pH 7 and poorly above pH 8 — so pH control often decides whether disinfection actually happens. See effects of pH on biocides.
Plate heat exchanger 板式熱交換器
A stack of thin metal plates that transfers heat between two water circuits. The narrow gaps between plates foul easily with scale and biofilm, which cuts efficiency long before anything visibly fails. Fouled exchangers are restored by chemical cleaning.
Potable water 食水
Water that is safe to drink. In a building, that safety depends on the tanks and pipework it passes through — which is why they are cleaned, disinfected and tested on a schedule.
R
Re-test 重驗
A follow-up sample taken after disinfection to confirm it worked. Under the Code of Practice, a high Legionella result starts a resample loop: the system must produce consecutive clean samples before it can return to routine monitoring. One good result is not the end of it.
S
Sampling 抽樣
Taking water from defined points in a system for laboratory analysis. How and where the sample is drawn affects the result as much as the laboratory work does — a badly taken sample produces a confidently wrong number. See laboratory testing.
Scale 水垢
Hard mineral deposit — mostly calcium carbonate — left behind on hot surfaces as water evaporates or heats up. Scale insulates heat transfer surfaces, wastes energy and shelters bacteria. Whether your water tends to scale is predicted by the LSI.
Sodium hypochlorite 次氯酸鈉
Liquid chlorine, the workhorse disinfectant for pools, tanks and water systems. It is dosed by pump or by hand and measured as free chlorine in the water. Gotech supplies it locally in Hong Kong.
Stagnant water 死水
Water that sits without moving — in idle cooling towers, dead-leg pipes and rarely used outlets. Warm stagnant water is where Legionella multiplies, which is why restarting an idle system needs care, not just a switch.
Strainer 隔濾器
A mesh screen in the pipework that catches debris before it reaches pumps, valves and heat exchangers. Strainers are checked and cleaned during maintenance visits; a blocked strainer is often the first visible sign of a dirty system.
T
Trichlor 三氯異氰尿酸
Trichloroisocyanuric acid — slow-dissolving chlorine tablets widely used in swimming pools. Trichlor contains a built-in cyanuric acid stabiliser, so the cyanuric acid level rises over time and must be watched. Gotech supplies chlorine tablets in Hong Kong.
Turbidity 濁度
How cloudy water is, caused by fine suspended particles. Rising turbidity in a tower or tank usually means dirt, rust or biological growth is building up — a visible early warning that the system needs attention.
W
Water tank 水缸
A building's storage tank for fresh or flushing water. Tanks collect sediment and can grow biofilm, so they need periodic cleaning and disinfection to keep the water leaving them as safe as the water entering them.
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