Cooling Tower Cleaning & Circulating Water Treatment

Abstract: Circulating cooling water is concentrated step by step as it evaporates inside the tower, so carbonate scale settles on the fill, the distribution piping and the basin, while algae and biological slime block the distribution holes and coat the fill with a slippery layer, and rust and sludge take up basin volume. The result is a narrower cooling water temperature difference, a higher condenser temperature, and more power and make-up water consumed, together with structural corrosion and a hygiene risk. This article covers the typical deposits found in cross-flow and counter-flow towers and how to identify them on site, when on-line cleaning is enough and when a shutdown clean is required, the slime-dispersion, biocide, descaling and passivation route with its control points, and the acceptance criteria, safety requirements and an anonymised case study.

1. Why Cooling Towers Have to Be Cleaned at Regular Intervals

A cooling tower is the heat-rejection end of a circulating cooling water system. Water is sprayed into films or droplets and meets air directly, part of it evaporates and carries heat away, and what remains is concentrated cycle after cycle, so hardness, alkalinity and dissolved solids keep rising. The higher the concentration ratio, the more readily components such as calcium carbonate, whose solubility falls as temperature rises, precipitate onto the fill, the distribution piping and the basin as a dense scale layer.

The tower is also the most biologically active point in the system. The fill stays wet, it sees daylight, and water temperature sits in the range algae and bacteria favour, while make-up water and air keep supplying nutrients and spores. Algae, bacteria and fungi build up as biological slime on the fill. The slime traps dust and corrosion products, adheres strongly, and is not shifted by blowdown, so it simply thickens.

Once deposits stack up the effects are broad. Blocked fill channels restrict the air and water paths, the cooling water temperature difference narrows and the condenser temperature rises, which lifts the compression ratio and power consumption. Blocked distribution holes and piping cause maldistribution, so parts of the fill dry out, age faster and break up in sheets. Basin sludge and rust are carried into the condenser and cause tube blockage and under-deposit corrosion, while corrosion products from the tower shell and pipework become fresh deposit. Cooling tower drift is also a recognised transmission route for Legionella and similar organisms, so a tower that is never cleaned carries a hygiene risk as well.

On site, the decision to clean is made from a handful of direct signals: the cooling water temperature difference has fallen well below design or historical values; condenser temperature is higher than the same period last year; make-up water consumption and fan current have risen; opening the access door shows brown slime or clumps of algae on the fill; sludge in the basin is obvious and blowdown is frequent. Two or more of these normally means the tower is due.

2. Deposit Types and On-Site Identification

Deposits in a cooling tower and its circulating water system are rarely of a single type. The common families are carbonate scale, sulphate and silicate scale, biological slime and algae, rust and sludge, and oil, with fill embrittlement being an equipment-life issue rather than a cleaning issue. Sampling and identification come first, following the qualitative methods of Table 1 in GB/T 25146-2010: effervescence with dilute acid points to carbonate scale, a burnt smell on ignition points to organic deposit, and material that will not dissolve in acid points to sulphate or silicate scale. A practical field check is to take a small piece of fill, immerse it in clean water and see whether the slippery layer detaches and whether there is a noticeable odour.

One point deserves emphasis: the temperature tolerance of the fill decides the process. Common PVC and PP fill has limited heat resistance, normally not to be taken much beyond 50-55 °C, and must not be washed with steam or hot water. Alkaline or acid cleaning temperature and chemical concentration have to be controlled according to the fill material, otherwise the fill deforms and collapses after cleaning. Fill that has already become brittle and broken is not saved by cleaning; it should be assessed for replacement with standard manufactured fill.

3. Choosing Between On-Line and Shutdown Cleaning

On-line cleaning needs no shutdown. A slime dispersant and a descaler are dosed into the circulating water system, and side-stream filtration with heavier blowdown carries the loosened deposit out of the system. Its strength is whole-system coverage: tower, pipework and condenser are treated together and production is unaffected. Its limitation is that basin sludge and hard scale on the fill cannot be removed completely, while chemical concentration and pH are constrained by system volume, materials and operating conditions.

Material compatibility and coupon testing must come first. Establish whether the system contains copper, galvanised or stainless steel components: copper-bearing systems avoid ammonia-bearing and strongly oxidising agents and receive a BTA-type copper inhibitor, while chloride is held at no more than 25 mg/L wherever stainless steel is present. During cleaning, pH, turbidity and iron are monitored more frequently and side-stream filtration runs throughout, so that loosened slime does not settle again inside the condenser.

A shutdown clean deals with what on-line cleaning cannot reach. The basin is drained and sludge removed by hand, the fill is washed layer by layer with low-pressure high-flow water, distribution headers and nozzles are cleared hole by hole, and the basin, strainers and return channel are cleaned out, after which the system is descaled, passivated and pre-filmed before it goes back into service. Wash pressure on the fill must stay low with the nozzle kept at a distance, so that sheets are not punctured or deformed, and distribution holes must not be forced open with metal tools, because enlarged holes cause permanent maldistribution.

The two approaches complement each other; in practice a plant runs an annual shutdown clean with on-line maintenance in between. Where dosing is well controlled, make-up water is stable and side-stream filtration is working normally, the shutdown interval can be extended. Where make-up water is hard, the surroundings are dusty and the tower sits in full sun, it should be shortened.

4. Process Route and Control Points

Cooling tower and circulating water cleaning is organised into six stages, and if an earlier stage is not done properly the effect of the next one is cancelled. The classic mistake is descaling without dispersing the slime first: the slime covers the scale, acid never reaches the deposit, chemical is wasted, and the organic material that does come off simply re-attaches somewhere in the system.

Dosing ratios and quantities are set from the deposit type, system volume and coupon test results, calculated on site by the engineer, and are not listed here. The important point is that cooling tower cleaning is not one chemical flushed through: dispersion, descaling and passivation use different chemical systems, and mixing them usually achieves neither.

5. Verification and Performance Assessment

Acceptance runs on two tracks. One is cleaning quality itself, judged against the scale removal, corrosion rate and passivation film criteria of GB/T 25146-2010. The other is operating performance: whether the cooling water temperature difference, condenser temperature and energy consumption have come back. A tower that merely looks whiter is not evidence.

Handover documentation covers the cleaning procedure and process card, agent certificates, coupon corrosion monitoring records, process records (pH, turbidity, iron and time), rinse water reports, pre-filming records and waste disposal notes. Those records are also the basis for setting the next cleaning interval.

6. Safety, Environment and Waste

Two risk families must be agreed in advance: confined space and biological exposure. The fill area, basin and below-tower sump are all confined spaces, so before entry the water flow and fan supply are isolated, the space is force ventilated, oxygen and hydrogen sulphide are tested, a permit is issued and an attendant stays outside. Slime that has accumulated for years may carry pathogens, so high-pressure air must never be used to blow dry sludge out of the tower, because it creates aerosols; operators wear protective masks, goggles and gloves, wash and disinfect afterwards, and the tower is disinfected once cleaning is complete.

Chemicals are stored with acids and alkalis separated and issued to named staff, dilution always adds the chemical to water rather than the reverse, and a wash-down point with first-aid supplies is kept at the mixing area. Waste streams are segregated: biocide-bearing waste and acid waste are collected separately, never mixed with oil-bearing waste, neutralised to pH 6-9 and discharged according to the plant treatment requirements or handed to a licensed contractor, while oil-bearing waste is collected on its own to keep emulsified oil out of the biological stage.

7. Case Study: Circulating Water Cooling Tower Cleaning at a Fine Chemical Plant

In August 2026 Blue Star Cleaning cleaned three cross-flow cooling towers and the associated circulating water system at a fine chemical plant in East China, working inside a four-day plant shutdown window. Three problems were reported before the work started: condenser temperature higher than the previous year with a clearly narrower cooling water temperature difference; large areas of brown slime with patches of green algae on the fill once the access door was opened; and a thick sludge layer in the basin with partly blocked distribution holes and uneven water distribution.

Work ran on two fronts, tower and system. On the tower side the basin was drained and sludge removed by hand, the fill was washed layer by layer with low-pressure high-flow water, and distribution headers and nozzles were cleared one by one before a distribution trial. On the system side a slime dispersant and biocide were circulated, blowdown was increased once dispersion was complete and side-stream filtration was brought into service, after which the system was descaled and passivated with a pre-film. A BTA-type copper inhibitor was dosed throughout for the copper-bearing parts, chloride was held at no more than 25 mg/L where stainless steel was present, and same-material coupons were hung under clause 5.2.11 of GB/T 25146-2010 to determine corrosion rate. After cleaning every item passed: scale removal, coupon corrosion rate, passivation film and water distribution, with discharge pH, turbidity and iron back close to make-up water levels. Once the plant restarted, the cooling water temperature difference recovered, condenser temperature came back into its normal band, and fan current and make-up water consumption fell; quarterly on-line maintenance and an annual shutdown clean were then written into the plant's equipment management procedure.

8. Frequently Asked Questions

Does a cooling tower have to be shut down to be cleaned?

Not always. Slime, algae and light carbonate scale can be treated on line by dosing a slime dispersant and a descaler while keeping circulation running, with side-stream filtration and heavier blowdown doing the removal work, so production is not interrupted. Basin sludge, hard scale on the fill and blocked distribution holes do require a shutdown and tower entry. Blue Star Cleaning surveys the tower, takes water and deposit samples first, and then proposes a combined on-line and shutdown plan; call 18952832843.

Can algae and slime be removed from the fill with a pressure washer?

Yes, but with low pressure and high flow, layer by layer, and with the nozzle kept at a distance so that the fill sheets are not punctured, deformed or stripped away in sheets. PVC and PP fill has a limited temperature tolerance, so steam or hot water must not be used directly on it. Where slime is well attached, soften it with a dispersant first and the wash goes much faster. Blue Star Cleaning inspects fill strength during washing and reports any embrittlement or breakage so the customer can decide about replacement with standard manufactured fill.

Will the cleaning chemicals attack the tower structure or the fill?

Not when the chemical system is matched to the materials and an inhibitor is dosed with it. Carbon steel towers and pipework are acid cleaned with an inhibitor, copper-bearing parts additionally receive a BTA-type copper inhibitor, and chloride is held at no more than 25 mg/L wherever stainless steel is present. Cleaning temperature and chemical concentration are limited according to the fill material. Blue Star Cleaning hangs corrosion coupons of the same materials and controls corrosion rate to the limits in Table 2 of GB/T 25146-2010, keeping the records for handover.

How often does a circulating water system need cleaning?

There is no fixed interval; water quality and running behaviour set it. Systems on hard make-up water, in dusty surroundings or run at a high concentration ratio commonly clean the tower once a year with quarterly on-line maintenance in between, while stable systems can run longer provided slime dispersant and biocide dosing continue as routine treatment. The signals that a clean is due are a narrowing cooling water temperature difference, a rising condenser temperature and higher make-up water consumption. Blue Star Cleaning can recommend an interval from your operating data; call 18952832843.

How is cooling tower cleaning priced?

By tower type and workload: the tower scope is priced on the number of towers and the fill area, the circulating water system scope on system volume and pipework size, with chemicals and side-stream filtration services quoted separately. Blue Star Cleaning surveys the site and analyses a deposit sample free of charge, issues a scope and quotation before work starts, and invoices after acceptance; call 18952832843.

All cleaning processes and acceptance criteria in this article follow GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment.

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