Why has cooling water treatment chemical usage increased?

This answer covers open evaporative cooling towers on a supplier's treatment program. Closed loops and boilers lose chemical through other paths and need their own check. A higher invoice is not higher usage: the question here is the quantity fed, not the price paid.

What are the most likely causes of higher cooling water chemical usage?

The most likely causes of higher cooling water chemical usage are more water through the tower, lower cycles of concentration, a change in makeup water quality, unmetered water losses, and feed errors. The first four change what the system needs; the last changes what the pumps deliver without any change in need.

More heat load, more makeup

More production, longer run hours or a hotter season mean more heat rejected, more evaporation and more makeup. Feed that follows makeup rises with it, at the same dose.

Lower cycles of concentration

When the tower bleeds more water per unit of evaporation, more treated water leaves, and the program feeds more chemical to hold its target residual.

Changed makeup water quality

A new water source, harder or more alkaline makeup, or a higher pH can raise the inhibitor or biocide needed per cubic metre.

Leaks, overflow and drift

Water lost through a leaking basin, an overflowing fill valve or a damaged drift eliminator carries treated water out of the system, and the program replaces the chemical with it.

Feed errors

A wrong pump setting, a pump that siphons, a controller fault or a stock count error raises the recorded usage while the system's real need stays the same.

How can you tell the causes of higher cooling water chemical usage apart from the records?

The causes of higher cooling water chemical usage are told apart by comparing the same records for the two periods: makeup and blowdown meter totals, cycles from conductivity, production or run hours, makeup water analysis, feed pump settings and calibrations, and inventory against feed logs. Each cause leaves a different pattern, and the table below decides which cause the records confirm.

The key row is chemical per 1,000 m3 of makeup: if that figure is flat while total usage rose, the plant used more water, not more chemical per cubic metre, and the answer to the customer starts with heat load or cycles. If it rose while makeup and cycles stayed flat, look at the feed equipment before anything else. Work down the table in order, because a confirmed heat load or cycles change may explain the whole increase, and the later rows then only need to account for what remains.

How much does higher production or heat load raise cooling water chemical usage?

Higher production or heat load raises cooling water chemical usage roughly in proportion to the extra evaporation, when cycles of concentration stay the same and feed follows makeup. One industry rule of thumb puts evaporation at about 26.55 US gallons, or roughly 100 litres, per day for each ton of cooling load (WC&P, 2017), so evaporation tracks the heat the tower rejects.

At constant cycles, makeup and blowdown are both fixed multiples of evaporation, and inhibitor products are typically added "in direct proportion to the fresh water makeup" (Solenis, 2018). A 10% rise in heat rejected therefore raises makeup, blowdown and makeup-paced chemical feed by about 10% (illustrative). The dose per cubic metre of makeup does not change, and that is the record that shows it.

Heat load can rise without more production. US federal guidance notes that fouled air handler coils add load to the chilled water system, which "also increases the load on the evaporative cooling process, which uses more water" (US DOE FEMP, accessed 2026).

To confirm heat load as the cause, put production tonnage or run hours beside makeup volume for both periods, month by month. If both rose by a similar share and cycles held, heat load explains the increase.

How do lower cycles of concentration raise treatment chemical usage?

Lower cycles of concentration raise treatment chemical usage because more treated water leaves the tower as blowdown for each cubic metre evaporated, and the chemical in that water has to be replaced. Blowdown equals evaporation divided by cycles minus one (WC&P, 2017), so the effect grows quickly as cycles fall.

An illustrative tower evaporating 100 m3 per day blows down 25 m3 per day at 5 cycles and 50 m3 per day at 3 cycles. Makeup rises from 125 to 150 m3 per day, a 20% increase (illustrative). Because the treatment chemical leaves with blowdown, the feed needed to hold the same residual roughly doubles with blowdown in this example; the mass balance approach rests on the same idea, since "if the feed rate of the treatment chemical and the cooling blowdown rate are known, the treatment dosage can be calculated" (Pyxis Lab, 2016).

Cycles fall for a few recorded reasons: the conductivity set point was lowered, for example after a scaling event; the blowdown valve leaks or sticks open; or the makeup changed so the program can no longer run as high. US federal guidance says that if a system is not at or near its target cycles, check the "conductivity controller, make-up water fill valve, and blowdown valve" (US DOE FEMP, accessed 2026). Compare average cycles from the monthly conductivity ratio for both periods before naming this cause.

Can a change in makeup water quality increase cooling water chemical demand?

Yes, a change in makeup water quality can increase cooling water chemical demand, because the cycles a tower can run and the dose it needs both depend on what comes in with the makeup. US federal guidance says the number of cycles a system can handle "depends on the make-up water quality and cooling tower water treatment regimen" (US DOE FEMP, accessed 2026).

Harder or more alkaline makeup can force a lower conductivity set point, which lowers cycles and raises blowdown. A higher pH weakens chlorine: "the effectiveness of bleach as a biocide drops dramatically as the pH of the cooling water increases" (POWER, 2008), so oxidizing biocide use rises to hold the same control. And a switch between water sources, or several sources in use, makes makeup-paced feed less reliable, since "consistent inhibitor levels may not be maintained if the conductivity of the water makeup varies, there are multiple water supply sources, and/or treatment pumps lose prime" (Solenis, 2018).

Airborne dirt adds demand: particles drawn into the tower "soak up and inactivate chemical treatments" (POWER, 2008), so new dust sources matter.

To confirm a water quality cause, compare makeup analyses for the two periods. UK guidance gives typical makeup checks of hardness and conductivity monthly and alkalinity and pH quarterly (HSE HSG274 Part 1, 2024), so a supplier following it has the data.

How do leaks, drift and overflow losses show up in chemical usage?

Leaks, drift and overflow losses show up in chemical usage as makeup and chemical feed that rise while the blowdown meter does not, because treated water leaves the tower by a path nobody meters. US federal guidance gives the balance as "Make-Up = Evaporation + Blowdown + Drift" and states that "Properly operated towers should not have leaks or overflows" (US DOE FEMP, accessed 2026).

Drift is usually small; the same guidance calls drift loss "small compared to evaporation and blowdown" (US DOE FEMP, accessed 2026). A damaged drift eliminator, a basin leak, an overflowing fill valve or a hose used for washdown can be much larger. Water treatment guidance calls these losses "unintentional blowdown" and notes that cycles worked out from meters will be wrong when they are not recorded by the blowdown meter (Solenis, 2023).

The record check is a comparison of two ratios. Divide metered makeup by metered blowdown, then divide tower water conductivity by makeup conductivity. US federal guidance says "If both ratios are not about the same, check the tower for leaks or other unauthorized draw-off" (US DOE FEMP, accessed 2026). When the meter ratio shows higher cycles than conductivity, water is leaving without passing the blowdown meter, and the chemical in it leaves too. Until someone finds the loss on site, report it as a likely but unconfirmed cause.

How do you tell the customer which causes the records confirm and which are unconfirmed?

Tell the customer the measured change first, then each cause in two lists: causes a record confirms, with the record and its figures, and causes still unconfirmed, with the record that would settle each one. Never estimate a share for a cause with no record behind it.

Start with quantity, not spend. A higher invoice can come from price alone: POWER reported in 2008 that prices of common cooling tower phosphonates had "doubled or, in some cases, tripled" within a year (POWER, 2008). Compare kilograms or litres fed, from feed logs and stock counts, for the same months of both periods.

Then use the records the program already keeps. UK guidance lists water treatment chemical usage among the records for an evaporative cooling system, and where a non-oxidizing biocide cannot be measured in the water, it says the quantity added should be checked and recorded weekly (HSE HSG274 Part 1, 2024). A confirmed cause reads like this (illustrative): makeup rose 18% with production while cycles held at 4.0, so inhibitor use rose 17%. An unconfirmed one reads: a basin leak is possible because the meter and conductivity ratios disagree; it is unconfirmed until the basin is inspected. Lubinpla lays out a customer's project product-use records and worklogs by period, so each figure in such an answer traces to a record, and leaves a number blank when no record supports it.

How do you keep cooling water chemical usage from creeping up again?

Cooling water chemical usage stays under control when it is tracked per cubic metre of makeup every month, beside cycles and makeup volume, so a change shows within weeks instead of at the annual invoice. US federal guidance advises keeping "a log of make-up and blowdown quantities, conductivity, and cycles of concentration" and monitoring trends (US DOE FEMP, accessed 2026).

Four habits catch drift early. Read the makeup and blowdown meters on a fixed day and compare their ratio with the conductivity ratio each month. Clean and calibrate the conductivity probe on a schedule, since the controller acts on that one reading. Check the feed pump setting and drawdown against the logged feed at each service visit. For larger towers, the same guidance recommends automated chemical feed on systems above 100 tons, controlled "based on make-up water flow or real-time chemical monitoring" (US DOE FEMP, accessed 2026); tracer-based monitoring is one such method (Pyxis Lab, 2016).

Report usage the way federal buyers are told to compare vendors, on "cost to treat 1,000 gallons of make-up water" (US DOE FEMP, accessed 2026), but in quantity per 1,000 m3 of makeup. A customer who sees that figure each quarter rarely needs the question answered at renewal, and showing technical service value before contract renewal covers how that record supports the account.