High‑efficiency drift eliminators and higher cycles of concentration (COC) are cutting cooling tower makeup by double digits, while treated effluent is stepping in as alternative feed. Case studies point to 27% demand reductions and millions of cubic meters saved per year.
Cooling towers in pulp and paper mills are under quiet, relentless optimization. On the mechanical side, new drift eliminators slash the mist of recirculating water escaping up the stack. On the chemistry side, operators are stretching cycles of concentration (COC—how many times dissolved solids are “recycled” before blowdown) to the highest level that avoids scale. And increasingly, mills are feeding towers with treated effluent rather than fresh intake—provided they manage quality.
In one utility example, upgrading drift controls on a 1,200‑ton tower can take losses from roughly 8 gpm at 0.005% drift to less than 1 gpm at ≈0.0005%, a shift worth tens of thousands of cubic meters per year in saved water ([studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=2,flow%20cooling)). In a separate pulp‑mill classic, routing internal condensates and filtrates to a cooling tower both stripped ~10,000 lb/day of BOD (biochemical oxygen demand) and cut overall intake by ~8–10 million gallons per day (~30–40×10^3 m³/d) ([nepis.epa.gov](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101IGVW.TXT#:~:text=Pulp%20mill%20condensates%2C%20decker%20filtrate%2C,investigated%20the%20BOD%20%20removal)).
Trials with treated sewage effluent (TSE—sewage treated to a non‑potable standard) echo the trend: a 1,200‑ton evaporative cooler that boosted COC and disinfected the water cut makeup ~27% versus a groundwater baseline; scaled to 6,000 tons that’s ~82,500 m³/year saved (and ~82,500 kWh energy saved), with all viral pathogens successfully removed by disinfection ([www.watertechonline.com](https://www.watertechonline.com/water-reuse/article/14187302/use-of-treated-sewage-effluent-as-cooling-tower-makeup-water-a-pilot-study-print#:~:text=Application%20of%20the%20TSE%20could,energy%20savings%20is%2082%2C505%20KWh)).
High‑efficiency drift eliminators
Drift—liquid droplets carried out with exhaust air—looks small on paper but adds up. A typical counter‑flow tower with an ordinary eliminator might lose ≈0.005% of flow as drift ([studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=2,flow%20cooling)). In one utility case, drift measured just 1.82 m³/h (8 gpm) out of 681 m³/h makeup (0.27%) ([studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=Evaporation%20loss%20%3D%20611%20m3%2Fh,that%20deposits%20water%20or%20minerals)).
Advanced eliminators push that loss down by an order of magnitude: “new developments” have driven drift to ≈0.0005% of flow, with ASHRAE 189.1 requiring ≤0.002% for counterflow towers ([studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=2,flow%20cooling); [jmpcoblog.com](https://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-9-controlling-cycles-of-concentration#:~:text=thresholds%20listed%20in%20the%20standard,flow%20towers)). Less drift keeps fresher water in the loop, enabling higher COC and conserving makeup—while also preventing salt/chemical spray deposition that can corrode or foul nearby equipment.
On a 1,200‑ton tower, cutting drift from ~0.005% to ~0.0005% trims loss from ~8 gpm to under 1 gpm; the same CTI‑referenced example suggests reducing drift from 1.82 m³/h to ~0.18 m³/h saves ~15,000 m³/year ([studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=2,flow%20cooling)). Towers meeting modern standards (drift <0.002%) recirculate >99.998% of their water ([jmpcoblog.com](https://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-9-controlling-cycles-of-concentration#:~:text=thresholds%20listed%20in%20the%20standard,flow%20towers); [studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=2,flow%20cooling)).
Chemical programs that protect heat‑transfer surfaces complement drift controls, and mills often evaluate targeted inhibitors via scale‑inhibitors.
Cycles of concentration targets
COC is the ratio of dissolved solids in recirculating water to those in makeup. Higher COC reduces blowdown (intentional bleed to limit solids), sharply cutting freshwater needs. One study found that increasing COC from 6.5 to 9.0, for the same heat duty, saved over 1.1×10^6 m³ of makeup per year ([www.researchgate.net](https://www.researchgate.net/publication/311752798_Reducing_Water_Consumption_by_Increasing_the_Cycles_of_Concentration_and_Considerations_of_Corrosion_and_Scaling_in_a_Cooling_System#:~:text=orthophosphate%20scale%20in%20the%20condenser,the%20same%20amount%20of%20cooling)).
In practice, many facilities run just 3–4 cycles by default; with proper treatment they can reach 6–10 cycles. “Water efficiency”—the share of makeup water that evaporates rather than is bled—rises from ~75% at 3 COC to ~90% at 5 COC and ~97% at 8 COC ([watertechnologyreport.wordpress.com](https://watertechnologyreport.wordpress.com/blog/page/4/#:~:text=Cycles%20of%20Concentration%20%20,8%20%20%7C%2097.4)). Most towers “operate fine” up to roughly 4–6 cycles without special treatment, and studies suggest 10–12 COC is achievable while preventing scale with advanced programs ([jmpcoblog.com](https://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-9-controlling-cycles-of-concentration#:~:text=For%20the%20record%2C%20most%20cooling,variables%2C%20including%20the%20quality%20of); [watertechnologyreport.wordpress.com](https://watertechnologyreport.wordpress.com/blog/page/4/#:~:text=cycles%20of%20concentration,free%20heat)).
As a rule of thumb, each extra cycle reduces blowdown by the same incremental fraction; for example, 5 COC versus 7 COC yields roughly (1–5/7)=29% fewer blowdown losses. Attainable COC is ultimately set by makeup quality (hardness, alkalinity, silica, etc.) and the treatment chemistry ([www.watertechonline.com](https://www.watertechonline.com/water-reuse/article/14187302/use-of-treated-sewage-effluent-as-cooling-tower-makeup-water-a-pilot-study-print#:~:text=Open,1%2C%202)).
Scaling limits are real: raw water with 100 ppm CaCO₃ hardness typically permits ~3.5–4.5 COC before calcium carbonate scale forms; softening the makeup can remove this limit ([watertechnologyreport.wordpress.com](https://watertechnologyreport.wordpress.com/blog/page/4/#:~:text=As%20mentioned%20previously%2C%20calcium%20in,the%20makeup%20to%20remove%20the)). Beyond ~10 COC, diminishing returns and risks appear as windage losses, small leaks, and uncontrolled carryout begin to dominate ([watertechnologyreport.wordpress.com](https://watertechnologyreport.wordpress.com/blog/page/4/#:~:text=bleed%20increases%20the%20cycles%20and,cycles%2C%20a%20point%20of%20diminishing)).
When hardness is the pinch point, plants often consider ion‑removal steps such as softeners or lower‑pressure hardness control via nano‑filtration.
Accurate chemical feed matters at higher COC, which makes equipment like a dosing pump a common part of the toolkit.
Alternative makeup sources and pretreatment
Using non‑potable water sources reduces freshwater withdrawal. Treated mill effluent, municipal reclaimed water, condensate, or rainwater can serve as cooling tower makeup if quality is managed; in many studies such sources are found “chemically superior” to raw supply, allowing fewer cycles and lower chemical usage ([www.prochemtech.com](https://www.prochemtech.com/recycling-treated-wastewater-through-cooling-tower-or-process-systems/#:~:text=Economic%20benefits%20include%20reduction%20and%2For,pretreatment%20requirements%20in%20process%20uses)).
Pulp mill effluent has precedent. In an EPA demonstration (1971) at an 850 tpd kraft mill, condensates and filtrates were fed to a cooling tower, which stripped volatile organics (especially methanol), removing ~10,000 lb/day of BOD and cutting overall water intake by ~8–10 MGD (~30–40×10^3 m³/d). The system effectively used the cooling tower itself as a condenser/stripper—treating wastewater on‑the‑fly while conserving makeup ([nepis.epa.gov](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101IGVW.TXT#:~:text=Pulp%20mill%20condensates%2C%20decker%20filtrate%2C,investigated%20the%20BOD%20%20removal)).
Reclaimed municipal water is gaining traction. In a Saudi Aramco pilot, partially treated sewage effluent used as tower makeup in a 1,200‑ton evaporative cooler, combined with higher COC and disinfection, cut makeup flow ~27% versus groundwater; scaled up to a 6,000‑ton cooling plant, the savings are ~82,500 m³/year and ~82,500 kWh, with all viral pathogens successfully removed by disinfection ([www.watertechonline.com](https://www.watertechonline.com/water-reuse/article/14187302/use-of-treated-sewage-effluent-as-cooling-tower-makeup-water-a-pilot-study-print#:~:text=Application%20of%20the%20TSE%20could,energy%20savings%20is%2082%2C505%20KWh)).
Other sources are possible, including groundwater, stormwater, condensate, or even seawater (for non‑potable cooling). Metsä Fibre’s new Kemi mill, for instance, uses water purified by its own plant in a closed cooling loop and avoided any increase in river intake despite doubling production ([www.metsagroup.com](https://www.metsagroup.com/metsafibre/news-and-publications/news-and-releases/stories/2023/Cooling-water-towers-minimise-the-need-for-water-intake-and-the-heat-load-to-the-sea/#:~:text=The%20mill%27s%20internal%20water%20circulation,water%20has%20to%20be%20run)).
Pretreatment is non‑negotiable with alternative feeds. Suspended solids, organics (e.g., lignin, oils), and salts can deposit or feed biological growth; oils and cutting fluids can “cement” deposits and explosively drive microbiological fouling ([www.prochemtech.com](https://www.prochemtech.com/recycling-treated-wastewater-through-cooling-tower-or-process-systems/#:~:text=presents%20some%20really%20unique%20problems,jump%20by%20a%20factor%20of)). That is why reuse programs recommend a full inventory of wastewater streams and chemistries before design; corrosion inhibitors and anti‑scalants may need adjustment, and cooling chemistry should be tailored to hardness, silica, organics, and nutrient levels ([www.prochemtech.com](https://www.prochemtech.com/recycling-treated-wastewater-through-cooling-tower-or-process-systems/#:~:text=SUPPLY%3A)).
For solids control ahead of the tower, mills commonly consider clarification steps such as a clarifier.
Fine particle removal and polishing frequently points to ultrafiltration or in‑line options like a cartridge filter.
Disinfection strategies range from oxidizing chemistries to physical barriers; many programs pair oxidants with biocides, and some add ultraviolet for non‑chemical microbial control.
Where total dissolved solids (TDS) management is required on reclaimed sources, some operators evaluate brackish‑water RO.
Measured outcomes and operating envelope
The data stack up. Raising COC from ~3.5 to ~9 can cut blowdown by over 60%, saving millions of cubic meters annually ([www.researchgate.net](https://www.researchgate.net/publication/311752798_Reducing_Water_Consumption_by_Increasing_the_Cycles_of_Concentration_and_Considerations_of_Corrosion_and_Scaling_in_a_Cooling_System#:~:text=orthophosphate%20scale%20in%20the%20condenser,the%20same%20amount%20of%20cooling)). Using high‑efficiency drift eliminators and optimized COC together can make a tower 95–99% “water efficient,” with only 1–5% of water leaving as blowdown ([watertechnologyreport.wordpress.com](https://watertechnologyreport.wordpress.com/blog/page/4/#:~:text=Cycles%20of%20Concentration%20%20,8%20%20%7C%2097.4)).
Replacing raw water with effluent or condensate can move systems toward nearly “zero discharge,” as demonstrated by reductions on the order of 8–10 MGD ([nepis.epa.gov](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101IGVW.TXT#:~:text=Pulp%20mill%20condensates%2C%20decker%20filtrate%2C,investigated%20the%20BOD%20%20removal)) or 82,500 m³/year ([www.watertechonline.com](https://www.watertechonline.com/water-reuse/article/14187302/use-of-treated-sewage-effluent-as-cooling-tower-makeup-water-a-pilot-study-print#:~:text=Application%20of%20the%20TSE%20could,energy%20savings%20is%2082%2C505%20KWh)).
Relevant industry studies and EPA guidelines provide quantitative backing for these strategies, including standards on drift thresholds, field cases on water reuse, and analyses of COC limits and returns ([studylib.net](https://studylib.net/doc/27266299/why-every-air-cooled-system-condenser-needs-a-cooling-tow...#:~:text=2,flow%20cooling); [www.researchgate.net](https://www.researchgate.net/publication/311752798_Reducing_Water_Consumption_by_Increasing_the_Cycles_of_Concentration_and_Considerations_of_Corrosion_and_Scaling_in_a_Cooling_System#:~:text=orthophosphate%20scale%20in%20the%20condenser,the%20same%20amount%20of%20cooling); [nepis.epa.gov](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101IGVW.TXT#:~:text=Pulp%20mill%20condensates%2C%20decker%20filtrate%2C,investigated%20the%20BOD%20%20removal); [www.watertechonline.com](https://www.watertechonline.com/water-reuse/article/14187302/use-of-treated-sewage-effluent-as-cooling-tower-makeup-water-a-pilot-study-print#:~:text=Application%20of%20the%20TSE%20could,energy%20savings%20is%2082%2C505%20KWh); [www.prochemtech.com](https://www.prochemtech.com/recycling-treated-wastewater-through-cooling-tower-or-process-systems/#:~:text=Economic%20benefits%20include%20reduction%20and%2For,pretreatment%20requirements%20in%20process%20uses); [jmpcoblog.com](https://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-9-controlling-cycles-of-concentration#:~:text=thresholds%20listed%20in%20the%20standard,flow%20towers); [watertechnologyreport.wordpress.com](https://watertechnologyreport.wordpress.com/blog/page/4/#:~:text=Cycles%20of%20Concentration%20%20,8%20%20%7C%2097.4)).
