Treated wastewater for cooling towers: when is RO needed?

Start the balance at the tower

The US Department of Energy expresses makeup as evaporation plus blowdown and drift. Evaporation leaves salts in the circulating loop; blowdown removes some of them. Changing makeup source without recalculating concentrations of calcium, silica, chlorides and other constituents can raise scaling or corrosion risk even if the water looks clear.

For an initial assessment, meter current makeup and blowdown and compare their conductivity. Then balance limiting constituents at several cycle ratios. Include variability from the treatment plant and any nutrients or organics that could affect microbiological control.

Choose treatment for the tower

Filtration can remove solids that foul heat exchangers, but does not alone remove dissolved salts. Softening may suffice when hardness is the dominant constraint. RO may help if several dissolved species prevent desired cycles or makeup quality, provided RO pretreatment and concentrate management are feasible.

Calculate whole-site water recovery: high-quality RO permeate can require additional feed and creates concentrate. Also test blending permeate with another source and operating at moderate cycles if further salt removal has a high marginal cost.

• Agree quality limits with the cooling-water treatment specialist.
• Plan conductivity, blowdown, corrosion and microbiological monitoring.
• Check local requirements for reuse and tower emissions.

Trial and acceptance

Test the proposed makeup across representative variations, not from a single grab sample. Specify permeate quality if RO is used, actual tower cycles, cleaning frequency and chemical use. Judge the project by freshwater displaced, operational reliability and total cost, rather than membrane rejection alone.