“Free and total chlorine in pad-and-fan systems should be measured at four points”

Few pieces of equipment in a greenhouse get less attention than the cooling pads, right up until they start growing something of their own. A pad-and-fan system pulls outside air through a wall of wet pads, and the water that keeps them wet runs in a loop, from the sump to the top of the pads and back, warm, aerated and in constant contact with whatever the air brings in. That makes the loop a comfortable home for algae and biofilm, and a lot of growers keep it in check by dosing hypochlorous acid into the water. The pump runs, the amount goes in the log, and the job looks done. “Whether that chlorine is still active by the time the water reaches the far end of the loop is something very few facilities check,” says Xavier Gaya, founder of Avitas Global.

“Hypochlorous acid dosing in pad-and-fan systems should never be treated as simply adding a fixed quantity to the sump,” he continues. The dose should enter where mixing is rapid and uniform, ideally in proportion to the make-up or recirculating flow, and only after confirming chemical compatibility with the pad manufacturer. “Some cellulose-pad manufacturers explicitly prohibit chlorine, while compatible plastic media may tolerate a properly controlled residual,” he points out. “The correct dose is therefore the lowest validated concentration that maintains microbial control throughout the operating cycle without degrading the pads, damaging equipment or creating an uncontrolled aerosol exposure.”

© Avitas Global

Finding that concentration comes down to testing. “Verification requires a DPD test, not reliance on the stock-solution strength, pump setting or ORP alone. ORP indicates disinfection potential rather than confirming actual ppm concentration at a specific point in the loop, which is why DPD verification at the sampling points still matters,” says Xavier. The test follows the water through the whole system. “Free and total chlorine should be measured immediately at the untreated supply, after injection and adequate mixing, in the sump during operation, and at the most hydraulically remote return point,” he says.

The moment the samples are taken is part of the method as well. “Measurements should be collected after start-up stabilization, after major make-up-water additions, bleed-off events or changes in dosing,” says Xavier. “pH and water temperature belong beside every chlorine result because both influence how much of the free chlorine remains in the more active hypochlorous-acid form.”

The difference between total and free chlorine reveals the chlorine demand and the combined residual, according to Xavier. Free chlorine that declines across the loop may expose organic loading, biofilm, poor mixing or an underperforming injection points. “A system can therefore receive HOCl every day and still provide almost no meaningful control at the pad surface. Many facilities are recording what they add; very few are proving what survives the full circuit, something I explicitly recommend.”