Reverse Osmosis (RO)
Reverse osmosis is a water purification technology that uses pressure to force water through a semi-permeable membrane so fine that it blocks dissolved minerals, bacteria, and most organic molecules while letting pure water pass. In pressure-washing and window-cleaning applications, RO is the second filtration stage: it takes municipal tap water from 250-400 PPM of Total Dissolved Solids (TDS) down to roughly 5-20 PPM before the water hits the deionization resin.
In depth
RO systems waste water — typically two to four gallons of "concentrate" drain for every gallon of purified "permeate" produced — so they are sized carefully to the service's flow demands. In cold-water climates like Madison, RO membranes also lose efficiency below 55°F, which is why we pre-heat or insulate lines in early spring. The advantage of RO is that it dramatically extends the life of downstream DI resin (the expensive part of the stack) by handling the bulk of the mineral load up front.
How this shows up on our jobs
Our window-cleaning trucks carry a 4-stage carbon-RO-DI system. RO is the workhorse stage that does the heavy lifting so our DI resin lasts months instead of days.
Services where this matters
Related terms
TDS (Total Dissolved Solids)
TDS stands for Total Dissolved Solids — the concentration of dissolved minerals, salts, and metals in water, measured in parts per million (PPM). A TDS meter is the single most important tool in professional window cleaning because it determines whether rinse water will dry spot-free. Standard Madison tap water measures roughly 250-400 PPM, which leaves visible mineral spots on glass when it dries. For streak-free, squeegee-free results with a water-fed pole, rinse water must be at or below 10 PPM, and ideally 0-2 PPM.
Deionized Water
Deionized water — often shortened to DI water — is water that has had essentially all of its dissolved mineral ions removed by passing it through specialized ion-exchange resin beads. Positive-charge ions (sodium, calcium, magnesium) are swapped for hydrogen ions, and negative-charge ions (chloride, sulfate, bicarbonate) are swapped for hydroxide ions. The result is near-pure H2O with a TDS reading at or near zero.
Water-Fed Pole (WFP)
A water-fed pole is a lightweight telescoping pole — typically carbon fiber, 20 to 45 feet long — with a soft brush and water jets at the tip, fed by a hose connected to a pure-water filtration system. The technician agitates the glass with the brush while pure water rinses away the loosened dirt, leaving zero mineral residue to dry into spots. When done correctly, windows, frames, and screens air-dry crystal-clear without ladders or squeegees.