Deionized Water System Sizing Calculator

What Deionization Does

Deionization removes dissolved salts by ion exchange. Cation resin trades hydrogen ions (H+) for the positive ions in the water: calcium, magnesium, sodium, iron. Anion resin trades hydroxide ions (OH−) for the negative ones: chloride, sulfate, bicarbonate, nitrate, and the weakly ionized carbonic acid and silica. The H+ and OH− combine into water, and what comes out has almost nothing dissolved in it.

Every ion removed uses up a bit of resin capacity. When the capacity is spent, the resin has to be regenerated with acid (cation) and caustic (anion). That is why a DI tank is sized in grains, like a water softener, but against a very different load. A softener only counts hardness. A water deionizer counts everything.

Divide the total by 17.1 to get grains per gallon. To convert other readings, use the water hardness converter.

Mixed Bed vs Two-Bed DI Resin

A two-bed system runs water through a cation tank, then an anion tank. The resins are separate, so they are easier to regenerate and hold more usable capacity per cubic foot, but the water leaves at lower purity. Larger two-bed plants often put a degasifier between the tanks to strip CO2 before it reaches the anion resin.

A mixed bed tank blends cation and anion resin in one vessel. The water passes through thousands of tiny exchange stages in a row, so it comes out much purer. Mixed bed resin is the standard for portable exchange DI tanks and for final polishing after RO or two-bed.

The calculator defaults to 12,000 grains per cubic foot for mixed bed and 18,000 for two-bed. Real ratings depend on the resin grade, the regeneration level and the purity you run to, so type in your supplier's figure.

Why DI Usually Goes After RO, and the CO2 Catch

Reverse osmosis removes most dissolved salts for the cost of power and drain water. Resin removes them at the cost of an exchange. So on any meaningful volume, RO does the bulk work and the DI tank polishes what is left. Size the RO with the commercial reverse osmosis sizing calculator.

The catch is carbon dioxide. CO2 is a dissolved gas, and RO membranes let it straight through. After a 97% RO, 300 ppm of TDS drops to about 7.7 ppm as CaCO3, but 10 ppm of CO2 still adds 11.4 ppm. CO2 is now about 60% of what the resin has to remove, and it all lands on the anion side. That is why RO + DI systems often exhaust the anion resin first, and why a TDS reading on the RO product water makes a DI tank look like it will last longer than it does.

Worked Example: Car Wash Spot-Free Rinse

A car wash uses 150 gallons a day of spot-free rinse water. City water tests 300 ppm TDS on a handheld meter (as NaCl), with about 10 ppm CO2. The dealer is quoting a 2.5 cu ft mixed bed DI tank rated at 12,000 grains per cubic foot, so the tank holds 30,000 grains. Peak rinse flow is 5 gpm.

The flow check passes: 5 gpm ÷ 3 gpm per cu ft = 1.67 cu ft minimum, and the 2.5 cu ft tank handles about 7.5 gpm.

DI only means a tank swap roughly every two weeks. With RO in front, the same tank lasts about 14 times as long, and at 179 days capacity is no longer what limits it. Idle resin can foul or grow bacteria, so many dealers still swap on a fixed schedule. The customer's choice is between frequent exchanges and buying an RO unit, and the calculator puts both numbers side by side.

Knowing When a DI Tank Is Spent

Pure water barely conducts electricity, so purity is measured as resistivity (megohm-cm) or its inverse, conductivity (microsiemens/cm). Theoretically pure water at 25°C reads about 18.2 megohm-cm. A portable exchange tank often runs through a quality light that changes color when resistivity drops below its set point. Labs and medical users usually want an inline meter with a logged reading.

As resin exhausts, the weakly held ions leak first: silica and CO2 on the anion side, sodium on the cation side. A TDS meter is too coarse to catch this early. Set the swap point from the equipment maker's water spec, since a spot-free rinse tolerates far lower purity than a lab analyzer does.

Where Deionized Water Systems Are Used

• Car washes: spot-free final rinse, usually RO + DI, with DI tanks alone at smaller washes.
• Laboratories: reagent and rinse water to a stated resistivity, often RO feeding a mixed bed polisher.
• Autoclaves and sterilizers: dental, medical and veterinary equipment that needs feed water to the manufacturer's spec.
• Battery watering: forklift and backup battery banks, where minerals shorten battery life.
• Window and solar panel washing: pure water poles that dry without spots, typically a small DI filter on the cart.

For Water Treatment Dealers: Exchange Service Is a Route

Portable exchange (sometimes called service deionization) keeps acid and caustic off the customer's site. The dealer rents the DI tanks, swaps exhausted ones for fresh ones and regenerates them at a central plant. In-place regeneration needs chemical handling, neutralization and trained staff, so most small commercial customers choose exchange service.

Dealers typically price it per exchange, sometimes with a monthly tank rental on top. Days per tank tells you how often each account goes on the route, and cost per 1,000 gallons turns the exchange price into something the customer can compare against an RO quote. A customer on 2.4 exchanges a month is a strong RO + DI prospect. After the RO goes in, the account keeps paying for prefilters, membrane service and the polishing tank.

That recurring work only pays if every swap is scheduled. Software for water treatment dealers handles the quote, the exchange route and the service plan in one place. For the wider picture, see the commercial water treatment guide and water softener vs reverse osmosis.