Commercial Reverse Osmosis System Sizing Calculator

Frequently Asked Questions

How do you size a commercial reverse osmosis system?

Start from the gallons of RO water needed per day, then correct for three things. First, the hours the system can run: an RO that can only run 20 hours a day has to be rated for 24 divided by 20 times the need. Second, temperature: RO systems are rated at 77 degrees F, and colder feed water cuts output, to about 58 percent of rated at 50 degrees F. Third, a design margin for fouling and membrane aging, commonly about 15 percent. Round the result up to a standard commercial size, then size storage for the peak draw and check the feed water flow and drain capacity.

Why does cold water reduce reverse osmosis output?

Cold water is more viscous, so less of it passes through the membrane at the same pressure. RO ratings assume 77 degrees F feed water. Using the temperature correction factor membrane manufacturers publish, output falls to roughly 72 percent of rated at 60 degrees F, 65 percent at 55 degrees F and 58 percent at 50 degrees F. A system that meets demand in August can come up short in February on well or surface water, which is why commercial RO is sized for the coldest month.

What recovery rate should a commercial RO system run at?

Recovery is the share of the feed water that becomes product water; the rest carries the rejected minerals to drain. Small light-commercial units often run 25 to 50 percent. Larger systems with softened or antiscalant-treated feed commonly run 65 to 75 percent. Pushing recovery higher saves water but concentrates hardness and silica on the membrane, so the safe recovery depends on the feed water chemistry and the pretreatment. Membrane makers publish projection software for the final design.

What pretreatment does a commercial RO system need?

Almost always sediment filtration and chlorine removal. Thin-film composite membranes are damaged by chlorine, so city water needs a carbon filter ahead of the RO, and chloramine needs catalytic carbon with more contact time. Hard water usually needs a softener or an antiscalant feed to stop scale on the membrane, especially above 50 percent recovery. Iron, manganese and high silt levels need their own treatment first. Pretreatment is where most commercial RO failures start.

How big a storage tank does a commercial RO system need?

Large enough to cover the biggest draw the RO cannot keep up with in real time. Take the gallons used in the peak period, subtract what the RO makes during that period, and the difference is the usable storage needed. Then divide by how much of the tank is actually usable: an atmospheric tank with a repressurization pump delivers most of its volume, while bladder pressure tanks deliver only part of theirs. A car wash that uses most of its spot-free water on Saturday afternoon is a storage problem as much as a membrane problem.

How much water does a commercial RO system send to drain?

At 50 percent recovery, one gallon goes to drain for every gallon of product water; at 75 percent recovery, one gallon for every three. The drain has to handle the concentrate flow continuously while the RO runs, and in some jurisdictions the concentrate discharge needs approval. The calculator shows the concentrate flow in gallons per minute and per day so the drain and the water bill can both be checked.

What TDS will a commercial RO system produce?

A thin-film composite membrane in good condition typically rejects around 97 to 99 percent of dissolved solids, so 400 ppm feed water becomes roughly 4 to 12 ppm product water. Rejection falls as membranes foul or scale, so rising product TDS on a monitor is the usual first sign a membrane needs cleaning or replacement. Espresso and brewing programs often remineralize RO water afterward to hit a target profile.