Proper Sizing of a Commercial Reverse Osmosis System

It is the purchaser's responsibility to ensure that their feedwater falls within the values these required parameters before purchasing a system. Feedwater outside the stated parameters will require to add additional pre-treatment.

  • Maximum Hardness: 15 Grains Per Gallon, however the system will function best with "soft" water which is ZERO Grains Per Gallon
  • Maximum TDS: 2000 ppm
  • Chlorine Tolerance: < 0.1 ppm, but ZERO is best
  • Oxidizer Tolerance: ZERO
  • Maximum Hydrogen Sulfide: 0 ppm
  • Maximum Iron: 0.5 ppm, but ZERO is best
  • Maximum Manganese (Mn): 0.05 ppm
  • Maximum Silica: < 1 ppm
  • [Barium or Strontium] + Sulfate (SO4): is detremental to Reverse Osmosis Membranes. Use a water softener to remove these when both are present.
  • Maximum Feedwater Turbidity: 1 NTU

NOTE: Pre-Treatment is required if any of the above values are exceeded or your system will be damaged.

  • Minimum Feed Pressure: 35 PSI
  • Nominal NaCl % Rejection: 98.5%
  • Minimum NaCl % Rejection: 96%
  • Typical Operating Pressure: 150 PSI standard. Aprox. 100 PSI with Extra Low Energy Membranes
  • pH Range Continuous Operation: 3-11
  • Maximum Temperature: 105° F.
  • Minimum Concentrate Flow Rate: 5:1
  • Protect units from freezing temperatures.
  • Permeate flow and salt rejection is based on the following test conditions: 550 PPM TDS treated tap water, 150 PSI, 77° F., pH:7
NOTE: Permeate Flow (Gallons Produced Per Day) may vary +or- 15% with each membrane, and also varies according to water temperature. Colder water slows down the membrane performance but not water quality. It is the purchasers responsibility to assure that the water temperature will be high enough to deliver the desired quantity.

Flexeon & Axeon Reverse Osmosis Systems are designed for higher recovery rates, minimum energy consumption, continuous commercial duty, and ease of use, as well as easy maintenance and operation. Axeon & Flexeon RO Systems have all of their components located for easy access and inspection. All Axeon & Flexeon systems come with a built-in booster pump, and a 1 Year Manufacturer's Warranty.

Reverse Osmosis is the best available technology for "full spectrum" water purification from organic contamination and inorganic contamination such as toxic metals, salts, toxic substances (such as nitrates, metallic salts, and cyanides), as well as all kinds of particulates, asbestos, PCBs, pesticides, herbicides, drug metabolites, pharmaceuticals in drinking water, and even microrganisms. Ultraviolet Disinfection is always recommended, especially, if you have a problem with microorganisms.

Carbon filtration alone does not remove inorganic contamination (when present) effectively, which is where Reverse Osmosis excels. On the other hand, Reverse Osmosis is a much more "hands-on" technology compared to carbon filtration, and requires that you take a close look at your water chemistry to see see if you have any water quality issues that will foul the RO Membrane. Reverse Osmosis also requires a certain amount of water pressure in order to work.

Typical Design of Atmospheric System:

A Flexeon or Axeon Reverse Osmosis System (after any pre-treatment) discharges the purified water into a atmospheric storage tank (typically 75 to 1,500 gallons), at zero pressure, while also discharging the waste water to drain. A Float Switch will turn off the unit when the tank gets full. A Repressurization Pump is installed just downstream of the storage tank, and an UV Disinfection System may be installed anywhere between the Atmospheric Tank and the repressurization pump.

Sometimes Pre-Treatment of the Raw Feed Water is required: Hard Water with a hardness higher than 15 Grains will usually need a water softener or anti-scale injection. Other water quality issues (such as Barium, Silica, Strontium, etc) may neccessitate the installation of an Anti-scale chemical injection system.

High levels of Iron, Manganese, and Sulfide may necessitate the installation of an Oxidation Unit ahead of the reverse osmosis system.

Additionally, many RO systems can be properly pretreated with CHEMICAL INJECTION of an ANTI-SCALANT. This method is very viable and is becoming increasingly popular among persons who do not want to use salt-based water softeners.

Feed water that is not properly disinfected (well water and/or tropical customers) may end up fouling your system with bacteria and virus unless you install a UV Lamp or Chlorination system upstream.

Customers who plan on processing large amounts of water may also opt to use a Backwashing Carbon unit upstream of the unit - instead of using (and replacing) the carbon block cartridges that comes with the RO unit, as the cost of operation using just the carbon cartridges would be prohibitive. You still use these cartridgers, but they are simply "fail-safe" devices.

Water Chemistry Issues:

You should always obtain a good water analysis of your water. If you do not have one available, you can obtain one HERE. It is best if you e-mail it to us atspan or you can phone it in at 800-608-USWA. This is critical. Do not buy any RO system unless you have at least a Basic Water Analysis.

Specifically, we need to know the following: Total Hardness, TDS, Iron, Sulfide, Manganese, pH, Chloride, Silica, Sulfate, and [Water Temperature during winter]. Barium and Strontium are also a serious problem when combined with Sulfates (SO4). Please inform us of any other water problems or special concerns that you know of, such as: high TSS, Radon, or unsafe levels of any contaminants such as Arsenic. If you are treating unsafe drinking water, it is is your responsibility to work with an on-site Water Treatment Specialist to ensure you are getting the proper results. Laboratory confirmation and periodic monitoring is a part of that. If you have high Chlorides and/or acidic water, you should upgrade the Titan's pump to stainless steel.

RO will typically lower your pH to some degree. You do not want acidic (pH < 6.9) RO water, especially with copper pipes. You may need to raise your water pH (after the RO system) to avoid leaching copper or metal pipes and faucets. This can be done simply by adding a calcite cartridge filter. There are also health concerns about long term consumption of acidic water. US Water offers inexpensive healthy solutions for that problem, such as a Calcite Mineral Tank, or Calcite mixed with 50% Magnesium Oxide.

If you have water that is too hard you will either have to install a water softener upstream of the RO system, or you may be able to install an AntiScale Chemical Injection System upstream of the RO unit. If you have too much Iron, Sulfide, or Manganese you will have to remove them with a well designed Oxidation Unit placed upstream of the RO system.

Sizing your RO System:

How many gallons per day do you require? You should add about 20% to this number to account for variation and margin of error. Additionally, colder water will also produce much less gallons per day than the stated production capacity, which is based on 77 F. degree water. You need to base system size on the coldest water temperature your system will experience, such as the water temperature during winter.

In order to save electricity and reduce wear on the pumps and motors, it is recommended that you size the RO system so that the pump only has to run no more than 8-12 hours per day. This saves you money on electricity and motor replacements. While this is not necessary, it is highly recommended. A good rule of thumb is to figure what your maximum daily capacity will be, add the 20% as a "fudge factor" and then double that number (so that the system only has to operate up to 12 hours per day).

Remember This: Reverse Osmosis Systems are rated at their respective gallons per day at 77 degrees F. If you live in a cold water climate where the water could be considerably below 77 degrees, you need to realize that you can lose approximately 2% production for every degree below 77 degrees F.

HOMEOWNERS: For whole house applications, each household occupant "typically" consumes 80-100 gallons per person per day, on average. So 4 people will typically need about 400 gallons per day. Anticipate holidays and guests. Take into account swimming pools, hot tubs, or any additional water-consuming activities you may have. If your water is not so bad you may ask your installer to leave the garden hoses and swimming pool supply as Untreated (not RO purified). The National Average according to the International Water Works Association is 106 gallons per day.

System Options:

For higher efficiencies (less waste water going down the drain) add an optional Concentrate Recirculate Feature and Recirculate Flow Meter to your system. This will typically boost your efficiency (less waste water) by around 20-40% or so, but will also make your waste water more concentrated with whatever was in your water to begin with. This will increase the "workload and stress " on your RO membrane. Do not add a Recirculate unless you actually have a need for it.

You can also upgrade your PVC Membrane Housings to Fiberglass or Stainless Steel if you wish.

Water Storage Tanks:

Most RO applications involve the purified water slowly filling a [zero pressure] open Atmospheric Storage Tank. Atmospheric Storage Tanks have no pressure, and require a 1" Bulkhead Fitting at the bottom so you can hook up a suitable Repressurization Pump. Atmospheric Storage Tanks also require you to add a Float Switch, in order to automatically shut off the RO system when the water tank gets full. Without this, "catastrophic flooding" will occur. The electrical wire on this is about 20 feet long, and requires a nearby electrical outlet.

Atmospheric Storage Tank (at zero pressure) vs. a Pressure Tank:

The atmospheric tank puts no "back pressure" on the RO Membrane, which will give you less waste water going down the drain. You may add a UV Disinfection System between the water tank and the re-pressurization pump. This must be properly sized to meet or exceed the flow rate of the Re-pressurization Pump.

Pressure Tanks are another option. They are totally sealed and do not allow any air, microbes, or impurities to get into the water. They are much smaller, empty faster, and do cost more.

Using a Pressure Tank will usually require you to purchase a larger sized RO system to keep up with your flow requirements, while a much much larger Atmospheric Storage Tank will allow you to buy a smaller RO system.

Pressure Tanks require you to add a High Pressure Switch to your RO system.

A High Pressure Switch automatically starts the RO system when the tank pressure falls below 40 PSI and stops the system when the tank pressure reaches 60 PSI.

RO systems generally put out a fairly low water pressure of about 40 PSI. Repressurization of the purified water supply may still be necessary.

All RO systems generate a Wastewater that will have concentrated levels of whatever was in your water to begin with. This will need to be directed to an appropriate drain. Remember that your waste water will typically be about 2-5 times more volume than the purified water you produce each day! This is how RO technology carries away the contaminants from the membrane and keeps it clean.

There are no refunds for commercial RO systems due to improper sizing because of lower water temperature or higher TDS levels. It is the customers responsibility to make US Water aware of potential temperature, pressure or water quality issues which may adversely affect the operation of a Reverse Osmosis System.

  • A commercial reverse osmosis system functions at its highest level when it is fed into an open-air or atmospheric tank. With no back-pressure from the bladder of a pressurized storage tank, a commercial RO will deliver water that is extremely low in TDS; it's not unusual for the TDS to be near ZERO when the RO is discharged into a atmospheric pressure tank. In food service applications or other applications where extremely low or ZERO TDS is not critical, a bladder tank can be utilized, but if a manufacturing process calls for extremely low TDS water, then the atmospheric tank is far and away the best solution.
  • Proper pre-treatment is necessary for successful and economical operation of a commercial RO system. The water needs to be as soft as possible or a scalant needs to be fed ahead of the system to prevent membrane scaling. In most cases, a water softener and backwashing carbon filter are the preferred and most cost effective method of pretreatment.
  • An atmospheric tank is required for storage of the RO water, and US Water offers atmospheric tanks in sizes from 20 gallons to 9,000 gallons, as well as every size in between. Additionally, the water needs to be re-pressurized out of the atmospheric tank(s) so several pumps of high purity water are offered as well as ultraviolet disinfection, UV, which kills any airborne or waterborne bacteria which could intrude into the atmospheric tank.
  • Additionally, US Water offers Deionization, DI, of the water for ultrapure applications, such as laboratory water and semi-conductor manufacturing, as well as instrumentation and monitoring.
  • Below is a complete typical system, including pre-treatment, tank, pump, UV and DI:

If you need any design or sizing help, US Water has the Engineering and Technical Support Staff to handle any application. Call us at 800-608-USWA or e-mail us at