Point-of-use reverse osmosis (RO) systems are seeing a resurgence in popularity, as they remain capable of broad-spectrum contaminant reduction with many of the existing and emerging drinking-water issues, such as PFAS, lead, microplastics, and pharmaceuticals. To ensure your customers’ RO systems achieve optimal performance, efficiency, and protection, make sure the systems are set up properly and installation parameters have been accommodated.

In this article, I will explain how specific pumps can enhance an RO system’s output, efficiency, and water quality.

Running with Proper Pressure and Rejection

To properly reject contaminants and output high-quality water, RO membranes must be placed under pressure. How much pressure is needed will vary depending on a few key parameters that often get overlooked.

RO manufacturers typically state an operating pressure range of 40 pounds per square inch (psi) to 100 psi on their specification sheet. In reality, these systems can function on pressures below 60 psi, although not very well. The membrane’s output and rejection ratings are typically based on operating pressure of 60 psi, so a system working with 40 psi will not work as expected.

Osmotic Pressure

Another factor that affects applied pressure is the total dissolved solids (TDS) level. The TDS level of incoming water creates osmotic pressure. Pressure applied to the membrane after adjusting for osmotic pressure is called net driving force. For every 100 parts per million (ppm) of TDS, you lose 1 psi of net driving force on the membrane. A TDS of 500, for example, drops 5 psi from the net driving force.

When pressure is too low, it drastically affects the rejection rate. This chart illustrates an example of the changes in rejection across a spectrum of pressure. Imagine a feed of 40 psi with TDS of 500 ppm. It has now dropped to a 35 psi net driving force. A 35 psi net driving force on the membrane drops rejection to an unacceptable percentage.

We run across 40 psi quite often in locations where TDS can run a little high, such as in private wells. Well pumps typically are controlled by a 30/50 pressure switch. In this scenario, the RO system will never see 50 psi and is always seeing less than optimal pressure.

Low pressure and high TDS cause a significant drop in RO performance. Depending on the application, these conditions could also mean that important contaminant reductions are not being made. Nitrates, for example, reject only in the 80 percentiles with good pressure, not in the 90s. As a result, a system operating with less than optimal net driving force will not adequately reject some contaminants, leaving customers at risk.

Booster Pump

A booster pump is a simple solution to low feed pressure and osmotic pressure. Boosting pressure to the RO by 30 psi is a game changer for system performance. In the example used above, adding 30 psi increases our net driving force to 65 psi. Adequate net driving force keeps the membrane producing water and rejecting contaminants at sufficient rates.

A booster pump must be capable of keeping up with the membrane output size. With the wide variety of residential RO systems on the market capable of 25 gallons to 150 gallons per day, it is important to select a pump that can provide adequate water flow at pressure.

There are a few important guidelines to note when setting up a booster pump on the RO system. These pumps require a power supply and tank pressure shut-off switch. The switches are available in preset shut-off pressures, such as 40 psi, 60 psi, and 80 psi. Tank pressure of 60 psi sounds great, but it is possible to create a situation in which the RO system’s built-in automatic shut-off (ASO) will actuate before the tank reaches the switch’s shut-off pressure. This results in the pump never shutting off and eventually overheating. ASOs shut off inlet water when the tank pressure reaches roughly two-thirds of the feed pressure. If the booster pump provides 80 psi, the ASO closes at a tank pressure of around 52 psi. If the pressure switch is set at 60, it will never shut off the pump.

Storage Tank Pressure

Another drain on net driving force is the pressure that builds up in the storage tank as it fills. When calculating net driving force, we subtract osmotic pressure and the precharge pressure of the storage tank from the feed pressure. Doing so helps us make sure that the RO system will do the job under specific conditions. What it doesn’t account for is the pressure in the tank when it is one quarter full, half full, and full, and at all the points in between.

When the tank is empty, the pressure is around 5 psi. This is the empty tank precharge. When it is full, the tank may be at 35 psi or 40 psi based on feed pressure. As the last few ounces go into the tank, the net driving force has lost the same 35 psi or 40 psi. This drop in water quality creates what is commonly referred to as TDS creep and can contribute to the inability to meet the contaminant-reduction goals.

The booster pump will raise net driving force when we need help due to low pressure, high TDS, or both. Unfortunately, a booster pump does not mitigate TDS creep. To eliminate TDS creep, the storage tank must stop pushing back on the membrane. We could use a nonpressurized tank, but then we would need a delivery pump to move the water out of it to the point of use.

Permeate Pump

A permeate pump solves the problems created by the pressurized storage tank by preventing the tank’s building pressure from pushing back on the membrane. Regardless of how full the tank is, the permeate pump allows the membrane to work at the 5 psi tank pressure reduction from net driving force. This solution prevents TDS creep from interfering with contaminant reduction.

The permeate pump also provides another important benefit: efficiency. The amount of water sent to the drain is drastically reduced when a permeate pump is used. Without it, as the tank fills and increases in pressure, the amount of RO water going into the tank slows down due to the resistance created by the pressure. The concentrate flow to drain, however, is a fixed rate and does not slow down, decreasing efficiency. Many of the residential membranes are specified at a 25 percent recovery (for every gallon of RO water to the tank, three gallons go down the drain). The recovery can fall as low as 5 percent (20 gallons to drain for every gallon in the tank) due to the slowing of RO water to the tank versus the fixed flow rate to the drain.

In an application with sporadic and low water use, the RO is always running at the worst point of production, quality, and efficiency. The permeate pump can save 75 percent to 80 percent of the water typically going to drain. With the U.S. Environmental Protection Agency’s WaterSense program being developed to include RO systems, using the permeate pump may be the only way to reach the efficiency goals being proposed.

Conclusion

Residential RO systems can make very high-quality water for consumption or use in many other applications. They can also fall very short of the quality goals if installation site parameters are not adequately accommodated. When pressure is low or TDS is high, adding a booster pump and permeate pump can make the RO system an efficient, quality water producer. My rule of thumb is that a booster pump should be considered whenever TDS is over 500, especially on a private well with a 30/50 pressure switch on the well pump.

Be mindful that pretreatment is still necessary when water conditions are poor enough that even the best-equipped RO will fail. Contamination such as excessive hardness, iron, manganese, and hydrogen sulfide must be addressed before the water reaches the RO system’s inlet.

About the Author

John Woodard, MWS, is the Technical Support and Training Manager for Fresh Water Systems.  He began his 30+ year water treatment industry career as sales director for a reverse osmosis system manufacturer.  In the years since, working for manufacturers, dealers, and wholesale distributors, John has gained a great deal of experience with RO systems, applications, and unusual issues.   In his role at Fresh Water Systems, he also is the technical advisor for their service division that supplies water treatment systems to over 16,000 pharmacies across the US. 

About the Company

Fresh Water Systems celebrates its 35th year providing water treatment solutions to residential and commercial customers.  Primarily a wholesale distributor, Fresh Water was one of the first successful companies to present products and replacement filters online when they launched freshwatersystems.com in 1997.  Now with distribution centers in Greenville, SC and Salt Lake City, UT, FWS can ship goods anywhere in the continental US in two days.  In 2012, Fresh Water established NeoLogic Inc. to provide a clear distribution channel for OEMs, dealers, and distributors across the country.

 

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