Reverse Osmosis: Residential RO Reimagined

Jan. 28, 2016
New membrane technology designs waste less water

About the author: Mike Glodowski is product manager, POU systems, for Pentair Water Purification. Glodowski can be reached at [email protected]. Tedd Schneidewend is mechanical engineer for Pentair Water Purification. Schneidewend can be reached at [email protected].


The point-of-use (POU) residential reverse osmosis (RO) system has remained a saturated, commoditized segment within the water filtration market with few technological advancements in system performance for nearly two decades. Yet, RO technology produces high-quality drinking water for consumers, making it essential to the success of water treatment businesses. 

Consumer demand for more efficient products continues to increase as the public becomes more aware of and the environmental impacts of water waste. This visibility is quickly creating opportunities for water treatment professionals to embrace new equipment that offers improved efficiency and performance without sacrificing overall quality. 

The Race to Efficiency 

Many POU RO element designs have remained static since the introduction of the TLC membrane in the late 1970s and early 1980s; these systems dispense nearly 70% to 90% of feedwater to the drain. This lack of efficiency often is used as a selling point to consumers against RO systems. Given the high level of competing water filtration technologies, it can be difficult to make end users look beyond the comparatively lower water recovery rates of RO systems to see their added benefits, such as fluoride and arsenic reduction, making it more challenging to sell RO systems over other technologies. 

Recent advances in residential RO technology have begun to address the relative inefficiency concerns. These advances have challenged design engineers to develop new technologies that allow equipment to remain competitively priced, retain ease of installation and afford retrofit capabilities. 

The heightened awareness regarding the water recovery rates of RO technology, coupled with the increasing scarcity of freshwater, have challenged the industry with reimagining residential RO systems. This challenge can be converted into new opportunities. 

In order to better to understand these opportunities, we must first take an in-depth look at the current general design of rolled RO elements. 

Standard Design 

A standard residential RO element features feed and permeate spacers, a permeate tube and a membrane. Often, residential element design is no different from commercial element design—although residential and commercial applications can be quite different. The feed spacer, permeate spacer and membrane are assembled together and rolled onto the permeate tube until it looks like a wound-up scroll. 

Untreated feedwater enters the membrane from one end of the scroll. While moving through, a small amount of this pressurized water passes through the membrane into the permeate spacer and then out the permeate tube. The remaining water exits through the other side of the scroll. This water is concentrated, because it has the same amount of contaminants but less water. Residential RO systems without pressurized tanks are capable of recoveries of about 20%, while pressurized tank system recoveries are typically about 10%. 

The feed spacer creates a channel within the membrane so water can flow through the scroll. It is designed to promote mixing as water flows through. This keeps salts and other contaminants away from the membrane surface so they do not affect filtration or obstruct the membrane. The velocity of the water that flows across the membrane must remain the same to maintain its performance and lifespan. If the water velocity through the feed spacer is not turbulent enough to mix the feedwater, the membrane will clog. 

New Design 

In order to increase residential RO system efficiency while maintaining performance, engineers needed to rethink the water flow within the membrane across the scroll. The biggest challenge was determining how to maintain the velocity of the feedwater across the membrane while reducing the volumetric flow rate of the concentrate. 

If you unwind a residential POU RO scroll, it resembles a rectangular sheet of paper, as shown in Figure 1. In a standard design, the water flows perpendicular to the widest section. In newer designs, the water instead flows either toward or away from the core tube, along the full length of the rolled membrane. 

This design creates similar flow velocities in the feedwater channel at lower volumetric flow rates, which creates a higher-recovery RO element without affecting the life of the membrane. Systems without a pressurized tank that utilize this new RO design can provide up to 60% recovery under typical operating conditions; while pressurized tank systems are capable of providing up to 25% recovery without a pump. 

Making a Difference 

This new RO technology has the potential to impact the perception of residential RO treatment systems. Manufacturers are constantly challenged to develop solutions that address customers’ drinking water concerns. New advances in technology that can be quickly applied or retrofitted into conventional equipment offer product differentiation to the water treatment professional in the commoditized POU market. 

OEMs and dealers that embrace new technology will be poised to position themselves favorably in the professional distribution channel, providing consumers with more efficient RO alternatives. Educated consumers concerned about their drinking water will seek those in the water treatment industry who can provide solutions to their concerns.

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