Jan 08, 2016

POU: RO in a Nutshell

An overview of RO system configuration, sizing, installation & maintenance

For decades, reverse osmosis (RO) has been a key player in the point-of-use (POU) and point-of-entry water quality improvement market. Natural osmosis results in the passage of a fluid from a solution with a low concentration to a solution with a higher concentration until equilibrium is achieved. Reversing this process, RO uses pressure, whether from the influent water supply or a booster pump, to overcome the natural osmosis process and force the opposite transition of high-concentrate solution to low concentrate. 

Typical RO systems use pressure to force water molecules through a semipermeable membrane that removes most dissolved ionized salts at a rejection rate of more than 90%. There is an array of sizes, configurations and designs, but the basics are substantially similar. When properly configured, sized, installed and maintained, RO is the ideal technology for manifold applications. Whether used for a car wash, hydroponics, residential drinking water, irrigation blending or any of the nearly countless other applications, RO is continuing to grow in popularity in today’s water filtration market.  

Configuration

RO system configurations vary widely even within specific sub-categories. Systems can operate using the influent line pressure or with booster pumps to increase pressure to overcome high total dissolved solids (TDS) loads. The correct applied pressure is dependent on factors such as the TDS level of the feedwater, the desired contaminant rejection rate and overall system design. Higher pressure generally results in higher contaminant rejection, but comes with a related price tag by way of increased costs for energy to drive the pump and basic system component expenditures. 

Residential POU systems are available in many formats, but there is no single system that is right for all end uses. Some applications require pretreatment for sediment and chlorine. Others may have no sediment issues, but must treat for chloramines. Because RO systems non-selectively remove contaminants, it is of the utmost importance to know what is in the water supply and how it will impact long-term system operation. Pre- and post-treatment can only be properly configured by knowing the requirements and goals of the application, contaminant obstacles involved, and operational parameters available. A comprehensive water report should be a fundamental part of any RO system configuration. 

Sizing

Sizing is contingent on several key factors, including peak flow requirements, storage capacity and influent water temperature. Membranes typically are rated based on silt density index, TDS, specified applied pressure (at the inlet of the membrane) and many other parameters, but the most important is water temperature. 

Membranes commonly are rated for production based on a water temperature of 77°F. A typical membrane rated for 2,000 gal per day (gpd) at 77°F will produce about 1,200 gpd at 50°F. Compensating for temperature is a must, and also needs to be taken into consideration when installing in a location with significant annual water temperature fluctuations. An application that requires 2,000 gpd in a 24-hour time period will be substantially different from an application that requires 2,000 gpd in an eight-hour time period. An application requiring 2,000 gpd in an eight-hour period with little space for storage and 50°F water temperature may require a 10,000-gpd system to function adequately. The same 2,000-gpd 50°F application with space for a large storage tank may opt for a 5,000-gpd system. Account for the key factors, then build in some additional capacity to account for anomalies and naturally diminished production due to component degradation.

Installation

Installation of RO systems, from simple under-the-counter POU systems to more complex commercial systems, varies significantly, but many of the fundamentals are similar. Familiarize yourself with the installation location, any space restrictions, and customer and application requirements. Proactively secure any required permits and follow all applicable codes and regulations. 

RO systems require frequent maintenance, so system design and installation should take this into consideration. Spend extra time and resources to add helpful gauges, valves, meters, gadgets, widgets or other equipment that will simplify future maintenance, troubleshooting and repair. The “pay me now or pay me later” scenario applies in this situation, as it takes far more time and expense to modify the installation at a later date. 

Maintenance

RO system maintenance can be done annually, quarterly, monthly, daily or even more frequently. Some applications may have stringent quality and production requirements that must be maintained for the business to operate. In these applications, it is not uncommon to have redundant systems, onsite maintenance/repair capabilities or pre-arranged emergency response available. Significant water temperature fluctuations may dictate specific seasonal adjustments or modifications. 

When it comes to typical residential POU systems, the common maintenance interval is six to 18 months. For these systems, the maintenance program should be designed to maximize component life, provide consistently high-quality sanitary product water and offer a level of service that meets the customers’ expectations. Check TDS rejection, adjust storage tank pressure, change filters and correct any potential leak issues. 

Follow a pre-determined schedule to replace wear items like O-rings, flow restrictors, check valves, leak detectors, automatic shutoff valves and other volatile components. Taking the time to thoroughly inspect and maintain the system is far less expensive than dealing with a return call or damage claim. Using a graduated cylinder, confirm that permeate and concentrate flow rates are within the expected parameters. It is time consuming, but the importance of an adequate complete system sanitization procedure cannot be overstated.

As technology continues to improve, increased production-to-waste rates are being achieved, solidifying RO as a leading mechanism for economically feasible substantial TDS reduction. Whether used to improve the quality of a POU drinking water supply or to provide critical medical or industrial needs, RO is ideal for myriad applications.

About the author

Jerry Horner, MWS, CI, is operations director for Impact Water Products. Horner can be reached at [email protected] or 909.939.8008.

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