By By Lawrence R. ‘Larry’ Zinser
The primary production technology for commercial and industrial water treatment applications is reverse osmosis (RO). The reason for this is that RO is chemical-free and it is the “great equalizer” for dissolved solids. Across a wide range of feed water levels of dissolved solids, the RO product waters will lie within a relatively small range:
This is the reason why regional and national beverage franchises use the RO as a common water treatment. No matter the local feed water dissolved solids level, the product beverage will have the identical taste at each franchise location.
Commercial reverse osmosis units are rarely sized by production rate (gallons per minute) because the rate of cost increase with increased RO size is prohibitive. Figure A shows a rough comparison of increased capital purchase cost versus increased RO production rate.
Additionally, with increased RO size comes a necessary increase in the size of pretreatment flow rate and their required assemblies. This does not even include the increased operating and maintenance costs of electrical power and membrane replacements. Consequently, commercial RO systems are most often sized by the RO production over a 24-hour period, taking into account the gallons per day (GPD) rather than the gallons per minute (GPM). However, since the water use requirement may at times exceed the production flow of the RO, commercial ROs are typically matched with a storage tank as a reservoir to provide distribution of the RO product water as required by the application. Water from the storage tank is then distributed with a pump to the point of use as required. This article will describe a suggested procedure to determine the best combination of RO and storage tank.
The balance relationship is pictured in Figure B: cost versus space available. The recommended procedure for specifying the combination of RO and storage tank sizing follows three parts including eleven steps.
Part One: Specify the RO Size.
We will assume feed water from a source which meets the USEPA drinking water standards.*
1. Select minimum RO size.
The minimum size for an RO unit Is determined by the total volume of treated water required by the application over the most demanding 24-hour period. This volume will be application specific. However, before selecting the RO, an allowance must be made for both maintenance down time, and effects of temperature upon RO performance. For our example, we will use a 24-hour production demand of 4000 GPD.
2. Allow for maintenance time.
RO units are sized based upon their production over a 24-hour period. However, I suggest that at least 2 hours per day be allowed for maintenance downtime. The maintenance time will provide a time window for the pretreatment (hard water bypass) filters and softeners to regenerate. Other maintenance considerations, such as a Clean-In-Place protocol may warrant extension of this allowance. For our example, this means that the base production volume must be multiplied by 24/22 or 1.09. So we modify our demand to 4360 GPD (4000 X 1.09). We will need a larger RO in order to produce the required volume in only 22 hours rather than 24.
3. Allow for temperature effects.
RO manufacturers size their units based upon their production with a standard feedwater temperature of 77 degrees Fahrenheit. However, unless tempered water is used, the feed water will normally be less than 77. Typically, for most temperate climates, municipal feed water will be at 65 degrees and well water will be at 55 degrees. The exceptions for municipal water will be in the Gulf and Southwestern states, where the feedwater temperature will be higher, and the Midwest where winter temperatures may be lower. Due to the significance of this factor, it is advisable to consider the actual local feedwater temperature. These temperature effects are specified by the RO membrane manufacturer. A typical temperature conversion table is at Figure C. Assuming a municipal feed source, we estimate a feedwater temperature of 65 degrees, and so modify our sizing demand to 5450 GPD (4360 X 1.25). Again, a larger RO is required to produce the required volume with feedwater colder than 77 degrees.
4. Select candidate RO.
Commercial reverse osmosis units are specified in production steps of about 1500 GPD since most use the 4”D X 40” L membranes, and each can produce about 1500 GPD (at 77 degF) with higher production rates with larger systems. Typical RO sizes include 1500 GPD, 3000 GPD, 4500 GPD, 6000 GPD, 8000 GPD, 10000 GPD, 11500 GPD. For industrial-sized systems requiring in excess of 25,000 GPD, RO systems with 8”D X 40”L membranes may be preferred. These membranes produce about 6000 GPD at 77 degF. In our commercial case, in order to ensure that the product will be at least 5450 GPD, we would select a four-membrane (4”D X 40”L) unit with a production estimate of 6000 GPD.
5. Apply the temperature effects for actual performance by the candidate RO.
Application of the temperature effects requires division by 1.25. This further reduces the RO production to 4800 GPD (6000 / 1.25)
Part Two: Balance of RO Size with the Storage Tank.
1. Calculate the hourly production estimate for the RO.
Divide the RO 24-hour production rate by 24 hours to specify the hourly production rate (GPH). In our test case that will be 200 gallons per hour (4800 GPD / 24 hours).
2. Specify the use profile of the application.
This step is critical. The actual estimated consumption by the application must be specified for each of the 24 hours of the most demanding day. This use profile will be added to a production profile. See Figure D for the suggested format. The projected hourly water use volumes are added in the USE row.
3. Calculate the projected RO production quantities based upon the water use profile.
See Figure E for an example of a completed form. Begin the procedure with 0 NET water, and only “operate” the RO when the water USE requires it. When operating, the RO can produce up to its hourly maximum (200 GPH). At each hour when the system uses water (i.e., USE), subtract the USE quantity, add the PRODUCE quantity, and add the NET for the hour on the next line. The form is completed for the entire 24-hour period.
4. Select the storage tank size.
Select the highest (most negative) NET over the 24-hour production profile. In this case it is 2500 gallons. Therefore, for this use profile, a solution will be 6000 GPD RO coupled with a 2500-gallon storage tank.
Part Three: Confirm specification of the RO and storage tank.
1. Test the storage tank size.
A typical 2500-gallon storage tank varies between 8.5’D X 6.6’Hand 7.5’D X 8.8’H. If either of these dimensions will fit the available space, then the recommendation would be a 6000 GPD RO with a 2500-gallon storage tank.
2. Modify the combination if necessary.
If this 2500-gallon tank is too large for the available space, then, select the next larger RO: 8000 GPD. Applying the same process, we apply 6400 GPD (8000 / 1.25 for temperature) and 266 GPH to our use profile which results in a storage requirement of 1872 gallons. This sizes up to a 2000-gallon storage tank, can measure either 5’ 3” diameter by 10’ high or 9’ diameter by 6’ high.
This process may be applied multiple times to fit a specific available space.
*For non-standard feed water, recommend consult with Peter Cartwright, RO System Design Workshop, Water Quality Association.
About the author 
Following an education in chemistry (BS Degree) at Georgetown University, graduate work at Wayne State University and a 27-year career with the US Marine Corps, Lawrence R. ‘Larry’ Zinser has served an additional 27 years in design, manufacture, education and troubleshooting of residential, commercial and industrial water treatment systems. He has provided numerous technical courses throughout the country and internationally, which have been accredited by the Water Quality Association, the Pennsylvania, North Carolina, Maryland, Virginia, and Delaware Ground Water Associations, the American Nephrology Nurses Association and the Lehigh-Carbon County Community College. Zinser can be reached at [email protected] or cell phone, (215) 421-7115.
About the company
Master Water Conditioning, founded in 1967, is based in Pottstown, PA. It offers residential, commercial and light industrial products and systems to address a wide range of water issues. The company’s products include POE water softeners, filters and ultrafiltration systems, which are sold under the Alliance, Clarifier, MasterFusion, Satin, PuroPro, UltraPro and Clear Reflections brands, among others. The company’s team of professionals work hard for the industry and are proud to be a respected market leader for innovative, high-quality products and market knowledge. Master Water Conditioning proudly joined the A. O. Smith family in 2021 as part of A. O. Smith North American Water Treatment.