July 2001
Volume 43 Number 7
 

An Open-Loop Recirculation Flow Pattern: Eliminating Water Waste in Membrane-Based POU Applications, Part 1 of 2
by Ted Kuepper, Robert Lovo and Mark Silbernagel   Pages: 

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Summary: Even the most widely used water technologies in the POU/POE industry have drawbacks. Chief among these often is the amount of water wasted in the process. A no-waste membrane system design has been introduced as a practical alternative. Some of the system's positives relate to its open-loop recirculation flow pattern.

Point-of-use (POU) reverse osmosis (RO) drinking water systems and point-of-entry (POE) ion exchange water softeners are two of the most universal water treatment technologies used today for residential, commercial and industrial applications. These systems, however, have operational characteristics that create significant concerns if they're installed in water-scarce regions of the world. These concerns include:

* Salt brine created by ion exchange softeners has increased the salt concentration -- salinity -- of domestic wastewater making water reuse options for municipalities more expensive and complex, and

* Water waste by membrane-based systems, such as RO, is high and limits these applications.

Salinity and efficiency

This is illustrated by an investigation conducted by the city of San Diego and the San Diego County Water Authority. The study found the increase of salinity, due to salt brine regeneration of conventional water softeners, to be 234 milligrams per liter (mg/L) in the wastewater of a community of 6,731 households.1,2 It was concluded that water softeners contribute 44 percent of the total salt increase in this particular test community’s domestic wastewater (534 mg/L total salt increase was found in the wastewater evaluated). Such results may vary depending upon softener penetration in the market served. (See NOTE before "References")

A salinity increase in a community’s wastewater -- regardless of the source -- directly impacts the cost to reuse and recycle wastewater in the future. In June 1999, the Metropolitan Water District of Southern California and the U.S. Bureau of Reclamation produced a Salinity Management Report that estimated costs associated with salt removal from domestic wastewater and recycled water.3 Based on this report, the annual cost to remove 234 mg/L salt from the domestic wastewater of the community investigated in the San Diego study was estimated at $475,000 and $650,000. Therefore, a membrane-based water softener, which doesn't increase the salt content of domestic wastewater, has the potential for significant cost savings for those communities that wish to implement water reuse options in the future.

Membrane-based demineralization systems do an excellent job of softening water without the addition of salt. They do concentrate salts in the waste stream (also referred to as the brine or concentrate stream) that are generally discharged to a drain -- but they do not increase them. Membrane-based systems, however, aren't used for water softening applications today primarily due to the quantity of water wasted by conventional membrane processes, such as RO and nanofiltration. This paper describes a membrane process design that doesn't waste water and thus has the potential to provide a real alternative to conventional salt-regenerated water softening equipment. In addition, the no-waste design also saves significant amounts of water when it's used in drinking water purification applications.

Membrane separation

All cross-flow membrane-based water treatment technologies -- RO, nanofiltration, ultrafiltration and microfiltration -- create two streams of water as a result of their process. Using RO as an example, the two streams exiting a membrane element are: 1) desalinated (or demineralized) product water that has passed through the membrane; and 2) concentrate or brine that has flowed across the membrane surface. This waste stream is necessary to flush salts and minerals away from the membrane so they don't accumulate and cause fouling of the membrane surface. A buildup of salts and minerals in the feed water to an RO membrane must not be allowed to occur continuously or dissolved substances can concentrate, precipitate and form a solid on the membrane's surface. If this occurs, the membrane can become irreversibly fouled and may have to be replaced. As previously noted, this characteristic of the RO membrane process poses a significant challenge to reducing waste effluent. It's also the primary reason why RO membrane systems aren't used in several POE applications (for example, water softening) as they waste relatively large quantities of water relative to the amount produced.

To reduce the amount of water used in the RO process, several techniques have been created. One is the "closed loop recirculation flow pattern." This technique recycles a percentage of the waste brine into the feed water of an RO system. By doing this, the overall recovery of the system is increased while the percentage of water wasted can be decreased depending upon the circumstances. In addition, the flow rate across the RO element remains at or above the membrane manufacturer’s recommended minimum flow rate for dynamic (and in some cases turbulent) flow. The recovery of an RO system is defined as:

Membrane System Recovery = Product Water Flow Rate x 100 percent

Feed Water Flow Rate

Water waste factors

As concentrate/brine is recycled back to the feed water of the RO elements, less new feed water is required and consequently system recovery increases. Because of concerns over fouling from precipitating salts and minerals concentrated on the membrane surface with higher recovery and the resulting need to replace elements, many small RO systems use a conservative nominal recovery of 20-to-30 percent (a nominal three to five gallons of water wasted for every gallon of demineralized water produced). This level of recovery has shown to be suitable for drinking water systems for residential and commercial applications, but has kept membrane systems from being used in other applications that require higher water production rates, such as water softening.

Another factor that greatly exacerbates water waste, particularly in POU drinking water RO systems, is that these units typically store product water in an air pre-charged, pressurized water tank. Product water delivered to the tank must overcome the pre-charge pressure (initially) and, more significantly, overcome an ever-increasing back-pressure in the tank as water fills the storage tank. It's been estimated that this characteristic of non-pumped RO systems causes the actual amount of water wasted in actual usage to be much more than the nominal three to five gallons for each gallon of demineralized water produced.4 This is largely because most small RO systems are built for simplicity and low cost and understandably must sacrifice efficiency to achieve these goals. An important aspect of investigation for this article was to document the actual quantity of water wasted in a conventional POU residential RO system to determine the problem's magnitude.

Design description

The process design described here is an evolution of the conventional, closed-loop recirculation flow pattern created to produce a membrane-based system that has an apparent 100 percent recovery capability. It also can operate most efficiently to purify a municipal water source in various applications where conventional system efficiencies are low and subsequent water waste is high.

This development is called an "open-loop recirculation flow pattern" and is shown in Figure 1. This new flow pattern allows an RO system’s concentrated salts and minerals to accumulate inside a recirculation tank, which is also connected to piping that feeds other water use fixtures. The recirculation tank provides a buffer volume to permit continuous production of demineralized water even when water isn't being used by water fixtures.

Also shown in Figure 1, product water is delivered into a flexible bladder that occupies the top of the recirculation tank. Since the bladder is sitting in a tank pressurized by municipal line pressure, product water can be delivered without the need for an additional re-pressurization pump. This "water-on-water" feature allows delivery of membrane-produced demineralized water to be consistent at municipal line pressure without fluctuations common with re-pressurization systems and air pre-charged storage tanks.

No-waste positives

An important characteristic of this no-waste membrane design is that the volume a full product water bladder occupies in the recirculation tank effectively sets the recovery of the RO system even when a location’s water fixtures aren't being used. To explain this feature further, if a full product water bladder occupies 50 percent of the re-circulation tank volume, then recovery for the system is limited to 50 percent even if no water is being used by water fixtures (and as long as additional purified water isn't required). This ability to determine and set the recovery of a membrane system that doesn't waste water greatly reduces the likelihood of membrane fouling. This feature of the design is an important advantage over previous attempts to re-use water with POU membrane processes.

Use of a bladder to store and dispense product water in the design functions the same as a water-on-water tank. But use of a bladder inside a recirculation tank is unique because it sets the recovery rate as water is produced without water being used in the building. Prior attempts to create a no-waste system by placing concentrate in the piping weren't successful because water had to be used continuously to prevent premature membrane fouling. This wasn't realistic since a normal residential situation, for example, calls for a drinking water system to operate (to fill a storage tank) while no water is being used in a building (such as when everyone goes off to work in the morning). The no-waste system described here operates effectively under those conditions because the recirculation tank volume dilutes salt and mineral concentrate enough to allow water production without additional dilution. Eventually, residents come home and use water that further dilutes tank contents to start the process over again.

In a no-waste design, this use of water fixtures permits periodic dilution of concentrated salts and minerals that build up during an RO membrane process. In this way, effluent from a membrane system doesn't go directly to drain, but instead is diluted and re-used throughout a location’s plumbing system for other than the highest purified water applications. This usually means purified water is used for drinking, cooking, ice making and dishwashing, while diluted concentrate is used for toilets, sink faucets, showers, ice-making equipment cooling, and landscape irrigation. In an open-loop recirculation flow pattern process, this separation of water usage is inherent in the design and is performed without making plumbing changes to a building’s piping system. This is another important distinction from previous attempts to create a no-waste membrane system design.

In addition, the open-loop recirculation flow pattern design creates an RO system with extremely flexible features that can accommodate a wide variety of applications and product water flow requirements. This is demonstrated by the fact that the design can create small capacity no-waste undersink drinking water systems for residential use, as well as large capacity no-waste water softeners and purified water systems suitable for large commercial businesses such as office buildings and hotels.

Conclusion

In this article, we've laid out the concept of an open-loop recirculation flow pattern and its benefits in improving the water efficiency of membrane systems such as POU RO units. Part 2 next month will discuss limitations and test results in applying such a concept in the field in California.

Editor's note: Readers should bear in mind that in 1999 compromise legislation passed in California, Senate Bill 1006 reinstated a community's ability to ban household water treatment equipment -- namely softeners and ROs -- but added caveats that require communities first assess total contributors to increased wastewater salinity and address reductions from all contributors. As part of the compromise, softener manufacturers agreed to significantly improve the salt efficiency of their equipment. It is WC&P's and the Water Quality Association's position that homeowners should have the right to improve the quality of water in their homes.

References

1. Rancho Bernardo Water Softener Impact Study, San Diego County Water Authority and the City of San Diego, March 1989.

2. Rancho Bernardo Water Softener Impact Study-Phase 2, San Diego County Water Authority and the City of San Diego, February 1990.

3. Salinity Management Study-Final Report, Metropolitan Water District of Southern California and the U.S. Department of the Interior, Bureau of Reclamation, June 1999.

4. Conversation with Bob Riley, Research chemist and owner of Separations Systems Technology, San Diego, September 1998.

About the authors

Ted Kuepper, Robert Lovo and Mark Silbernagel developed the Zero-Waste Membrane System described in this article.

Kuepper has a master's degree in environmental engineering from the University of Southern California and a bachelor's degree in ocean engineering from Florida Atlantic University. He's a Registered Environmental Manager with the National Registry of Environmental Professionals; managing director of the Seawater Desalination Test Facility in Port Hueneme, Calif.; a project manager with the humanitarian organization, Global Water; and a member of Pacific Research Group in Ventura, Calif. He can be reached at (805) 985-3057, (805) 985-3688 (fax) or email: ted@isle.net

Lovo has a bachelor's degree in chemistry from the University of Oklahoma. He's project manager and senior engineer with Pacific Research Group, Ventura, Calif., and the Seawater Equipment Test Facility in Port Hueneme, Calif.

Silbernagel has a bachelor's degree in chemical engineering from the University of California, Santa Barbara. He's test director at the Seawater Desalination Test Facility in Port Hueneme, Calif.; a technical advisor with the humanitarian organization, Global Water; and a member of Pacific Research Group in Ventura, CA. He designed and performed construction management for the first municipal seawater reverse osmosis (SWRO) desalination facility on the West Coast for the U.S. Navy on San Nicolas Island.


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