Membranes in reverse osmosis (RO) elements can become fouled by biological matter, mineral scale, colloidal particles and insoluble organics. A loss in normalized permeate flow, normalized salt rejection or both can occur from deposit buildup on membrane surfaces during normal operation. Most element manufacturers prescribe that elements should be cleaned when one or more of the following parameters present themselves:
1. The normalized salt passage increases 5 to 10 percent
2. The normalized permeate flow drops 10 percent
3. Feed pressure minus concentrate pressure (the normalized pressure drop) increases 10 to 15 percent.
If you wait too long cleaning may not restore the membrane element performance successfully and you may basically end up with a patient that’s dead on arrival. In addition, the time between required cleanings may become shorter as the membrane elements will foul or scale more rapidly if not maintained at appropriate intervals. While there is no guarantee that all membranes will be able to be cleaned up to manufacturers’ specifications, most of the time they will, again, if cleaned within appropriate time intervals.
The economics
As a service provider in the high purity water market, when a customer’s reverse osmosis membrane does not meet the required element flow and rejection properties, you have the choice of either buying a new membrane from your favorite supplier or having it cleaned back to within manufacturer's specifications for continued work in the field; this, of course, assumes you have the capability in your facility to do this cleaning, or you have access to a vendor that can perform this service on an outsourced basis.
This decision is not unlike that made in the DI resin world where sending resin out to be regenerated and reused is always going to save significant money versus buying new (with an understanding that certain applications do require virgin resin).
In a world that is always on the lookout for cost savings and an environmentally preferred solution in extending the life of useful materials, cleaning or regenerating comes up a winner in most cases. It should be emphasized that frequent cleaning is not required when the membrane is a component of a properly designed and properly operated RO system.
From a purely economic point of view, your customer will save money if the membranes are cleaned and then returned for service, while you as the service provider can still make a good profit on this.
While you may make more money per membrane in the short run by buying and reselling a new one to your customer, the competitive environment may put you at a disadvantage when comparisons are made with other service providers for either maintenance contracts or “billed as serviced” charges.
Doing what is best for the customer usually pays off in the long run monetarily and it should also help you sleep better. Also, if you do indeed provide all inclusive maintenance contracts to your customers, this approach will help you lower your cost of providing service without sacrificing water quality. The typical cost of cleaning a membrane will reduce your cost in half or less versus buying a new one in most circumstances.
Customer clean-in-place versus off-site
While some of the larger customer facilities may still have legacy clean-in-place skids for membrane cleaning in their facilities, this is becoming increasingly rare particularly in new facilities due to water discharge permitting issues, on-site chemical storage and other compliance requirements.
Even if they do have these skids in-house, they are typically designed with membrane housings in series, creating a much less than ideal environment to clean multiple membranes at once as each successive membrane in the cleaning series is rinsed with an ever deteriorating water quality. This can be compared to showering using the water from the person before you.
Off-site cleaning, with membranes cleaned in individual housings in a parallel format, will provide the best possible environment for rinsing up membranes to manufacturers’ specifications, along with other controls in the cleaning environment which are described below.
Preferable process
When evaluating an effective approach to membrane cleaning, the following areas should be considered:
1. As mentioned previously, the membrane cleaning rack should preferably be designed with parallel, individual housings for each membrane to be cleaned.
2. The rinse water being used should be RO permeate water or ideally deionized water, and there should be control over the flow rates, water temperature and cleaning pressures.
3. The chemical solutions being used in the cleaning process should be from a known high quality producer of cleaning agents.
4. Pre-test and post cleaning results of each membrane should be available for analysis and information that may determine where the membranes are re-installed.
5. Upon completion of the cleaning process, each membrane should be separately sealed, preferably in a poly bag. If the membranes are not going to be put into service within a relatively short time period, it is recommended that a preservative be used for the storage period. It is also recommended that if the membrane is not going to be placed in service within six months that they are re-tested prior to going back into service. Storage should be in a cool, dry place.
In conclusion, for those who are already having membranes cleaned, you are likely aware of the overall benefits to you and your customers. For those of you that have not begun introducing this as a service to your customers, it may be a good opportunity for you to broaden your service offerings and introduce a new revenue stream into your P&L.
Matthew Unger has been the operations manager of Region-X, LLC, Taunton, Mass. for the past six years. Region-X regenerates DI resin, cleans and conditions RO membranes and provides other related services for water treatment service providers. He is responsible for the design and build of the company’s current regeneration and membrane cleaning stations. Prior to Region-X, Matt gained experience in field service and installation of High Purity Water Systems in the New England region.
Norm Marowitz joined Region-X this past year, and before that was the president of Atlas Watersystems Inc., a full service design, installation and service firm in the water treatment industry, where he worked for the past 16 years. Prior to this, he gained experience as a senior executive in several industries including the surgical laser industry, metals and others, as well as work as a CPA and management consultant. Norm also serves on the Advisory Board of Water Technology magazine.
