BACKGROUND OF THE INVENTION

This invention relates to reverse osmosis membrane units, and, more particularly, to the cleaning of reverse osmosis membrane units that have been previously fouled in service.

Reverse osmosis (sometimes termed RO) is used to purify fluids that contain dissolved and undissolved impurities, in a variety of industrial, commercial, and home applications. In reverse osmosis, a fluid to be treated is passed over a reverse osmosis membrane in a cross flow manner. Some of the fluid, termed the "permeate", passes through the reverse osmosis membrane and is collected as the purified fluid that is the objective of the procedure. The remainder of the fluid, now having a higher concentration of the impurities and termed the "concentrate", is discarded or further processed. The permeate has a lower concentration of impurities than does the concentrate. Although much of the impurity material leaves the reverse osmosis unit in the concentrate, some remains adhered to the surface of the reverse osmosis membrane as a colloidal solid. Undissolved impurities include substances found widely in process fluids such as clays, silica, iron and aluminum hydroxides, and organic debris, and also substances that may be peculiar to a particular application such as paint pigments, proteins, high-molecular-weight alcohols, and bacterial and yeast cells. Flocculents may be added to the impure fluid upstream of the reverse osmosis unit, and prior to its passing through filters or clarifiers, to cause colloidal solids to form, so that some of the colloidal solids may be removed by the filters or clarifiers. Inevitably, however, some of the colloidal solids reach the reverse osmosis unit and lead to fouling of the unit during service.

The colloidal solids adhering to the reverse osmosis membrane gradually build up over time, eventually forming layers of colloidal solid material that interfere with the operation of the reverse osmosis membrane and reduce its performance. This gradual reduction of operational performance is termed

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"colloidal fouling" or simply "fouling". The result of such fouling is that the permeate flow rate is gradually reduced and eventually becomes unacceptably low.

When the mass of colloidal solids on the reverse osmosis membrane becomes so thick and dense that the permeate flow is reduced to an unacceptably low level and the reverse osmosis unit becomes inefficient, the reverse osmosis unit is removed from service and either discarded or, more preferably, cleaned and later returned to service. The unit is typically designed as an integral tubular housing containing the reverse osmosis membrane in a rolled-up form. The fouled unit is removed from the reverse osmosis system, replaced by a spare unit so that the system continues to operate, and sent to a cleaning operation. ("Fouled" as used herein means that the unit cannot be used further in the process without cleaning— it does not mean that the unit or the fluid flowing from the unit is necessarily toxic or injurious.) Alternatively, the fouled unit may be removed from service and cleaned in place. In the cleaning operation, accumulated colloidal solids are removed from the reverse osmosis membrane and the unit is otherwise reconditioned. The cost of each reverse osmosis unit is such that cleaning is economically attractive as compared with discarding the fouled unit each time the colloidal fouling becomes excessive. In the cleaning operation, a cleaning fluid is flowed through the reverse osmosis unit. Some of the colloidal foulant is removed by the cleaning operation, so that the performance of the reverse osmosis unit is improved after it is returned to service. A number of cleaning fluids have been developed and tested, and some promote improved cleaning. However, experience has shown that, even with the best cleaning techniques and cleaning fluids now available, the colloidal foulant is not removed completely from the reverse osmosis membrane and the cleaned unit does not achieve the performance level of a new unit. When the cleaned unit is returned to service, it fouls faster than does a new unit and therefore must be recleaned sooner than a new unit must be cleaned. There is a need for an approach that improves the cleaning of reverse osmosis membrane units so that the cleaned units more closely approach the performance of new units. The present invention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a method for cleaning a fouled reverse osmosis filter unit and a reverse osmosis filter unit cleaned by the method. The approach uses conventional cleaning equipment in a new way. It does not require additional cleaning time. The approach is operable with existing cleaning fluids, and serves to improve the results obtained with the conventional cleaning fluids. The approach of the invention achieves improved performance of the cleaned reverse osmosis units, approaching or equaling that of new units, with no added cleaning cost. In accordance with the invention, a method for cleaning a reverse osmosis membrane unit includes the providing of a fouled reverse osmosis membrane unit comprising a reverse osmosis membrane with a forward flow direction corresponding to the direction of flow of a prior fouling flow of fluid past the reverse osmosis membrane and a reverse flow direction substantially opposite to the forward flow direction. A cleaning fluid is flowed through the fouled reverse osmosis membrane unit in a plurality of cycles of flow in the forward flow direction and the reverse flow direction. In each cycle a cleaning flow ratio of a reverse cleaning fluid flow time in the reverse flow direction to a forward cleaning fluid flow time in the forward flow direction is greater than 1.0, preferably from about 1.5 to about 4.0. Each cycle has a total cycle duration of a total of forward cleaning fluid flow time and reverse cleaning fluid flow time of no more than about 100 seconds, preferably from about 15 seconds to about 100 seconds, most preferably about 75 seconds.

The reverse osmosis unit is typically in the form of a tubular container having the reverse osmosis membrane rolled up therein in jelly-roll fashion. The container has an inlet and an outlet, with the forward flow direction being from the inlet to the outlet, and the reverse flow direction being from the outlet to the inlet. The cleaning is preferably continued for about 15 minutes in a continuous flow-reversal pattern. For such a most preferred embodiment, there are 12 cycles. each lasting about 75 seconds. In each cycle, the cleaning flow ratio is about 2.

The present invention provides substantially improved reverse osmosis membrane performance after the unit is returned to service, as compared with

conventional cleaning operations wherein either the cleaning fluid flow is continuous in the forward direction or the cleaning fluid flow reverses with a ratio of 1.0 and a cycle duration of typically about 75 seconds. Depending upon how fouled the unit was when removed for service for cleaning, after cleaning its performance may approach that of a new unit and in any event is much better than that of the conventionally cleaned unit. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a tubular reverse osmosis membrane unit;

Figure 2 is a sectional view of the reverse osmosis membrane unit of Figure 1 , taken along lines 2-2; Figure 3 is a schematic illustration of a typical service application of a reverse osmosis membrane unit;

Figure 4 is a schematic illustration of a preferred cleaning apparatus according to the invention;

Figure 5 is a block diagram of a preferred cleaning method according to the invention;

Figure 6 is a graph of flow improvement during service of a cleaned reverse osmosis membrane unit as a function of the reversal ratio/cycles; and

Figure 7 is a graph of flow improvement during service as a function of cycle time, for a cleaning flow ratio of 3.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates a reverse osmosis membrane unit 20 in the preferred tubular form. The unit 20 includes a tubular, preferably cylindrical housing 22 with closed ends 24, with a reverse osmosis membrane 26 rolled therein in jelly- roll fashion, see Figure 2. The unit 20 further includes three openings between the

exterior and the interior: an inlet 28 at one end, and a permeate outlet 30 and a concentrate removal port 32 at the opposite end.

Figure 3 schematically depicts a typical circuit in which the unit 20 is used. A commercial, industrial, or home process operation 40 utilizes a fluid, typically water, for process functions such as cleaning, cooling, or the like. The process 40 is depicted genetically, as many different such processes are utilized. The process 40 produces an outflow 42 of water that is sent to a holding tank 44. From there it is pumped by a pump 45 back to the process 40.

Raw feed water is required to fill the holding tank 44 initially or as makeup water during operation. The raw feed water, usually well water or city water, typically contains soluble impurities and insoluble impurities such as colloids. These impurities are removed by the reverse osmosis unit 20 before the feed water is placed into the holding tank 44. Feed water is pumped by a pump 46 through a filter 48 that removes the largest of the insoluble impurities. Optionally, a flocculent 50 is added prior to passing the fouled water through the filter 48 to cause agglomeration of small insoluble species to a size sufficiently large that most are captured by the filter 48.

The filtered water enters the reverse osmosis unit 20. In this unit 20, the water passes in a cross-flow manner across the face of the reverse osmosis membrane 26. Some of the water passes through the reverse osmosis membrane 26 as the permeate, and flows out of the unit 20 through the permeate outlet 30. In most cases, such as the one illustrated, the permeate is of sufficiently good quality that it may be provided to the holding tank 44 for use in process applications. The reverse osmosis membrane 26 is constructed so that many contaminants cannot pass therethrough and are retained in the water that does not pass through the reverse osmosis membrane 26. This water, the concentrate, becomes even more fouled with the contaminants, and leaves the unit 20 through the concentrate removal port 32 to be cleaned or disposed of elsewhere. Processes such as that of Figure 3 are well known in the art, as are the construction and use of reverse osmosis membranes 26 and units 20. For reference, a forward flow direction 54 is defined as the flow direction from the inlet 28 to the outlet 30, the direction of flow in the fouling process of Figure 3. A reverse flow direction 56 is defined as the flow opposite to the forward flow direction 54, from the outlet

30 to the inlet 28.

After a period of service, some of the solid contaminants of the water that pass through the unit 20 accumulate on the side of the reverse osmosis membrane 26 that contacts the fouled water entering the unit 20 through the inlet 28. The layer of these solid contaminants gradually thickens, resulting in increasing pressure drops between the inlet 28 and the concentrate removal port 32, a lower flow rate of the permeate leaving the permeate outlet 30, and less efficient rejection of the soluble contaminants so that the quality of the permeate falls. At some point, when the performance of the unit 20 becomes unsatisfactory, it is removed from service. Typically, a replacement unit is stocked and is placed into service to replace the fouled unit. The fouled unit may be discarded but, more preferably due to its cost, the fouled unit is cleaned. ("Fouled" as used herein means that the unit cannot be used further in the process without cleaning— it does not mean that the unit or the fluid flowing from the unit is necessarily toxic or injurious.)

Figure 4 depicts a preferred cleaning apparatus 60 used to clean a fouled unit 20. (A single unit 20 may be cleaned, or several units 20 may be connected in series for simultaneous cleaning.) A cleaning fluid, typically an acidic or basic aqueous solution, is provided in a tank 62. The unit 20 is placed into a piping system 63 with four solenoid valves 64, 66, 68, and 70 that permits the cleaning fluid to be passed through the unit 20 in either direction. When the cleaning fluid is to be passed in the forward direction 54, valves 64 and 70 are opened and valves 66 and 68 are closed. When the cleaning fluid is to be passed in the reverse direction 56, valves 66 and 68 are opened, and valves 64 and 70 are closed. The sequence of opening and closing of the valves is controlled by a timer 71. The forward and reverse pumping of the cleaning fluid are alternated, with each pair of forward and reverse pumping segments being a "cycle". The cleaning fluid is pumped from the tank 62 into the piping system 63 by a pump 72. The cleaning process may be accelerated by introducing air bubbles 74 into the cleaning fluid before it passes through the unit 20.

In the past, it has been the standard practice to pump the cleaning fluid through the unit 20 either continuously in the forward direction or, alternatively, for equal times in the forward direction 54 and the reverse direction 56 during

each cycle. In the latter approach, a cycle typically lasts 60 seconds, and there are a total of 15 cycles in the cleaning operation. The conventional approaches clean the reverse osmosis units 20 moderately well. In most cases, the process users of the units 20 keep them on line as long as possible, so that the units 20 are quite fouled when they are taken off line for cleaning. With the conventional approach, it has not generally been possible to clean the units 20 sufficiently well so that they are returned to the performance of the new units.

According to the present invention, the ratio of the times of reverse flow and forward flow of the cleaning fluid is made greater than 1 , preferably from about 1.5 to about 4, and most preferably about 2. That is, the flow time in the reverse direction is always greater than that in the forward direction. The duration of each cycle (i.e., one pair of forward and reverse flows) is less than about 100 seconds, preferably from about 15 to about 100 seconds, and most preferably about 75 seconds. Figure 5 illustrates the cleaning method according to the invention. The fouled reverse osmosis membrane unit 20 is provided, numeral 80. It is cleaned in the apparatus 60 of Figure 4, with the timer 71 set to produce the flow times and ratios just discussed, numeral 82.

Tests were conducted using the apparatus of Figure 4, except modified so that two different cleaning solutions could be pumped sequentially through the unit 20, to clean a number of similarly fouled reverse osmosis membrane units 20. The units were standard types, each 8 inches in diameter and 40 inches long. The operating conditions were 200 pounds per square inch net driving pressure, 25°C operating temperature, and 35 gallons per minute flow rate. The cleaning operation included 15 minutes of cleaning with an aqueous solution of a commercial acidic cleaner, Bioclean 103 A, followed by 15 minutes of cleaning with an aqueous solution of a commercial basic detergent cleaner, IP A 41 1.

Within each 15 minute cleaning period, the duration of cycles and the ratios of the durations of the reverse flow to the forward flow in each cycle were varied. The flow properties of the units were measured before and after cleaning, and the change due to cleaning presented as a percentage improvement (or dis- improvement. if the value is preceded by a minus sign). The following Table I presents the data obtained:

Table I

Cycle Duration Flow Ratio Improvement. %

(seconds ^' ) ( forward :reverse Permeate Flow Diff. Press. Salt Rej.

75 1 : 1 80.8 36 5.1

1 :2 126 44.6 14.6

1 :4 1 18. 40 5.22

1 :6 57.9 30 1.34

140 1 :3 19 21.7 2.97

180 1 : 1 4.9 -9.4 1

1 :2 5 0 2

1 :4 7.6 0.9 3.2

1 :6 10.1 -14.1 2

Permeate flow, differential pressure, and salt rejection were substantially improved for flow ratios (ratio of reverse flow time/forward flow time) of greater than 1 , and most significantly in the range of about 1.5 to about 4.0, and particularly for the cycle duration of 75 seconds. By inteφolation, it is determined that cycle times of less than about 100 seconds provide substantially improved performance, as compared with longer cycle durations. The cycle duration should not be less than about 15 seconds, as a full flow reversal cannot be practically obtained in lesser times.

Figure 6 is a graph of the percent of permeate flow increase as a function of a quantity "reversal ratio/cycles" determined as 1/CN, where C is the ratio of the reverse flow time to the forward flow time in a cycle, and N is the number of cycles of duration D accomplished in a 15 minute period. This quantity serves to unify the interpretation of the data such that the best performance is obtained for 1/CN of from about 0.015 to about 0.4, for any period of time. The best results are obtained for a value of 1/CN of about 0.03, and for a duration D of 100 seconds or less. Figure 7 illustrates the semi-logarithmic dependence of the percent of permeate flow improvement after cleaning as a function of the total duration of each cycle.

Although a particular embodiment of the invention has been described in

detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.