Comprehensive Guide to Diagnosing and Resolving High Salt Passage and Permeate Flow Issues in RO Systems

Any RO System malfunction manifests itself in a loss of salt rejection, a loss of permeate flow, and an increase in differential pressure, respectively or collectively. Then It needs maintenance.

If one of the three parameters or combined ones deviates slowly from the normalized value, it may indicate normal fouling and scaling which can be removed by proper cleaning.

A fast or immediate performance decline indicates a defect or misoperation of the system. It is essential in this case that the proper corrective measure is taken as early as possible because any delay decreases the chance of restoring the system performance and also may create other problems.

Locating High Salt Passage (Low Salt Rejection)

A loss in salt rejection may be uniform throughout the system or it could be limited to the front or the tail end of the system. It could be a general system failure or it could be limited to one or few individual vessels.

The location of the high salt passage can be isolated by following the three steps.

• Check the individual vessel permeate TDS values.
  • Probe the suspected vessel.
  • Individually test each element in the vessel.

The system contains a sample port located in the permeate stream from each vessel. Care must be taken during sampling to avoid mixing the permeate sample with permeate from other vessels. All permeate samples are then tested for their concentration of dissolved solids with a TDS meter. Notice that from one array to the next the next average permeate

TDS usually increases, because the second array is fed with the concentrate from the first array. To determine the salt passage of all vessels from their permeate TDS. The TDS of the feed stream to each array must also be measured.

The salt passage is the ratio of the permeate TDS to the feed TDS.

If one pressure vessel shows a significantly higher permeate TDS than the other vessels of the same array, then this vessel should be probed. Probing involves the insertion of a plastic tube(ex. Teflon tube, 1/4”) into the full length of the permeate tube.

While the RO system operates at normal operation conditions, water is diverted from the permeate stream of the in question. A few minutes should be allowed to rinse out the tubing and allow the RO system to equilibrate.

The TDS of the permeate sample from the tubing can then be measured with a hand-held meter and the data be recorded.

This measurement should reflect the TDS of the permeate being produced by the RO membrane at that location.

High Salt Passage and High Permeate Flow

(1) Membrane Oxidation

A combination of high salt passage (low salt rejection) and a high permeate flow is a typical symptom of the damaged membrane oxidized by oxidizing chemicals including chlorine, bromine, and ozone. Other oxidizing chemicals such as peracetic acid, hydrogen peroxide, and N-chloro compounds are less aggressive but still can damage the membranes when they are present in excessive amounts or coexist with transition metals. In the case of chlorine and bromine, a neutral to alkaline pH favors the attack on the membrane. At the early stage of oxidation, the front-end elements are usually more affected than the rest.

The oxidation damage can be made visible by a dye test on the element or membrane coupons after an autopsy of the element. All damaged elements must be replaced.

(1) Leak

A leak of feed or concentrate to the permeate through mechanical damage of the element or the element or of permeate tubing can cause high salt passage and high permeate flow. The contribution of the leak to the permeate flow may depend on the magnitude of the damage usually caused by high pressure and vibration.

The types of damages include leaking O-rings, cracked tubes, telescoping, punctured

membranes, and centerfold cracking.

High Salt Passage and Normal Permeate Flow

(1) Leaking O-ring

Leaking O-rings can be detected by the probing technique(See Section 2-5.5.1). Inspect O- rings of couplers, adaptors, and end plugs for correct installation and aging condition. Replace old and damaged O-rings.

O-rings may leak after exposure to certain chemicals, or to mechanical stress, e.g. element movement caused by a water hammer. Sometimes, they have been improperly installed or moved out of their proper location during element loading.

(2)Telescoping

Telescoping is caused by excessive pressure drop from feed to concentrate. Telescoping damage can be identified by propping. The operating conditions leading to excessive pressure drop are detailed in the section of High Differential Pressure. For example, when the pressure pump is started before a drained system has time to fill, the front-end elements will be exposed to higher-than-normal water velocities. This can hammer the elements to telescoping which can be prevented by opening the throttling valve slowly.

(3) Membrane Surface Abrasion

The front-end elements are typically most affected by crystalline or sharp-edged metallic suspended solids in the feed water. Check the incoming water for such particles. Microscopic inspection of the membrane surface will also reveal the damage. No corrective action is possible. The pretreatment must be changed to cope with this problem. Ensure that no particles are released from the high-pressure pump.

(4) Permeate Backpressure

When the permeate pressure exceeds the feed/concentrate pressure by more than 0.3 bar at any time, the membrane may tear. The damage can be identified by probing. Upon autopsy of the damaged element, the outer membrane typically shows creases parallel to the permeate tube, usually close to the outer glue line. the rupture of the membrane occurs mostly in the edges between the feed-sided glue line, the outer glue line, and the concentrate-sided glue line.

(5) Centerfold Cracking

The regular process making a spiral wound element requires folding a leaf of membrane sheet in the center (centerfold).

The creased (folded) membrane can break at the centerfold under certain conditions. Then the salt passage increases with or without an increase in the permeate flow. Centerfold cracking may be caused by ;

☞ Hydraulic shock during start-up, e.g. by air in the system ;

☞ Too fast pressure increases;

☞ Increased shear stress ;

☞ Abrasion by scaling and fouling ;

☞ Permeate backpressure.

Centerfold cracking typically occurs only after one year or more of improper operation, and only at systems with a high start and stop frequency.

High Salt Passage and Low Permeate Flow

High salt passage combined with low permeate flow is the most commonly occurring system failure, usually induced by colloidal fouling, metal oxide fouling, and scaling.

Colloidal fouling

Colloidal fouling occurs predominantly in the first array. The problem can be more easily located when permeate flow meters have been installed in each array separately. SDI should be checked more frequently to identify the pretreatment upset.

Inspect SDI filters and cartridge filters for deposits. Clean the elements according to the cleaning procedure, and correct the pretreatment process accordingly.

Metal Oxide Fouling

Metal oxide fouling also occurs predominantly in the first array. Check feed water for levels of iron and aluminum.

Check the materials may filter for deposits. Clean the membranes with an acidic cleaning solution. Correct the pretreatment and/or material selection.

Scaling

Scaling will involve deposits starting on the last array, and then gradually moving to the upstream arrays. Analyze the concentrate for levels of calcium, barium, strontium, sulfate, fluoride, silicate, pH, and LSI. Try to calculate the mass balance for those salts, analyzing also feed water and permeate. Scaling occurs slowly because of the low concentrations involved except CaCO₃.

The crystalline structure of the deposits can be observed under the microscope. The type of scaling is identified by chemical analysis or X-ray analysis. Cleaning with acid and/or an alkaline EDTA solution with subsequent analysis of the spent solution may also help to identify the type of sealant. In the case of carbonate scaling, adjust the pH of the pretreatment. For the other salts, either use an appropriate scale inhibitor or other suitable pretreatment techniques or lower the recovery. Make sure that the design recovery is not exceeded.

Low Permeate Flow and Normal Passage

Biofouling

Biofouling of the membrane occurs predominantly at the front of the system and affects permeate flow, feed flow, feed pressure, differential, and salt passage in the way as shown below ;

☞ Permeate flow decreases when operated at constant feed pressure and recovery.

☞ Feed flow decreases when operated at constant feed pressure and recovery.

☞ Feed pressure has increased if the permeate flow is maintained at constant recovery. Increasing the feed pressure will invoke a worse situation, since it increases the fouling, making it more difficult to clean later.

☞ Differential pressure increases sharply when the bacterial fouling is massive or when it is combined with silt fouling.

☞ Since pressure drop across the pressure vessels is a sensitive indicator of foiling, installing pressure monitoring devices is strongly recommended for each array.

☞ Salt passage is normal at the beginning, but may increase when fouling becomes massive.

☞ High counts of microorganisms in water samples from the feed, concentrate, or permeate stream indicate the beginning or the presence of biofouling.

☞ Corrective measures require disinfection of the whole system including pretreatment, and optimization of the pretreatment system to cope with the microorganism growth in the raw water.

☞ An incomplete cleaning and disinfection will result in rapid regrowth of the microorganisms.

1. Aged Preservation solution

Elements of RO systems preserved in a bisulfite solution can also become biologically fouled if the preservation solution is too old, too warm, or oxidized by oxygen. An alkaline cleaning usually helps to restore the permeate flow.

Incomplete Wetting

Elements that have been allowed to dry out, usually give a very low permeate flow with a normal salt passage.

The lost permeate flow may be recovered by soaking the elements in a 50: 50 mixture of alcohol and water for one or two hours followed by soaking in water.

Low Permeate Flow and Low Salt Passage

Compaction

Membrane compaction usually results in low permeate flow and low permeate salt passage. Typical RO membrane does not undergo compaction at normal operation, but significant compaction may occur at high feed pressure, high water temperature (> 45℃), and water hammer.

The water hammer can occur when the high-pressure pump starts with air in the system and the full opening of the throttle valve.

The compaction can induce intrusions of the membranes into the permeate channel spacer fabric, which is visible.

Thus, the permeate flow is not only restricted by the compaction of the polyamide or polysulfone layer but also by the reduced cross-section of the permeate spacer that is available for permeate flow.

Organic Fouling

Organic matter in the feed water can deposit in the membrane surface to cause flux loss usually in the first array.

The deposited organic layer could act as an additional barrier for dissolved solutes, or plug pinholes of the membrane, to increase salt rejection.

Organics with hydrophobic characters or cationic groups can produce such an effect. Examples are hydrocarbons, cationic polyelectrolytes, cationic surfactants, nonionic surfactants, and humic acids. Analyze the incoming water for oil organic matter, and check the SDI filter and the cartridge filter for organic deposits. Conduct SDI and TOC measurements on a more frequent basis. Improve the pretreatment accordingly.

An oil fouling can be removed with an alkaline cleaning agent, for example, NaOH (pH 12).

Cationic polyelectrolytes may be cleaned off at an acid pH, if it is not a precipitation product with other compounds, e.g., antscalants.

Cleaning with alcohol has also proven effective in removing adsorbed organic films.

High Differential Pressure

High differential pressure, also called pressure drop from feed to concentrate, generates a high force pushing the feed side of the element in the flow direction. This force impacts the permeate tubes and the fiberglass shells of the elements in the same vessel. The stress on the last element in the vessel is the highest since it has to bear the sum of the forces from the pressure drops of all prior elements.

The upper limit of the differential pressure per multi-element vessel is 4.1(Four point One) bar, for single element 1.4(One point Four) bar. When these limits are exceeded, even for a very short time, the elements might be mechanically damaged to result in telescoping and/or breaking the fiberglass shell. This type of damage may not disturb the membrane performance temporarily, but eventually cause flux loss or high salt passage.

An increase in differential pressure at a constant flow rate usually arises from the accumulation of debris, foulants, and scale within the element flow channels (feed spacer). It usually decreases the permeate flow. An exceeding the recommended feed flow, and building up the feed pressure too fast during start-up (water hammer).

A water hammer, a hydraulic shock to the membrane element, can also happen when the system is started up before all air has been flushed out. This could be the case at initial start-up or restart-ups after the system has been allowed to drain. Ensure that the pressure vessels are not under vacuum when the plant is shut down. In starting up a partially empty RO system, the pump may behave as if it had little or no backpressure. It will suck water at great velocities, thus hammering the elements. Also, the high-pressure pump can be damaged by cavitations.

The feed-to-concentrate differential pressure is a measure of the resistance to the hydraulic flow of water through the system. It is very dependent on the flow rates through the element flow channels and on the water temperature.

It is therefore suggested that the permeate and concentrate flow rates be maintained as constant as possible to notice and monitor any element plugging that is causing an increase in differential pressure.

The knowledge of the extent and the location of the differential pressure increase provides a valuable tool to identify the cause of a problem. Therefore it is useful to monitor the differential pressure across each array as well as the overall feed-to-concentrate differential pressure.

Some of the common causes and prevention of high differential pressure are discussed below.

Failure of Cartridge Filters

When cartridge filters are loosely installed in the housing or connected without using interconnectors, they can shed debris and particles to block the flow channels in the front-end elements. Sometimes cartridge filters deteriorate while in operation due to hydraulic shock or the presence of incompatible materials.

Media filter Breakthrough

Fines from the A/Carbon filter may set loose and enter into the RO feed water. Sometimes some of the coagulated colloids can pass through the channeling of the filters. The channeling could occur in very old filters. Cartridge filters should catch most of the larger particles. Smaller particles and pass through a 5(five) micro nominally rated cartridge filter to plug the lead elements.

Pump Impeller Deterioration

Most of the multistage centrifugal pumps employ at least one plastic impeller. When a pump problem such as misalignment of the pump shaft develops, the impellers have been known to deteriorate and throw off small plastic shaving. The shavings can enter and physically plug the lead-end RO elements.

Scaling

Scaling can cause the tail-end differential pressure to increase. Make sure that scale control is properly taken into account, and clean the membranes with the appropriate chemicals. Ensure that the designed system recovery will not be exceeded.

Brine Seal Damage

Brine seals can be damaged or turned over during installation or due to hydraulic impacts. A certain amount of feed water will flow through the chasm in the damaged seals to bypass the element, resulting in less flow and velocity through the element. This will cause to exceed the limit for maximum element recovery to increase the potential for fouling and scaling.

As a fouled element in the multi-element pressure vessels becomes more plugged, there is a greater chance for the downstream elements to become fouled due to insufficient concentration

flow rate within that vessel.

The brine seal damage causing an increase in differential pressure could happen randomly in any pressure vessel.

Early detection of the increase in differential pressure is important for an easy correction of system malfunctions.

Biological Fouling

Biological fouling is typically associated with a marked increase in the differential pressure at the lead end of the RO system.

Biofilms are gelatinous and quite thick, thus creating a high flow resistance.

Precipitated Antiscalants

When polymetic organic antiscalant come into contact with multivalent cations like aluminum, or with residual cationic polymeric flocculants they can heavily foul the lead elements.

Repeated applications of an alkaline EDTA solution may clean the fouled elements with some difficulties.