RO Part III: Mitigating scale formation and membrane fouling

This is the third part of a five-part series on the design, operation, and maintenance of reverse osmosis systems for powerplants compiled by Wes Byrne, U.S. Water’s consultant on membrane technologies. Parts I, II, IV, and V are identified below; the final two segments will appear in upcoming issues of CCJ ONsite.

There is typically at least one salt in any natural water source that will concentrate beyond its solubility and potentially form scale. Preventing scale formation should not be a major challenge unless the water source has an unusually high concentration of a slightly soluble salt, or unless the RO is being operated with an unusually high-percent permeate recovery.  

Scale formation may be prevented by injecting an acid into the inlet water, by softening the water, or by injecting a chemical scale inhibitor. Usually the least expensive method is a scale inhibitor, which slows the rate at which the salt crystals grow when their solubility is exceeded.  

Acid injection prevents calcium carbonate scale formation, but leads to an extremely high concentration of carbon dioxide, which is not removed by the RO system and will place a high removal demand on downstream ion exchange processes. Also, acid injection alone will not offer much protection against the formation of sulfate or certain other scales.  

Softening offers several advantages, but suffers from high capital and operating costs, unless there are particularly low concentrations of calcium and magnesium hardness in the water. The softener also will remove other potential scale-forming ions, like strontium and barium, and will remove metals that would otherwise foul the RO system (for example, iron, manganese, and aluminum). But the softening resin will also foul with the metals and then require periodic chemical treatment.  

Scale-inhibition chemical suppliers often will use software programs to estimate the potential for scale formation. These programs will predict the concentrations of salts that will be present in the RO concentrate stream, as well as its pH, to determine how much scale-inhibition chemical will be needed.  

The potential for silica scale is common with certain ground-water sources in the West. Inhibition formulations have shown varied success. Maintaining warmer water temperature will improve silica solubility, as will changing the water pH. Increasing pH is a common strategy, although the water must first be softened to prevent hardness scale from forming when a caustic chemical is injected to raise the pH.

When using a scale inhibitor, it is critical to rinse the RO system of its increased concentration of dissolved salts whenever the RO shuts down. Otherwise, scale particles will grow and stick to the membrane surfaces during the shutdown. This rinsing process should be automated and is often performed with low-pressure inlet water. Low pressure reduces the RO permeation that tends to concentrate the dissolved salts.

A better rinse might be performed with pressurized permeate water if a line can be plumbed back to the RO from a permeate storage tank system. The permeate is biostatic. Its use will reduce the formation of biological solids within the RO while shut down.

Fouling will not necessarily reduce RO membrane life if the RO is effectively cleaned. If the RO is allowed to foul too severely and cleaning is not effective, then the membrane will likely continue to lose performance.       

It is common to include a filter housing on the RO inlet that contains 2.5-in.-diam cartridge filters whose pore size is nominally rated. The actual ability for removing smaller particles can vary greatly. Some (regardless of rating) will only protect the RO against large particles that might get caught within the membrane flow channels or damage the high-pressure pump. These are inexpensive and may last weeks before an elevated pressure drop indicates the need for replacement.

Tighter porosity filters that might remove more of the incoming suspended solids are more expensive and will also require more frequent replacement. Therefore, the use of these tighter filters becomes more economically viable if the concentration of suspended solids in the water has been minimized by upstream treatment.

Suspended solids often can be effectively reduced to reasonable concentrations for the downstream RO system with just a multimedia filter. Its inclusion in the RO water system might be sufficient to prevent a high RO fouling rate that could result in unmanageable cleaning requirements. Multimedia filters contain granules of two or more different types or sizes of sand, crushed rock, or anthracite (hard coal). Such a filter can be successful at removing most of the particles that make up the suspended solids if:

      • It is sized for a downward flow velocity approaching 2 ft/sec.

      • It has a lower collection lateral system designed to obtain uniform flow distribution across the media when the filter is operated at low flow velocity, while also allowing the entry of a sufficient backwash flow rate for a 40% bed expansion.

      • The filter is backwashed before its previously removed smaller/fine particles are shed, which may occur before there is an appreciable buildup in filter pressure drop.

      • After backwashing, the filter is forward-rinsed at its service flow rate until its effluent quality is acceptable (such as based on effluent iron concentration, turbidity, or SDI).  

The preceding points do not provide all of the filter design requirements, but were chosen because these particular guidelines often are not followed (mostly because they would increase the filter’s cost).  

Some water sources may contain unusually high concentrations of fine particles. In these cases, it may be necessary to send the water through large reaction tanks intended to give the particles more time to coagulate into larger particles that can then be more easily filtered.  

An inorganic chemical coagulant (never a cationic polymer) may be added to the water upstream of the tank to speed the coagulation process. The coagulant will be most effective if it is first well-mixed with the suspended solids. If soluble metals (like iron or manganese) are present in the water source, some percentage will be oxidized by allowing the water to contact atmospheric air in the tank, although this percentage will be small.  

A chemical oxidizer like chlorine (bleach) can be added to the water to oxidize the metals into their insoluble oxides (actually into their hydroxides when present in water) prior to coagulation.

Membrane filtration is becoming more common in various applications including pretreatment for RO systems. Membrane filtration often can provide water that is more consistently low in its concentration of suspended solids than that provided by a pressurized multimedia filter. Therefore, these systems may be used as an alternative to multimedia filtration, or possibly downstream to further polish the water and minimize RO fouling.  

The most common configuration is hollow-fiber technology. Fibers of an inert polymer are extruded with a hollow internal region called the lumen. The fibers may be relatively fine/small in diameter where the inlet water passes through the outside of the fibers, and the fiber lumen. It then moves toward one end of the module for collection.  

Because the fibers are tightly packed, flow movement around them is not uniform. Feedwater particles will come out of suspension on the membrane surface as the water goes through the fiber. They are not concentrated within a passing stream as the particles mostly would be with spiral-wound reverse osmosis, so there is no concentrate stream. The systems are simply operated at 100% recovery, except for the water losses from frequent backwashing with filtered water, resulting in an overall recovery of 90% to 95%.   

There also are modules with larger fibers that use an inside-out service flow direction. The fatter fibers offer improved membrane surface flow characteristics for better distribution of the fouling solids, while the finer fibers offer the cost advantages of more membrane surface area in the modules.  

The membrane filtration systems should be sized to keep the fiber pressure differential transmembrane pressure (TMP) relatively low to prevent compaction of solids against the fiber and into the fiber pore structure, and to reduce the potential for fiber breakage. This may mean sizing the fiber for a filtrate flux rate of 30 gal/ft²/day or less.  

The fiber modules are backwashed using the filtrate water at a frequency of roughly once every 30 minutes, again to try to keep the solids from compacting and to prevent particles from getting forced into the pores and subsurface structure. Some manufacturers will reduce the backwash volume by knocking the solids free with compressed air.  

Backwashing alone may not fully restore the original TMP. A chemically enhanced backwashing may then be required. If this fails to restore original performance, a circulated cleaning for an extended period of time may be needed.

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