CES only uses RO for removing salt or sugar from water. Our standard systems don't employ RO and are not blinded by oil, soap, grease, bacteria scum, algae or metal hydroxides. Any of these will foul a RO system. 

An elementary prerequisite for users of the RO process in industrial water-treatment applications is to understand the RO membrane’s tendency to become fouled by the contaminants it is removing from the feed water.

By Stan Lueck

A prior article entitled “RO-system architecture” covered the basic configuration of reverse-osmosis membrane devices used in industrial water-treatment applications. By way of review, a brief description of spiral-wound membrane water treatment elements follows.

Fouling Happens

The elements of an RO system are placed end-to-end in a cylindrical pressure vessel (PV) (See Figure 1.) The permeate tubes of each element are connected to form a channel, which allows the permeate from the collective elements to exit one end of the PV.

RO Membrane System

Feed water enters one end of the vessel and concentrate exits from the other. A typical industrial element is of 8 in diameter by 40 in. long. PVs may hold one to eight elements, with six being average, and are arranged as shown in the figure. Entire membrane systems may contain from one to hundreds of PVs.

Fouling Promoter: Configuration of Membrane Units

Feed spacers mounted inside the element can cause problems. They limit the velocity of the feed water flowing through the element and reduce the cleaning ability of the cross flow. These spacers also provide locations where colloidal particles can be retained and bacteria can grow, increasing the tendency of a typical spiral-wound element to foul.

As the feed channels become fouled, the feed water flow through the element becomes uneven. The feed water seeks the path of least resistance, resulting in lower cross-flow velocities downstream of the fouling. This can produce even more fouling, especially if microbiological growth is present. At some point the system shut down and cleaning become necessary.

Three Kinds of Fouling that Reduce Membrane Performance

A membrane treatment system can be fouled by virtually anything present in the water being fed to the unit. However, in common treatment systems such as reverse osmosis, the fouling materials generally may be categorized as inorganic, organic, and biological.

Inorganic compounds that cause fouling of membrane modules include inorganic salts with low solubility. They can enter the treatment system in particle form, or they may precipitate inside the system as a result of concentration changes occurring in the feed water as permeate is recovered through the membrane. The highest concentration of dissolved solids occurs immediately adjacent to the surface of the membrane in the treatment module (Figure 1).

If the feed water contains salts of low solubility, it is likely that these salts will precipitate on the surface of the membrane to form scale. Salts such as calcium carbonate (CaCO3) and calcium sulfate (CaSO4) are common in most feed waters. Other salts such as barium sulfate (BaSO4), strontium sulfate (SrSO4), and calcium fluoride (CaF2) also may be in solution.

In many feed-water sources these salts are present at or near their solubility limits and will precipitate as the concentration of the feed water in the system increases. Although this precipitation can be controlled with proper pretreatment, fouling from these salts frequently occurs because of operator error or unknown changes in feed-water quality.

Metal hydroxides are other inorganic compounds that cause fouling. The most common culprits are iron hydroxide, [Fe(OH3)] and aluminum hydroxide, [Al(OH3)]. As in the case of inorganic salts, these hydroxides may enter the system as suspended particles or they may form inside the system. Unlike the inorganic salts however, metal hydroxides do not deposit a hard crystalline scale; rather it is a soft, gelatinous layer.

Clay, silt and other silica-based materials can cause fouling if the particles are not removed by upstream pretreatment equipment. In some feed-water sources, clay occurs as very finely divided (1 to 5 microns) particles.

These small colloidal particles can be very difficult to remove with conventional equipment. Silica also may enter the membrane system in the dissolved or reactive form. This low molecular form of silica will polymerize as the feed-water concentration increases at the surface of the membrane. The resulting solid silica deposit on the membrane can be extremely difficult, if not impossible, to remove.

Organic compounds make up the second category of fouling materials. Surface-water sources like rivers and lakes may contain such naturally occurring organics as humic acids. Clarified water may contain residual polymers, and wastewater influents may contain any number of organic compounds.

The mechanism of organic fouling depends upon the size and chemical nature of the specific substance causing the fouling. High molecular-weight compounds may act more as particles and mechanically plug the feed spacer in the membrane element. This plugging may be worsened if inorganic particles, such as clays and metal hydroxides, also are present.

Low molecular-weight organics may foul the surface of the membrane through chemical interaction. For example, chlorinated phenols will adhere to the surface of an RO membrane by means of hydrogen bonding. In such a situation, a small concentration of the chlorinated phenol in the feed water can cause a large loss of flux in the treatment system.

Biological organisms tend to foul membrane surfaces. Although they are technically organic, biological organisms demand special consideration. Concern is primarily because of single-cell organisms, including bacteria, algae, and fungi. Of them, bacteria cause the majority of problems in membrane water-treatment systems—for a variety of reasons.

First, many types of bacteria can adapt to the environment inside the membrane modules. Unfortunately, a great number of these species are found in typical feed waters, particularly water from a surface source, such as a river or lake.

Second, since the membrane rejects the bacteria, they end up on its surface. Although their presence of itself is of appreciable concern, their food, consisting of organic matter, also is being concentrated at the membrane surface. And the reality is that when bacteria are in a livable environment with sufficient food, they multiply rapidly. Thus even more bacteria end up on the membrane surface.

Finally, bacteria have a number of defense mechanisms that add to their fouling ability. Several have small hair-like appendages, called fimbriae, that protrude from all sides of the cell. These allow the bacteria to attach themselves, and remain attached, to the surface of the membrane or to the feed spacers. In addition, bacteria secrete a mucous capsule, or slime, which coats the cell and protects them from any harsh elements entering their environment.

Contact: RODI Systems at 936 Highway 550, Aztec, NM 87410; Tel. 505-334-5865; Fax. 505-334-5867.

About the Author: Stan Lueck is president of RODI Systems Corporation.