Reverse Osmosis Membrane

Fouling of reverse osmosis membranes is defined operationally herein as the reduction in water transport per unit area of RO membrane (flux), caused by a substance or substances in the feed water that accumulate either on or in the membrane. While there are several common causes of RO membrane fouling (5), this article will focus primarily on biological and colloidal fouling, which are less well documented than other foulants such as inorganic scales.



Premature failure of reverse osmosis (RO) membrane elements due to identified or unidentified membrane fouling substances costs thousands to millions of dollars each year. Membrane fouling is becoming widely accepted as the single largest cause, if not the only cause, of permeate flux decline at normal operating pressures and temperatures in brackish water systems.



While RO fouling may not actually be the "final" frontier as the title asks, it is clearly a frontier, an incompletely known territory with many uncharted areas.



Typed of Membrane Fouling



1) Scale formation

2) Biological slime formation

3) Suspended solids

4) Colloids

5) Metal oxides

6) Oil and grease deposition



Biological contamination, known as bio-fouling, occurs most often during and Reverse Osmosis and nano filtration processes. This is because the membranes cannot be disinfected with chlorine, in order to kill bacteria. Bio-fouling in nano filtration or Reverse Osmosis membranes is probably the least comprehended contamination that can occur in membrane systems. This can be ascribed to the complex growth of microbiological bacteria. These microorganisms have damaging, often irreversible effects on Nano Filtration and Reverse Osmosis systems.

The types of microorganisms, their growth factors and concentration in a membrane system greatly depend on critical factors, such as temperature, the presence of sunlight, pH, dissolved oxygen concentrations and the presence of organic and inorganic nutrients.

Microorganisms can enter the system through water or air, or both.

Aerobic (oxygen-dependent) bacteria usually live in an environment of warm, shallow and sunlit water, with a high dissolved oxygen content, a pH of 6.5 to 8.5 and an abundance of organic and inorganic nutrients.

Anaerobic bacteria (oxygen-independent), on the other hand, are usually present in closed systems with little to no dissolved oxygen and become active when a sufficient amount of nutrients is present. This can be organic matter or the remains of dead algae.

Both types of bacteria can be present within the same system. There are bacteria that can switch between aerobic and aerobic conditions and vice versa. Their nature depends on the state of the water.

One of the most abundant types of bio-fouling originates during pre-treatment of Reverse Osmosis systems and in parts of membrane systems that can promote the growth of algae. Membrane system parts that are exposed to sunlight or contain still water can cause the growth of algae to expand.



Sunlight plays a major role in the photosynthesis process for the growth of algae. The amount of sunlight determines the amount of oxygen that is produced. Aerobic bacteria, which are oxygen-dependent, need the oxygen produced by algae when the dissolved oxygen content in the feed water does not suffice the amounts they need to perform metabolism.

While the algae die off they become a good food source for bacteria, because they release organic nutrients that bacteria need for growth in a membrane system.



Another type of bio-fouling in a membrane system is the attachment of bacteria to the inner walls of pipelines. Corners and dead-ends are locations in a pipeline that bacteria can absorb to.

After bacteria have absorbed to a wall, the first parts of a bio film are formed. The bio film will increase in size while bacteria keep multiplying and while dead organic matter absorbs to the bio film structures. Despite the fact that bio films influence the water flow, it still attracts small suspended solids and microorganisms. The bio film deposits become a strong, coherent whole that is very hard to remove. Eventually, parts of the bio film will be released and spread through the system components, including the membranes. When they are attached to the membranes, microorganisms start multiplying, using nutrients that are present in the feed water. As a result a bio film will develop on the membranes, which encumbers the feed water flow through the RO membranes. This results in a higher pressure, which causes higher system costs and irreparable damage to the membranes.

It even occurs that some membrane materials are suitable environments for microorganisms to grow, which will cause the RO membranes to be completely destroyed in a short period of time.

For more information about membranes cleaning chemicals, please visit our website: http://www.pure-aqua.com/ro-chemicals.html .



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