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Particles, scaling and biofouling

Membrane technology

Particles

Particle membrane pollution can be defined as suspended and colloidal matter, which absorbs to the membrane. Solids will plug the membrane, causing that the water that needs treatment is no longer able to pass through. When the membrane is plugged, more pressure is necessary to be able to perform the usual water treatment. This causes energy costs to rise.

Scaling

Scaling means the deposition of particles on a membrane, causing it to plug. It is an unwanted effect that can occur during nano filtration and Reverse Osmosis processes. Scaling causes a higher energy use and a shorter life span of the membranes, because these will need cleaning more often.

Nano filtration and Reverse Osmosis are processes that are often applied for drinking water preparation from groundwater or surface water. During these processes a high conversion is desired, because this will limit the loss of raw materials and energy. Dependent on conversion, about 75 to 90 percent of the feed water will be converted to the desired product.
During the process, the membrane concentrate absorbs salts. Inorganic salts, such as calcium carbonate and barium sulphate, which are water-insoluble, can become over-saturated. This causes them to precipitate. The precipitation of water-insoluble salts on the membrane is more likely to occur when conversion is high.

Scaling causes the nominal flux to decrease. The consequences are, as has been noted before, a higher energy use, an increase of the cleaning frequency and a shorter life span of the membranes. This will cause the membrane water treatment process to become much more expensive.
Adding acids or anti-scalents to the system can prevent the precipitation of salts. Acids decrease the over-saturation of calcium carbonate, while anti-scalents decrease precipitation levels.
A membrane filtration unit performs optimally at maximum conversion and a minimal dose of acids and anti-scalents, without the occurrence of scaling.

Biofouling

Biological contamination, known as biofouling, occurs most often during nano filtration and Reverse Osmosis processes. This is because the membranes cannot be disinfected with chlorine, in order to kill bacteria. Biofouling 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 microrganisms have damaging, often irreversible effects on nano filtration and Reverse Osmosis systems.
The types of microrganisms, 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.
Microrganisms 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 biofouling 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 an important 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 food source for bacteria, because they release organic nutrients that bacteria need for growth in a membrane system.

Another type of biofouling 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 microrganisms. 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, microrganisms 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 membrane. 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 microrganisms to grow, which will cause the membrane to be completely destroyed in a short period of time.

Membrane technology

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