Membrane cleaning methods

Membrane technology

There are several different membrane cleaning methods, such as forward flush, backward flush and air flush.

· When forward flush is applied, membranes are flushed with feed water or permeate forward. The feed water or permeate flows through the system more rapidly than during the production phase. Because of the more rapid flow and the resulting turbulence, particles that are absorbed to the membrane are released and discharged. The particles that are absorbed to membrane pores are not released. These particles can only be removed through backward flushing.

· Backward flush is a reversed filtration process. Permeate is flushed through the feed water side of the system under pressure, applying twice the flux that is used during filtration. When the flux has not restored itself sufficiently after back flushing, a chemical cleaning process can be applied.

· During a chemical cleaning process, membranes are soaked with a solution of chlorine bleach, hydrochloric acid or hydrogen peroxide. First the solution soaks into the membranes for a number of minutes and after that a forward flush or backward flush is applied, causing the contaminants to be rinsed out.

· A newer cleaning method is the so-called air flush or air/ water flush. This is a forward flush during which air is injected in the supplier pipe. Because air is used (while the water speed remains the same), a much more turbulent cleaning system is created.

Forward flush

When forward flush is applied in a membrane, the barrier that is responsible for dead-end management is opened. At the same time the membrane is temporarily performing cross-flow filtration, without the production of permeate.
The purpose of a forward flush is the removal of a constructed layer of contaminants on the membrane through the creation of turbulence. A high hydraulic pressure gradient is in order during forward flush.

Backward flush

When backward flush is applied the pores of a membrane are flushed inside out. The pressure on the permeate side of the membrane is higher than the pressure within the membranes, causing the pores to be cleaned. A backward flush is executed under a pressure that is a bout 2.5 times greater than the production pressure.
Permeate is always used for a backward flush, because the permeate chamber must always be free of contagion. A consequence of backward flush is a decrease in recovery of the process. Because of this, a backward flush must take up the smallest possible amount of time. However, the flush must be maintained long enough to fully flush the volume of a module at least once.

Air flush or air/ water flush

Fouling on the membrane surface needs to be removed as effectively as possible during backward flush. The so-called air flush, a concept developed by Nuon in cooperation with DHV and X-flow, has proved to be very useful to perform this process. Using air flush means flushing the inside of membranes with an air/ water mixture.
During an air flush air is added to the forward flush, causing air bubbles to form, which cause a higher turbulence. Because of this turbulence the fouling is removed from the membrane surface.
The benefit of the air flush over the forward flush is that it uses a smaller pumping capacity during the cleaning process.

Chemical cleaning

When the above-mentioned cleaning methods are not effective enough to reduce the flux to an acceptable level, it is necessary to clean the membranes chemically.
During chemical cleaning chemicals, such as hydrogen chloride (HCl) and nitric acid (HNO₃), or disinfection agents, such as hydrogen peroxide (H₂O₂) are added to the permeate during backward flush. As soon as the entire module is filled up with permeate, the chemicals need to soak in. After the cleaning chemicals have fully soaked in, the module is flushed and, finally, put back into production.

Cleaning methods are often combined. For example, one can use a backward flush for the removal of pore fouling, followed by a forward flush or air flush.
The cleaning method or strategy that is used is dependent on many factors. In practise, the most suitable methods is determined by trial and error (practise tests).

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