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