On account of their typical mechanical resistance ceramic membranes can be subjected, in particular conditions, to pressure pulses occurring in a counter-current condition with respect to the permeate flow, so as to prolong the cycles of operation between two chemical cleanings.
In normal conditions of cross-flow filtration, a dynamic film forms on the surface of the membrane composed of the same constituents of the solution treated and it is not appropriate for this to be disturbed, altering the fluid-dynamic conditions. Consequently, it is necessary to proceed feeding the solution inside the channels at a constant flow rate, possibly compensating for the decrease in permeability with a sole increase in transmembrane pressure (TMP). On reaching excessively high values of TMP, the decision will be taken to proceed with a chemical cleaning of the membrane under equi-current conditions.
However, when dealing with solutions with a high load of contaminants or with particles of small dimensions (e.g. colloids) which tend to migrate inside the pores, it may be appropriate to subject the membrane to a periodical pressure pulse under the counter-current condition. This is achieved by means of pulses of pressurized air sent from the permeate side, after having closed the outlet valve of the permeate itself; they are transmitted to the layer of contaminant accumulated on the wall of the ceramic membrane and detach it, so that it can be evacuated together with the concentrate. It is thus possible to slow down the decrease in permeability that would occur naturally and prolong the operational cycles.
Sepra has developed a fouling control system, applicable directly at the permeate output in its housings for membrane elements, making an indirect use of compressed air in a chamber that is external with respect to the permeate, so as to avoid wasting liquids and at the time same to avoid introducing gas on the membranes. A solenoid valve is applied at the chamber output which intercepts the permeate during the counter-current pulse.
The system is equipped with a control unit allowing the user to set the frequency and duration of the pressure pulses independently, allowing for adaptation with respect to all applications.