device 11 to allow the liquid level to reach the top end space 37 inside the pump device 11 during operation in this case the injection of gas must be at or near the highest point inside the pump device 11 so that all or nearly all the gas pocket 37 can be vented the gas connection point 38 shown in fig7 is an example depending on the sludge quality the venting can be performed periodically at varying frequency to prevent the creation of any permanently dried environment inside the pump device it was also noted in operation of the pump device 11 that the liquid level a in fig7 can vary according to the gas flowrate the higher the gas flowrate the less the gas pocket formation inside the pump device 11 accordingly another method which may be used is to periodically inject a much higher air flow into the pump device 11 during operation to break up dehydrated sludge depending on the design of the device the gas flowrate required for this action is normally around 30 or more higher than the normal operating gas flowrate this is possible in some plant operations by diverting gas from other membrane tanks to a selected tank to temporarily produce a short much higher gas flow to break up dehydrated sludge alternatively a standby blower not shown can be used periodically to supply more gas flow for a short duration the methods described above can be applied individually or in a combined mode to get a long term stable operation and to eliminate any scum sludge accumulation inside the pump device.