An apparatus includes a foreline for conveying a waste stream which is drawn from a process chamber to a pumping mechanism. The foreline comprises an isolation valve for selectively isolating the pumping mechanism from the process chamber and a bypass positioned around the isolation valve. The apparatus further comprises a controller for actuating the isolation valve. The controller is configured to cause the isolation valve to be closed when the waste stream is initially drawn from the process chamber, during which time the waste stream is conveyed to the pumping mechanism via the bypass. The bypass includes means for inhibiting propagation of a flame front therethrough. The controller is configured to cause the isolation valve to be opened once a pressure within a region upstream of the isolation valve has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.