output terminal t 3 of the processing circuit 10 e g steps 25 26 28 30 or 25 26 29 22 24 32 33 30 on the other hand in a case where the cumulative operating time t 1 of the vacuum pump 2 is determined as t 2 gt t 1 i e where the gas liquid ratio is relatively small the vacuum pump is started by the and circuit 19 when the liquid level of the waste water in the water collecting tank 1 has fallen to lwl output from the lwl sensor 6 e g steps 27 35 37 and the vacuum pump 2 is to be stopped by the or circuit 13 just after the differential time period t 2 t 1 has elapsed by the timer 23 e g steps 39 40 30 in this case it should be noted that once the liquid level of waste water in the tank 1 rises to hwl the booster pump 3 is driven by the signal from the and circuit 7 thereby causing the liquid level to fall to the lwl also once the vacuum pump 2 is started by the and circuit 19 the operation of the vacuum pump 2 is continued for the differential time period t 2 t 1 to increase the gas.