A pressure alarm for sewage treatment systems such as waste-water or sewage treatment tanks which require water level control. In a preferred embodiment the pressure alarm includes a pressure sensing apparatus which includes a tee or elbow located in a tank or system aerator air line, a nipple set into the tee leg of the tee or the elbow and a ball float fitted with a shaft or neck extending through the nipple and receiving an air deflector. According to the method of this invention, the air deflector projects into the run of the tee or the elbow responsive to floating of the ball float upwardly in a high water condition in the treatment tank or system. Extension of the air deflector into the run of the tee or the elbow causes a pressure drop through the tee or elbow as air is deflected from the run of the tee through the tee leg or elbow. A control system including a pressure-sensitive switch, is provided, typically on a conventional compressor or blower for sensing this pressure drop and sounding an audible and/or a visual alarm.