Problem: In the winter of 2014-2015, the vacuum sewage collection system of Plum Island froze, forcing the evacuation of 180 residents. In a vacuum system, once one part fails, the vacuum is broken and the whole system fails.

Analysis: AirVac sewage pits are divided into two halves: warm (unfrozen!) sewage accumulates in the lower half and the valve and actuator sit in the cold and dry top half. Somehow, moisture was getting up into the actuator, freezing and jamming the system open.

The sewer system’s Superintendent Jamie Tuccolo helped to identify three possible locations where moisture could be freezing first: 1) The Sensing Line – moisture can get into the controller unit through a pressure sensing line which comes up from the sewage pit; 2) The Makeup Air – the valve opens using the vacuum pressure which removes air from the top of the actuator, to close the spring-loaded piston draws moist air from the sewage pit into the top chamber of the actuator; 3) The Displaced Air  – to open the valve a fixed volume of air is temporarily required to fill the lower chamber of the actuator, it is dispelled when the valve closes again and currently is drawn from the sewage pit.

Solutions: For each possible source of the problem we proposed and prototyped a solution:

1) The Sensing Line – we proposed either to solve 100% of the problem using a flexible vapor barrier, or to solve 90% of the problem using a float with wipers.

2) The Makeup Air – we proposed to stop taking makeup air from the moist valve pit, and to instead run a dedicated line from the air intake – avoiding the moisture source.

3) The Displaced Air – we proposed to stop using the moist valve pit as a capacitor for the lower piston chamber, and to instead replace it with a dedicated capacitor supplemented by a connection to the dedicated dry air line.

Jamie Tuccolo, the system’s superintendent combined our solutions 2&3 and modified them, bringing dry air from a separate dedicated air vent without the capacitor. The system has been installed in four locations in Plum Island as a trial.

The local paper described the project ominously as “It’s Mother Nature vs. MIT engineers”