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Vacuum Sewer System Problems

Although presenting advantages over typical gravity-flow sewer systems in particular applications, vacuum sewers nonetheless have unique attributes that can cause significant problems if not timely discovered. Chief among the concerns is the need for absolute and complete integrity of the wastewater lines to maintain the vacuum and avoid exfiltration of wastewater. Because vacuum sewer lines are shallowly installed, often in the same trench with fresh water mains or storm water lines and in water protection areas, the potential adverse consequences of exfiltration is far more devastating than the consequences of leakage in traditional gravity-flow lines.

Loss of vacuum typically results from excessive sewage surge flow, causing a sewer main to become waterlogged and lose vacuum pressure even when its vacuum station is operating properly. At times of vacuum loss, only the airtight and watertight integrity of the waterlogged sewer mains prevent sewage from leaking into the groundwater. Although vacuum sewer system components are sized according to peak factor tables based on Ten State Standard Formulas or other regulations, the formulas are still just design estimates and do not account for all situations of sewage surge flow, such as those created by special community events or storm water infiltration.

A symptom of an overloaded sewer main is a low vacuum level or loss of vacuum at the valve pits. When vacuum levels decrease at the pits, there is a corresponding decrease in the Air-to-Liquid Ratio ("A/L") of sewage entering the sewer mains. This is due to the lower vacuum levels causing slower sump evacuation times, lowering the A/L. Lowering the A/L leads to more liquid than air in the sewer mains and a waterlogging of the sewer main. This process is continues - causing the vacuum to drop more and decreasing the A/L even more. Most water and wastewater systems can recover from a surge flow situation over time, but a vacuum sewer system may not recover on its own.

In essence, it is impossible to know if a lift section underground is waterlogged without the aid of monitoring equipment and designing a new system or upgrading an existing system to have a massive amount of extra capacity to handle irregular high surge flows is very inefficient and expensive. Because the adverse consequences from such an event are greater, the need for an efficient and reliable solution is greater.