When choosing between conventional gravity or vacuum sewer collection, most utilities compare total capital costs, location, and constructability. Until recently, they had no way of knowing how each system’s energy use impacts operational costs.
Vacuum sewers are constructed utilizing a sawtooth profile based on a 0.2% slope, requiring 3-to-5-feet-deep trenches, compared to up to 20 feet for gravity sewers. In addition, vacuum sewers are typically installed in the right-of-way, requiring fewer road closures (and less pavement restoration) than gravity-based systems.
The heart of a vacuum system is the vacuum pump station. It’s similar to a gravity lift station, as it has a wet well and two sewage pumps, but it also has two or more vacuum pumps. Thus, when a given area could be served by one gravity lift station or one vacuum station, the vacuum system’s energy requirements are greater. But because of the shallow trench depths, it’s not uncommon for a single vacuum station to replace six or seven major lift stations.
In that case, figuring out which solution will use less energy overall isn’t so obvious.
One utility, both technologies
In 2012, vacuum sewer supplier Airvac of Rochester, Ind. (now Bilfinger Airvac Water Technologies Inc.), retained me to conduct an independent study to answer this question.
I gathered and compared three years of energy consumption data on gravity and vacuum systems serving a similarly sized area, choosing Key West Resort Utilities Corp. (KWRU) in Stock Island, Fla., as the study’s subject. It’s one of the few U.S. utilities that operates both.
KWRU is a private utility that serves a 1-square-mile residential community in the lower Florida Keys east of Key West. Its gravity and sewer systems are about equal in size in terms of number of service connections and have been in operation for 10 or more years. More importantly, the utility maintains detailed operational records for each.
There was one initial concern.
Vacuum sewers are inherently watertight, while gravity sewers are prone to inflow and infiltration (I/I). Because the latter’s energy requirement could include I/I transmission, I feared the analysis would inherently favor vacuum. Over the last decade, though, KWRU and the county implemented a sewer lateral inspection program and routine salinity monitoring to detect and fix leaks. Research ultimately confirmed that, as a result, the gravity system has no major I/I problems.