KWRU utilizes Airvac’s standard municipal vacuum system featuring a vacuum station with four 25 hp Busch Model 630 rotary vane vacuum pumps and two 25 hp Yeomans Model 6153 submersible centrifugal sewage pumps. Number of equivalent residential connections (ERCs): 1,309.
An equivalent section of the gravity sewer system was selected consisting of nine lift stations—six small and three large—serving a total of 928 ERCs. (See Figure 1 for lift station layout.) Three convey wastewater via force mains that discharge at the treatment plant’s headworks. The vacuum pump station is at the plant.
Both systems’ service areas include typical south Florida flat topography.
KWRU’s actual January 2010 through May 2012 runtime data for each vacuum pump and each sewage pump in the vacuum pump station was used for the analysis. To complete a total three-year period of record, data was extrapolated for the second half of 2012. Sewage accounted for about 50% of the station’s annual wastewater flow into the treatment plant.
I repeated the process for each of the nine gravity lift stations.
Pump voltage and amperage information were used to calculate a total kilowatt usage for all 15 stations. KWRU’s electric utility is Keys Energy Services, which imports nearly all of its power supply and charges about $0.14 per kilowatt-hour. Each station’s total kilowatt usage was multiplied by this figure to calculate annual electricity costs (see Figure 2).
Here is what the figures revealed: Gravity lift stations with fewer than 100 ERCs spend less than $1,000/year on electricity; medium-size stations (LS-2A and ForceMain) spend the most at $6,000 to $8,000/year. Little variability in energy consumption from year to year suggests the system doesn’t have a major I/I problem.
Figure 3 quantifies the vacuum pump station’s energy consumption. Average annual sewage pump cost: $2,000; vacuum pump: $10,000 to $14,000. The station’s electricity bill is $12,000 to $16,000 per year.
Figure 4 shows how the two systems compare for the same number of connections: $11/year/ERC (vacuum) and $21/year/ERC (gravity).
The bottom line
While this study compared a gravity system of nine lift stations with one vacuum station, only three of the gravity stations are large. In fact, the opposite configuration—six or seven large lift stations and two or three small stations—is more typical. That means gravity-system costs are probably higher than those reported here.
Also, the vacuum station is located at the treatment plant. Lower-than-normal sewage pump energy consumption is the result of the shorter-than-normal distance sewage is being pumped.
For this study, these two factors were thought to be somewhat offsetting. Overall, however, increasing the number of large gravity lift stations will probably consume more energy, and thus cost more both initially and over the long term, than placing a vacuum station farther from the treatment plant.
Daniel G. Burden is a senior associate in the Palm Bay, Fla., office of engineering firm Wade Trim. E-mail email@example.com; visit www.wadetrim.com.