For many years, gravity sewers were the only choice engineers had when considering a wastewater collection system. This changed 40 years ago when the U.S. Environmental Protection Agency challenged the industry to develop alternative collection systems by providing special funding for such endeavors. One such system is vacuum sewers.
There are construction advantages as well as operation advantages to using vacuum sewers, each contributing to the Triple Bottom Line (people, planet, profit). Vacuum sewers reduce impacts from construction, reduce hazardous situations for operators, reduce pollution and waste from operations, conserve energy, and enhance community well being.
Vacuum sewers use smaller diameter pipes (typically 4”, 6”, 8” and 10”) and shallow burial depths than do conventional gravity sewer systems. The resulting narrow, shallow trenches greatly reduce the excavation, dewatering effort, surface disruption and the danger associated with larger, deeper trenches.Reducing the construction footprint minimizes disruption, helps conserve existing natural areas which provides habitat and protects biodiversity.
The 1-piece rotationally molded PE valve pit package consists of the 3” vacuum valve, an upper chamber and lower chamber, the in-sump breather unit, and the air intake. The valve pit serves as the conduit through which sewage from homes enters the vacuum sewer system, flowing via gravity into the sealed sump that is part of the pit package.
Sewage level sensing is remarkably simple. As the sewage rises, air trapped inside the empty 2” (50 mm) diameter sensor pipe pushes in a diaphragm in the valve’s controller/sensor unit, signaling the valve to open. When 10 gallons (38 liters) of sewage accumulates in the sump, the valve automatically opens. The differential air pressure propels the sewage at velocities of 15-18 ft/sec (4.5 to 5.5 m/s), disintegrating solids while being transported to the vacuum station. The valve stays open for four to six seconds during this cycle.
Valve pit package
The valve pit provides the interface between the vacuum system and the house. The 1-piece valve pit is rotationally molded and made entirely of PE. It consists of two (2) separate chambers; an upper chamber that houses the vacuum valve and a lower chamber that is the sump into which the building sewer is connected. These two chambers are sealed from each other. The 1-piece valve pit includes an integral anti-buoyancy collar and has an H-20 traffic load rating.
Each valve pit can accommodate sewage from up to four homes, although the most common configuration is one valve pit serving two adjacent houses. Houses connected to the vacuum sewers do not need any special plumbing fixtures.
Located in the upper chamber is a vacuum interface valve, which is pneumatically controlled and operated. Vacuum from the sewer line opens the valve and outside air from a breather pipe closes it. The interface valve is a full-port 3” (75 mm) valve and designed for handling 3” (75 mm) solids.
The upper chamber, made of a PE material, houses the vacuum valve and controller and is suitable for H-20 traffic loading..
Lower chamber (Sump)
The sump portion of the valve pit is used to accept the wastes from the house. Elastomer connections are used for the entry of the building sewer. Holes for the building sewers are field cut by the contractor at the position directed by the engineer.
In-Sump breather unit
The controller requires a source of atmospheric air to the actuator chamber permitting spring assisted closing of the vacuum valve. Without this air, the valve will remain in the open position.
Early vacuum systems used an external breather for this source of air. This has been replaced by the in-sump breather, which uses atmospheric air from the sump. In the event of low vacuum conditions where the valve would not open, floats in the in-sump breather protect the controller from unwanted liquid.
Atmospheric air used for transport enters through the 4” (100 mm) screened air-intake on the homeowner’s gravity line. There are no odors at this air-inlet due to the small volumes of sewage and short detention time in the sump.
When 10 gallons of wastewater collects in the sump, the valve opens and differential pressure propels the contents into the vacuum main.
Vacuum sewers use small diameter PVC pipe installed in shallow trenches. Vacuum mains are slightly sloped towards the vacuum station. In flat terrain with no unusual subsurface obstacles present, a vacuum main can typically be extended to a length of 10,000 feet from the vacuum station. Click here for more information regarding line sizes, slopes and line lengths.
Profile changes, called lifts, are used to minimize trench depth. The line profile is designed to ensure that sewage will not block the pipe at low flow periods when the sewage is at rest.
Vacuum mains are typically installed within 1-2 ft of the minimum trench depth required by code.
There is a potential vacuum loss associated with every lift in the saw tooth profile. This limits the length of each vacuum main to about 10,000 ft (3 km) in flat terrain. Elevation changes along the route can extend or reduce this range.
Vacuum mains are slightly sloped (0.20%) towards the vacuum station. Unlike gravity sewers that require a minimum slope for a given pipe diameter to obtain the 2 ft/sec scouring velocity , vacuum sewers have no such requirement as the velocity in a vacuum main is independent of both slope and pipe diameter. The pressure differential results in velocities of 15 to 18 ft/sec.
Line sizes are typically 4”, 6”, 8” and 10“ (100 to 250 mm) SDR 21 gasketed PVC pipe. PE pipe can also be used.
Wastewater travels at 15 to 18 feet per second in the vacuum main to the vacuum station. The vacuum main is laid in a sawtooth fashion to ensure adequate vacuum levels at the end of each line.
At the vacuum station, vacuum pumps cycle on and off as needed to maintain a constant level of vacuum on the entire system. Wastewater enters the collection tank and when the tank fills to a predetermined level, sewage pumps transfer the contents to the treatment plant via a force main.
vacuum station is similar in function to a gravity sewer lift station. Sewage is collected in an enclosed wet-well (collection tank) and sewage pumps then transfer the sewage through a force main to the treatment plant. Vacuum pumps create a vacuum on the collection tank which is then transferred to the entire piping network via the vacuum mains which are connected to the collection tank.
vacuum stations consist of two or more vacuum pumps, two sewage pumps, a collection tank and controls . Units arrive at the job site pre-piped, pre-wired and factory tested to minimize construction costs.
The vacuum station skid is housed in a protective structure designed to fit the characteristics of the neighborhood. A standby generator keeps the vacuum system in operation during extended power outages.
The vacuum pumps maintain the system vacuum in the 16” to 20” mercury vacuum (-0.5 to –0.7 bar) operating range. Vacuum pumps do not run continuously; rather the operation is cyclical. Typical run times are 2 to 3 hours per day per pump (4 to 6 hours total).
As sewage and atmospheric air enter the system during valve cycles, system vacuum will gradually decrease from 20” to 16” Hg. The vacuum pumps are sized to bring the vacuum level back to 20” Hg within 3 minutes time. Typical vacuum pump sizes are 10, 15 and 25 horsepower (7.5, 11 and 18.7 Kw). Rotary vane pumps are typically used. .
Two, non-clog, dry- pit, horizontal, centrifugal sewage pumps each capable of pumping design peak flow, are used. Dry pit submersible pumps can also be used.
The collection tank is steel or fiberglass and is sized according to peak design flow. Typical sizes range from 1000 to 6000 gallons (3.8 to 22.7 m3). The incoming vacuum mains individually connect to the tank, effectively dividing the collection system into zones.
The electrical controls are housed in a NEMA Type 12 enclosure which generally is mounted on the equipment skid. Control panels can use either relay-logic or PLC logic. The panel includes motor starters, control relays, pilot lights, hand-off-auto (HOA) switches and hour-run meters.
To monitor system performance, a 7-day chart recorder is installed in the enclosure. In addition, an automatic telephone alarm dialer is provided to alert the operator of alarm conditions.