BY ROBERT P. MADER
OF CONTRACTOR'S STAFF
The old plumbing joke is that hot is on the left, cold is on the right, poop flows downhill and don't bite your fingernails. The exception to those rules is that effluent will flow uphill in a vacuum plumbing system.
Nature abhors a vacuum, which is the principle on which vacuum plumbing systems work. In a vacuum plumbing system, wastewater and effluent fill one side at atmospheric pressure. On the other side of a vacuum plumbing system, vacuum pumps keep the waste pipe at about one-half atmospheric pressure.
In between the two sits an "extraction valve." As the waste fills up an "accumulator" in a low spot of the system, it increases the air pressure in that part of the waste pipe. That increase in air pressure triggers the extraction valve — pneumatically, mechanically or electrically. The extraction valve opens, exposing the built-up effluent in the pipe to a partial vacuum and, whoosh, out it goes, uphill in many cases. The waste can go straight up as much as 25 ft. before it gets to a horizontal run.
Vacuum plumbing systems were invented in the 19th century in suburban Paris, where the first system served about 500 buildings. Although the plumbing system in that community functioned satisfactorily, the system fell into oblivion over the course of time.
It wasn't until 1959 that the first practical vacuum waste system was conceived by Swedish engineer Joel Liljendahl, who perfected the vacuum toilet in an effort to solve some of the challenges posed by the climate and terrain of a residential district north of Stockholm. Pumping systems in Liljendahl's region weren't economically feasible because they would require an independent pump at each point of use. It only seemed reasonable to consider a collection system that would use differential air pressure to move the waste to its disposal point by pumping a partial vacuum at one central point. Thus began the development of vacuum sewer systems.
Supermarkets and prisons
Vacuum plumbing systems have migrated from Europe to North America over the past couple decades and the systems typically serve two major markets over here: supermarket refrigerated display cases and prison wastewater systems.
Vacuum plumbing systems are used to remove condensate generated by refrigerated cases. The systems allow refrigerator and freezer cases to be located and relocated anywhere in a store because the concrete slab doesn't need to be trenched for drain lines.
The advantage of vacuum plumbing systems in prisons is that inmates have figured out ways to pass contraband through conventional DWV systems, but the interface valve between the atmospheric and vacuum sides blocks the way. If prison officials are planning a sweep through the cells, they can shut off the vacuum pumps beforehand so the prisoners can't flush contraband.
In a prison application, the vacuum system removes waste from toilets, sinks, showers and any other water-using application. Moreover, the toilets only need a 0.5-gal. flush because of the vacuum assist. The low-water consumption makes vacuum plumbing systems eligible for points from Leader-ship in Energy and Environmental Design in green building applications.
While the two big U.S. markets for vacuum plumbing systems are super-markets and prisons, in truth, they can be used anywhere that site conditions make them attractive, such as historic buildings where rehab would require drilling through feet of stone walls or concrete floors.
An existing building might have a post-tensioned slab or the slab might be covered with a water or vapor seal that can't be punctured. Or, the ground might not be trenchable because it's bedrock or coral, such as Key Largo, Fla., or the Big Island of Hawaii.
Site conditions make vacuum plumbing systems more attractive, but first cost is not a huge impediment even in "conventional" applications. The system consists of pipes, valves and pumps, much like any other commercial/institutional plumbing system. A vacuum plumbing system can be cheaper in some aspects, because it doesn't require venting and pipe sizes are typically smaller.
Essentially, vacuum plumbing and drainage systems work via differential air pressure created by vacuum pumps, operating only as needed, to maintain constant vacuum inside a closed waste-piping network. The vacuum-piping network is connected to fixtures such as toilets, urinals, hand-wash basins, showers, mop sinks and floor drains by means of interface valves. At rest, the interface valve is closed and separates atmospheric pressure at the fixture from vacuum pressure within the piping network. When the valve is opened, it allows atmospheric air pressure to move wastewater into and through the piping network toward a central collection point.
At the central collection point, the wastewater is then automatically discharged to either a municipal sewer connection or treatment facility. The wastewater may be collected in a tank in the mechanical room and then moved to the sewer with a regular sewage ejector pump.
A vacuum plumbing system is not a siphon system — there is no full pipe transport of wastewater. Air pressure forces the waste from the fixture into the vacuum piping network where it expands as it moves to a lower pressure, thus adding velocity to the sewage.
For the system to work properly, the plumbing contractor must make sure that the entire installation is airtight. The inside of the pipe must be smooth and free of burrs. PVC is ideal, but copper may also be used.
Because vacuum plumbing systems use the interface of vacuum and atmospheric pressures, transport of wastewater is no longer solely dependent on gravity. Vacuum waste piping is therefore typically of smaller diameter than its gravity counterpart (typically 1 1 /2-in.) and can be routed vertically or horizontally from the fixture and throughout the piping network, offering opportunities for greater flexibility in building design and layout. This can be a benefit in facilities where space planning or tenant turnover concerns are a design priority, such as malls, airports or medical office buildings.
From a design perspective, the intermittent transport of waste propelled by air pressure eliminates the need for continuous waste piping slope and lift stations throughout or between buildings. This makes relatively shallow but long piping runs possible and can eliminate the need for dewatering and trench stabilization.
Moreover, since air can leak in, but wastewater can't leak out, sewage can't leak into the environment if there's a hole in the pipe.
Vacuum plumbing and drainage systems offer solutions to maintenance issues associated with frequent line blockages in facilities such as public restrooms or correctional institutions. While toilets and waste piping systems in these types of installations are subject to many abuses, the rapid movement of wastewater created by the differential air pressures that exist in a vacuum system reduces waste build-up and blockages. When blockages occur, they are typically at the fixture, rather than downstream where they are tougher to locate.
From a health and sanitation perspective, because vacuum toilets draw water, waste, and surrounding air into the piping network, bacterial aeration from the toilet flush is significantly reduced.
How fixtures work
A toilet in such a system would have to be a vacuum toilet designed for the application. Any other fixture, such as sinks, may be conventional.
When the toilet it flushed, it sends a signal to a controller that opens up the extraction valve to expose the waste line to the vacuum. The controller also opens a water valve that releases 0.5-gal to flush and wash down the fixture. In toilet applications, the waste passes through a sewage grinder on its way to the vacuum center.
For sinks or showers, water drains by gravity into an accumulator. An accumulator, in some applications, may be a PVC box supplied by the manufacturer of the vacuum plumbing system. In other applications, the accumulator is just the low spot in the pipe.
The pipe, typically PVC if allowed by code, would include a tee and nipple containing a sensor that runs back to the controller. The sensing device is usually low-tech rubber tubing that goes back to the controller to pneumatically signal an increase in pressure as waste flows into the accumulator.
A rise in the level of accumulated wastewater within the pipe accumulator activates the controller, which opens the normally closed waste extraction valve. The accumulated wastewater is then pushed or lifted by the interface of the vacuum and atmospheric pressures into the vacuum piping network and toward the vacuum center.
The vacuum center is an engineered product supplied by a manufacturer. The mechanical contractor sets, pipes and wires the vacuum center, which has been sized to handle the waste-producing capacity of the facility.
A vacuum center includes pumps, collection tanks, electrical control panel, valves and fittings necessary to provide a fully integrated automatic system. Dual tanks and multiple pumps are used for redundancy. One manufacturer builds vacuum centers with pumping capacities ranging from 1.5-HP to 10-HP. Manufacturers also will supply sizing tables showing how many fixtures can be served by each vacuum center configuration.
As long as the fixture loads are prop-erly calculated and the correct vacuum center is selected, vacuum plumbing systems can handle the same loads as a conventional DWV system. And hot should still be on the left and it's still inadvisable to bite your fingernails.