A vacuum sewer or pneumatic sewer system is a method of transporting sewage from its source to a sewage treatment plant. It maintains a partial vacuum, with an air pressure below atmospheric pressure inside the pipe network and vacuum station collection vessel. Valves open and reseal automatically when the system is used, so differential pressure can be maintained without expending much energy pumping. A single central vacuum station can collect the wastewater of several thousand individual homes, depending on terrain and the local situation. Vacuum sewers were first installed in Europe in 1882. Dutch engineer Charles Liernur first applied negative pressure drainage to sewers in the second half of the 19th century. Technical implementations of vacuum sewerage systems began in 1959 in Sweden. Historically, vacuum sewers have been a niche product, used only in trains, airplanes, ferries and flat areas with sandy soils and high ground water tables. Gravity sewers were used for most applications, because although vacuum sewers were cheaper to install, they were more expensive to maintain. In the 20th century, vacuum sewer technology has improved significantly: fault-locating sensors have reduced operation and maintenance costs, and some operators now consider that vacuum sewers can be cheaper to run than conventional gravity sewers.
Basic elements
The main components of a vacuum sewer system are a collection chambers and vacuum valve parts, sewers, a central vacuum station and monitoring and control components. Some vacuum systems have vacuum toilets which are connected directly to a vacuum line, which requires less water for flushing (less than a quarter of a liter per flush). Others use standard gravity drainage for the first phase of collection; sewage flows by means of gravity from each house, as in a standard system. It discharges into a collection sump that might collect sewage from 2-6 houses and is located in a public area. Vacuum technology is based on differential air pressure. Rotary vane vacuum pumps generate an operation pressure of -0.4 to -0.6 bar at the vacuum station, which is also the only element of the vacuum sewerage system that must be supplied with electricity. Interface valves are installed inside the collection chambers. They work pneumatically. After a certain fill level inside this sump is reached, the interface valve opens. The impulse to open the valve is transferred by a pneumatically mechanical controlled controller unit. No electricity is needed to open or close the valve. The energy is provided by the vacuum itself.
While the valve is open, the resulting differential pressure between atmosphere and vacuum becomes the driving force and transports the wastewater and air towards the vacuum station. Besides these collection chambers, no other manholes, neither for changes in direction, nor for inspection or connection of branch lines, are necessary. High flow velocities keep the system free of any blockages or sedimentation. Large systems with numerous collection chambers benefit from the provision of a monitoring system for remote monitoring of the vacuum valves and sump pits. Such systems allow much faster troubleshooting and easier preventive maintenance of collection chambers and valves. However, monitoring systems are optional systems and not required for operation of vacuum sewer systems.
Vacuum sewer systems are considered to be free of ex- and infiltration which allows their use even in water protection areas. For this reason, vacuum sewer lines may even be laid in the same trench as potable water lines (depending on local guidelines). In order to ensure reliable transport, the vacuum sewer line is laid in a saw-tooth (length-) profile. The whole vacuum sewers are filled with air at a pressure of -0.4 to -0.6 bar. The most important aspect for a reliable operation is the air-to-liquid ratio. When a system is well designed, the sewers contain only very small amounts of sewage. The air-to-liquid ratio is usually maintained by collecting liquid/air simultaneously or controller units that adjust their opening times according to the pressure in the system. Sewers can be laid in flat terrain, and parts may flow uphill (within limits). A saw-tooth profile keeps sewer lines shallow; in frost-free climates, trench depth can be about 1.0 – 1.2 m. By contrast, gravity sewers need a monotonically falling slope of at least 0.5 - 1.0%, which can mean that expensive trenching and pumping stations are needed. Once the wastewater arrives in the vacuum collection tank at the vacuum station, it is pumped to the discharge point, which could be either a gravity sewer or the treatment station. As the dwell time of the wastewater inside the system is very short and the wastewater is continuously mixed with air, the sewage is kept fresh and any fouling inside the system is avoided (less H2S).
Advantages
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