Vacuum sewer


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, 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. 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.
File:The vacuum station.jpg|thumb|left|upright|A vacuum station for a student dorm in Norway, providing suction for all the vacuum toilets in the dorm
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.
In order to ensure reliable transport, the vacuum sewer line is laid in a saw-tooth 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. 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.

Advantages

  • closed, mechanical/pneumatical controlled system with a central vacuum station. Electrical energy is only needed at this central station
  • no sedimentation due to self-cleansing high velocities
  • spooling and maintenance of the sewer lines is not necessary
  • manholes are not required
  • Usually only a single vacuum pump station is required rather than multiple stations found in gravity and low pressure networks. This frees up land, reduces energy costs and reduces operational costs.
  • investment costs can be reduced up to 50% due to simple trenching at shallow depths, close to surface
  • flexibile piping allows obstacles to be over- or underpassed
  • reduced installation time
  • small diameter sewer pipes of HDPE, PVC materials; savings of material costs
  • aeration of sewage, less development of H2S, with its dangers for workers, inhabitants, as well as corrosion of the pipes may be avoided; sewage is kept fresh
  • no odours along the closed vacuum sewers
  • no stormwater infiltration, therefore less hydraulic load at the wastewater treatment plant
  • no sewage leakages, which is not only an ecological/public health benefit, but also makes repairs to the pipes easier and more sanitary for repair workers
  • sewers may be laid in the same trench with other mains, also with potable water or storm-water, as well as in water protection areas
  • Lower cost to maintain in the long term due to shallow trenching and easy identification of problems
  • In combination of vacuum toilets it creates concentrated waste streams, which makes it feasible to use different waste water treatment techniques, like anaerobic treatment

    Disadvantages

  • vacuum systems are not capable of transporting sewage over very long distances, but can pump long distances from the vacuum station to the next sewage treatment plant or main gravity sewer.
  • vacuum sewerage systems are only capable of the collection of wastewater within a separated system
  • the lines can only reach up to 3–4 km laid in flat area )
  • systems should be designed with help of an experienced manufacturer
  • external energy is required at the central vacuum station; an emergency power supply, such as a backup generator, is needed for power outages.
  • odours close to the vacuum station can occur, a biofilter may be necessary
  • integrity of the pipe joints is paramount
  • grease can clog the sensor tube thus requiring preventative maintenance cleaning
  • vacuum valves can get stuck open leading to pressure drops in the entire system. However, a monitoring system can help identifying stuck open valves immediately

    Applications

Vacuum sewer systems may be the preferred system in the case of particular circumstances, such as:

Transport

Trains, aircraft, buses, and many ships with plumbing generally have vacuum systems with vacuum toilets. The lower water usage saves weight, and avoids water slopping out of the toilet bowl in motion. Aircraft toilets may flush with blue disinfectant solution rather than water. A portable collection chamber is used; if it is filled from an intermediate vacuum chamber, it need not be kept under vacuum.

Dry areas

Lack of water in many countries and drastic water savings measures have led to difficulties with aging gravity networks with solids blocking in the pipes. Vacuum systems save water.

Boggy, rocky, or permafrost terrain

Flat terrain, unfavorable soil, or a high groundwater table can make gravity sewerage systems much more expensive. Vacuum sewers are small in diameter and leak inwards, and in frost-free areas, they can be laid close to the surface in small trenches.

Water protection areas, environmental use

Vacuum sewers can pass through water protection areas and areas with sensitive high ground water tables, because there is no danger of spoiling groundwater resources. Vacuum systems are used in many environmentally sensitive areas such as the Couran Cove Eco Resort close to the Barrier Reef in Australia. They've also been used to replace septic tanks to reduce nitrogen levels in ground/surface water.
Vacuum systems have also been applied to collect toxic wastewater from the environment.

Seasonally sub-freezing climates

If the temperatures in an area dip below freezing in winter, the vacuum line is buried below the frost line, in ground that stays unfrozen year-round. Valves, collection pits, intake vents, and control systems need to be designed to keep functioning despite cold, snow and ice. Temperature-monitoring sensors are also standard, so problems can be noticed early.
In the case of Plum Island, the island was prone to freezing temperature and excessive snowfall that initially made it difficult to locate a potential problem. Changes to their Pit setup and monitoring at the valve pit has helped with maintenance.
Many Nordic Countries utilize vacuum sewers, it is helpful to have some type of marker or monitoring to locate valves when they are buried under the snow for extended periods.