4.2 Pump station with mounting of pumps in dry position

The building part consists of a substructure, made according to size and amount of the pumps and a built-on superstructure for which mostly a prefabricated garage is used.
In the superstructure the vacuum pumps, sound feeding and measuring plants, wash basin and toilets are located.

Building site of the technical outfitting

Vacuum pumps

Self-lubricating pumps have proved to be worthy that create the vacuum by means of a liquid ring. The do not require lubricating within the working space. The air which is sucked out of the vacuum tanks is blown out through a pipe above the station's roof level, which results in less odour emission. If the installation is situated at the centre of a location, the used air is led through an absorber shaft which is filled with suction and muffling materials. The condensate can drain and no further emission is noted.

Sewage pumps:

Of the numerous types on the market, those sewage pumps with a clear passage of 75 mm have proved to be the most reliable. They cause no blockages or built up of debris and they operate safely.

Vacuum tanks:

They consist of a 10 mm thick steel sheet and are protected against corrosion both inside and outside. Access and cleaning is guaranteed through a manhole.

Size and Amount of Vacuum Pumps, Sewage Pumps and Vacuum Tanks

The number and size of the vacuum pumps, sewage pumps and vacuum tanks depend on the amount of residents using the system.
The installation is chosen in a way that one vacuum pump and one sewage pump each serve one vacuum tank.
Pump stations consist of at least two vacuum tanks with the corresponding pumps. The calculated transport demand is divided to several pumps with a single capacity of 3 and 5 KW power demand in order to avoid current peaks. These pumps are bolt against another so that only one of the both pumps can run at the same time. The long term reservation costs for peak current are thus reduced to 50 %. If an installation consists of several vacuum and sewage pumps, considerable long-term reservation current costs can be saved by using a balanced control switch.

Effectivity of the pumps

Liquid ring vacuum pumps dispose of a very good effectivity as they only operate in the area up to 0.7 bar vacuum.
The current costs for the production of the necessary amount of air for transporting each cbm sewage amount to 0.15 DM or for each resident = 40 x 0.15 DM = 6.00 DM /year.
In contrary to that the sewage pumps dispose of a bad effectivety. This can mainly be explained by the fact that the suction is effected from a vacuum area of 0.6-0.7 bar that is only reduced by 0.06 bar by the static water pillar averaging to 0.6 m.
The pump costs for each cbm sewage amount to 0.30 DM or
for each connected resident =
40 x 0.30 DM = 12.00 DM per year
transport costs amount to approx. 18.00 DM per resident and year.

4.3 Pump station with mounting of the sewage pumps in wet position:

Contrary to the mounting of pumps in dry position the sewage pumps are mounted directly as submersible pumps in the vacuum tanks, that are located vertical. The equipment of the sub contraction remains like the mounting in dry position.
The pump is permanently flushed by water. It pumps from a vacuum area of 0.6 to 0.7 bar vacuum but this area is reduced by the static pressure of the water pillar that is on average 2.0 m high in the vacuum tank so that effectively a transport from an area of 0.4 bar vacuum is effected.
The pump mounted in wet position has priority. With this approximately 20 % of the transport costs of the sewage pump can be saved.


Image 2. Sewage pump in wet position

Emergency power unit

Emergency power units have been mounted into pump stations very rarely. According to the EVU's the current supply net is connected by means of even in case of a snow catastrophe in Schleswig - Holstein the longest power failure would be less than 4 hours.

Pipe net

Main and auxiliary pipes are formed in a branched way. Pipes PN 10 made of PVC with a diameter of DN 65 to DN 150 are used.

Way of bedding

The pipes are bedded in a sufficient depth to prevent freezing in winter. In order to transport the water by air a complete filling in some sections of the pipe is necessary. This results in a systematic formation of peaks and throughs which vary according to the diameter of the pipe.
Pipe revisions are located in the throughs.
They serve the determination of leakages in the pipe net and the air condition when the transport system breaks down.

4.7 Flow velocities in the pipe network

According to the vacuum pressure flow velocities of 1 and 7 m per second are measured in the short peak areas.

Blockages, sedimentation, maintenance of the network

No cases of blockages or sedimentation have ever been reported at any installations in normal operation. The network requires no maintenance.

4.9 Pressure testing of pipe network

The pipe network is checked regarding leakages. The test is considered passed if within an hour the vacuum pressure of 0.6 bar does not lose more than 10 %.

4.10 calculation procedure

So far the pipe net was dimensioned according to empirically gathered facts. There are no calculation procedures except the procedure being published in the working sheet draft of the ATV.

4.11 Interface valve at the house connection

In the cellar or in the domestic shaft an interface valve is located. It consists of a membrane valve that is connected to a control box with 2 pipes. The liquid waste arising in the house runs down to the interface valve and blocks. Via a pitot tube it gets into the control box and causes a pressure on the located membrane. This opens and closes the suction valve via 2 control lines. It is set to a certain opening time in which the water can drain and a certain amount of air can flow in afterwards.
If the pressure is reduced under 0.3 bar the valve manages during the opening phase just the water drain. But the necessary air for the transport does not flow in after that. If in that time several valves open without air coming in afterwards, the channel runs full of water and it drowns. The system breaks down. In this case the emergency stowage space being either built in the shaft or arranged on the house gets into action and stores the sewage until the re-conditioning of the system. For preventing such breakdowns air-conditioning-stations are mounted are installed at the ends of problematic, long channel ways. They shall add air via a magnet valve, prestostat or time relais when needed. Trials trying to add air together with a membrane function via a tube with adjusting cock into the channel net were not successful because an adjustment of air inlet might be possible in high vacuum areas but would then lead to a lack of sufficient air inlet in dangerous lower areas. If the adjustment is made at the lower value, too much air gets into the channel net when a high vacuum is caused and that would cost energy.

5.0 Development of a new and operation-safe transport system

5.1 General

Due to the insecurities in the present system a complete vacuum sewerage plant was built with crystal pipes. Several examinations and measurements led to entirely new findings ant to the development of an operation-safe transport system. In the following you can see all details of the findings that were made in vacuum
sewerage.

5.2 Pump station

Time / pressure diagrams (Image 3) that were taken with vacuum pump operation show an even increase of the energy needs up to a vacuum of 0.55 bar. From 0.6 bar an increasing energy demand is necessary for increasing the vacuum. Practice showed that for plants being operated with only 0.6 bar vacuum, a saving of approx. 30% of the energy costs is possible.



5.3 Pipe network

The most favourable way of bedding the pipes turned out to be the arrangement of peaks and throughs in regular intervals. For upgrade as well as for downgrade flow the same flow velocidity of 4 - 5 m/sec. were measured.



The air could entirely be used for the transport of water. Even with a vacuum of only 0.1 bar the transport of liquid waste could be effected without problems.



5.4 Control shafts

Control shafts are arranged at junctions, but mostly at distances of 200 m from one another, in order to monitor the network. Stop valves as well as suction valves are already built into these shafts at the time of manufacture to deal with the potential problem of rainwater penetration.
The network may be inspected by unflangeing the connection so that access to all pipes will be permitted.



5.5 Pressure test

During a fault of the operation a considerable pressure could be put on a lower located sewage area by a higher located area and thus overstrain the pipe network more than in vacuum operation.
The leak detection of the pipes should therefore not be effected by a vacuum but with a pressure of 20 to 30 m water pillar. In this pressure area a compressed pebble bed is enough, abutments of concrete do not have to be arranged.
The pressure testl will be effected by the suction valves being located in the control shafts in short distances or throughout the entire canal net.
The domestic valves do not have to be secured individually, as they are allowed for PN 10.

5.6 Domestic shaft with interface valve

Main item of the investigation was the domestic shaft with the interface valve, as in this area the greatest insecurity in the vacuum system was found.
The operation safety and construction was to fulfil the following:
For the valve:
Free penetration in order to prevent the loss of energy
opening independent from pressure
closing without extern energy
transport of sewage and at the same time adding of air, supervision and report of the operation function in the connected house
long lasting durability and no need for maintenance
For the domestic shaft:
water tightness
buoyancy safety
frost safety
simple mounting
Trials with valves from the market did not lead to a satisfying result. A new valve made of PVC was manufactured having the following details and functions.
The valve disposes of an inflow diameter of 50 mm and an outflow diameter of 65 mm. A Chamber for water and air is located in the middle. During admission, air flows into the chamber, becomes mixed with the liquid waste and leaves the valve as a mixture of water and air. The valve is opened hydraulically and closed by means of a steel spring at a force of approximately 400 kg.

The control device is activated electronically and requires no contact with the liquid waste. The valve function is monitored and transmitted from the shaft to the house via cable. In the event of several villages being connected, it is possible to monitor a domestic drain on a central monitor via telephone cable.
The valve was permanently tested by adding textiles, foam material, paper and sand. 200.000 (in words: two hundred thousand) connections were executed without any wear and tear being noted. It can be taken as a fact that the valve has to operate approximately 4.000 connections per year.


Valve and control are already built into the watertight concrete shaft at the manufacturer. No mounting or electronic works are necessary on the building site.