értekezés - Szent István Egyetem

8. SUMMARY
A large part of our country is flat having high ground water table levels, conditions that
make traditional gravity sewer systems difficult and expensive to install and operate. Laid
in shallow trenches vacuum sewerage systems offer an excellent alternative for these areas,
thus more than 50 modern, environmentally friendly vacuum sewerage systems operate
already in Hungary serving altogether more than 200000 people. Taking into
consideration that 60% of the country is canalized we can expect sewer system
installations in many villages in the next decade according to the Wastewater Directive of
the EU.
Certain questions of this special field regarding sizing and operation are still unsolved or
mostly treated based on the different experiences of the innovative predecessors often
lacking solid theoretical background. I carried out researches on the hydrostatics of
vacuum sewerage systems, their evacuation time and their energy demand for collecting a
unit of fluid volume based on theory as well as experimenting on a vacuum sewerage pilot
plant test rig designed and built by me.
I found that the use of the so called vessel evacuation formula based on concentrated
parameters for determining the pump down time and the required vacuum pump
performance of vacuum sewerage systems leads to inaccurate results. I approached the
evacuation process with a model of divided parameters and for the determination of the
evacuation time a software can be worked out based on the worked out theoretical
background.
For the determination of the real evacuation time I suggest that a correction factor be
introduced based on the length of a system.
For the determination of the theoretical maximum of the static vacuum loss in a vacuum
pipe I worked out a relation that takes into consideration the open and closed lifts, their
geometry and their combination with horizontal or back fall pipe sections. I found that the
static vacuum loss depends on the used air/water ratio. With the increase of the air surplus
the static vacuum loss monotonously decreases but there is a certain limit above which
there is no point in increasing the air/water ratio because it will not cause the static vacuum
loss to decrease any more. I ascertained that increasing the vacuum level in the vacuum
vessel monotonously decreases the static vacuum loss. I introduced the concepts of the
static vacuum loss eliminating and the waterlogging boundary curves. The reduction of
the air/water ratio over the bottom boundary leads to such an increase of the static vacuum
loss that the vacuum interface valve is not able to open any more and the system becomes
waterlogged, thus its proper operation stops.
I suggest that the expected maximum static vacuum loss of the vacuum mains be
determined at the design phase of vacuum sewerage systems. In case of commissioned
vacuum sewerage systems I suggest complementing the existing requirements of the
standard with the testing of static vacuum losses by continuously adding water to the
vacuum chamber located at the end of the vacuum main and in the meantime measuring
the pressure levels and air/water ratio during the operation of the system.
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