Sewer aeration breaks new grounds
Faced with an odour nuisance from the main sewer in the low-lying areas to the north-east of the River Danube, Vienna's sewer engineers decided to use a series of chimneys to vent waste gases to atmosphere, solving the problem. Dipl.-Ing. Robert Nowak, Head of Planning Division, Municipal Department 30 - Sewerage, explains.
Chimneys along the Old Danube are built at a higher level than in the city centre.
This measure profoundly changed the river landscape and set off a development of the city which is still continuing today. For city residents, the Old Danube became one of the most popular recreational areas in Vienna.
Planning for the construction of a main sewer alongside the Old Danube was started in the mid-1980s. The sewer collects effluent from the small gardens and allotments along the left bank of the Old Danube as well as connecting adjoining rural areas and settlements near the Danube to the public sewer network.
However, the Viennese sewer system often emits poor quality odours, particularly in the low-lying areas to the north-east of the River Danube. These odours can affect the quality of life of neighbouring residents with the result that more and more complaints have been lodged in recent years.
This odour nuisance is the result of an insufficient sewer gradient. To counteract the phenomenon, the main sewer was restructured slightly to increase the flowrate. But since only low gradients can be used, the problem has not been fully solved. To completely eliminate the problem in the critical area of the Old Danube, the city's sewer engineers had to come up with a new approach to sewer aeration.
The obvious solution used in many European cities - sealing the sewers with manholes - had to be ruled out because of the high investment costs of adapting a 2500km-long public sewer system. After analysis of the available alternatives, a system of artificial aeration was developed.
In the conventional sewer aeration system in Vienna, fresh air is admitted through aeration grates. Waste air from the sewer exits through a series of chimneys - a system which works well even in the densely populated areas of the city centre.
However, the area along the main Old Danube sewer is almost entirely made up of allotments and housing developments built at a slightly higher level than the aeration apertures in the sewer. In these cases, venting odours through chimneys is not sufficient.
The main goal of the new exhaust air scheme for the Old Danube area is to extract exhaust air from the sewer through higher chimneys which blow the air into a higher strata of the atmosphere.
Upper air strata are more influenced by a stronger air circulation, so the deposition of undispersed and undiluted sewer air at the source of emission is no longer a problem.
In the first phase, 14 exhaust units (chimneys with a fan), extending over a length of 2200m, were installed. Chimneys were spaced at a distance of 100-200m.
The exhaust-air chimneys - plastic poles with a diameter of 200mm at the top and a net length of 10m - double up as street lights so they had to be positioned at every path junction and square as well.
Through a side aperture at the bottom, the sewer air is blown into the chimney by an axial blower positioned in the appropriate aeration pit. The aeration units (chimney and blowers) are connected with each other and with the main sewer by an underground pipeline.
Blowers are controlled by a sensing device that measures the air flow. The sewer can be aerated naturally under favourable temperature and pressure conditions. However, as soon as the air flowrate falls below 0.5m/s, the appropriate fan is automatically switched on.
In case the limit value of methane is exceeded, the flow sensors are automatically over-amplified and the fans of the affected sewer section switched on.
Trenchless sewer renewal
One Austrian company has developed a technique enabling repair and renewal of Vienna's ageing public sewer system without the need for open-trenching.
Using a profile cutter, developed by Vienna-based Wibeba, old sewers can be reamed to a wider diameter allowing new pre-assembled elements to be installed while preserving the original sewer section.
An old brickwork sewer built around the turn of the century beneath a street section of the Burggasse, near the centre of Vienna, badly needed restoration. The Burggasse connects the inner city with the outer districts and must therefore cope with high traffic loads.
Using the profile cutter, the old brickwork was reamed to a wider diameter and a pre-assembled inliner installed in the sewer. Obstructions caused by the work were restricted to two parking lanes.
The sewer remained in service throughout the construction period and, by using a pumping system, sewage was diverted section-by-section.
The profile cutter is equipped with a diamond saw blade (cutting head) which provides accurate non-destructive reaming of the old profile.
Two Austrian companies -Wibeba and Wienerberger - have also developed a technique for the application of inliners in trenchless sewer renovation. The process consists of introducing pre-assembled polymer parts with a 30mm wall thickness into the sewer. The inliners are fitted into each other with an epoxy resin. The gap between the old sewer and the new profile is then filled with a cement mortar.