Main Wastewater Treatment Plant of Vienna

The main wastewater treatment plant of the city of Vienna is located in Simmering, the topographically lowest point of Vienna, near canal mouth of the Danube. This is where all the municipal wastewater of Vienna is treated. That amounts to more than 500.000 m³ on dry weather days. The amount is comparable to a channel flow of a medium-sized river. It takes about twenty hours for the wastewater to pass through one mechanical and two biological treatment stages at the plant, before running off in a purified condition into the Danube.

The main wastewater treatment plant of Vienna was expanded during the time from 2000 to 2005. The expansion consisted of the installation of a second biological treatment stage for eliminating nitrogen (nitrification and denitrification), which is connected to the biological treatment stage dating back to 1980. By using the latest technology, an increase in cleaning performance makes an essential contribution to Vienna wastewater management, so that Vienna can continue to be designated as an international "environmental model city".

Mechanical treatment stage

Coarse materials, such as gravel and grit, are filtered out of the wastewater flowing through the main ducts.

Spiral pumps lift the wastewater up to plant level so that it can flow through in a natural gradient.

Floating and suspended solids are separated by strainer screen in the screening plant. The debris is drained and transported to incineration in closed containers. In the following grit channel, fine solids, sand and ash are removed. The grit chamber trappings are also burned after drainage.

The last stage of the mechanical treatment of wastewater takes place in the primary clarifiers, each of which is accessible through an inlet channel. By decreasing the speed of flow, the floating solids that remain in the wastewater settle to the bottom of the tank as primary sludge. This sludge is pushed through scrapers in collecting channels and pumped to the thickeners. The mechanical treatment of the wastewater removes up to 30% of the contaminants.

First biological treatment stage

The following biological treatment corresponds to the model of the biological self-cleaning of natural waters. In the activated sludge tanks microorganisms, called "activated sludge", degrade the dissolved, predominantly organic contaminants. The oxygen needed for the degradation process is provided by aerators.

Phosphate elimination with ferric sulphate takes place by dosing in the activated sludge tanks.

The separation of the activated sludge from the pre-cleaned wastewater takes place in intermediate clarifiers. The microorganisms settle together with the collected dirt as activated sludge. Part of the sludge (return sludge) is led back into the activated sludge tank to speed up the concentration of microorganisms and to keep the degradation process intact. Excessive sludge is pumped into the thickener.

The wastewater flows through toothed thresholds out of the intermediate tanks into a channel which leads to the second biological treatment stage.

Second biological treatment stage

The flow of the first activated sludge stage and the return sludge from the second activated sludge stage are fed to the intermediate pumping station, from where the wastewater and sludge are pumped  with propeller submersible pumps into a channel equipped with mixing elements.

Because of the dividing of wastewater flow after pre-treatment, phosphate elimination with ferric sulphate also takes place in this section.

The wastewater is divided evenly into 15 activated sludge tanks of the second stage, collected in three blocks. Each activated sludge tank of the second stage consists of three cascades. The first cascade forms an upstream, unaerated denitrification tank, downstream to the throughflow circulating tanks (cascades 2 and 3) in which simultaneous nitrification and denitrification take place. Propeller-type agitators provide for regular flow and uniform mixing in the activated sludge tank. The activated sludge tanks are equipped with deep fine-bubble aerators and it is possible to adjust the aerated volume to the temperature and organic loads.  There is a vent zone connected to each activated sludge tank, from which the corresponding secondary settling tank is loaded.

The separation of the treated wastewater from the activated sludge takes place in 15 round secondary settling tanks. Every secondary settling tank is equipped with double scrapers and an immersed drain pipe, and has a diameter of 64 metres. Part of the sludge is led back into the activated sludge tank (return sludge) to speed up the concentration of microorganisms and to keep the degradation process intact. Excessive sludge is pumped into the thickener. The run-off of the secondary treatment flows through the outlet channel into the Danube canal. A part of the nitrate-containing run-off is led back into the first stage in dry weather, to take advantage of the denitrification capacity there.

Wastewater sludge treatment

In the wastewater sludge thickeners, primary and excessive sludge thicken together, under the influence of gravity, to a dry substance content of approx. 4% and is pumped on to sludge treatment. After dewatering in centrifuges, the wastewater sludge is burned in fluidised-bed furnaces at 850°C .

Exhaust air treatment

To avoid odour nuisance, the gravel filter, spiral pumps, grit chamber and wastewater sludge thickeners, as well as the sludge channels, are covered.

Co-workers make their contribution to environmental protection

The qualified personnel of the main wastewater treatment plant of Vienna ensure proper function of the plant and control of the complex processes round the clock. The co-workers in the corresponding labs monitor adherence to permitted emission levels as prescribed by law.

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