Wastewater purification

Vienna’s main wastewater treatment plant is located in Simmering, one of Vienna’s topographically lowest zones close to where the Danube Canal flows into the Danube. All of Vienna’s wastewater is cleaned here. On dry-weather days, more than 500,000 m³ of wastewater - the water flow of a medium-sized river - pass through the plant’s mechanical stage and both biological treatment stages. After around 20 hours, the purified water flows via the Danube Canal into the Danube River.

Mechanical cleaning

The wastewater delivered by the sewer mains is first rid of coarse mineral matter, such as gravel and grit, in the gravel trap. Six screw pumps lift the wastewater up to plant level, so it can flow through the plant driven by gravity alone. In the screening chamber, fine bar screens filter out the floating and suspended particles. These screenings are dewatered and transported off in closed containers for disposal. Fine-grained solid particles, sand and ashes are left to settle in the downstream sand trap. The last stage of mechanical cleaning takes place in the primary clarifiers or sedimentation tanks, into which the wastewater flows through the connecting channels. The reduced flow rate allows suspended particles remaining in the wastewater to settle to the bottom of the tanks as primary sludge. This sludge is forced into sludge troughs by means of scrapers and pumped into the thickening tanks. Up to 30 percent of the pollutant load is eliminated from the wastewater by mechanical cleaning.

Biological cleaning

The following biological cleaning process is modelled on the biological self-cleaning capacity of natural bodies of water. In the aeration tanks, microorganisms, which are also referred to as “activated sludge”, break down the dissolved, mostly organic pollutants. The oxygen needed for the degradation process is blown in by disk-aerators. Phosphate precipitation is achieved by dosing iron(III)sulphate into the aeration tanks. In the intermediate clarifiers, the activated sludge is separated from the precleaned effluents. Microorganisms settle down along with the pollutants they take up as activated sludge. A portion of the sludge (return sludge) is recirculated into the aeration tanks to accelerate microbial growth and keep the degradation process on track. Excess sludge is pumped into the consolidation tanks. From the intermediate clarifiers, the wastewater flows over dentated sills into a channel that leads to the second biological cleaning stage. 

In the years between 2000 and 2005, Vienna’s main wastewater treatment plant was substantially enlarged. The upgrade consisted in two additional biological cleaning stages for nitrogen removal (nitrification and denitrification). Thanks to the new cutting-edge technology, the plant’s cleaning efficiency has been significantly improved and vitally helps to protect Vienna’s environment.

The outflow from the first aeration stage and the return sludge from the second aeration stage are transported into the intermediate pumping station, from where the wastewater and the sludge are pumped into a combined channel - equipped with mixers -  by means of submersible propeller pumps. Since the wastewater flow is split up after preliminary sedimentation, phosphate is precipitated by using iron(III)sulphate also in this stage.

Inside the distribution chamber, the wastewater is segregated into identical streams which flow into the 15 aeration tanks of the second biological cleaning stage. These tanks are grouped into three blocks of five tanks each. Each secondary-stage aeration tank consists of three cascades. The first cascade, an upstream non-aerated denitrification basin, is followed by two downstream serial circular basins (cascades 2 and 3) where nitrification and denitrification take place simultaneously. The aeration tanks are equipped with propeller-type mixers which assure a balanced flow and optimal mixing of the effluents. The bottom of the aeration tanks is covered with fine bubble diffusers, and there is the option to adjust the aeration volume to temperature and pollution load. Each aeration tank is followed by a downstream degassing zone. From here the effluents flow into the secondary clarifiers.

In the 15 circular secondary clarifiers, the purified water is separated from the activated sludge. Each clarifier is equipped with double scrapers and submerged draining pipes and has a diameter of 64 m. A portion of the sludge (return sludge) is recirculated into the aeration tanks to accelerate microbial growth and keep the degradation process on track. Excess sludge is pumped into the consolidation tanks. The effluents that leave the secondary clarifiers pass through the on-site laboratory for analytical testing, and further on to the outlet channel into the Danube Canal. A portion of the nitrate-containing effluents are recirculated into the first stage under dry weather conditions to use the denitrification capacities of that stage. 

Sewage sludge treatment

Primary and excess sludge end up in the sludge thickening tanks, where they are thickened by gravity to a dry matter content of approximately 4%. The combined sludge is then pumped through underground pipes to the sludge treatment unit at Simmeringer Haide (operator: Wien Energie). After centrifugal dewatering, the sewage sludge is incinerated at a temperature of 850°C in fluidised bed furnaces.

Exhaust air cleaning

To avoid unpleasant smells, the grit chamber, screw pumps, sand trap, sludge consolidation tanks and sludge troughs are roofed in. This system is equipped with biofilters for cleaning the exhaust air.

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