A biological wastewater treatment plant draws on the biodegradation processes which permanently occur in natural environments. In the biological treatment stage - the centrepiece of such plants -, living microorganisms actively degrade pollutants in the water. Specific knowledge of the living conditions of such simple organisms is required to assure that the degradation process inside the treatment plant takes place in a carefully controlled manner.
The bare essentials which microorganisms thrive on - water and organic pollutants - are available in abundance in wastewater. Oxygen, which they also need to build their cellular substance, is dose-fed into the wastewater through aerators. Under such favourable conditions, microorganisms rapidly proliferate to twice or even four times their amount per hour. The metabolic process that takes place while they build their cellular structure serves to break down the available pollutants and contaminants into their components. How the degradation process develops and what decomposition products arise clearly depends on the composition of the substances contained in the wastewater. To decompose the various pollutants as completely and effectively as possible, a huge variety of different microorganisms combine to form a symbiotic living community.
The carbon contained in the organic compounds is broken down into gaseous carbon dioxide. One group of microorganisms converts the organic nitrogen compounds to ammonia and then to nitrates (nitrification). Under appropriate conditions, other microorganisms can further convert these intermediate products to nitrogen gas (denitrification). Since phosphates can only partly be degraded by the microorganisms during their metabolic process, ferrous salts need to be added to facilitate the precipitation of phosphate.
In the intermediate and secondary clarifiers, the microorganisms settle down as sewage sludge. Part of this activated sludge is refed into the aeration tanks to maintain a steady-state cleaning process (return sludge). The excess sewage sludge is thickened, dewatered and incinerated. The purified water flows into the Danube.
ebswien hauptkläranlage uses a technology with two different operating modes, which were developed in conjunction with experts from the Vienna University of Technology. Depending on the pollution level, water temperature, season of the year and water amount, the wastewater is distributed throughout the system by a fully automated process with dosed oxygen supply in both modes. This process is controlled by a sophisticated online measuring system. The aim is to assure optimum operational safety and process stability throughout the entire purification process.
In the by-pass-mode the wastewater is precleaned and then split up in two streams, with one stream directly flowing into the 2nd aeration stage. The idea is to provide the 2nd aeration stage with a sufficient amount of easily degradable carbon for the denitrification process while lowering the hydraulic feed of the 1st stage to allow nitrate-containing discharge water from the 2nd stage to be recirculated to the 1st stage for denitrification under low flow conditions (dry weather). This helps to optimise the cleaning capacity of the 1st stage.
Another possibility to optimise cleaning performance is to change the aeration volume in the 2nd stage. Between 15% and 85% of the aeration tank volume of the 2nd stage can be supplied with oxygen. A portion of the nitrate-containing discharge water from the 2nd stage is recirculated to the 1st biological stage for denitrification. The excess sludge from the 2nd stage is recirculated to the 1st stage. The nitrificating bacteria benefit from the high ammonia concentration in the 1st stage. Due to the high decomposition rate of the activated sludge, the resulting nitrate is quickly decomposed in the anoxic zones.
The hybrid-mode can only be used under dry weather conditions. The entire precleaned wastewater flows to the 1st aeration stage. This results in a high sludge burden and the activated sludge has a high oxygen demand. A portion of the sludge is pumped into the discharge water from the 1st stage, which means that high-carbon biomass is fed into the 2nd stage. The wastewater is subsequently cleaned in the same way as in by-pass mode.