How does a sewage treatment plant work?

Mechanical purification

In mechanical pre-treatment, the wastewater flows through a screening and raking system to remove coarse refuse. In the course of this primary clarification for example debris like leaves, stones and hygiene articles are removed. The screenings are mechanically washed, pressed, thermally recycled (incinerated), composted or deposited on a landfill.

Grit removal

The purpose of the grit trap is to remove mineral impurities like sand, fine stones, gravel or glass splinters from the wastewater. This debris would otherwise heavily strain the mechanical parts of the sewage treatment plant through abrasion (scraping) or clog them. Often the sand trap is combined with a grease trap in one building.

Primary clarifier

Here the fine organic matter is removed from the wastewater by sedimentation. Sedimentation is achieved by reducing the flow velocity in the primary clarifier (sedimentation tank). In this way materials that could not be removed in the previous treatment steps are allowed to settle to the bottom. Faeces or paper settle in the primary clarifier as "settleable materials" or float on the surface. In this way about 30 percent of the organic matter can be removed from the wastewater. The settled solids are also known as primary sludge.

Biological treatment

In this process step bacteria and other microorganisms decompose organic wastewater components with the help of oxygen. Specific bacteria break down nitrogen compounds. With respect to the Biochemical Oxygen Demand (BOD) the purification performance of sewage purification facilities with combined mechanical and biological treatment amounts to 90 - 95 %. The purification performance results from the difference between the (contaminant) material load of the inflow and the load of the treated wastewater (effluent).

Aeration tank

The aeration tank is a biological reactor to which oxygen is supplied by technical equipment. The aeration tank contains the so-called activated sludge, a sludge enriched with bacterial mass. The bacteria decompose the contaminants contained in the wastewater, i.e. the organic carbon compounds. Nitrogen is first split off from the organic compounds as ammonium (= hydrolysis) and then oxidized with oxygen to nitrate with the help of special bacteria (nitrification). The oxygen bound in the nitrate can, through special bacteria, be used for further decomposition, and for the actual removal, of the nitrogen compounds from the wastewater (denitrification).

Most urban wastewater treatment plants are operated according to the activated sludge process described above.

Secondary clarifier

Via the return sludge circuit, the secondary clarifier forms a process unit with the activated sludge tank. In the secondary clarifier (final clarification tank) the activated sludge is separated from the treated wastewater by settling. Part of the settled sludge is returned to the aeration tank (return sludge) to enrich the bacteria specialised in wastewater purification in the aeration tank. The excess sludge, i.e. the increase in biomass no longer required, must be removed from the system. It is thickened for further treatment and usually pumped into a digestion tower together with the primary sludge. The digestion tower is a separate tank in which controlled degradation processes are carried out without oxygen. This produces methane gases that can be used thermally.

Chemical treatment stage

In municipal plants this treatment stage removes above all phosphorus (phosphorus precipitation). This process is of great importance to prevent the eutrophication of water bodies. This means that it must be prevented that an excess of nutrients reaches the receiving water - the water body into which the treated wastewater is discharged.

Phosphorus is found in wastewater dissolved and in particulate form. In order to remove phosphorus from the wastewater, it is brought into a form in which it can be removed as sludge in the chemical treatment stage.  Part of the phosphorus is incorporated into the biomass during the decomposition of the organic carbon compounds, the rest has to be converted into an undissolved form that can settle as sludge. This is usually done by adding iron or aluminum salts.

In industrial wastewater treatment plants also problematic substances like heavy metals or salts are removed in this purification stage.

Sludge treatment

During the individual purification steps of the wastewater treatment process, different types of sludge are produced, which must be subjected to special treatment. It is important to achieve a stabilization of the sludge, which means that organic carbon contained in the sludge should be decomposed as much as possible, so that odour impairments can be avoided as far as possible. Another purpose of the sludge treatment is to reduce the volume of the sludge to be able to further use or dispose of it.

Austria’s largest wastewater purification plant is the Main Waste Water Treatment Plant Wien / Simmering. It purifies the entire municipal wastewater of Austria’s capital. In dry weather, i.e. without additional rainwater, this amounts to more than 500,000 m³ of wastewater per day.

Designed in 1980 for originally two million population equivalents (PE₆₀), the plant had to be extended by a second biological treatment stage in 2005. Wastewater amounting to four million PE₆₀ can now be treated. The sewage sludge produced - more than 900,000 m³ per year - is dewatered and incinerated in fluidized bed furnaces at the Simmering disposal companies. From 2020 onward, the sewage sludge is to be treated anaerobically before dewatering and incineration. During this digestion process, sewage gas is produced, which will be used to generate electricity and heat.