How can carbon be separated from pollution

How does a sewage treatment plant work?

Mechanical cleaning

In the mechanical pre-cleaning, coarse dirt is removed using a ballast trap and rake. During this primary treatment, leaves, stones and hygiene items, for example, are removed. The screenings are machine washed, pressed, thermally recycled (incinerated), composted or deposited in a landfill.

Sand trap

The task of the sand trap is to remove mineral impurities such as sand, fine stones, gravel or glass splinters from the wastewater. Otherwise, these substances would stress or clog the mechanical parts of the sewage treatment plant due to abrasion (scraping). The sand trap is often combined with a grease trap in a building.

Primary clarifier

Here the fine organic substances are removed from the wastewater by sedimentation. The sedimentation takes place by reducing the flow velocity in the primary clarifier. This ensures that substances that could not be removed in the previous treatment steps settle. Feces or paper settle in the primary clarifier as "settling substances" or float on the surface. About 30 percent of the organic substances can be removed from the wastewater. The solid, separated components are also referred to as primary sludge.

Biological cleaning

In this process step, bacteria and other microorganisms break down organic wastewater constituents with the help of oxygen. Special bacteria break down nitrogen compounds. In relation to the biochemical oxygen demand (BOD), the cleaning performance of the sewage treatment plants with a combination of mechanical and biological cleaning is 90-95%. The cleaning performance results from the difference in the concentration of (pollutants) substances from inlet to outlet.

Aeration tank

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

Most of the municipal sewage treatment plants are operated using the activated sludge process described.

Secondary clarifier

The secondary clarifier forms a process unit with the activated sludge tank via the return sludge circuit. In the secondary clarifier, the activated sludge is separated from the treated wastewater by settling. Part of the settled sludge is returned to the activated sludge tank (return sludge) in order to enrich the bacteria that specialize in wastewater treatment in the activated sludge tank. The excess sludge, i.e. the increase in biomass that is 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 container in which controlled degradation processes are carried out without oxygen. This creates methane gases that can be used thermally.

Chemical cleaning stage

During this purification stage, phosphorus is primarily removed in municipal systems (phosphorus precipitation).

This process is of great importance in order to prevent water eutrophication. This means that an excess of nutrients must be prevented from entering the receiving water. This is the body of water into which the treated wastewater is discharged.

Phosphorus occurs dissolved in wastewater and in particulate form. In order to remove the phosphorus from the wastewater, it is brought into a form in the chemical cleaning stage, in which it can be drawn off as sludge. Part of the phosphorus is incorporated into the biomass when the organic carbon compounds break down, 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 sewage treatment plants, problematic substances such as heavy metals or salts are also removed in this purification stage.

Sludge treatment

The individual cleaning steps in wastewater treatment produce different types of sludge that have to be subjected to special treatment. It is important to achieve a stabilization of the sludge, i.e. organic carbon contained in the sludge should be broken down as far as possible so that odor disturbances can be avoided as far as possible. Another aim of sludge treatment is to reduce the volume of the sludge so that it can be used or disposed of.

The largest sewage treatment plant in Austria is the main sewage treatment plant Vienna / Simmering. It cleans all municipal wastewater in the federal capital. In dry weather, i.e. without additional rainwater, that is more than 500,000 m³ of wastewater per day.

1980 for originally two million population equivalents (EGW60), the system had to be expanded to include a second biological purification stage in 2005. Wastewater to the extent of four million PE60 can now be cleaned. The sewage sludge that arises - over 900,000 m³ annually - is dewatered and incinerated in fluidized bed furnaces belonging to the Simmering waste disposal company. From 2020, the sewage sludge is to be treated anaerobically before it is dewatered and incinerated. During this digestion process, sewage gas is produced, which will be used to generate electricity and heat.