Chriwa Group - Water matters. We care.

Aerobic biological processes

Activated sludge tank

The aerobic treatment of wastewater in the activated sludge tank is the most widely used treatment process in wastewater purification and has proven its worth over decades. In this process, organic and chemical impurities in the wastewater (BOD, COD, nitrogen and phosphorus) are degraded by aerobic biological processes. Various microorganisms, which need elemental or chemically bound oxygen for their oxidative metabolism, join together to form sludge flocs and convert the unwanted pollutants in the water into CO2 and further biomass.

The advantages for our customers are:

  1. High process reliability, simple and low-maintenance process technology, low investment and operating costs, precise control and adaptation of the purification process to changing influent values.
  2. In sufficiently dimensioned and carefully operated mechanical-biological wastewater treatment plants, wastewater can be purified to such an extent that fish can live in control aquaria for quality assurance purposes before being discharged into the receiving water.


SBR (Sequencing Batch Reactor)

An SBR is an activated sludge tank that is filled discontinuously. All processes of biological treatment such as aeration, mixing and secondary sedimentation take place in one and the same tank.

The process can be divided into 5 successive process stages:

  1.     Filling: The reactor is filled with wastewater and substrate to be decomposed. Aeration/mixing can already take place.
  2.     Aeration/mixing: The biomass in the reactor converts the mainly organic wastewater ingredients. By mixing alone, carbon and nitrogen decomposition can take place, if necessary iron salt is added for phosphorus precipitation.
  3.     Settling: The separation of biomass from wastewater takes place by sedimentation in the same tank. No additional secondary settling tank is necessary.
  4.     Effluent: The treated wastewater that is on the surface is drawn off to create capacity again for the next filling process.
  5.     Idling/sludge removal: Downtimes can occur - especially with several SBRs connected in series or in series. During this time, excess sludge can be removed from the basin, for example.

It is a robust, stable-running and easy-to-operate process that is very suitable for the treatment of wastewater from the soft-drink industry, among other things.


Biological phosphorus elimination

Phosphorus is an essential element for cell growth. Bacteria consist of about 1% of their dry mass of phosphorus. Some microorganisms can store phosphorus in their cells in the form of polyphosphates, allowing them to reach a phosphorus dry matter content of up to 7%. By using a system that stimulates the growth of these "polyphosphate-accumulating organisms (PAOs)", a large part of the contained phosphorus can be removed from the wastewater. The stimulation takes place through a rapid sequence of anaerobic and aerobic phases


Nitrification and denitrification

Similar to phosphorus, nitrogen is needed by all living organisms for cell growth. In nature, the nitrogen available to organisms is often a limiting factor. In order to prevent overfertilisation (eutrophication) and the associated uncontrolled growth of algae in the receiving water, certain nitrogen levels must be maintained in the treated clear water. In the case of high loads, targeted biological nitrogen elimination can be useful. This consists of two separate biological steps: Nitrification and denitrification.

Most of the nitrogen found in wastewater enters the treatment plant as ammonium (NH4) or bound in organic material. During nitrification, this nitrogen is converted into nitrate (NO3) under aerobic conditions and partially bound in the activated sludge. The nitrate produced during nitrification can be converted into gaseous nitrogen (N2) in an anoxic process (excluding dissolved oxygen). The nitrogen is removed from the wastewater in the excess sludge or as a gas and high clear water requirements can also be met. Over the years, many different methods have been developed for combining the two treatment steps. Biological nitrogen reduction can be carried out in one or more activated sludge tanks or SBRs. In this way, we align the plant perfectly to your needs during the planning phase.


MBBR (Moving Bed Biofilm Reactor)

In the MBBR process, the living biomass adheres to plastic growth bodies located in the bioreactor. A high specific surface area of the plastic bodies between 300 and 5500 m²/m³ allows a high active biomass density and thus smaller tank volumes. The plastic bodies are kept in suspension by aerators or agitators for optimal contact between biomass and substrate. Targeted nitrogen elimination is possible due to the high sludge age. Any excess sludge produced leaves the bioreactors in suspended form and is separated from the wastewater by lamella separators, flotation plants or conventional secondary clarifiers.

Further advantages of MBBR are a robust biological biocoenosis that withstands pH, temperature and organic load fluctuations well, as well as reduced sludge growth, which leads to lower disposal costs of the excess sludge.

The MBBR process is also very suitable for retrofitting existing wastewater treatment plants. By adding growth bodies to the activated sludge tank, either the effluent quality can be improved or the treatment capacity of the existing treatment plant can be expanded.


Rotating disk immersion reactor (RSR)

In a rotating immersion disk reactor, as in the MBBR, a biofilm growing on growth bodies is used for wastewater treatment. The biofilm is located on large horizontally fixed discs, about 1/3 of which are submerged in the wastewater. By slowly rotating these discs, the biofilm is regularly lifted out of the wastewater and then submerged again. Due to the contact between air and biofilm, the wastewater does not have to be additionally aerated to stimulate aerobic processes. This keeps the energy consumption and operating costs of an RSR low. However, since a large biofilm surface must be ensured, an RSR is particularly suitable for smaller wastewater volumes.


Trickling filter reactor

In a trickling filter reactor, as in the MBBR, a biofilm (often lava slag or plastic bodies) growing on growth bodies is used for wastewater treatment. Unlike the MBBR, however, the growth bodies are not immersed in the wastewater, but the wastewater is sprinkled over a bed of trickling filters. The wastewater slowly percolates to the bottom of the reactor where it is drawn off. Because of the contact with the ambient air, a trickling filter reactor does not need additional aeration and costs are saved. Disadvantages are a large space requirement and comparatively high investment costs.