Aerobic biological treatment

Aeration, denitrification and separation

  • Energy efficiency
  • Continuous discharge possible
  • Bacteria separation


After the waste water has been purified of coarse particles, greases and emulsions in an earlier stage, it may, for various reasons, be desirable to apply biological treatment. The purification yield can be increased considerably through biological water purification processes as they decompose dissolved organic substances as well 24 hours a day, 365 days a year. Furthermore, it is a first step for the re-use of waste water.

The main process involves air (oxygen) being introduced by an aeration system into a mixture of wastewater combined with organisms to develop a biological floc which reduces the organic content. The bacterial material, which in healthy sludge is a brown floc, is largely composed of saprotrophic bacteria but also has an important protozoan flora. The bacteria will only decompose all pollutants in harmless products if the amount of pollutants (food) per bacteria is kept very low. Under these conditions the bacteria will almost starve and will decompose all the food as much as possible in order to release sufficient energy to keep their life support systems running. Therefore in the process they will hardly get enough food to grow or multiply. This system of continuous starvation of bacteria is called a low loaded activated sludge system. The mixed liquor is kept from separating by turbulence caused by the aeration system and/or separate mixers. During aeration periods the pollutants will be gradually decomposed (oxidized) by the bacteria. Eventually almost all the pollutants will be converted into carbon dioxide, water and nitrate. These components do not contribute to the BOD and COD level.

Nitrate in high concentrations is not desired in the effluent as it is an excellent fertilizer and can cause severe algae growth, resulting in extinction of flora and fauna in the stream. Converting the nitrate into nitrogen gas can reduce the nitrogen concentration. This process is called denitrification and takes place in absence of oxygen, in either a separate basin or in the aeration basin during periods when the aeration has been switched off.

Separation can be done means of settling in a BioClar® clarifier or in the aeration tank (BioArt® SBR), by air flotation in a BioFlot® system or by Membrane Bio Filtration (MBR).
Part of the settled material, the sludge, is returned to the head of the aeration system to
re-seed the new wastewater entering the tank. This fraction of the floc is called return activated sludge (R.A.S.). Excess sludge is called waste activated sludge (W.A.S) and is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance


There is a choice of two systems:

  • Depth aeration - Air is introduced into the water through aeration elements that are installed on the bottom of the basin. The advantage of this system is that it is very energy efficient.

  • Surface aeration - Water is pushed up and transferred through the air. This way, oxygen is introduced and the water is mixed.

2) Denitrification is the conversion of nitrate to gaseous nitrogen. This process is also referred to as removal of nutrients. The advantage of denitrification is that it reduces the amount of oxygen required for the digestion of organic compounds in the aeration basin.

Marel Water Treatment disposes of three different systems:

  • Filtration
    Marel’s BioBrane is a compact membrane bioreactor (MBR) system that works with high sludge concentrations. Since high concentrations of bacteria are used, the system can be built more compact. In addition, a very high-quality effluent is obtained by the filtering action of the membranes.

  • Flotation
    Marel’s BioFlot® system separates bacterial colonies, and can maintain high bacteria concentrations so that small basins will suffice. BioFlot® is virtually insensitive to filamentous bacteria and therefore ideal for wastewater with largely dissolved pollution. The advantage of this system is the higher dry matter percentage of the separated sludge, reducing disposal costs.

  • Sedimentation

Bacteria can also be separated by sedimentation. The sedimentation separation can be applied in the aeration tank via Marel’s SBR system BioArt, or a separate clarifier called BioClar.


Marel offers a range of service solutions to prevent failures, maximize performance or in case of unexpected breakdowns remote or onsite assistance.

Marel has offices in 30 countries across all regions, and a global network of highly skilled professionals that provide remote as well as onsite service support. We provide quality spare parts and customized service agreements for our customers to ensure optimal performance of Marel equipment.

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