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Header image to illustrate the page: Classical wastewater treatment: conventional activated sludge process
Activated sludge plant, aerial view
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Biological processes

Biological treatment is used for the removal of organic carbon and nutrients (N and P) removal. They can be aerobic, anoxic or anaerobic. Aerobic processes are the widely employed, most commonly as the conventional activated sludge (CAS) process.

Key biological process parameters are the hydraulic and solids retention times, which determine the mixed liquor suspended solids (MLSS) concentration and food to micro-organism (F:M) ratio.

Graphical image to demonstrate biotreatment process configurations in membrane bioreactors

Biotreatment process configurations

The biochemical action of a biological process is dependent on whether oxygen is present, and in what form. Oxidative ‘aerobic’ processes require dissolved oxygen to convert organic carbon to carbon dioxide and ammonia to nitrate.

Biotreatment process configurations in membrane bioreactors

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Nutrient removal: N&P

Nitrate formed from nitrification of ammonia can be converted to nitrogen gas by biochemical reaction with organic carbon. This is an example of an 'anoxic' process and known as ‘denitrification’.

MBR operation and maintenance − nutrient removal

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Sludge characteristics: biological

MBR sludge is characterised by two different types of microorganism - those responsible for carrying out biochemical functions and pathogens.

Sludge characteristics: biological
Foaming sludge

Sludge characteristics: rheological

Sludge rheology relates to how it flows. This is in turn characterised by its viscosity, which is defined as the relationship between the shear stress and the shear rate.

Sludge characteristics: rheological