Biological Treatment



Biological treatment is the most commonly used method for additional treatment of wastewater to remove high amounts of organic matter and nutrients following mechanical treatment and is one of the largest and most expensive elements at a wastewater treatment plant.

The activated sludge process that is most common in biological treatment utilises the growth of specific floc-forming bacteria that live suspended in the wastewater. To create optimal conditions for the bacteria, a certain retention time for wastewater and sludge is necessary for the biological processes to take place.

During biological treatment, bacteria circulate between different process zones which provide them with specific living conditions. The different zones use aerobic, anoxic and anaerobic conditions utilising different biological processes for the removal of organic matter and nutrients, such as Nitrogen and Phosphorous.

For circulation of wastewater between these zones the Grundfos SRP recirculation pumps keep vast flows at low head moving reliably.  

Where tank design doesn’t allow the use of flowmakers, Grundfos AMD and AMG mixers offer a robust design with corrosion resistant materials, easy service and maintenance, and trouble-free operation in difficult environments.

Recirculation of sludge from the secondary clarifier to the aerobic tank is a key process of the activated sludge process. This can be reliably handled with the Grundfos ranges of SE/SL, S, and KPL/KWM pumps, which are specifically designed for durable use and flow management, and can be installed dry or submerged. Variable speed drives (Grundfos CUE) can be added, depending on the specific control strategy at the treatment plant.

Grundfos can supply complete dosing pump systems for large or small volumes, based on different technologies. These pumps can be used for pH adjustment in the process tank and for substrate dosing, for example to keep biological phosphorous removal and de-nitrification running.

Secondary clarification

Following biological treatment, the treated water must be separated from the activated sludge, which is done in the secondary clarifiers. The activated sludge is slightly heavier than water and separation is achieved by letting the sludge flocs settle by gravity, which is aided by design of the clarifier to ensure low turbulence and velocity.

The Grundfos ranges of SE/SL or S pumps with a vortex impeller can handle a dry matter concentration of up to 5% for the return pumping of sludge. Variable speed drives (Grundfos CUE) can be added, dependent on the specific control strategy at the treatment plant.

The majority of the settled sludge is then returned to the process tank to be utilised one more time, and an amount equal to the daily production is pumped to sludge treatment. Recirculation of sludge ensures that a certain sludge age is attained which accounts for its ability to clean the wastewater effectively.

Chemicals and poly-electrolytes can be added, to induce coagulation and improve sludge settling. Grundfos can supply complete dosing pumps systems for large or small volumes and based on different technologies for precipitation and flocculation and PLC-controlled, fully automatic systems to handle preparation and the reliable dosing of organic coagulants or poly-electrolytes.

Grundfos has the specialist expertise to help meet your performance objectives, from the initial identification of needs, to the selection, installation, operation, and maintenance of the solution for your process tank. Computational Fluid Dynamics (CFD) simulations can be used to optimise the total hydraulic solution for aeration and mixing. Grundfos commissioning agreements ensure correct installation and service agreements cover all eventualities, from routine maintenance to spare parts.

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