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Sewage sludge thickening with Flottweg decanter centrifuges

The thickening of excess sludge is often not given the attention it actually deserves. Thickening is an important process stage that affects the removal of sludge from the secondary clarification plant as well as the feeding of the digester. Sludge removal from the secondary clarificaiton plant should be carried out in line with the technical requirements of secondary clarification. In other words, after the sludge has accumulated or also to prevent the sludge from being discharged in heavy rain. Digester feeding should be done at the most consistent rate possible, both hydraulically as well as in terms of the solid mass flow. These objectives are often contradictory.
The task of sewage sludge thickening is then to reconcile these two requirements. This can be achieved with a Flottweg thickening plant, fully regulated and optimized for unattended 24/7 operation.

Our decanter centrifuges for sludge thickening

The sewage sludge thickening process with Flottweg separation technology

An OSE decanter is the central element—OSE stands for the German phrase for “optimum sludge thickening.” This machine is optimized for this particular task. Thickening is achieved with the help of a centrifugal field (i.e., a physical process). Very small quantities of flocculant are usually used to smooth the centrate (liquid phase). However, the key to resolving the conflicting goals of removing sludge and feeding the digester is controllability. A sensor measures the concentration in the thickened sludge. This signal is used to control the machine so that the concentration always remains at the value defined by the operator (e.g., 6%). This also keeps the hydraulic quantity at a constant level. The digester is supplied on a constant and continuous basis. This control setting compensates for fluctuating inflow conditions for thickening due to changing conditions in the sludge discharge. This can happen, for example, when there is a heavy rainfall. An additional sensor determines the concentration in the inflow and regulates the addition of polymers. This saves polymers and ensures a consistently high degree of separation.

But why all this?

The focus here is on three key aspects: reliability , increase in efficiency, and cost-savings.

The Flottweg thickening plant guarantees operational safety thanks to a highly reliable machine and effective controls. It serves as a reliable buffer between upstream and downstream process steps, especially during unmanned hours in the sewage treatment plant. The increase in efficiency results from a consistently high degree of separation, which prevents reverse contamination.

Are you facing a particular challenge or an unusual process? From the treatment of mixed sludge to primary sludge, we support you with individual solutions to optimize your process.

Overall sludge treatment process with Flottweg decanter centrifuges Overall sludge treatment process with Flottweg decanter centrifuges

The advantages offered to you through sewage sludge thickening


Automated and reliable operation

The thickening plant operates nearly independently and requires no intervention by the operating personnel. Continuous 24/7 operation is also possible without problems—there is also no need to shut down for rinsing processes. Flottweg decanters are also designed to require little maintenance in the long term.


Compact design

A Flottweg decanter for thickening purposes stands out thanks to its very small space requirements and its high throughput rates at the same time. This often allows for integration and installation into existing buildings. It is also possible to operate the plant when the sludge conditions change. Adjustment to the changed process conditions can also be done quickly.


Automatic monitoring of centrate quality

The degree of separation is an important indicator for sludge thickening. The higher the degree of separation and the cleaner the centrate, the lower the reverse contamination for the sewage treatment plant. With the OSE decanter, a separation efficiency of >95% can be achieved permanently, often even >98%. For control purposes, a sensor monitors the centrate quality and enables rapid intervention in the event of deviations.


Closed design

Flottweg thickening plants have a closed design to prevent odor emissions. Biologically contaminated aerosols are not released into the environment. This design and the low rotor speed mean noise emissions are also kept low and ensure very quiet operation.


Minimal polymer requirement

Only very small quantities of polymers need to be added during sewage sludge thickening. Depending on the sludge properties, this might not be necessary at all. After the initial setup, the control system automatically regulates the the precise addition of polymers. Complete flocculation is not required.
Good to know: In addition to the usual synthetic polymers, biodegradable polymers based on chitin or starch can also be used!


Optimized process control of the sewage treatment plant

Sludge treatment with Flottweg decanter centrifuges ensures optimized process control in your sewage treatment plant. The constant feeding of the digester leads to an optimized digestion process. The result is an increased digestion rate accompanied by an improved gas yield. This improves the sewage treatment plant’s cost balance.

Decanter centrifuges for sewage sludge treatment in Fritzens

The sewage treatment plant started operations in 1995 and is part of the Abwasserverband Hall in Tirol – Fritzens (Wastewater Association of Hall in Tyrol – Fritzens). The association treats wastewater from 16 municipalities with around 60,000 inhabitants as well as some industrial and commercial zones. A change in the waste treatment process made it necessary to compensate the dry substance loss. This task is now performed by a Flottweg OSE decanter, which was specially designed for thickening.

In the following video, you can find out which challenges the sewage treatment plant in Fritzens has overcome and how sewage sludge treatment is now carried out successfully:

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