Activated sludge process for small sewage treatment plants

The activated sludge process is a method of biological wastewater treatment. It is widely applicable and provides good cleaning performance. The activated sludge process also has some disadvantages.


In the activated sludge process, a bacterial mass dissolved in the water purifies the water and removes organic matter from the wastewater. The mass of bacteria is called activated sludge.

The supply line can either be set continuously in continuous operation or at individual intervals. These intervals are called "sequence batch".

Process procedure Bel Activated sludge from a previous clarification is already stored in the aeration tank. The wastewater runs on the activated sludge layer. Thereafter, aeration of the waste water to increase the oxygen content and to promote the bacterial activity.

The water-sludge mixture is then transported to the secondary clarifier, where the sludge settles. The purified water is then removed. For the relief of sludge residues, it usually runs over an edge or a small weir.

A part of the sediment is removed to keep the bacteria concentration constant. The largest part is transported as return sludge back to the aeration tank.

Sequence Batch Operation

In Sequence Batch mode, all processes take place consecutively. This only requires a single basin.

Benefits of the activated sludge process

High cleaning performance

  • Particularly good reduction of nitrogen, chemical oxygen demand and biochemical oxygen demand
  • Generally usable
  • Disadvantages of the activated sludge process

High investment costs

  • Sensitivity to hydraulic overload but also under load
  • Higher energy and maintenance costs than other technologies
  • Possible Problems with the activated sludge process

Formation of bulking sludge by special microorganisms, especially in the case of insufficient utilization

  • Floating sludge formation by certain microorganisms and too high a content of surfactants in the waste water
  • Also waste water fungus can occasionally be a problem
  • Tips & Tricks Even in the case of very small-sized plants, even cheaper ones can be offered and less maintenance-intensive alternatives for a small wastewater treatment plant. But this is always dependent on the required cleaning result.
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