Minimising whole life costs of mixing Anoxic and Anaerobic Tanks

Höfken, M., Steidl, W., Huber, P. and Björndal, J.,  INVENT Umwelt- und Verfahrenstechnik AG, Germany


In the field of wastewater treatment, efficient mixers are needed to suspend solids and homogenise in large mixing and equalization tanks, especially in anaerobic and anoxic tanks for full nutrient removal in the activated sludge process. The layout and design of these mixing systems must also take into account process and reactor design. Otherwise the plant will not perform properly, and will not comply with the specification.

This paper describes the basic demands on a mixing system. It then details the layout and design process for large anaerobic basins at large-scale wastewater treatment plants, from basic design to lab scale testing, CFD simulation and large scale testing after the start-up of the plant.


Water purification is one of the most important environmental tasks today. This includes the physico-chemical treatment of natural water for the supply of drinking water and the biological treatment of wastewater before it is discharged. The status of wastewater treatment varies from country to country. Most of the industrialized countries have a very good coverage in wastewater treatment plants with more than 90 % of the inhabitants connected; whereas less developed countries continue to suffer from lack of sewage collection systems, water and wastewater treatment plants. The quality of the wastewater treatment can be segmented into the following types of treatment:

  1. Primary Treatment: Mechanical treatment using screens and primary sedimentation to separate solid matter.
  2. Secondary Treatment: Includes the oxidation of biodegradable organics, usually referred to as BOD[1] and COD[2] by means of additional oxygen. The most commonly applied process is the so-called activated sludge process.
  3. Tertiary Treatment: Includes the biological reduction of BOD and COD plus full nutrient removal. In this case nitrates and ammonia are also removed by using anaerobic processes. In these processes the wastewater needs to be mixed without the addition of oxygen.

In the EU tertiary treatment for full nutrient removal is obligatory for all member states since May 1991 (EEC, 1991). This is a standard which currently has not yet been reached in all member states. Many plants still have to be built and many have to be upgraded – a situation which is typical for the status of wastewater treatment in many countries in the world. This is the reason why the activated sludge process including mixing and aeration has become a key topic in wastewater treatment.

The activated sludge process stands for a large variety of continuous, space and time-oriented periodic processes. As mentioned before, the activated sludge process is used for BOD-removal and complete nitrogen removal. Well-known variants are the preliminary nitrification/denitrification and the post-denitrification (Metcalf and Eddy, 1991). Well-known batch process variants are the Sequencing Batch Activated Sludge Reactor (SBASR), the Cyclic Activated Sludge System (CASS) and the Intermittent Cycle Extended Aeration System (ICEAS) which are covered in detail in the literature: Irvine, 1971; Wilderer and Schroeder, 1986 or Metcalf and Eddy, 1991. All these biological purification processes as well as the known standard activated sludge process include two major operations: mixing and aeration. Without mixing and aeration an effective reduction of BOD and nitrogen is impossible. Since approximately 70% of the total energy consumption is used for these two processes (Höfken et al., 1995) a proper choice and design of such systems is indispensable. Hence, only innovative and energy-efficient equipment should be used.

The above reasons clarify the importance of discussing the role and impact of mixing and aeration processes on the biological treatment process. The paper at hand focuses on mixing and presents the basic design rules for the selection and layout of mixers for activated sludge treatment plants. Using the example of the design of mixers for a large wastewater treatment plant it is shown how an intelligent selection and design of mixers can help to improve the reactor behavior and reduce the investment and operating costs of a plant.

[1] BOD: Biological Oxygen Demand

[2] COD: Chemical Oxygen Demand

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