Mixers do not account for a significant share of the overall budget of a biogas plant. Perhaps that is why investors often underestimate their importance and leave the selection of the mixing technology entirely to contractors. This is a mistake, as the mixing system plays a crucial role in the efficiency and profitability of a biogas plant, says Mirosława Dyjak-Wojciechowska, SUMA’s new Area Sales Manager for Central and Eastern Europe, including Serbia.

Agitators are standard equipment in every biogas plant. What role do they play in digesters, and how do they affect operating costs?

Agitators ensure thorough homogenisation of the substrate while distributing heat and nutrients evenly throughout the digester. This creates optimal conditions for the fermentation bacteria, directly influencing biogas production and process stability.

The efficiency of the mixing system also determines the required mixing time and, consequently, the plant’s own electricity consumption. In addition, the service life of agitators depends largely on their operating hours. Since mixers operate under demanding conditions, especially with substrates containing high levels of dry matter, maintenance and servicing can become a significant operating cost.

Incorrect mixer selection often results in recurring problems, including frequent repairs, equipment failures, membrane roof removal, tank cleaning and, most importantly, production losses.

What does proper mixing in biogas tanks mean?

Proper mixing starts with a detailed analysis of the available feedstock, including its physicochemical properties, temperature, pH and dry matter content. Based on this information, engineers can determine the most suitable type of agitator (high-, medium- or low-speed) as well as the appropriate materials and design.

Another important aspect is planning the substrate preparation process before feeding it into the digester. Once the tank dimensions are known, the required watts per cubic metre can be calculated. This parameter is essential for determining the number of agitators required, together with their motor power and thrust.

The characteristics of the substrate, including its tendency to form floating layers or sediment, also determine the optimal positioning of the agitators.

At the design stage, these assumptions should be verified using Computational Fluid Dynamics (CFD). CFD simulations make it possible to predict the behaviour of the substrate inside the tank, verify whether the planned mixing system achieves the required average flow velocity throughout the digester, analyse different filling levels and identify potential dead zones that reduce the effective tank volume.

The simulation can also evaluate mixing performance under various operating conditions and demonstrate the benefits of adjusting the position of the agitators vertically or horizontally during operation.

Does mixing optimisation involve only selecting the right equipment?

No. When choosing a mixing system for biogas digesters, investors should also consider installation, maintenance and long-term service requirements.

It is advisable to review the manufacturer’s maintenance recommendations to understand what service work will be required and how frequently it must be carried out to ensure reliable operation.

This may lead to selecting agitators with easier service access, such as side-entry or central mixers, or submersible agitators equipped with service boxes that allow maintenance without interrupting plant operation.

The objective is to minimise downtime and ensure continuous mixing, because efficient mixing is ultimately about preventing disruptions to the biological process.

What advice would you give to investors entering the biogas industry regarding mixing technology?

A biogas plant is, in many ways, a sophisticated system that converts organic waste into renewable energy. However, the biological process is neither maintenance-free nor inexpensive. Achieving high biogas yields requires careful planning and a thorough understanding of the technology long before construction begins.

A properly designed mixing system should provide investors with trouble-free operation throughout the plant’s lifetime. Ideally, once the mixers have been selected and installed, they should operate reliably with minimal intervention.

To achieve that, however, investors need to invest time in understanding the fundamentals of mixing technology before making purchasing decisions.

Biogas can be an excellent business – but only when every component of the process, including mixers, has been carefully planned and optimised.