Warum die Rührwerkstechnik maßgeblich über die Effizienz einer Biogasanlage entscheidet

In discussions about the efficiency of biogas plants, the focus is often on issues such as substrate quality, feeding strategy or biological process control. However, one crucial factor is often underestimated: agitator technology.

An optimally designed agitator not only ensures that the contents of the digester are thoroughly mixed, but also directly influences biological activity, the plant’s energy consumption and, ultimately, the economic viability of the entire process. At the same time, the agitator is one of the largest consumers of electricity in a biogas plant. Even minor improvements to flow patterns or positioning can therefore lead to significant energy savings over the plant’s operational lifespan.

Why agitation is necessary at all

During the anaerobic digestion process, different layers of material form inside the digester. Light fibrous materials and plant matter tend to rise to the surface and form floating layers. Heavier mineral components or sand, on the other hand, settle at the bottom of the tank and form sediment layers.

Without continuous or demand-driven mixing, areas develop where biological activity is virtually non-existent. The usable fermenter volume decreases, substrates are converted unevenly and gas production may fall.

An effective agitator system therefore fulfils several tasks simultaneously:

• Homogenisation of the entire fermenter contents
• Uniform distribution of microorganisms
• Improvement of mass and heat transfer
• Prevention of floating and sinking layers
• Release of trapped gas bubbles
• Support for uniform biological conversion

The aim here is not to maximise movement within the fermenter, but to create a targeted flow that reaches all areas of the vessel and avoids dead zones.

Dead zones – an underestimated problem

In almost every fermenter, areas with low flow velocity develop. These so-called dead zones are often found in the corners of the vessel, behind internal fittings or in the lower part of the vessel.

Solids tend to accumulate in these areas. At the same time, fewer microorganisms and fresh substrate reach these areas. The result is deposits, a reduced reaction volume and poorer utilisation of the available fermenter capacity.

By selecting the correct propeller geometry, installation position and angle of attack, such dead zones can be significantly reduced.

The challenge: Different substrates require different mixing approaches

Modern biogas plants now operate with a wide variety of feedstocks. In addition to maize silage, farmyard manure, solid manure, whole-crop silage and residues from the food industry are increasingly being used.

Each substrate has different rheological properties.

Whilst thin slurry can be mixed relatively easily, both the viscosity and the power requirements of the mixing system increase as the dry matter content rises.

Fibre-rich substrates, in particular, place high demands on flow patterns within the digester. Here, high motor power alone is not sufficient. Rather, the decisive factor is the combination of propeller diameter, rotational speed and positioning.

Flow rather than power

A common misconception is to judge the performance of an agitator solely on the basis of motor power.

In fact, however, it is primarily the flow generated that determines the quality of the mixing.

Large propellers operating at an appropriate speed can often generate a significantly higher flow rate than smaller, high-speed systems – whilst consuming less energy.

For operators, this means:

It is not necessarily the highest connected load that leads to the best mixing results, but rather the flow-dynamic design optimised for the specific fermenter.

Energy efficiency as an economic factor

Depending on the plant design, agitation technology accounts for several per cent of a biogas plant’s total self-consumption of electricity. Over an operating life of 15 to 20 years, this results in considerable energy costs.

For this reason, optimising existing agitators is becoming increasingly important.

Possible measures include, for example:

• Adjustment of the propeller geometry
• Optimising the installation position
• Use of energy-efficient drives
• Intermittent operation instead of continuous operation
• Adjusting the speed to actual process requirements

Even minor optimisations can reduce specific energy consumption without compromising biological process stability.

Modern agitator designs rely on bespoke engineering

There is no one-size-fits-all agitator that is equally suitable for every biogas plant.

Key factors include:

• Tank diameter
• Fill level
• Fermenter volume
• Dry matter content
• Substrate mixture
• Feeding method
• Number of fermenters
• Operating strategy

These parameters determine the optimal mixing concept.

Depending on the application, different agitator systems are used – for example, rod agitators, paddle agitators or submersible motor agitators. Often, it is only by combining different systems that the desired flow is achieved throughout the entire tank.

When is it worth modernising?

Many biogas plants were designed ten or fifteen years ago under different conditions.

Today, both the substrates used and the operating methods have often changed.

Typical indications of potential for optimisation include:

• recurring floating layers
• sediment build-up
• increasing electricity consumption
• unstable biological process
• declining gas yields
• high maintenance costs

In such cases, it may be advisable to review the existing agitation system. The aim is not necessarily to replace all components, but rather to make technical and economically viable adjustments to suit the current operating conditions.

Conclusion

Agitation technology is far more than just a mechanical auxiliary unit. It influences virtually all key processes within a biogas plant – from biological conversion and gas release right through to the plant’s own electricity consumption.

Careful design, taking into account digester geometry, substrate properties and operating mode, lays the foundation for stable process control and cost-effective plant operation.

Those who regularly review their agitation technology and adapt it to changing requirements where necessary can not only optimise energy consumption but also make better use of existing digester capacity and enhance operational reliability in the long term.