Modern waste management concepts naturally include the separate collection and recycling of biological rubbish. In addition to composting, anaerobic fermentation is especially suitable for wet material with a low structure content.

The waste treatment process is divided into five stages. Initially, the biological waste delivered is gently chopped in a worm mill and any ferrous metals are removed by a magnetic separator. After this, a wet treatment follows. By adding water the organic components are pulped in a special waste decomposition container. Interfering materials such as plastic, textiles, wood, glass and mineral components are separated by the organic fraction and removed. No additional manual removal of the foreign substances is necessary. Thus a waste suspension is formed with a high share of dissolved organic materials.

In the purification process, the suspension is heat-treated at 70 °C for over an hour. This kills off micro-organisms, weed seeds and worm eggs. Heat exchangers integrated in the system enable a maximum heat recovery.

In the subsequent hydrolysis, the organic solids are decomposed in several decomposition stages to compounds which can be converted to methane, such as acetic acid. Through the continuously operated sold/fluid separation process, the decomposed products enter the methane reactor. Any solids are sent back to the hydrolysis reactor to undergo further treatment. The drainage of the solids which cannot be decomposed takes place in a centrifuge from where they are passed on to a composting plant for retreatment.

The dissolved organic compounds are anaerobically decomposed further through bacteria in a packed bed methane reactor. During this process a biological gas is produced with a methane concentration of 60 to 65%. A decomposition degree of 60 to 70% is achieved. This corresponds to a gas exploitation of approx. 120 Nm3 per tonne biological waste. Excess water is mechanically and biologically subjected to a sewage treatment so that it can be fed to municipal sewage plants.

The gas produced in the methane reactor is fed to a block power station for the generation of energy (electric power/heat) and to the heating facilities to heat the process water. Approx 25% of the energy produced is used for self-sufficient operation. The remainder is available for other uses.

To measure gas volume flows, a sturdy, reliable and accurate measurement method has to be used, as on the one hand continuous operation has to be ensured and on the other hand the quantities produced are contractually agreed upon.

An Ex-type Höntzsch Vortex measurement sensor is fitted in each of the supply lines to the block power station, the heating facilities and the gas flare. The Vortex measurement sensors enable the measurement in damp gasses whereby the flow speed is correctly recorded even if the composition of the gas mixture fluctuates.

The measurement principle is based on a Karman vortex street being formed in the Vortex sensor head at a vortex shedding element such as a triangular rod.

The vortices modulate a ultrasonic field located downstream behind the rod which is generated by a piezo transducer. The modulated ultrasonic signal is picked up by another piezo ceramic piece functioning as a microphone. As a constant ratio exists between the vortex shedding frequency and the flow speed in a broad speed range, the frequency signal indicates the flow speed.

The great advantage of ultrasonic scanning of the vortex street compared with other scanning processes, which for example measure the force effect on the interference body, is that very low flow speeds can be measured even from 0.5 m/s upwards!

As the measurement sensor has to be intrinsically safe for the application described, the frequency signal proportional to the flow speed is converted into a resistance decoupled signal by an Ex-type transducer LDX. This signal is then transformed in a transducer U2a to a flow proportional analogue signal which is available as measurand for the control and monitoring of the plant.