Measuring flow rate at the digestion tower’s exit and in before the combined heat and power plant

Sewage gas / biogas quantity measurement in wastewater treatment



Process data

Measuring task:
Flow velocity for determining flow rate

Measuring point:
Directly at the digestion tower’s exit and on the other side before the combined heat and power plant and before the flare

Measuring range:
Up to 40 m/s

Process pressure

Process environment
ATEX Zone 1

ATEX Zone 0


Anaerobic degradation processes happen in the digestion tower during wastewater treatment in a sewage plant, causing biogas (also sewage or fermentation gas) to arise, which mainly consists of methane and carbon dioxide. 

Exact measuring of biogas quantity is one oft he most challenging applications in flow measuring technology. Biogas is very moist, tends to condensate and is particle-laden. Its composition varies and is quite aggressive. Combined with air it can turn into explosive mixtures which is why it’s important to take according safety measures. Applied instruments have to be certified for the defined ex zone

Fermentation gas is measured directly at the digestion tower’s exit and on the other side before the combined heat and power plant. Flow rate towards the flare is also partly measured.

Your advantage:

Safe measurements even in hazardous areas

Once installed, the sensor hardly needs any maintenance

Our vortex sensor is especially suited for condensate-laden fermentation gas that can also be particle-laden

Recommended products

Vortex VA40 ZG8 Ex-d / ATEX
Standard compact unit for hazardous applications, variable insertion depth

To the product Product data sheet

Vortex VA40 ZG4 with separate transducer
Variable insertion depth, wide selection of sensor material, max. +240 °C

To the product Product data sheet

Measuring flow rate at the digestion tower and before the combined heat and power plant

Sewage plants are a pivotal part of our wastewater treatment. Processing requires a lot of energy mainly for the biological treatment processes (ventilation). Municipal sewage plants are the greatest consumer of energy with about 20 % of the entire municipal use. There’s great potential for possible savings. Part of the required energy can be self generated in the sewage plant with the sewage gas from wastewater treatment.

How is fermentation gas created from sewage sludge?

Sewage sludge is collected in the digestion tower. Bacteria decompose it in a anaerobic degradation process (without oxygen) causing different gases in different amounts to occur.

Biogas, sewage gas, fermentation gas

are different names for the created gas in a digestion tower. Depending on the sewage sludge’s composition and the envorinmental circumstances each gas has its own consistency and therefore also another density. Main components are methane, carbon dioxide and hydrocarbon.

Methane is flammable and there for ideal for energy production. It is fed to a combustion engine which in return creates electricity and as well as heat. The heat can then also be further used energetically.

How can we measure the flow rate of biogas?

Due to its variable composition, the possible condensate and other contaminations many measuring instruments are not suited or need a lot of cleaning and maintenance.

Höntzsch vortex flow sensors measure flow rate irrespective of the gas‘ density. It has no moving parts making it unsusceptible against parts of condensate and other possible particles in the gas. Hardly any maintenance is required – „simply install and forget“.

Continuing operation in a sewage plant

The amount of sewage gas, occuring in the digestion tower varies. Therefore the gas is temporarily stored / buffered in a storage from where it can be fed to the combined heat and power plant.

Occurring sewage gas is measured directly after the source, being the digestion tower and before the combined heat and power plant to regulate power generation. In many plants flow rate is also measured towards the flare.

Many operators add natural gas to the biogas to ensure continuous generation of power. Höntzsch also offers fitting measuring equipment.

Making the measuring of sewage gas quick

For short term measurements to check the current status of a plant at a certain point our mobile sensor with a handheld device is the perfect option. The system is also available for ATEX zone 1.