The technical aspects of the project

From wastewater to biogas…

With a treatment capacity of 1,000,000 inhabitant equivalents, the Wantzenau Wastewater Treatment Plant (WWTP) is the fourth largest WWTP in France. The biogas produced by the digesters of this facility, in a volume of about 2.5 million cubic meters per year, is currently fully utilised on site for combined generation of heat and power and as a substitute for fuel oil for the heat treatment of the sludge.

The main stages of the biogas extraction process from wastewater:

  • Settling the wastewater and extraction of the primary sludge.
  • Biological purification and clarification steps resulting in extraction of the biological sludge.
  • After thickening, some of the primary and biological sludge is conveyed to two digesters. This is where an airless (anaerobic) fermentation process takes place, releasing biogas.
  • The resulting biogas is stored in a gas holder. In the current configuration of the wastewater treatment plant, this gas is utilised on site, primarily for heating the digesters and the buildings and for the heat treatment of the sludge.

…and from biogas to biomethane:

In-house utilisation of the biogas has significantly reduced the environmental footprint of the Strasbourg-Wantzenau wastewater treatment plant by preventing release of greenhouse gas emissions into the atmosphere. However, another technology is opening up even better prospects in terms of energy efficiency and environmental relevance, i.e., the injection of biomethane into the natural gas distribution network.

To reach this goal, the existing anaerobic digestion facilities will be expanded to include a dedicated biogas filtration and purification system as a prerequisite for its injection into the distribution network. This purification process allows converting ‘raw’ biogas into a gas that is similar to natural gas with a methane content of 97% or higher. This gas will therefore be fully compliant with the requirements of the distributor (Réseau GDS) and current regulations. These regulations define very precisely the characteristics required of gas that can be injected into the distributor’s network, irrespective of its source.

It is this process of converting biogas into biomethane that lies at the core of the Biovalsan project.

The project team is currently conducting qualification testing for several purification techniques.

Getting the biomethane in the pipes: odorisation, compression and injection

Before being sent to the compression and injection station for feeding into the network, the biomethane undergoes a final process of odorisation to meet the distributor’s requirements. At this point, nothing can distinguish it from standard natural gas that flows through the distribution network.

In accordance with its mission as Distribution Network Operator, Réseau GDS monitors the quality of the gas and controls the injected amount and metering of the volume of gas at the feed-in point.

Recycling biogenic CO2

Although the main purpose of the Biovalsan method is producing high-quality biomethane, it also enables the full recovery of the CO2 contents of the primary biogas.

This is not a waste by-product. Known as biogenic CO2 (or Bio-CO2), this gas could be used for local purposes (for example, as a natural accelerator of photosynthesis for greenhouse crops). During their growth period, plants would thus convert CO2 into vegetable matter, according to the model of the short carbon cycle.

In fact, the project team is currently examining various sectors in which bioCO2 could be utilised.

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