The mass balance is a mandatory accounting document that tracks the flow of sustainable material throughout the biomethane value chain. It ensures that the volume of renewable gas claimed by an end-user corresponds precisely to the quantity of verified sustainable feedstock processed.

At the production stage, the biomethane producer maintains a mass balance to reconcile feedstock inputs with gas outputs. This record accounts for conversion efficiencies and internal energy use, ensuring the volume of sustainable gas injected into the grid does not exceed the energy content of the compliant organic waste. This quantitative log is the prerequisite for issuing a Proof of Sustainability (PoS), which provides the verified environmental data for that specific batch.

Throughout the midstream, grid operators and traders maintain their own mass balance records to track these "virtual" volumes as they move through the pipeline infrastructure. These logs are reviewed by third-party auditors and recorded in national registries or the Union Database (UDB). This continuous chain ensures that the sustainability attributes remain attached to the gas volume even as ownership changes, preventing the double-counting of renewable attributes across different markets.

Ultimately, the supplier provides the verified mass balance data and the associated PoS to the end-user, such as a transport fleet or an industrial plant under the EU Emissions Trading System (ETS). This documentation allows the consumer to legally "zero-rate" their emissions or meet mandatory renewable fuel targets. By providing a transparent audit trail from feedstock to consumption, the mass balance document maintains the regulatory integrity of the European biomethane market.