Mass Balance in Sustainable Supply Chains

How Does Mass Balance Work in Practice?

The core idea of mass balance application is bookkeeping: for a given factory or supply chain, keep a record of sustainable inputs and make sure you don’t allocate more “sustainable output” than you put in. Here’s a high-level look at the process: 

Mass balance example: Consider a factory that uses a mix of 30% certified sustainable feedstock and 70% conventional feedstock in its production. All the material gets mixed together in the process. Through the mass balance approach, the operator assigns that 30% sustainable portion to the outputs in an equivalent way.  

In practice, this could be done in two ways:  

1. They might label every product as 30% sustainable content, or; 

2. They could certify a certain subset of the output as 100% sustainable (up to that same 30% of total volume) and the rest as 0%.  

Both approaches reflect the same overall reality – that 30% of the raw materials were sustainable. What they cannot do is claim more than that 30% across the outputs. In our example, after accounting for any process losses, the company could claim, say, 270 kg of product as “100% recycled content” or 540 kg as “50% recycled,” but not magically certify all output as recycled. Even if a particular item off the line contains only conventional material, another item will contain the equivalent sustainable share, such that across the whole production run, the math adds up. 

The crucial point is that the total sustainable input equals the total sustainable output claimed. It might be impossible to tell which molecules of plastic in a wrapper came from bio or circular feedstocks, but we know that the manufacturer used, for example, 1 tonne of circular feedstock for every 3 tonnes of plastic they made, so effectively one-third of their production by weight displaced fossil-based material. In other words, the environmental benefit is real, the fossil feedstock was replaced by the sustainable feedstock upstream, even if the mix is homogeneous. 

To implement mass balance, companies set up internal systems to track materials. Typically, they maintain a mass balance ledger (record) at each site. Every time they receive certified sustainable material, they add it to the ledger; every time they ship out product and want to claim some sustainability attribute, they deduct it.  

Conversion factors are applied to account for yields and losses, for instance, if 100 kg of bio-feedstock yields 90 kg of product, they can’t claim more than 90 kg of “bio-based product” from that input. These rules ensure the accounting is honest and conservative. 

Critically, independent certification bodies audit these records. A company can’t just self-declare; they are usually part of a certification scheme that lays out the mass balance rules. Auditors will verify that the inputs, outputs, and claims all line up (often checking over a defined period, like quarterly or annually). The process is governed by standard, for example, ISO 22095:2020. It is a chain-of-custody standard that outlines how to do mass balance bookkeeping, and many certification schemes follow similar principles.  

Companies also issue or receive ‘proof of sustainability’ declarations with shipments: when sustainable material is transferred between sites or companies, a document travels with it declaring the quantity and characteristics that are sustainable. This paper trail means that if you follow the chain upstream, you can find where and how the sustainable portion originated. 
 

Example of Carbon Central's Proof of Sustainability Certificate.

In summary, mass balance in practice works like a controlled accounting system for sustainability.  

Examples of Mass Balance in Action 

Mass balance is applied in many sectors. Here are three prominent examples: 

• Biogas / Renewable Gas: Biogas (or biomethane when purified) is a renewable gas produced from organic waste, and it can be injected into existing natural gas pipelines. The challenge is that once in the grid, the biogas mixes with fossil natural gas. Mass balance principles are used to handle this: gas producers inject a certain volume of biomethane into the network and issue certificates or credits for that amount.  
  Customers (like a factory or home) at the other end can purchase those credits and claim they used that equivalent volume of renewable gas, even though physically the molecules they burn may be mixed. What matters is that the overall balance is respected – no one claims more green gas than was actually put in. This system, often called a book-and-claim, underpins many Green Gas certification schemes. It ensures renewable energy usage can scale up using the existing gas infrastructure. For compliance with emissions targets (e.g. in EU climate policies), each unit of biomethane must be tracked.  Certification schemes (like ISCC or national biogas registries) certify the sustainability of the biogas and use mass balance accounting to link injections to end-use claims. This way, a city bus fleet can proudly say it runs on renewable gas, given that the equivalent biogas was fed into the grid on their behalf and no one else is claiming it. 
  
  

• Sustainable Aviation Fuel (SAF): Aviation is embracing mass balance for bio‑jet fuel. SAF production often blends green hydrogen, bio‑oils or synthetic kerosene with conventional jet fuel. Because the final jet fuel is chemically uniform, the industry uses a book‑and‑claim chain-of-custody. As the IATA guidance notes, book-and-claim allows “organisations to claim and trade the environmental benefits” of SAF via certificates, separate from the physical fuel. In effect, airlines or fuel blenders can buy SAF credits from certified producers and claim the corresponding fuel as renewable, even if the actual kerosene delivered is a mix. This system ensures that if an airline credits using 100 tons of SAF, then 100 tons of genuine biofuel were produced and entered the market, keeping the carbon accounting honest. 
  
  

• Bio-based / Circular Feedstocks (Chemicals & Plastics): The chemicals industry uses mass balance to incorporate recycled or plant-based inputs. For example, chemical recycling converts plastic waste into pyrolysis oil, which is then fed into an existing petrochemical cracker. BASF’s ChemCycling® program illustrates this: mixed plastic waste is turned into pyrolysis oil and co-processed with naphtha. The resulting polymers are the same as conventional plastics, but BASF claims an equivalent share as “recycled content.” 
  
  Similarly, many refineries co-process waste cooking oil or vegetable oil with crude: if 5% of the total feed is bio-oil, they can label 5% of the fuel output as renewable diesel or bio-jet. These mass-balanced bio-attributed products (often certified by ISCC PLUS or equivalent) allow companies to market “X% bio-based content” without any physical segregation. This is vital for meeting regulations (e.g. recycled plastic quotas) and for gradually building a circular raw materials market. 

Each example follows the same principle: mix green and conventional inputs, but account for the green share in the final mix. The result is that end users can claim sustainable content with confidence, knowing independent certification verifies the math.