STISUSMAN › O5. Residuals intensity
Mass balance approach
O5: Residuals intensity (mass balance) =
(Weights of all inputs + Weight of fuel consumed – Weight of all products)
/ Normalisation factor
O5: Residuals intensity (waste output) =
(Weight of releases [from production processes and, if available, overhead] to air
/ Normalisation factor
Unit of the indicator: tonnes/normalisation factor
Why this indicator is important
Ideally, a manufacturing facility would operate like a closed-loop ecosystem where nothing is wasted and all outputs are inputs to another production processes. Residuals represent a cost to the company and often have negative environmental impacts. Reducing residuals will improve profitability by ensuring that a higher proportion of the purchased materials are used.
There are two approaches to calculating residuals intensity. The “mass balance” approach calculates the weight of all materials consumed in overhead and production then subtracts the weight of all products. The “waste output” approach adds up the quantities of residuals actually generated. Both approaches are applied since any differences show that the waste output approach may have excluded an important residual.
Although the categories for the accounting approach are the same as those used for Pollutant Release and Transfer Registries (PRTRs), the substances and quantities controlled by regulations governing PRTRs are not always comprehensive. For example, in Canada, pesticides are treated under other regulations and therefore not tracked by the National Pollutant Release Inventory (NPRI). Furthermore, according to many regulations, only quantities above a certain threshold need to be reported. For this indicator, all significant releases and transfers should be included.
In addition, PRTRs generally track only certain chemicals and metals. For the purposes of this Toolkit, releases and transfers also include any significant materials (such as paper, plastics and organics).
Although it might be difficult or insignificant to track all releases, it is recommended to track the residuals from overhead as well as production processes to be consistent with the calculation by the mass balance approach.
Additional GHGs released from the production process (from chemical processes) and overhead (such as from air-conditioning and refrigeration) are included here rather than in the Indicator O6: Intensity of residual releases to air. This is because GHGs are accounted for separately but are required here for calculating the mass balance.
Since this is an intensity indicator, if it is normalised by the total value added of production (total factory gate price), it could be sensitive to price fluctuations. As with other intensity indicators, it should also be a priority for the facility to decrease its total residuals output even as production increases.
A review of the detailed data will show which processes are most intensive in terms of residuals. It may be possible to reduce residuals by changing materials or fuels, reusing materials and components rather than recycling them outside the facility, reducing spillage and emissions by improved maintenance of equipment.
This is an aggregate indicator. Its purpose is to provide an accounting checkpoint to ensure that most residuals are accounted for. It does not capture the relative impacts of the residuals. Other two indicators highlight the intensities of residual releases to air (O6) and surface water (O7).