O7. Intensity of residual releases to surface water



O7: Intensity of pollutant releases to surface water =

Weight of releases [from production processes and, if available, overhead] to surface water

 / Normalisation factor

Unit of the indicator: tonnes/normalisation factor


Why this indicator is important

Residuals released in water can affect the economy, human health and the environment. Excess residuals can directly affect the health of aquatic species and enter the food chain to eventually be consumed by humans.

This indicator is the measured or estimated quantity of pollutants released on-site to surface water over the reference year. Although it might be difficult or insignificant to track, it is recommended to track the releases to surface water from overhead as well as production processes.

As with the intensity of residuals released to air indicator (O6), the Toolkit recommends expanding the definition of materials covered beyond those legislated in the national PRTR.



The first and most effective means of reducing residuals released is to prevent them from occurring in the first place. This can be done by choosing less polluting materials and improving process efficiency. The second line of defence is wastewater treatment. Municipal wastewater treatment is generally not capable of removing specific industrial pollutants. Furthermore, the efficiency of wastewater treatment plants to remove common pollutants (BOD, COD, solids) varies greatly.

Although water bodies can assimilate a certain amount of pollution, exceeding this carrying capacity will reduce water quality. If the receiving body becomes polluted, this will increase source water treatment costs for downstream users.

As with the other intensity indicators, it is important to reduce the intensity of residual releases to surface water but also to reduce the absolute amounts during times of production increases.

Related issues

It is important for the facility to also report and track discharges of its individual water pollutants of concern, e.g., BOD, TSS, phosphorus, nitrogen, toxics, and/or other pollutants that are priorities for the state, region, watershed, locality and/or public interest groups. These may be pollutants that are regulated or permitted. The facility should be in a position to identify which water pollutants to prioritise and confirm this list with its community, regional or state officials. This level of disaggregation may inform decision making on how to most efficiently or effectively improve environmental performance. For example, the facility may find that it is better to reduce a more toxic but lower volume pollutant than a higher volume pollutant.

This indicator also does not take into account the relative impacts of the mix of pollutants released. As with air pollutants, methods are available that index individual pollutants based on their impacts but these are either non-standard or too complex for the current Toolkit. The individual pollutants may be of interest locally and could be assessed independently of the Toolkit. For example, if the facility is looking to reduce its suspended solids effluents, this value could be considered along with the more aggregate indicators in the Toolkit.