Life cycle inventory practices for major nitrogen, phosphorus and carbon flows in wastewater and sludge management systems

Nitrogen, phosphorus and carbon originating from wastewater and sludge can, depending on their partitioning during wastewater treatment, either become available as potential resources or leave as emissions. Several reviews have highlighted the dependence of life cycle assessment (LCA) results on the inventory data. To provide a foundation for future assessments of systems in which resources are utilised from wastewater or sludge, this paper identifies common practice and highlights deficiencies in the selection and quantification of nitrogen, phosphorus and carbon containing flows.

Inventories of major direct flows containing nitrogen, phosphorus and carbon in 62 studies on wastewater and sludge management operations have been reviewed. A special focus was put on flows of nitrogen, phosphorus and carbon originating from the wastewater and sludge and on how these are either leaving the system as emissions and hereby contributing to environmental impacts, or how potential resource flows of these elements are accounted for, in particular when sludge is used in agriculture.

The current study shows a large variation between studies regarding what resource and emission flows were included in inventories on wastewater and sludge treatment, the type of data used (primary or secondary data) and, when flows have been modelled rather than measured, how the modelling has been done. Except for nitrogen and phosphorus emissions via the effluent, which were generally quantified using measured data or data modelled to represent the specific situation, direct emissions to air from the water and sludge lines at the wastewater treatment plant were mostly estimated using secondary data, sometimes of poor data quality. In systems where resources were recovered through agricultural application of sludge, studies often credited the system for avoided use of mineral fertiliser, but the considered replacement ratio differed.

The current review identified increased completeness and specificity in the modelling of the evaluated flows as particularly relevant for future studies and highlighted a need for improved transparency of data inventories. The review can be used as a support for LCA analysts in future studies, providing an inventory of common practices and pinpointing deficiencies, and can thereby support more conscious and well-motivated choices as regard which flows to include in assessments and on the quantification of these flows.