A model and tool to calculate life cycle inventories of chemicals discharged down the drain

Purpose

The aim of this article is to present a new model and tool to calculate life cycle inventories (LCIs) of chemicals discharged down the drain. Exchanges with the technosphere and the environment are attributed for based on the predicted behaviour of individual chemicals in the wastewater treatment plant (WWTP) and following discharge to the aquatic environment, either through the treated effluent or directly when there is no connection to WWTP. The described model is programmed in a stand-alone spreadsheet, WW LCI.

Methods

The model includes treatment in a modern WWTP and sludge disposal as well as the greenhouse gas (GHG) and nutrient emissions from degradation in the environment. The model fundamentals are described, and its application is shown with six industrial chemicals: sodium carbonate, ethanol, tetraacetylethylenediamine (TAED), diethylenetriamine penta(methylene phosphonic acid) (DTPMP), zeolite A and sodium tripolyphosphate (STPP). This application considers two scenarios: Germany, with full connection to WWTP, and a generic direct discharge scenario. The scenario with WWTP connection is assessed with WW LCI as well as with the wastewater treatment model developed for ecoinvent. Results are presented for key LCI flows and for life cycle impact assessment (LCIA), focusing on GHG emissions, freshwater ecotoxicity and marine and freshwater eutrophication.

Results and discussion

GHG emissions predicted by WW LCI differ to those predicted by the ecoinvent model, with the exception of sodium carbonate. For zeolite A and DTPMP, WW LCI predicts GHG emissions 330 higher and 12.5 times lower, respectively. Eutrophication scores are lower for WW LCI as the German scenario considers more optimistic nutrient removal rates than the default ones from the ecoinvent model. Freshwater ecotoxicity is mainly driven by the magnitude of the USEtox characterization factors; however, the ecoinvent model cannot accommodate chemical-specific toxicity assessments. When WW LCI is used to compare a direct discharge scenario with the German scenario, differences are found in all three impact categories.

Conclusions

WW LCI provides a comprehensive and chemical-specific inventory, constituting an advance over previous models using generic descriptors such as biological oxygen demand. This level of detail comes at the price of an increased effort for collecting input data as well as the need to identify individual chemicals in wastewater prior to the assessment. The LCIs generated through this model can then be applied in the context of LCA studies where each chemical contributes to the total life cycle impacts of a product or service.

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