Revealing the hidden burden: A life cycle assessment (LCA) of water treatment sludge disposal

• Life cycle impacts of four sludge disposal methods were evaluated in detail. • Disposal impacts exceeded treatment impacts in all the categories. • Land application showed the highest total burden due to drying and metal emissions. • Direct discharge caused severe toxicity despite low energy and material inputs. • Site-specific sludge inventories and Monte Carlo analysis improved impact accuracy. This study presents a detailed life cycle assessment of four widely used water treatment sludge disposal strategies – lagooning, direct discharge, landfilling, and land application. Previously, sludge disposal was considered to have a very limited impact and was often ignored when conducting LCA analysis of water treatment facilities. However, this work revealed that sludge disposal contributed more to total environmental burden than the treatment stage in most of the impact categories. For instance, the land application scenario accounted for the human health damage of 1.43E-05 DALY (disability adjusted life years) compared to only 4.49E-07 DALY of the treatment stage, or 96.7% of the facility total, while landfilling was responsible for the ecosystems damage of 2.63E-09 opposed to 4.86E-10 species/year of the treatment stage, or 84.4% of the facility total. Lagooning and discharge to river accounted for less, but still significant share of impact in those categories – 2.73E-07 DALY and 2.01E-10 species/year for lagooning and 1.36E-06 DALY and 2.20E-09 species/year for discharge. Further, Monte Carlo simulation results showed that endpoint impacts can vary by up to a factor of 10 depending on energy profiles, metal concentrations, and system configuration. Specifically, the highest sensitivity was observed for land application and landfilling scenarios, while lagooning and discharge scenarios were proven to be relatively stable under varying conditions. These results thus make it clear that fate-specific sludge disposal emissions can dominate facility-level damages and therefore should be incorporated into environmental assessments and regulatory planning for drinking-water treatment systems.