Comprehensive life cycle inventories of alternative wastewater treatment systems

Over recent decades, the environmental regulations on wastewater treatment plants (WWTP) have trended towards increasingly stringent nutrient removal requirements for the protection of local waterways. However, such regulations typically ignore other environmental impacts that might accompany apparent improvements to the WWTP. This paper quantitatively defines the life cycle inventory of resources consumed and emissions produced in ten different wastewater treatment scenarios (covering six process configurations and nine treatment standards). The inventory results indicate that infrastructure resources, operational energy, direct greenhouse gas (GHG) emissions and chemical consumption generally increase with increasing nitrogen removal, especially at discharge standards of total nitrogen <5 mgN L−1. Similarly, infrastructure resources and chemical consumption increase sharply with increasing phosphorus removal, but operational energy and direct GHG emissions are largely unaffected. These trends represent a trade-off of negative environmental impacts against improved local receiving water quality. However, increased phosphorus removal in WWTPs also represents an opportunity for increased resource recovery and reuse via biosolids applied to agricultural land. This study highlights that where biosolids displace synthetic fertilisers, a negative environmental trade-off may also occur by increasing the heavy metals discharged to soil. Proper analysis of these positive and negative environmental trade-offs requires further life cycle impact assessment and an inherently subjective weighting of competing environmental costs and benefits.