Previous studies have highlighted the trade-offs between eutrophication and global warming impact
categories caused mainly by effluent discharge, sludge treatment and disposal, and electricity use [2,8].
Driven by more stringent wastewater discharge standards aimed at improving the aquatic environmentby alleviating eutrophication arising from anthropogenic nutrient source, biological nutrient removal
(BNR) is being increasingly applied at WWTPs. The overall trend, therefore, is toward increasing energy
consumption and chemical dosage per unit of wastewater treated [9]. On the other hand, under the big
backdrop of global and national actions to achieve carbon neutrality or even carbon negative development,
the Chinese wastewater treatment sector will inevitably need to follow the lead to reduce GHG emissions
and cut energy consumption to mitigate its climate change impact. Carbon footprint accounting thus
should be adopted as a tool to measure climate change impact in developing technology roadmaps for
wastewater treatment and in the decision-making process for taking precautionary measures [10]. With
the development of urbanization in China, more WWTPs will be constructed and operated in small and
medium cities in the near future; therefore, plants with an average flow rate of 20,000 cubic meters
per day (m3/day) will be the mainstream scale of newly built WWTPs. Assessment of carbon footprints
of mainstream treatment technologies to be adopted in these new plants are therefore important for
decision-making on process design, operation strategies and performance evaluation to achieve carbon
neutrality. Meanwhile, the management of raw sludge is also a critical problem in close relationship with
wastewater treatment. The treatment and disposal of raw sludge with increasing production from
wastewater treatment process not only brings about potential pollution to the environment, i.e., metals
and trace pollutants contained might pollute underground water and soil, but also require energy and
chemicals during the process. In previous studies on GHG accounting, most are focused on the GHG
emissions from the liquor treatment [4,10–13] or sludge treatment [14–16] separately; less attention has
been paid to quantify GHG emissions to a larger extent, with the integration of both wastewater and
sludge treatment in the system boundary. The measurement of GHG emissions including both the
wastewater and sludge treatment process could shed light on the water–energy–GHG nexus relationship
and contribute to the GHG mitigation efforts in WWTPs with synergy effects. To meet this objective,this study aims to estimate and compare carbon footprints of mainstream liquid and sludge treatment
portfolios to be considered for new WWTPs built in China, by using plant design data and following
the life cycle approach. Extensive analyses and discussion of all treatment scenarios are presented
to highlight the variation of carbon footprints of different treatment scenarios and factors that affect
GHG emissions.