Abstract
Municipal wastewater treatment plants (WWTPs) could become valuable contributors to a circular economy by implementing the 3R principles (reduce, reuse, and recycle). While reducing the pollution load of sewage is the primary objective of a WWTP, this process generates several potentially valuable byproducts including treated effluent, biogas, and sludge. The effluent can be reused in various end use applications and biogas can be reused as a fuel (for electricity generation, transportation, and cooking) or a chemical feedstock. The sludge can either be directly recycled as soil conditioner or via thermochemical/biochemical processing routes to recover material (e.g., hydrochar), energy (e.g., heat, and syngas), and resource value (phosphorus). This work presents a five-layered assessment framework for quantitatively evaluating the sustainable value of municipal WWTPs by using life cycle assessment (LCA) and life cycle costing assessment (LCCA) tools. In addition, indicators reflecting potential benefits to stakeholders and society arising from investments into municipal WWTPs such as the private return on investment (PROI) and the environmental externality costs to investment ratio (EECIR). The framework is validated in a hypothetical case study where the sustainable value of a circularly managed municipal WWTP is evaluated in situations involving multiple byproduct utilization pathways. Four future circular options (FCOs) are examined for a 50,000 m3/d capacity WWTP treating sewage up to tertiary standards. The FCOs mainly differ in terms of how biogas is reused (to meet the WWTP's internal energy demands, as cooking fuel, or as fuel for city buses after upgrading) and how sludge is recycled (as soil conditioner or by producing hydrochar pellets for electricity generation). The FCO in which treated effluent is reused in industry, biogas is used as cooking fuel, and sludge is used as a soil conditioner provides the greatest sustainable value (i.e., the lowest private costs and environmental externality costs (EEC) together with high revenues), the highest PROI, and the lowest EECIR. The strengths and limitations of the proposed assessment framework are also discussed.
Highlights
The value of the global water and wastewater market was estimated to be USD 263.07 billion in 2020 and is projected to reach USD 500 billion by 2028 (Statista, 2021)
The life cycle assessment (LCA) results for FCO1 and FCO2 suggest that the environmental benefits realized by replacing kerosene with biogas as a fuel for cooking in FCO2 or replacing diesel for transit buses with compressed biomethane (CBM) in FCO3, cannot offset the environmental burden caused by wastewater treatment plant (WWTP) operations
This is because external utilization of biogas leads to a situation in which the municipal WWTP must rely completely on electricity drawn from the Indian electrical grid mix, which increases the overall environmental burden of FCO2 and FCO3
Summary
The value of the global water and wastewater market was estimated to be USD 263.07 billion in 2020 and is projected to reach USD 500 billion by 2028 (Statista, 2021). Examples of LCA studies focusing on performance measurement include works quantifying the environ mental impacts of advanced tertiary treatment systems for removing emerging contaminants (Li et al, 2019; Rahman et al, 2018), evaluating the reuse of treated effluent as cooling tower makeup in industrial fa cilities (Theregowda et al, 2014) or for irrigation (Kanaj et al, 2021), comparing the use of biogas as an energy source to meet the internal energy demand of WWTPs to using it as vehicle fuel for freight trans portation (Shanmugam et al, 2018), recovering phosphorus from sewage sludge (Egle et al, 2016), and producing hydrochar pellets from sewage sludge for use as biocoal to replace fossil coal for electricity generation (Mohammadi et al, 2020).
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