Energy use and carbon footprints differ dramatically for diverse wastewater-derived carbonaceous substrates: An integrated exploration of biokinetics and life-cycle assessment

Yanbo Li,Jiuhui Qu,Junxin Liu,Xu Wang,David Butler

doi:10.1038/s41598-017-00245-1

Abstract

Energy neutrality and reduction of carbon emissions are significant challenges to the enhanced sustainability of wastewater treatment plants (WWTPs). Harvesting energy from wastewater carbonaceous substrates can offset energy demands and enable net power generation; yet, there is limited research about how carbonaceous substrates influence energy and carbon implications of WWTPs with integrated energy recovery at systems-level. Consequently, this research uses biokinetics modelling and life cycle assessment philology to explore this notion, by tracing and assessing the quantitative flows of energy embodied or captured, and by exploring the carbon footprint throughout an energy-intensive activated sludge process with integrated energy recovery facilities. The results indicate that energy use and carbon footprint per cubic meter of wastewater treated, varies markedly with the carbon substrate. Compared with systems driven with proteins, carbohydrates or other short-chain fatty acids, systems fed with acetic acid realized energy neutrality with maximal net gain of power from methane combustion (0.198 kWh) and incineration of residual biosolids (0.153 kWh); and also achieved a negative carbon footprint (72.6 g CO2). The findings from this work help us to better understand and develop new technical schemes for improving the energy efficiency of WWTPs by repurposing the stream of carbon substrates across systems.

Highlights

Energy neutrality and reduction of carbon emissions are significant challenges to the enhanced sustainability of wastewater treatment plants (WWTPs)
Wastewater treatment plants (WWTPs) in the United States consume ~15 GW annually[4], and 50–70% of the electricity consumption is as a result of the aeration processes[5,6,7], adding an unintended burden that increases energy use and carbon footprints
There is a need to better understand the potentials of energy neutrality and carbon reduction in aerobic systems such as activated sludge processes (ASPs); and subsequently, to develop strategies to promote a sustainable reform in the role of WWTPs, from carbon-degradation oriented to carbon-recovery intensive infrastructures[10]

Summary

Introduction

Energy neutrality and reduction of carbon emissions are significant challenges to the enhanced sustainability of wastewater treatment plants (WWTPs). The type of carbon substrate is recognized as one significant factor that affects the energy depletion and harvesting of traditional ASPs11 It was observed in previous studies that the conversion of short-chain fatty acids (SCFAs) in the aeration processes was dominated by substrate storage and polyhydroxyalkanoate (PHA) synthesis, rather than aerobic degradation to CO2, thereby promoting energy reduction in the water processing line[12]. Little of the literature delivers extended information on the effect and metabolic characteristics of carbonaceous substrates, in relation to the energy and carbon footprints associated with ASPs. mathematical models have been widely applied to describe, predict, and evaluate the performance of an expanded range of wastewater treatment alternatives[14,15,16,17]. This work was intended to inform researchers, design practitioners, utility managers, and planners on how the carbonaceous substrate alters the energy use and carbon footprints of wastewater solutions; and on the associated implications for energy recovery practices

Objectives

Methods

Results