Life cycle assessment of sewage sludge pretreatment for biogas production: From laboratory tests to full-scale applicability

Highlights

• Pretreatment technologies were investigated to enhance sludge methane production.

• Low temperature thermal pretreatment (75 °C) showed the highest CH₄ increase.

• LCA highlighted a significant difference between laboratory and full-scale results.

• Ultrasonication led to annual net profits despite the high capital costs.

• Biochar breakeven cost to yield profit was calculated as 6.5 €/ton.

Abstract

In this work, different pretreatment technologies (low temperature thermal treatment, ultrasonication, alkali treatment, combined alkali-thermal treatment, icing-thawing) were investigated together with biochar addition to increase biogas yield from sewage sludge. The aim of the work was to develop a standardized protocol, including physicochemical characterization, biochemical methane potential (BMP) tests, digestate characterization, life cycle assessment (LCA) and economic analysis, to verify the techno-economic suitability and the related environmental impacts of upgrading conventional full-scale anaerobic digesters. Low-temperature (65–85 °C) thermal pretreatment showed the best performances in enhancing sludge solubilization and methane yield (up to 110%), followed by ultrasonication (up to 53%) and biochar addition (about 16%). The solubilization rate obtained after each pretreatment was similar for municipal and industrial sludge; however, it did not correspond to a proportional increase in methane yield. LCA showed that ultrasonication led to the worst environmental impacts at laboratory scale, due to the huge electricity request; however, at full-scale conditions, green-house gases (GHG) emissions could be reduced by 22%, fossil depletion by 25% and stratospheric ozone depletion by 18%, when compared to the baseline scenario (raw sludge anaerobic digestion). The economic analysis proved that the high ultrasonication capital costs could not be recovered in the considered timeframe (15 yr), while biochar price should be significantly lowered (down to 6.5 €/ton) to give an overall positive outcome. Finally, the thermal scenario would be convenient only when considering waste heat recovery. The proposed protocol, including physicochemical sludge characterization, BMP tests, digestate characterization, LCA and economic analysis, could be extended to other existing digesters to compare alternative pretreatment strategies, enhancing renewable energy generation in biogas form.

Introduction

Quantitatively, sewage sludge is the most important by-product generated by wastewater treatment: the produced sludge amounts can be estimated as 2% of the influent wastewater flow (Zhen et al., 2017). Regarding the qualitative aspects, sewage sludge includes organic compounds, solids, nutrients, microbial aggregates, bacteria, extracellular polymeric substances (EPS), heavy metals, emerging contaminants (Khakbaz et al., 2020).

Anaerobic digestion (AD) is a widely employed strategy for sludge stabilization that produces renewable energy in biogas form (Mainardis et al., 2019b), and has been proved to be an effective, efficient and environmentally friendly technology (Khanh Nguyen et al., 2021). Hydrolysis is the rate-limiting step in sludge AD (Gonzalez et al., 2018), due to the complex and slowly biodegradable high-molecular weight compounds present in the substrate, that hinder a full exploitation of AD potentialities (Xu et al., 2020). Considering that sewage sludge is commonly applied to the soil for fertilization purposes, AD can stabilize organic matter with beneficial effects; however, antibiotic and metal resistance genes present in sludge have to be carefully monitored, as they can accumulate in the soil and subsequently enter the food chain (Markowicz et al., 2021).

In literature, several mechanical, physical, chemical and biological pretreatment technologies have been tested on sewage sludge (Cho et al., 2014). However, only few techniques appear to be technically and economically applicable at commercial scale, and some critical aspects (e.g., pathogen removal, microbial dynamics, micropollutants) still need to be solved (Taboada-Santos et al., 2019). In literature, most of pretreatment techniques have been applied at laboratory or pilot-scale conditions; the obtained results in terms of methane yield increase depend on sludge nature (i.e., primary, secondary or mixed sludge) as well as on peculiar operating conditions (i.e., batch or continuous mode) (Elalami et al., 2019). Energy evaluations must be conducted together with experimental assays to properly assess the optimal pretreatment conditions, as a trade-off between pretreatment energy consumption and methane generation increase, including both heat and electricity (Atelge et al., 2020). Moreover, the applicability of each pretreatment solution to a selected sludge stream has to be carefully assessed considering the eventual generation of toxic by-products (Cheah et al., 2020). Thus, techno-economic analysis should always be conducted together with experimental trials and eco-toxicological assessments (Kamusoko et al., 2019).

Among the various pretreatment solutions, thermal hydrolysis, ultrasonication and alkali pretreatment appear particularly promising for full-scale applications. Innovative approaches involve the upgrading of existing digesters in wastewater treatment plants (WWTPs) with a thermal hydrolysis pretreatment to increase organic matter solubilization and biogas yield (Liu et al., 2020). Improved volatile solids (VS) reduction, augmented sludge dewaterability and reduced odor generation are commonly observed after applying sludge thermal pretreatment (Zhang et al., 2020).

Beside thermal hydrolysis, ultrasonication (US), belonging to advanced oxidation processes (AOPs), is a mechanical pretreatment involving the generation of powerful ultrasonic waves that propagate in the substrate (Mainardis et al., 2019b). US pretreatment was shown to increase sludge digestibility by disaggregating complex organic molecules (Mainardis et al., 2019b), despite being characterized by a significant electric energy consumption, especially at laboratory scale (X. Li et al., 2018).

Alkali treatment has also been proposed as a potential way to enhance methane yield from sewage sludge, particularly when coupled with a thermal pretreatment, aimed at reducing the inhibitory effects of the added chemicals on the AD process (Zou et al., 2020). Sludge alkali pretreatment (using KOH) was coupled in literature with macroalgae and olive pomace co-digestion, boosting an enhanced methane yield and leading to a digestate that could be reused in agriculture (Elalami et al., 2020). In another study, Ca(OH)₂ addition enhanced sludge dewaterability of hydrothermally treated sludge, realizing an energy self-sufficient system at pilot-scale (C. Li et al., 2017). When applied alone, however, despite favoring sludge solubilization, AD system overload could be observed, with anaerobic trophic chain destabilization (Zahedi et al., 2017). (Khanh Nguyen et al., 2021) revised the different possibilities for alkali sludge solubilization, and stated that the effectiveness of the alkali chemicals used for sludge pretreatment is in the following order: NaOH > KOH > Mg(OH)₂ > Ca(OH)₂.

A part from sludge pretreatment, several additives have been tested to enhance AD performances, such as silver nanoparticles (Grosser et al., 2021), essential trace metals (Linville et al., 2016), biochar (BC) (Yan et al., 2020), activated carbon and magnetite (Xie et al., 2020). BC addition to sewage sludge AD is particularly interesting, as BC is a carbonaceous material obtained from pyrolysis of biomass sources (Mainardis et al., 2019a) that can lead to local circular economies by exploiting low-cost residual biomass (Xiao et al., 2020). BC has been extensively investigated in the latest years as an AD additive, improving electron transfer through the so-called direct interspecies electron transfer (DIET) mechanism (Jiang et al., 2020), enhancing biogas yield (Mainardis et al., 2019a) and ameliorating digestate quality (Yan et al., 2020).

To evaluate the effects of sludge pretreatment and additives, laboratory assays are normally conducted through biochemical methane potential (BMP) tests. BMP tests are a standardized method to assess methane production potential during the AD process of a given substrate (Mainardis et al., 2019b). The optimum inoculum-to-substrate (I/S) ratio to be applied in BMP tests (normally evaluated on VS basis) depends on substrate characteristics as well as on inoculum health, in terms of microbial population activity (Li et al., 2020; Y. Li et al., 2018).

As previously introduced, energy and environmental analysis should always be performed when testing AD pretreatment technologies, to broadly analyze the up-scale feasibility of the proposed solutions. To assess the environmental impacts of a product, process or system over its lifetime, a life cycle assessment (LCA) can be performed (European Commission and Joint Research Center, 2010). In literature, a significant number of studies has been focused on LCA application to sludge treatment technologies (Table S1), to select the most environmental-sound solutions and compare different alternatives. The utilization of economic analysis alongside LCA can further improve decision-making process, considering both environmental and economic performance.

However, at the best of our knowledge, limited studies were made on combined LCA and economic analysis application to sludge pretreatment before AD (Table S1): the existing studies were either focusing only on LCA (Carballa et al., 2011; Wang et al., 2020) or analyzed a single pretreatment technology (H. Li et al., 2017; Mills et al., 2014). Also, LCA studies were mostly based on literature, rather than on experimental data (Arias et al., 2021). Moreover, no study coupled a preliminary laboratory investigation with LCA (at laboratory and full-scale conditions) and economic assessment.

In this work, different pretreatment solutions (low temperature thermal hydrolysis, US, icing/thawing, alkali treatment, combined alkali + thermal treatment) were investigated to improve sewage sludge AD. BC addition to AD was investigated as well. Two different kinds of sludge (municipal and industrial sludge) were considered. The laboratory investigation was aimed at establishing the best pretreatment solutions. The second phase of the work included digestate characterization, LCA modelling at laboratory and full-scale conditions, and an economic analysis (at full-scale conditions), considering the municipal sludge as a reference. To the best of our knowledge, this is the first study that thoroughly evaluates the feasibility of applying different pretreatment technologies to sewage sludge AD, coupling laboratory investigations, LCA and economic analysis. This work can lead the path to a robust evaluation method for water utilities and researchers to properly evaluate the best technical solutions to improve energy yield from existing digesters, fitting the circular economy perspective.