Life Cycle Assessment and Energy Balance of a Novel Polyhydroxyalkanoates Production Process with Mixed Microbial Cultures Fed on Pyrolytic Products of Wastewater Treatment Sludge

Luciano Vogli,Paola Galletti,Diego Marazza,Cristian Torri,Chiara Samorì,Serena Righi,Stefano Macrelli

doi:10.3390/en13112706

Abstract

A “cradle-to-grave” life cycle assessment is performed to identify the environmental issues of polyhydroxyalkanoates (PHAs) produced through a hybrid thermochemical-biological process using anaerobically digested sewage sludge (ADSS) as feedstock. The assessment includes a measure of the energy performance of the process. The system boundary includes: (i) Sludge pyrolysis followed by volatile fatty acids (VFAs) production; (ii) PHAs-enriched biomass production using a mixed microbial culture (MMC); (iii) PHAs extraction with dimethyl carbonate; and iv) PHAs end-of-life. Three scenarios differing in the use of the syngas produced by both pyrolysis and biochar gasification, and two more scenarios differing only in the external energy sources were evaluated. Results show a trade-off between environmental impacts at global scale, such as climate change and resources depletion, and those having an effect at the local/regional scale, such as acidification, eutrophication, and toxicity. Process configurations based only on the sludge-to-PHAs route require an external energy supply, which determines the highest impacts with respect to climate change, resources depletion, and water depletion. On the contrary, process configurations also integrating the sludge-to-energy route for self-sustainment imply more onsite sludge processing and combustion; this results in the highest values of eutrophication, ecotoxicity, and human toxicity. There is not a categorical winner among the investigated configurations; however, the use of a selected mix of external renewable sources while using sludge to produce PHAs only seems the best compromise. The results are comparable to those of both other PHAs production processes found in the literature and various fossil-based and bio-based polymers, in terms of both non-biogenic GHG emissions and energy demand. Further process advancements and technology improvement in high impact stages are required to make this PHAs production process a competitive candidate for the production of biopolymers on a wide scale.

Highlights

According to the European Commission [1], the transition to a more circular economy is an essential contribution to the efforts to develop a sustainable, low-carbon, resource-efficient, and competitive economy
The initial steps of the PHAs production process were tested at the laboratory scale by the Chemistry Lab of CIRI FRAME of the University of Bologna, Ravenna Campus; a hypothesis of process upscaling at the industrial scale was carried out
The process for PHAs production involving mixed microbial cultures (MMCs) consists of several phases (Figure 1) that can be summarized into three main steps: (1) Biomass feedstock pre-treatment through pyrolysis, followed by anaerobic digestion of organic carbon to produce mixtures of volatile fatty acids (VFAs); (2) PHAs-enriched microbial biomass production; and (3) PHAs extraction using organic solvents

Summary

Introduction

According to the European Commission [1], the transition to a more circular economy is an essential contribution to the efforts to develop a sustainable, low-carbon, resource-efficient, and competitive economy. Bio-based materials, i.e., those derived from renewable resources, such as wood, crops, or fibers, have various applications in a large variety of industries (e.g., construction, furniture, packaging, coatings, textiles, cardboard, chemicals, etc.) and energy uses (e.g., biofuels). Their characteristic of being made of organic carbon, which can be recycled and reused many times, in many ways, goes towards the principles of the waste hierarchy and, more generally, results in better overall environmental performances [1]. They have the advantage, over traditional plastics, of diminishing the use of non-renewable resources and decreasing the environmental impact related to fossil resources’ consumption [3,4]

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Results