Life cycle assessment of bio-based materials : environmental impacts of the value chain from cyanobacteria to PHB
Creators: name: Frehner, Alena
name: Itten, René
name: Stucki, Matthias
Publisher: name: ZHAW Zürcher Hochschule für Angewandte Wissenschaften
Year: 2022
Subjects: Life cycle assessment
Cyanobacteria
PHB
Biopolymer
572: Biochemie

description: Considering the severe impacts of synthetic polymers on the environment as well as the human health, the development of biopolymers is becoming increasingly important. While biopolymers such as Bio-PA or Bio-PET address the issue of fossil resource depletion, they are not automatically biodegradable and can cause significant environmental impact in their respective value chains. One alternative to petroleum-based polymers while also being biodegradable is Polyhydroxybutyrate (PHB). Cyanobacteria is one of various bacterial strains capable of producing PHB. The aim of this study was to conduct a life cycle assessment (LCA) of cyanobacteria production on a thin-layer photobioreactor (PBR) pilot plant, and to extrapolate the data to an industrial-scale production scenario.
Primary data was collected on a pilot-scale thin-layer PBR, where cyanobacteria are cultivated using three different cultivation media being a Z medium (a standard mineral medium) and two variants of Z medium where the nitrogen source is replaced with either water from an aquaculture system or pre-processed liquid digestate. The functional unit was chosen as 1 kg of cyanobacteria biomass (dry matter). Processes included in the LCA reach from construction of the thin-layer PBR and its operation to the centrifugation of the cyanobacteria biomass. The environmental impacts of the cyanobacteria production were analysed using a selection of environmental impact categories according to the environmental footprint method, greenhouse gas (GHG) emissions according to the Intergovernmental Panel on Climate Change (IPCC) and the total environmental impact according to the ecological scarcity method.
Due to higher yields, cyanobacteria cultivation in the Z medium showed lower environmental impacts across all categories analysed. Main contributors to the environmental impacts of the pilot plant production were found to be the CO2 fed to the cultures and electricity consumption of the pumps and centrifugation, across all three cultivation media. Extrapolation of the data from the pilot plant to an industrial-scale production scenario, where yield is increased and CO2 input is decreased, showed a reduction potential of > 80 % across all environmental impacts analysed, leaving electricity consumption as the distinct primary environmental hotspot. In terms of climate change, the industrial-scale scenario showed a reduction of GHG emissions from 35 kg CO2-eq to 4 kg CO2-eq per kg of cyanobacteria for the Z medium. Regarding the aquaculture water and liquid digestate, GHG emissions per kg of cyanobacteria were reduced from 90 kg CO2-eq to 8.7 kg CO2-eq and 8.5 kg of CO2-eq, respectively.
The present study thus shows, that from an environmental perspective, cyanobacteria production using waste streams would have to achieve an increase of productivity rates by a factor 3, if it were to compete with production using a Z medium. Regardless of the medium used, focus for reducing the environmental impact of PHB production from cyanobacteria should lie on reducing the CO2 input while aiming to maximise the areal productivity rates as well as decreasing electricity consumption, in order to compete with PHB produced from other feedstock as well as petroleum-based polymers.