Methodological issues in life cycle assessment of mixed-culture polyhydroxyalkanoate production utilising waste as feedstock

• Cradle-to-grave life cycle assessments are preferable for the studied technology.
• Multifunctionality creates an allocation problem – we suggest a solution.
• Due to the electricity intensive technology the results show regional dependency.
• Water use has not been assessed in previous studies but is potentially relevant.

Assessing the environmental performance of emerging technologies using life cycle assessment (LCA) can be challenging due to a lack of data in relation to technologies, application areas or other life cycle considerations, or a lack of LCA methodology that address the specific concerns. Nevertheless, LCA can be a valuable tool in the environmental optimisation in the technology development phase. One emerging technology is the mixed-culture production of polyhydroxyalkanoates (PHAs). PHA production by pure microbial cultures has been developed and assessed in several LCAs during the previous decade. Recent developments within mixed-culture PHA production call for environmental assessment to guide in technology development. Mixed-culture PHA production can use the organic content in wastewater as a feedstock; the production may then be integrated with wastewater treatment (WWT) processes. This means that mixed-culture PHA is produced as a by-product from services in the WWT.This article explores different methodological challenges for LCA of mixed-culture PHA production using organic material in wastewater as feedstock.LCAs of both pure- and mixed-culture PHA production were reviewed. Challenges, similarities and differences when assessing PHA production by mixed- or pure-cultures were identified and the resulting implications for methodological choices in LCA were evaluated and illustrated, using a case study with mixed- and pure-culture PHA model production systems, based on literature data.Environmental impacts of processes producing multiple products or services need to be allocated between the different products or services. Such situations occur both in feedstock production and when the studied system is providing multiple functions. The selection of allocation method is shown to determine the LCA results. The type of data used, for electricity in the energy system, is shown to be important for the results, which indicates, a strong regional dependency of results for systems with electricity use as an environmental hot spot. The importance of assessing water use, an environmental impact not assessed by any of the reviewed studies, is highlighted.