From wastewater to bioplastics
This project focuses on producing polyhydroxyalkanoates (PHA) in UK wastewater treatment plants as a substrate for innovative composite materials.
The project aimed to assess if wastewater treatment plants in the UK are producing polyhydroxyalkanoates, a biopolymer with increasing commercial interest, under current operation conditions. Moreover, it was explored if the biopolymer could be used in innovative applications as a biobased material.
The microbial communities used in our wastewater treatment (WWT) processes inadvertently produce bacterial polymers known as polyhydroxyalkanoates (PHAs). These are biodegradable and biocompatible polymers and due to their thermoplastic properties, offer an interesting bioderived alternative to the fossil-produced polyesters.
PHA synthesis from an external carbon source has been identified in a great number of different bacteria, which utilise this polymer as a carbon, energy and reducing equivalents storage product, when facing stress or limiting conditions (e.g. lack of nutrients or oxygen). They accumulate PHA inside the bacterial cells as discrete granules that are often in the form of co-polymers, e.g., poly(3-hydroxybutyrate-co-3-hydroxyvalerate). The diversity of the monomer units and their specific arrangements, including as co-polymers, provide PHAs with very interesting mechanical properties albeit at a cost: that PHAs are still not competitive with conventional oil derived plastics.
Therefore, the prospect of producing these polymers from waste substrates using mixed microbial cultures and as a by-product of WWT is still being developed as an attractive means to reduce costs, capitalize infrastructure and contribute to the notion of “Circular Economy”.
Hence, this project focuses on two main questions:
- What is the potential for UK WWT Plants to produce PHA?
- Could this PHA be used in producing innovative composite materials?
The first question focused on surveying the current PHA production yields in UK WWT plants performing biological treatment, even in un-optimised processes. Samples of activated sludge from 11 WWT plants located in the south of the UK were collected and the PHA content analysed. All biomass samples contained the biopolymer and the hydroxybutyrate and hydroxyvalerate monomers were quantified using gas chromatography. An estimation of PHA production capacity can be calculated for each plant, giving indication of the system potential to promote biopolymer accumulation.
The second question focused on the application of PHA in sustainable composite materials. As there is a wide range of PHAs produced naturally by activated sludge and these are likely to be extracted in small quantities only, commercially available poly(3-hydroxybutyrate-co-3-hydroxyvalerate) was used to answer this question. As a preliminary study, it was only possible to demonstrate the production of PHA microbeads with a size ranging from 0.12μm to 7.16μm. Further work can include the combination of PHA with cellulose to form a biocomposite with enhanced properties.
The determination and quantification of PHA accumulated by microbial cultures in WWT plants gives evidence of the production of a value-added product in these systems. Such production can be further explored and improved to increase the PHA yield and eventual recovery of the polymer. Moreover, a proof of concept for production of PHA microbeads was demonstrated.
The production of this polymer from renewable resources, such as wastewater, can make PHA production more cost-effective and promote its chance to compete with the conventionall petroleum-based materials. Moreover, the recovery of PHA from wastewater treatment systems takes advantage of the already existing facilities and a continuously available feedstock.