From poo to bioplastic

22 June 2018

Biosolids from wastewater treatment plants can be converted into bioplastics.

Scion engineers and biochemists have been exploring the potential of using biosolids (sewage sludge) and organic waste treated by sub-critical wet oxidation as feedstocks for the production of PHA (polyhydroxyalkanoate) bioplastics.

PHAs can be thought of as ‘bacterial bodyfat'. Some bacteria make and store them when food is readily available and utilise them up when food become scarce. PHAs are fully biodegradable and have properties similar to polypropylene and polyethylene made from fossil fuels.

PHA manufacture and processing is not quite ready to replace conventional polymers as the industrial fermentation process is around 5-10 times more expensive than the fossil fuel-based process.

One way to reduce cost is to use waste as a feedstock. Sub-critical wet oxidation, a process that uses temperatures below the critical point of water (374°C) in the presence of oxygen, breaks organic compounds into acetic and other short chain organic acids, CO2 and water.

The acetic and propionic acids in the wet-oxidation ‘liquor’ from biosolids are an excellent food source for PHA-producing organisms.

Another way to reduce costs is to use mixed, rather than pure, cultures. In this work, the PHA-producing bacteria content of biological nutrient removal sludge from the Rotorua wastewater treatment plant was enriched using repeated feast and famine cycles. The most numerous bacteria in the final microbial community were of the genus Thauera, which are known to produce PHA.

The enriched culture grown in the wet oxidation liquor accumulated 40% PHA of cell dry weight within seven hours. This is the first time wet oxidation liquor has been used to produce PHA, to the best of the authors’ knowledge. It is also one of the highest reported yields for a nitrogen- and phosphorus-rich hydrothermal liquors.

Three quarters of the recovered PHA was polyhydroxybutyrate and the remainder was polyhydroxyvalerate (by mass). The presence of hydroxyvalerate alters the properties of the final polymers by lowering melting temperatures, decreasing stiffness and increasing flexibility and toughness, for example.

Producing bioplastics from waste organic materials will not only help minimise biosolid disposal, but also recover carbon in the form of a high value product. More cost-effective ways to produce biopolymers will also aid the transition from petrochemicals to a carbon neutral economy

Wijeyekoon, S., Carere, C. R., West, M., Nath, S., & Gapes, D. (2018). Mixed culture polyhydroxyalkanoate (PHA) synthesis from nutrient rich wet oxidation liquors. Water Research, 140, 1-11.