Polyhydroxyalkanoates (PHA) production from fermented crude glycerol in mixed microbial consortia

Polyhydroxyalkanoates (PHA) are microbial polyesters, which present similar properties to polyethylene and polypropylene, and therefore they have been proposed as a possible renewable and biodegradable bio-based alternative to these petroleum-derived plastics.

PHA are already at commercial scale with a global production of 35 000 tons in 2014, which represented a 2.1% of the total share of bio-based polymers (1.7 million tons in 2014). PHA competitiveness, however, is limited by its elevated production costs and limited development of market applications.

As a response to the elevated production cost of PHA, the trends in the sector are mainly focused in three areas: 1) the reduction in the production costs by the use of waste substrates and/or mixed microbial consortia; 2) the research in new applications for native or modified PHA. ; 3) the development of sustainable and economical PHA recovery methods. This PhD project focuses on areas 1 and 3.

Currently, PHA is produced at industrial scale using isolated or recombinant strains and pure substrates. A possible way to reduce the costs of the process would be the use of mixed microbial consortia (MMC) and ecological selection principles, where bacteria are selected by the operational conditions imposed on the biological system.

The reduction of costs would mainly come from the possibility of operation under non-sterile conditions and the consequent energy savings, as well as the higher adaptability of these cultures to the waste substrates.

Crude glycerol is among the substrates tested for PHA production. This by-product of the biodiesel industry represents 10% of the biodiesel output and contains on average 70% glycerol, among other impurities such as methanol, salts or fats.

Direct production of PHA using crude glycerol has the drawbacks of i) side production of glycogen besides PHA, ii) reduced molar mass of the polymer due to glycerol chain termination and iii) production of HB as only monomer, which has less desirable thermal properties than PHB copolymers.

Similar limitations have been overcome in other substrates by the fermentation of the original substrate into volatile fatty acids (VFA), recognized preferred substrates for PHA production. However, the fermentation of glycerol in MMC often leads to the production of 1,3-propanediol (1,3-PDO).

This alcohol has already many market applications and as a 3-carbon substrate, 1,3-PDO has the potential of increasing the carbon efficiency of the process as well as to bring interesting attributes to the PHA monomer composition.

The objectives of this PhD project (as a part of the GRAIL European project) are:

  • Evaluate 1,3-PDO as a possible substrate for PHA production, by the application of different MMC enrichment strategies.
  • Test the possibility of enriching MMC with selective consumption of VFA over 1,3-PDO with the objective of recovering 1,3-PDO while converting the volatile fatty acids into PHA. Moreover, the possibility of applying the same strategy with other bioproducts such as butanol, lactate or ethanol will also be evaluated.
  • Characterize the microbial communities in MMC as well as study the stability of the cultures.
  • Assess different feeding strategies in order to increase the PHA productivity in MMC.

Investigate ammonia treatment as a method for PHA recovery and purification.


Main supervisor: Hariklia Gavala

Co- supervisor 1: Ioannis Skiadas

Co-supervisor 2: Anders Egede Daugaard


Hariklia Nikolaos Gavala
Associate Professor
DTU Chemical Engineering
+45 45 25 61 96


Anders Egede Daugaard
Associate Professor
DTU Chemical Engineering
+45 23 65 21 52