Process engineering for improved marine biosurfactant production

UoM administered thesis: Phd

  • Authors:
  • Sara Bages Estopa

Abstract

Trehalolipid biosurfactants are commercially attractive because of their range of interesting properties. For instance, immunomodulatory activity and low toxicity make them suitable for the biomedical industry and high emulsification properties for the petroleum and oil industry and for bioremediation applications. However, the production of these biosurfactants has not yet reached commercialisation because of the prohibitive production costs associated with difficult bioprocessing due to the production in a heterogeneous and dynamic system, low titers and expensive recovery methods. Foam fractionation is a potentially cheap and green separation method that uses foam to separate surface active compounds from a solution. It has been successfully demonstrated for a range of compounds including the antimicrobial peptide nisin and the biosurfactants rhamnolipids, surfactin and hydrophobin protein HFBII. This thesis presents the production of trehalolipid biosurfactant by a marine bacterium, Rhodococcus sp. PML026. A study of the foaming characteristics of trehalolipid in fermentation broths and the utilisation of foaming to enable its recovery was carried out. A novel integrated production and separation process using foam fractionation was developed. Foam formation was first evaluated in a 2 L and 10 L bioreactor. Fermentations were conducted with hexadecane as the main carbon source. Hexadecane is an antifoam agent that suppresses foam during fermentation and this is an advantage during the growth and production phases. A suitable medium formulation and process conditions to deplete the hexadecane substrate by the end of the fermentation, to enable foaming to occur and product separation by foaming, were achieved. Results show that at a threshold of biosurfactant and residual amount of hexadecane, vigorous foaming occurred in the bioreactor. Foam overflowed through the bioreactor condenser and separated 23-58% w/w of the total trehalolipids produced. Trehalolipid separation using foaming was further explored using foam stability and foam fractionation experiments. Minimum trehalolipid and maximum hexadecane concentrations to produce foam were determined, and foam stability and bubble size distribution were investigated using four sintered discs. A series of foam fractionation experiments were first carried out in simple batch mode and then in continuous stripping mode to find the best condition for the separation. In batch mode, a maximum enrichment of 6 and recovery of 94% w/w were obtained. In continuous mode, the maximum recovery was 37% w/w and the maximum enrichment was 9, though these were achieved under different conditions. Finally, a novel trehalolipid production and separation process was developed by integrating the production in the bioreactor in fed-batch mode with separation using foam fractionation. Experiments consisted of three production and separation cycles, with the introduction of fresh medium and hexadecane after each separation and re-start of the trehalolipid production. This allowed the production time to be extended by up to three times that in a normal batch experiment, and prevented uncontrolled foaming, yielding a concentrated product with enrichments between 1.3 and 2.8 and recoveries between 56% and 63% w/w. This work demonstrates for the first time the potential for foam fractionation as an inexpensive and environmentally friendly method to separate trehalolipids from an emulsified fermentation broth and goes a step towards the commercialisation of these biosurfactants.

Details

Original languageEnglish
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Supervisors/Advisors
Award date1 Aug 2018