Prototyping of fully continuous process for production of PHB by mixed microbial culture (MMC)

PHAs are bio-based and biodegradable polyesters with properties close to petrochemical plastics and the most investigated is the homopolymer PHB. PHAs can be produced by microorganisms that can use different carbon sources as feedstocks, thus reducing environmental impact and costs of polymer production. Mixed microbial cultures (MMC) have been proposed as an alternative to pure cultures to limit the operational costs. PHA production by MMC is usually divided in two steps: biomass enrichment (selection of PHA-storing bacteria) and PHA accumulation. The former is usually operated under feast and famine regime, while the latter is performed under excess of substrate to boost PHA storage. Other parameters can influence the PHA yield (e.g. pH, aerobic/anoxic conditions ...), hence it's important to understand how their variation affects the plant production. This research has the purpose to implement and optimize a fully continuous MMC pilot plant for PHA production using simulated fermented wastewater (acetic acid + nutrients). The major improvement of fully continuous system compared to batch (or fed-batch) is the possibility to produce PHB continuously. To achieve this goal, feast and famine regime was adopted, but the distinction between the two phases was by space: reactors were connected together to allow both phases to occur simultaneously, reducing time needed to obtain PHB. PHB content was assessed by pyrolysis. Soluble COD, remaining acetate and pH measurements were performed. Results showed that SRT, HRT and temperature were important parameters for the stability of the plant, self-balanced pH was obtained and no substrate inhibition effect was observed. PHB concentration as high as 86% was achieved with an average volumetric productivity of 14%. Predominant bacterial strain was Thauera. Continuous biomass harvesting, the non-use of toxic solvents and pyrolysis allow production of PHB with relatively low cost and reduce environmental burden.