Abstract

Polyhydroxyalkanoates (PHAs) are fully biodegradable bioplastics synthesized by microorganisms from renewable resources. In the last decade, PHA implementation has been limited by high production costs when compared with conventional plastics. Low cost processes for PHA production, such as the use of mixed microbial cultures (MMCs), have been recently developed. Their use offers several advantages compared with pure cultures process, as sterile conditions are not required and cheap or even free substrates, like industrial wastes or by-products, may be used. Given its availability, large diversity, lack of valuable alternative utilizations, and low cost, the use of wastes or industrial by-products as substrate is the most promising strategy to reduce the cost of biopolymers produced by microbial processes Feast and Famine (FF) strategies have been used as the selection pressure for the enrichment of MMC capable of storing high levels of PHA. Using this strategy and industrial by products, such a as sugar cane molasses, accumulation levels as high as 75% PHA have already been achieved Furthermore, most of the wastes/by-products are seasonal and, in practice, PHA producing industries will have to comply with this feedstocks seasonal availability. The effect of different real feedstocks alternation on PHA production bioprocess and consequent implications for process operational stability and final product characteristics was evaluated using cheese whey and sugar cane molasses. The link of reactor performance and microbial structure was studied in order to better understand the process. A further reduction in operational costs of PHA process was evaluated using photosynthetic mixed cultures (PMC) that, unlike the traditional mixed microbial cultures, do not require aeration. A new process where PHA accumulating cultures were selected under non-aerated illuminated conditions in a feast and famine regime was developed. The metabolism of the PMC was studied and correlated with process performance. The use of fermented wastes as substrates for PHA production, combined with this new photosynthetic technology led to a more cost-effective and environmentally sustainable PHA production process.