Food waste valorization through the production of polyhydroxyalkanoates by mixed microbial cultures

Dissertação para obtenção do Grau de Mestre em Engenharia Química e Bioquímica

Polyhydroxyalkanoates (PHAs) are polyesters of hydroxyl fatty acids, which are accumulated in microbial cells as carbon/energy reserves. PHAs are bio-based and biodegradable and display a wide range of thermoplastic properties, being a promising alternative to conventional plastics. Presently, industrial PHA production was primarily based on pure microbial cultures. Although this process has high PHA production efficiency, it presents high costs associated with the use of chemically-defined feedstocks, and to the need for sterility. An attractive feature of mixed microbial cultures (MMCs) PHAs production is the ability to use waste/surplus feedstocks. Many industrial wastes are seasonally produced making it necessary find the best method of utilization of this feedstock on PHA production process. Two different approaches might be taken account: (1) stock of industrial wastes during their production for their use throughout the year. However, the high fermentability of these agro-industrial wastes makes them susceptible to degradation during storage period; (2) the use of different feedstocks over the year according its availability. It is thus important to study MMC’s response to different feedstocks. The aim of this work is study how MMC PHA production process is affected by a feedstock shift, using cheese whey (CW) and sugar cane molasses (SCM) as model feedstocks. The use of waste based feedstock by MMCs requires a previous conversion of sugars to organic acids (OAs), which is achieved through anaerobic fermentation. In this study, a three-stage MMC PHA process was used, comprising: (1) anaerobic fermentation of surplus feedstocks to produce OAs in a membrane bioreactor (AnMBR); (2) PHA accumulating culture selection in a sequencing batch reactor (SBR) under feast and famine conditions using fermented feedstocks; and (3) PHA production using the selected cultures and the OAs produced in the earlier stages. Initially the effect of both feedstocks (CW and, SCM) in the acidogenic fermentation (stage 1) was assessed. Firstly, the AnMBR was operated under steady state with CW. When the feedstock was changed to SCM an adaption period of about 10 to 15 days was observed. When SCM was replaced by CW a faster adaptation response, approximately 7 days, was observed. The AnMBR reached similar OAs profiles in both phases when CW was fed (% g-COD HAA/g-COD OAs): 65% acetate, 10% propionate, 22% butyrate, 2% valerate, and 1% lactate. These results demonstrate that the system’s performance is reproducible. On the other hand, the anaerobic fermentation of a different feedstock, SCM, resulted in a different OAs profile (%g-COD HAA/g-COD OAs): 24% acetate, 38% propionate, 19% butyrate, and 19% valerate. In a second phase, different fermented feedstocks were used in the selection of PHA-storing organisms under a feast and famine regime in a SBR (stage 2). Initially the SBR was fed with a synthetic OAs solution; then fermented SCM (fSCM) and fermented CW (fCW) were subsequently fed as they were produced in the AnMBR. The adaption of the MMC to fSCM was faster than the adaptation to fCW. Whenever steady state was reached, PHA accumulation tests were performed using the enriched MMC fed with the corresponding feedstock (stage 3), namely synthetic OAs solution, fSCM, and fCW. Storage yields of 0.74, 0.49, and 0.73 C-mol PHA/ C-mol OAs were obtained with synthetic OAs solution, fermented molasses, and fermented CW, respectively. The culture reached a maximum PHA content of 60%, 56% and 65%, when feedstock fed were synthetic OAs solution, fSCM and fCW, respectively. A direct relation between the used feedstock and the polymers composition was observed, which was related with the different OAs profile. Even though, the shift of complex feedstock in three-stage MMC PHA process is still at a very early stage of development, this work illustrates the advantage of favoring the selection of cultures with the capacity to adapt its metabolism to different feedstocks. This will offer the possibility of using numerous substrates and improving strategies to optimize acidogenic fermentation, culture selection and polymer production.