Abstract

The use of microbial enrichment cultures allows the production of polyhydroxyalkanoates (PHA) under non-axenic conditions and, thereby, the use of organic waste as substrate. Agro-industrial waste streams, possibly fermented, often contain compounds such as volatile fatty acids (VFAs) and lactate, which are highly suitable for PHA production. To create a selective environment for PHA-producing bacteria, the reactors are operated under so-called feast-famine conditions. In 2009, Johnson et al. thus enriched a microbial culture with a superior PHA storage capacity of 89 wt% of total suspended solids (TSS), using acetate as substrate. The enrichment culture was highly dominated by Plasticicumulans acidivorans, a species that was later also enriched when propionate or butyrate was used as substrate. When lactate was used another, novel but related species was found – Plasticicumulans lactativorans – with a PHA storage capacity of 92 wt% of TSS. Both species show great potential for the successful implementation of waste-based PHA production. To study the production of PHA by microbial enrichment cultures using a real, variable waste stream, a pilot study was performed at the Mars chocolate factory in Veghel, The Netherlands. The pilot plant was set-up similar to the previous lab-scale studies and designed as a 3-step process. In the first step, the anaerobic fermentation, the fraction of VFAs in the wastewater was increased to more than 60% of the total chemical oxygen demand (COD). The second step is the enrichment of PHA-producing bacteria, and was performed in a 200-L reactor operated as a sequencing batch reactor (SBR) under aerobic conditions (12 h cycles, 1 d solids retention time). The temperature was maintained between 28 and 32°C, and the pH between 6.5 and 7.5. The same organism as obtained at lab-scale, P. acidivorans, was found as dominant species and after the final, accumulation step, biomass containing 70 wt% PHA (of volatile suspended solids) was obtained. The PHA content was lower than observed in the lab due to the presence of solids in the wastewater and bacteria growing on the non-VFA fraction of the substrate. Overall, the results demonstrate the efficient production of PHA from waste at pilot-scale, and show a great prospect for the scale-up and optimization of waste-based PHA production.