Enhancing polyhydroxyalkanoate productivity with cell-retention membrane bioreactors

[Display omitted] •High PHA productivity (0.87–1.44 g/(L.h)) was reached with membrane bioreactors (MBRs).•Hollow fibers and ceramic filters showed a stable flow during a 24 h fed-batch.•The use of MBRs significantly reduced the working volume (up to 82 %).•Ceramic filters presented lower fouling but higher footprint than hollow fibers.•VFA diffusion was not enough to satisfy the culture needs in a diffusion-based MBR. One of the factors limiting the economic viability of polyhydroxyalkanoates (PHA) is the low volumetric productivity obtained with second-generation feedstocks, resulting from their low carbon concentration. In the present study, the use of membrane bioreactors (MBRs) was evaluated as a strategy to retain the microbial cells in the reactor and to enable a repeated supply of substrate without increasing the reactor volume. Two immersed MBR systems were studied: classical pressure-driven MBRs (hollow fibers and ceramic filters), and a novel diffusion-based MBR. In the latter, the rate of volatile fatty acid (VFA) diffusion across the membranes was lower than the VFA consumption rate of the culture, and thus, not suitable to attain high productivities. Possible research directions to increase substrate diffusion are suggested. On the other hand, pressure-driven configurations led to high values of productivity (0.87–1.44 g/(L.h)) during a fed-batch PHA accumulation using mixed microbial consortia. No flux reduction was observed in a 24 h fed-batch process, which allowed for a reduction of up to 82 % of the reactor volume, demonstrating the potential of this strategy. Hollow fibers and ceramic filters offered similar results during the fed-batch, but they presented different limitations and advantages.