Sludge retention time impacts on polyhydroxyalkanoate productivity in uncoupled storage/growth processes

Highlights: • Pilot scale polyhydroxyalkanoate production with uncoupled carbon/nitrogen feeding. • Similar biomass volumetric productivities for different sludge retention times. • Lower sludge retention time favours growth rate and yield. • PHA storage yield directly linked with relative abundance of putative PHA-storers. • Higher sludge retention time led to higher storage kinetics/accumulation capacity. The process involving mixed microbial cultures (MMCs) and waste-based substrates emerged as an alternative solution to reduce the market price of polyhydroxyalkanoates (PHAs). The selection of an efficient MMC that displays a significant PHA accumulation potential and a high growth rate is considered a key factor for the MMC PHA production feasibility. This study used a pilot plant to investigate the dynamics of growth vs storage in a mixed culture fed with fermented fruit waste under uncoupled carbon and nitrogen feeding. Varying sludge retention times (SRTs) (2 and 4 d) and organic loading rates (OLRs) (from 2.6 to 14.5 gCOD.L⁻¹.d⁻¹) were imposed for this purpose. Results showed that, regardless of the OLR imposed, cultures selected at lower SRT grew faster and more efficiently using stored PHA. However, they had inferior specific storage rates and accumulation capacity, resulting in lower PHA productivity. Additionally, the polymer storage yield was independent of the SRT, and was directly linked with the abundance of putative PHA-storers in the MMC. The high PHA productivity (4.6 ± 0.3 g.L⁻¹.d⁻¹) obtained for the culture selected at 4 d of SRT was 80% above that obtained for the lower SRT tested, underlining the importance of achieving a good balance between culture growth and accumulation capacity to increase the viability of the PHA-producing process from wastes.