Enriching a biomass with polyhydroxyalkanoate-storing organisms (PHA-storers) is an essential step in upgrading organic carbon from municipal wastewater to PHAs (bioplastics). A major challenge is to create selective growth conditions favourable to PHA-storers while using continuous reactors. Our study investigates high influent COD to phosphorus ratios (COD:P ratios) as a tool for robust selection of PHA-storers in a single continuous-flow stirred-tank reactor (CSTR). Therefore, we operated five CSTRs in parallel, fed with synthetic wastewater (50% acetate - 50% propionate) with different COD:P ratios (200-1000 gCOD gP-1), and performed a detailed analysis of the microbial communities over long-term (30-70 solid retention times). Our study demonstrates that efficient and robust selection of PHA-storers can be achieved in a single CSTR at high influent COD:P ratios. The selective advantage for PHA-storers increases with the influent COD:P ratio, but only as long as growth conditions remain limited by both carbon substrate and P. Indeed, selection performance deteriorates when COD:P ratios are too high and growth conditions are limited by P only. At an optimal COD:P ratio of 800 gCOD gP-1, a stable microbial community consisting of >90% PHA-storers and dominated by Pannonibacter sp. was selected in the long-term. Finally, our results suggest that high COD:P ratios provide a selective advantage to microorganisms with low cellular P requirements, explaining why different PHA-storers (i.e. Xanthobacter sp. vs. Pannonibacter sp.) were selected depending on the influent COD:P ratio (i.e. 200 vs. 800 gCOD gP-1). Overall, we propose a new CSTR-based approach for selecting PHA-storers as a relevant alternative to the conventional approach relying on the use of sequencing batch reactors.
