Raw used cooking oil valorization into polyhydroxyalkanoates by mixed microbial cultures: evaluation of one- and two-unit configuration

• Raw used cooking oil was successfully valorized into PHBV by MMCs. • MMC enrichment was achieved using a feast and famine strategy. • Urea as nitrogen source improved pH stability and enhanced PHBV accumulation. • Optimal PHBV yield and productivity were achieved at 48 h HRT in one and two units. • One-unit approach enables enrichment and accumulation, simplifying PHBV production. The production of polyhydroxyalkanoates with untreated used cooking oil (UCO) as substrate represents an interesting strategy to valorize this residue into a value-added product. Three sequencing batch reactors (R1, R2, and RA) were operated, using mixed microbial cultures (MMCs) fed with raw UCO. R1 and R2 operated as enrichment units, with withdrawal at the end of the feast and famine phases, respectively. Enrichment was achieved in both within 30 days, reaching similar accumulations of the copolymer poly(3-hydroxybutyrate -co- 3-hydroxyvalerate) (PHBV) (17.76 wt% in R1 and 12.47 wt% in R2). To improve process stability and reduce chemical consumption for pH control, urea was evaluated as nitrogen source, resulting in a reactor less dependent on pH control and increasing PHBV content from 16.7 to 25.7 wt%. Then, the accumulation unit (RA) was operated in series with R1 to evaluate the maximum PHBV production of the biomass and compare the one-unit (R1) and two-unit (R1 + RA) configurations. Different hydraulic retention times (HRTs) were studied for the one-unit (24 and 48 h) and the two-unit (48 and 96 h) configurations. The best overall performance was observed at an HRT of 48 h in both cases, with similar accumulations (44.8–49.1 wt% PHBV), yields (230–250 g PHBV/kg UCO) and productivities (0.010–0.013 g PHBV/(L·h)), showing that the one-unit was the best strategy for its operational simplicity. These results demonstrate the feasibility of enriching MMCs to produce PHBV using raw UCO, highlighting the potential of one-unit configuration to perform enrichment and accumulation steps in the same reactor.