Processing recommendations for using low-solids digestate as nutrient solution for poly-ß-hydroxybutyrate production with Synechocystis salina

- Evaluating growth and PHB production of S. salina in digestate fractions.
- The highest growth of S. salina (1.12 d‐1) was obtained in digestate supernatant.
- The highest PHB concentration (95 mg L-1) was reached in diluted supernatant (1/3).
- Successfully up-scaled PHB production (89 mg L-1) in a 200 L photobioreactor.

Within the last decades, environmental pollution with persistent plastics steadily increased; therefore the production of biodegradable materials like poly-ß-hydroxybutyrate (PHB) is essential. Currently, PHB is produced with heterotrophic bacteria from crops. This leads to competition with food and feed production, which can be avoided by using photoautotrophic cyanobacteria, as Synechocystis salina, synthesizing PHB from CO2 at nutrient limitation. This study aims to increase the economic efficiency of PHB production with cyanobacteria by using nutrients from anaerobic digestate. First, growth and PHB production of S. salina in digestate fractions (supernatant and permeate, with/without precipitating agents) and dilutions thereof and then the scale-up (photobioreactor, 200 L working volume) were evaluated. With precipitated and centrifuged digestate diluted 1/3 the highest biomass (1.55 g L-1) and PHB concentrations (95.4 mg L-1), being 78% of those in mineral media, were achieved. In the photobioreactor-experiments biomass (1.63 g L-1) and PHB concentrations (88.7 mg L-1), being 79% and 72% of those in mineral medium, were reached, but in a cultivation time 10 days longer than in mineral medium. The possibility to use digestate as sustainable and low cost nutrient solution for microalgae cultivation and photoautotrophic PHB production, instead of applying it on fields or processing it to achieve discharge limits, makes this application a highly valid option.