Assessing the potential of Chlorella vulgaris for valorization of liquid digestates from agro-industrial and municipal organic wastes in a biorefinery approach

Abstract

Effluents from anaerobic digesters, digestates, represent promising substrates for cultivation of microalgae, resulting in efficient digestate treatment and production of a valuable feedstock for bio-based products. However, further research is needed until a large-scale application is established. Under this scope, the purpose of the present study was to investigate cultivation of the microalga Chlorella vulgaris in different types of digestates, derived from agro-industrial and municipal organic waste respectively, as well as under various culture conditions, and to explore the potential uses of the produced biomass. Cultivation in agro-waste digestate initiated from 10% (v/v) loading, while enrichment with 15% cheese whey (CW) increased both maximum specific growth rate, μmax and biomass production, reaching 0.84 d−1 and 1.63 g L−1, respectively. Increasing digestate concentration up to 50% decreased μmax, while 80% loading totally inhibited microalgal growth. In contrast, digested municipal organic wastes could be effectively used up to 100% resulting in biomass production of 1.24 g L−1. In terms of digestate remediation, removal of organic carbon, total nitrogen and phosphorus reached up to 92%, 77% and 94%, respectively depending on the applied conditions. Furthermore, culture conditions and digestate type determined composition of C. vulgaris’ biomass, which was mostly rich in proteins and carbohydrates. Photosynthetic pigments increased with increasing digestate concentration, while lipids, mostly rich in polyunsaturated fatty acids (PUFAs) decreased. Lastly, biomass extracts inhibited growth of Bacillus subtilis, with minimum inhibitory concentrations (MIC) between 0.3 and 2.85 mg mL−1, revealing the potential of digestate-grown microalgae for pharmaceuticals production.