Valorising nutrient-rich digestate: Dilution, settlement and membrane filtration processing for optimisation as a waste-based media for microalgal cultivation

• Published in: Waste management (Elmsford) 2020-12, Vol.118, p.197-208
• Main Authors: Fernandes, Fleuriane, Silkina, Alla, Fuentes-Grünewald, Claudio, Wood, Eleanor E., Ndovela, Vanessa L.S., Oatley-Radcliffe, Darren L., Lovitt, Robert W., Llewellyn, Carole A.
• Format: Article
• Language: English
• Subjects: Chlorella vulgaris; Digestate; Membrane filtration; Microalgae; Pilot-scale; Settlement and dilution
• ISSN: 0956-053X; 1879-2456
• DOI: 10.1016/j.wasman.2020.08.037

[Display omitted] •Dilution, settlement, and membrane filtration to valorise digestate.•Digestate optimised as a waste-based medium for Chlorella vulgaris cultivation.•Mechanical treatment separated liquid and solid fractions and recovered nutrients.•C. vulgaris performed better with low concentrations of processed digestate (2.5%)•C. vulgaris growth on digestate is scalable and converts excess nutrients to biomass. Digestate produced from the anaerobic digestion of food and farm waste is primarily returned to land as a biofertiliser for crops, with its potential to generate value through alternative processing methods at present under explored. In this work, valorisation of a digestate resulting from the treatment of kitchen and food waste was investigated, using dilution, settlement and membrane processing technology. Processed digestate was subsequently tested as a nutrient source for the cultivation of Chlorella vulgaris, up to pilot-scale (800L). Dilution of digestate down to 2.5% increased settlement rate and induced release of valuable compounds for fertiliser usage such as nitrogen and phosphorus. Settlement, as a partial processing of digestate offered a physical separation of liquid and solid fractions at a low cost. Membrane filtration demonstrated efficient segregation of nutrients, with micro-filtration recovering 92.38% of phosphorus and the combination of micro-filtration, ultra-filtration, and nano-filtration recovering a total of 94.35% of nitrogen from digestate. Nano-filtered and micro-filtered digestates at low concentrations were suitable substrates to support growth of Chlorella vulgaris. At pilot-scale, the microalgae grew successfully for 28 days with a maximum growth rate of 0.62 day−1 and dry weight of 0.86  g⋅L−1. Decline in culture growth beyond 28 days was presumably linked to ammonium and heavy metal accumulation in the cultivation medium. Processed digestate provided a suitable nutrient source for successful microalgal cultivation at pilot-scale, evidencing potential to convert excess nutrients into biomass, generating value from excess digestate and providing additional markets to the anaerobic digestion sector.