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A liquid digestate rich in ammonium nitrogen (8.3 g L−1) was collected from an agricultural biogas plant and supplied to microalgae cultures as their only nitrogen source. Chlorella vulgaris was cultivated for up to 21 days, both under controlled conditions in laboratory-scale glass-column photobioreactors as well as outdoors in an open pilot-scale thin-layer photobioreactor. By systematically addressing issues associated with the use of liquid digestate (i.e., turbidity, nutrient imbalance, ammonium toxicity, and acidification), microalgae were robustly cultivated at a high density and cultures achieved a net biomass dry weight of between 10 and 14 g L−1, and a productivity of up to 0.93 g L−1 d−1 (93% of maximum expectation). Cultivation in the thin-layer photobioreactor achieved areal productivities between 7 and 10 g m−2 d−1. Water acidification due to the uptake of ammonium by microalgae was prevented by a controlled addition of NaOH. A detailed mass balance showed that, despite high removal efficiencies (approximately 3% of the supplied nitrogen remained in the medium), microalgae assimilated only 40–60% of the supplied nitrogen and, consequently, a large amount of nitrogen was lost to the atmosphere.

Microalgae present promising green economy applications in the energy and biorefinery sectors. The work concerns a pilot study on the integration of anaerobic digestion with microalgae cultivation for managing at the same time emissions and digestate from the dry anaerobic treatment of organic waste. Biogas produced was used to feed Solid Oxide Fuel Cell after a filtering step for removing toxic compounds. The exhausts and digestate were used for providing carbon and nutrients for microalgae growth. The experimental workflow includes the characterization of both for defining their suitability in the microalgal growth (Chlorella vulgaris) tests. The exhausts of Solid Oxide Fuel Cells showed relatively stable concentration of CH4 (4–7%) and CO2 (93–96%) and low concentrations (sub ppm(v)) of sulphur, carbonyl and carboxyl, and aromatic compounds and terpenes, making it particularly suited for algae growing as compared with internal combustion engines. The challenging growing conditions are a compromise between carbon recovery and use of digestate. A good microalgae growth has been obtained (22.31 mm3 mL−1 of biovolume corresponding to 151 dry mg L−1 day−1) exploiting ammonia and phosphate from dilute digestate (removal efficiency 94% and 30% respectively) as well as a good carbon recovering (310 mg CO2 L−1 day−1). Based on our data, the integration of microalgae growth and anaerobic digestion process seems a viable solution to achieve (i) reduced emissions due to carbon recovery; (ii) optimum integrated management of anaerobic digestion waste and (iii) biomass production by low-cost nutrients and carbon.

Nutrient recycling is essential to make microalgae cultivation sustainable at industrial scale. To this aim, lab-scale anaerobic digestion experiments of Chlorella vulgaris after lipid extraction were carried out to evaluate biogas yield and nutrients recovery in the liquid digestate. Then, this liquid fraction was used as source of nutrients for cultivation of a closely related C. vulgaris species, in order to assess the possibility of re-cultivating microalgae towards a closed-loop nutrient recycling process. The biological methane potential tests resulted in biogas production of about 347 NmL gVS−1 with 43% v/v methane content. In re-growth experiments, the liquid digestate showed an insufficient amount of soluble sulfate and phosphorus. However, by amending these two nutrients, the specific growth rate and final biomass concentration increased to about 2 d−1 and 2 g L−1, respectively, which were comparable to those obtained in a defined control medium. The low content of soluble phosphorus in liquid digestate was mainly due to precipitation and removal with the solid phase. Several techniques were hence tested to enhance phosphorus solubilisation, and highest recoveries of up to 41% were obtained when using NaHCO3. Finally, C. vulgaris was grown in such treated digestate, obtaining a final biomass production comparable to that of the control, without the need of external phosphorus supply.

In this study, zeolite was employed for the separation and recovery of P from synthetic wastewater and its use as phosphorus (P) source for the cultivation of the green microalga Chlorella vulgaris and the cyanobacterium Arthrospira (Spirulina) platensis. At P-loaded zeolite concentration of 0.15–1 g/L, in which P was limited, the two species displayed quite different behavior regarding their growth and biomass composition. C. vulgaris preferred to increase the intracellular P and did not synthesize biomass, while A. platensis synthesized biomass keeping the intracellular P as low as possible. In addition under P limitation, C. vulgaris did display some little alteration of the biomass composition, while A. platensis did it significantly, accumulating carbohydrates around 70% from about 15%–20% (control). Both species could desorb P from zeolite biologically. A. platensis could recover over 65% and C. vulgaris 25% of the P bounded onto zeolite. When P-loaded zeolite concentration increased to 5 g/L, P was adequate to support growth for both species. Especially in the case of C. vulgaris, growth was stimulated from the presence of P-loaded zeolite and produced more biomass compared to the control. ispartof: International Journal of Molecular Sciences vol:16 issue:2 pages:4250-4264 ispartof: location:Switzerland status: published