Table 5.

Alternative uses of digestate.

Products	Valorization type	Description	Major concern	Scale	Ref
Fertilizers	Agricultural purposes	Liquid fraction from raw digestate mechanical separation	Relative low nutrient concentration, salinity, pathogens, chemical and organic contaminants	Commercial	(Al Seadi and Lukehurst, 2012; Nkoa, 2014)
		Struvite (magnesium ammonium phosphate, K, and calcium phosphate) obtained from crystallization in raw digestate liquid fraction mainly	Nutrient accessibility can be low especially in alkaline soils. In addition, it could contain heavy metal	Commercial	(Vaneeckhaute et al., 2017; Yetilmezsoy et al., 2017)
		Incineration ashes product of combustion of dried digestate pellets	No nitrogen P-accessibility tends to be poor	N/A	(Christel et al., 2014)
		Microalgae and macrophyte biomass from liquid fraction pre-treated digestate	High operational cost	Pilot	(Uggetti et al., 2014; Xia and Murphy, 2016)
Soil improver	Agricultural purposes	Solid fraction fibers, cake, obtain from raw digestate mechanical separation	It could contain inert material as stones, plastics, glass, and metal parts. Low nutrient concentrations and it is not as stable as compost	Commercial	(Al Seadi and Lukehurst, 2012; Teglia et al., 2010)
		Compost from solid fraction composting digestate	Heavy metal and inert material (stones, plastics, glass, and metal parts) contents	Commercial	(Dahlin et al., 2015; Teglia et al., 2010)
		Vermi-compost from solid fraction digestate	Heavy metals and pathogens contents.	Commercial	(Quintern and Morley, 2017)
		Biochar obtained from pyrolysis of dried digestate pellets	Low nutrient accessibility	Commercial	(Christel et al., 2014)
		Humic-like substances obtain from raw digestate alkaline extraction	R&D needed	Laboratory conditions	(Montoneri, 2017)
Biofuel/Biomass	Energy	Dried pellets from raw digestate or solid fraction	High ashes content and low calorific value	Pilot/Commercial	(Kratzeisen et al., 2010).
		Bio-ethanol from post-treatment for digestate fermentation	Research needed	Laboratory conditions	(Monlau et al., 2015)
		Bio-oil from pyrolysis of dried digestate pellets.	Removal of tars	Industrial/Pilot	(Balat et al., 2009; Wei et al., 2018)
		Bio-hydrogen from post-treatment for digestate fermentation	Research needed	Laboratory conditions	(Uggetti et al., 2014)
		Bio-diesel from microalgae harvesting and extraction	Research needed	Pilot	(Uggetti et al., 2014)
		Bio-methane from post-treatment for digestate post-digestion or recirculation	Minimum 35% methane in biogas needed for combustion in Stirling engines	Commercial.	(Monlau et al., 2015)
		Syngas from pyrolysis or gasification of the dried pellets	Necessary gas conditioning, especially the removal of tars. Fermentation: low gas solubility	Commercial	(Balat et al., 2009)
Biomaterials	Other industrial	Biopesticide from raw digestate or solid fraction as growing media after inoculation of the B. thuringiensis.	Research needed to Solid State Fermentation upscaling process and reactor design	Laboratory conditions	(Rodríguez et al., 2019)
		Duckweed or microalgae biomass, from phototropic raceway pound with pre-treated liquid fraction	Low protein content	Not clear	(Uggetti et al., 2014; Vaneeckhaute et al., 2017)
		Mushrooms from composting digestate or solid fraction with the addition of bulking material	R&D needed	Not clear	(Stoknes et al., 2016)
		Biochar (raw or activated) from dried pellets pyrolysis or gasification	Biochar commercial production is still mainly	Commercial	(Hagemann et al., 2018; Wu et al., 2017)
		Bioplastics (poly-vinyl alcohol-co-ethylene) obtain from hydrolysis and extraction of organic matter complex soluble of the digestate	R&D needed	Laboratory conditions	(Franzoso et al., 2016)
		Biosurfactants (the glycolipid, sophorolipids) from digestate by using a yeast (Starmella bombicola) inoculum under aerated conditions	High operational cost	Laboratory conditions	(Cerda et al., 2019)

Note: R&D = Research and Development.