Anaerobic co-digestion of agro-industrial cashew nut wastes with organic matters for biogas production: case of cashew nut hull and cashew almond skin

Abstract

The cashew nut wastes (CNW), more released in the environment by cashew nut transforming units, are underused based on bioenergy. This study aimed  at evaluating the biogas production by the anaerobic codigestion of CNW with various organic feedstocks. Physicochemical parameters and lower  heating value (LHV) of cashew nut hull (CH) and cashew almond skin (CS) were determined. Batch cultures of each waste, supplemented to pig manure  (PM), cow dung (CD), fermentation sludge (FS), activated sludge (AS) and Macrotermes bellicosus termite gut homogenate (TGH), were performed in  anaerobic and mesophilic conditions. The biogas and methane (CH4) were measured. CNW were acid (pH 5.86-6.44), with 86.80-92.38% volatile solids (VS)  and slightly high carbon/nitrogen ratio (31.94-37.79). The LHV was 23.094 MJ/Kg and 18.113 MJ/Kg for CH and CS, respectively. The highest yields  were retrieved with the co-digestion PM-CH (189.16 L biogas.kg^-1 VS, 130.89 L CH4.kg^-1 VS, and 69.2% CH4) and CD-CS (159.85 L biogas.kg^-1 VS, 99.58 L  CH4.kg¹ VS, and 62.3% CH4). Therefore, to boost biogas and biomethane yield, it is necessary to adjust the C/N ratios to the optimal values, and maintain  the pH to the neutral, and combine PM and CD as inoculum during the digestion process of CNW.