Biostimulation of food waste anaerobic digestion supplemented with granular activated carbon, biochar and magnetite: A comparative analysis

Highlights: • Anaerobic digestion enhancement with three conductive materials were investigated. • CM have physicochemical and morphological properties that favor AD process. • Process stability and methane yield is further enhanced with the use of magnetite. • Electrical conductivity is not the determining factor in process improvement. • Lower doses of CM (GAC/BC = 1 g/L and magnetite = 0.1 g/L) show higher methane yield. The biostimulant effect of three conductive materials in anaerobic digestion (AD) of food waste (FW) was investigated. These materials included granular activated carbon (GAC) and biochar (BC) (10, 5 and 1 g/L) and magnetite (Fe₃O₄) (1, 0.5, 0.1 g/L). All three materials have physicochemical properties that enhance AD process. The favorable features include a basic character that improves the buffering capacity of the system and avoids inhibitions due to VFA accumulation, a suitable morphology for the colonization by microorganisms and the formation of biofilms. Conductive materials (CM) also have surfaces with alkaline and alkaline earth metals (mainly K⁺, Na⁺, Mg²⁺ and Ca²⁺) which provide trace elements and could cause CO₂ carbonation due to the effect of metal oxides. In addition, carbonaceous materials (GAC and BC) possess quinone groups that have a suggestive redox activity to enhancing direct interspecies electron transfer (DIET). Experimental tests, modified Gompertz model and statistical analysis show that the increase in biogas production is not exclusively attributed to the microorganisms using the materials as immobilizing matrix and although electrical conductivity is important, it is not the determining factor of the improvement, since the increase in yield is not directly proportional to the value of this parameter. The materials and doses with the best results were Fe₃O₄ 0.1 g/L and BC 1 g/L with an increase in methane yield of 30.1% and 20.3%, respectively. In the range of doses analyzed, all three materials share that at lower doses, methane production is higher.