Enhanced food waste anaerobic digestion: An encapsulated metal additive for shear stress-based controlled release

Abstract

Eco-friendly delivery carrier of trace metals is very significant to appropriate metal supplementation to improve metal bioavailability and prevent metal precipitation in anaerobic digesters. A novel encapsulated form of dry water (DW) additive was synthesized by the high-speed mixing of nickel ions solution with hydrophobic silica nanoparticles and was characterized using scanning electron microscope (SEM), nuclear magnetic resonance (NMR) and particle size analyzer. Nickel (Ni) was chosen as a model trace element. The encapsulated structure of DW additive and the feasibility of controlled release of Ni ions from DW additive in pure water and anaerobic digesters were verified. The optimal mixing speed for the highest methane yield of digesters with DW additive was determined as 800–1000 rpm. By varying stirring speed, controlled release of trace metal element (Ni) into digester was easily achieved during anaerobic digestion of food waste. The results showed that nickel ions delivery via DW additive significantly enhanced the metal bioavailability by 18.9–42.6% and process stability during 59 days of operation, resulting in an increase of 11.6–31.8% in methane yields compared to the controls. Pyrosequencing analysis indicated that bacterial communities were dominated by phyla Firmicutes, Bacteroidetes, Proteobacteria, Chloroflexi and Spirochaetes while methanogenic genus Methanolinea, Methanoculleus, Methanothrix, Methanomassiliicoccus and Methanospirillum were more abundant in archaea communities. Further, phyla Bacteroidetes, Proteobacteria and Methanoculleus were selectively enriched by the DW additive. Results indicated that it was feasible to apply the method of DW additive supplementation to enhance anaerobic digestion of food waste for biogas production by improving metal bioavailability.