Fate and effect of silver on the anaerobic digestion process

Abstract

Laboratory assays were conducted to assess the anaerobic biodegradability of a silver-bearing, waste activated sludge as well as the effect of silver compounds on the anaerobic digestion process. All assays were performed at 35°C in the dark. The ultimate biodegradability of a silver-bearing waste activated sludge (5.0 g silver/kg sludge dry solids) was 61% as compared to 59% for the control (i.e., silver-free) sludge. The rate and extent of methane production was similar for both sludge samples. Addition of either silver nitrate or silver sulfide to methanogenic, mixed cultures up to an equivalent concentration of 100 mg Ag/l did not affect the rate and extent of methane production. Silver thiosulfate when tested at an equivalent concentration of 100 mg Ag/l (and 1000 mg S/l), resulted in accumulation of ca. 28 mM of fatty acids (mainly acetate), 90% inhibition of methanogenesis and 39% inhibition of acidogenesis. However, when using silver-free, thiosulfate-amended controls, it was concluded that the observed inhibition in the silver thiosulfate-amended cultures was not attributed to the silver but rather to the excess thiosulfate (used as an alternative electron acceptor resulting in the production of soluble sulfide at inhibitory levels). Computer simulations under typical anaerobic digestion conditions using the geochemical equilibrium speciation program MINTEQA2 resulted in extremely low concentrations (<10 −14 M) of free silver ions (Ag +). The two predominant insoluble silver species were Ag 2S and Ag°. Therefore, due to the high complexing capacity of the anaerobic digester mixed liquor as well as the reduction to elemental silver, relatively high levels of silver (at least up to 100 mg Ag/l) can be tolerated by anaerobic digestion systems. The results of this study have important implications on the biological treatment and management of photoprocessing wastewaters.