FATE AND EFFECT OF SILVER ON THE ANAEROBIC DIGESTION PROCESS

摘要

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 hiodegradability of a silver- hearing waste activated sludge (5.0 g silver:/kg sludge dry solids) was 6l/100 as compared to 59/100 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 l00 mg Ag/I did not affect the rate and extent of methane production. Silver thiosulfate when tested at an equivalent concentration of l00 mg Ag/l (and l000 mg S/l), resulted in accumulation of ca. 28 mM of fatty acids (mainly acetate), 90/100 inhibition of methanogenesis and 39/100 inhibition of acidogenesis. However. when using silver-free. thiosulfate-amended controls, it was concluded that the observed inhibition in the silver thiosulfte-amended cultures was not attributed to the silver hut 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 (< l0~-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 us thc reduction to elemental silver. relatively high levels of silver (at least up to l00 mg Ag/I) can be tolerated by anaerobic digestion systems. The results of this study have important implications on the biological treatment and management of photoprocessing wastewaters.