/L), Ni2+ (0.5-2 mg/L), Co2+ (0.1-0.5 mg/L), and Se4+ (0.005-0.05 mg/L) had drastic negative impacts on methane production rates. Mo6+ (2-20 mg/L) was the only TE ion that did not significantly affect SMA rates. On the other hand, with the low Fe sludge, the same ionic TE concentrations (Se4+ concentrations ranged from 0.1 mg/L to 0.8 mg/L) did not affect the methane production rate but 5 mg Mo6+/L, 20 mg Mo6+/L, and 400 mg Fe2+/L addition increased the SMA rates by 28%, 22%, and 20%. Single and mixed ionic TE supplementation to the batch reactors with the Fe-rich sludge led to the same values of SMPRmax and Kh for TE dosed and control reactors, except for Fe2+ and Co2+ which reduced the Kh as well as Mo6+ which decreased the SMPRmax. Soluble ionic TE concentrations decreased significantly during the experiments. The estimated amount of free sulfides (S2-) was well below the S2- concentration required to precipitate all the TE ions i.e. Fe2+, Ni2+, Co2+, Mo6+, and Se4+, therefore co-precipitation and adsorption onto iron sulfide complexes potentially played a significant role in reducing the soluble ionic TEs. In the second experiment with low Fe sludge, Ni2+, Co2+, Mo6+, Se4+ and Fe2+ increased the biogas yield by 28%, 25%, 21%, 29%, 13%. In BMP tests however, Ni2+, Co2+, Mo6+, Se4+, and Fe2+ supplementation increased the methane yield (CH4 per gram VSFW) by 27%, 23%, 31%, 32%, 22%. This positive effect of methane production was never observed with AD of FW with Fe-rich sludge. Maximum specific methane production rates (based on the modified Gompertz model) as well as hydrolysis rate constants (Kh) resulted in the same values for all ionic TE dosed and control reactors. Exceptions were for Co2+ at 0.1 mg/L which reduced the Kh value by 33% as well as Fe2+ which at 100, 200, and 400 mg/L increased the Kh by 74%, 57%, and 42%, respectively. Moreover, all TEs decreased the digestion time (for 60% COD degradation) from 6.5 days (controls) to 2.5-4.5 days. Ni2++Co2++Se4+, Ni2++Co2++Mo6+, Mo6++Se4+, and Ni2++Co2+ increased the methane yield by 6%, 9% 12%, and 16%, respectively. Maximum specific methane production rates based on modified Gompertz model and Kh values were the same for TE added and control reactors. A comparison between the performance of anaerobic digestion of FW with the two types of sludge indicates that methanogenic activity, methane yields (in all cases), and hydrolysis rate constant (in the Fe2+ case) were improved significantly when TEs were added to the batch reactors with low Fe inoculum, potentially due to the very different levels of Fe in the two inoculums which increase TE bioavailability. Thus, supplementing TEs in AD should be accompanied with a trace element background check in the sludge to make sure that Fe concentrations are not at the levels to promote co-precipitation and/or adsorption of ionic TEs onto the abundant Fe sulfide precipitates.
Yazdanpanah, Andisheh, "Impact of Trace Element Ions and Fe2+ on Anaerobic Food Waste Digestion" (2018). Electronic Thesis and Dissertation Repository. 5475.