Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Chemical and Biochemical Engineering

Supervisor

Nakhla, George

Abstract

In Canada, about $31 billion worth of food is wasted annually. This amount of food waste (FW) ends up in landfills where it is naturally broken down by bacteria and releases methane to the atmosphere, a powerful greenhouse gas (GHG) which is 21 times more harmful to the environment than carbon dioxide (CO2). This process can also be done in a controlled environment at wastewater treatment plants, in a process known as anaerobic digestion (AD). Diverting food waste from landfills to wastewater treatment facilities allows for the capture of the methane, which can be used as an energy source. Although AD is a relatively reasonable technology to treat food waste, digesters that take FW as the sole feedstock have been facing unstable performance and even process failure mainly because of the accumulation of volatile fatty acids (VFAs) which is linked to trace elements (TEs) deficiencies. TEs play a key role in stimulating the enzymatic activity of the methanogenic micro-organisms and enhancing metabolic pathways.

In this study, the importance of trace element (TE) background levels, especially Fe in the inoculum utilized for the mesophilic FW anaerobic digestion was assessed, while single and mixed ionic TEs were supplemented to batch reactors. Primarily, series of specific methanogenic activity (SMA) tests was carried out with acetate as substrate to observe the effects of TE ions supplementation on methanogenic activity. Subsequently in the biomethane potential tests (BMP), where FW was inoculated with the sludge and the effects of TE ions supplementation on the methane yield, maximum specific methane production rate (SMPRmax), and hydrolysis rate constant (Kh) were determined. The experiments were conducted first with an Fe-rich sludge and then repeated with a low-Fe sludge. Finally, the results of the two studies were compared in terms of methane yield and digestion kinetic parameters. SMA tests with the Fe-rich inoculum showed that Fe2+ (50-400 mg/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.

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