| Microbial technology that could convert wastes into high quantity of methane(CH4) and hydrogen(H2) for energy could reduce the use of fossil fuels, which are limited and contribute towards carbon dioxide(CO2) emission. Biogas digester represents anaerobic digestion(AD) technology that could convert waste into renewable energy in the form of biogas that comprised approximately 60% CH4 and 40% CO2. Recently, attention has also been paid to bio-hydrogen production from dark fermentation(DF), and microbial electrolysis cell(MEC) as clean energy sources and potential alternatives to fossil fuels. Dark fermentation is the first stage of anaerobic fermentation(acidogenesis) and requires controlling p H, during which H2 and CO2 are produced, in addition to volatile fatty acids(VFAs), while MEC is a technology that uses electricity to initiate bacterial oxidization of organic matter to produce H2. In this dissertation, combinations of the three types(DF, MEC, and AD) of technology are studied to optimize the production of H2 and CH4.Chapter 2 was focused on using DF and MEC as two different stages, with the main objective of determining if using these two technologies in two stages could increase in H2 production. Five different treatments were used during the experiment: A(MEC-only without plastic separator between electrodes); B(MEC-only with plastic separator between electrodes); C(DF for 24 h, followed by treatment with MEC without plastic separator between electrodes); D(DF for 24 h followed by treatment with MEC containing plastic separator between electrodes); and E(DF only). The substrate used during the experiment was a mixture(8% TS) of corn, bio slurry, and wastewater, moreover the p H during the experiment were between 4.9- 5.5. Reactor A produced the highest average quantity of H2(1.89 m3/m3/day), with an average energy efficiency based on electricity(?e) of 254%. Reactor C produced approximately the same amount of H2 as Reactor A(1.8 m3/m3/day), but had ?e that was 51% higher than Reactor A. Moreover, Reactor C used less electric energy(22% of total energy) in first 24 h than Reactor A since MEC was not applied during the first 24 h in Reactor C. Reactor E, which only had DF, produced the lowest average daily H2 quantity(1.49 m3/m3/day). Compared to Reactors A and C, Reactors B and D that contained plastic separators between the anode and the cathode produced lower average daily H2 quantity(1.69 m3/m3/day, and 1.5 m3/m3/day, respectively), and had lower values for ?e(141% and 114%, respectively). Compared to DF, using MEC resulted in higher H2 production, but the use of MEC after DF(Reactor C) resulted in higher energy recovery efficiency(?e) than the MEC only treatment(Reactor A). Additionally, Reactor C also resulted in higher gas production(cumulative biogas) than DF. Thus, we recommend the combined use of DF and MEC, with DF as the first stage before the MEC.In Chapter 3, DF and MEC were used in a two-stage process, but the focus was on varying treatments, heating(80°C for 15 minutes) and voltage(2.5 V for 15 minutes) after injecting fresh substrate in each cycle(total of five cycles). The idea was to use heat and voltage treatments to control the methanogenic community by increasing the temperature to 80°C for 15 minutes and then back to the normal temperature(35°C), and increase the voltage from 0.7V to 2.5V for 15 minutes and then back to 0.7 V. The experiments were carried out with three main groups: Group A(DF for 24 h followed by MEC), Group B(MEC only), and Group C(blank; no MEC and no DF). Group A and Group B each had four reactors, with each reactor having different treatments: 1(without voltage or heat); 2(heat only); 3(heat and voltage); 4(voltage only). Group C had one reactor(blank). The results showed that in Group A, Reactor A3(treatment with heat and voltage) produced the highest quantity of H2 gas, with a cumulative biogas production of 3989 m L after 280 hours(5 cycles), followed by A2(heatonly treatment)(3800 m L), A4(voltage-only treatment)(3500 m L), and A1(without voltage or heat)(3311 m L). In Group B, Reactor B3 produced the highest cumulative biogas of 3878 m L after 280 hours(5 cycles) with average H2 1.28 m3/m3/day followed by B4(3830 m L, with average H2 1.2 m3/m3/day), B2( 3438 m L, with average H2 0.746 m3/m3/day), and B1(2718 m L, with average H2 0.407 m3/m3/day). These results showed that using both treatments of heat and electricity(A3 and B3) could considerably increase in H2 production compared to the other treatments within Group A and Group B. Comparison of Groups A and C showed that A3 in Group A produced more H2(1.5 m3 H2/m3/day, 0.017 m3CH4/m3/day) than treatments in Group C(0 m3 H2/m3/day, 0.38 m3CH4/m3/day). The ?e for reactor A3(250%) was compared to reactor B3(175%). The result showed that using DF followed by MEC(reactor A3) produced a higher quantity of H2 than when only MEC was used(reactor B3), while reducing overall electricity needed by approximately 45%. The use of heating and voltage treatments also resulted in an increase in H2 gas production.The period of dark fermentation is very short and there is a need to increase MEC efficiency, so the main objective of Chapter 4 was to determine the effect of combining anaerobic digestion(AD) and MEC(Reactor A) on energy production when compared with a normal digester without MEC(Reactor B). In reactor A, a single chamber MEC(150 m L) was placed inside a 10 L digester(80% active volume) on Day 10 after starting the experiment. Cattle manure was used as substrate in both reactors, and an inoculum to substrate ratio(volatile solids basis) of 2:1 was used. Cumulative H2 and CH4 production during the batch test(272 h after starting MEC) in A(2.43 L H2, 23.64 L CH4) was higher than the quantity observed in B(0.01 L H2, 10.9 L CH4). Hydrogen concentration in biogas within the first 24 h after placing the MEC went up to 20%, after which, it reduced as CH4 concentration increased from 50% to 64%. Additionally, after accounting for energy produced in reactor B, the efficiency of electrical energy recovery(?e) in MEC was 324%(?e max.), 73.1%(?e min.), with an average of 170% over time. COD conversion efficiency was also observed to be higher in reactor A(7.09 KJ/g COD removed) when compared with reactor B(6.19 KJ/g COD removed). Chapter 4 showed that combining AD with MEC could raise overall energy production from the digestion of cattle manure.The objective of Chapter 5 was to study the effect of performing AD combined with MEC(Reactor A) and effect of using MEC for only a short time(120 h) followed by normal digestion(Reactor B) on energy production. Food waste was used as substrate, and an inoculum to substrate ratio(volatile solids basis) of 1:1 was used. Using MEC continuously in the digester reduced CO2 concentration to approximately 5% in reactor A, while increasing in the concentration of H2 and CH4 to a maximum of 95.0% and an average of 82.7% during the experimental period. Cumulative H2 production was higher in Reactor A(480 h of combined MEC and digestion treatment) compared to Reactor B(120 h of MEC followed by 120 hours of normal fermentation) or Reactor C(without MEC)(4.3 m3/m3, 2.25 m3/m3, and 0.2 m3/m3, respectively). Moreover, increased COD removal(82 % reduction) was observed in Reactor A compared to Reactor B(74.6% reduction) and Reactor C(68% reduction in COD). Therefore, we recommend adding MEC to AD for continuously supplying voltage to the MEC, in order to increase in energy production and COD removal.Chapter 6 was written to present solutions for improving the existing digesters design by combining digesters with MEC to maximize energy production as future design. An environmentally friendly way of supplying electricity to the MEC was also considered within Chapter 6. The chapter was divided into three parts. The first part studied the use of solar energy as a source of electricity for MEC, and the results showed in case we had more than one MEC or multi-electrodes, it is better to connect the MECs or a pair of electrodes to solar cell as a panel of MECs. Due to the voltage of solar cell higher than the target voltage of MEC, so the study recommended, first connect a number of MECs in the series, because the voltage divided by the number of MECs in series connection until reaching to the optimum MEC voltage(0.2-1.3 V), and then connect the rest MECs to the solar cells in parallel. The second part of the study focused on simulating heat exchange of an underground digestion system that had a coverall greenhouse with solar cell, an inner greenhouse for inlet heating, and MEC as a separate stage before digestion(System A). Results were compared to the heat balance of a similar digestion system without any heating source(System B). The third part of the study was focused on predicting the geographic locations where System A's design could be used. This study recommends the use of biogas digester combined with MEC powered by solar energy as a source of heating for the digester. |
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