Enhancement of Bıogas Productıon From Cattle Manure Usıng a Combıned Mıcrobıal Electrolysıs Cell and Anaerobıc Dıgester
Özet
It is assumed that fossil fuel sources which accounts for 80 % of the total energy production in the world, will be depleted in the near future. Because of the depletion of fossil fuel sources and environmental concerns, the attentions have been canalized to renewable energy resources. Being one the most utilized renewable energy resource in the world, biomass is also used extensively in anaerobic digestion processes for methane production. Recently a new technology called Microbial Electrolysis Cell (MEC) was introduced as an alternative and sustainable approach to harvest hydrogen, methane and other valuable chemicals from the organic materials and to treat waste and wastewater simultaneously. The studies conducted on MECs revealed that the newly technology is superior to conventional anaerobic digestion (AD) in terms of methane production and organic removal efficiency. Therefore, combined/integrated MEC+AD systems have been introduced to overcome the limitations of anaerobic digestion such as unstable process, insufficient treatment, low rate methane production, etc.
So far, organic materials used as substrate in MECs included synthetic wastewater, acetate, waste activated sludge, leachate, food waste, pig slurry and other wastes. And yet the operating conditions such as hydraulic retention times (HRT) and organic loading rates (OLR) were not chosen in the range of that would force the limits of the reactors. Thereby, cattle manure which to our knowledge have not been applied to MEC reactors before was chosen as the substrate in this study. Also, because it was stated in many studies that MEC technology was superior to conventional AD technology, it was thought that combining MEC and AD could overcome the challenges of conventional AD technology and enhance the treatment and methane production performances. As a result the main objective of this thesis was determined as to enhance methane production from cattle manure in a combined MEC+AD reactor operated at different conditions. Firstly Biochemical Methane Potential (BMP) of cattle manure was investigated in MEC+AD and control reactors to determine the differences in terms of methane production, degradation efficiency and treatment time. BMP tests were conducted at the start-up period and at the acclimatized conditions of the reactors. Then MEC+AD and control reactors were operated on semi-continuous mode by feeding with manure at fixed content (3 % VS, 4.15 % TS, 30 g VS/L) and HRTs from 6 days to 1 day in descending order. The feeding corresponded to OLRs from 5 to 30 g VS/L/d. After that, the reactors were operated at fixed HRT of 2 days by feeding with manure at content of 4.5 % VS and 6 % VS corresponding to OLR of 22.5 and 30 g VS/L/d respectively. Meanwhile MEC+AD reactors operated at different HRTs and OLRs were supplied with external voltages of 0.3, 0.6 and 1.0 V as well. Biogas productions, methane yields, organic removal rates and current productions of the reactors were observed during the entire study.
The results showed that biogas productions increased consistently in MEC+AD reactors from the lowest OLR of 5 g VS/L/d to highest OLR of 30 g VS/L/d. Biogas productions in MEC+AD reactors changed between 1.23 L/L/d (HRT:6 day, OLR:5 g VS/L/d, 0.3 V) and 5.11 L/L/d (HRT:2 day, OLR:30 g VS/L/d, 1.0 V) depending on HRT and OLR. Methane yields of the MEC+AD reactors changed between 0.09 and 0.24 L CH4/g VS, decreasing by the increase in OLR. The highest methane yield of 0.24 L CH4/g VS was obtained at OLR and HRT of 5 g VS/L/d and 6 days respectively in MEC+AD with supplied voltage of 0.6 V. Methane content of the biogas produced from MEC+AD reactors were in the range of 75-80 % at all operational conditions. The methane content of biogas was totally independent of the input voltage and the applied HRTs and OLRs in this study. Input voltages of 0.6 and 1.0 V were significantly effective on biogas productions at OLRs of as high as 20-30 g VSL/d. During the entire study biogas productions and methane yields of MEC+AD reactors were superior to control reactors at all HRTs (6, 4, 3 days). Energy assessments of the reactors showed that (MEC+AD)0.3V reactor exhibited the highest energy efficiency according to the energy input and energy output. The energy content of methane obtained from (MEC+AD)0.3V reactor were 200 folds of the energy supplied to the reactor. Highest COD, TS and VS removal efficiencies obtained in MEC+AD reactors were observed at HRT of 6 days and OLR of 5 g VS/L/d. The highest removal efficiencies were between 41.4 and 44.9 % for COD, 26.1 and 29.5 % for TS and 34.3 and 37.7 % for VS respectively. Current productions in MEC+AD reactors were strictly depended on the supplied voltage amount. Highest and lowest current productions were generally in the range of 4 - 6 mA/L and 1 - 2.5 mA/L at supplied voltages of 1.0 and 0.3 V respectively.
