Anaerobic digestion (AD) is a well-known process for not only treating organic waste but also producing biogas as renewable energy. However, In organic waste, sulfur-containing amino acids (cysteine, methionine) exist as the organic sulfur compounds which are converted to major sulfur-containing gases ($H_2S$, methyl mercaptan, dimethyl sulfide) by sulfur-reducing bacteria (SRB) during anaerobic digestion. Among them, hydrogen sulfide ($H_2S$), a corrosive gas to pipeline and engine in the biogas-utilization process, should be removed in biogas before the application. For producing desulfurized biogas, two-stage anaerobic digestion (TSAD) which consists of first-stage digester for sulfidogenesis and second-stage digester for methanogenesis has been studied in this field. In TSAD, the sulfur is converted to gaseous sulfur in first-stage resulting in a low concentration of hydrogen sulfide in biogas produced in the second stage. However, the TSAD has critical limitations in terms of losing the carbon source and slow conversion rate in the first-stage digester. In this study, the strategy for maximizing conversion of organic sulfur compounds to gaseous sulfur with minimizing loss of carbon source in first stage digester was investigated by the change of pH condition and addition of conductive material with revealing the conversion pathway of organic sulfur compounds during anaerobic digestion.
The sludge which was cultivated with COD/S ratio (100 (w/w)) for 300 days was utilized for batch experiments.
In chapter 3, for revealing the conversion pathway of organic sulfur compounds and gaseous sulfur, the conversion of each sulfur compounds (cysteine, methionine and sodium sulfide) was monitored by batch experiment at pH 7. As a result, the conversion from cysteine to methionine appeared before the production of gaseous sulfur in the reactor in which cysteine was injected as a sulfur source. Moreover, the homocysteine which is well-known as a precursor of methionine was analyzed in the reactor. In the case of the reactor in which sodium sulfide was injected, the methyl mercaptan and dimethyl sulfide which are products of methylation from hydrogen sulfide were analyzed in the produced gas. In conclusion, the conversion from cysteine to methionine was occurred during anaerobic digestion with methylation from hydrogen sulfide to methyl mercaptan and dimethyl sulfide.
In chapter 4, for investigating the impact of pH on the conversion of organic sulfur and loss of carbon source, the initial pH was adjusted to 5.5 and 7 by 7% hydrogen chloride solution in the batch experiment. As a result, the loss of COD decreases by about 46% at pH 5.5 by non-methanogenesis. However, the conversion efficiency of the organic sulfur compound to gaseous sulfur at pH 5.5 was 39% lower than pH 7 due to the low degradation rate of sulfur-containing amino acids in the condition that methanogen was inhibited. In conclusion, although the loss of carbon source decreases at pH 5.5 by inhibition of methanogens, the conversion of organic sulfur compounds to gaseous sulfur also decreases.
In chapter 5, for enhancing the conversion from organic sulfur compounds to gaseous sulfur at pH 5.5, the impact of multi-walled carbon nanotubes (MWNTs) was investigated in the batch experiment. As a result, MWNTs enhanced the conversion efficiency of organic sulfur compounds to gaseous sulfur about 38% by facilitating the degradation of amino acids. Besides, the maximum conversion rate from organic sulfur compounds to gaseous sulfur increases about 64% by MWNTs. Moreover, the production of hydrogen gas increase about 55% and organic acids especially, butyric acid produced about 9% more by acidogen. In conclusion, the conversion of organic sulfur compounds to gaseous sulfur increases by MWNTs. It is supposed that the phenomenon was caused by facilitating the electron transfer between SRB and acidogen.
In conclusion, the addition of MWNTs at pH 5.5 can enhance the conversion of organic sulfur compounds to gaseous sulfur with minimizing the loss of carbon source during anaerobic digestion.