Abstract
The methanogenic activity is an important indicator to assess the efficiency of high-solid anaerobic digestion. However, it is not yet elucidated clearly how to detect the parameter rapidly and reliably in the rice straw feeding reactor. Co-inoculated with ruminal digesta and anaerobic sludge, the digestion performance was studied at three different organic loading rates (OLRs). The excitation emission matrix–parallel factor analysis (EEM–PARAFAC) was used to detect dynamic changes in the characteristic of fluorescence components. Our results revealed that CH4 productivity reached 280.90 mL/g volatile solid (VS) with a 54.39% CH4 content under the OLR of 2.26 g/(L⋅d), which amount to 80.29% of its theoretical value. At the OLR of 2.47 g/(L⋅d), the average accumulated NH4+ concentration was 1082.63 mg/L, which resulted in the hydrogenotrophic Methanobacteriales decreasing from 1.70 × 109 to 1.04 × 106 copies/g in the solid residues, whereas the acetotrophic Methanosarcinales increased from 7.89 × 106 to 9.44 × 106 copies/g. The dynamics of the methanogenic community consequently influenced the bioconversion efficiency of rice straw, and CH4 productivity was reduced to 256.54 mL/g VS. The three fluorescent components, at the excitation/emission wavelength of 420 nm/470 nm, 340 nm/430 nm, and 280 nm/340 nm, were decomposed by PARAFAC model in the digestate. Fluorescence intensities of coenzyme F420 and NADH reflected the dynamic changes of CH4-producing activity and anaerobic digestion efficiency, respectively. The coenzyme F420, unique to hydrogenotrophic methanogens, was correlated with methane yield, suggesting they played a dominant role in the anaerobic reactor. This study demonstrates that the EEM–PARAFAC combined with Q-PCR can be used to characterize methanogenic activity variation during the high-solid anaerobic digestion of rice straw with 15% total solid (TS).
Highlights
Rice straw, which consists mostly of cellulose, hemicellulose, and lignin, is among the organic components characteristic of plant materials
From day 10 to 18 of the first organic loading rates (OLRs), there was a peak in biogas productivity ranging from 208 to 218.5 L/d
After day 50, the methane-producing activity was enhanced, and the CH4 productivities gradually remained stable accompanied by the hydrolysis of rice straw
Summary
Rice straw, which consists mostly of cellulose, hemicellulose, and lignin, is among the organic components characteristic of plant materials. CH4-producing plants could lower straw incineration and acquire later use as a natural fertilizer. The commercial high-solid anaerobic process displays signs of “acid-intoxication” (Liotta et al, 2016) and ammonia limitation (Hu et al, 2014), among others, which affects the methanogens and subsequently their methane-producing activity. It would seem prudent to investigate the correlation between CH4 production and methanogenic activity. The performance of anaerobic digestion had been investigated in a horizontal reactor; this work revealed that the CH4-producing efficiency is influenced by changes in the methanogens’ composition induced by detrimental metabolites (Deng et al, 2017b). How to reliably detect and distinguish among methanogenic activity has yet to be extensively elucidated in real time during the entire digestion process
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