Abstract
High-pressure anaerobic digestion is an appealing concept since it can upgrade biogas directly within the reactor. However, the decline of pH caused by the dissolution of CO2 is the main barrier that prevents a good operating high-pressure anaerobic digestion process. Therefore, in this study, a high-pressure anaerobic digestion was studied to treat high alkalinity synthetic wastewater, which could not be treated in a normal-pressure anaerobic digester. In the high-pressure reactor, the pH value was 7.5 ~ 7.8, and the CH4 content reached 88% at 11 bar. Unlike its normal-pressure counterpart (2285 mg/L acetic acid), the high-pressure reactor ran steadily (without volatile fatty acids inhibition). Furthermore, the microbial community changed in the high-pressure reactor. Specifically, key microbial guilds (Syntrophus (11.2%), Methanosaeta concilii (50.9%), and Methanobrevibacter (26.8%)) were dominant in the high-pressure reactor at 11 bar, indicating their fundamental roles under high-pressure treating high alkalinity synthetic wastewater.
Highlights
Biogas produced by conventional anaerobic digestion (AD) is primarily composed of methane (CH4, 50 ~ 70%) and carbon dioxide (CO2, 30 ~ 50%), and the ratio mainly depends on the substrate and pH of the fermentation process in the AD reactors (Wahid et al, 2019)
In phase 1, the increase of pressure in the high-pressure anaerobic digestion (HPAD) reactor and the production of biogas in the normal-pressure anaerobic digester (NPAD) reactor came from the residual biomass in the inoculum sludge
When the pressure became stable in the HPAD reactor and daily biogas production (DBP)/daily methane production (DMP) became zero in the NPAD reactor, the synthetic wastewater was added into both reactors
Summary
Biogas produced by conventional anaerobic digestion (AD) is primarily composed of methane (CH4, 50 ~ 70%) and carbon dioxide (CO2, 30 ~ 50%), and the ratio mainly depends on the substrate and pH of the fermentation process in the AD reactors (Wahid et al, 2019). Before injecting biogas into the natural gas grid or used in other high-grade applications (CH4 > 90%, CO2 < 8%), upgrading technologies are necessary (Omar et al, 2018; Wahid et al, 2019). External upgrading technologies, such as cryogenic, chemical absorption, membrane separation, pressure swing adsorption, water scrubbing, are applicable only for biogas flows higher than 100 m3/h (Angelidaki et al, 2018; Li et al, 2017; Lindeboom et al, 2011). There is a demand for the development of new technologies that improve the biogas quality at a small scale
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