Methane potentials of wastewater generated from hydrothermal liquefaction of rice straw: focusing on the wastewater characteristics and microbial community compositions

Background: Hydrothermal liquefaction (HTL) has been well studied for the bio-oil production from biomass. However, a large amount of wastewater with high organic content is also produced during the HTL process. Therefore, the present study investigated the methane potentials of hydrothermal liquefaction wastewater (HTLWW) obtained from HTL of rice straw at different temperatures (170-320 degrees C) and residence times (0.5-4 h). The characteristics (e.g., total organic content, organic species, molecular size distribution, etc.) of the HTLWW were studied, and at the same time, microbial community compositions involved in AD of HTLWW were analyzed. Results: The highest methane yield of 314 mL CH4/g COD was obtained from the sample 200 degrees C-0.5 h (HTL temperature at 200 degrees C for 0.5 h), while the lowest methane yield 217 mL CH4/g COD was obtained from the sample 320 degrees C-0.5 h. These results were consistent with the higher amounts of hard biodegradable organics (furans, phenols, etc.) and lower amounts of easily biodegradable organics (sugars and volatile fatty acids) present in sample 320 degrees C-0.5 h compared to sample 200 degrees C-0.5 h. Size distribution analysis showed that sample 320 degrees C-0.5 h contained more organics with molecular size less than 1 kDa (79.5%) compared to sample 200 degrees C-0.5 h (66.2%). Further studies showed that hard biodegradable organics were present in the organics with molecular size higher than 1 kDa for sample 200 degrees C-0.5 h. In contrast, those organics were present in both the organics with molecular size higher and less than 1 kDa for sample 320 degrees C-0.5 h. Microbial community analysis showed that different microbial community compositions were established during the AD with different HTLWW samples due to the different organic compositions. For instance, Petrimonas, which could degrade sugars, had higher abundance in the AD of sample 200 degrees C-0.5 h (20%) compared to sample 320 degrees C-0.5 h (7%). The higher abundance of Petrimonas was consistent with the higher content of sugars in sample 200 degrees C-0.5 h. The higher Petrimonas abundance was consistent with the higher content of sugars in sample 200 degrees C-0.5 h. The genus Syntrophorhabdus could degrade phenols and its enrichment in the AD of sample 320 degrees C-0.5 h might be related with the highest content of phenols in the HTLWW. Conclusions: HTL parameters like temperature and residence time affected the biodegradability of HTLWW obtained from HTL of rice straw. More hard biodegradable organics were produced with the increase of HTL temperature. The microbial community compositions during the AD were also affected by the different organic compositions in HTLWW samples.