Efficient anaerobic digestion of hydrothermal carbonization wastewater via an innovative multistage anaerobic hythane reactor (MAHR): Organic conversion and microbial evolution

Seeking continuous treatment of downstream wastewater is a desperate requirement for the scale-up of hydro -thermal technology. In this study, a novel multistage anaerobic hythane reactor (MAHR), which was constructed with an internal downflow biofilter bed and an external up-flow sludge bed, was used to dispose of hydrothermal carbonation wastewater (HTCWW), a by-product of the hydrochar preparation process of microalgae. To verify the performance of MAHR for the actual complex wastewater treatment, a conventional up-flow anaerobic sludge bed reactor (UASB) was used as a comparison. The two reactors were both operated independently in parallel with a hydraulic retention time (HRT) of 12 h and an organic load rate (OLR) of 20 g SCOD/L/d. Both MAHR and UASB achieved 84% COD removal rate and 7.0 & PLUSMN; 0.6 and 5.5 & PLUSMN; 0.3 L/L/d of methane production rate, respectively, when the proportion of HTCWW was increased to 100% of the original wastewater. Specifically, the hythane-producing area (MH) of MAHR was gradually transformed into methane producing area in the process of feeding HTCWW. High-performance liquid chromatography (HPLC) analysis revealed that more small molecule compounds including organic acids and inhibitors were degraded in MAHR. Fourier transform ion cyclone resonance mass spectrometry (FT-ICR MS) analysis demonstrated that more soluble proteins and lipids were degraded to some extent, while the lignin and condensed aromatic compounds increased especially in UASB. A dramatic change was observed in the dominating microbial communities in MAHR and UASB from the Strep-tococcaceae associated with carbohydrates degradation to the Renkenellaceae involved in both proteins and carbohydrates degradation. At the archaeal level, hydrogenotrophic methanogens replaced acetoclastic metha-nogens. Redundancy analysis (RDA) further verified the relationship between dominant functional communities and the anaerobic digestion performance of the reactors. The above results indicate that MAHR is a promising configuration for the anaerobic valorization of hydrothermal wastewater.

Automated summary

In this study, a novel multistage anaerobic hythane reactor (MAHR) was used to treat hydrothermal carbonation wastewater (HTCWW) and compared with a conventional up-flow anaerobic sludge bed reactor (UASB). The results showed that MAHR achieved better degradation performance and methane production in treating HTCWW. It was also found that the methane-producing area in MAHR gradually transformed in the process of feeding HTCWW. Additionally, the microbial communities in MAHR underwent significant changes. Overall, this study demonstrates that MAHR is a promising configuration for the treatment of hydrothermal wastewater.