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

Abstract

Seeking continuous treatment of downstream wastewater is a desperate requirement for the scale-up of hydrothermal 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 ± 0.6 and 5.5 ± 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 Streptococcaceae associated with carbohydrates degradation to the Renkenellaceae involved in both proteins and carbohydrates degradation. At the archaeal level, hydrogenotrophic methanogens replaced acetoclastic methanogens. 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.