Ozone enhances biological treatment of hydrothermal liquefaction aqueous stream from municipal sludge

Hydrothermal liquefaction (HTL) is a promising alternative to anaerobic digestion for municipal sludge valorization due to biocrude oil and nutrient-dense hydrochar production, smaller footprint, enhanced micropollutant degradation, and minimized residual biosolids. However, integrating HTL into wastewater treatment plants necessitates on-site treatment of HTL aqueous, which is 80 % of byproducts by volume. HTL aqueous exhibits inhibition on downstream biological treatment due to high chemical oxygen demand (COD), total phenolics, ketones and nitrogen heterocyclics. This study evaluated the efficacy of ozone (O₃) pretreatment on HTL aqueous characteristics and subsequent biodegradability under anaerobic and aerobic conditions. HTL aqueous was obtained from dewatered sludge cake at 350 °C with 15 min retention time. Various dissolved O₃ doses (0.03, 0.12, 0.14, 0.16, 0.18 g O₃/g COD of HTL aqueous) were investigated, with the maximum dose of 0.18 g O₃/g COD achieving 43 % COD reduction, 72 % removal of total phenolics, 69 % removals of selected nitrogen heterocyclics, while retaining volatile fatty acids (VFA). The optimal dissolved dose for anaerobic and aerobic treatment were 0.14 and 0.18 g O₃/g COD, respectively. Under mesophilic (35 °C) and thermophilic (55 °C) conditions, the pretreated HTL aqueous at the optimum dose of 0.14 g O₃/g COD demonstrated 98 % and 89 % improvements in specific cumulative methane yields, compared to non-pretreated HTL aqueous (control), respectively. The maximum improvement in 5-day biochemical oxygen demand (BOD) was 90 % at 0.18 g O₃/g COD, compared to the control. The enhanced biodegradations were attributed to the removal of total phenolics and nitrogen heterocyclics, ultimately enhancing carbon recovery from municipal sludge.