Characterization and Biodegradability Assessment of Hydrothermal Liquefaction Aqueous Phase Derived from Different Municipal Sludge Streams

INTRODUCTION The municipal wastewater treatment sector has been working towards carbon neutrality and circular economy. Current sludge treatment technologies are re-evaluated as part of this effort. Municipal sludge, with around 70% of the carbon (C) from wastewater, is an important feedstock for biochemical and thermochemical processes to recover energy and nutrients. Hydrothermal liquefaction (HTL), a thermochemical process suitable for wet biomass processing such as sludge, can achieve over 70% energy recovery in the form of a liquid biofuel (bio-crude) at high temperatures (280-374 degrees C) and pressures (8-22 MPa), while recovering N and P from its by-products (Liu et al., 2022). However, HTL produces a wastewater (HTL aqueous) with high environmental risks if untreated (Basar et al., 2021). This creates a major bottleneck for scaling-up. Metro Vancouver has been working towards building North America's first continuous flow HTL demonstration facility, featuring 10 ton sludge cake/d capacity and co-located at an operating wastewater treatment plant (WWTP).This study was conducted to generate lab-scale feasibility data to support design, construction and operation of the demonstration facility. The specific objectives of the study were: (1) To reveal the characteristics and biodegradability of HTL aqueous derived from sludge with different primary sludge (PS) to secondary sludge (SS) ratios and anaerobically digested cake from different plants, (2) To correlate HTL feedstock characteristics (carbohydrate, protein, lipid, ash content) with the composition and the biodegradability of their HTL aqueous streams. METHODOLOGY Feedstock preparation and batch HTL operation: Dewatered PS and SS from two municipal WWTPs (WWTP 1 and WWTP 2) in British Columbia, Canada were used. Sludges were blended to simulate high-PS, average, and high-SS ratios by wt.% total solids (TS), based on the plants' operational data. Mixed sludges were then centrifuged to achieve 20 wt.% TS. Additionally, digested sludge (DS) cakes from mesophilic and thermophilic anaerobic digesters of the plants, with TS levels of 26 and 23.3 wt.%, respectively, were used as HTL feedstock. Carbohydrate, protein, lipid and ash contents of sludges were measured using the methods of phenol-H2SO4, nitrogen content multiplication by 6.25, and Standard Methods 3540, and 2540, respectively. For HTL, 500 g of sludge cake was processed in a 1 L Parr(R) 4570 high temperature/pressure reactor at 332 degrees C and 16.9 min following the previous findings (Liu et al., 2022). After the reactor cooled down, products were filtered through a 0.45 µm membrane to separate HTL aqueous from bio-crude and hydrochar. HTL aqueous characterization and biodegradability: Chemical oxygen demand (COD), ammonia, dissolved proteins, and phenolic compounds were determined using Standard Methods 5220, 4500-NH3, Lowry method, and Folin-Ciocalteu procedure, respectively (APHA, 2005; Lowry et al., 1951; Singleton et al., 1965). Concentrations of fatty acids, N-heterocyclic compounds, and ketones were measured using gas chromatography (GC-FID and GC-MS). Further details can be found in a previously published study (Basar et al., 2023). The aerobic biodegradability was assessed by biochemical oxygen demand (BOD), whereas mesophilic (35 degrees C) and thermophilic (55 degrees C) anaerobic biodegradability was assessed by biochemical methane potential (BMP) assays. Aerobic biodegradability (%) was represented by the BOD/COD ratio while anaerobic biodegradability was compared to the theoretical yield of 350 mL CH4/g CODremoved (0oC and 1 atm). FINDINGS There was a discernable variation in HTL aqueous of WWTP 1 for different feedstock PS/SS ratios, whereas the impact was less pronounced for WWTP 2 (Fig. 1). Higher SS in mixed sludge was correlated to higher HTL aqueous COD for WWTP 1 (Table 1, Fig. 1a). This was caused by the protein degradation products remaining in HTL aqueous. Similar trends were observed for ammonia, fatty acid (Fig. 1c), and N-heterocycles (Fig 1e), as they are all products of protein degradation. The higher the protein content of feedstock, the greater the rate of protein degradation during HTL, resulting in lower proteins in HTL aqueous (Fig. 1b). The concentration of phenolic compounds was not affected by the PS:SS ratio to the same extent due to phenolics being degradation products of carbohydrates, proteins, and lipids (Fig. 1d). The variation in HTL aqueous also impacted the biodegradability (Figs. 2, 3). The biodegradability of HTL aqueous ranged from 69-78, 58-70, and 42-56% under aerobic, mesophilic anaerobic and thermophilic anaerobic conditions. The BOD and BMP of HTL aqueous from WWTP 1 sludges increased with an increase in SS ratio in the feedstock. This effect was discernable only in BOD of HTL aqueous for WWTP 2, while the BMP was not affected, suggesting that an increase in SS ratio resulted in organics that are only aerobically biodegradable. According to the Pearson correlation test, the protein content of feedstock was positively correlated to BOD and BMP performances (Fig. 3b). CONCLUSIONS The findings of this study suggest that for mixed sludge, HTL aqueous characterization and downstream biodegradability will vary depending on the PS to SS ratios. Protein content of sludge was found to have the greatest impact on WWTP integration/design. There was only a slight variation in biodegradability between HTL aqueous derived from mesophilic and thermophilic DS.