(469c) Hydrothermal Liquefaction of Municipal Sludge: Inhibition of Heterocyclic N-Containing Organic Compounds from Aqueous Co-Product on Autotrophic Nitrifying Bacteria

Waste-to-Energy (WtE) technologies hold significant importance in today’s world, enabling the conversion of various types of waste into high-value products. Different feedstocks can be applied to a technology called Hydrothermal liquefaction (HTL). As the starting material undergoes HTL, it generates a mixture of solid, liquid, and gaseous products. Understanding the application of each HTL-resulting product is intrinsic and a highly investigated topic. The liquid fraction produced during an HTL reaction is called Aqueous-Co-Product (ACP), and an appropriate application has not been found yet due to its high organic composition and potential inhibitory compounds. Recycling the ACP back to the headworks of a Wastewater Treatment Plant (WWTP) is a potential effort, but understanding how that would affect the microbial community in terms of its inhibition potential remains an ongoing area of research interest. While previous research has indicated inhibitory effects of HNOCs on heterotrophic growth, their impact on autotrophs remains unclear. In this study, ACP produced from municipal sludge HTL has been characterized for its biological toxicity, considering the presence of heterocyclic N-containing organic compounds (HNOCs). This investigation delved into the inhibitory effects of HNOCs on autotrophic nitrifying bacteria by determining the half-maximum inhibitory concentration (IC₅₀) of ACP on nitrification. IC₅₀ indicates the potency of a certain compound or substance in inhibiting a particular biological activity. Predominant HNOCs in ACP from municipal sludge were identified as 2-pyrrolidinone, 2-pyrazine, and 2-piperidinone and their derivatives, present at concentrations of 8.31, 8.22, and 0.96 mM, respectively. Individual testing revealed that 2-pyrrolidinone and 2-pyrazine hindered nitrifying bacteria by impeding ammonia uptake. Conversely, 2-piperidinone did not exhibit inhibitory effects on nitrification. Furthermore, testing of an ACP sample showed inhibitory effects on nitrification, with an IC₅₀ of 0.08% (v/v), when the ACP was stored for 1 week after HTL reaction. Storage duration emerged as a significant variable, with ACP stored for 15 weeks showing reduced inhibitory effects on the nitrifying community. Specifically, the concentrations of HNOCs in the 1-week study decreased from 8.31, 8.22, and 0.96 mM to 6.36, 6.21, and 0.85 mM, respectively. The IC₅₀ for the 15-week ACP was found to be 0.63% (v/v) which shows a clear reduction in its inhibitory effects on nitrifiers. In summary, this investigation underscores the inhibitory impact of ACP on nitrification, offering valuable insights for devising management strategies for the aqueous stream in municipal sludge HTL.