Inhibition of Nitrifying Bacteria from Heterocyclic N-containing Organic Compounds from Municipal Sludge Hydrothermal Liquefaction

Adrian D Romero ¹ João V Poli ² Sean Larson ² Susan Stagg-Williams ² Ray Carter ¹ Belinda SM Sturm ^1*

• ¹ Civil, Environmental & Architectural Engineering, University of Kansas, Lawrence, KS, United States
• ² Chemical & Petroleum Engineering, University of Kansas, Lawrence, KS, United States

Hydrothermal liquefaction (HTL) is a thermochemical technology that converts wet biomass into biochar and biocrude at high temperatures and pressures. HTL can be utilized within municipal wastewater treatment to convert waste activated sludge into valuable resources, but HTL by-products include an aqueous co-product (ACP) that has been characterized for its biological toxicity, high ammonia, and presence of heterocyclic N-containing organic compounds (HNOCs). This study evaluated the inhibitory effects of HNOCs on autotrophic nitrifiers present in activated sludge. 2pyrrolidinone, pyrazine, and 2-piperidinone and their derivatives were the most prevalent HNOCs in ACP from waste activated sludge at concentrations of 8.98, 6.05, and 0.40 mM respectively. These compounds were tested to determine the inhibitory concentration that reduces ammonia uptake by 50 percent (IC50). The IC50 of 2-pyrrolidinone and pyrazine was 5.2x10 -5 and 2.0x10 -3 mM, respectively. The IC50 of the ACP was 0.08% (%v/v). This corresponded to concentrations of 2-pyrrolidinone, pyrazine, and 2-piperidinone of 7.52x10 -3 , 5.07x10 -3 , and 3.36x10 -4 mM, respectively. This provides guidance for how ACP recycle can be incorporated at a wastewater treatment plant without inhibiting nitrification. The impact of ACP storage was also tested. ACP stored for 15 weeks exhibited less inhibitory effects on the nitrifying community compared to ACP stored for 1 week. The % maximum ammonia uptake rate was reduced by 23% for the 15-week stored ACP, in contrast to 51% reduction for ACP stored for 1 week. Overall, this study shows the inhibitory effects of the ACP in activated sludge nitrification and will help determine management strategies of the aqueous stream in solids processing of municipal sludge HTL.