(91c) Hydrothermal Liquefaction of Waste Activated Sludge: Initial Composition Effects on Product Distribution and Nutrient Recovery

Municipal sludge is a widely available feedstock that is often discarded through composting, landfilling, or agricultural land application, without energy recovery. Recent concern about the toxicity of land applying biosolids has driven research into new technologies for valorizing municipal sludge. Hydrothermal liquefaction (HTL) is a thermochemical process that occurs in the presence of an aqueous environment that can be used to recovery energy and nutrients from municipal sludge. The process conditions for HTL include temperatures ranging from 200 °C to 450 °C and pressures ranging from 4 MPa to 35 MPa. Under these conditions, four product streams are obtained including biocrude, biochar, gas (primarily CO₂), and an aqueous stream. To gain a better understanding of the viability of implementing HTL conversion of municipal sludge at wastewater treatment facilities, it is necessary to understand the impact of the wastewater treatment process on the sludge composition and ultimately the product properties. Specifically, the ability of the aqueous stream to be recycled back to the mainstream wastewater process is critical to the implementation of HTL at a wastewater treatment facility. Furthermore, the solids produced from HTL must have reduced toxicity compared to the biosolids and be suitable for land application.

This study aims to identify the effects of initial sludge composition on the HTL product distribution and product composition. Specifically, waste-activated sludge (WAS) was sampled from two different municipal facilities: one with and one without enhanced biological phosphorous removal (EBPR). In addition, the WAS had different calcium concentrations, due to differences in lime addition at the two wastewater treatment facilities. HTL was performed on both feedstocks and the HTL products, including the potential for nutrient recovery, are presented. Preliminary results show that WAS from EBPR processes have higher biocrude yields compared to the process without nutrient removal. Furthermore, The Ca content of the municipal sludge can change the biochar yield significantly and promotes a different product distribution in terms of elements such as P and Mg. WAS from the EBPR processes has a high feedstock P content, and > 94% of the P was recovered in the biochar after HTL. The storage of the sludge also plays an important role in nutrient distribution in the products. Sludge stored aerobically showed a lower recovery of P in the biochar phase, whereas anaerobically stored sludge showed a higher recovery of P in the biochar. The results indicate that P recovery is efficient for all WAS feedstocks, but the calcium content of the WAS affects the total mass of biochar produced as well as the P-recovery efficiency.