Feasibility Study for the Implementation of Hydrothermal Liquefaction in Southeast Michigan: Considering Environmental, Economic, and Social Aspects

Introduction: While some sectors can be fully decarbonized through the implementation of renewable energy sources, the decarbonization pathways for the wastewater sector are more complex. Resource recovery through sludge management can be an option to reduce GHG emissions in water resource recovery facilities (WRRFs). Of all the sludge produced in the U.S., 22% is being landfilled, 16% is being incinerated, and 52% is being land applied (with 80% of the sludge being digested before land application).[1] New regulations and concerns about emerging contaminants such as per- and polyfluoroalkyl substances are creating challenges and increasing costs for land application, the most used disposal strategy. Thus, it is critical to implement new processes to maximize resource recovery and remove emerging contaminants without increasing overall costs and GHG emissions. HTL is a promising technology that can reduce sludge volume by 90%, recover phosphorous, remove emerging contaminants, and convert 65% of the energy content in the sludge into biocrude oil, a precursor for aviation fuel.[2,3] A large-scale adaption of HTL for WRRFs requires an evaluation of the economic, environmental and social impacts of HTL. Here we premier the first evaluation of technical feasibility and assessment of economic, environmental and social impacts related to the implementation of HTL in the WRRF of the City of Detroit (GLWA), which operates one of the largest WRRF in the nation. The feasibility study consisted of three tasks. Task 1 assessed the availability and detailed characterization of waste streams. Task 2 conducted community engagement (hearing from the community and professional stakeholders). Task 3 involved Economic and environmental analysis. GLWA generates 320 dry ton of sludge per day. 240 dry ton/day of sludge is being converted in class A-EQ biosolids for land application through a thermal process and the rest (80 dry ton/day), is incinerated. Our feasibility assessment focused on identifying three promising scenarios for replacing incineration at GLWA with HTL. Furthermore, one of the scenarios was compared from an environmental and economic perspective with two other sludge management options (current GLWA sludge management practices and the replacement of the incinerator with AD). Current GLWA sludge management practices GLWA generates 320 dry ton of sludge per day. 240 dry ton/day of sludge is being converted in class A-EQ biosolids for land application through a thermal process and the rest (80 dry ton/day), is incinerated. Our feasibility assessment focuses on replacing the incinerator with HTL. Task 1: Availability and detailed characterization of waste streams The number of multiple types of organic wastes was quantified in the City of Detroit and its surroundings (Table 1). Samples of different types of wastes were characterized by Pacific Northwest National Laboratory (PNNL) to evaluate them as a substrate for HTL. For each waste PNNL evaluated its pumpability and the solids and ash percentage. Furthermore, Proximate, FAMES and ICP analyses were conducted on the different wastes collected. This data was used for subsequent tasks related to HTL modeling, and economic and environmental assessment. Task 2: Community engagement (hearing from the community and professional stakeholders) We performed two separate workshops: (1) a community workshop with neighbors from Detroit and (2) a professional workshop with personnel from wastewater utilities and refineries, consultants and waste haulers. These workshops collected relevant data for our environmental and economic analysis. The community workshop identified sustainability metrics that were relevant for the neighbors of the WRRF in Detroit. The workshop also revealed the community's perspective on the HTL co-substrates. For example, the community groups preferred food waste to be diverted to composting instead of HTL, which was taken into consideration when determining the three best scenarios to implement HTL. Furthermore, a survey was sent to the neighbors in Delray (neighborhood close to WRRF) to know their opinion about resource recovery from wastes. Overall, the survey showed that neighbors have a positive perception about resource recovery and HTL (Figure 1). The professional stakeholders expressed benefits and risks HTL can have for their type of industry. The benefits are related to economic incentives of biocrude, the reduction of solids and the potential to destroy emerging contaminants. The risks they expressed were related to the willingness of refineries to accept the crude and the safety and operation of high temperature and pressure systems. Specially, having personnel able to work with such systems are not abundant in WRRFs. Task 3: Economic and environmental analysis Task 3.1: Three most optimal waste-to-energy scenarios involving HTL Based on wet waste resources and logistic analyses in the Detroit area, communication with project team, community groups and stakeholders, and economic and environmental analyses, three blending scenarios made through our selection of optimal waste-to-energy scenarios (see Table 2). Table 3 and Figure 2 show the economic and environmental indicators for the three blending scenarios. As per the GREET model, the potential avoided emissions for food waste and sludge are 1238 and 735 kg CO2e/dry ton, respectively. In the LCOD calculation of scenarios involving food waste, a $36/wet ton tipping fee is assumed for food waste along with the transportation costs associated of delivering food waste to the HTL plant gate. In Scenario 3, 35 dry tons per day of food waste are mixed with 110 dry tons of sludge to generate biocrude. Increasing the utilization of food waste leads to enhancements in environmental indicators like GHG, WI, NRRU, and SW, albeit at the expense of a higher LCOD. Task 3.2: Comparison among current practices, AD and HTL For this task we compared scenario 1 from task 3.1 with GLWA current practices and with a scenario where GLWA replaces the incineration with AD. Table 4 and 5 show the life cycle inventories and the sustainability indicator values, respectively. Compared to AD alternatives, HTL offers better energy efficiency and production, as well as lower levelized disposal costs and water intensity. However, the AD scenario has the lowest greenhouse gas (GHG) emissions among the three scenarios because part of the generated biogas is assumed to heat the digesters, eliminating the need for additional natural gas in the AD process.