Comparative Analysis of Mass and Energy Balances in Incineration, Anaerobic Digestion, THP, Drying, Pyrolysis, and Gasification Processes for Municipal Biosolids Treatment

Problem Statement: Wastewater treatment plants (WWTPs) are increasingly challenged to manage biosolids efficiently while meeting stringent regulations, especially for emerging contaminants like PFAS (per- and polyfluoroalkyl substances). Traditional biosolids disposal methods, such as landfill and land application, face restrictions due to environmental concerns. Rising energy costs further compel WWTPs to adopt treatment solutions that maximize energy recovery, reduce emissions, and align with regulatory standards. This study provides a mass and energy balance evaluation of six biosolids treatment technologies-drying, incineration, mesophilic anaerobic digestion (MAD) with post-digestion drying, thermal hydrolysis pretreatment (THP) followed by MAD and drying, and drying prior to pyrolysis or gasification-focusing on energy efficiency, life cycle costs, carbon sequestration, environmental impacts, and PFAS removal potential. Drying of biosolids from 20% to 90% total solids (TS) achieves a 90% reduction in mass, improving biosolids handling and transport for downstream processes. However, drying is energy-intensive, requiring approximately 1,000--1,200 BTU per pound of water removed, totaling around 1.76 MMBTU per ton of biosolids (Kreuger et al., 2020). Incineration does not require pre-drying and provides high energy recovery of around 6--8 MMBTU per ton of wet biosolids (Samolada & Zabaniotou, 2014). Operating at temperatures above 800°C, incineration shows promise for PFAS breakdown, though studies are ongoing regarding PFAS fate in ash and emissions. Incineration achieves 90% mass reduction with ash as the final residue. MAD operates with biosolids at 20--30% TS, producing approximately 7.9 MMBTU per ton in biogas energy (Zhou et al., 2019). Post-digestion drying requires an additional 1.76 MMBTU per ton, resulting in a net energy output of around 6.14 MMBTU per ton. While MAD is effective for moderate energy recovery and carbon sequestration, it has minimal impact on PFAS, making it less suited for facilities with strict PFAS requirements. THP + MAD enhances biogas production to approximately 9.0 MMBTU per ton by pre-treating biosolids with high-temperature thermal hydrolysis, which partially breaks down organic matter (Carrere et al., 2016). Combined, THP + MAD yields a net energy output of around 6.54 MMBTU per ton and reduces final digestate mass by 85%. Both pyrolysis and gasification require drying upto 90% TS. Pyrolysis decomposes biosolids into biochar, bio-oil, and syngas, providing stable carbon storage but resulting in a net negative energy balance due to drying (Winchell et al., 2022). Gasification, producing primarily syngas and ash, yields 0.8--1.0 MMBTU per ton and a 90% mass reduction. The ongoing studies need to be finalized to assess and understand the fate of PFAS byproducts in pyrolysis and gasification. Findings and Implications: MAD and THP+MAD are effective for facilities prioritizing moderate energy recovery and carbon sequestration. Incineration offers high energy output with significant emissions control costs. Pyrolysis and gasification, while energy-intensive, provide carbon sequestration and emerging PFAS removal capabilities. This analysis supports WWTPs in selecting biosolids treatment technologies aligned with regulatory, environmental, and energy objectives. Additional Insights on Life Cycle Costs, Environmental Impacts, PFAS Removal, and Carbon Sequestration Potential 1. PFAS Removal Potential - Incineration: Operating at temperatures above 800°C, incineration shows promise for PFAS breakdown. Studies are ongoing to assess the fate of PFAS and resulting compounds in ash and emissions (Samolada & Zabaniotou, 2014). - Pyrolysis and Gasification: High temperatures in pyrolysis and gasification may break down PFAS, but research is investigating the environmental fate of resulting byproducts in biochar (Winchell et al., 2022). 2. Life Cycle Cost Analysis - Drying: High operational cost due to drying energy demand (1.76 MMBTU/ton). Costs can be reduced with heat recovery systems (Kreuger et al., 2020). - Incineration: High initial capital and emissions control costs, but the significant energy output can provide a return on investment in high-throughput facilities (Samolada & Zabaniotou, 2014). - MAD + Drying: Moderate capital and operational costs; biogas revenue can offset drying energy costs (Zhou et al., 2019). - THP + MAD: Higher initial costs with THP, which improves biogas yield, making it favorable for facilities facing stricter regulations (Carrere et al., 2016). - Pyrolysis/Gasification: High operational costs for drying and emissions control, with net negative energy due to drying demands. Suited for facilities prioritizing PFAS destruction and carbon sequestration. 3. Environmental Considerations and Carbon Sequestration Potential - Carbon Sequestration: Pyrolysis produces biochar, offering stable carbon storage for soil. MAD and THP+MAD provide moderate carbon sequestration through digestate land application (Winchell et al., 2022). - Emissions Control: Incineration and gasification generate emissions requiring control systems, while gasification and pyrolysis emit lower greenhouse gases due to reduced oxidation (Samolada & Zabaniotou, 2014).