Life Cycle Greenhouse Gas Emissions Modeling of Wastewater Residuals

Introduction The U.S. EPA estimates that water resource recovery facilities (WRRFs) emit nearly 40 million metric tons of carbon dioxide equivalent (CO2eq) greenhouse gas (GHG) each year, representing nearly 1% of all domestic emissions sources. Landfills, which are interlinked with WRRFs by residuals management especially for small to moderate WRRFs, represent an additional 1.5% of all U.S. GHG emissions. WRRFs and landfills may also represent the single largest sources of GHGs in municipalities that own and operate these systems. To mitigate concerns about the long-term economic and environmental sustainability of landfill disposal, WRRFs are investigating technologies that reduce the mass and volume of biosolids before end-use. Mass reduction, such as anaerobic digestion or thermal drying, is a strategy to lower hauling and disposal costs, which typically are the largest line items in a WRRF's biosolids management budget. Several moderately sized WRRFs (<10 million gallon per day [MGD]) are considering thermal drying systems that use heat to remove moisture from dewatered sludge and can represent a 70-80% decrease in the mass of material hauled. Though capitally intensive, the process can represent a significant reduction in operating costs due to the mass reduction and the higher quality biosolids product (Class A) produced. The process is especially economically attractive in areas where natural gas prices are low and biosolids are hauled long distances, such as Ohio. Widespread adoption of sludge drying can potentially impact the GHG footprint of WRRFs and the municipalities that own them. Increased use of natural gas will undoubtedly raise direct GHG emissions at the facilities. However, dried biosolids will require fewer trucks, reducing fossil fuel demands associated with truck traffic. Additionally, high quality biosolids have been shown to improve soil health upon land application and contribute to carbon sequestration. It is currently unclear what net impact sludge drying will have on GHG emissions. Quantification of these emissions is critical to better understand the environmental impacts of the technology. Study Objectives The goal of this study was to quantify impacts of sludge drying technology and anaerobic digestion on greenhouse gas emissions at WRRFs. The goal was met by completing the following objectives: - Develop a mass and energy balance model to quantify direct and indirect GHG emissions for sludge treatment and disposal at a moderate sized (<10MGD) WRRF. - Compare GHG emissions profiles for the following sludge treatment trains: 1.Sludge Dewatering with Landfilling of Undigested Sludge 2. Sludge Dewatering with Drying and Land Application of Class A biosolids 3. Sludge Thickening, Mesophilic Anaerobic Digestion, Digestate Dewatering, and Land Application of Class B Biosolids. Biogas combusted at combined heat and power (CHP) for renewable electricity and heat generation. 4. Sludge Thickening, Mesophilic Anaerobic Digestion, Digestate Dewatering, Drying, and Land Application of Class A Biosolids. Biogas combusted at CHP for renewable electricity and heat generation. Heat requirements not met by CHP fueled by Natural Gas. Scenario 1 was considered a baseline condition because it is a common procedure for small to moderate size biological nutrient removal (BNR) WRRFs in Western Ohio. Scenarios 2 through 4 are common treatment trains evaluated during biosolids master planning efforts at moderate size facilities. Methodology The mass and energy balance model (Microsoft Excel based) allows users to input facility sludge flow data and select common sludge handling unit operations such as sludge thickening, dewatering, digestion, drying, and others. Mass flows are resolved around the whole process and individual unit operations. Energy requirements (electricity and heat) are calculated based on typical parameters provided by equipment vendors. The baseline model was developed around a ~10 MGD BNR WRRF that does not include primary treatment and produces a waste activated sludge (WAS) flow of 100,000 gpd with 10,000 lb/d of total suspended solids (TSS). This treatment train is common to many small to moderate sized WRRFs in the state of Ohio. The boundary of the analysis only includes sludge treatment operations, residuals transportation (via diesel truck) and end-use (e.g., landfill, land application). Emissions for unit processes outside of the solids treatment and end-use boundary were not included (e.g. liquid train treatment). Direct GHG emissions were quantified by multiplying modeled utility consumption (kWh/yr; MMBTU/yr) by US EPA and USEIA emissions factors for the region of interest. Indirect GHG emissions were primarily attributed to the residuals end-use method. End use emissions were quantified using the Biosolids Emissions Assessment Model (BEAM) model version 1.1 from the Canadian Council of Ministers of the Environment (CCME). Total GHG emissions for a particular case were calculated as the sum of net direct (consumption - production) and indirect GHG emissions (residuals end use). Results and Discussion Figure 1 shows the impact of sludge treatment train and residuals end-use on GHG emissions. The results show that sludge dewatering and landfilling was associated with the most GHG emissions (>5,000 T CO2eq/yr), mostly from uncaptured methane emissions at landfills. The integration of sludge drying, and Class A land application reduced GHG emissions by nearly 80% compared to dewatering and landfilling. This is attributed primarily to limited methane release in farm fields (due to low moisture content coupled with aerobic oxidation) and somewhat to net negative emissions from Class A land application GHG emissions were lowest for Scenario 3 (anaerobic digestion, Class B land application option) because of the potential to recover renewable power and heat. In fact, more heat and power is produced than needed, resulting in negative emissions for Scenarios 3 and 4. Scenario 4 (anaerobic digestion with drying, Class A land application) was comparable to Scenario 2 (Sludge Dryer Only). Figure 1 clearly shows that technologies for mass and volume reduction are likely to reduce GHG emissions associated with sludge treatment and end-use. However, many WRRFs typically make technology decisions based on a cost-benefit analysis. Table 1 summarizes approximate capital expenditures and GHG emissions offsets compared to Scenario 1. The results show that Scenario 2 & Sludge Drying provides the greatest net reduction in GHG emissions normalized to spent capital. This indicates that sludge drying may provide a cost-effective means to improve the GHG emissions profile at small to moderately sized WRRFs. Conclusions and Future Work WRRFs require cost-effective and environmentally sustainable solutions for sludge treatment and end-use. The results from this work show that technologies for mass and volume reduction are likely to reduce GHG emissions associated with sludge handling. This modeling work showed that land application of Class A and Class B biosolids provides significant benefits including soil carbon sequestration and fertilizer offsets compared to landfilling. Sludge drying was shown to provide the most greenhouse gas emissions offsets per unit of capital expenditure, indicating the technology may be a cost-effective method to improve the environmental footprint of municipalities that own and operate small to moderate sized WRRFs. This is significant because sludge drying may become a technology to prepare for higher cost end-use outlets caused by emerging contaminant regulations (e.g. PFAS). Sludge drying is also a potential precursor process to PFAS destruction technologies such as high-temperature incineration, gasification, and pyrolysis. Future applications of this work include an evaluation of the GHG emissions profile of PFAS destruction technologies and more sustainable landfilling practices, such as enhanced methane capture for renewable natural gas (RNG).