Sustainability Within Collection Systems: Methane Mitigation as a Result of Odor and Corrosion Prevention Programs Utilizing Calcium Nitrate

Introduction Greenhouse gases (GHGs) such as methane and carbon dioxide are common byproducts of chemical reactions within collection systems but are less commonly considered by municipalities during sustainability analyses. Understanding the scope of the impact of these GHGs requires an understanding of their relative effects on global warming. The Intergovernmental Panel on Climate Change (IPCC) created the Global Warming Potential (GWP) metric to compare different GHGs via an equivalency to carbon dioxide (CO2e), the reference gas. Methane has a GWP of 28, meaning that it has 28 times the impact on global warming as compared to carbon dioxide. After outlining and studying the theoretical reactions of sulfide generation in wastewater collection systems and calcium nitrate dosing for sulfide prevention, we hypothesize that dosing calcium nitrate for odor and corrosion control significantly reduces the overall impact of GHGs emitted by wastewater within collection systems. Objective In the proposed paper, we will set the groundwork to understand the theoretical mitigation of methane generation resulting from calcium nitrate dosing in US wastewater collection systems, as well as test the theoretical mitigation with a use case in Manatee County, FL. Theoretical Methodology In an untreated collection system, an anaerobic BOD reduction occurs in the wastewater as follows (all reactions assume methanol as carbon source for simplicity and consistency in comparison): SO4= + 4CH3OH → S= + 4H2O + 2CH4 + 2CO2 In a collection system being dosed with calcium nitrate for sulfide prevention, the intent is to drive an anoxic BOD reduction in the wastewater instead of anaerobic, which occurs as follows: 6NO3- + 5CH3OH → 5CO2 + 3N2 + 7H2O + 6OH- While more CO2 is generated in this reaction versus the anaerobic reaction, no methane is produced. Based on the comparison of these reactions and the GWP of methane, for every 1 lb of sulfide prevented, 25.25 lbs of net CO2e is prevented. Assuming that all GHGs are fully released from the wastewater into the atmosphere, we calculate that a maximum of 11.83 lbs CO2e can be prevented per gallon of 3.5 lb NO3-O/gal solution dosed into collection systems as part of an odor prevention program. In fiscal year 2021, theoretically a maximum of 71,887 metric tons of CO2e could be prevented as a result of all the chemical shipped to customers in the US by Evoqua for use in odor prevention. While it is recognized that a municipality choosing an odor prevention program will be incurring additional GHG emissions via transportation, manufacturing, provision of service, etc, the analysis of the use case study will focus explicitly on changes in GHG emissions specifically related to the theoretical reactions described above in relation to calcium nitrate dosing into the collection system. Application Manatee County Utilities (MCU) operates an expansive collection system involving over 800 lift stations (LS) with many long retention time force mains. The collection system conveys over 22 million gallons per day (MGD) of flow across three service areas to a corresponding wastewater reclamation facility for each service area. To significantly reduce corrosive conditions, MCU set a target atmospheric hydrogen sulfide (H2S) control objective of 20 ppm average and 50 ppm peak. MCU utilizes multiple solutions within their program, including calcium nitrate and pH shift technologies, to achieve these goals. For additional knowledge and to establish a baseline, we decided to also test the methane mitigation from dosing other chemicals by testing sites treated with magnesium hydroxide and calcium hydroxide. Four portions of the collection system were selected for sample testing. Two portions, N4B force main to N1B LS and Greenbrook 2 force main to Lakewood Ranch Riverwalk LS, are treated with calcium nitrate. Another section, Pope Road Master LS, receives flow treated with magnesium hydroxide. The final section, Tara 20 LS, receives flow treated with calcium hydroxide. A portable H2S gas monitor was deployed at each injection point and each control point to log continuous sulfide measurements. Grab samples were taken for dissolved sulfides, pH, temperature using gas detector tubes and a calibrated Hach portable pH probe. Gas samples were collected from the headspace of the wet wells using tedlar bags and CO2/CH4 concentrations analyzed via GC/MS by a certified third-party lab. Samples were collected at periodic intervals based on route efficiency and record keeping. Results and Discussion From six sample sets taken from the N4B force main to N1B LS, five showed a decrease in methane concentration from the injection point to the calcium nitrate-treated control point, with an average decrease in methane concentration of 38%. Half the samples showed an increase in CO2 emissions at the control point. Similarly, five of the six sample sets collected at the Greenbrook 2 force main to Lakewood Ranch Riverwalk LS saw a decrease in methane concentration with an average decrease of 13%. CO2 results from Greenbrook 2 mostly decreased in concentration, but only by small fractions. More data is needed to draw conclusions regarding CO2 results. Overall, methane was broadly reduced with nitrate treatment, with an average of 24% reduction in methane emissions during nitrate dosing. The CH4 concentrations at Pope Road Master LS treated with magnesium hydroxide were similar to that of the control points treated with calcium nitrate but showed lower CO2 values. The CH4 and CO2 concentrations at Tara 20 LS treated with calcium hydroxide were similar to the results obtained from the control points of N4B force main to N1B LS and Greenbrook 2 force main to Lakewood Ranch Riverwalk LS. This paper will go further into the analysis of the results, discussing outliers, testing challenges, and comparing test results to theoretical assumptions. We will also discuss the immediate and future steps to support the project, such as utilizing a continuous method of data collection for more robust data to decrease standard deviation. Another municipality has expressed interest in continuing the research within their collection system, which will provide more information on methane mitigation regarding different collection system designs, climates, and wastewater characteristics. We will also discuss the long-term sustainability target of evaluating GHG emission impacts from the full life cycle of calcium nitrate dosing for odor control, including manufacturing and transportation impacts. Benefits The theoretical mitigation calculation method would allow any municipality to calculate the methane mitigation they are achieving as a result of their odor control program utilizing calcium nitrate, making it easier to quantify the sustainability benefit of the program and communicate it with their community. As sustainability metrics become more important, including GHG metrics, these calculations can play a critical role in quantifying the overall GHG impact of the wastewater treatment and collection system and providing an additional method of GHG reduction. In the case of Manatee County (the County), the benefits are less theoretical. The County conducted a GHG Audit that was completed in June 2020, and while it took into consideration the wastewater treatment plants and overall utility operations, it did not examine GHG emissions from the collection system. This audit recommended a reduction of nearly 3,000 metric tons of CO2e a year to reach net zero by 2040. Understanding GHG emissions and particularly methane mitigation of the odor prevention program could allow MCU to demonstrate a more robust view of the wastewater contributors towards GHGs for the County as well as provide potential solutions towards the recommended reduction goal through a method that neither party may have considered beforehand. Whether or not wastewater authorities or utilities have their own GHG goals, they can provide benefit to their municipalities that are more likely to have them, as well as build public support through communicating the sustainability impacts of their operations.