Reduction of Nitrous Oxide Emissions from Biological Nutrient Removal Processes by Thermal Decomposition

PURPOSE The purpose of this research is to demonstrate the concept of thermal destruction of nitrous oxide as a technology that fits into the current goal of resource recovery and utilization at wastewater treatment plants while simultaneously providing increased flexibility with which to operate a given treatment process. This paper will present the computer simulated results indicating the thermal decomposition of nitrous oxide emissions from a biological nutrient removal (BNR) process. INTRODUCTION Two separate sources of gaseous emissions at wastewater treatment plants are those from the aeration basins for the biological treatment and from the combustion processes within the facility. The combustion processes may include one or more of the following: boilers, operating on fossil fuel or natural gas, combined heat and power (CHP) units, operating on digester biogas or a sludge incinerator. The objective of this research is to link the gaseous emissions of the biological processes to the air intake of the existing combustion processes with the net effect of reducing the total emissions of harmful gases from the facility. The emission of nitrous oxide (N2O) from wastewater treatment plants is of concern due to its impact as a greenhouse gas [i.e 298 times that of carbon dioxide (CO2)] and is a consideration in the ongoing research regarding the nitrite shunt and Anammox processes. By using the emissions from the biological reactors as the inlet gas stream to the combustion process the nitrous oxide is removed by thermal decomposition; see Figure 1. Thermal decomposition of N2O occurs at approximately 565 ºF (296 ºC), well below the combustion temperature, as N2O splits to nitrogen (N2) and oxygen (O). The oxygen is consumed as an oxidant. Thermal decomposition of nitrous oxide has been implemented to reduce the unavoidable N2O emissions from the manufacture of adipic acid for the nylon industry (Reimer et al. 1994 and Shimizu, Tanaka and Fujimori 2000). In addition to the reduction of nitrous oxide, any hydrogen sulfide (H2S), methane (CH4) or VOCs that may be emitted is also removed. A secondary benefit of using the gaseous emissions from the biological processes as the inlet to the combustion process is that the CO2 acts as a thermal sink thereby reducing the thermal NOx emissions produced by the combustion process. This technology removes any constraints on how the biological process may be operated because it eliminates the need to consider the N2O emissions. For example, the goal of maximum pollutant removal, lowest energy consumption or resource recovery may be pursued without concern for the gaseous emissions that result. For this study, gaseous emissions from the aerobic portion of a BNR process treating a flow of 30 million gallons per day (MGD) with influent parameters of biochemical oxygen demand (BOD5) = 250 mg/l and NH3-N = 35 mg/l were estimated. Six cases, for which, the nitrous oxide emissions were estimated as a percentage of the influent ammonia value were simulated. The values of 0.2, 0.5, 1, 2, 5, and 10% were chosen based on published literature (Kampshreur, et al. 2009). Methane, hydrogen sulfide and VOCs emissions were also estimated. Estimates of methane emissions were based on data from a municipal plant treating wastewater at a rate of 1 MGD that showed the average annual emission rate from the aeration tanks was 2.2 x 105 g-CH4/year (Czepiel, Crill and Harriss, 1993). Hydrogen sulfide emission estimates were based on data collected from three municipal treatment plants (Devai and DeLaune, 1999). The VOC emissions were estimated based on the EPA 40-City Study, (1982) and the Joint Emissions Inventory Program (JEIP), (1993). Computer simulations using STANJAN© to run chemical equilibrium calculations were conducted (Reynolds 1986). Each of the six cases (i.e. the gaseous emissions with different concentrations of N2O) was run at different fuel to air ratios (equivalence ratios). The primary objective of the study was to determine the fate of the nitrous oxide emissions from the biological process. The secondary objective was to determine the effect of carbon dioxide (CO2) and nitrogen gas (N2) on the production of thermal NOx (i.e. NO and NO2) formed in the combustion process. The formation of nitrogen oxides (NOx) primarily nitric oxide (NO), by the oxidation of atmospheric nitrogen is a function of flame temperature and is termed thermal NOx (Turns 1996 and Warnatz, Maas and Dibble 2001). The final objective was to determine the fate of several other gaseous constituents that may be present: methane (CH4), hydrogen sulfide (H2S) and ten of the most common VOCs found in municipal treatment plants. RESULTS In all six cases nearly complete thermal decomposition of N2O was achieved. Equilibrium conditions were assumed given the adiabatic flame temperature of methane is approximately 2000 K. In Figure 2 the influent and effluent mass flow rates of N2O, for three of the gas streams, (those with 0.1, 0.2 and 0.5% of NH3-N as N2O) are plotted against the equivalence ratio. The effluent mass flow rate (shown in red) is negligible indicating significant thermal destruction of nitrous oxide. The results are similar for the higher concentrations, not shown. In Figure 3 thermal NOx produced, for each of the individual gases, the complete gas stream and the standard gas stream (air) is plotted against equivalence ratio. The results indicate that combustion with the gas stream from the biological process, (i.e. complete gas stream) reduces the thermal NOx emissions by 18.4% over combustion of methane with air (i.e. standard gas stream). The reason for this is that the carbon dioxide (CO2) produced from the biochemical reactions acts as a thermal sink. This is analogous to exhaust gas recirculation (EGR) technology used to reduce NOx emissions from internal combustion engines. Figure 3 also indicates that the nitrogen gas (N2) produced from the thermal decomposition of the N2O had no effect on NOx emissions when compared to the standard gas stream. This is due to the small quantity of N2 compared to the amount of CO2 produced by the biological reactions. Figure 3 also indicates that the equilibrium concentration of NOx is high at temperatures near stoichiometric combustion and decreases rapidly from that point (Flagan and Seinfeld 1988). Most combustion process used for power generation are operating slightly at an equivalence ratio between 0.8 -0.95, near the peak of the plot in Figure 3. CONCLUSIONS The results indicate that the thermal decomposition process reduces N2O emissions as well as methane, hydrogen sulfide, and VOC emissions (results, not shown here). A secondary benefit is the reduction in thermal NOx emission from the combustion process. This process mitigates greenhouse gas emissions without significant impact on the existing WWTPs. It also provides more flexibility in the process decisions by removing the constraints that results when considering the gaseous emissions that result from the biological process.