Estimation of greenhouse gas emission from domestic wastewater treatment plants

Anthropogenic greenhouse gas emissions lead to global warming, and consequent changes in the precipitation regime, thus increasing the occurrence frequency of extreme weather events, such as heat waves, cold waves, drought, and flood. With population, production and consumption rapidly increasing since the Industrial Revolution, it is estimated that irreversible side effects on the environment will be observed, if emissions related to anthropogenic activities will continue to increase with the same speed. Between years 1880 and 2012, global mean surface temperature increase is reported to be 0.9°C. Scientific studies on global warming at the international level indicate that the global mean temperature rise caused by human-induced greenhouse gas (GHG) emissions should be limited to 2°C above the increase in human-induced GHG emissions prior to the Industrial Revolution. In order to achieve this goal, every economic sector needs to fulfill its duty to reduce emissions with the help of increasing technological opportunities. Within the context of the United Nations Framework Convention on Climate Change (UNFCCC), the economic sectors identified by the IPCC are; economy, energy, industrial processes and product use, agriculture, forestry and other land use, and the waste sector. Greenhouse gas emissions from the waste sector account for approximately 3% of global human-induced emissions, while wastewater treatment accounts for the 20% of the waste sector. The greenhouse gases released from domestic wastewater treatment plants are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). The global warming potentials of CH4 and N2O are 28 and 265, respectively. According to the Greenhouse Gas Protocol, the emission of CO2 gas, which is the result of microorganism activities (biogenic) in wastewater treatment, is not considered among human-induced greenhouse gas emissions. CH4 occurs in biological treatment of organic matter in anaerobic conditions in wastewater collection channels and treatment plants, while N2O gas is formed as an intermediate product by the nitrification and denitrification of ammonia oxidizing bacteria (AOBs) in the aeration tank (low dissolved oxygen & high NO2-N concentration) and in heterotrophic denitrification processes in anoxic tanks. The release of dissolved greenhouse gases from the liquid phase to the atmosphere is due to the ventilation and current turbulence. AOBs, which play a role in nitrification at wastewater treatment plants, obtain energy when aerobic conditions oxidize "N" "H" _"4" ^"+" to "N" "O" _"2" ^"-" by nitrification reaction, and N2O is formed as an intermediate product in this reaction chain. The amount of N2O produced by AOBs in nitrification increases with low dissolved oxygen concentration. When low concentrations of O2 and sufficient "N" "O" _"2" ^"-" are present in the nitrification zone, AOBs use "N" "O" _"2" ^"-" as an e- receptor and perform partial denitrification. It can be said that N2O is the final product in AOBs denitrification since there is no N2O reducing enzyme in the AOBs. It is suggested that the largest contributor of N2O emissions released from biological wastewater treatment plants is the AOB denitrification reaction. According to the Greenhouse Gas Protocol, emissions of non-biogenic greenhouse gases arising from wastewater treatment within the boundaries of the facility are reported in Scope 1 as greenhouse gas emissions directly emitted from the facility. In Scope 2, the external energy-related greenhouse gas emissions are reported. While in Scope 3, emissions of greenhouse gases arising from the production of all materials purchased from outside, from use of produced product, from employees and transportation activities in the supply chain are reported. The amount of greenhouse gas emissions emitted from domestic wastewater treatment plants can be calculated in accordance with the IPCC (2006) National Greenhouse Gas Inventory Guideline, which includes the emission factors selected by the IPCC for the purpose of determining the annual national greenhouse gas emissions of the United Nations Framework Convention on Climate Change parties. In addition, direct greenhouse gas emissions from the treatment plants can be estimated more precisely by taking into account the mathematical models and computer simulations as well as the specific conditions. Within the scope of this study, direct greenhouse gas emissions from 9 major biological wastewater treatment plants in Istanbul (namely; Ambarli, Atakoy, Buyukcekmece, Canta, Pasakoy, Selimpasa, Silivri and Tuzla II advanced biological wastewater treatment plants and Tuzla I primary wastewater treatment plant) is estimated using the IPCC (2006) Guideline and GPS-X simulation model, based on the population and pollution loada of 2020 projections. According to the IPCC (2006) Guideline, the direct emission factor is 3.2 g N2O/cap/year while the indirect emission factor is 5 g N2O/g Ndischarge. In the study, it is accepted that industrial wastewater enters the municipal wastewater network. The amount of nitrogen in the discharged water is calculated for indirect N2O emission after discharge. In order to do this, the TN concentration specified in the discharge standards was multiplied by the plant flowrate to achieve the discharge nitrogen load. While calculating the CH4 emission amount according to the IPCC (2006) Guideline, the facility's input COD load was accepted as the total amount of organic matter in the wastewater. Accordingly, CH4 emission factor was found by multiplying the maximum CH4 production capacity and the CH4 correction factor (MCF), where the maximum CH4 production capacity was selected as 0.25 kg CH4/ kg COD and the methane correction factor is selected to be 0.8 for anaerobic digesters. The GPS-X simulation program developed by Canadian Hydromantis Firm defines basic plant processes that can be used in different configurations of water and wastewater treatment. In the GPS-X model, steady state and dynamic simulations can be run selectively, and the standard temperature-sensitive biokinetic models, such as ASM1, ASM2d, ASM3, and 2-stage nitrification (Mantis 2) are also available. The Mantis3 module is used for carbon footprint calculations of wastewater treatment plants. With the GPS-X model, the total emission amount for each unit of greenhouse gas emission and Scope 1, 2, and 3 can thus be calculated. The amount of methane recovered from anaerobic processes is also included in the model and the Mantis3 module includes energy calculations as well. According to the IPCC (2006) Guideline, the total N2O and CH4 emission potential of the plants are 6779 and 218593 t CO2 eq/year respectively. The total greenhouse gas emission calculated according to the IPCC (2006) Guideline is 225372 t CO2 eq/year. The total N2O and CH4 emission potential of biological treatment plants calculated using the GPS-X model are 690114 and 37429 t CO2 eq/year respectively and the total amount of greenhouse gas emissions calculated with the GPS-X model is 687357 t CO2 eq/year. Total greenhouse gas emissions per unit input nitrogen load of the plants were calculated as 0.09% N2O-N/Ninput and 9% N2O-N/Ninput with IPCC (2006) Guideline and GPS-X model respectively. Literature states that the unit of GHG emissions from full scale BNR plants is in-between 0.003-25% N2O-N/Ninput. As the results calculated by the above-mentioned methods in this study are compared with the literature figures, it is concluded that the results are compatible with previous studies. The reason for the difference between the result of two methods is that the direct N2O emission factor (3.2 g N2O/cap/year) used in the calculations made with respect to the IPCC (2006) Guideline is based on a wastewater treatment plant without biological nutrient removal. The amount of N2O emissions calculated by the GPS-X model includes the nitrification and denitrification mechanisms that play a role in producing N2O. Therefore, as the N2O emission due to the nitrogen removal mechanisms was not considered, the N2O emission amount calculated by the IPCC (2006) Guideline is less than the result based on the GPS-X model. In addition, the amount of CH4 gas emissions from biological treatment plants calculated according to the IPCC (2006) Guidelines is more than the value calculated by the GPS-X model. In the GPS-X model, it is assumed that the amount of released CH4 emissions is due to fugitive emissions of ~0.1 of the amount of CH4 produced in the anaerobic digester. The GPS-X model assumes that there is no CH4 emissions from biological treatment tanks. As the maximum CH4 production capacity (0.25 kg CH4/kg COD) defined in the IPCC (2006) Guidelines gives the amount of CH4 resulting from anaerobic digestion of the organic matter under optimum conditions with the highest yield, the CH4 emission factor values calculated by this method exceed the literature values. When the maximum CH4 production capacity is selected in between 0.16-0.18 kg CH4/kg COD, it is concluded that the obtained CH4 emission factor values are in accordance with the literature. Critical factors affecting direct greenhouse gas emissions from biological wastewater treatment plants have been investigated using the GPS-X model for the Ambarlı Advanced Biological Wastewater Treatment Plant Final Stage. Within this context, Scenario 1 presumes that the COD/TKN ratio is changed with the varying TKN values, and examines the possible impact of the wastewater COD/TKN ratio on the greenhouse gas emissions from the facilities. On the other hand, only the sludge age parameter in Scenario 2 and only the dissolved oxygen level in the aeration tank in Scenario 3 was altered in order to investigate the effect of plant operating conditions on greenhouse gas emissions. Finally, Scenario 4 examines the effect of hydraulic conditions on the direct GHG emissions due to climatic conditions, with shock loading. In this study, it was concluded that N2O emissions were higher in plants with higher TKN concentrations. When the concentration of NO2-N in the nitrification tank is high, the denitrification activities of the AOBs increase. Furthermore, the results show that the N2O emission is increased when the COD/TKN ratio is low. This is because when COD/TKN is low, carbon becomes the limiting factor in the sludge, and the NO2-N concentration increases with the incomplete nitrification. With increasing NO2-N concentration, AOB denitrification and hence N2O emissions are increasing as well. It is recommended to keep the COD/TKN ratio at a level that does not affect nitrification and denitrification processes by reducing the activity of the primary sedimentation, or by adding an external carbon source, in order to prevent the increased N2O emission. When the influence of sludge age on direct GHG emissions from biological wastewater treatment plants is examined, direct GHG emissions are observed to decrease with increasing sludge age. The reason is that as the growth rate of autotrophic nitrification bacteria is slow, it is unable to complete nitrification at the time of low sludge age and accumulation of NO2-N concentration as a result of incomplete nitrification. When the concentration of NO2-N in the reactor is increased, the denitrification activities of the AOBs and thus the N2O emissions are increased. Since low sludge age results in increasing GHG emissions, to complete the nitrification it is recommended that the sludge age be kept long enough to prevent partial nitrification, which is caused by. In this study, it is observed that the nitrification efficiency increases and the N2O emission from the aerobic tank decreases, when the dissolved oxygen level in the aeration tank of the treatment plant increases. This is due to the decrease in the denitrification of AOB with increased oxygen level. However, excessive ventilation caused the transfer of dissolved oxygen to the anaerobic and anoxic tank by the recycled sludge, and the inhibition of denitrification enzymes in these tanks resulted in increased N2O emissions. Nevertheless, since the amount of N2O emissions from the aeration tank is considerably higher than the amount emitted in the anaerobic and anoxic tank, it is concluded that the total greenhouse gas emission of the plant is reduced with increasing dissolved oxygen level. In order to control ventilation-related GHG emissions, plants should be operated with optimal oxygen level in the aerobic area (<3 mg/L). Finally, the scenario in which the Ambarli Advanced Biological Wastewater Treatment Plant is exposed to 50% shock load is examined via the GPS-X model. It was observed that direct greenhouse gas emission and discharge concentrations increased suddenly with increasing flow, however they returned to regular values after the flow returned to normal as well.