| Summary: | 碩士 === 國立中山大學 === 環境工程研究所 === 103 === Greenhouse gas (GHG) emission has become an important issue due to the global warming in these decades. Wastewater treatment plants (WWTPs) are known to be one substantial GHG emission source. Understanding the mechanism behind and developing the associated control strategies are the issues worth investigating and the objective of this study. In this study, a WWTP in southern Taiwan that mainly treats wastewater from metal processing industries (Plant A) was selected for investigation in this study. The emissions of three GHGs including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) were continuously monitored during two sampling periods in winter and summer. After identifying the biological treatment process was the main contribution source, lab-scale bioreactor batch experiments were conducted in-situ to simulate the full-scale treatment with respect to the effects of different operational parameters including different aeration intensity, mixed liquid suspended solid (MLSS) concentrations, and reaction contact time on three GHG emissions. The results were studied with different analytical tools and theories so the science and mechanism behind the findings can be understood and used to develop possible strategies to control the future GHG emissions from WWTPs.
The study contains four sub-topics: First, the GHG emissions from the WWTP of interest were continuously investigated by analyzing the air-phase GHG concentrations every six hours in a one-week monitoring period in winter and summer. The treatment units analyzed includes the mixing tank, primary clarifier, aeration tank, sludge digestion tank, and final settling tank. In the results, the main emission unit of three GHGs in both winter and summer was the aeration tank, except in winter, the emission of CH4 in the final settling tank was relatively higher. The average ir-phase concentrations of CO2, CH4 and N2O in winter and summer were 3284 mg/m3 and 4363 mg/m3, 98.13 mg/m3 and 137.52 mg/m3 and 9.98 mg/m3 and 50.84 mg/m3, respectively. In winter, the emission factors of CO2, CH4 and N2O were 0.0225 kg CO2/kg COD~0.0719 kg CO2/kg COD, 0.000855 kg CO2/kg COD~0.00297 kg CO2/kg COD and 0.000222 kg N2O/kg N ~0.000872 kg N2O/kg N. In summer, the emission factors of CO2, CH4 and N2O were 0.0184 CO2/kg COD~0.0748 CO2/kg COD, 0.00113 kg CO2/kg COD~0.00355 kg CO2/kg COD and 0.000152 kg N2O/kg N ~0.000301 kg N2O/kg N, respectively. The numbers of the emission factors estimated in this study was close to the numbers estimated by a study in Japan. The mass emitted per day of CO2, CH4 and N2O were 83 kg/day,2.2 kg/day and 0.99 kg/day, respectively. In summer, the mass emitted per day of CO2, CH4 and N2O were 55 kg/day, 2.7 kg/day and N2O 0.25 kg/day, respectively. From the viewpoint of the total mass emitted per day, the mixing tank was more dominant as compared to the other units. The possible explanation was that the mixing tank was the first unit in the WWTP, which faces the source water with significantly higher concentrations enhancing the GHG emissions as compared to those in the subsequent treatment processes.
Next, the carbon and nitrogen mass balance were studied to understand the contributions of GHG emissions on the carbon and nitrogen loss from each treatment unit. The regression analyses were also conducted to investigate the possibility of predicting the GHG emissions by knowing the water quality information. The results showed that the contributions of GHG emissions on the the total carbon or nitrogen losses from different treatment processes were mostly below 10%. In the results of regression analyses, two degree polynomial equations that contain multiple variables were capable of predicting the GHG concentrations in the air phase, with the water quality parameters including temperature, dissolved oxygen and nitrite being considered. The R2 values of the CO2, CH4 and N2O concentration prediction results were 0.99, 0.84 and 0.59, respectively.
Third, the lab-scale experiments simulating the aerobic activated sludge were conducted, as mentioned previously. At the ends of the experiments, the gaseous and aqueous samples were collected to analyze the GHG concentrations in the air and water phases. Selected water quality parameters including the suspended solids (SS) and chemical oxygen demand (COD) were also monitored to understandin the effect of changing certain operational strategies on the treated water quality. Three findings were concluded from the experimental results in this stage: (1) It is possible to slightly reduce the sludge concentration without significantly affecting the treated water quality, minimizing the energy consumption and associated emissions of GHGs, notably CO2; (2) Lower reaction contact times reduced the CO2 emissions without substantially affect the emissions of N2O with a higher global warming potential (GWP). However, the treated water quality was more sensitive to the variation of this operational parameter, indicating that changing the reaction contact may not be an appropriate strategy for the GHG control; (3) Although decreasing the aeration intensity increased the GHG emissions, a lower aeration intensity represents a lower operational cost. Given that the treated water quality was less significantly affected by different aeration intensities in the experiments, increasing the aeration intensity to a limited degree was possibly suitable for controlling the GHG emissions from the aerobic biological process.
Forth, the results of the batch simulation experiments were used to estimate the fugacity, emissions flux, mass transfer rate, and to models the concentration variations of three GHGs in the air phase over the experimental time assuming a close system and a steady wastewater concentration in the water phase. The results are summarized as follows: (1) By estimating the fugacity, the GHGs all transferred from the water to air phase in both winter and summer. Mass transfer potentials were stronger in summer than in winter, attributed to the effect of temperature; (2) The estimated mass transfer rate of CO2, CH4 and N2O were 0.124±0.041 m/d, 0.103±0.017 m/d and 0.096±0.012 m/d, respectively. (3) The estimated the flux of three GHGs decreased in the order: CO2&;lt;CH4 &;lt;N2O. (4) By assuming a steady wastewater concentration and a close reactor system, of the times needed for three GHGs to achieve equilibrium between the air and water phases were approximately 120 hours. (5) With the life cycle assessment, even though a lower aeration intensity increased the GHG emissions, the reduced GHG emissions associated with the lower aeration intensity and operational burden would compensate the increase of GHG emissions resulted by the treatment process itself, indicating that lowering the aeration intensity is still a possible strategy to reduce the GHG emissions in an aerobic biological treatment process. The findings from both the field monitoring and lab-scale simulation experiments help understand the GHG emissions in a typical WWTP and investigate the effects of different operational strategies on the GHG emissions from its main emission unit, providing important insight into the development of possible strategies to minimize the GHG emissions from WWTPs in the future.
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