Investigation of optimum glycol ratio and biogas recovery during anaerobic treatment of the wastewaters contaminated with aircraft de-icing fluid.

Airway transportation has become the most preferred transportation option today with the development in technology. In both human (passenger) and cargo transportations; the demand for the aviation sector is increasing day by day because of its economic benefits and comfort. When air transportation statistics from Turkish Statistical Institute (TUIK) are examined, total domestic and international traffic was reported as about 376.041 in 2002 and about 1.452.995 in 2016. According to this, about 4-fold increase was observed in 14 years and it is estimated that this increase rate will be higher in the coming years. But there are disadvantages as well as the advantages of the air transportation industry. It is predicted that this increase in air transportation will lead to an increase in the amount of the pollutants occured at the airports. Especially, de-icing/anti-icing fluids used to provide flight safety in cold weather conditions are causing significant environmental problems. Icing problem, one of the situations that threatens flight safety, became an important issue after an accident of a flight in March 1992 in New York City during takeoff. Following this accident, the Federal Aviation Administration (FAA) introduced strict standards for de-icing operations. Ice formation can affect flight mechanics during aircraft landings and take-offs. Therefore, when temperature values are observed at 0°C or below and when precipitation events such as snow, sleet, and frost fog occur, deicing operations should be applied to the aircrafts. Aircraft Deicing Fluids (ADF) are generally contain with propylene glycol (PG) or ethylene glycol (EG). Propylene glycol is widely preferred since it has been found in the past years that ethylene glycol was toxic to mammals.There are four types of fluids that are used in the de-icing application process. The main difference between these different types of fluids is their viscosity values. Most preferred types of ADF at the airports are Type I and Type II. Type I fluids have lower viscosity compared to other types of fluids. Type I fluids flow away faster from the surface of the aircrafts and they provide relatively short protection. Type II fluids are in pseudoplastic form and they contain the polymeric thickener. For this reason, Type II fluids do not flow away quickly from the surface of the aircraft and they provide much longer protection. De-icing operation takes approximately 20 minutes and nearly 4000 liters ADF might be spent during this operation depending on the size of the aircraft. ADF is supplied in pure (concentrated) form from the related companies. These fluids are sprayed under pressure immediately after diluted with hot water (55-80°C). Following this operation, ADFs are discharged through the grids/channels found in the special parking areas at the airports (de-icing pads) and the spent ADF is commonly stored in a storage tank. After a certain period, these ADF storage tanks need to be emptied/cleaned. Hence, they might contaminate domestic wastewaters and they might reach to the nearest municipal Wastewater Treatment Plant (WWTP) at certain ratios. In addition, these fluids might also reach to the sewerage system due to snow melting and runoff. However, domestic wastewater when contaminated with ADF causes serious environmental problems in receiving water bodies as well as in municipal WWTPs. It is reported that such wastewaters will deplete DO amount and cause eutrophication. It was also reported that the various additives in ADF might have toxic effects on some living organisms in aquatic ecosystems. Therefore, it is very important to treat/manage/dipose the wastewaters contaminated with ADF by sustainable methods. Various types of ADF might have different effects on the biological treatment processes at WWTPs. Besides, the ratio of ADF in domestic wastewater should be determined before it reaches to municipal WWTP because Chemical Oxygen Demand (COD) concentrations of domestic wastewater contaminated with ADF could be increased significantly.Hence, it is necessary to know the mixing ratios of these fluids in the influents of the biological systems of WWTPs. For example, in the samples taken from a storage tank at an airport, the total COD (tCOD) concentration was measured up to 95000 mg/L. It is reported that the most suitable treatment method for these wastewaters with such high COD concentrations is anaerobic treatment. With this treatment technology, wastewaters containing high organic pollution loads can be treated at lower costs compared to aerobic treatment methods. During anaerobic treatment, it is also possible to recover biogas, which is a renewable energy source and which makes this treatment method relatively sustainable as compared to the energy-consuming treatment technologies. It is also reported that anaerobic treatment technologies are appropriate for the sustainable management/treatment/disposal of these special wastewaters generated at the airports. According to previous studies on the anaerobic treatment of wastewaters containing glycol at high concentrations, about 0.3 m3/kg COD biogas yield with 77% methane content could be obtained. Within the scope of this thesis; anaerobic treatability and biogas yield of the domestic wastewater having about 0,9-12,5% ADF were investigated in anaerobic batch reactors operated at mesophilic temperature (35°C). Moreover, optimum (appropriate) ADF ratios of Type I and Type II ADF were determined in the domestic wastewater by the biogas yields (m3/kg sCODfeed) obtained during anaerobic treatment. Within the scope of the study, which was carried out in two different parts, namely Type I – domestic wastewater and Type II ADF – domestic wastewater studies, the change in the ratio of propylene glycol (PG) during anaerobic treatment was also comparatively investigated. In addition, another experimental setup with only distilled water (Type I – distilled water and Type II – distilled water) at the most appropriate ADF ratios was conducted based on the data from the experiments carried out with domestic wastewater. Hence, the operational problems that might be encountered in field-scale anaerobic systems treating such special wastewaters and their biogas potentials could be foreseen. In the first part of this thesis study, the purpose, scope and the importance of the study are explained. In the second part, there is a detailed literature survey for anaerobic biotechnology, anaerobic treatment of wastewaters contaminated with ADFs, current status of ADFs in Turkey and in the world, and the studies related to this subject. In the third section, 'Material and Methods' part is presented. In this section, especially characteristics of the raw domestic wastewater, inoculums, and two different types of ADF (Type I and Type II) are presented. In addition, operational conditions of the batch reactors are presented in detail. Analytical methods and the equipments used throughout the study are also included in this section. In the 'Results and Discussion' part in the fourth chapter; the findings of the study are presented evaluating the changes in the conventional parameters (pH, alkalinity, tCOD, sCOD, VFA) as well as in biogas productions and in PG are presented. Moreover, overall evaluation and comparison of the results for all batch studies are also available in this part. In the 'Conclusion and Recommendation' section of the study; the results were comparatively discussed for two types of ADF (Type I and Type II). Accordingly, it was observed that high biogas productions could be achieved if domestic wastewater is contaminated with ADF up to a specific ratio in anaerobic batch reactors. The highest biogas yield in the Type I ADF – domestic wastewater study was found approximately 0.23 m3/kg sCODfeed in the reactor with 0,9% ADF ratio. In Type II ADF – domestic wastewater study; the highest biogas yield and sCOD removal were obtained as approximately 0.26 m3/kg sCODfeed and 63%, respectively in the reactor with 1,8% ADF ratio. Thus, 1-2% of Type I and Type II ADFs in domestic wastewater would not cause meaningful problems during the operation of anaerobic systems if these special wastewaters reach to the nearest municipal WWTP through the grids/channels at the airports or through runoff due to snowmelt. However, a significant reduction in the biogas yields occurred at reactors with ADF ratio ≥ 3%. This might be due to the fact that the decrease in pH with the increase of VFAs occurred due to high organic loadings in the reactors and this could cause an inhibition effect on the methanogenic microorganisms. The operation of the reactors prepared with Type I %1,371 and Type II %1,764 ADF added in distilled water lasted for 67 days. In this scope, the biogas yields were obtained as, 0,238 m3/kg sCODfeed for Type I and 0,169 m3/kg tCODfeed for Type II. The results of Type I and Type II ADF studies did not show meaningful changes in PG content at the end of the incubation period when compared to the PG ratios at the first operating day. All of these findings are summarized and recommendations were given in the fifth section of this study.