Treatability of aircraft deicing fluids (ADFs) by high-rate anaerobic systems: An investigation on biogas potential and energy recovery

Air transportation is the most preferable alternative due to the increase in world's population because it provides convenience to people in many ways. International Organization of Standardization (ISO) examined that the airline transportation has increased remarkably compared to previous years. For example, total number of domestic and international flights, which were approximately 595,000 in 2006, was recorded as 1,450,000 in 2016 (TUIK, 2017). In order to ensure flight safety in winter conditions when heavy snowfall occurs at the airports around the world, Aircraft Deicing Fluids (ADFs) are applied at the special places namely the 'deicing pads' to remove ice, frost and snow accumulated on the outer surfaces of the aircraft. There are four types of ADFs used during deicing applications. The main component of these fluids is 'ethylene glycol (EG)' or 'propylene glycol (PG)'. Compared to EG, PG is less harmful to the environment and it is easier to be treated. In addition, some of the ADFs (i.e., Type II) has a structure that prevents ice formation. Four types of ADFs are named as Type I, Type II, Type III, and Type IV. Among them, Type I ADF consists of 8% water, 90% glycol and less than 2% chemical substances. Type II and Type IV ADFs are the reformulated types of the Type II ADF; and it consists of 33% water, 65% glycol and 2% alcohol-based chemicals. Since such ADFs contain more water than Type II ADF, a dilution does not need before their usage. The types of ADFs vary according to the size of the aircraft. The main difference between these fluids is their viscosity. ADFs are produced in accordance with the standards set by ISO (EPA, 2000). Among them, Type II ADF is widely used in all over the world for deicing purpose and this type of ADF will be also used at the airport that is still under construction in Istanbul. During their applications; ADFs are sprayed on the surfaces of the airplanes after being diluted with hot water. Then, they are discharged/transported to the storage tanks by the channels at the airports. It is known that the storage tanks at the airports might be discharged to the nearest municipal wastewater treatment plant (WWTP) at certain ADF ratios. Moreover, ADFs might eventually reach to the municipal WWTPs via surface runoff through raining and snow melting. However, studies have shown that the organic loading rates (OLRs) of the biological systems at the WWTPs i.e., chemical oxygen demand (COD) are significantly increased by the wastewaters contaminated with ADFs and this might lead to lower treatment efficiencies which are not compatible with design parameters. Besides, the effluents of the WWTPs also do not provide the required standards given in the related regulations. ADF application is done in Turkey only in winter time (i.e., about 3 months). A portion of the ADF is collected at several airports in Turkey and it is moved away from the storage tank by a tanker. On the other hand, some portion is given directly to the sewerage system or to the receiving water bodies without any treatment. Since ADF contains high organic matter, it leads to environmental pollution and/or human health risks in the case of its direct discharge. The main impacts of wastewaters contaminated by ADFs are as follows (Vasilyeva, 2009): • Decreased amount of dissolved oxygen in the receiving water body, • Increased nutrient concentration and accordingly eutrophication in the receiving water body, • Creation of dead zones for the aquatic organisms, • Change and/or decrease in the variety of living beings in the environment, • Contamination of groundwater and drinking water sources, • Change in the physical conditions of the environment and surrounding areas (e.g., odour, foam problem, etc.), • Respiratory health problems, skin and eye irritation. Preliminary studies reported that anaerobic systems provide more effective treatment performance than aerobic systems and operating costs of anaerobic systems are lower in the treatment of wastewaters contaminated especially with Type II ADF. For this reason, it is more feasible to treat/manage the wastewaters contaminated with these fluids by anaerobic systems at the source of the pollution. Since the wastewater contaminated with ADFs have high COD concentrations that might seriously affect the biological systems at the WWTPs; ADF ratios should be at optimum/appropriate rate inside wastewater in order to treat easily. The aim of this study is to investigate anaerobic treatability of domestic wastewaters contaminated with ADFs (i.e., including high organic matter) at certain ratios by a high-rate anaerobic reactor namely the upflow anaerobic sludge bed (UASB) reactor. Besides, the biogas yield and the change in the ADF were also evaluated during anaerobic treatment next to conventional parameters such as total COD (tCOD), soluble COD (sCOD), pH, alkalinity, etc. UASB reactor occupies an important place among energy technologies. In these systems, wastewaters with high organic content could be treated, less space is needed for treatment and biogas can be obtained as a renewable energy source. Within the scope of the study, it is also aimed to investigate the change in microbial species in the biomass samples taken at certain intervals during the operation to understand the effect of these types of ADFs having high PG contents on the microbial cultures during anaerobic treatment. In the thesis, first of all, the purpose, scope and importance of the study was defined. The second part includes a literature review by giving general information about the subject and anaerobic biotechnology, the main characteristics of the ADFs, and the treatment methodologies of the wastewaters contaminated with ADFs. Moreover, the previous studies about the related subject were evaluated and main conclusions were given in this part. After this part; materials and the methods used during the study are given in detail in the third section. Also, in this section, the characteristics of the raw domestic wastewater, pure Type II ADF, and domestic wastewater contaminated with Type II ADF as well as the operating conditions of the bioreactors (batch and UASB) are given. In the fourth 'Results and Discussion' section; conventional parameters pH, alkalinity, tCOD, sCOD, total suspended solids (TSS), volatile suspended solids (VSS), biogas productions, PG, and volatile fatty acids (VFAs) are measured. In the last 'Conclusion' part; the main findings/remarks obtained within the scope of this study are presented and necessary future works are evaluated. According to the results, high biogas productions occur during anaerobic batch treatment of domestic wastewaters contaminated with Type II ADF until a certain ratio whereas biogas production from the UASB reactor indicated relatively lower yields. In the first study with batch system, a total of 9 reactors were run with Type II ADF in the domestic wastewater with the following ratios; 0% (control), 0.83%, 1.01%, 1.20%, 2.24%, 2.83%, 3.08%, 3.45%, and 4.20%. Results in terms of biogas yield and tCOD removal, the optimum ADF ratio was found as as 0.83% and 1.01%, respectively. The removal efficiencies of tCOD and sCOD at these ADF ratios changed between 90-97%. In addition, the highest biogas yield in the reactor with 0.83% ADF was found as about 0.404 m3/kg sCODinfluent. Moreover, biogas yields decreased dramatically in the reactors where ADF ratios were ≥ 1.20% which might be due to the fact that high ADF ratios caused an increase in VFAs so that the methanogens could be inhibited at pH values lower than optimum range. After the determination of the optimum ADF ratio in the domestic wastewater as about 0.83 and 1.01% in the batch study, a high-rate UASB reactor was operated at semi-continuous mode for about 8 months in the second part of this thesis study. During the start-up of the UASB reactor, some of the sludge taken from the batch reactors was used as the inoculum which has been already adapted to Type II ADF. Moreover, raw granular methanogenic sludge was also added into the bioreactor which was supplied from the mesophilic anaerobic reactor of a paper/cardboard industry. The total volume was 1300 m. At start-up; the UASB reactor was operated only with the raw domestic wastewater as the substrate and the ADF ratio was between 0.36-0.54% in the domestic wastewater for about two months (i.e., adaptation on ADF). Afterwards, the ADF ratio in the domestic wastewater was increased to 1.05% as operation continued between 68-154 days (about three months) which then decreased to 0.19% during the last two months due to low biogas productions. Despite of relatively lower biogas production; high COD and TSS removals were obtained during the study. Accordingly, the biogas yields were obtained as 3.9 L/kg sCODinfluent for the first period (ADF=0.36-0.54%), 1.4 L/kg sCODinfluent for the 2nd period (ADF=1.05%), and 1.8 L/kg sCODinfluent for the 3rd period (ADF=0.19%). Moreover, average tCOD and sCOD removals changed between 73-88% and 84-90%, respectively for each period. Hence, findings of this study indicated that Type II ADF might cause inhibitory effect above a certain ratio ≥ 1.05% which was observed more clearly in the high-rate anaerobic system operated at semi-continuous mode. In addition to these results, the microbial communities were identified in order to understand the microbial relationships between bacteria and methanogens as well as how these cultures affect the stability and the performance of anaerobic reactors especially when treating wastewaters contaminated with special pollutants like ADFs. Accordingly, the phyla namely Actinobacteria, Bacteroidetes, Chloroflexi, Firmicutes, Fusobacteria, Proteobacteria, Spirochaetes, Synergistetes, Thermotogae and Verrucomicrobia were detected at the last operating days of the 1st period at 0.36-0.54% ADF (t=67 day) and the 2nd period at 1.05% ADF (t=154 day). Among them, Bacteroidetes and Firmicutes were the dominant phyla during the operation of the UASB reactor at increased ADF ratios (from ca. 25% to 30% and from ca. 31% to 32%, respectively). Moreover, three dominant archaeal phyla were detected namely Methanomicrobia, Methanobacteria, and Thermoplasmata and while the relative abundances of Methanomicrobia and Methanobacteria increased, the relative abundance of Thermoplasmata decreased. Additionally, a conceptual design of one module UASB reactor was also done for the year 2019 in order to reflect field-scale conditions better at airports. Not only the findings of this study were used but also the reported aircraft numbers that de-icing application had been done in previous years were taken into consideration during conceptual design. Results indicated that high-rate anaerobic treatment of ADF-contaminated domestic wastewaters could be applicable at controlled feeding and operating conditions with reasonable biogas and energy productions. Accordingly, about 920 m3/day biogas production equivalent to ca. 4340 kWh total energy (electricity and heat) generation could be obtained by the UASB reactor treating about 0.5% Type II ADF included domestic wastewater.