Removal of Hydrogen Sulde in Biogas From Wastewater Treatment Sludge by Real Scale Biotrickling Filtration Desulfurization Process

High sulfur content in excess sludge impacts the production of biomethane during anaerobic digestion, mean-while leads to hydrogen sulde (H 2 S) formation in biogas. This study aims to reveal the eciency of the real-scale Biotrickling Filtration Process (BTF) in the removal of H 2 S in the biogas formed in the anaerobic digester. The biogas was produced by stabilization of the treatment sludges formed in the processes of Urban Wastewater Treatment Plant with mesophilic anaerobic sludge digesters. It was determined that the anaerobic stabilization unit of the treatment plant was operated eciently and the biogas with a high ow (18,123-21,383 m3/day) was formed during the operation of the plant. The H 2 S concentration in the biogas at the inlet of the BTF was 3,632 ppmv on average (2,900-4,400 ppmv) and 16 ppmv at the outlet. The elimination capacity of the system reached a maximum of 52.71 gH 2 S m-3h-1. As a result, a real scale BTF unit was found to provide a sucient removal eciency (97.84-99.90%) for H 2 S in the biogas.


Introduction
Anaerobic digestion (AD) is commonly used in the treatment of organic waste, such as agricultural waste, sewage sludge and organic form of municipal solid waste. During this process, approximately 95% of the organic matter and 95% of the energy present in the substrate are restrained in the biogas. (Guerrero et al. 2015). The biogas produced through anaerobic digestion is an environment friendly and important renewable energy resource (Oztürk 2007;Khoshnevisan et al. 2018). The produced biogas can be burned directly in combined heat and energy conversion plants and can be used as transportation fuel. Biogas is produced through anaerobic digestion of the treatment sludge originating from the wastewater treatment plants. The most important ingredients in the biogas produced by the digestion of the treatment sludges through anaerobic processes are: 60-70% methane (CH 4 ), 30-35% carbondioxide (CO 2 ), 1-2% hydrogen sulphide (H 2 S) and 0.3-3% other gases (Al Mamun and Shuichi Torii 2015;Rulkens 2008). The components of the biogas can vary depending on the used substrate for the production (Rasi et al. 2007). If the substrate used for biogas production contains sulphur, the formation of hydrogen sulphide (H 2 S) is inevitable. (Chaiprapat et al. 2015, Dumont 2015. The concentration of H 2 S in biogas varies from a few hundred to ten thousand ppm depending on the amount of bioavailable sulfur compounds in the feedstock and the outcome of the competition among sulfate-reducing bacteria, acetogens and methanogens for the organic substrates (Stams et al. 2005). The presence of high concentration of H 2 S causes corosion on the equipment and increases the maintenance costs. Especially, due to the corrosive effect on the gas engines, engine life is shortened, the service/maintenance costs increase, and the conversion of biogas to electricity decreases. (Rasi et al. 2011). For this reason, H 2 S must be removed from the produced biogas. By the removal of the H 2 S, higher quality biogas is produced increasing electricity production and extending the life of equipment used. In addition, H 2 S should be removed in terms of health and security (Deublein and Steinhauser 2008).
Actually, H 2 S is produced under anaerobic conditions because sulphate (SO 2 4-) acts as an electron acceptor while organic compounds are decomposed biologically. H 2 S is produced by anaerobic degradation of sulfur-containing compounds (mainly proteins) and reduction of anionic species (especially SO 4 2-) in the feedstock of the digester (Ramos et al. 2013). Kuenen (1975) proposed the mechanism of HS removal that occurs through a series of physico-chemical processes and biological reactions, summarized by Equation (1)-(4) below.
All of the processes lead to changes in terms of pH, dissolved oxygen and oxidation-reduction potential (ORP), which can be used to follow and control process performance (Janssen et al. 1998). There are also other reactions that have been reported, including non-biological oxidation of H 2 S to thio-sulphate and the further biological oxidation of thio-sulphate to sulphuric acid (Fortuny et al. 2011).
For the removal of H 2 S in biogas; solid phase adsorption, liquid phase absorption, membrane seperation, chemical, biological, and thermal methods are used (Abatzoglou and Boivin 2009;Wellinger and Linberg 2000;Rasi et al. 2011;Lin 2013;Angelidaki et al. 2018;Diaz et al. 2011;Peluso et al. 2019). The biological desulphurisation of biogas can be performed in additional units mainly using bio-lters and bio-trickling lters during digestion process and by applying microaerobic conditions directly in anaerobic digestors (Ramos et al. 2013). This biological desulphurisation treatment method for the cleaning the contaminated biogas is a relatively new trend and is of great interest. On the other hand other gas desulphurisation methods have high operation costs and produce wastes that must be disposed. Biological desulphurisation method is economically more advantageous and more environment friendly than the other methods. Biological desulphurisation of biogas takes place under low temperature and pressure and can proceed with limited reactive consumption or no reactive consumption (Alverez 2003;Syed et al. 2006). This treatment method is also more useful because the gas stream contains biodegradable or biconvertable compounds (Gabriel and Deshusses 2003;Tomas et al. 2009).
In bioreactor systems, rst hydrogen sul de in the gas phase is dissolved into sulfur oxidizing bacteria (SOB) containing microbial media, followed by the oxidization of hydrogen sul de by bacteria with oxygen in the liquid phase (Duan et al. 2006;Park et al. 1999). High elimination capacity and stability in the presence of severe operating conditions are required for bioreactor systems to be able to apply biological methods for the removal  (Ramos and Fdz-Polanco 2012). The differences between these systems are the phase of the biomass (suspended or xed), the state of the liquid phase ( owing or stationary) and the state of having or not having a carrier material (Ramirez et al. 2009). BTF, the waste air stream passes through a bed which is packed and which has pollutant-degrading organisms immobilized in the form of bio lms. The contaminant either passes from gas phase to liquid phase and later to the bio lm, or directly from gas phase to the bio lm, where it is eventually degraded biologically to harmless compounds (Gabriel and Deshusses 2003). The usage areas of BTF are large-scale gas applications to control and other odorous emissions from WWTPs and other industries (Khanongnuch et al. 2019). Its major advantages are having low operation cost, requiring low-energy and chemicals and having high removal e ciencies (REs), mostly above 99% (Aita et al. 2016). Thus, the aim of the present study is to eliminate H 2 S from biogas generated in Konya advanced biological urban wastewater treatment plant sludge through anaerobic processes with the use of real scale biotrickling ltration desulphurisation method.

Real-Scale Biotrickling Filtration (BTF) Process
This study performed at Konya advanced biological urban wastewater treatment plant with an equivalent population of 1,000,000 and a ow rate of 200 00 m 3 /day. BTF was used for the puri cation of H 2 S in the biogas collected at the anaerobic digester output used for sludge stabilization. In this process, the H 2 S is removed from biogas and biogas is cooled to condense the moisture in it and the condensate is disposed.
Biogas collected from anaerobic sludge digesters is transferred to the feeding chamber at the bottom of the closed tower where the BTF unit is located. The biogas moves from bottom to the top and in the tower that contains layers of polypropylene media lling circles (Table 1) where desulphurisation occurs. A complexed culture of sulfur oxidizing bacteria (SOB) dominated by Acidithiobacillus thiooxidans acclimated from activated sludge was used as the bacterial strain and a bio lm was formed. In order to supply the substrate for the SOB, treated wastewater was feeded to the feeding chamber at the bottom of the tower. The feeding water was passed through heat exchangers to adjust the temperature to 35-36 o C and it was sprayed to the media material from the top of the tower. Some authors reported for similar sul de-oxidizing microorganisms, an optimum growth temperature at around 30 °C (Ravichandra et al. 2006;Sanchez et al. 2014). Operation of H 2 S bio ltration reactors report 100% removal e ciency at 30-50 °C, but only 20% at temperatures below 10 °C (Yang and Allen 1994). At the entrance point of the desulphurisation unit 1.5-3.5% air was added to the biogas. In this process, O 2 /H 2 S ratio was 2/1. The end product of oxidation, sulfate (high O 2 /H 2 S ratio in bio lm) or elemental sulfur (low O 2 /H 2 S ratio), should vary depending on the availability of oxygen for microorganisms in the bioreactor. If the oxygen is more than the stoichiometric requirement, the formation of elemental sulfur decreases (Buisman et al. 1991). The treated biogas was passed through cooling units to decrease the temperature and moisture before it was feeded into the gas conversion engines. The ltrate collected at the bottom of the unit was discharged into the sulphur fertiliser tank. The sludge layer accumulated on the polypropylene material was disposed from the system by back-washing. The ow diagram of Biotrickling Filtration process is given in Figure 1. Real scale BDP design criteria are given in Table 2. The produced biogas consits of 65% methane (CH 4 ), 34% carbondioxide (CO 2 ), and 1% H 2 S and other gasses. The process was designed for biogas average temperature to be 30 ºC and the dilution water average temperature to be 15 ºC.

Monitoring and Analytical Methods
The pH is an important parameter affecting the process e ciency and the system was operated in the pH range of 1.5-3.5.The optimum pH should be in the range of 2-3.5 for activities of sulphate oxidising Acidithiobacillus thiooxidans bacteria (Syed et al. 2004;Montebello 2013;Rodriguez et al. 2014). Kim and Deshusses (2005) reported that the biological activity of microorganisms was inhibited due to the low pH and high sulfate content (at pH 2 the sulfate content in the water was 1,900 ppm). In order to monitor and control of the environment conditions of sulphur bacteria taking active role in the system, full otomation (SCADA) system was used. In this biological desulphurisation process; biogas ow meter, air ow meter, circulation liquid ow meter, pH and temperature measurement devices, dilution (addition) liquid indicators, biogas oxygen analysis system, sulphur removal tower, tank level indicator, gas detector, pressure indicator, and other instruments were used. In order to compare the ability of bio lters on the same basis, the elimination capacity (EC) was used. It represents the ability in removing pollutants in gaseous form compared to the incoming pollutant mass, expressed as the mass of pollutant removed per unit time per bed volume. The parameters used in this study to describe the operating conditions and for the determination of the removal performances are given in Table 3. Table 3. Process control parameters used in this study H 2 S removal e ciency of real scale BTF system was monitored for twelve months between January 2017 and December 2017 and the performance of the process was evaluated. During this period, the ow rate of biogas produced in the anaerobic sludge digesters, minimum, maximum, and average values of H 2 S level in the biogas and at the process outlet were monitored on a monthly basis to determine the H 2 S removal e ciency of the process. During this study, biogas ow was measured by ow meter (Drager) and H 2 S concentration was measured by H 2 S measurement tubes (Rea) and analyzed by colourmatic method (TS EN 1231: 2000).

Anaerobic Digester and Biogas Production
The ow rate of biogas and the H 2 S concentration in the biogas were measured for the e cient process operation. The operational parameters of the mesophilic anaerobic sludge digester (pH, organic loading rate, sludge feeding rate, ambient temperature, volatile organic acid concentration, sludge retention time) during the operation of the biological desulphurisation process, were given in Table 4. The characteristics of sludge at the inlet and outlet of sludge digester (total solid material, chemical oxygen demand, protein, alcalinity) were given in Table 5. The most important indicator showing the e cient operation of the anaerobic sludge digesters is the biogas production. During the working period, the ow rate of biogas produced in anaerobic sludge digesters varied between 18,123-21,383 m 3 /day and an average of 19,519 m 3 /day (Table 6) ( Figure 2). However, the range of percentage composition of the biogas produced from AD processes is dependent upon several factors including the digestibility of organic matter, digestion kinetics, digester retention time, and the digestion temperature (Dobre et al. 2014).  1998). At the outlet of the BTF process, H 2 S concentration varied between 4-63 ppm and an average of 16 ppm.
( Figure 3). No relation was determined between the biogas ow rate produced in anaerobic sludge digester and the H 2 S concentration in the biogas. It is thought that H 2 S is produced depending on the other factors (protein and sulphate concentrations in wastewater, etc.) completely independent from the produced biogas quantity. Since the produced biogas is used in the production of electrical energy, H 2 S needs to be removed due to the corrosive effect of H 2 S on gas engines and other auxiliary equipment. For this reason, H 2 S concentration should be reduced up to the limit value (≤ 260 ppm) determined for gas engines before biogas is given to gas engines.
The recommended level of H 2 S in the produced biogas is in the range of 0.02 to 0.05% (w/w) (200 to 500 ppm) while H 2 S-free biogas is more desirable (Rodriguez al. 2014). During the working period, the H 2 S removal e ciency ranged between 97.84-99.90% and an average of 99.55 %. (Table 6). In January 2017, when the performance of the process started to be monitored, H 2 S removal e ciency was observed to be 97.8% and increased during operation to 99% (Figure 4). It was determined that the H 2 S concentration at the outlet of BTF process was well below the determined limit value.
The elimination capacity (EC) and RE as functions of the load supplied to the system were analyzed for BTF reactor. Figure 4 shows the removal e ciency and elimination capacity of H 2 S monthly. EC changes as a function of EBRT and LR values. In BTF process, EBRT values were between 6.3-7.95 min, LR values were between 33.35-52.83 g H 2 S m -3 h -1 , EC values were between 33.21-51.71 g H 2 S m -3 h -1 (Figure 4). The average H 2 S removal was 99.9% at EBRT of 7.39 min (i.e., a LR of 41.38 g H 2 S m -3 h -1 ). In addition, the elimination capacity and H 2 S removal e ciency of this study BTF proces performance well when compared to the previous studies (Table 7). negatives which can use sul de and thiosulfate as an energy source. Due to their ability to tolerate a pH swing between 1.5 and 3.5, SOB Acidithiobacillus thiooxidans was found to be a major microorganism group in our bio lter in the present study. This bacterium is thought to be an ideal inoculum for the bio ltration of H 2 S in biogas and it is the most acidophilic SOB (Aita et al. 2016;Ramirez et al. 2016). It has a pH range between 0.5 and 5.5 and an optimum at pH 2-3 for growth

Conclusion
In this study, the removal of H 2 S from the biogas that was produced at real scale anaerobic sludge digester by BTF process was investigated. Average biogas ow rate produced in mesophilic anaerobic sludge digester varied between 18,123-21,383 m 3 /day and H 2 S concentration vaeried between 2,923-4,400 ppm v . It was observed that the H 2 S concentration in the produced biogas is completely independent from the biogas ow rate. The removal of high concentartions of H 2 S in biogas was accomplished by real scale BTF process with SOB bacteria (Acidithiobacillus Thiooxidans) which active at acidic environment (pH 1.5-3.5.). BTF process was operated at; pH:1.5-3.5, O 2 /H 2 S:1/2, EBRT:6.3-7.95 minutes, LR:33.35-52.83 g H 2 S/ m -3 h -1 . The H 2 S removal e ciency (RE) varied in the range of %97.84-99.90 and H 2 S elimination capacity (EC) varied in the range of 33.21-52.71 gH 2 S m -3 h -1 . The process e ciency was found to be independent of inlet H 2 S concentration. The average H 2 S values in biogas desulphurized by BTF process ranged between 4-63 ppm. As a result, BTF process regardless of the biogas ow and inlet H 2 S concentration was found to be an effective and e cient process for the removal of H 2 S from biogas produced in the real scale anaerobic sludge digester.

Declarations
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