IJERD – International Journal of Environmental and Rural Development (2021) 12-1

Research article
erd

Effects of Air Injection and Iron Oxide Pellet Addition on
I
Hydrogen Sulfide Removal
and Biogas Production
7de
LYTOUR LOR*n
Biogas Technology and Information Centerand Faculty of Agricultural Engineering,t

Royal University of Agriculture, Phnom Penh, Cambodia
Email: lor_lytour@yahoo.com
4.
7
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
2

MACHITO MIHARABUNTHAN NGO

Royal University of Agriculture, PhnomwPenh, Cambodiaor
LYHOUR HIN
dBiogas Technology and Information Centerand Faculty of Agricultural Engineering,
Royal University of Agriculture, Phnoms Penh, Cambodia

DYNA THENG
Biogas Technology and Information Center and Faculty of Agricultural Engineering,
Royal University of Agriculture, Phnom Penh, Cambodia

CHAN MAKARA MEAN
Biogas Technology and Information Center, Royal University of Agriculture, Phnom Penh,
Cambodia

CHHORTH CHHIENGPUTHEAVY
Faculty of Agricultural Engineering, Royal University of Agriculture, Phnom Penh, Cambodia

BART FREDERIKS
Independent Bioenergy Consultant and Biogas Technology and Information Center,
Royal University of Agriculture, Phnom Penh, Cambodia
Received 22 January 2021

Accepted 30 June 2021(*Corresponding Author)

Abstract Hydrogen sulfide (H2S) in biogas resulting from anaerobic digestion process isunwanted trace compound, because it is toxic and can corrode biogas engine. This study aimed
to compare the 2 desulfurization methods, air injection and iron oxide pellets addition, on H2S
removal and quantity and quality of biogas. The experiment was carried out with two floating
drum digesters (1 m3 each) constructed at Royal University of Agriculture, Phnom Penh, fromJanuary to August 2020. Three levels of air injection at 2%, 4%, and 6% of O2 regarding thedaily biogas production and iron oxide pellets application at 1 kg, 2 kg, and 4 kg per m3 of
biogas were applied to remove H2S in biogas from different raw materials of pig manure andfood waste. The amount of daily biogas production was quantified by gas flow meter, also gas
quality was measured using a GEM5000 gas analyzer. The experimental results indicated thatfood waste had higher daily biogas production comparing to pig manure in both desulfurization
methods. Biogas from food waste increased from 544 L d-1 without iron oxide pellets addition(0 kg) to 657 L d-1 with 4 kg iron oxide pellets addition; and to 566.5 L d-1 with 2% of injected
O2. To the contrary, desulfurization for pig manure with 2% of O2 and 1 kg of iron oxide
showed high daily biogas yield of 348 L d-1 and 340 L d-1, respectively. For raw materials ofⒸ ISERD

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pig manure, in both desulfurization methods, had higher CH4 content than food waste. Air
injection was more effective in H2S removal than iron oxide for both substrates, but higher levelof H2S reduction was observed with pig manure. Accordingly, it was concluded thatdesulfurization methods with air injection and iron oxide pellets addition were effective inbiogas production as well as H2S removal, but a clear trend appeared in the raw material of foodwaste.
Keywords floating drum digester, anaerobic digestion, food waste, pig manure, H2S removal

INTRODUCTION
Biogas is a product generated from the anaerobic digestion of organic substances by appropriate
microorganisms through four metabolic stages; namely hydrolysis, acidogenesis, acetogenesis, andmethanogenesis (Dumont, 2015). It is considered one of the renewable energy sources that can provide
both heat and electricity for use in households, in farms, or in industries. Gas compositions containedin biogas include methane (CH4), carbon dioxide (CO2), and other trace elements such as ammonia(NH3), water vapor (H2O), and hydrogen sulfide (H2S). The majority of them are CH4 (60-70%) andCO2 (30-40%) (Okoro and Sun, 2019), but the only energy source is CH4, which has both benefits anddrawbacks. If released into the atmosphere unused, CH4 is a greenhouse gas which has 28 times more
powerful than CO2, which actually accelerates global warming (National Geographic, 2019). Whenburned or used for internal combustion engine, its harmful effects are reduced and at the same time, its
beneficial energy can be harnessed. Nevertheless, the presence of H2S in biogas may causes problems
in terms of health hazard and economic aspects (Pinate et al., 2017).
H2S is an unwanted gas mixed in biogas because it is toxic to humans at low concentrations andcorrosive to engines. High concentrations of H2S can corrode engines or metal parts, and lead to faster
degradation of engine lubricant oil. A maximum recommended level for generator operation ranges
from 200 to 500 ppm (Rodrighez et al., 2014). If it is used without treatment, oil lubricants must bechanged more often, or the lifespan of a generator is reduced, resulting in high investment andoperating costs. Furthermore, H2S limits for gas heating boilers, combined heat and power (CHP), fuelcells, and national gas upgrade are 1,000, 1,000, 1, and 4 ppm, respectively (Choudhury et al., 2019).
However, different substrates used for biogas production produce different concentrations of H2S.
Substrates that contain high-level protein produce higher H2S content than those contain carbohydrate
and lipid due to the presence of more sulfurous elements in it. Biogas produced from organic waste
may contain H2S in the range of 10-20,000 ppm, but about 10-40 ppm is found in biogas produced
from sewage and 50-300 ppm from landfill (Dumont, 2015). Moreover, Huertas et al. (2020) statedthat H2S produced from organic waste such as pig manure can be as high as 30,000 ppm.
To reduce its concentration to an acceptable level, desulfurization techniques can be applied
physically, biologically, and chemically. Biologically removal is done by air injection into digesterwith regulated amounts of oxygen (O2) between 0.3 and 3% of produced biogas; however, this reaction
may result in sulfur build-ups in the digester space (Hines et al., 2019). Many kinds of chemical
methods have been studied. Pinate et al. (2017) proposed a small batch test for H 2S removal byabsorbent granules soaked in ferric chloride (FeCl3) and in sodium hydroxide solution (NaOH), theremoval efficiency was around 90%. Zulkeflia et al. (2016) stated that H2S removal could be done byactivated carbon soaked in NaOH, potassium hydroxide (KOH), and potassium carbonate (K2CO3), but
added that the last chemical element was the most effective. Biological H2S removal methods are also
increasing popular and considered highly effective. Those biological methods include biofilters,
biotrickling filters, bioscrubbers, and activated sludge (Barbusiński and Kalemba, 2016). However,
H2S removal techniques are limited, or have not been well documented in Cambodia, which makes it
difficult for promotion of biogas use inside the country. Therefore, proposing an applicable
desulfurizing technique is highly valuable for economic reasons.
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Pig manure is a major waste from animal productions in Cambodia. According to MAFF (2020),
the number of pigs was around 2.18 million heads in 2019 of which more than half raised incommercial farms. On the other hand, kitchen waste amounted from 63.30% to 80.46% of all solid
wastes from households (Sour, 2017). Proper management of these two wastes by converting them tobiogas will provide both environmental and economic benefits.
OBJECTIVE
The objectives of this study were (1) to compare the effects of air injection and iron oxide pellets on
H2S removal and quantity and quality of biogas produced from two different substrates, food waste andpig manure; and (2) to identify relations between biogas production with CH4 content in the two
substrates.
METHODOLOGYThis research was conducted at the pilot biodigesters belonging to the Biogas Technology andInformation Center (BTIC), at the Royal University of Agriculture, Cambodia, starting from January toAugust 2020. Two floating drum biodigesters (1 m3 each) were used for this experiment. Two different
substrates, food waste and pig manure, were daily fed into the systems. At its full production, biogas
was then treated by two desulfurizing methods: air injection and iron oxide pellets. Air injection wasapplied directly into the gas holders (the floating drum) on top of the digesters at O2 levels of 2%, 4%,
and 6% of daily produced biogas. Meanwhile, iron oxide pellets were placed in a gas treatment
container through which raw biogas was passed through and treated before being quantified. The levelsof iron oxide pellets were applied at 1, 2, and 4 kg m-3 of daily produced biogas. Effectiveness of thedesulfurizing methods was also determined by comparing the results with raw untreated biogas.
Materials
Two floating drum biodigesters used in this experiment had concrete bases and walls constructed under
the ground. The whole volume was 1 m3, one third of which was for gas storage. The gas holders werehigh-grade polyethylene plastic water tanks that were cut in half and used as covers for the digesters.
They can move up and down, depending on the biogas pressure produced inside.

Fig. 1 Technical drawing of a floating drum biodigester used in this experiment
Substrates used in this experiment were food waste and pig manure. Their physical and chemical
properties were analyzed in advance. Still, there might be some variations due to everyday freshcollection of the substrates. To reduce errors, substrate collection was done at the same placethroughout the experimental period.
Food waste was daily collected from the university canteen. It contained 21.9% dry matter, 1.4%
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ash, 5.6% protein, 3.2% lipid, and a C/N ratio of 12.6:1. For high efficiency, bones and other organic
materials that were slowly biodegradable were removed. Before being fed into the digester, food waste
was thoroughly mixed with water at a ratio of 1:1, or 5 kg each. Pig manure used in this experiment
was collected from two small-scale pig farms nearby the university. It contained 25% dry matter, 8.1%
ash, 10.4% protein, 0.5% lipid, and a C/N ratio of 6:1. It was freshly collected, mixed with water, andfed into the system on daily basis at a ratio of 1:1, or 5 kg each.
Sampling Methods
Feeding time was done in the morning and repeated regularly every day for 30 days, which was
assumed as a minimum hydraulic retention time (HRT) for the digesters. Then, volume of untreatedbiogas was measured for 7 days to determine average daily biogas production, followed by air injection,
which lasted 7 days for each O2 level. In total, the whole process of air injection lasted one month.
After this process was completed, the untreated biogas was measured another 7 days before applicationof iron oxide pellets in filter for H2S removal. Experiments with each level of iron oxide lasted 7 days,
which took 1 month in total. The iron oxide pellets used in the experiment were proprietary ones which
had been imported from China.
To measure the quality of biogas produced from the two substrates, a GEM5000 gas analyser was
utilized. It was a product supplied by Geotech, UK, and was capable of measuring biogas quality suchas CH4, CO2, and O2, all in percentage; and H2S in ppm with the maximum concentration of 5,000 ppm.
Biogas quality was measured three times a day for each treatment after it was treated with air injection
a day before, or went through the desulfurizing container on the day of the measurement.
To quantify daily biogas production, a gas flow meter was used. When the biogas was used forboiling water, the flow meter started to record the consumption at the average outdoor temperature of
28-33 ℃.
Data Analysis
Analysis of Variance (ANOVA) were used for the data analysis, and when significant differences wereobserved, a post-hoc LSD-test was then used to show significant differences among the treatments at α
= 5%. Besides that, the data for air injection and iron oxide pellets were presented separately because
the experiments were done separately.
RESULTS AND DISCUSSION
Daily Biogas Production
Daily biogas production was compared among the two substrates, when treated with air injection at
different levels of O2 (Fig. 2a). Significant differences were detected among the treatments, and it wasobserved that biogas produced from food waste was higher than from pig manure. Daily biogas
quantity was similar for both substrates, when measured without application of air injection. It was
about 484 L d-1. For food waste, daily biogas production increased slightly with 2% of injected O2 toalmost 500 L d-1, but decreased continuously at O2 levels of 4% and 6%. Similar results were obtainedfor pig manure. Daily biogas quantity decreased more and more, as levels of air injection increased.
Daily biogas production from pig manure was 384, 260, 227 L d-1 at injected O2 levels of 2%, 4%, and
6%, respectively. This finding indicates that putting more air into the digester may affect biogas
production because its ideal condition is anaerobic. Hines et al. (2019) proposed a maximum air
injection level of 3%.

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Fig. 2 Comparison of biogas production with air injection (a) and with iron oxide pellet (b)Daily produced biogas quantity was compared among the two substrates when using iron oxidepellets (Fig. 2b). It was not affected by different levels of pellet, but by different substrates. Higher
daily biogas production was observed with food waste at all levels of applied iron oxide pellets. Its
average value was 585 L d-1. In contrast, daily biogas production from pig manure was quite low,
averaging 320 L d-1. Moreover, it was surprising to see the lowest daily biogas production with 4 kg ofiron oxide pellets. The reason why biogas quantity was higher for food waste used as a substratebecause it contained more lipid. Lipid has more carbon atoms than protein, thus producing more biogas(Morales-Polo et al., 2018).
Methane Content
CH4 content contained in biogas was compared among the two substrates with air injection at different
levels (Fig. 3a). It was found that there were significant differences among the treatments and CH4content was higher for pig manure, when compared with food waste. CH4 fluctuated for pig manure atdifferent levels of inject O2; however, the average value ranged from 57 to 61%. Meanwhile, CH4content for food waste increased, as the levels of O2 increased. The highest value was only around 56%.
Similar results were found for food waste when iron oxide pellets were applied at different levels (Fig.
3b). CH4 increased, as the amounts of pellets increased for both food waste and pig manure. Still, theaverage value was 60% for pig waste and 56% for food waste.

46Fig. 3 Comparison of CH4 content with air injection (a) and with iron oxide pellet (b)Hydrogen Sulfide Content
H2S cotent was compared between the two substrates under different levels of injected O2 (Fig. 4a).
Significant differences in H2S was observed among the treatment, and the higher level of O2 wasinjected, the more H2S was reduced. Greater H2S reduction was observed with the use of pig manure.
Without air injection, the level of H2S was 840 ppm and 1,000 ppm for pig manure and food waste,
respectively. this level decreased sharply for pig manure, as levels of injected O2 increased. H2SⒸ ISERD

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content was 522 ppm at O2 level of 2%, and decreased to 351 ppm at 4% and then to 271 ppm at 6%.
For food waste, H2S content decreased from 923 ppm at O2 level of 2% to 626 ppm at 4%, and thenslighly increased to 674 ppm at 6%. Higher levels of injected O2 reduced H2S, but also affected overal
biogas production (Fig. 2a). Thus, it should be applied at recommended levels only. Nevertheless,different levels of pellets did not affect H2S content, but different substrates did (Fig. 4b). It wasobserved that H2S content for pig manure was higher than for food waste, and varied greatly fromabout 800 to over 3,000 ppm, though treated with high levels of iron oxide pellets. Such unsual
increase was due to using pig manure from a different, as the first one emptied the barns. However, atendency for H2S removal was observed at 4 kg of pellets, as H2S decreased to above 2,500 ppm. For
food waste, iron oxide did not affect H2S content, and its average ranged from 1,000 to 1,100 ppm.

Fig. 4 Comparison of H2S reduction by air injection (a) and by iron oxide pellets (b)CONCLUSION
Pig manure and food waste were fed into two floating drum digesters for biogas production andinspection of biogas quality, along with H2S removal techniques by air injection and ferrous oxidepellets. It was observed that food waste produced more biogas than pig manure, but less CH4 content.
H2S contents were similar for both substrates, and the use of air injection tends to be more effective
than ferrous oxide in this study.
ACKNOWLEDGEMENTS
The study was made possible thanks to the project “Reduction of Greenhouse Gas Emission throughPromotion of Commercial Biogas Plant in Cambodia” implemented by United Nations Industrial
Development Organization (UNIDO) that not only provided funding, but also continuously assisted instrengthening the research team.
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