A Project Report
On
Hybrid reactor design for enhanced food waste Biomethanation
using Granular Activated Carbon
BY
Sahil Varman
2019B1A31017H
Aman Raj Singh
2019B1A31483H
Under the supervision of
P. Sankar Ganesh, Ph.D.
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF
BIO F-217
BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE PILANI (RAJASTHAN)
HYDERABAD CAMPUS
(MAY 2023)
ACKNOWLEDGMENT
Firstly, I would like to thank my professor for guiding me throughout
this process and providing valuable feedback that helped me improve
my work.
I want to use this chance to express my sincere gratitude to Dr. P. Sankar
Ganesh for his support, invaluable guidance, and illuminating views on
issues throughout the course of the project.
I would also like to thank Sir for his support, comments, and
constructive suggestions during the project work.
Lastly, I am also grateful to my institute BITS Pilani, for giving me this
chance to gain knowledge about such an interesting topic and apply it
practically.
Thank you all for your valuable contributions.
Birla Institute of Technology and Science-Pilani,
Hyderabad Campus
Certificate
This is to certify that the project report entitled “Hybrid reactor design
for enhanced food waste Biomethanation using Granular Activated
Carbon” submitted by Mr Sahil Varman (ID No. 2019B1A31017H) & Mr.
Aman Raj Singh (ID No. 2019B1A31483H) in fulfillment of the
requirements of the course BIO F217, embodies the work done by him
under my supervision and guidance.
Date: 3rd May 2023
(P. Sankar Ganesh)
BITS-
Campus
Pilani,
Hyderabad
ABSTRACT
The biomethanation of organic-rich food wastes is a promising approach for
producing biogas and reducing waste volumes. Characteristics of food wastes such
as high moisture content and readily degradable organic components make them an
efficient substrate for biomethanation. However, the accumulation of total soluble
products (TSP) during the process can inhibit microbial activity and often lead to
decreasing pH, ultimately reducing biogas yield. Granular Activated Carbon
(GAC) favors the forward reaction of VFAs conversion to methane due to its large
surface area, providing a porous environment for colonizing sensitive
methanogens. Due to their high electrical conductivity, GAC accelerates direct
interspecies electron transfer (DIET) between acetogens and methanogens and
improves pH buffering. Experiments were conducted in borosilicate glass reactors
of total and working volumes of 135 and 105 mL to comprehend the
biomethanation process of food waste and monitored for 30 days. A hybrid reactor
is designed to maintain the integrity and achieve better results by inserting a
perforated plastic tube containing GAC into the substrate. The perforated tube
arrangement prevents the complete mixing of GAC with food waste and aids
in efficiently removing GAC after the adsorption process. The results indicated
high methane generation, which proves GAC reduces acidification by utilizing
VFAs during the methanogenic phase. The addition of GAC during the treatment
of easily hydrolysable food waste prevents inhibition of methanogens leading to
enhanced process stability and providing optimal pH for high bio-methane
production. Subsequently, a biorefinery can be developed from the TSPs extracted
during the process, which contributes to developing a circular economy and a
sustainable environment.
CONTENTS
Title page………………………………………………………………….
Acknowledgments……………………………………………………….
Certificate……….…………………………………………………….......
Abstract……………………………………………………………….......
1. Introduction….………………………………………….………….......
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Anaerobic Digestion of Food Waste
Four Phases of Anaerobic Digestion
Intermediate Products
Carbon-Based Conductive Material
GAC Provides Resistance to Unfavourable Conditions:
Effect of VFA Concentration on Biogas Production
2. Materials and Methods………………………………………………….
3. Steps…………………………………………………………………….
4. Results…………………………………………………………………..
5. References……………………………………………………………...
INTRODUCTION
Annual production of food waste increases along with population growth and
urbanization. Improper disposal of food waste has had a significant negative
impact on the environment and the global economy. With increasing population
growth and urbanization, the amount of food waste produced annually has risen,
resulting in significant problems related to its handling. This has led to wastage of
resources and environmental hazards. Therefore, it is important to manage food
waste effectively to protect the environment and promote profitability. Landfilling
is a low-cost and simple method for managing food waste, but it has negative
impacts such as greenhouse gas emissions, which contribute to global warming and
climate change. Landfills also cause ozone depletion and pollution. Incineration is
not suitable for food waste with high moisture content, and it can cause air
pollution and negative health effects. Anaerobic digestion is a cost-effective
technology for producing methane-rich biogas from food waste, which can be
beneficial. However, the accumulation of volatile fatty acids during the process can
inhibit anaerobic digestion, but they can also be used for the synthesis of valuable
products. Biomethanation is an established method for food waste treatment since
food waste contains macromolecular organic matter which is suitable for anaerobic
microbial growth. Anaerobic digestion is the process in which organic waste is
biologically degraded into methane-rich biogas. Some intermediates of anaerobic
digestion that inhibit the product should be controlled and recovered if
economically viable.
.
Anaerobic Digestion of Food Waste
Anaerobic digestion is a sequential biochemical process:
Complex organic components such as polysaccharides, proteins, and lipids are
hydrolyzed or broken down into intermediate products Intermediates are later
oxido-reduced into methane and carbon dioxide. Anaerobic digestion requires less
energy and produces less sludge compared to aerobic treatment.
The anaerobic digestion of organic-rich food wastes is a promising approach for
producing biogas and reducing waste volumes. Characteristics of food wastes such
as high moisture content and readily degradable organic components make them an
efficient substrate for anaerobic digestion. However, the accumulation of total
soluble products (TSP) during the process can inhibit microbial activity,
accumulation often leads to decreasing pH ultimately resulting in reduced biogas
yield. Granular Activated Carbon (GAC) favors the forward reaction of VFAs
conversion to methane due to its large surface area providing a porous environment
for the colonization of sensitive methanogens
Syntrophic relationships between different microorganisms play a crucial role in
anaerobic digestion
Intermediate Products:
Total soluble products (TSP) are produced during the anaerobic digestion of food
waste. TSP includes acids, alcohols, and solvents.
Volatile fatty acids (VFA) are one of the key TSP that is used for the production of
high-value-added products. The accumulation of VFAs in reactors reflects a kinetic
imbalance between production and consumption rates, causing acidification.
Carbon-Based Conductive Material:
The conductive materials can also serve as conduits for electron conveying, with
carbon materials, and provides habitats for microorganisms.
It also offers additional buffer capacity.
Carbon materials commonly used for bio methanation enhancements such as:
Biochar/hydrochar, granular activated carbon (GAC), carbon felt, carbon fibers,
graphene, graphene-oxide, and carbon nanotube.
The large surface area and honeycomb porous structure of GAC enhance the VFA
consumption by methanogens.
GAC improves the syntropic relationship between microbial consortia and serves
as electron acceptor and donor during bio methanation process due to their:
● High electrical conductivity
● Specific surface chemistry
There is currently a lot of interest in using carbon-based conductive materials to
promote the consumption of volatile fatty acids (VFA) during anaerobic digestion.
These materials are thought to facilitate the interaction between micro-organisms,
rather than allowing the accumulation of inhibitors or biofilm formation on their
surfaces.
Activated carbon, in both granular and powdered forms, has been extensively
developed to improve reactor performance by enhancing the rate at which VFA is
utilized, resulting in faster methane production. Activated carbon is beneficial
because of its high surface area and porous structure, as well as its high electrical
conductivity, which allows it to act as an electron acceptor for anaerobic
respiration. This, in turn, promotes direct interspecies electron transfer (DIET), a
faster and more efficient mechanism for transferring electrons compared to
microbial interspecies electron transfer (MIET). Using activated carbon materials,
therefore, supports DIET and prevents the accumulation of VFA during the
anaerobic digestion process.
GAC Provides Resistance to Unfavourable
Conditions:
Anaerobic reactors supplemented with GAC show:
● Resistance to high organic loading conditions than conventional reactors
● A shortened lag period of anaerobic digestion even under conditions of high
hydrogen partial pressure
● Resistance to low pH for attached cells on biomass support media
● Enhancement in methane yield
● Enhancing system stability
.
Materials and Methods:
Figure 5- Porous tube filled with GAC in the anaerobic
lab-scale reactor experimental setup.
Fig: 4- Anaerobic lab-scale reactor experimental setup.
Food waste substrate (S) was collected from the BITS Pilani Hyderabad campus
hostel mess. The inoculum (I) was collected from a full-scale anaerobic plant,
treating food waste, and fed in a 1:1.6~2 (I:S) ratio.
To improve the efficiency of anaerobic digestion of food waste 0.05g/TSS of GAC
was added. Experimental Setup and operating condition
130ml serum bottles were operated in mesophilic conditions (35+ 2 °C).
Figure 6- Porous tube filled with GAC in the anaerobic lab-scale reactor experimental setup.
STEPS:
1. We have collected Food waste from the Mess/Food Hall in our colony and have
ground the food to obtain a thick liquid.
2. We then added the appropriate amount of (Inoculum ~20% + Food Waste~80%
=100%).
3. We then poured the mixture into serum bottles and then sealed them with a
rubber plug and a metal cap to make sure it was airtight and thus, aid in carrying
out the anaerobic process. A Syringe was then inserted into the exposed rubber
plug to ensure that the gas does not explode the bottle when uncapped and allows
us to remove air from the bottle on a daily basis (Since it is an anaerobic process
gas/methane production is very common).
4. For the creation of a Hybrid reactor we made use of a makeshift Syringe
cylinder was sealed with M-seal and around 6*4 holes were made in the
mechanical workshop of our college in a +Sign with around 3mm diameter to
allow the GAC to interact with the mixture in the serum bottles. The GAC was also
segregated by using an electronic shaker and sieves of different sizes (>3mm) to
have uniformity. We tied a string around the syringe to allow us to remove it from
the serum bottle once they were sacrificed and also plugged the hole by using
Mseal (sealant).
Fig: 7- Sanding syringes with holes made in the mechanical workshop to close mouth with M-seal. A string is
attached on the top for removal and insertion easily into the serum bottles.
Fig: 8- Anaerobic lab-scale reactor experimental setup with GAC.
5. We then take the PH readings and sacrifice the bottles one at a time( daily; we
have 29 bottles without GAC and 15 with GAC) to analyze the contents and the
relative changes that have occurred. Other parameters such as BOD, COD, TS, VS,
TKN, TP, etc are also measured.
6. This data will be interpreted further and help us assess the effects of GAC in
anaerobic digestion.
Results
TS & VS
The proximate analysis focuses on determining the ratio of a substance that
undergoes combustion as a gas (referred to as volatile solids, VS), as a solid (fixed
carbon), and the portion that remains as inorganic residue (ash). This analysis is
useful in calculating the potential for biogas energy from biomass. VS represents
the amount of organic material available for microbes to use as an energy source in
biological treatment. Table-1 reports the analysis of total solids (TS) and VS
content of food waste in an intermittently fed reactor over a period of 30 days.
Food waste is known to have a high VS content ranging from 78% to 97%, which
makes it particularly suitable for biological treatment compared to other organic
waste.
BOD & COD:
The rate at which microbes utilize a substrate is significantly influenced by the
results of an analysis that includes pH, chemical oxygen demand (COD), and
biological oxygen demand (BOD). The pH is a measure of the acidity or alkalinity
of the substrate, and for anaerobic digestion, it is a highly influential parameter due
to the varying levels of sensitivity of different groups of organisms towards pH. In
this reactor, the pH range of the substrate is between 6.8 to 4.4, with an initial pH
of 6.8 that gradually decreases over the next few days. Figure 8 illustrates the
variation in pH value over the course of 30 days, which is caused by the formation
of acids during the acetogenesis and acidogenesis process. Acidogens have a pH
tolerance range of 4.0 to 8.5, while most methanogens require a narrow window of
pH values between 6.5 to 7.2. Beyond this range, methanogens fail to function, and
they are the main group of microorganisms responsible for producing the desired
methane-rich biogas product.
COD is a significant parameter in anaerobic digestion that measures the quantity of
organic matter present and available for oxidation. It determines the quantity of
oxygen required for the chemical oxidation of organic matter, utilizing a potent
oxidant such as potassium dichromate, through a reflux process. On the other hand,
BOD determines the amount of oxygen required by microorganisms to break down
organic materials using a modified Winkler's method. These tests are commonly
performed to assess treatment plant efficiency. In the anaerobic digestion of food
waste, the COD ranges from 9,000 mg/l to 88,000 mg/l as depicted in Figure 8.
The COD reduction percentages for the first to fourth cycle are 80.9% (day 0- day
9), 66.19% (day 9 – day 18), 70.5% (day 18 – day 21), and 85.15% (day 24 – day
30) with the addition of GAC in the fourth cycle. Similarly, the BOD ranges from
9600 mg/l to 38400 mg/l, as shown in Figure 4, and the BOD reduction
percentages for the first to fourth cycle are 65.6% (day 0- day 9), 46.67% (day 9 –
day 18), 64.28% (day 18 – day 21), and 73.3% (day 24 – day 30) with the addition
of GAC in the fourth cycle.
In the early phases of anaerobic digestion, complex organic matter is decomposed by
microorganisms into simpler substances, including sugars, amino acids, and fatty acids.
Subsequently, these substances are transformed into volatile fatty acids, which serve as
the primary substrate for methane production. As the digestion progresses, the volatile
fatty acids are further degraded into methane and carbon dioxide.
However, if the conditions in the anaerobic digester are not optimal, the microorganisms
may not be able to completely degrade the volatile fatty acids. This can result in the
accumulation of intermediate compounds such as alcohols, aldehydes, and organic acids,
which have a higher COD than the initial substrate. Therefore, the COD of the digester
effluent increases as the digestion progresses
The increase in Biological Oxygen Demand (BOD) of food waste during anaerobic
digestion is primarily due to the accumulation of intermediate compounds and the
incomplete degradation of organic matter. The increase in BOD of food waste during
anaerobic digestion is a complex process that can be influenced by various factors such as
substrate composition, hydraulic retention time, temperature, pH, and microbial
community composition.
Figure 9: COD Analysis without GAC
Figure 10: COD Analysis with GAC
Figure 11: COD Analysis side by side Comparison of without GAC vs with GAC.
Figure 12: COD Analysis Reduction values side by side.
When GAC is added to a sample for COD analysis, it can adsorb some of the organic compounds that
contribute to COD. This can result in lower COD values than the actual COD of the sample. Therefore,
the presence of GAC can lead to an underestimation of the COD in the sample.
Figure 13: BOD Analysis without GAC
Figure 14: BOD Analysis with GAC.
Follows a similar trend as without GAC but reaches lower BOD values slightly faster than without GAC.
Figure 15: BOD Analysis side by side Comparison of without GAC vs with GAC.
Figure 16: BOD Analysis Reduction values side by side.
BIOGAS Production
The amount of biogas produced daily was measured using the liquid displacement
method. The daily biogas production ranged from 0.2 m3/Kg VS to 0.03 m3/Kg
VS. At first, there was an increase in biogas production, up to 0.292 m3/Kg VS,
but as the pH decreased, biogas production gradually decreased to 0.03 m3/Kg VS.
The acidification hindered the biogas production, and the daily biogas readings for
30 days are shown in Figure 14. To maintain the pH constraint of food waste in a
single-phase reactor, granular activated carbon was added as a buffering agent,
which helped in DIET and increased biogas production. After the addition of GAC
on day 23, biogas production increased, as shown in Figure 14, and no prominent
decline in biogas reading was observed.
Figure 17: Biogas production without GAC
Figure 18: Biogas production with GAC
Figure 19: Biogas production without GAC vs with GAC
pH:
pH is a way to determine how acidic or alkaline a solution is by measuring the
concentration of hydrogen ions (H+). It is represented on a scale of 0 to 14, where
7 is considered neutral. Values < 7 =>Acidic (lower numbers indicate more H+
ions hence stronger acidity) while values> 7 => alkalinity (higher numbers indicate
more OH- ions hence stronger alkalinity). The pH scale is logarithmic, meaning
each unit represents a tenfold difference in acidity or alkalinity. pH plays a
significant role in the behavior and properties of substances, chemical reactions,
enzyme activity, and the overall equilibrium of ecosystems. Maintaining
appropriate pH levels is essential for the proper functioning of many biological
processes.
Fig: 20- pH variation for 30 days without GAC addition
Figure: 21- pH variation with GAC addition
Figure: 22- pH variation without GAC addition vs with GAC addition
The addition of GAC to food waste undergoing anaerobic digestion can help to stabilize the pH
of the system, which can improve the efficiency of the process. Food waste that is not treated
with GAC may be more susceptible to pH fluctuations, which can negatively impact the
efficiency of the anaerobic digestion process.
Conclusion
Food waste is a major global issue, with over 1.3 billion tons of food wasted
annually. However, it can be a valuable substrate for anaerobic digestion due to its
easily biodegradable organic content and potential to contribute to sustainability.
The anaerobic digestion of food waste can generate significant economic energy
yield, but there are challenges to overcome, including the VFA inhibition threshold
and acidification due to the accumulation of VFAs. The addition of GAC is an
effective and economical method to enhance the rate of VFA consumption by
methanogens, which are sensitive to changes in pH. This approach enriches
syntropic bacteria, which degrade the organic products into methane, resulting in
less pH variation, minimal lag phase, and increased biogas productivity.
Furthermore, the COD and BOD reduction percentages are higher compared to
conventional anaerobic food waste digestion. Overall, the addition of GAC
accelerates the methanogenesis process and improves the performance of the
anaerobic reactor by reducing acidification caused by intermediate products during
the process.
References:
● Anaerobic digestion
● Anaerobic digestion of food waste – Challenges and opportunities
● Comparative effects of GAC addition on methane productivity and microbial
community in mesophilic and thermophilic anaerobic digestion of food waste
● Enhancing the Performance and Stability of the Co-anaerobic Digestion of
Municipal Sludge and Food Waste by Granular Activated Carbon Dosing