Implementation of a Small-Scale Anaerobic Digester
Valentine Geze, Brooke McDonald, and Anna Zachary
Sponsor: Plant Chicago
Faculty Advisor: Dr. Gajan Sivandran
Abstract
Testing
The team, comprised of three
Senior Environmental
Engineering students from Loyola
University Chicago, is working
with the non-profit organization
Plant Chicago to design an
anaerobic digester (AD) system
that focuses on circular economy
and waste diversion. Anaerobic
digestion is a microbial process
that is used to convert agricultural
and organic waste into liquid
digestate and biogas. Liquid
digestate can be a useful fertilizer,
and biogas can be used in both
Figure 1: Plant Chicago firehouse.
Heat and electricity applications.
In order to create a system that is efficient and useful for Plant Chicago’s
operations, the team executed a series of experiments to study anaerobic
digestion on a small scale as well as explore different design options. The
experimental systems were given inoculum from a local wastewater treatment
plant and studied in terms of the microbial colony health, biogas yield, and
digestate yield. Controls such as temperature and continuous mixing will help
inform the best environment for the system the team will recommend to Plant
Chicago. Using all this data and research, the team will be able to determine an
AD design for Plant Chicago that will best suit their operations and bolster their
waste reduction efforts.
In order to gather data for the construction of the small-scale anerobic digester
at Plant Chicago, the team conducted two rounds of experiments to further test
the biogas yield and wellness of the microbe colony.
Experiment Type 1:
First, the team set up a tabletop AD named
Ronan. The digester was set on a stir plate and
was temperature controlled using a heat wrap
and temperature sensor, keeping it between
35-37ºC. Ronan was filled with 1.5 L of
inoculum and fed approximately 174 ml of
blended food waste. A gas bag was attached to
the top of the AD, and data collection ensued
to study the yield of biogas. Overnight, Ronan
began producing gas. This is indicative of a
healthy microbe colony and supports the team’s
plans to design an AD of larger size that will
successfully produce biogas for Plant Chicago.
The findings from Experiment Type 1 provided
enough positive results to lead the team to
having confidence in the final design solution
Figure 4: Lab AD reactor.
Experiment Type 2:
The team used an acrylic plug-flow reactor tank to test for hydraulic retention
time that could be scaled to the size of the final design. Plug-flow reactors are
used to increase hydraulic retention time, which subsequently increases the
time organic waste would spend in the AD, which would ensure complete
digestion. This testing aided the team in determining maximum feeding rate for
the solution design for the ideal hydraulic retention time of 30 days. To carry out
this experiment, the tank was filled completely with water dyed green, then was
inputted with a constant flow rate of fresh water on the input end, and solution
was pumped from the output at the same rate as the input. An ultraviolet-visible
light spectrometer was used to analyze the hue of the water as if moved through
the system, indicating the diffusion throughout the tank. The results of this test
determined that the digester design will allow for a 30-day hydraulic retention
time and produce viable digestate since the AD will fully digest organic waste.
The curve of absorbance values with time is seen in Figure 5.
Solution
The final design seen in Figure 2 is a physical representation of the unit
processes shown in the System Description section. The physical representation
consists of PVC piping, PVC connectors, an IBC tank, a funnel, a macerator, an
activated carbon filter, an aerator, and multiple storage containers. The DIY
solution is accessible to anyone who wishes to build it. After Plant Chicago
constructs the final design solution, they can use their experience to teach
others to do the same, enriching the education aspect of the design. The DIY
solution follows regulations and project constraints.
Results
The results of the two types of experiments mentioned are as follows.
For Experiment Type 1:
• An understanding of the types of organic waste the reactor can use
successfully
• An approximation of how often a reactor of that size should be fed, which
can be scaled up for the full-sized solution design
• 1.188 L of gas was produced by 174 ml of organic waste which can be
scaled and approximated for larger reactors. The 174 ml of organic waste is
the same ratio of organic waste expected from
Plant Chicago in a month
• The scaled gas production for the
design solution in Figure 2 is 218.06 gal
• The smell of the biogas is unpleasant and
mitigation techniques must be tested
• Assuredness that methane can be
produced from our system through the
successful burning of biogas, seen in Figure 6
For Experiment Type 2:
• The hydraulic retention time of the experiment
was 37.24 mins which was determined using
the MATLAB code seen below:
Q = 300/20 %mL/s input and output
Volume = (13.3 * 28 * 90) %cm3 = mL,
volume of reactor
HRT1 = (Volume/Q)/60 %(cm3/(ml/s)/60)
= min
Result: HRT1 = 37.2400
Figure 6: Flame produced
by burning biogas that
reactor Ronan, Figure 4,
created.
• Hydraulic retention time for solution design will be 275 days, when scaled
appropriately, as seen from the MATLAB code below:
Q = 1; %gal/day
Volume = 275; %gal, volume of IBC Tank
HRT = Volume/Q %gal/(gal/day) = day
Result: HRT = 275
• This result means that Plant Chicago could increase their waste
input to 9 gallons per day and still be within the ideal range for
hydraulic retention time of the system
• Determined the amount of diffusion that can be expected in a reactor with
the calculated hydraulic retention time
• Gave the team an idea of how much diffusion can be expected
within the solution design due to feeding rate
Conclusion
Figure 2: Plant Chicago AD solution design.
Figure 5: Time versus absorbance graph from plug-flow hydraulic retention time
experiment, with the blue points being data and the red line being the line of
best fit.
System Description
In conclusion, the team finds that the suggested design for Plant Chicago will
be the most efficient system to comply with their mission and operations. Its
size will allow for organic waste to be redirected from a landfill while avoiding
wasteful amounts of output. Through the selection of off-the-shelf parts that
are easily accessible and of high quality, the anaerobic digester system will be
reliable and easy to repair if the situation should arise. Plant Chicago will be
able to dispose of any agricultural and organic waste in this system and able to
put the resulting digestate and biogas towards their operations. The liquid
digestate, which is a nutrient-rich slurry, can be used as a fertilizer for
agriculture growing on site at the firehouse. The biogas can be converted into
electricity or heat, and this application can fluctuate through time as the needs
of Plant Chicago may vary seasonally and year to year.
Since the system is designed and built independently by the Loyola team and
Plant Chicago, there are vast opportunities for using the digester as an
educational tool. Education is a cornerstone of Plant Chicago’s mission, and
this design will make it easy for others to learn about how anaerobic digestion
works and the potential it can carry in the realm of sustainability and clean
energy alternatives.
Figure 3: AD System diagram.
Shown in Figure 3, above, are the various components of the solution to the problem proposed in the Abstract section. Organic waste enters the pretreatment step,
maceration. After the organic waste is macerated, it enters the anaerobic digester where anaerobic digestion occurs. Sludge produced from the anaerobic digestion
processed is periodically drained out of the tank. Excess effluent is recycled through the system to maximize microbe efficiency. Effluent leaves the digester as
digestate, is tested by the sensor network, and enters the post treatment step, where it is aerated, nitrified, and diluted. After dilution, the digestate is tested again
and then stored. Gas leaves the digester, is treated using a carbon filter, and is stored where it is ready for use in a gas grill. Excess gas is flared.
Acknowledgements
• Loyola University
Chicago Engineering
Department
• Jay N. Meegoda
•
•
•
•
Michael Fagbohungbe • Jan Allen
Greg Palmer
• Richard Freudenberger
Zach Waickman
• Sherman Galla
Michael Smith