2. 1
Project Title: __ Design of an Energy Producing Waste Treatment System Utilizing Anaerobic Co-
Digestion of Organic Wastes Coupled with Algae Cultivation _
Abstract: The goal of this project was to design a full-scaled system capable of treating organic wastes
that would result in a net energy production, while retaining valuable nutrients that could be utilized as
an environmentally-friendly fertilizer. Specifically, this project, performed by a University of Arkansas
(UA) senior Biological Engineering team, designed a wastewater treatment/energy production system
that coupled anaerobic digestion to algae cultivation technology. A system was designed to treat all
biological wastes produced by the UA Swine Finishing and Poultry Units, located in Washington County,
Arkansas. The feedstock characteristics were improved by supplementing the agricultural wastes with
carbonaceous waste materials generated by UA facilities and by coupling the digestion system with two
periphytic algae cultivators (PACs).
A prototype was constructed in order to test the developed concept and generate data critical for a full-
scale design. The prototype consisted of a 1000 L anaerobic digester and two 9” by 10’ PACs. The
digester was maintained at 37°C, in the mesophilic regime, using a thermostat controlling a heat
exchanger. The pH of the digestate was also monitored using a pH electrode interfaced to LabVIEW, to
maintain the reactor in a pH range from 6 to 8. Data on methane production rate, specific methane
yield, and volatile solids conversion was collected, and used to calculate the size and operation of a full-
scale system. The specific methane production for the prototype was 0.477 m3
CH4 kg-1
VScon.
The full-scale system was designed to uphold the objectives and constraints specified by the client.
Economics, functionality, and safety were important parameters considered in order to optimize the
effectiveness of the designed system.
Acknowledgements:
Dr. Thomas Costello – Faculty Mentor
Dr. Julie Carrier – Faculty Mentor
Dr. Jun Zhu – Faculty Consultant
Jerry Jackson – Staff Technician and Contractor for Prototype Room
Lee Schrader – Staff Technician
Julian Abram – Staff Technician and Electrician for Prototype Room
Charles Maxwell – Client, Swine Finishing Unit Supervisor, Provided Information and Materials
David McCreery - Broiler Unit Manager, Provided Information and Materials

3. 2
Table of Contents
1. Introduction……………………………………….……………………………………….……………………………………… 3
1.1 Background Information……………………………………….……………………………… 3
1.2 Problem Statement……………………………………….……………………………………… 5
1.3 Goal……………………………………….……………………………………………………………… 5
2. Preliminary Design……………………………………….……………………………………………………………………. 5
2.1 Design Objectives……………………………………….………………………………………… 5
2.2 Design Constraints……………………………………….………………………………………. 5
2.3 Approach……………………………………….…………………………………………………….. 6
3. Design, Fabrication, and Testing of a Prototype………………………………..………………………………. 6
3.1 Prototype Design……………………………………………………..…………………………… 6
3.2 Safety……………………………………….…………………………………………………………… 8
3.3 Prototype Testing and Results………………………………….………………………….. 9
4. Full-Scale System Design………………………………………………….………………………………………………… 11
4.1 Digester Sizing…………………………………………………….…….…………………………… 11
4.2 Algae Cultivators……………………………………………………………………………………. 12
4.3 Storage………………………………………………………………………………………………….. 12
4.4 Gas Utilization……………………………………………………………………………………….. 12
4.5 Economic Analysis……………………………………….……………………………………….. 13
5. Summary of Proposed System……………………………………….…………………………………………………. 14
6. Conclusion……………………………………….……………………………………….…………..………………………….. 15
References……………………………………….……………………………………….………………………………………. 16
Appendix……………………………………….……………………………………….…………………………………………. 17

4. 3
1. Introduction
1.1 Background Information
1.1.1 Current Agricultural Waste Treatment Practices
As the projected population increases to 9 billion by 2050, the production and allocation of resources to
produce food and energy will become increasingly challenging. Thus, the development and use of high-
density farming units for both plant and animal based food production will become more prevalent in
order to sustain increased food production. Existing modern agro-industrial complexes produce large
amounts of biologically reactive wastes. Instead of being viewed as a nuisance for the farmers,
biological wastes can be considered as a resource to be used for the production of energy and fertilizer.
For cattle and swine waste streams, manure is typically stored/treated in either a holding pond or a
lagoon prior to land application, which can lead to the volatilization of carbon into carbon dioxide (CO2)
and methane (CH4), contributing to greenhouse gas emissions. In the poultry industry, some bedding
and manure are used to compost bird carcasses, but all of the wastes are ultimately applied to the land.
If proper best management practices (BMP’s) are not followed, over application to the land and
insufficient treatment of the waste prior to application can result in eutrophication and fecal
contamination of surface waters through enriched runoff.
Anaerobic digestion (AD) is an alternative method of agricultural waste treatment, which results in the
mitigation of organic carbon and its ensuing components. Although this process can be somewhat
complicated and have high construction capital costs, it is becoming the preferred method of waste
treatment due to: 1) the production and utilization of CH4 as a fuel, and 2) the generation of sludge that
can be used as a soil amendment. It is possible that the use of this technology could result in a reductionof
greenhouse gas emissions.
1.1.2 Anaerobic Digestion
Anaerobic digestion is a biological process where organic matter is reduced in an anoxic environment
into smaller key components through several biological reactions (Capareda, 2014). The product of
these biological reactions is biogas, which is composed of approximately 50-70% CH4, 50-30% CO2, and
trace gases. Organic matter is decomposed into carbohydrates, fatty acids and amino acids through
hydrolysis, which is the rate limiting reaction of anaerobic digestion. The products of hydrolysis undergo
acidogenesis, resulting in the production of a variety of organic acids. During acidogenesis, the pH of the
mixture will decrease due to homoacetogenic bacteria, which will double every 14 h. The next step is
acetogenesis where the acids formed in acidogenesis are converted into acetic acid and other volatile
fatty acids. The final process is methanogenesis, where 70% of the methane is produced by acetotrophic
methanogens by converting the carbon in acetic acid and other organic acids into methane. The other
30% of the methane produced by hydrogenotrophic methanogens is accomplished by using carbon
dioxide as a carbon source and hydrogen as a reducing agent. The production of methane requires a pH
in the range of 6-8 and is also sensitive to ammonia (NH3) and hydrogen sulfide (H2S). Typically, biogas
produced from AD is combusted for heat or processed in a combined heat and power unit (CHP) for the
sustainable production of both electricity and heat. Biogas can also be purified to produce pure
methane gas. Substrates used in anaerobic digestion are typically biological waste products, such as
manure. As the AD process progresses, a thickened material known as sludge, which is mainly composed

5. 4
of spent microbial biomass, accumulates at the bottom of the reactor. Sludge is rich in nitrogen (N) and
phosphate (P) and can serve as fertilizer with reduced concentrations of biological oxygen demand BOD
and fecal coliforms in comparison to the original waste material.
As a biological system, anaerobic digestion is a sensitive process that requires systematic control to
obtain biogas production with methane content greater than 60%. The sensitivity of the process results
in the need of automated control systems to maintain the reactor within optimal operating parameter
ranges in terms of temperature (close to 37o
C), pH (6-8), realistic solids and hydraulic retention times
(SRT and HRT), and carbon to nitrogen ratio (C:N ratio of 20-30:1), as described by Spanjer et al. (2006).
High nitrogen content can result in ammonia inhibition of methanogens, leading to a less efficient
digestion of the biomass and decreased methane yields. Poultry waste, including litter, has an inherently
low C:N ratio 3-10:1 (Kelleher et al., 2002). Blending poultry litter feedstock with other biomass sources
(co-digestion) addresses this barrier, as described by Kelleher et al. (2002). Substrate combinations can
increase methane production by enabling positive synergism and adding nutrients to support microbial
growth (El-Mashad and Zhang, 2010). One successful example of co-digestion is the blending of swine
waste and poultry manure, which resulted in higher biogas yields than digesting the two streams
separately ( Magbanua et al., 2001). Yen et al. (2007) reported on successful co-digestion of algae, with
a C:N ratio of 7, and of waste paper, resulting in a 104% increase of specific CH4 production. Thus, it was
reasonable to postulate that combining agricultural wastes and cellulose could result in an increased CH4
production, increasing the likelihood for implementation of AD technology.
1.1.3 Algae Cultivation in Periphytic Algae Cultivator (PAC)
A periphytic algae cultivator (PAC) is a system where wastewater containing high amounts of nutrients
can be circulated over a bed of algae, resulting in the reduction of dissolved N (primarily ammonia) and
P through uptake of these nutrients by the algae. The ultimate effect of this process in relation to
biomass characteristics is a net increase in C:N ratio. Research has demonstrated how a PAC system can
be used for the treatment of swine waste resulting in an average dry matter production of 10.6 g m-2
d-1
(Costello, 2015). Markou and Georgakakis (2011) reported that algae can be grown in a PAC from
agricultural wastewaters upstream or downstream of AD, serving the dual purpose of waste treatment
and energy production from algal biomass.
1.1.4 Combination of AD and PAC
A study conducted by Sialve et al. (2009) reported that AD of the entirety of the algal biomass is
preferred over lipid extraction followed by conversion to biodiesel, specifically when algal cells have dry
matter lipid content less than 40%. Griffiths and Harrison (2009) reported that only 14 out of 42 species
tested in the laboratory could reach a lipid content of 40% or greater when grown in ideal conditions.
From a practical standpoint, AD is a much simpler conversion process than lipid extraction. Due to the
fact that CH4 yields are higher in AD when C:N ratios are greater than 20:1, AD feedstock could be
amended using PAC grown algae. Such a co-digestion AD system could result in an increased CH4 yield
and a more efficient recovery of nutrients in the sludge to be used as a soil amendment.

6. 5
1.2 Problem Statement
There is a need to design a system for the UA Swine Finishing and Broiler Units that can fully utilize the
energy production potential of the biological wastes that are being produced, while reclaiming and
recycling nutrients needed for soil fertilization. The design and construction of such a system could act
as a model to be applied in other areas of the world, utilizing waste as a renewable resource to reduce
greenhouse gas emissions.
1.3 Goal
The design team set out to maximize the energy production potential and economic benefit of an
anaerobic digestion system for the UA farm whilst adhering to the objectives and constraints specified
by the manager of the UA farming operation, our client.
2 Preliminary Design
2.1 Design Objectives
The design objectives were the desired features and functions of the designed system from the client’s
perspective. It was not necessary that all objectives be met in the final design, but an optimization of the
system required a maximization of the number of design objectives met. The system should:
 Treat all biological wastes being produced by the UA
 Produce liquid effluent with reduced biologically-active pollutants that can be more safely
applied to the land (reduced BOD, P, and N concentrations)
 Produce a sludge to be transported off farm and used as a soil amendment
 Maximize energy production
 Utilize nutrients as much as possible for algal growth to maximize methane production
 Be simple and easy to operate, requiring little training for employees
 Emit no unpleasant odors transported to neighboring areas
 Have reliable design; low maintenance/labor requirements
 Exhibit carbon neutrality
 Minimize energy costs through electricity and methane production
2.2 Design Constraints
The design constraints were the critical features and functions of the designed system. It was absolutely
necessary that all constraints be met in the final design. The system must:
 Be placed on existing UA property
 Include safety mechanisms (pressure release valves, high rated materials, sound construction,
etc.) due to the hazardous nature of methane
 Provide sufficient energy savings to cover capital and maintenance cost over the life of 10 years

7. 6
2.3 Approach
2.3.1 General Approach
The team decided that coupling a PAC with an AD could greatly increase the methane production and
efficiency of the process leading to higher profitability. The goal of this project became one to confirm
this concept through prototyping and to develop a design for a full-scale system based on the confirmed
concept and prototyping data. The general concept of the system is illustrated in Figure A5.
Feedstock of the proposed AD system consisted of swine waste, poultry waste, algae, and carbonaceous
materials such as leaves and paper towels. Due to the fact that swine waste contains a high level of
ammonia, which could be detrimental to the digestion process, it should be treated by a PAC (algae lane
1, Figure A5) before entering the preparation tank. In the AD, feedstock goes through four biological
conversion steps, which result in the production of gasses, liquid effluent with a reduced solids content,
and nutrient-rich sludge. The liquid effluent, contains dissolved NH3, P, and CO2, which is further
removed by PAC post-treatment (algae lane 2, Figure A5). Energy is used by the system for heating the
digester and running the PACs. Energy is introduced into the system as potential energy in the feedstock
and as solar energy. Usable energy is produced by the AD system in the form of CH4.
2.3.2 Prototyping
A prototype was developed in order to obtain data related to the rate of CH4 production, specific yield,
and volatile solids conversion of the AD process. This data was used to perform calculations related to
the sizing and operation of the UA full-scaled system. The prototype was operated in a manner to be
representative of the the full-scaled system. Specifically, the waste materials were mixed at the same
ratio as would be required in the full-scale system. Finally, the digester was operated in a manner that
allowed the SRT to have a longer duration than the HRT.
2.3.3 Full System Scale-up
The team approached the design for the full-scale system to fulfill all the design objectives whilst
adhering to the constraints. Various available digester technologies were considered, and engineering
and economic analyses were performed to determine which existing technology would be most cost
effective in performing the required functions. Prototyping data was used to model and size the major
components for the system and to determine the major costs associated with its construction. Economic
analysis along with mass and energy balances were used to determine the overall profitability and
feasibility of the system.
3. Design, Fabrication, and Testing of a Prototype
3.1 Prototype Design
The prototype was designed within given constraints (section 2.2) and resources. We utilized existing
components already available at the UA to construct the prototype; specifically, the mini PACs, two one-
horsepower pumps, a 1 kW heating coil, a pH electrode, thermocouples, a 30 gal impermeable polyester
bag, and storage barrels for the feedstock materials. Decisions had to be made with respect to designing
the small-scaled digester, the control system, and the housing for the prototype.

8. 7
3.1.1 Digester Type
It was determined that there were three main categories of anaerobic digesters for agricultural
applications: passive systems, low-rate systems, and high-rate systems as described by Hamilton (2010).
It was decided that a high-rate system would be needed in order to accomplish the objective of
separating the HRT from the SRT. This is an important feature of the design because the HRT/SRT
separation results in higher and faster conversions and allows for the nutrient rich sludge to be
harvested from the process. It was decided that a sequencing batch reactor would be ideal for the
prototype system because of its simpler design and operation as compared to the other two high-rate
systems. A sequencing batch reactor is a reaction vessel that operates in a four-step cycle: 1) the system
is operated with a fixed volume of liquid, being mixed in order to assure that the microbes come in
contact with the substrate; 2) the mixing is turned off and the solids are allowed to settle for a given
period of time; 3) a fraction of the liquid is decanted from the top; and, 4) a volume of feedstock equal
to the volume removed is added to the digester.
3.1.2 Mixing
It is important that the contents within a sequencing batch reactor be mixed when in operation to
ensure optimal conversion rates and efficiencies. Three common methods of mixing anaerobic digesters
are: 1) pumping the produced gas into the bottom of the container; 2) pumping the liquid out from one
point of the container and into another; or 3) by using a mechanical stirrer. For the sake of simplicity,
safety, and convenience, it was decided that mixing by liquid displacement would be acceptable for the
prototype. The mixing system of the prototype was constructed using 3/4'’ PVC and a 1/50 HP
centrifugal pump drawing liquid from the bottom of the digester and discharging into the top.
3.1.3 Digester Size
The team had two options for containers: a 50 gal drum or a 1000 L (about 275 gal) water tank. The
team decided to use a 1000 L water tank in order to prototype the largest working volume to better
model the full-scale digester. Moreover, the 1000 L water tank already had a drainage valve at the
bottom and a threaded cap, minimizing the number of holes that would have to be created in the tank
for installation, ensuring that the digester would be air and water-tight.
3.1.4 pH Measurement System
The pH sensor could either be placed inside the digester or within the piping system that would be used
to mix the digestate. It was decided to mount the pH sensor in the piping of the mixing system. pH data
from the sensor would be acquired by a data acquisition module (myDAQ) integrated with LabVIEW
software on a laptop computer. Although the team initially assumed that pH control would be required,
it was shown during the operation of the prototype that the system could maintain an acceptable pH
without any addition of buffer. Thus, the LabVIEW program was used only to monitor and record pH
values.
3.1.5 Temperature Control System
The team had to decide on the mechanisms for monitoring and controlling the temperature within the
digester. Since a 1 kW immersion heater was already available, it was decided to adjust the ambient
temperature in order to use the available heater to maintain the digester at 37°C. The heater could

9. 8
either be placed directly into the digester or be used to power an external heat exchanger. Placing the
coil directly in the digester would have been the easiest option, but contact with the high-temperature
element could have increased mortality of the microbial consortia. It was decided to construct an
external heat exchanger consisting of a 1 kW immersion heater placed in a water bath, a copper coil
connected to the mixing system for the digester immersed in the water bath. A thermostat was installed
to control the heater based on the temperature of the digestate, in order to maintain the digester at
37o
C. This system ensured mixing and heating would be taking place continuously, using the same pump
and piping system.
Since heating was restricted to 1 kW of power supplied, calculations had to be performed to determine
the minimum temperature of ambient air such that the digester could be maintained at 37°C. This was
determined by performing heat transfer (Fourier’s law) and fluid mechanics (Bernoulli’s equation)
calculations. The operating point of the pump with the piping system was determined using Figure A1 in
the appendix. Using the flow rate of 300 gph specified by the operating point in Figure A1, the
temperature of the fluid entering the digester from the mixing/pumping system was calculated to be
37.8°C, in order to supply 1 kW of heat to the digester. In other words, the temperature of the digestate
at the exit of the water bath needed to be 37.8°C such that the overall temperature of the digester
would be maintained at 37°C. Further calculations were performed to determine the temperature of the
water bath required to heat the digestate to a temperature of 37.8o
C; this temperature was estimated
to be 37.9°C. Finally, it was determined that the maximum allowable heat loss of 1 kW from the digester
would take place at an ambient temperature of 60°F or 15.6°C. It was decided to use the thermostat for
control since it was already available and easy to implement, saving time and money. The capillary bulb
of the thermostat was installed into the piping system using a T connected to a brass fitting that the
bulb was sealed into using JB weld.
3.1.6 Biogas Collection and Measurement
A collection system needed to be design for the collection of the biogas produced. A 30 gal polyester
bag was already available at the UA for this purpose. A PVC gas line installed to the cap of the 1000 L
tank was connected to clear Tygon tubing through a brass fitting. The tubing was run through a PVC pipe
installed into the ceiling of the enclosure, exiting the building wall and into the 30 gal polyester bag so
that gas could be collected in the open air. Faculty consultant, Dr. Zhu graciously allowed us to use the
gas chromatography machine in his laboratory to quantify CH4 composition of our produced biogas.
3.2 Safety
The team had to minimize the chance of CH4 being leaked into the room and ensure that there would
not be an excessive NH3 or hydrogen sulfide concentrations in the air coming from the PACs and waste
materials. Two measures were taken to ensure this did not happen. Gas was piped outside through the
ceiling to allow gas to be collected in the open air. Also, a vent fan was installed to provide sufficient
ventilation. A photograph of the prototype digester is shown in Figure 1.

10. 9
Figure 1: (left) Periphytic algae cultivator (PAC). PAC 1 is to reduce ammonia content of swine upstream
of the digester. PAC 2 is to recover nutrients in downstream digester effluents. (right) 1000L digester
mixed with circulating digested in heat exchanger (blue insulated box) and feedstock loading barrel (blue
barrel on top of digester).
3.4 Prototype Testing and Results
Throughout most of the prototyping phase, the pH was monitored daily on a digital display. The pH
ranged from 6 to 7.5 during the monitoring period. The temperature was monitored for a period of 2
days in order to determine the effectiveness of the heat exchanger design. Typical temperature data is
presented in Figure 2. At the beginning of the monitoring period, the ambient air dropped below 60°F
(as shown by the red arrow), which resulted in a sharp decrease in the temperature of the digester and
the digestate exiting the heat exchanger. This confirmed the robustness of the design calculations
performed for the heat exchanger, which showed that the heater would not be able to maintain the
digester at 98o
F while being operated in an ambient temperature below 60°F. After the initial drop in
temperature, the ambient air was increased to 72°F, after which the digester was maintained at an
average value of 86.5°F. This is significantly lower than the optimal temperature of 98.6°F, but
fluctuating water bath temperature suggested that the heater was cycling on and off. This suggested
that the temperature control error was most likely due to an inaccuracy in the thermostat. The effect of
the lower temperature on the gas output was taken to be insignificant, however, because it could only
result in a slight underestimation of CH4 potential of the system.

11. 10
The waste ratios of the feedstock
needed to be representative of the
feedstock to be used in the full-
scale system, which was based off
of the total annual wastes
produced by the UA swine finishing
and poultry units. The total mass of
poultry litter and liquid swine
waste produced was found to be
approximately 500 tons per year
and 16,000 tons per year,
respectively. These values were
based on information provided by
Dr. Charles Maxwell, who oversees
the swine research facility, David
McCreery, who manages the UA
broiler unit. Gaps in information were filled in using the USDA Agricultural Waste Management
Handbook. The mass ratio of liquid swine waste to poultry litter was determined to be approximately
32:1 (see Figure A7 of the Appendix). Leaves, grass clippings, and waste paper towels were added in
order to increase the C:N ratio of the feedstock to a value within the range of 20-30:1. The mass ratio of
poultry litter, swine waste, and leaves and/or paper towel were 1:32:1.2. The amount of algae added
was not considered in the feedstock ratio calculation due to uncertainty in C:N ratio of the algae
produced. This ratio resulted in a slurry of 5% total solids, which is acceptable for anaerobic digestion.
To start off the anaerobic digestion process,
400 L of sludge from the anaerobic digester
at the wastewater treatment facility in Little
Rock, Arkansas was placed in the reactor.
This ensured that the reactor would have
the right methanogens and other anaerobic
bacteria to break down the biomass leading
to the production of CH4. The prototype was
operated in the mesophilic regime as a
sequencing batch reactor to save energy and
to allow solids to separate from the liquid so
that the SRT or the digester was longer than
the HRT. The prototype was used to
estimate the specific yield in terms of
volume of CH4 produced per unit mass of
volatile solids (VS) consumed. Eight distinct
trials runs lasting at two days were
50
60
70
80
90
100
110
120
0 0.5 1 1.5 2
Temperatue(°F) Time (days)
Ambient
Digester
HXout
Water Bath
Figure 2: Graph of temperature versus time of the digester, digestate
exiting the heat exchanger, ambient air, and water bath in the heat
exchanger
y = 477x - 406
R² = 0.94
0
500
1000
1500
2000
2500
3000
3500
4000
0 2 4 6 8 10
MethaneProduction(L)
VS con (kg)
Figure 3: Plot of the methane produced by each batch versus
the mass of volatile solids consumed.

12. 11
conducted. Four of the eight batches were allowed to proceed until nearly all the digestible materials
had been converted in order to develop kinetic models for the process.
The line fitted to the gas production data versus mass of VS consumed is shown in Figure 3; the slope
was determined to 477 L CH4 kg-1
VScon, which corresponded to specific yield of the process. Compared
to literature values, the specific yield produced by the prototype is on the high end of the typical range
for anaerobic digestion (Monnet, 2003). Batch data were fitted to a first-order kinematic model shown
by Equation 1:
( ) ( )
Equation 1
G(t) is the cumulative methane production
at time t, G∞ is the ultimate methane
production, and k is the first-order rate
constant in d-1
. An example of fitting the
kinetic model to the prototype data is
shown in Figure 4. The average value of k
for the four batches analyzed was
determined to be 0.6 d-1
.
Feedstock preparation for the prototype
was somewhat strenuous because all the
grinding and transferring of materials had
to be performed by hand. Thus, the full-scale system was designed to reduce these labor requirements
through a more integrated design and automation. Throughout prototyping, both PAC exhibited a
continuous production of algal biomass, which was integrated into the feedstock. It was found that the
NH3 concentration of the swine waste was decreased by 86% after a PAC treatment period of 2 days
(Carter, 2015).
4. Full-Scale System Design
The full-scale system was designed to have the same major components as the prototype (two PACs, an
anaerobic digester, a control system, and storage tanks for feedstock). The system is to be located on-
site at the UA swine finishing unit, which contains the vast majority of the mass of waste to be treated.
4.1 Digester Sizing
A sequencing batch reactor and up-flow anaerobic sludge blanket were compared using a weighted
objectives table based on performance criteria, and it was determined from this analysis that an up-flow
anaerobic sludge blanket would be most suited for the full-scale system. An up-flow anaerobic sludge
blanket is a type of reactor in which influent and effluent are continuously flowing in and out at equal
rates. The solid concentration inside the reactor decreases as the fluid travels from the bottom to the
top.
0
50
100
150
200
250
300
0 5 10 15 20
VolumeCH4(L)
Time (days)
Cumulative Gas Production (Batch 8)
Figure 4: A first-order kinematic model fitted to the data for
batch 8

13. 12
Data produced by the prototype was used
to estimate the rate of production for a
digester of a given volume. The
relationship between CH4 production, HRT,
and digester volume is illustrated in Figure
5 where it is shown that the maximum
rate of CH4 production is 676 m3
d-1
. Since
the volume of waste that the digester
must treat annually is equal to the total
volume of waste produced by the UA
client, the flow rate of feedstock into the
digester is fixed. Thus, the HRT of the
digester is directly related to the volume
of the digester. The minimum value shown
in Figure 5 corresponds to a HRT of 7 days,
a volume of 285.8 m3
, and a CH4
production of 665 m3
d-1
. Since these parameters would result in a production equal to 98.5% of the
maximum value produced by the mathematical model and a relatively small digester volume, it can be
assumed that an HRT of 7 d is the optimal value for the design (anaerobic digesters are not typically
operated at an HRT less than 7 d in order to prevent wash out).
4.2 Algae Cultivators
The PACs for the full-scale system are already present at the UA swine finishing unit. The other main
components of the designed system can be easily integrated into the existing farm operation.
4.3 Storage
Storage will be needed on site to contain the waste materials to be used as feedstock. Swine
wastewater is already stored on site in a holding pond, which can be pulled from directly. Poultry litter
can be stored on site in a stacking shed. Storage will also be required to hold reserve carbonaceous
materials (paper towels, cardboard, etc.), which was designed to hold 50 m3
of solid material or
approximately three days’ worth of solid material.
4.4 Gas Utilization
Three different options were considered when determining how the gas produced by the digester
should be utilized, each with a different monetary benefit and feasibility of implementation. Two of the
three options involved sizing a CHP to only meet requirements for heating the digester, which was found
to be a heat loss of 8.9 kW. This would leave 95% of the methane produced (8.2 million cubic feet per
year) available for other uses. Option 1 was to use the remainder of methane to offset natural gas
consumption and option 2 was to use the remainder of methane to offset gasoline and diesel
consumption. Option 3 was to use a CHP capable of combusting all of the biogas produced and using the
electricity generated by the CHP to offset electricity consumption. Estimated monetary benefits of
offsetting consumption of the various energy sources considered are shown in Table 1.
0 20 40 60
500
550
600
650
700
0 500 1000 1500 2000 2500
Hydraulic Retention Time (days)
MethaneProduction
(cubicmetersperday)
Digester Volume (cubic meters)
Figure 5: Relationship between rate of methane production
and volume of digester.

14. 13
Table 1: Estimated monetary equivalences of methane produced for three options.
(1) Natural Gas Monetary Equivalence Based on $0.43/CCF $35,000
(2a) Gasoline Monetary Equivalence Based on gasoline price of $2.35/gal $154,000
(2b) Diesel Monetary Equivalence Based on diesel price of $3.00/gal $174,000
(3) Electricity Equivalence Based on electricity price of $0.07/kWh $57,000
Both option 1 and option2 would require cleansing of the biogas to produce nearly pure methane (95 –
99%). Option 1 would also require a large storage and distribution capacity in order to use the methane
when and where it was needed. The methane is most valuable when used to offset vehicle fuels
(gasoline and diesel) but option 2 would also require much more planning and larger associative cost
than the other two options because of the location of the system, the conversion and/or purchasing of
vehicles capable of running on compressed natural gas, and the many considerations that must be made
in order to construct a fueling station for the vehicles. Although it is possible that option 2 would be the
most profitable, it is likely that the large increase in scope of the project, capital needed, and planning
required related to option two would cause it to be found less favorable by the client. The main
component required for option 3 would be a CHP capable of combusting the entirety of the gas
produced. Also, a CHP designed specifically for biogas would not require the biogas to be cleansed.
Option 3, however, also has a fairly low monetary value placed on the biogas compared to option 2.
It was recommended by the design team that the client utilize the methane produced by the anaerobic
digester by combusting in a CHP and sending excess electricity to the grid for energy credits with the
electric company. The usable heat produced by the CHP can be used to maintain the digester at 37o
C, to
heat swine houses in the winter, to maintain the PAC’s at optimal temperature throughout the year, and
to provide hot water to the residence on site, which will help to offset some natural gas costs.
4.3 Economic Analysis
An economic analysis was performed based on predicted performance of the digester and the gas
utilization method chosen to determine the viability of the proposed system. Table 2 gives the
estimated capital costs of the project and Table 3 gives the estimated annual costs and benefits of the
project.
Table 2: Estimated capital costs of project
Item # Units Description Cost
Hammer Mill 1 15 HP, 1000 lb/hr; for grinding solid feedstock $4,300
CHP 1
51 gpm input biogas capacity, 190 kW electric output
Generates heat and electrical energy from biogas $200,000
Tank 1 308 m3
steel; digestion vessel $60,000
Heating NA 8.9 kW; heat exchangers for heating digester $8,000
Stacking Shed 1 To store 125 tons of poultry litter; quarterly amount $13,000
Building Cost N/A Construction of system $20,000
Total $305,300

15. 14
Table 3: Annual costs and benefits of project
Item Description Annual Amount
COSTS Annual costs associated with the system
Capital Capital costs spread out over 10 years with 5% interest $40,000
Operation Labor and Maintenance $10,000
Total Capital + Operation $50,000
BENEFITS Annual benefits of system designed
Electricity Offset electricity costs $58,000
Heating Offset natural gas costs for heating $16,000
Total Electricity + Heating $74,000
NET TOTAL Benefits – Costs $24,000
The capital costs associate with the design amount to approximately $300,000 and the gross annual
benefits amount to $74,000. This gives a simple payback period of 4 years and an internal rate of return
of 20% based on a 10-year payment period. The net annual benefit of the system designed is $24,000
based on a 10-year payment period and 5% interest. After the 10-year payment period, the benefit of
the system would amount to approximately $64,000.
5. Summary of Proposed System
The digester for the system was sized to have a hydraulic retention time of 7 d and a 2 ft freeboard,
which corresponds to a volume of 308 m3
. The digester is to be a cylinder with a conical bottom having a
diameter of 6.9 m (22.6 ft) and a height of 7.5 m (24.6 ft). The digester will produce approximately 1,025
m3
d-1
(25 ft3
min-1
) of biogas with a 65% methane content and is to have a floating conical top capable
of storing approximately 2 d worth of gas produced. The system will be located at the swine finishing
unit on the UA farm where there are already four 5’ by 200’ PAC’s. Two of the PAC’s will be used to treat
swine wastewater before entering the digester and the other two will be used to grow more algae from
the digester effluent. Poultry litter from the UA broiler unit and carbonaceous materials from campus
and other UA facilities will be truck in and stored at least every two days, and will be processed by a
hammer mill before entering the digester. The system will be automated and have an alert system in
order to reduce labor costs and to ensure safety. There will also be an up-spout coming from the bottom
of the digester so that sludge can be removed to be applied to pasture land in the various areas around
the farm. Table 2 shows the capital costs and Table 3 shows the annual costs and benefits associated
with the system designed.
The major components of the proposed waste treatment/energy production system are:
 Swine Operation Capacity: 250 head
 Poultry Operation Capacity: 80,000 head
 Algae Cultivators Area: 4,000 ft2
 Digester Vessel Volume: 80,000 gal
 Combined Heat-Power Unit: 190 kW electricity (51 gpm biogas input)
 Hammer Mill: 15 HP (1000 lb/hr solid input)
 Heat Exchanger: 9 kW

16. 15
 Stacking Shed: 60’ by 20’ stacking shed (125 tons, 5 ft deep)
 Total Estimated Capital Cost: $305,000
The estimated feedstock inputs of the system are:
 Poultry Litter: 500 tons/yr (2,700 lb/d)
 Swine Wastewater: 4 million gal/yr (10,000 gal/d)
 Carbonaceous Material: 600 tons/yr (3,200 lb/d)
Estimated outputs of the system are:
 Biogas: 86,000 CCF/yr (methane equivalent)
 Electricity: 820,000 kWh/yr ($58,000/yr)
 Thermal Energy: 1,700 Mbtu/yr ($16,000/yr)
Estimated net benefits of the system are:
 Net Monetary Savings: $24,000/yr
 Greenhouse Gas Reduction: 600 tons CO2/yr
6. Conclusion
Anaerobic co-digestion coupled with algae cultivation technology was shown to be a cost effective
investment in animal waste treatment that provides fossil fuel and reduced carbon emissions. The
system will not require any additional water inputs and will produce a nutrient rich sludge that can be
more efficiently applied to the land than swine wastewater. The system can be automated and an alert
system can be installed in order to minimize labor requirements and safety concerns. It is expected that
some odor emissions may be alleviated by the system designed given that a large portion of the
wastewater will be contained in the digester and ammonia will be removed by the algae cultivation
lanes.
It should be noted that the monetary benefits are dependent upon the net metering electricity policies
in Arkansas. The system is capable of meeting the maximum allowable power production requirement
of 300 kW placed on commercial applications, but arrangements would have to be made to use the
energy credits created by the system to offset electricity costs of other areas of the UA farm. Also, the
CHP of the system was sized to combust a day’s production of biogas in a 12 hour period so that
arrangements could be made to supply energy to the grid only during peak hours if required by the
electric company. This also allows flexibility in the actual biogas production that will result from the
system.

17. 16
References
Angelidaki, I., M. Alves, D. Bolzonella, L. Borzaconni, J. L. Campos, A. J. Guwy, S. Kalyuzhnyl, P. Jenicek, J.
B. van Lier. 2009. Definiing the biomethane potential (BMP) of solid organic wastes and energy
crops: a proposed protocol for batch assays. Water Science and Technology 59.5: 927-934.
Capareda, S. C. 1997. Introduction to Biomass Energy Conversions: 249-287. Boca Raton, Florida: CRC
Press.
Carter, J. B. 2015. The effects of algae pre-treatment on the biomethane potential of swine waste.
University of Arkansas Honors Thesis, Fayetteville AR.
Costello, T., A. 2015. Personal communication. Faculty member in Biological and Agricultural Engineering
at the University of Arkansas.
El-Mashad, H. M, Zhang, R. 2010. Biogas production from co-digestion of dairy manure and food waste.
Bioresource Technology 101(11): 4021–4028.
Griffiths, M. J., Harrison, S. T. L. 2009. Lipid productivity as a key characteristic for choosing algal species
for biodiesel production. Journal of Applied Phycology 21: 493-507.
Hamilton, D. W. 2010. Anaerobic Digestion of Animal Manures: Types of Digesters. Stillwater, OK:
Oklahoma Cooperative Extension Service.
Kelleher, B. P., Leahy J. J., Henihan, A. M., O’Dwyer, T. F., Sutton, D., Leahy, M. J. 2002. Advances in
poultry litter disposal technology – a review. Bioresource Technology 83: 27-36.
Liua, C., Yuana, X., Zenga, G., Lia W., Lib J. 2008. Prediction of methane yield at optimum pH for
anaerobic digestion of organic fraction of municipal solid waste. Bioresource Technology 99(4):
882-888.
Markou, G., Georgakakis D. 2011. Cultivation of filamentous cyanobacteria (blue-green algae) in agro-
industrial wastes and wastewaters: A review. Applied Energy 88: 3389–3401
Monnet, F. 2003. An introduction to anaerobic digestion of organic wastes.
http://www.biogasmax.co.uk/media/introanaerobicdigestion__073323000_1011_24042007.pdf
(accessed December 2014).
Richard, T., Trautmann, N. 2014. C/N Ratio. Cornell Composting Science and Engineering. Available at:
http://compost.css.cornell.edu/calc/cn_ratio.html. Accessed 12 December 2014.
Sialve, B., Bernet, N., Bernard, O. 2009. Anaerobic digestion of microalgae as a necessary step to make
microalgal biodiesel sustainable. Biotechnology Advances 27: 409 – 416.
Spanjers, H., van Lier, J.B. 2006. Instrumentation in anaerobic treatment – research and practice. Water
Science & Technology 53(4-5): 63–76.
USDA. Agricultural Waste Management Handbook. Available at: http://www.nrcs.usda.gov
/wps/portal/nrcs/ detailfull/national/water/?&cid=stelprdb1045935 accessed September 26,
2014.
Yen, H., Brune, D. E. 2007. Anaerobic co-digestion of algal sludge and waste paper to produce methane.
Bioresource Technology 98: 130 – 134.
Angelidaki, I., M. Alves, D. Bolzonella, L. Borzaconni, J. L. Campos, A. J. Guwy, S. Kalyuzhnyl, P. Jenicek, J.
B. van Lier. 2009. Definiing the biomethane potential (BMP) of solid organic wastes and energy
crops: a proposed protocol for batch assays. Water Science and Technology 59.5: 927-934.
NRCS. 2009. Anaerobic Digester. Code 366. Accessed on May 1, 2015. Available at:
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_026149.pdf

18. 17
Appendix
Figure A1: Pump/system curve for the prototype system heating/piping system. The point at which the
two lines intersect gives the operating flow rate of the system. This flow rate was used to estimate the
convection coefficient within the heat exchanger in order to determine the minimum ambient
temperature at which the 1kW heater could maintain the digester at a temperature of 37o
C.
Figure A2: LabVIEW block diagram for pH monitoring system.
head(ft) flowrate(gph)
1 490
2 456
4 370
6 275
8 116
head(ft) flowrate(gph)
0.370753 70
2.575585 205
5.190542 300
0.702134 100
13.553811 500
Pump
System(1/2'' copper, 3/4'' pvc)
y = -3E-05x2 - 0.0025x + 8.658
R² = 0.9997
y = 5E-05x2 + 0.003x - 0.0809
R² = 1
0
2
4
6
8
10
12
14
16
0 100 200 300 400 500 600
Head(ft)
Flowrate (gph)
Pump/System Curve
Pump curve
System Curve
Poly. (Pump curve)
Poly. (System Curve)

19. 18
The probe reads the pH of the tank. The single is filtered from surrounding noise and is changed to
calibrated settings. The signal is then displayed numerically and on a waveform chart. This information is
recorded every 900,000 ms (every 15 minutes).
Figure A3: Instrumentation and control system for prototype

20. 19
Heat Exchanger
Mixing
Swine Waste
Urea (CO(NH2)2)
Ammonia (NH3)
Ammonium (NH4)
Carbohydrates
Animal LIpids
Anerobic Digestion
1.Hydrolysis
2.Acidogenesis
3. Acetogenesis
4. Methanogenesis
Poultry Waste
Urea (CO(NH2)2)
Ammonia (NH3)
Ammonium (NH4)
Carbohydrates
Animal LIpids
Leaves/Paper Towels
Carbohydrates
Plant Lipids
Liquid Effluent
Ammonia
Phosphate
Carbon Dioxide
Algae Lane 2
Treatment of Digester Effluent
Volatilization of Ammonia
Phosphate Recovery
Light
Algae Lane 1
Treatment of Swine Waste
Volatilization of Ammonia
Sludge
Gases
Methane (65%)
Carbon Dioxide(34%)
Ammonia
Hydrogen Sulfide
Sulfur Oxides
Algae Only
Figure A4: Process flow diagram of proposed system
Figure A5: Illustration of room housing the prototype system

21. 20
Formulas
%C = %N x C/N
Mass of material ( )
Moisture goal (G)
Moisture content (%) of material n ( )
R= Goal(C: N ratio)
Carbon % ( )
Nitrogen %( )
Moisture content (%) of material n ( )
Mass of material ( )
Figure A6: Feedstock Spreadsheet. This excel sheet is design to have our feedstock reach a target carbon to nitrogen ration of 20:1. The
instructions are provided in the sheet. The feedstock being mix are poultry, swine, and leaves. The mass of these values are taken from the
feedstock information spread sheet and gives us the carbon nitrogen ratio and moisture content of the combination. Since algae are also being
digested, we can add the information into the feedstock sheet the spreadsheet will calculate a desired range.

22. 21
Figure A7: Aerial schematic of system designed

23. 22