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UofA Algae Digester

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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 m3CH4 kg-1VScon.

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

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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

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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 reduction of

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

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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 37oC), 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.

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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

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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.

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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

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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

37oC. 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 37.8oC; 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.

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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 98oF 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.

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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 2Te

mp

era

tue

(°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

Me

than

e P

rod

uct

ion

(L)

VS con (kg)

Figure 3: Plot of the methane produced by each batch versus the mass of volatile solids consumed.

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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

Vo

lum

e C

H4

(L)

Time (days)

Cumulative Gas Production (Batch 8)

Figure 4: A first-order kinematic model fitted to the data for batch 8

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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)

Me

than

e P

rod

uct

ion

(c

ub

ic m

ete

rs p

er

day

)

Digester Volume (cubic meters)

Figure 5: Relationship between rate of methane production

and volume of digester.

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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 37oC, 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

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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

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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.

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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

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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 37 oC.

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 + 8.658R² = 0.9997

y = 5E-05x2 + 0.003x - 0.0809R² = 1

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500 600

He

ad (

ft)

Flowrate (gph)

Pump/System Curve

Pump curve

System Curve

Poly. (Pump curve)

Poly. (System Curve)

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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

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Heat Exchanger

Mixing

Swine WasteUrea (CO(NH2)2)Ammonia (NH3)

Ammonium (NH4)CarbohydratesAnimal LIpids

Anerobic Digestion1.Hydrolysis

2.Acidogenesis3. Acetogenesis

4. Methanogenesis

Poultry WasteUrea (CO(NH2)2)Ammonia (NH3)

Ammonium (NH4)CarbohydratesAnimal LIpids

Leaves/Paper TowelsCarbohydrates

Plant Lipids

Liquid EffluentAmmoniaPhosphate

Carbon Dioxide

Algae Lane 2Treatment of Digester Effluent

Volatilization of AmmoniaPhosphate Recovery

Light

Algae Lane 1Treatment of Swine WasteVolatilization of Ammonia

Sludge

GasesMethane (65%)

Carbon Dioxide(34%)Ammonia

Hydrogen SulfideSulfur Oxides

Algae Only

Figure A4: Process flow diagram of proposed system

Figure A5: Illustration of room housing the prototype system

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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.

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Figure A7: Aerial schematic of system designed

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