TREATMENT OF DAIRY MANURE FOR
ENERGY PRODUCTION AND NUTRIENT RECOVERY
By
SHUNCHANG YANG
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
ACKNOWLEDGMENTS
I would like to express my deepest appreciation to my advisor and committee chair, Dr. Pratap Pullammanappallil for generous inspire and continual guidance during my research at the University of Florida. Without his consistent help and
encouragement my dream would not come true. I am grateful to Dr. Spyros Svoronos for his consistent support and thoughtful advices throughout my doctoral program. I would like to thank Dr. Edward J. Phlips, Dr. Gao Bin and Dr. Ben Koopman for serving on my committee and their valuable suggestions and lectures when it came to my research. I would like to thank Mr. Daniel Preston and Mr. Patric Rush for the heartful help and technical support during my research. I give thanks to Dr. Beily Trump and Dr. Leslie Landauer for kindly assistances on the instruments training. I acknowledge financial support for my study on University of Florida by Department of Energy, Blue earth Technologies, Inc.
I would like to thank to my lab members, Na Wu, Yingxiu Zhang, Yikan Liu, Kyle Griffin, Clement Tseng, Samriddhi Buxy, Patric Dube, Cesar Moreira. I give thanks to my research project volunteers who were introduced by Dr.Svoronos for the past years. With their considerate help and suggestions, I had such enjoyable and wonderful lab experiences. I would like to also thank everyone in the Agricultural and Biological Engineering department who gave me help when I needed at any atmosphere.
TABLE OF CONTENTS page ACKNOWLEDGMENTS ... 4 LIST OF TABLES ... 8 LIST OF FIGURES ... 9 ABSTRACT ... 12 CHAPTER 1 INTRODUCTION ... 14 Background ... 14
Treatment of Dairy Manure ... 17
Problem Statement ... 21 Research Methods ... 23 Objective 1 ... 23 Objective 2 ... 26 Objective 3 ... 26 Objective 4 ... 27
2 BIOCHEMICAL METHANE POTENTIAL OF DAIRY MANURE ... 29
Introduction to Dairy Manure as Feedstock ... 29
Disadvantage of Current Methods Utilization of Dairy Manure ... 30
Materials and Methods... 32
Dairy Samples ... 32
BMP Assays Experiment ... 33
Analysis ... 36
Results and Discussion... 38
Characterization of the Different Dairy Manure Feedstocks ... 38
Nutrient Balance of BMP Assays ... 39
Methane Potential ... 39
BMP of fresh dairy manure ... 41
BMP of separated manure fiber ... 43
BMP of scraped dairy manure slurry ... 46
3 PILOT SCALE INDUCED BED REACTOR FOR THREE DIFFERENT DAIRY UNIT WASTE TREATMENT ... 48
Introduction to Pilot Scale of Anaerobic Digestion ... 48
Materials and Methods... 51
Induced Bed Reactor ... 51
Results and Discussion... 56
IBR Operation of Dairy Manure from Dairy Research Unit (DRU) ... 56
IBR Operation of Dairy Manure from Flushed Dairy (Bell dairy) ... 60
IBR Operation of Dairy Manure from Scraped Dairy (North Holstein Dairy Farm) ... 63
4 LOW-COST, ARDUINO-BASED, PORTABLE DEVICE FOR MEASUREMENT OF METHANE COMPOSITION IN BIOGAS ... 66
Introduction of Current Methane Measurement ... 66
Materials and Methods... 68
Methane Measurement Device ... 68
Operation of Device ... 69
Device Validation Tests ... 70
Gas Chromatograph (GC) Measurements... 71
Application of the Device ... 72
Results and Discussion... 72
Device Integrity ... 72
Linearity of Pressure Sensor ... 73
Effect of Humidity and Pressure on Baseline Sensor Signal ... 73
Response of MQ-4 to Injections of Different Volumes of Samples of Standard Biogas ... 74
Response of MQ-4 to Injections of Constant Volume of Standard Biogas Samples with Different Methane Content ... 75
Methane Measurements of Actual Biogas Samples ... 77
Device Cost ... 77
Conclusions on Methane Measurement Device ... 78
5 INLINE MEASUREMENT OF METHANE PRODUCTION RATE FROM ANAEROBIC DIGESTER ... 88
Introduction to Inline Methane Measurement ... 88
Material and Methods ... 90
Modification of Offline Methane Sensor ... 90
U-tube Gasmeter ... 94
Digester with All Part Together ... 95
Results and Discussion... 95
Inline Methane Measurement Device Integrity ... 95
Response of Inline Measurement to AFBR ... 96
Gas Chromatograph (GC) Measurements Vs Inline Methane Measurement. .. 98
Accuracy of inline measurement system ... 98
Inline measurement system response to interruption of AFBR digester .... 99
Methane Production Rate ... 100
Inline Methane System Cost ... 100
Conclusions of Inline Methane Measurement System ... 101
Background of Phosphorus Recovery ... 102
Materials and Methods... 107
Pilot Scale IBR ... 107
Phosphorus Form in IBR ... 108
Sequential Batch Reactor for Phosphorus Removal ... 114
Effect of magnesium concentration on phosphorus removal in sequential batch reactor ... 115
Effect of seed concentration on phosphorus removal in sequential batch reactor ... 121
Results and Discussions ... 122
Efficiency of pH Adjustment by Aeration ... 122
Effect of Magnesium Concentration on Phosphorus Removal ... 125
Effect of Seed Addition on Phosphorus Residue Removal ... 132
Bottom Sludge Composition ... 133
Conclusions of Phosphorus Recovery from Magnesium ... 134
LIST OF REFERENCES ... 135
LIST OF TABLES
Table page
1-1 Phosphorus concentration in manure slurry, digester feed and treated
effluent ... 27
2-1 BMP assays feedstock characteristics ... 40
2-2 Methane yield of different feedstock based on BMP ... 41
3-1 disadvantages of several onsite pilot scale digester ... 50
3-2 The energy recovery efficiency of different pilot scale anaerobic digester ... 50
3-3 Characteristics of different dairy manure samples ... 57
4-1 Cumulative volume of injected air and corresponding gauge pressure in device ... 79
4-2 Methane composition measured by device and GC for various mixtures of standard biogas sample and air ... 80
4-3 List of individual components to assemble device and purchase price ... 81
5-1 Current methods of inline methane measurement ... 89
5-2 Inline methane system cost ... 101
6-1 Characteristics of feeding dairy manure ... 112
6-2 Characteristics of Digested dairy manure ... 112
6-3 Phosphorus concentration within digester ... 113
6-4 Phosphorus precipitation with aeration only ... 118
6-5 The effect of aeration on total phosphorus recovery ... 124
6-6 Effect of MgCl2 loading ... 127
LIST OF FIGURES
Figure page
1-1 Schematic diagram of a flushed dairy. ... 17
1-2 Schematic diagram of a scraped dairy. ... 17
2-1 Schematic of BMP assay ... 36
2-2 Cumulative methane yield of total fresh manure solids from DRU and Bell Dairy ... 42
2-3 Cumulative methane yield of volatile fresh Manure solids from DRU and Bell Dairy ... 43
2-4 Cumulative methane yield of total separated fiber from Bell Dairy ... 44
2-5 Cumulative methane yield of volatile separated fiber solids from Bell Dairy ... 44
2-6 Cumulative methane yield of total pond water solids from Bell Dairy ... 45
2-7 Cumulative methane yield of volatile pond water solids from Bell Dairy ... 46
2-8 Cumulative methane yield of total solids of scraped dairy manure pit ... 47
2-9 Cumulative methane yield of volatile solids of scraped dairy manure pit ... 47
3-1 Schematic of IBR digester system. ... 53
3-2 Physical characteristics of dairy manure from DRU ... 58
3-3 IBR performance of dairy manure from DRU ... 59
3-4 Physical characteristics of dairy manure from Bell dairy ... 61
3-5 IBR performance of dairy manure from Bell dairy ... 62
3-6 Physical characteristics of dairy manure from North Holstein dairy ... 64
3-7 IBR performance of dairy manure from North Holstein dairy ... 65
4-1 Offline methane measurement device ... 82
4-2 Baseline sensor signal in air versus relative humidity at different chamber pressures. ... 83
4-4 Comparison of sensor methane measurements to measurements by GC of
gas in device chamber after injecting variable volumes of biogas standard ... 85
4-5 Comparison of sensor methane measurements to measurements by GC of gas in device chamber after injection of fixed volume (10 ml) of mixtures of biogas standard and air in various ratios ... 86
4-6 Comparison of methane measurements by sensor to measurements by GC of biogas samples from a laboratory scale fluidized bed anaerobic digester. ... 87
5-1 Completed inline methane measurement connected to lab scale reactor ... 93
5-2 Photo of real inline methane measurement system. ... 94
5-3 Leakage test on the inline methane measurement system ... 96
5-4 Response of inline methane measurement chamber to AFBR biogas composition. ... 98
5-5 Comparison of inline methane measurements by sensor to measurements by GC of biogas samples from a laboratory scale fluidized bed anaerobic digester. ... 99
6-1 The commercial fertilizer usage in U.S. agriculture from 1960 to 2014 ... 106
6-2 Prices of crops paid by U.S. farmers for fertilizer and prices received by farmers for all crops from 1990 to 2016 ... 107
6-3 XRD pattern of dried sludge drained from bottom of digester ... 113
6-4 XRD pattern of ash produced from dried sludge drained from bottom of digester ... 114
6-5 Schematic flowsheet of phosphorus precipitation ... 115
6-6 Schematic of continuous sequential batch reactor ... 118
6-7 Photograph of a PPR Sequential Batch Reactor ... 120
6-8 Schematic diagram of a PPR Sequential Batch Reactor ... 121
6-9 pH adjustment by aeration in digested effluent without addition of seeding fiber ... 124
6-10 Total phosphorus content in 24 hours settling after CO2 stripping ... 125
6-12 5 g MgCl2 dosage for soluble phosphorus removal ... 129
6-13 5 g MgCl2 dosage for total phosphorus removal ... 129
6-14 2.5 g MgCl2 dosage for soluble phosphorus ... 130
6-15 2.5 g MgCl2 dosage for total phosphorus removal ... 130
6-16 1 g MgCl2 dosage for soluble phosphorus removal ... 131
6-17 1 g MgCl2 dosage for total phosphorus removal ... 131
Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
TREATMENT OF DAIRY MANURE FOR ENERGY PRODUCTION AND NUTRIENT RECOVERY
By
Shunchang Yang August 2018 Chair: Pratap Pullammanappallil
Major: Agricultural and Biological Engineering
Anaerobic digestion is a biochemical process that mineralizes organic materials to biogas (which is a mixture of methane and carbon dioxide) through the action of a syntrophic association of microorganisms under anaerobic conditions. Biogas upgraded to pure methane is referred to as renewable natural gas. Among alternative fuels, renewable natural gas is the most attractive due to its high-energy content and cleaner burning potential. This research deals with the operation of novel reactors for
conversion of dairy manure to biogas and the subsequent treatment of digested effluent for removing and recovering phosphorus.
scrapped manure, which were 66.4% and 85.2% of biochemical methane potential of fresh dairy manure respectively.
To ensure stable operation of anaerobic digesters, it is necessary to monitor biogas composition and methane production rate. Currently, instrumentation available to make these measurements are expensive, costing at least $2,000 to 3000. A low cost, arduino based device was constructed, tested and validated for making offline biogas composition measurements. The device measured methane within an average absolute error of 0.69 ± 0.55% when compared to gas chromatograph (GC)
measurements. Using 10 ml biogas sample size, methane content as low as 18% by volume could be reliably measured by the device. By increasing sample to 90 ml, methane content as low as 2.4% could be analyzed. Device was assembled for a cost of US$37. A field version that includes an LED display and power pack could be assembled for under US$50. This device was then modified for automatic, inline measurement of biogas composition, and fitted to a laboratory scale fluidized bed reactor. Inline measurements were within 1.36 ± 1.11 % of offline GC measurements.
CHAPTER 1 INTRODUCTION
Background
Climate change and environmental concerns related to usage of traditional fossil fuel resources, like coal, oil and natural gas, has diverted attention to alternate energy sources. Finding economically viable cleaner energy sources will be very important. Innovative energy sources being studied, include solar, wind, nuclear, geothermal, hydrogen syngas, fusion, biodiesel, bioethanol, and renewable natural gas.
Considerable attention is being devoted towards obtaining renewable energy from biomass resources which include dedicated energy crops, and residues and wastes from agricultural and forestry operations. Among alternative fuels, renewable natural gas is the most attractive due to its high-energy content and cleaner burning potential. Biogas is produced by microbial degradation of organic compounds under anaerobic conditions. Biogas typically contains 60 – 70% methane and can be purified to ~100% methane. Biomethane produced in this manner is usually referred to as renewable natural gas (RNG). This research deals with the conversion of livestock wastes, specifically dairy manure, to biogas and the treatment of effluents resulting from this conversion process. Livestock wastes are an ideal feedstock for renewable energy production as the wastes from many livestock operations can be easily collected in large quantities.
United States has been through significant changes in the past decades. Large dairies with more than 2000 heads of cows are becoming the norm due to economic
considerations. For example, dairies with 500 or more cows increased from 2795 to 3350 from 2001 to 2009, but operations with more than 2000 heads, more than doubled increasing from 325 to 740 during the same period. Small dairy units with less than 500 heads of cow declined by more than 35% during this period (NASS database, 2010). Total number of dairy cows was 9.14 million generating about 1.2 billion metric tons (fresh) manure annually (NASS database, 2009). Improperly managed livestock
Typically, large dairies may employ one of two methods for manure collection and storage, namely flushed and scrapped systems (Kirk and Faivor,2014). In both types of dairies the cows are housed in free stall barns with alleys separating rows of cows. In flushed dairies, water (groundwater or recycled water) is periodically pumped at the head of the alleys to flush the manure. The manure along with bedding (usually sand) is incorporated into water stream, which then collects in a secondary pit. Most of the sand settles out as the water flows over a concrete path into the secondary pit. Further separation of sand occurs in the pit. Wastewater from the secondary pit is pumped into a lagoon after separating the fibers from the manure using screens. The wastewater may pass through a series of aerated and non-aerated lagoons for
treatment. Figure 1-1 below shows the schematic diagram of a flushed system. The wastewater after treatment from the lagoons may be applied to the fields, as a means of final disposal or partly recycled for flushing the barns. Flushed systems are commonly operated with enormous quantities of water. The second type of dairies are scraped systems (Figure 1-2). The scraped dairies utilize physical tools like blade or the cut-away of a truck tire attached to the front of a tractor or a pickup truck to push the manure into a pit. Scraped systems are mostly used in northern and eastern areas of the country where climate is cold and wet. The waste pushed to the pit is a mixture of manure, sand bedding, urine and other debris. The scraped collection system
and total phosphorus are high as no water is used to dilute the manure. Waste from the pit is pumped into tankers and spread on the fields for final disposal. Disadvantages of this system are that there is no treatment of manure before disposal, so there is a high likelihood of overloading the soil with nutrients. Lack of odor and pathogen control may also cause concerns.
Figure 1-1 Schematic diagram of a flushed dairy.
Figure 1-2 Schematic diagram of a scraped dairy.
Treatment of Dairy Manure
produces 80.0 lb per day manure, including 0.45 lb nitrogen and 0.07 lb Phosphorus (USDA Natural Resources Conservation Service, Agricultural Waste Management Handbook, 1992). Many techniques have been applied to treat dairy manure such as, composting, lagoons, incineration and anaerobic digestion.
Compost piles treat the solids separated from the manure wastewater. These solids include fibers and sand, and are mixed with other organic residues like straw. Decomposition occurs at thermophilic temperatures of 45 to 71 degrees Celsius (SSSA, 1997) which controls odor and pathogens (Grewal et al., 2006; Larney et al., 2006). After composting the residue can be used as fertilizer or soil amendment to benefit crop growth. Compost piles also provide a method of disposing animal carcasses. The disadvantage of composting is the frequent turning of piles for aeration. Depending on size of piles, specialized equipment may be required for turning and moisture addition.
Manure can also be managed by incineration. (Miller and Moyle, 2014; Kaikake et al., 2009; Kalmykova and Fedje, 2013; Szogi et al., 2013) The manure is dried and then burnt, with the energy released being utilized for farm heating operations.
However, emissions from manure incinerators do not meet air quality regulations unless expensive clean up technologies are implemented. Also, the manure must be dried before incineration, which significantly increases the cost of the units. Solar drying is not practical in humid climates.
Lagoons are commonly used for livestock waste management. (Barth and Kroes, 1985; James and Houston, 1996; Mukhtar et al., 2004). Solid separation can help
employed. After treatment, the wastewater is used for irrigation or recycled for flush systems. Despite lagoons being the easiest way for preventing the surface and ground water contamination it has its disadvantages. Methane emissions from anaerobic
lagoons are not captured therefore contributing to greenhouse gas and loss of a valuable resource. Lagoons require large area. Over time solids like bed material (sand) and fibers will build up in the lagoon and has to be removed which increases the operational costs of the dairy.
Anaerobic digestion (AD) is a useful process to recover energy from wet waste streams like livestock waste. AD is a natural biological process wherein a mixed culture of microorganisms (like carbohydrates, proteins and fats) through a stable,
2007; Powers et al., 1997; Kennedy and Vandenberg, 1982; Lansing et al., 2010; Page et al., 2008)
Covered lagoons are an improvement over anaerobic lagoons. (James and Houston, 1996; Safley and Westerman, 1989; Safley and Westerman, 1992).
Anaerobic lagoons are covered with a flexible plastic, usually high-density polyethylene, sheet to capture the biogas produced from anaerobic microbial activity occurring within the lagoon. Following the covered anaerobic lagoon the wastewater flows into another lagoon (which may or may not be aerated) before being utilized for irrigation purposes. About 87% COD reduction occurs in these systems (MASSE and Masse, 2000). Except for biogas (methane) recovery other disadvantages associated with lagoon also persist with covered anaerobic lagoons.
Plug flow digesters are large sealed concrete tanks, with a mixing tank up stream (Martin, 2005). These digesters are usually constructed below ground. The dairy
manure (with or without solid separation) is first collected in the mixing tank, where it is homogenized and then directed into the digester. Walls within the digester can serve to increase the residence time by forcing the wastewater to a more circuitous route
through the digester. Hydraulic retention time (HRT) varies between 15-90 days. Even though inexpensive and producing a better quality effluent than covered anaerobic lagoons, one of the disadvantages of the system is the lack of mixing within the digester and settling of sand. The maintenance cost of these type of reactor design are also very high.
agitator to keep its contents mixed. Manure may be added to the tank continuously or semi-continuously with effluent being withdrawn to maintain a constant level. The tank is usually constructed above ground and is heated and insulated. Stirred tank digesters are simple to operate. However, these suffer from problems associated with scum formation and sand accumulation due to inadequate mixing.
Fixed film reactors have been employed for treating dairy manure (Ramasamy and Abbasi, 2000; Powers et al., 1997; Samson et al., 1984; Trinet et al., 1991) . The reactor or vessel is packed with materials to provide a solid surface for supporting microbial biofilm growth, at the same time there is sufficient void space between the packing to allow unimpeded liquid flow. Different types of packing materials have been used for example potter's clay (Kennedy et al., 1981), needle-punched polyester
(Vandenberg and Kennedy, 1983) and Bio-Pac Random Trickling Filter Media (Powers et al., 1997). Manure wastewater is usually pumped from above and it trickles over the packing materials to the bottom of the tank where it is collected and discharged. Operational efficiency of this type of digester depends on the efficiency of solid liquid separation up stream. Any solids (fiber or sand) introduced into the fixed film digester can clog the flow of liquid. Moreover, digesting only the liquid part of the manure can diminish methane productivity.
Problem Statement
are several technologies commercially available for anaerobically digesting dairy manure, there is need to improve these designs. An ideal design for treating dairy manure should occupy a small footprint, be able to handle solids in manure, and provide means to easily discharge solids accumulating within the system.
Anaerobic digestion process mineralizes carbonaceous organic compounds in waste to biogas. However, other pollutants like ammonia and phosphates are not removed in the process. In some instances, this can be advantageous as land application of the digested effluent will return the nitrogen and phosphorous nutrients back to the soil. But in many states, there are strict limits on land application of nitrogen and phosphorus. Based on the nature of soil, permissible rates of application are
determined. Many dairy farms are constrained in how much they can expand its
operations. Adding more heads of cow may entail purchasing more land for application of manure. This land may not be available or may be expensive. Therefore, in addition to removing carbonaceous compounds and recovering energy in the form of biogas, there is a need to treat the digested effluent to remove or recover nitrogen and phosphorous.
The research carried out as part of this dissertation attempts to address the above waste management needs of the dairy industry. The research objectives are as follows:
Objective 1. To investigate the feasibility and efficiency of utilizing an improved
Objective 2. To build low-cost, arduino-based, portable device for methane
measurement.
Objective 3. To modify offline methane measurement device to inline methane
measurement system.
Objective 4. To recover phosphate from digested manure by chemical
precipitation.
Research Methods
In this section methods and analytical measurements employed to address the above research objectives is discussed.
Objective 1
In this study, the induced bed reactor (IBR) was applied for treatment of dairy manure. The IBR technology was developed by Professor Conly Hanson and
collaborators at Utah State University (Dustin, 2010; Castrillón et al., 2013). For
purposes of testing this technology in the State of Florida, a mobile, trailer-mounted 600 liter pilot scale unit complete with a control room and office space was constructed by Blue Earth Technologies, Clearwater, FL. This unit was parked at the Department of Agriculture and Biological Engineering field site on SW 23rd Street, Gainesville. The IBR is designed to handle wastewater slurries with high suspended solids content. A schematic diagram of the pilot scale IBR system is showed in Figure 3. The system consists of a mixing tank, feed tank, IBR, and effluent tank. The mixing tank is
homogenized mixture to feed tank. From the feed tank the manure slurry is fed into the bottom of the IBR using a peristaltic pump. The IBR is an insulated cylindrical reactor with a conical bottom, and heated with electrical tape. The digester contents are maintained at 37 deg C. The effluent overflows by gravity via an inverted V-shaped weir. To break up any floating-scum formation, the surface of the liquid in the digester is continuously agitated by a paddle wheel attached to a motor mounted on the top of the digester. There is no other moving parts within the reactor. Biogas from the
digester is passed through a condensing coil placed in a refrigerator to remove moisture before being metered using an Alicat biogas mass flow meter.
Microbial degradation of manure components and the nature of operation of the IBR promotes the development and retention of a dense sludge bed in the bottom of IBR. As manure is fed into the bottom, it passes through the sludge bed bringing it in contact with the active bacteria which initiates rapid degradation.
For meeting objective 1, the research was divided into several tasks. These tasks included
Task 1.1. Long term operation and monitoring of pilot scale using manure from
both flushed and scraped dairies.
Task 1.2. Characterization of manure and digested effluent for total solids,
volatile solids, ammonia, total Kjeldahl Nitrogen (TKN), total phosphorous, soluble chemical oxygen demand (sCOD) and total volatile fatty acids (tVFA)
Task 1.3. Optimization of hydraulic retention time (HRT) and organic loading rate
Task 1.4. Determination of biochemical methane potential (BMP) of dairy manure
to compare methane yields obtained from IBR to maximum methane yield. For this study, fresh manure was collected from barns as soon as it was produced before it was mixed with bedding.
Three different manure waste streams were fed to IBR. Feedstock I was
obtained from University of Florida Dairy Research Unit (DRU). DRU is flushed system. The flushed manure flows into a pit from which it was withdrawn to operate IBR.
Feedstock II was acquired from Bell Dairy which also runs flush system. This site was equipped with a mechanical sand separator. The manure for operating the IBR was taken after the separator. Feedstock III was taken from North Florida Holstein Dairy which runs scraped system. The manure was collected from the pit into which scraped manure is dumped. A biochemical potential assay was conducted on all three
feedstocks separately by mixing the substrate (the material for which the methane potential is being determined) with inoculum and nutrients, and incubating the mixture in a sealed 4 liter bottle at predetermined temperature for a period of time to allow
incubator maintained at 37 deg C. The contents of the bottle were stirred continuously using a magnetic stirrer. The assays were conducted until methane production ceased.
Objective 2
In this objective, researching for a novel idea, and low cost for methane measurement was focused. Compared with current developed device or ideas on methane measurement, it was either too expensive or difficult on measuring the methane composition of sample. This objective was using a cheap methane sensor MQ-4, assisted by micro-computer arduino, to carry out methane measurement both laboratory and field.
Task 2.1. Constructed a low cost, portable device for methane composition
measurement.
Task 2.2. Validate the accuracy of offline methane measurement device.
Objective 3
As running the continuously anaerobic digestion in IBR, convenience and accuracy become our major concerns for monitoring the performance of the anaerobic digester. Encountered many problems like humidity, temperature interruption, failure of recording biogas production by real-time and so on, meanwhile, the expensive cost would be a concern if inline measurement was integrated in the anaerobic digestion system, a novel and low cost device for methane measurement was investigated in this research. Since the inline measurement for the methane composition and biogas
To complete this objective, the following had been accomplished by investigating:
Task 3.1. Combined the offline device and the anaerobic digester, constructed
an inline methane measurement system.
Task 3.2. Integrated the real-time biogas production rate measurement device
into the inline methane measurement system, ultimately optimized the fermentation by automating the loading rate by the principle of constant methane yield.
Objective 4
As mentioned previously anaerobic digestion process does not remove nutrients like nitrogen and phosphorous from digested dairy manure. These pollutants that
remained in the digested effluent should be removed or reduced prior to land application as there are limits imposed on soil nutrient loading. Phosphorus and nitrogen are the main contaminants that decreased the water resource quality with surface run-off and leaching causing eutrophication (Petersen et al, 2007). Table 1-1 listed the
phosphorous content of manure and digested manure. There is some reduction in phosphorous content when the manure is digested. It is possible that some of the phosphorus is precipitated within the digester and settles into the sludge bed. Recovery of these residual phosphorous from the manure effluent can further reduce impact on the environment. In this objective, it was investigated whether the phosphorous in the digested manure effluent can be decreased by chemical precipitation.
Digested manure effluent was collected from the IBR effluent tank. A 5 liter conical bottomed reactor was used for the precipitation process. This precipitation reactor was operated in batch mode with a reaction step followed by a settling step. Software Visual MINTEQ was used to determine the amount of chemical reagents to be added for optimizing struvite production. Struvite (NH4MgPO4) is the desired
compounds to be produced by precipitation. Struvite can be used as a slow release fertilizer. The following factors was investigated in this research. The effect of aeration on raising the pH of the manure. Struvite precipitation reaction occur at an optimum pH of 8.9 to 9.3.
Digested effluent pH is close to neutral so there is a need to raise pH to
encourage struvite precipitation. Aeration is less expensive than adding chemicals. The effect of fibers and other particulate matter in assisting settling of the precipitated
phosphorous was also studied. Phosphorus was measured by inductively coupled plasma- atomic emission spectroscopy (ICP) as outlined in USEPA method 200.7. Both soluble and total elemental phosphorus was measured. Total phosphorus was
CHAPTER 2
BIOCHEMICAL METHANE POTENTIAL OF DAIRY MANURE
Introduction to Dairy Manure as Feedstock
Among all the livestock waste, manure is the one of the most promising alternative biomass. It can be highly efficiently converted to environmental clean and high energy value fuels. The ascendant of taking advantage of this agricultural waste are obvious: 1) Dairy manure production are enormous. In USA, 80 pounds manure per day including 0.45 pounds nitrogen and 0.07 pounds phosphorus produced by 1000-lb dairy cows in 1992. The dairy industry had tendency to rise up the large dairies with more milk cow capacities. The significant changes of structure of dairy industry aids the possibility (operational feasibility and economical cost) of facilities treating waste from dairy and attained more and more attractions for utilizing the manure waste from milk cows. 2000 heads dairy unit can be holding 2,800,000 pounds manure waste per day. 2) Climate change and environmental concerns urge human to find alternative biofuels, ultimately reducing the carbon release by re-use the biomass for energy purpose. Comparing to traditional fossil energy, like coal, oil, and natural gas, dairy manure also can be utilized by incineration. The disadvantage of burning dairy manure is the high moisture content reduced the energy efficiency, meanwhile mass of fume formed and released causing environmental concerns. Composting technics also been used for utilizing the manure waste from dairies, thought the frequent turning over the
emission. Instead of physically store the manure waste, anaerobic digestion technics had already been widely applied for alternative fuels generation. The main product biogas produced by anaerobic fermentation most composing biomethane and carbon dioxide. The biomethane is considered as higher heating value and cleaner fuels. The higher heating value of methane is around 55 Mj/Kg which is higher than traditional fuel, like coal which has higher heating value of 32.5 Mj/Kg. The cleaner combustion product only including water and carbon dioxide.
Disadvantage of Current Methods Utilization of Dairy Manure
the slurry to the land, but also the viability of reducing the cost for maintaining the operation of the pumps.
Both methods require secondary pit for manure collection and storage. Along with the odor and pathogens control problem for human, the degradable organic matter lost is another concern. The longer retention time of flushed dairy lagoon and scraped dairy pit, the degradable manure organics stay longer in the secondary pit. The organic matter can be fermented to methane which is greenhouse emission. Methane has
In regarding to all the factors impacted on the manure fermentation, it is very important to determine the ultimate methane potential from the dairy manure waste. Biochemical methane potential (BMP) was determined by batch loading the feedstock into BMP assays, mixing with microbial consortia in inoculum. Under optimized
conditions, the microbial degradation consumed the organic matters of the feedstock. Total methane produced during the fermentation period contribute to the methane yield. As the BMP of the manure biomass was determined, with minimization of the
degradation or conveying lost, the efficiency of biofuel production from manure biomass can be enhanced by improving the methods of manure collection and storage. In this study, BMP research was conducted to establish the maximum potential methane yield of fresh manure as feedstock. Before the manure was diluted by enormous water or settled in a scraped pit for long time, the manure waste was directly loaded into anaerobic digester to produce biogas which including methane and carbon dioxide.
Materials and Methods
Dairy Samples
Three dairy operation units was chosen for BMP assays experiment. Fresh manure, manure slurry, manure fiber after sand separator were used as feedstock. Dairy manure waste was acquired from three different dairy operation sites: University of Florida Dairy Research Unit (DRU), Bell Dairy and North Florida Holstein. The DRU and Bell dairy ran flush system for the dairy manure conveyance and collection, while the North Florida Holstein applied scraped system as the method of collecting dairy manure. The fresh manure was obtained from the dairy cow barn without any
samples were taken. Fresh manure samples were taken from University of Florida Dairy Research Unit (DRU) and Bell Dairy. In purpose of getting the fresh manure clean and without diluted by the water on the ground, a hand hold shovel was used to catch the original milk cow feces. In the free stall barns, the opportunity for taking samples are time critical. Most of time the milk cow was laying in the bedding, only when they stand up the shovel was prepared right after the tail. The advantage of taking fresh manure sample by this method is that, minimized the interferences from the urine or sprayed water in the barn. Manure fibers was collected in Bell Dairy (BD). BD ran ground water to flush the barn, the weir was used for the temporary storage of the manure fibers, a sand separator was installed to separate the bedding materials. All the flushed manure waste in water stream was screened by the sand separator. The manure fiber was sorted out and water stream gravitational flowed into the lagoon. The separated manure fiber was used as BMP samples. DRU also used lot of sand for filtrate the flushed waste water. The float fiber was collected from the top of the weir. The manure slurry samples were withdrawn from Florida North Holstein Dairy (FNHD). FNHD used scrape truck for collecting the manure waste from the barn. Concentrated manure slurry was stored in the secondary pit due to minimized usage of water. The manure slurry sample was taken by scoop the slurry sample out of the pit. The sand or other bedding materials were settled at the bottom of the pit, so bottom samples are avoided, only middle layer samples were loaded into BMP assays.
BMP Assays Experiment
degradation of the substrate. Assays 1-6 were tested. Table 2-1 listed the contents of assays. Each assay was conducted in 5L modified pyrex glass digester tank with working volume of 4L at mesophilic temperature of 37°C. The digester tank had dimension of 0.406 m height and 0.061 m inner diameter. A glass flange was used for sealing the top of the digester tank. Rubber O-ring was fitted between the glass flange and top of digester which was clamped with stainless steel clamps. Assays contained substrate, inoculum and other supportive solutions (micronutrient, macronutrients, and pH buffer). The inoculum used was collected from a pilot scale anaerobic digester in which that had been digesting various biomass feedstocks and wastewater at
mesophilic (35 °C) temperature for over years. Nutrient stock solution was prepared according to Owens et al (1993). Before the feedstock was loaded into the BMP assays, 2 g/L sugar was added to initial the fermentation. After the produced methane yield reached to theoretical methane yield of sugar, another 2-3 days was used for stabilizing the digester. Without changing the inoculum in the digester, the feeding of prepared manure samples was then started. Different feedstock was not loaded into the digester tank simultaneously. Each feedstock was loaded following after the previous substrate been completed degradation. The digester tank was placed on stirrer hotplate (Thermo scientific,USA). The temperature on the hotplate was set up at 35 °C. Inside the digester tank, stirrer bar was used for mixing the inoculum and the substrate. The stirrer was turned on 24 hours 7 days. To better reduce the inferences from the temperature, the entire digester tank was set in a modified chamber as showed in figure 2-1. A nylon test tube went through one of the ports on the glass flange. One side of the tube was
tube left in atmosphere with a rubber stopper. The analytical sample was withdrawn by the nylon tube with a 50 ml syringe. The sample was taken periodically. The biogas (mainly methane and carbon dioxide) produced from the digester tank was pushed through a little mason jar. The mason jar filled with soda lime which was used for
Figure 2-1. Schematic of BMP assay
Analysis
(pore size 0.45 um). The processed samples were used for sCOD measurement. The tCOD was analyzed by original liquid samples. Both sCOD and tCOD samples were analyzed using HACH COD digestion vials which has detection range of 0-1,500 ppm (Method 8000), the vials were heated up in the Hach COD reactor for 2 hours around 150°C. Then the vials were measured by the colorimeter DR890 (Hach, USA ) for the COD values. The BOD was analyzed by Hach BOD Trak II in 5 days. The diluted BOD samples were mixed with polyseed inoculum in brown BOD bottles, and kept stirring during the measurement. The brown coat on the BOD bottles was used for blocking visible light. The capacity of the BOD bottles was 473 ml. On the top of the BOD bottles, rubber holder was used for lithium hydroxide. The lithium hydroxide was added to
remove the carbon dioxide released during the bacterial metabolism. Plastic cap fitted rubber pad was used for sealing the bottle bottles. Plastic cap also had tubing
connected to Hach BOD Track II. After 5 days, BOD value for each feedstock can be read from the Hach BOD Track II. Total volatile fatty acid and alkalinity concentration were measured by acid titration method (Anderson et al., 1992). This method had been calibrated using mixed acids: different acetic, propionic, butyric acid concentrations.
Nutrient components phosphorus and nitrogen were not consumed during the anaerobic digestion process. The balance of the nutrient composition of the loaded feedstock also had been measured. The samples were sent to University of Florida Analytical Service Lab (ASL) for phosphorus (P) and nitrogen (N) concentration
Results and Discussion
Characterization of the Different Dairy Manure Feedstocks
The total solids (TS) and volatile solids (VS) were analyzed under 105°C and 550°C condition. The dairy manure samples were processed around 2 days and 2 hours for TS and VS analysis, respectively. The equation 2-1 and 2-2 were used for
determining the TS and VS. TS =𝑀2 − 𝑀 𝑀1 − 𝑀 (2-1) VS =𝑀2 − 𝑀3 𝑀2 − 𝑀 (2-2)
Where M is the weight of the bolt, M1 is the weight of the manure sample with bolt, M2 is the weight of the dried manure sample with bolt under 105°C till the weight doesn't change, M3 the weight of the burned sample with bolt under 550°C, M, M1, M2, M3 was constant before recorded.
The moisture of the separated manure sample can reach up to 30%. The scraped dairy was collected from the scraped manure pit. Table 2 shows the
characteristics of waste samples from DRU dairy, BD and FNHD. The TS (w/w) and VS (w/w) of fresh manure varies from 16.1% to 18.4% and 66.0% to 86%. The differences may come from the diet recipe of each dairy. TS and VS from manure slurry samples varies from 5.5% to 21.2% and 62.7% to 93.3%. Sand was used as bedding of the barn, devoting high percentage to most of the collected dairy manure. Bell Dairy used sand separator to divide the manure fiber and the waste stream into manure pile and lagoon respectively. The VS composition raised up to 93.3%. The Florida North Holstein Dairy scraped the sand bed and the manure waste in to a concrete pit, without any
High solids dairy manure (49.10%) had been used for anaerobic digestion in leaching bed reactors (Demirer et al, 2008). Mixture of diary manure and the co-digestion of sugar beet had been used for anaerobic co-digestion with 2.6-3.9 g dry matter/L/day loading rate (Umetsu et al., 2006). Dairy manure (9.68% volatile solids) had co-digested with food waste with 0.67-3 g volatile solids/L/day loading rate (Agyeman and Tao, 2014). 17.2% w/w dry solid and 82.7% w/w volatile solids for the cattle manure was used for thermogravimetric analysis of anaerobic digestion (Otero et al., 2010).
Nutrient Balance of BMP Assays
The BMP feedstock contained some P and N which good nutrient amendments for soil were. 5 grams of each feedstock of BMP assays were sent to ASL for nutrient analysis. Total P and orthophosphate content were 458.78 mg/g and 245.61 mg/g in the fresh manure, respectively. Sand separated manure fiber was also analyzed for total P and orthophosphate which had corresponding results of 238.26 mg/g and 122.23 mg/g. The P and N cannot be removed during the anaerobic digestion process. Some of the P and N was precipitated due to the pH change or the inferences of other precipitation compounds formed. After the BMP assays completed, the sample in Pyrex glass tank was withdrawn and analyzed for P and N. The total P and orthophosphate were measured, 538.23 mg/L and 129.34 mg/L respectively.
Methane Potential
from different site including DRU and Bell dairy were analyzed. The dairy manure fiber which had been flushed and screened by a sand separator were also analyzed. The water after the separator which flew into the secondary storage lagoon were collected for BMP measurement. The last sample were obtained from scraped operation dairy farm. The manure slurry samples were pumped from the slurry pit using a heavy duty pump.
Table 2-1. BMP assays feedstock characteristics
Assays Substrate TS(𝒈
𝒈%) VS(
𝒈
𝒈%)
1 DRU Fresh Manure 16.1±1.1 86.0±2.6
2 Bell Fresh Manure 18.4±1.3 66.0±9.8
3 Separated Fiber 21.2±6.1 93.3±0.5
4 Bell Dairy Pond Water 1.8±0.4 95.6±0.6
5 Scraped Fiber 5.5±1.3 62.7±3.3
Table 2-2. Methane yield of different feedstock based on BMP
Dairy samples Methane yield
(L/g TS)
Methane yield (L/g VS)
DRU Fresh Manure 209.8±5.6 237.8±10.2
Bell Dairy Fresh Manure 176.0±16.3 268.8±18.6
Bell Dairy Separated Fiber 100.2±28.0 115.0±22.3
Bell Dairy Fiber Separated Pond Water 38.4±0.2 40.2±1.3
Scraped Dairy Manure Pit 113.1 190.7
BMP of fresh dairy manure
DRU fresh manure was conducted duplicated measurement in BMP assay A-1, 113.3 grams (TS: 17.06%, VS: 88.21% of TS) and 232.4 grams (TS: 15.05%, VS: 83.7% of TS) of the feedstock was loaded sequentially. The daily methane production was recorded by Alicat gas flow meter. The cumulative methane yield of DRU fresh manure was showed in figure 2-2 and figure 2-3. The cumulative methane production stayed unchanged which indicated the 1st BMP run was completed. The 2nd run was initiated consecutively after the 1st run. The total methane production was around 3.9 L and 6.9 L at STP condition respective for Run 1 and Run 2. The ratio of different
degradation efficiency was because of the microbial consortia was activated and
populated in the Run 1. When the second batch of feedstock was loaded, the lag time of the BMP assays was shortened. The Run 3 showed slower degradation rate due to the waiting period between Run 2 and Run 3. Without loading feedstock into assay, the microbial bacterial were inhibited lacking organic loading. Therefore, with the third batch of organic waste loaded, the methane was produced at a lower rate.
Figure 2-3. Cumulative methane yield of volatile fresh Manure solids from DRU and Bell Dairy
BMP of separated manure fiber
TS: 23.77%, VS: 94% of TS). The triplicated separated manure fiber BMP experiment showed average methane yield around 115 ml CH4/ g VSS.
Figure 2-4. Cumulative methane yield of total separated fiber from Bell Dairy
clean the livestock waste in the barn, most of the soluble organic matter was diluted and lost in the water. The BMP of the separated water was to confirm the methane balance from the dairy manure. As showed in figure 2-6 and figure 2-7, the methane yield of lagoon water was around 60 ml CH4/g VSS. The combination of separated manure fiber and separated water methane yield was only around 175 ml CH4/g VSS (around 71.5% of fresh manure methane yield) which was still lower than the methane yield of fresh manure. This confirmed the flushed operation system for the dairy manure would lost about 29% of useful organic matter into the lagoon water.
Figure 2-7. Cumulative methane yield of volatile pond water solids from Bell Dairy
BMP of scraped dairy manure slurry
production using anaerobic digestion. Although the high methane yield can be reached using scraped dairy manure from the pit, the high content of sand made it difficult to operate which including jamming problem and maintenance cost.
Figure 2-8. Cumulative methane yield of total solids of scraped dairy manure pit
CHAPTER 3
PILOT SCALE INDUCED BED REACTOR FOR THREE DIFFERENT DAIRY UNIT WASTE TREATMENT
Introduction to Pilot Scale of Anaerobic Digestion
digestion in semi-continuously lab scale reactors (3 L Erlenmeyer flask). The methane production efficiency of dairy manure was discovered by the small scale of anaerobic digestion (Dustin and Hansen, 2011; Page et al., 2015; Güngör et al., 2009; Vedrenne et al., 2008; Gavala et al., 1999). Based on the operations knowledge of laboratory scale and the optimized conditions for running the anaerobic fermentation, pilot scale had been developed for on-site treating dairy manure (Zemke et al., 2011; Zaher et al., 2008; Coats et al., 2012; Lansing et al., 2008; Hills and Mehlschau, 1984; Wright et al., 2004; Safley et al., 1992; Demirel et al., 2005; Martin, 2005). Pilot scale digester
including anaerobic digestion lagoon, plug flow digester, fixed film digester, induced bed reactor and so on.
Compared to other pilot scale digesters which listed in table 3-1, induced bed reactor (IBR) had several advantages which including high solid loading rate, preventing scum formation, short hydraulic retention time and so on. As table 3-2 listed, the energy recovered from biogas produced from anaerobic digestion process in pilot scale dairy farms differ from different operation of digesters. Induced Bed Reactor showed the best biogas production among these digester designs which had around 4.05 m3
biogas/cow-day. The methane composition was around 65.2%. Highlight of this
Table 3-1. Disadvantages of several onsite pilot scale digester
Methods Advantages Disadvantages
Covered lagoon Easy to collect the biogas. Very convenient for flushed
dairy design. Operation at ambient condition lower
the cost of operation.
Biogas production vary seasonally due to temperature change. Low
efficiency of degradation. Require large land area. Stirred tank reactor Simple to operate Sand accumulation in the
reactor, scum formation. Large volume required.
Temperature control required. Plug Flow Simple to operate. Usually
below ground level, so temperature fluctuations
are minimized.
Long hydraulic retention time, high maintenance cost, sand accumulation
problem. Fixed film Able to maintain higher
microbial concentration, so high organic loading rates and low hydraulic retention
time possible.
Require solid-liquid separation, easily clogged,
the maintenance cost is relatively high.
Table 3-2. The energy recovery efficiency of different pilot scale anaerobic digester
Methods Energy Recovery Efficiency
Materials and Methods
Induced Bed Reactor
The IBR system (Fig.3-1) consists of total three containers which are mixing tank, feed Tank, main anaerobic digester. A semi-continuous feed system was composed. The mixing tank was operating at size of 200 Liters. A circulating pump integrated with the cutter had been install in the mixing tank. The feedstocks come from the dairy waste consisting high amount content of long fiber. These fibers can easily jam the IBR
system. With the cutter pump, the waste solids were blended into fine waste stream, and 10 minutes’ circulation made the samples in the mixing tank identical. The mixed dairy samples then were transferred into the second tank - feed tank. The feed tank was also operating at size of 200 liters. The waste stream was temporary stored in the feed tank. A peristaltic pump was installed between the feed tank and the main anaerobic digester. The feed pipe was used for the waste stream transportation. The peristaltic pump was turned on periodically per the designed feeding rate. In our study the
hydraulic retention time was intending to optimize around 6 days. The dairy waste was kept well mixed in the feed tank by installed stirring motor. A sight glass level gauge was installed on the feed tank. When the level reached to certain point, the dairy waste sample will be prepared in the mixing tank, and then will be transferred in to the feed tank.
The main anaerobic digester was operating at size of 600 liters. The digester was maintained the temperature around 35°C to 40°C. The insulation was applied outside of the whole digester for less interruption from the environment. The heating oil was
applied around the feed pipe. The waste stream was heated up before they were pumped into the digester in case of temperature shock. A sight level gauge was also installed to check the level inside the digester. The overflow side-arm was designed in our study to maintain the volume of digester. The overflow digester effluent was
IBR Operation Performance
The improvement of the digester fermentation depends on several factors. Organic loading rate, hydraulic retention time, the characteristics of the sample and so on. Manure fiber from the University of Florida Dairy Research Unit (DRU) for around 8 months. Because of the sample can’t be consistent, too much sand was presented in the sample, the performance of the digester is not quite good. The weir in DRU was designed to overflow the flush water, but due to the mis-operation, the manure was pre-fermented in the weir. Thus, these caused reduction on the bio-gas production from the manure sample. Meanwhile, the phosphorus and ammonium were released into the waste water when the manure sitting in the weir. The detected amount of phosphorus and ammonium from the effluent of the digester is very low due to large amount of flushed water was used for running off the dairy farm barn.
Then the fresh manure was sampled from the barn directly, it was collected before it was flushed into the weir using lot of water. The total solids and volatile solids content is around 17% and 88% respectively. The sample still contained lot of sand. Pipe sand cleaning was operated by flush well water into the pipe system in the
BMP experiment results, the expected methane yield of fresh manure should be around 233.7 ml/g.
The temperature, pH, daily methane and daily biogas production, methane yield, chemical oxygen demand (COD), volatile fatty acid (VFA) were measured for the IBR. The results demonstrated the performance of the digester. The ph is one of indicators for overloading of the digester. The digester produced a lot fatty acid as intermediate product during anaerobic digestion. Maintain the pH is very important. Under optimal pH the bacterial in the digester can maximize the fermentation efficiency. The intermediate production would be converted to methane and hydrogen would be consumed in the reactor. Otherwise the anaerobic digestion will be inhibited. The environment of the IBR would become acidic. The methane production rate would reduce. Thus, daily pH test was applied. Another indicator is VFA, three main acid including Acetic Acid, Propionic Acid, and Butyric Acid. After the fresh manure was loaded, the VFA stays around 10 mM, which corresponding had COD around 4 g/L.
composition was able to be measurement once daily. With combination of biogas production and the methane composition, the methane production rate was able to calculate. This showed the digester could consistently produce around 600L biogas and 300 L methane per day for DRU samples. The methane production is low due to the solid content of the feedstock batch was relative low (3%). So, the solid content of the batch was increased gradually. The yield is from 159 ml/g to 273.49 ml/g, comparing to the fresh manure yield which is 233.7 ml/g is very close. Also, the error on the methane production from the Alicat need to be considered. The corrected results should be more close to the real biogas yield.
Results and Discussion
IBR Operation of Dairy Manure from Dairy Research Unit (DRU)
when the manure fiber mixed with lagoon water, and left within 2 hours soaked. Most of the fiber floated to the top of the drum, and sand settled to the bottom. As showed in 3-2, the leachate of the dairy manure mixed with lagoon water was used as the feedstock for the IBR digester. The most priority was to remove the long chain fiber from the dairy manure. And the scum formation problem periodically caused the piping system
clogged. Thus, after the dairy manure mixed with lagoon water, the mixture was left without disturbance for 2 hours, and removed after that. The leachate was pumped into the mixing tank directly without transfer the bottom sand. The daily biogas production continue to rise till day 180. Concern about the operation system of the dairy farm was flushed dairy, most of the volatile solid was wash out by the running water. The fresh manure was used as feedstock instead of the leachate of the dairy manure mixing with lagoon water.
Table 3-3. Characteristics of different dairy manure samples
Dairy sample Total solids (g/g) Volatile solids (% of TS)
DRU dairy 19.26±2.56 43.21±5.61
Bell Dairy 21.26±2.14 76.46±3.17
IBR Operation of Dairy Manure from Flushed Dairy (Bell dairy)
As showed in figure 3-4, the bell dairy manure sample had been used as
IBR Operation of Dairy Manure from Scraped Dairy (North Holstein Dairy Farm)
CHAPTER 4
LOW-COST, ARDUINO-BASED, PORTABLE DEVICE FOR MEASUREMENT OF METHANE COMPOSITION IN BIOGAS
Introduction of Current Methane Measurement
Methane is naturally produced as a result of microbially mediated processes under anaerobic conditions. Methane measurements are important due to methane’s role as a potent greenhouse gas and as a fuel. Natural sources of methane include ruminants, anaerobic sediments, sewer, manure pits, composting heaps and landfills (Sutaryo et al,2012; Edelmann and Schleiss, 2000; Ishler, 2017; Agostini et al, 2016; Liu et al, 2015a; Huber-Humer et al, 2008; Yamulki, 2006; VanderZaag et al, 2011).
Anaerobic digesters are engineered systems that convert organic feedstocks to biogas, which is a mixture of methane and carbon dioxide. These systems have been
2015). Drawbacks of these instruments are that either they are expensive or require large amounts of gas sample or both.
MQ-4 (Hanwei Electronics Group Corporation, Zhengzhou, China; MQ-4 sensor technical data, 2017) is a low cost (approximately US$ 6) methane sensor that can measure methane between 200 and 10,000 ppm and can be used if the environment temperature is between -10 and 50 C and the relative humidity is less than 95%. It can be connected to an Arduino circuit board for data logging and utilizing the data collected for triggering alarms, which makes this an inexpensive system for detecting natural gas leaks and for monitoring environmental methane. Ahmed et al (2017) proposed using an MQ-2 sensor (which is a forerunner of the MQ-4 sensor) to measure methane content of biogas from anaerobic digesters. However, the focus of their paper was on connecting the MQ-2 sensor to an Arduino board and the graphical user interface to display the results from the sensor on a personal computer in real time. Methane composition results presented in the paper varied between 100 and 1000 ppm, making it unlikely that the samples analyzed were biogas samples from an anaerobic digester. Methane content of biogas typically varies between 50 and 70% by volume on a moisture free basis. This poses a challenge in using the MQ-4 (or MQ-2) for biogas analysis as the methane content is above the measurement range of the sensor. Biogas requires dilution before the methane content can be measured. In addition, biogas has a high moisture content, which may affect the sensitivity of the sensor (Bârsan and Weimar, 2003; Anisimov et al,2007).
the sensor and to maintain appropriate environmental conditions for its optimum response, a sample injection port, an Arduino data logger and controller, and the requisite electrical connections, is described. The functionality of the device was validated by testing for leaks and by ascertaining the reproducibility of the
measurements, the effect of environmental conditions, the linear range of the sensor, and the response time of the sensor. The device was then used to measure the methane content of biogas produced from a laboratory scale fluidized bed anaerobic digester treating stillage from a cellulosic ethanol plant. In all cases, the methane measurements obtained from the device were compared to measurements using a gas chromatograph equipped with a thermal conductivity detector.
Materials and Methods
Methane Measurement Device
breadboard was connected to a programmable Arduino Uno R3 clone (Elegoo UNO R3 Board, 2017) outside the jar for collecting the data from the sensors. Two holes were drilled through the lid of the jar, one to securely pass electrical cables and the other for inserting a nylon adapter. The electrical cables were used for transmitting signals from the sensors and providing power for the breadboard and they were passed through a 1/8” male, brass, hex nipple fitting. The nylon adapter was fitted tightly into the hole and was connected to Tygon tubing on the outside. A rubber septum was inserted into the tubing for sample injection and sample extraction. The tubing was clamped with a pinchcock to prevent gas leakage. One minute instant mix epoxy (Loctite, USA) was used to seal the gaps between lid, brass fitting, and nylon adapter as well as to plug the hole in the brass fitting through which the cables were passed. A personal computer was used to download the data from the Arduino.
Operation of Device
Castro (2016). The methane content by volume of injected sample (X) was calculated as follows:
X = M(Vinj+Vchamber)/Vinj (Eq. 4-1)
where M = methane content measured by the MQ-4 sensor Vinj = volume of injected sample (ml)
Vchamber = volume of chamber and tubing into which the sample is injected = 405
ml
The response of sensors (temperature, relative humidity, pressure and methane content in the chamber) was logged every second and stored in an SD card connected to the Arduino. Subsequently, the data collected by the Arduino was downloaded to a PC for visualization and analysis.
Device Validation Tests
Several tests were carried out to validate the function of the device. After
assembly, the device was first checked for gas leaks. Six injections of air, each 10 ml, were introduced into the chamber every twelve minutes, and the pressure of the
chamber was monitored. These injections were done without returning the chamber to atmospheric pressure. After the sixth injection, the pressure was monitored overnight for twelve hours. The data collected for leak detection was also used to verify the linearity of the pressure measurements. In the next test, the effect of humidity on the baseline response of the sensor to injection of air was observed. Three different
volumes (15ml, 20 ml and 25 ml) of air were injected into the device. Sensor signal and relative humidity was monitored for 8 to 10 minutes after each injection.
Standard biogas samples ranging from 5 ml to 90 ml (in 5 ml increments), for a total of eighteen different samples, were injected into the device chamber. Methane, relative humidity, pressure and temperature were monitored for 8 to 10 minutes after each injection. Before venting the gas from the device chamber, a 1 ml sample was
withdrawn and injected into the GC. The chamber was then vented by opening the lid. The lid was closed when methane dropped to less than 0.05 volume %. After waiting until relative humidity dropped below 1%, the next sample of standard biogas was injected. Injections for each volume were repeated three times.
The above steps were repeated using a constant volume (10 ml) of gas sample. The methane composition of gas sample was varied by mixing standard 60:40
(methane: carbon dioxide) biogas with air in different ratios. 10 ml mixture of biogas standard and air, each containing the following volumes of biogas standard were used: 0.3, 0.4, 0.5, 1, 2, 3, 4, 5, 5.3, 6, 7, and 8 ml. As with the previous test, injections for each sample were repeated three times. Following these tests, samples of varying ratios of standard biogas and air were prepared; 10 ml were injected into the device and 1 ml into the GC. The methane measured by device was used to calculate methane content of injected sample using equation 4-1. Measurements on each sample were repeated three times.
Gas Chromatograph (GC) Measurements
A gas chromatograph (Gow-Mac series 580, Bethlehem, PA, U.S.A.) equipped with a thermal conductivity detector was used to measure methane content of injected samples as well as gas samples extracted from the measurement chamber. A
SUPELCO (Sigma-Aldrich Corp, St. Louis, MO, USA) analytical column 80/100
The GC can detect CH4, CO2 and Air (as sum of O2 and N2). The operating conditions of the GC were: column temperature 61°C; detector temperature 152°C; and injector temperature 81°C. The GC was calibrated using standard gases containing 60% CH4 and 40% CO2, and 30% CH4, 20% CO2, 11% O2 and 39% N2 purchased from Airgas. The injection volume was 1 ml.
Application of the Device
The device was used to measure the methane content of biogas produced from a laboratory scale anaerobic fluidized bed digester. The fluidized bed digester has been operational for over three years being fed with stillage from a cellulosic ethanol pilot plant at Stan Mayfield Biorefinery, University of Florida (Tian et al, 2013;
Pullammanappallil, 2013). Biogas samples were withdrawn from the digester every half-hour to one hour, a portion of which was analyzed using the GC and 10 ml was injected into the device. Feed flow rate to the digester was deliberately varied during the measurement period to produce changes in methane composition of the biogas. The digester was monitored over two days with gas measurements being conducted over a 12-hour period each day.
Results and Discussion
Device Integrity
calculated by subtracting the initial pressure of air in the chamber before the first
injection. The mean and standard deviation of gauge pressure were calculated for data collected between 2 and 10 minutes after each injection. As there were no significant pressure changes during the 12-minute intervals, the tests indicated that leakage was minimal. The sixth injection pressure, after the accumulated injection volume reached 60 ml, was monitored overnight for twelve hours. To compensate for changes in pressure due to changes in temperature, the ratio of absolute pressure to absolute temperature was calculated. This ratio decreased from 0.3543 to 0.3533, by only 0.001 (corresponding to 3 kPa), in 12 hours, which again indicated very low leakage.
Linearity of Pressure Sensor
As showed in Table 1, the average pressure increased with each injection. The average gauge pressure shows a linear trend with volume which was correlated as follows:
Pgauge = 0.2102V, where Pgauge = gauge pressure in kPa and V = volume in ml.
The equation can be rearranged as V = Pgauge / 0.2102. This means that it is not necessary to inject a known volume of gas to use in equation 4-1. The change in
pressure after injection of a sample, as measured by BME280 sensor, can also be used to calculate volume injected.
Effect of Humidity and Pressure on Baseline Sensor Signal
