Optimization of agricultural anaerobic co-digestion with diverse feedstocks

Final report for GNC21-334

Project Type: Graduate Student
Funds awarded in 2021: $14,978.00
Projected End Date: 06/01/2023
Grant Recipient: Purdue University
Region: North Central
State: Indiana
Graduate Student:
Faculty Advisor:
Dr. Jiqin Ni
Purdue University
Faculty Advisor:
Nathan Mosier
Purdue University
Expand All

Project Information

Summary:

Anaerobic digestion (AD) allows farmers to generate renewable natural gas (methane) from manure, earn carbon credits by reducing greenhouse gas emissions, and sustainably treat manure and other organic waste to reduce their environmental impact. Agricultural anaerobic digesters can increase production of renewable natural gas and profit by co-digesting agro-industrial feedstocks in addition to manure; however, the selection process for the type and amount of each feedstock is challenging. Previous researchers have shown that biomethane potential (BMP) tests on these feedstocks can predict methane production in full-scale digesters, but many anaerobic digestion operators do not have the knowledge or resources make this evaluation. In addition, BMP testing can be time-consuming and expensive, particularly if a third-party must be involved. There is a critical knowledge gap among digester operators in evaluating potential co-feedstocks and among scientists in performing this evaluation rapidly and efficiently. Our solution is to both increase AD operator knowledge of co-digestion and develop techniques to predict feedstock performance in full-scale farm AD.

 

The primary goal of this project is to increase adoption of anaerobic co-digestion in the North Central Region through 1) reducing risk of entry for farmers through by predicting co-digestion performance of feedstocks commonly available for agricultural digesters and 2) training farmers about co-digestion. Partnering with local farming company Bio Town Ag in Indiana and other farmers, I will work with faculty experts at Purdue University to study eight feedstocks used used in agricultural digesters. From this research, I will publish two academic journal articles, as well as use the results for my dissertation and present at an academic conference. A workshop training will be held on-site at Bio Town Ag for farmers interested in adopting or optimizing their use of co-digestion. Two webinars will be held prior to this event to give additional information on this and related topics. In addition, we will present our findings at the Waste to Worth Conference, reaching additional farmers and extension specialists. The workshop, presentation, and webinars will spread awareness of the benefits of co-digestion for agricultural digesters, enabling farmers to both increase profitability and improve environmental stewardship through increased generation of renewable energy, reduced farm carbon footprint, and improved air quality.

Project Objectives:

Learning Outcomes:

  • Outcome 1: Farmers will understand the economic value of several possible agro-industrial co-digestion feedstocks for use in their farm digesters. This will be measured by the number of feedstocks (target: eight) tested for their co-digestion methane yield and digestion rate and correlated to simple predictors that can be a first indicator of possible value. The results will be important for Bio Town Ag and other farmers considering co-digestion when selecting future feedstocks.
  • Outcome 2: I will develop a model that will predict how to improve yield from existing digesters using experimental results of laboratory tests. 
  • Outcome 3: Farmers and extension specialists will be able to evaluate to evaluate possible co-digestion substrates on-site through a training workshop located at Bio Town Ag, and a presentation in the Waste to Worth Conference in Ohio. These workshops will spread knowledge to agricultural AD operators in the North Central Region and provide a collaborative forum for networking with other digester operators.

Action Outcomes:

  • Outcome 1: Bio Town Ag and other livestock farmers will apply the research results to optimize digester feeding strategy, increase renewable natural gas production, and improve sustainability.

Cooperators

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Research

Materials and methods:

We conducted three different experiments to evaluate both mono-digestion and co-digestion performance of various feedstocks from third-party companies that our farmer partner regularly co-digests in their digester. These samples were provided by the farmer partner.

Overview of Experimental Procedure for Experiment 1

  • The tests were conducted using 30 1000-mL lab-scale anaerobic digesters at Purdue University.
  • The anaerobic digestion was performed at mesophilic conditions of 101°F (38.3°C), using temperature-regulated water
  • For most digesters, the test lasted for 30 days, until daily biogas production a small fraction of the cumulative biogas production. Eight digesters (2 × Blank, 3 × Slaughter-house waste, and 3 × Soapstock) ran for 37 days to allow additional biogas production as they were still producing substantial amounts of gas.
  • Each substrate was tested alone (mono-digestion) with inoculum in triplicate.
  • A single co-digestion treatment was tested in triplicate that combined all eight feedstocks, where the ratios of each are proportional to the amounts of each feedstock used in the digesters at Bio Town.
  • Controls included negative control (blank) inoculum-only digesters, tested in duplicate, alongside a single positive control digester (cellulose and inoculum).
  • Many of the digesters were too thick with the initial ratios and 26 experienced foaming, expansion, or excessive gas production that resulted in overpressure of the digester. Dilutions were made during the experiment to prevent additional explosions. The final dilution is shown in Table 1.
  • Table 1 contains details of the experiment, including both initial and final amounts of inoculum and feedstock. The working volume in all cases was 1000 mL, which means that the rest of the mass was made up by deionized water.
  • Additional details regarding digester set up, intermediate dilutions and tear down are contained in Appendix A. This includes dates and reasons for the intermediate dilutions.

BMP Tests

Table 1: Experimental digester treatment details. Average weight percent refers to the mass of the inoculum or feedstock out of the total digestate mass (approximately 1000 g). Deionized water was used to make up the digester volume to 1000 mL.

Treatment

Initial preparation (average wt %)

Final dilution (average wt %)

Number of digesters

Digester ID

 

% Inoculum

% Feedstock

% Inoculum

% Feedstock

 

 

Blank

100

0

86

0

2

1-2

Cellulose control

83

9

56

6

1

3

F1: Pad manure

63

38

31

18

3

4-6

F2: Starch

72

24

51

17

3

7-9

F3: Slaughterhouse waste

59

35

16

9

3

10-12

F4: Waste activated sludge

38

38

25

25

3

13-15

F5: Soapstock

9

91

3

32

3

16-18

F6: Filter press slurry

63

37

7

21

3

19-21

F7: Food waste DAF

42

31

23

17

3

22-24

F8: AR effluent

20

80

20

80

3

25-27

Mixture

48

50 (F1: 20, F2: 2.7, F3: 3.7, F4: 4.7, F5: 4.9, F6: 1.8, F7: 2.9, F8: 10.5)

25

26

3

28-30

 

Sampling and sample delivery process

Purdue employees collected substrate and inoculum from Bio Town Ag. The inoculum was collected one week prior to the substrate collection to allow it to de-gas prior to the beginning of the experiments. No grinding of the samples was done as it was determined that the feedstock particle size would not be further reduced in the BTA digester.

Biogas collection and analysis

Biogas produced from each lab-scale digester was individually collected into a 3- or 5-L gas bag for the experiment. The gas bags were connected to each digester at all times during the test. The gas production was measured daily initially and subsequently as needed, based on the biogas production rate. The biogas volume in the bags was measured with a syringe.

The biogas compositions, including concentrations of methane (CH4), carbon dioxide (CO2), oxygen (O2), and hydrogen sulfide (H2S) in collected gas bags was measured with a BIOGAS 5000 Gas Analyzer (LANDTEC North America, Inc., Colton, CA). This analyzer has measurement ranges of the four gases as: CH4 (0–100%); CO2 (0–100%); O2 (0–25%), H2S (0– 10,000 ppm) and balance (nitrogen) in biogas.

 

Experimental Summary for Experiment 2

Experiment 2 (E2): Experimental description

We conducted a co-digestion biomethane potential (BMP) test in September 2021. As manure contributes the greatest mass to the BTA digester, we combined the manure (F1) pairwise with some of the industrial feedstocks, specifically starch (F2), slaughterhouse waste (F3), soap stock (F5), and filter press slurry (F6). These combinations were made at two different ratios of the two feedstocks. The first set of treatments combined the manure and an additional substrate at a 1:1 ratio on a mass basis. The second set of treatments combined the feedstocks proportional to the amounts commonly used at BTA. Due to the extreme behavior of the starch and soap stock in the mono-digestion experiment, a final treatment pairing these two feedstocks at a 0.6:1 ratio was also included, which is proportional to the amounts used at BTA. The objective of the experiment was to evaluate potential synergism or antagonism from combinations of some of the feedstocks used in Experiment 1 (E1, conducted in April 2021). The experiment was conducted using 33 1000-mL lab-scale anaerobic digesters at mesophilic conditions of 101°F (38.3°C), using temperature-regulated water baths. One blank and a single mono-digestion digester for each feedstock was run to compare with the results from E1. Table 1 shows the treatment combinations of the feedstocks. The feedstocks are numbered the same as in E1 for consistency.

Table 1: Experiment 2 treatments.

 

Treatment

# digesters

S1 (g)

S2 (g)

I (g)

W (g)

S1/S2

I/S

Blank

1

--

--

1000

0

--

--

F1

1

101

--

399

500

--

0.5

F2

1

73

--

427

500

--

0.5

F3

1

111

--

389

500

--

0.5

F5

1

303

--

197

500

--

0.5

F6

1

94

--

406

500

--

0.5

F1+F2 Eq

3

42

42

415

500

1.0

0.5

F1+F3 Eq

3

53

53

394

500

1.0

0.5

F1+F5 Eq

3

76

76

349

500

1.0

0.5

F1+F6 Eq

3

49

49

403

500

1.0

0.5

F1+F2 Pr

3

83

13

404

500

6.3

0.5

F1+F3 Pr

3

85

18

397

500

4.7

0.5

F1+F5 Pr

3

92

24

383

500

3.8

0.5

F1+F6 Pr

3

91

9

401

500

10.6

0.5

F2+F5 Pr

3

52

88

360

500

0.6

0.5

Total:

33

F1 = pad manure, F2 = starch, F3 = slaughterhouse waste, F5 = soap stock, F6 = filter press slurry, I = inoculum, W = water; S1 = substrate 1 (first substrate in list), S2 = substrate 2; S1/S2 = substrate 1 to substrate 2 ratio (by mass); I/S = inoculum to substrate (total) ratio in terms of volatile solids; TS = total solids of combined substrates and inoculum; Eq = treatment has equal amounts (mass basis) of both feedstocks; Pr = treatment amounts are proportional to the amounts used at BTA

 

In addition to the testing of feedstock and post-digestion digestate samples for physical and chemical characteristics, samples were taken directly from the digesters prior to digestion after the inoculum and feedstocks had been mixed and were tested for the same characteristics, as shown in Table 2. All vial test kits from the Hach company (Loveland, CO, USA) were measured using the Hach DR3900 Benchtop Spectrophotometer. Dilutions were done on a mass basis when needed for samples to be within an acceptable range for the chemical analysis. No samples were taken from the digesters during digestion, although dilution of the digesters was needed due to digester foaming. Post-digestion results are corrected accordingly.

Table 2: Physical and chemical characteristics measured in Experiment 1 and Experiment 2.

Characteristic

Experiment 2

Analysis method

F

Pre

Post

Carbohydrates

X

X

X

Anthrone method

Proteins

X

X

X

Modified Lowry method

Lipids

X

X

X

Bligh and Dyer method

Total solids (TS)

X

X

X

Standard Methods of the APHA (APHA, 1992)

Volatile solids (VS)

X

X

X

Standard Methods of the APHA (APHA, 1992)

Soluble chemical oxygen demand (SCOD)

X

X

X

Hach TNTplus Vial Test, Ultra high range (TNT823)

Total chemical oxygen demand (TCOD)

X

X

X

Hach (TNT823)

Total nitrogen

X

X

X

Hach Simplified TKN TNTplus Vial Test (TNT872)

Total Kjeldahl nitrogen (TKN)

X

X

X

Hach TNT872

Nitrate + nitrite nitrogen

X

X

X

Hach TNT872

Ammonia nitrogen

X

X

X

Hach Ammonia TNTplus Vial Test, High range (TNT832)

Volatile fatty acids (VFAs)

X

X

X

Hach Volatile Acids TNTplus Vial Test (TNT872)

Alkalinity

X

X

X

Hach Alkalinity (Total) TNTplus Vial Test (TNT870)

F = Feedstock; Pre = Pre-digestion sample, removed from the digester after mixing inoculum and feedstocks but prior to digestion; Post = Post-digestion sample, removed from the digester following the conclusion of the BMP test.

 

Macromolecular assays

We tested the Bio Town feedstocks from a mono-digestion biomethane test E1 and feedstocks and pre- and post-digestion samples from a co-digestion biomethane potential test to establish the validity of using the following methods for characterization of macromolecules:

  • Anthrone method for carbohydrate characterization;
  • Bligh and Dyer method for lipid characterization; and
  • Modified Lowry method for protein characterization.

Each sample (8 feedstocks from E1; 5 feedstocks, 33 pre-digestion samples, and 33 post-digestion samples from E2) was tested in duplicate as described in Table 2.

Experimental Summary for Experiment 3

Experiment 3 (E3): Experimental description

The objective of the experiment was to continue evaluating potential synergism or antagonism from combinations of some of the feedstocks used in Experiment 1 (E1, conducted in April 2021). The results from the first co-digestion experiment (E2, conducted in September 2021) suggested that pairwise combinations of feedstocks resulted in synergy. This experiment was designed to evaluate whether balanced proportions of macromolecules impacted this synergy while using different combinations of feedstocks. Four treatments received approximately equal proportions of carbohydrates, proteins, and lipids in terms of mass added to the digester while varying the two of the three feedstocks. Three additional treatments received approximately the same proportions of lipids and proteins as each other but much more carbohydrates. A final treatment had similar proportions of macromolecules as the high carbohydrate treatment but restricted the amount of manure. Manure was used in all digesters due to its high prevalence in BTA waste streams and its favorable macromolecular characteristics.

We conducted the co-digestion biomethane potential (BMP) test in May 2022. The experiment was conducted using 33 1000-mL lab-scale anaerobic digesters at mesophilic conditions of 101°F (38.3°C), using temperature-regulated water baths. One blank and a single mono-digestion digester for each feedstock was run to compare with the results from E1 and E2. Table 1 shows the treatment combinations of the feedstocks. The feedstocks are numbered the same as in E1 and E2 for consistency. Each digester received a 2:1 substrate to inoculum ratio by volatile solids. Each digester was diluted by half to maintain total solids contents less than 7% to minimize foaming issues.

Table 1: Experiment 3 treatments.

Treatment

n

I (wt%)

S1 (wt%)

S2 (wt%)

S3 (wt%)

W (wt%)

%C/P/L (added)

Blank

1

50

--

--

--

50

--

F1

1

34

16

--

--

50

--

F2

1

40

10

--

--

50

--

F3

1

36

14

--

--

50

--

F5

1

34

17

--

--

50

--

F6

1

28

22

--

--

50

--

F1+F2+F3

3

37

5

2

6

50

33/33/33

F1+F3+F5

3

35

11

1

3

50

33/33/33

F1+F3+F6

3

34

9

3

4

50

33/33/33

F1+F5+F6

3

33

12

2

3

50

33/33/33

F1+F2+F3

3

38

3

6

3

50

66/17/17

F1+F2+F5

3

37

6

5

2

50

66/17/17

F1+F2+F6

3

35

7

3

5

50

66/17/17

F1+F2+F3

3

38

1

6

5

50

60/17/23

Total:

30

 

F1 = pad manure, F2 = starch, F3 = slaughterhouse waste, F5 = soap stock, F6 = filter press slurry, I = inoculum, W = water; S1 = substrate 1 (first substrate in list), S2 = substrate 2; S3 = substrate 3

 

In addition to the testing of feedstock and post-digestion digestate samples for physical and chemical characteristics, samples were taken directly from the digesters prior to digestion after the inoculum and feedstocks had been mixed and were tested for the same characteristics, as shown in Table 2. All vial test kits from the Hach company (Loveland, CO, USA) were measured using the Hach DR3900 Benchtop Spectrophotometer. Dilutions were done on a mass basis when needed for samples to be within an acceptable range for the chemical analysis. No samples were taken from the digesters during digestion, although dilution of the digesters was needed due to digester foaming. Post-digestion results are corrected accordingly.

Table 2: Physical and chemical characteristics measured in Experiment 1 and Experiment 2.

Characteristic

Experiment 2

Analysis method

F

Pre

Post

Carbohydrates

X

X

X

Anthrone method

Proteins

X

X

X

Modified Lowry method

Lipids

X

X

X

Bligh and Dyer method

Total solids (TS)

X

X

X

Standard Methods of the APHA (APHA, 1992)

Volatile solids (VS)

X

X

X

Standard Methods of the APHA (APHA, 1992)

Soluble chemical oxygen demand (SCOD)

X

X

X

Hach TNTplus Vial Test, Ultra high range (TNT823)

Total chemical oxygen demand (TCOD)

X

X

X

Hach (TNT823)

Total nitrogen

X

X

X

Hach Simplified TKN TNTplus Vial Test (TNT872)

Total Kjeldahl nitrogen (TKN)

X

X

X

Hach TNT872

Nitrate + nitrite nitrogen

X

X

X

Hach TNT872

Ammonia nitrogen

X

X

X

Hach Ammonia TNTplus Vial Test, High range (TNT832)

Volatile fatty acids (VFAs)

X

X

X

Hach Volatile Acids TNTplus Vial Test (TNT872)

Alkalinity

X

X

X

Hach Alkalinity (Total) TNTplus Vial Test (TNT870)

F = Feedstock; Pre = Pre-digestion sample, removed from the digester after mixing inoculum and feedstocks but prior to digestion; Post = Post-digestion sample, removed from the digester following the conclusion of the BMP test.

Research results and discussion:

We evaluated the mono-digestion feedstocks for suitability in anaerobic digestion. We also demonstrated that co-digestion can result in positive synergistic effects over mono-digestion. Our overall conclusion is that co-digestion can lead to higher overall biomethane yield. Detailed results from each of the three major experiments follow.

Results for Experiment 1

Experiment 1 Summary

Bio Town Ag Digestate Methane Potential

Summary

Approximately 7.6 L biogas/L raw digestate (5.6 L methane/L raw digestate, average 73.7% methane concentration) was produced over 45 days. More than 50% of the methane was produced during the first 7 days. All gas volumes are represented at standard temperature and pressure (32°F, 1 atm pressure).

 

Measurements

Digestate was collected from one of the BTA digesters seven days prior to the beginning of the BMP experiment to be used as inoculum in the BMP experiment. It was allowed to “de-gas” under controlled temperature conditions (38.3°C) for 7 days. Biogas volume and methane content was measured throughout this week-long period. At the end of the de-gas period, two “blank” digesters were set up with the de-gassed digestate (inoculum) only during the BMP experiment and were monitored for gas production volume and composition.

Results

We measured the cumulative gas production during the course of the de-gas and BMP experiment. Approximately 7.6 L biogas/L raw digestate (5.6 L methane/L raw digestate) was produced over 45 days (7 days of de-gas followed by 38 days of BMP experiment).

Discussion

The raw digestate collected on March 29, 2021 still contained a substantial amount of biogas. Approximately 51% of the biogas and methane was produced during the initial seven-day de-gas period. Bio Town Ag might consider methods of capturing this residual gas, either through operational changes (longer hydraulic retention time) or recycling the digestate (instead of just the ammonia recovery effluent) back into the digester(s). These two approaches would each require reduction of feedstock input to the digesters or additional digesters. Other options may include: 1. Expand working volume of digesters to improve performance by removing the built-up sludge; 2. Cover the digestate storage lagoons to recover methane.

 

 

Biomethane Potential (BMP) and Kinetics of Mono-digestion

Summary: Overall, most replicates gave similar biomethane potentials as measured by cumulative methane produced per unit volatile solids added. Most also exhibited fairly typical kinetic patterns, where the majority of the biogas is produced within the first few to several days, with a gradual tapering. Table 2 contains the biogas and methane potentials of each digester, corrected for inoculum. Zero values indicate that the inoculum likely contributed all of the methane measured during the experiment, and perhaps that there was some digestion inhibition. All gas volumes are represented at standard temperature and pressure (32°F, 1 atm pressure).

 

Table 2: Specific biogas and methane production per gram volatile solids added after inoculum subtraction.

Feedstock

Digester

Biogas (mL biogas/g VS added)

Methane (mL CH4/g VS added)

Individual digester

Feedstock average

Individual digester

Feedstock average

Blank

1

0

0

0

0

 

2

0

 

0

Cellulose

3

209

209

86

86

F1

4

316

373

202

228

 

5

363

 

228

6

440

 

254

F2

7

32

47

0

0

 

8

28

 

0

9

80

 

0

F3

10

75

135

51

100

 

11

238

 

179

12

93

 

71

F4

13

101

116

73

91

 

14

165

 

124

15

81

 

75

F5

16

193

212

159

173

 

17

168

 

134

18

275

 

226

F6

19

39

40

0

2

 

20

42

 

3

21

41

 

2

F7

22

188

183

134

133

 

23

202

 

143

24

158

 

120

F8

25

56

78

56

72

 

26

78

 

71

27

99

 

87

Mix

28

45

47

0

6

29

16

 

0

30

80

209

17

 

 

Cumulative Biogas and Methane Production by Treatment

For rapid comparison of digestion rate, Table 3 is also provided with cumulative specific biogas and methane production potential to reflect the approximate amount of methane recovery that could be expected at varied hydraulic retention times. These values have been corrected for inoculum contribution, with the exception of the blank, which is shown as the raw, uncorrected value. The blank also includes the one-week de-gassing period, rather than only showing the amount of gas produced since the beginning of the BMP test. There are a few cases where the cumulative gas production decreases over time. This is due to the method of calculating inoculum removal and likely indicates some inhibition of the digestion process. In contrast to Table 2, these values are calculated based on the feedstock mass rather than the volatile solids mass. The gas volumes are still calculated in terms of standard temperature and pressure (32°F, 1 atm pressure).

 

Table 3: Cumulative biogas and methane production potential per pound raw feedstock at three different hydraulic retention times.

Feedstock

Biogas (ft3/lb)

Methane (ft3/lb)

12-day

21-day

30-day

12-day

21-day

30-day

Blank

0.09

0.11

0.12

0.07

0.09

0.11

Cellulose control

3.30

3.19

3.21

1.40

1.31

1.32

F1: Pad manure

1.36

1.52

1.59

0.80

0.92

0.97

F2: Starch

0.23

0.28

0.33

0.01

0.00

0.00

F3: Slaughterhouse waste

0.18

0.25

0.29

0.00

0.04

0.11

F4: Waste activated sludge

0.17

0.19

0.20

0.13

0.15

0.16

F5: Soapstock

0.13

0.16

0.13

0.09

0.12

0.11

F6: Filter press slurry

0.25

0.22

0.21

0.04

0.02

0.01

F7: Food waste DAF

0.25

0.28

0.30

0.17

0.20

0.22

F8: AR effluent

0.03

0.03

0.04

0.03

0.03

0.03

Mixture

0.13

0.11

0.12

0.02

0.01

0.01

 

Comparison of Average Biogas Production

The soapstock (F5) and slaughterhouse waste (F3) treatments were both producing substantial biogas on a daily basis at the end of the originally planned 30 days. As a result, they were both maintained for another week to recover more biogas. The blank digesters were also maintained during that time so that the inoculum contribution could be accurately subtracted.

For each of the following treatments, a solid line is used to indicate the average for the treatment cumulative biogas production, and a dotted line is used to indicate the average for the treatment cumulative methane production.

Blank

The blank (inoculum-only) replicates (1-2) performed very similarly to each other, with a final coefficient of variation of 7% for average biogas production and less than 1% for average methane production. These results give us confidence in our calculations to remove the inoculum biogas and methane productions from the mono-substrate and mixture digesters.

Cellulose control

The positive (cellulose) control did not perform as well as expected (~350 mL methane/g VS cellulose). However, this may be the result of high starting total solids, which required substantial dilution during the experiment. Although these results control for that, it is possible that more VS were lost than calculated initially. Despite the low BMP, the curve for the cellulose control is reasonably shaped. A future experiment may explore other possible causes for the discrepancy between what we observed and what was expected.

F1: Pad manure

The pad manure digesters performed similarly and as anticipated based on previous manure results. The replicates are even closer in performance when using cumulative methane as a metric rather than biogas. As shown in Table 2, the pad manure had the highest BMP of all feedstocks tested.

F2: Starch

The starch digesters all rapidly produced essentially all of their methane at the very beginning of the experiment, within the first 2-4 days, and little to none the rest of the experiment. Although there is an unusual increase in biogas production at the end of the experiment from one of the digesters, very little of it was methane. The BMP was one of the lowest among the feedstocks.

F3: Slaughterhouse waste

The slaughterhouse waste digesters generated an unusual cumulative gas production curve. Initially, they started producing a small amount of gas then temporarily appeared to be tapering off. However, towards the end of the experiment, they all began to produce an exceptionally high amount of gas and began foaming to the point of clogging gas bags. These digesters were all run a week longer than the original proposal of 30 days in order to observe this increase. This very unusual behavior may be attributable to the low carbohydrate content and high protein and lipid content of the feedstock. This will be investigated further. This feedstock in particular may benefit from co-digestion with another feedstock that can balance out the unusual kinetic behavior shown here, but more research is needed.

F4: Waste activated sludge

This feedstock appears to be relatively normal in terms of rate of gas production. One of the digesters did generate an unusually high amount of gas compared to the other two, but this difference is expected occasionally in digesters as it is a biological process.

F5: Soapstock

The soapstock digesters all followed an unusual pattern. Each reached a plateau of gas production approximately three different times. The final time, the digesters all started producing a significant amount of foam as well as more than double their previous biogas production. These digesters ran one week past the original 30 days in order to more fully observe this behavior. Similar to the slaughterhouse waste, the soapstock composition has low carbohydrates but high proportions of protein and lipids, which may contribute to this behavior.

F6: Filter press slurry

The filter press slurry exhibited a very normal specific methane potential curve and all the digesters gave very consistent results. However, the BMP is one of the lowest exhibited.

F7: Food waste DAF

The food waste DAF had one of the highest BMPs and exhibited fairly normal kinetics.

F8: AR effluent

The ammonia recovery effluent experienced a slight delay at the beginning of gas production, but its later gas production curve appears normal. However, it is also important to remember that biomethane potential is gas potential per gram of volatile solids added, and the AR effluent has the least volatile solids of all the feedstocks.

Mixture

The mixture digesters did not perform as anticipated, where the BMP of each individual feedstock can be added together to give a total expected BMP. In fact, as shown in Figure 14, two of the digesters did not produce as much gas as would be anticipated from the inoculum alone. It is possible that the dilution required due to overpressure of the digesters at the beginning may have had adverse effects on these digesters. Alternatively, during the dilution the digestate may have been insufficiently mixed, which could have resulted in less of certain feedstocks (such as the manure) than anticipated, which would also skew the results. A similar test should be tried again to determine the causes.

Hydrogen Sulfide (H2S) Concentrations

For simplicity, the maximum H2S concentration from each digester and the average of each treatment is presented in the following table. Additional information regarding H2S production over time can be provided.

 

Table 4: Maximum hydrogen sulfide concentration observed at any point during digestion, for individual digesters and treatment average.

Feedstock

Digester

Maximum hydrogen sulfide concentration (ppm)

Digester

Feedstock average

Blank

1

1

2

 

2

2

Cellulose

3

570

570

F1

4

2529

2712

 

5

2536

6

3070

F2

7

329

2307

 

8

1591

9

5000

F3

10

84

63

 

11

92

12

12

F4

13

341

229

 

14

303

15

42

F5

16

290

418

 

17

467

18

496

F6

19

301

337

 

20

318

21

393

F7

22

139

129

 

23

137

24

110

F8

25

3

8

 

26

10

27

12

Mix

28

515

565

29

506

30

674

Physical and Chemical Composition of Feedstocks and Digestate

Summary

Total Kjeldahl nitrogen, ammonia nitrogen, volatile fatty acids, total and soluble chemical oxygen demand, alkalinity, total and volatile solids, and concentrations of carbohydrates, proteins, and lipids were all measured for the feedstocks. All of these were measured for the digestate as well except for the carbohydrate, protein, and lipid concentrations. Additional details are available for any of these measurements, but only the most relevant information has been included in this report.

 

Volatile Fatty Acids

In most cases, VFAs were reduced from the original amounts contained in the feedstock. However, exceptionally high VFAs (>10 g/L) were found in the cellulose, starch (F2), filter press slurry (F6), and mixture digesters. In addition, VFAs increased from the initial feedstocks for those same digesters listed as well as the slaughterhouse waste and soapstock digesters, although VFAs for those remained on average 3.7 g/L and 1.7 g/L, respectively. The accumulation of VFAs in these digesters may have led to methanogenesis inhibition, which could account for the low BMPs of the cellulose, starch, filter press slurry, and mixture digesters. Co-digestion of these feedstocks could result in higher BMPs if inhibition was the cause of the low BMP. Additional research would be needed here, and an additional mono-digestion or co-digestion test with these feedstocks would be recommended.

 

Alkalinity

Alkalinity increased in all the digesters except cellulose, starch, and filter press slurry. These alkalinity results are consistent with the VFA results. Mixture change in alkalinity was not calculated, but the average mixture alkalinity (4.2 g/L) was only slightly higher than that of the feedstocks listed (averages: cellulose: 3.6 g/L; starch: 2.5 g/L; filter press slurry: 2.9 g/L).

 

Volatile Solids Reduction

Volatile solids were measured in duplicate for both feedstocks and digestate. On average, solids reduction (calculated to include inoculum contribution and take subsequent dilutions into account) were high (>60%) as shown in Table 5. The blank and AR effluent saw the lowest reductions, but also started with the lowest quantity of volatile solids (3.4 g/g and 2.9 g/g respectively).

 

Table 5: Average volatile solids reduction per feedstock.

Feedstock

Days of test

Average VS% Reduction

Blank

38

60%

Cellulose

30

83%

F1: Pad Manure

30

85%

F2: Starch

30

89%

F3: Slaughterhouse waste

38

80%

F4: Waste activated sludge

30

72%

F5: Soapstock

38

76%

F6: Filter press slurry

30

86%

F7: Food waste DAF

30

70%

F8: AR effluent

30

63%

Mixture

30

81%

 

Carbohydrates, Lipids, and Proteins

One of the future goals of this project is to determine whether carbohydrates, lipids, and proteins can be used, in conjunction with other feedstock properties, as metrics for choosing feedstocks and even predicting biomethane potential or kinetics. More work will be needed before this is possible. However, these initial results may give some insight into the BMP and kinetics results from this experiment. Figure 15 shows that the three feedstocks that were the fastest to finish gas production (as represented by the percentage of BMP produced after the first four days) also had the lowest protein content (filter press slurry (F6), starch (F2), and cellulose). Filter press slurry and starch also had the lowest BMPs when the contribution of inoculum was removed. In the case of starch and cellulose, the low BMP could have been the result of rapid acidification due to the fast degradation of the carbohydrates. The slowest in terms of gas production were slaughterhouse waste (F3) and soapstock (F5), and these had the lowest percentages of carbohydrates and almost the highest percentages of lipids. More analysis will be done in this area prior to the next experiment, but the initial results are promising. Co-digestion of some of these feedstocks may balance out some of the unwanted BMP and kinetic effects seen here, such as low BMP or exceptionally slow digestion.

Experimental Results for Experiment 3

Experiment 2 Summary

Variability between batches

            In order to develop laboratory protocols for quantifying macromolecular composition of representative substrates used for agro-industrial anaerobic co-digestion, it was first necessary to verify whether the assays give replicable data. Accordingly, using the data acquired from the E2 samples, we calculated the coefficient of variation between sample replicates. The majority of the samples had a coefficient of variation <30% for each of the macromolecular assays.

We compared the results of the macromolecular characterizations of the feedstocks between E1 and E2 and found substantial differences between the two. Table 2 shows the percent difference between E1 and E2 for each of the macromolecular compositions. There does not appear to be any particular pattern to the differences other than the fact that a higher concentration of proteins and lipids was measured for all feedstocks, including the inoculum, in E1. However, the differences are so varied that there does not appear to be a single cause of this.

Table 3: Percent difference between E1 (April 2021) and E2 (September 2021) feedstocks. A negative number indicates an increase in concentration of the macromolecule from E1 to E2.

Feedstock

Carbohydrate

Lipid

Protein

Inoculum

-62%

53%

29%

F1

-7%

63%

25%

F2

25%

13%

22%

F3

66%

51%

74%

F5

-20%

25%

65%

F6

52%

62%

65%

 

If the feedstocks are regularly experiencing such large fluctuations, this could cause issues for BTA’s digester, such as causing unanticipated fluctuations in gas production or inhibition. It would also make it difficult to establish a reliable method of recommending a feeding strategy to BTA as the composition of the feedstocks would need to be assessed more regularly. More research is needed to determine the magnitude of these fluctuations as part of developing laboratory protocols for measuring macromolecule composition for the purposes of informing BMP tests.

Gas production

Comparison of mono-digestion results between E1 and E2

Figure 1 compares the specific biogas production of the mono-digestion of the individual feedstocks from E1 and E2. Although in some cases the general shape of the methane production curves are similar, the slaughterhouse waste, soap stock, and filter press slurry (Figure 1D, E, and F respectively) all exhibit sufficient differences to require us to use the mono-digestion results from E2 only to make comparisons between co-digestion treatments, rather than being able to extrapolate from the results of E1. This is important as it establishes the necessity of these mono-digestion treatments in future experiments as well. However, further research is needed to determine the cause of these differences.

Figure 1: Comparison of mono-digestion results between E1 and E2.

Synergistic effects of co-digestion

During E2, we observed both total yield and kinetic synergy in all treatments. Only one digester (digester 19, one of the replicates from the starch and manure proportional treatment) produced substantially less (<30%) methane than would be expected for an additive effect for more than one day. An additive effect can be calculated as a weighted average between the mono-digestion specific methane potential curves at each time point. This effect can be seen in Figure 2, which shows the cumulative methane curves (corrected for inoculum contribution and averaged over the three replicates) of the mono-digestion digesters for manure and starch individually and the curves for both co-digestion treatments using both manure and starch.

Figure 2: Cumulative specific methane production for manure (F1) and starch (F2). F1 + F2 Eq = 1:1 ratio of VS; F1 + F2 Pr = ratio of VS is proportional to what full-scale digester receives. Points indicate individual digesters (7-9 are Eq treatment, 19-21 are Pr treatment).

For each of the treatments, an additional figure will be included to show the average digester results (points) alongside the predicted co-digestion results based on the weighted average of the two mono-digestion results. Figure 3 shows that the equal parts manure and starch treatment significantly outperformed the prediction assuming no synergy for both kinetics and ultimate methane yield. The proportional treatment is not so definitive as one digester (Digester 19) underperformed the average significantly. Therefore, synergy cannot be assumed.

Figure 3: Co-digestion results for manure (F1) and starch (F2). F1 + F2 Eq = 1:1 ratio of VS; F1 + F2 Pr = ratio of VS is proportional to what full-scale digester receives. Points indicate treatment averages with error bars showing standard deviation. Dashed lines indicate treatment prediction assuming no synergy.

Figure 4 and Figure 5 show the same curves for the co-digestion of manure and slaughterhouse waste. These co-digestion treatments show that combining the feedstocks leads to a faster rate of methane production initially. They also show that co-digestion alleviates the lag phase experienced by the slaughterhouse waste.

Interestingly, while the same trend of improved kinetics and reduced lag phase holds true for the treatments of manure and soapstock, the F1 + F5 Eq treatment does experience a mild delay in methane production around day 3 as compared to the proportional treatment. This may indicate that the higher proportion of soap stock did have an impact on the methane production.

A similar phenomenon can be observed in the treatment with equal portions of manure and filter press slurry likewise experiencing a slight delay in gas production.

Finally, despite the rapid cessation of gas production in the starch and soap stock combination, they still performed better together than separately. It is possible that for both the starch mono-digestion treatment and the co-digestion treatment rapid degradation of readily digestible carbohydrates led to accumulation of acids, inhibiting further gas production. This could be a possible cause for concern for Bio Town Ag as they consider adding readily available carbohydrates such as starch to their digester in large quantities.

Experimental Results for Experiment 3

Experiment 3 Summary

Gas production

Comparison of mono-digestion results between E1, E2, and E3

Figure 1 compares the specific biogas production of the mono-digestion of the individual feedstocks from E1, E2, and E3. Although in some cases the general shape of the methane production curves are similar, the slaughterhouse waste, soap stock, and filter press slurry (Figure 1D, E, and F respectively) all exhibit sufficient differences to require us to use the mono-digestion results from E2 only to make comparisons between co-digestion treatments, rather than being able to extrapolate from the results of E1. This is important as it establishes the necessity of these mono-digestion treatments in future experiments as well. However, further research is needed to determine the cause of these differences.

Figure 1: Comparison of mono-digestion results between E1, E2, and E3.

 

 

Synergistic effects of co-digestion

During E3, we observed some total yield and kinetic synergy in a few treatments, where synergy is defined as exceeding the gas production anticipated from the additive effect. An additive effect can be calculated as a weighted average between the mono-digestion specific methane potential curves at each time point. The mono-digestion results from this experiment were used to calculate the predicted methane production assuming the yield is additive. Table 3 shows the methane yield results for each digester and overall treatment averages. Although there is some variability within each treatment, a clear pattern emerges overall. The digesters with a more balanced feed macromolecular composition either experienced overall yield synergy or at least (in the case of F1+F3+F6) approximately additive results. However, the high carbohydrate digesters all experienced rapid acidification, leading to digester collapse and total yield antagonism.

 

 

Table 3: Final cumulative biomethane potential of each digester in terms of volume produced per mass of volatile solids (mL CH4/g VS). The estimated contribution of inoculum is subtracted from the results. Prediction of gas production is estimated based on the mono-digestion control results. A positive percent improvement over prediction indicates that more methane per amount volatile solids was produced than estimated based on the mono-digestion results.

Digester

Treatment

Methane produced (mL/g VS)

Treatment average (mL CH4/g VS)

Percent improvement over prediction

1

Inoculum

0.0

 

 

2

F1 (pad manure)

321.3

 

 

3

F2 (starch)

18.2

 

 

4

F3 (slaughterhouse waste)

252.9

 

 

5

F5 (soap stock)

61.5

 

 

6

F6 (filter press slurry)

14.3

 

 

7

F1+F2+F3, 33% carbs added (33% protein, 33% lipid)

304.4

350.8

41%

8

393.5

82%

9

354.6

64%

10

F1+F2+F5, 33% carbs added

432.6

367.5

81%

11

332.3

39%

12

337.4

41%

13

F1+F3+F6, 33% carbs added

253.7

259.9

1%

14

284.9

13%

15

241.2

-4%

16

F1+F5+F6, 33% carbs added

234.7

314.3

-8%

17

340.8

34%

18

367.2

44%

19

F1+F2+F3, 66% carbs added (17% protein, 17% lipid)

16.8

17.5

-86%

20

16.0

-87%

21

19.8

-83%

22

F1+F2+F5, 66% carbs added

24.5

26.0

-82%

23

26.7

-81%

24

26.9

-80%

25

F1+F2+F6, 66% carbs added

14.8

17.5

-90%

26

20.8

-86%

27

17.1

-89%

28

F1+F2+F3, 60% carbs added (low manure)

10.9

10.0

-90%

29

9.1

-92%

30

9.9

-91%

 

In terms of kinetic behavior, the results are less clear. As Figure 2 shows, there was significant variability in the manure + starch + slaughterhouse waste treatment with 33% carbohydrates. However, the overall treatment digested faster and produced more methane per mass volatile solids added than the additive prediction.

Figure 2: Cumulative methane production curves for the three manure, starch, and slaughterhouse waste (F1+F2+F3) treatments. Error bars indicate standard deviation and points represent treatment averages. Dotted lines indicate prediction curves based on weighted average of mono-digestion controls.

The manure + starch + soapstock with 33% carbohydrates treatment appeared more regular as shown in Figure 3. The early kinetic results were very close to the additive prediction.

Figure 3: Cumulative methane production curves for the two manure, starch, and soap stock (F1+F2+F5) treatments. Error bars indicate standard deviation and points represent treatment averages. Dotted lines indicate prediction curves based on weighted average of mono-digestion controls.

Figure 4 shows that the manure, slaughterhouse waste, and filter press slurry treatment ultimately behaved quite similarly to the methane yield additive prediction, but there appears to be some sort of antagonistic effect that delayed the gas production kinetics.

Figure 4: Cumulative methane production curves for the manure, slaughterhouse waste, and filter press slurry (F1+F3+F6) treatment. Error bars indicate standard deviation and points represent treatment averages. Dotted lines indicate prediction curves based on weighted average of mono-digestion controls.

Figure 5 shows that the combination of manure, soapstock, and filter press slurry generally outperformed the additive prediction, although with the variability between digesters in the treatment it is difficult to say that synergy is definitely occurring in this treatment.

Figure 5: Cumulative methane production curve for the manure, soap stock, and filter press slurry (F1+F5+F6) treatment. Error bars indicate standard deviation and points represent treatment averages. Dotted line indicates prediction curve based on weighted average of mono-digestion controls.

Figure 6 shows that the high carbohydrate treatment was unsuccessful for manure, starch, and filter press slurry, similar to the other high carbohydrate treatments.

Figure 6: Cumulative methane production curve for the manure, starch, and filter press slurry (F1+F2+F6) treatment. Error bars indicate standard deviation and points represent treatment averages. Dotted line indicates prediction curve based on weighted average of mono-digestion controls.

Figure 7 compares the methane production curves for the low and high carbohydrate treatments. Three of the four low carbohydrate treatments have fairly similar curve shapes, and all four high carbohydrate treatments rapidly produced gas prior to quick failure. Upon closer inspection, it appears that both low carbohydrate treatments that included slaughterhouse waste experienced a lag phase. Although this lag phase was an improvement over that experienced by the slaughterhouse waste alone, it may be an indicator of some antagonism from substances in the slaughterhouse waste.

Figure 7: Cumulative methane production curves for the low (A) carbohydrate and high (B) carbohydrate treatments.

In conclusion, these results show that in this case, the balanced 33% carbohydrate, 33% protein, and 33% lipid mixtures generally performed at least as well as the prediction based on a weighted average of the results of the mono-digestion controls. However, the high carbohydrate digesters significantly underperformed expectations, likely due to acidification leading to rapid digester failure early in the digestion process. While a specific ratio of macromolecules does not guarantee a digester’s kinetic performance, the 33% carbohydrate, 33% protein, and 33% lipid treatments performed relatively similarly to each other even with varied feedstocks. It is also clear that high carbohydrate amounts can cause problems for a digester.

Participation Summary
1 Farmers participating in research

Educational & Outreach Activities

1 Consultations
2 Online trainings
2 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

17 Farmers participated
86 Ag professionals participated
Education/outreach description:

Consultations

Our farmer partner, Bio Town Ag, has been providing us with samples for testing. We have consulted with them about the results of our experiments. The reports for this consultation are included in the results and discussion section.

Online trainings

We conducted two online webinars on anaerobic digestion topics in October and November 2021. Our attendance statistics are shown in the table below.

Table 1: Summary statistics of the anaerobic digestion webinars.

 

Webinar 1

Webinar 2

Date

10/20/2022

11/17/2022

Topics

Anaerobic Digestion Part 1

Anaerobic Digestion Part 2

Speakers (n)

4

4

Registrants (n)

242

181

   States/provinces of registrants (n)

35

 

   Countries of registrants (n)

8*

5 (US, Canada, India, El Salvador, Malaysia)

Participants (n) (for > 30 min)

148

106

   States/provinces of participants (n)

30

 

   Countries of participants (n)

5**

1 (US)

   Average connection time (min/n)

137

140

   Indiana participants (n)

33

 

   Indiana participants (%)

22%

 

* US, Canada, India, Italy, El Salvador, Ireland, Morocco, China; ** US, Canada, India, Italy, Morocco

   

It was difficult to evaluate which attendees were specifically farmers/agricultural professionals, so estimates were used.

All proceedings from these webinars can be found on our website: https://engineering.purdue.edu/adt/wm/index_files/MM.htm

The combined views from all videos at these webinars totals 130 over the course of 7 months.

Workshop

We conducted a hybrid workshop at Bio Town Ag in March 2023. During this workshop, we had 8 speakers, including two of the team members on this grant. 130 total people registered, with the understanding that they would receive a link to the slides and recordings of presentations afterwards. Although attendance of only 61 was confirmed (30 online and 31 in-person), many of those will still have the opportunity to watch the presentations after the fact. During this workshop, two posters were displayed portraying more information, as well as a simple demonstration of a biomethane potential test. In addition, many extension publications were available as handouts. The feedback on the in-person event was very positive.

Between the workshop and the webinars, we had 315 confirmed participants.

Talks and presentations

This work was presented at the Waste to Worth Conference in April 2021 in Ohio.

This work was also presented at the Annual International Meeting of the American Association for Agricultural and Biological Engineers in July 2021 in Houston, TX.

Outreach in Progress

Two journal articles based on this work are in progress of writing and submission to journals.

Project Outcomes

Project outcomes:

This project has contributed to increased knowledge about anaerobic co-digestion. Improved adoption of anaerobic digestion and co-digestion practices can reduce odors from intensive livestock farming, add a revenue stream to the farm, reduce greenhouse gas emissions, and provide renewable energy to the farm.

Knowledge Gained:

We observed that anaerobic digestion and particularly anaerobic co-digestion has the ability to degrade diverse feedstocks, including livestock manure, to provide a nutrient-rich effluent usable for applications such as land-application while simultaneously producing biomethane which can be used as a renewable energy source. Synergy is possible in co-digestion, which can enhance digester performance and provide an additional source of income. However, there can be drawbacks to co-digestion as well, so farmers should carefully evaluate each feedstock prior to adoption.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.