screening co-digestion of food waste water with manure for biogas production

© 2008 Society of Chemical Industry and John Wiley & Sons, Ltd 11

Correspondence to: Yan Liu, Department of Biosystems & Agricultural Engineering, Michigan State University, 203 Farrall Hall, East Lansing,

MI 48824, USA. E-mail: [email protected]

In the Field

Introduction

The volume of waste water generated from vegetable-

and fruit-processing waste water in Michigan is

estimated to average 12 773 million liters per year

(Table 1) and contains an approximate 5-day biochemical

oxygen demand (BOD) of 67 ~ 4338 kg per year.1 Technologies

to dispose of this waste water are diverse. In rural envi-

ronments, land application with treatment by soil is oft en

practiced. High organic loadings and inappropriate soil

conditions, however, may result in exceeding a soil’s assimi-

lation capacity and cause surface and ground water quality

issues, and odor problems. A traditional aerobic waste-water

treatment system is energy-consuming and costly, especially

Screening co-digestion of food waste water with manure for biogas productionYan Liu, Steve A. Miller and Steven I. Safferman, Michigan State University

Received May 5, 2008; revised version received November 5, 2008; accepted November 10, 2008

Published online December 23, 2008 in Wiley InterScience (www.interscience.wiley.com); DOI: 10.1002.bbb.120;

Biofuels, Bioprod. Bioref. 3:11–19 (2009)

Abstract: Anaerobic digestion, an environmental protection technology for treating organic compounds in waste

water, produces biogas, resulting in a renewable energy source. A protocol including waste analysis, waste blending,

energy potential and energy balance calculations was developed to determine the energy production from blending

food and animal wastes. Fruit and vegetable waste water produced from crop commodity processing was charac-

terized in terms of quantity and 5-day biochemical oxygen demand (BOD). Often these wastes have high levels of

degradable carbon but lack buffering capacity and adequate nitrogen and other nutrients to meet the minimal C/N

ratio needed for optimal digestion. Blending food-processing waste water with high nutrient manure can enhance

the biogas production by optimizing nutrient levels and providing buffering capacity. The protocol shows the proce-

dure to determine the optimal blend and theoretical biogas production from the anaerobic digestion of that blend.

An energy balance technique that determines the lowest COD concentration required to close the energy balance

in the digester during different seasons is illustrated. A case study was conducted to determine the potential energy

production from anaerobically digesting blended waste water from the top 14 fruit and vegetable commodities in

Michigan. The resulting biogas production supports a substantial amount of the energy consumption needed for the

treatment process. This case study in Michigan can be extended to national level since the calculations were based

on the mean value of their typical range. © 2008 Society of Chemical Industry and John Wiley & Sons, Ltd

Keywords: food-processing waste; manure; anaerobic digestion; co-digestion

12 © 2008 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 3:11–19 (2009); DOI: 10.1002/bbb

Y Liu, SA Miller, SI Safferman In the Field: Co-digestion of food waste water with manure for biogas production

due to the oft en very high BOD of food-processing waste

water.2 Th e use of anaerobic digestion for at least part of the

treatment requirements provides benefi ts and revenues to

farmers and fruit/vegetable processors including substantial

odor reduction, production of a renewable energy source

(biogas), reduction of greenhouse gas (GHG) emissions,

potential pathogen reduction, and enhanced nutrient

management. Biogas produced from anaerobic digestion can

be used for heat production and/or electricity generation if

the investment in such equipment proves to be benefi cial.

Although simply fl aring biogas does not yield a renewable

energy, it does reduce GHGs and may qualify for green

credits.3 In addition to revenues that can be realized from

renewable electricity generation, there may be the oppor-

tunity for green tags from electricity, carbon credits, tipping

fee, renewable energy tax credits and fi ber sales, as bedding

material can be obtained from anaerobic digestion of a

manure-based feedstock. Table 23,4 shows potential revenues

associated with anaerobic digestion of dairy manure waste.

Using the current industry electricity rate of $0.053 per

kilowatt in Michigan, combining all the revenues together,

results in approximately $0.124 per kilowatt, which can

substantially off set the total digester installation and opera-

tion costs. Currently, in many government entities, including

Michigan, however, all of these revenues cannot be recov-

ered because of regulatory issues. Further, other sources

supporting the digestion installation and operation may

include cost-share, grant funding and support for renewable

energy development.5

Table 1. Characterization of vegetable and fruit waste water from Michigan.1

CropCrop processed

(103 kg/yr) Waste water

(106 L/yr) BOD

(103 kg/yr) COD* (mg/L) Energy potential

(106 kJ/yr)

Apples 263 000 2 389 2 149 1 889 17 142

Cucumbers 163 496 2 166 3 118 3 023 24 865

Potatoes 113 750 1 550 4 338 5 877 34 599

Squash 60 941 1 384 553 840 4 413

Cherries, Tart 92 500 1 365 1 403 2 157 11 187

Beans, Snap 62 780 998 428 900 3 410

Tomatoes 109 860 915 349 801 2 785

Grapes 81 000 460 331 1 511 2 640

Pumpkins 40 780 448 592 2 779 4 725

Carrots 35 760 447 487 2 290 3 885

Blueberries 24 300 322 210 1 367 1 672

Peppers 7 700 134 112 1 752 892

Cherries, Sweet 17 000 116 146 2 645 1 163

Peas – 80 67 1 773 537

Total – 12 773 14 282 29 604 113 914

* COD is calculated by dividing the annual waste water fl ow by the BOD produced per year and multiplying by the assumed, fi xed COD to BOD ratio of 2.1.

Table 2. Revenues from anaerobic digestion.4

Revenue source$/kwh from

Manure %Electricity sales 0.0350 28.23

Green tags from electricity 0.0150 12.10

Carbon credits 0.0075 6.05

Tipping fees 0.0500 40.32

Bedding saving 0.0120 9.68

Renewable energy tax credits 0.0045 3.62

Total revenues 0.124 –

© 2008 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 3:11–19 (2009); DOI: 10.1002/bbb 13

In the Field: Co-digestion of food waste water with manure for biogas production Y Liu, SA Miller, SI Safferman

Optimal anaerobic digestion occurs at a pH of 6.5

to 8.0; temperature of 25–45°C (mesophilic), 45–60°C

(thermophilic), < 20°C (psychrophilic); carbon/nitrogen/

phosphorous (C/N/P) ratio at 100–128/4/1.6 Total energy

from vegetable- and fruit-processing waste water alone may

not balance the energy required to maintain the temperature

of the anaerobic digester. Further, food-processing waste

water oft en does not have the optimal C/N/P ratio for anaer-

obic digestion. For many types of waste, the carbon content

is much higher than the available nitrogen and there may

not be adequate trace nutrients. Blending food-processing

waste water with manure, which has a high nitrogen and

carbon content, off ers the potential to improve the perform-

ance and biogas potential of anaerobic digestion.7

In this article, a protocol to determine energy produc-

tion from co-digestion of fruit- and vegetable-processing

waste water and manure was developed. To illustrate the

use of the protocol, a case study was conducted using wastes

in Michigan. Th eoretical biogas production via anaerobic

digestion and energy potential were calculated at diff erent

blending ratios and concentrations of manure and fruit- and

vegetable-processing waste water.

Protocol of co-digestion of fruit- and vegetable-processing waste with manure

A four-step protocol on co-digestion of fruit- and vegetable-

processing waste water and manure to produce biogas is

represented in Figure 1. Each step is explained below.

Step 1. Analysis: BOD, chemical oxygen demand (COD),

total solids (TS), volatile solids (VS), pH, total nitrogen

and phosphorous concentrations are measured to charac-

terize raw waste materials (food solids, food waste water

and manure). Th e waste materials are then separated into

two categories based on their characteristics – A: adequate

nutrients with pH buff er capability and high volatile solids;

B: defi cient nutrients or lack of pH buff er capability.

Step 2. Blending: Wastes in Category B can be blended with

those in Category A to reach the minimum nutrient require-

ment such as the C/N of 25 ~ 32:1 or the C/N/P of 100 ~

128:4:1.6 Blending may also be required to reach the desired

COD needed to provide enough energy to operate the digester.

Step 3. Energy production: Th e theoretical biogas and

energy production for blended wastes are calculated.

Experimental tests may be included to compare the theoret-

ical yield and experimental results. Th e detailed calculations

are described in the following case study.

Step 4. Energy balance: Based on the theoretical gas

production found in Step 3, an energy-balance analysis is

conducted to determine if anaerobic digestion generates

the energy required for heating the digester. If not, further

blending with a waste containing a high concentration of

COD is needed, if available.

Case study

To illustrate the protocol, a case study was conducted to esti-

mate the amount of dairy manure needed to enable the effi -

cient anaerobic digestion of the most commonly processed

vegetable- and fruit-processing waste water in Michigan.

Step 1: Analysis

Th e top 14 vegetable and fruit commodities in Michigan

were identifi ed by their mean production volumes. BOD

concentration and total annual amount of processing waste

water are presented in Table 1. Waste water from vegetables

typically have low C/N ratio and fruits have high ratios

(Table 3).8 Th e condensate of food waste (CFW) has a high

Figure 1. Protocol fl owchart of anaerobic digestion of food and animal wastes.

Food solid waste

Adequate pH buffer and enough nutrients

Deficient of nutrients or pH buffer

1. Analysis (Nutrients and

pH buffer)

Food waste water

2. Blending

Manure

Energy revenue ∆E>0

∆E<=04. Energy balance check

3. Energy production from anaerobic digestion (Theoretic calculation and experimental results)



organic concentration with relatively low nutrient content

(Total-COD/Total-N ratio = 159).9 Consequently, this waste

material falls under Category B, discussed above, as the

nutrient level is not adequate.

A typical 636-kg dairy cow produces 18 560 kg of manure

annualy.10 Th e properties are shown in the Table 4. Manure

has a relatively low C/N of 11:1, and high solid concentration

of 13%.11 Due to the sludge-like characteristics of manure,

diluted manure with 6 to 7% total solid is usually used in

anaerobic digestion. Dilution also reduces the inhibitory

eff ects of ammonia and hydrogen sulfi de and improves the

rheology in the anaerobic digestion.10 Much of this dilution

oft en occurs during collection and storage. Flush systems

to remove manure from dairy barns are widely used in the

USA. One to two hundred gallons per cow per day of dilu-

tion water are typical used to fl ush a barn. Table 5 presents

the characteristics of diff erent diluted manure based on the

daily manure production from a 636-kg milk cow. Although

manure from dairy cows is used for the case study high-

lighted in this article, other types of manure may also be

appropriate for blending.

Step 2: Blending

In order to maintain microbial activity during anaerobic

digestion, a minimal C/N/P of 100 ~ 128:4:1 or C/N of

25 ~ 32:1 is required.6,12,13 For vegetable waste, COD can

Table 4. Annual dairy manure production by a 636-kg milk cow.10

Characteristics Raw manure Manure (kg) 18 560

Manure (L) 18 652

Total solids (kg) 2 320

Volatile solids ( kg) 1 972

COD (kg) 2 072

TKN (kg) 104

Table 5. Characteristics of different kinds of manure.10,11

CharacteristicsRaw manure per

cow per dayManure with 378 liter per cow per day fl ush water

Manure with 756 liter per cow per day fl ush water

Manure (L) 51 429 807

Total solids (mg/L) 124 390 14 730 7 847

Volatile solids (mg/L) 105 731 12 521 6 670

COD (mg/L) 111 062 13 152 7 006

BOD (mg/L) 52 887 6 263 3 336

TKN (mg/L N) 5 598 663 353

Total phosphorus (mg/L) 871 103 55

Total carbon (mg/L) 60 750 – –

Table 3. Carabon and nitrogen content in food waste water.

CropNH4-N

(g/L) Organic N (g/L)

Inorganic C (g/L)

Organic C (g/L)

Total C to Total N Ratio

Potato8 0.19 0.48 0.10 5.91 9.0

Fruit8 0.02 0.26 0.00 6.58 22.8

Vegetable8 0.16 0.15 0.48 0.92 4.5

Grape18 0.00 0.08 0.02 6.11 81.1

Grape 28 0.03 0.34 0.03 24.2 65.0

CFW9 0.083* 13.20 159*

* The condensate of food waste (CFW) had total nitrogen of 0.083 g/L and total carbon counted as COD concentration of 13.20 g/L with pH value of 4.0.



conveniently serve as a surrogate for carbon, based on the

reported COD/N/P of 100/4.3/0.9.14 Th e waste water from

fruit/vegetable processing has a C/N from 4.5 to 77 (Table 3),

along with a relatively low COD concentration (less than

6 g/L) and low pH of 4.0 (Tables 1 and 3). Raw manure has

110 g/L of COD and a C/N of 11 (Table 5), which suggests

that it may serve as a concentrated nutrient source that can

be blended with food waste water to generate an optimal

feedstock for anaerobic digestion. Table 6 shows examples

that theoretically mixing of waste water with high C/N (or

using COD as a surrogate for C) and manure to produce

a desired C/N ratio of 30. Additionally, manure has very

strong buff er capacity that can control the pH of mixture

feedstock at the optimal value of 7–8 for anaerobic digestion,

which overcomes another inherent diffi culty associated

with waste water from fruit- and vegetable-processing

waste water.

Step 3: Energy production from anaerobic digestion

Th e energy potentials from fruit- and vegetable-processing

waste water can be estimated by Eqn 1.

Ead = abcd = 30.0 × abc (1)

Ead

: annual energy potential associated with each waste

(kJ/yr).

a: annual mean waste water BOD (kg/yr).

b: ratio of COD to BOD (diff erent waste water may have

diff erent COD/BOD; in this article b = 2.1 was used

however, determining the specifi c value for the waste

water is recommended).15

c: energy produced by destroyed COD, (c = 12 660 kJ/kg

COD destroyed via anaerobic digestion).16

d: fraction or yield of COD destroyed by anaerobic process.

(Diff erent sources of COD may have diff erent conver-

sion rates. In this article, a typical anaerobic digestion

conversion rate of 30% was used to calculate the poten-

tial energy.5) Th is yield is relatively conservative because

adding food waste to a dairy digester can increase COD

conversions to biogas and improve methane content

within the biogas.7 A more accurate conversion rate can

be obtained from experiments.

Based on Eqn 1, the potential energy from fruit- and

vegetable-processing waste water and dairy manure were

calculated as shown in Table 1 and Table 7, respectively.

In terms of energy generated per liter of feedstock, food-

processing waste water has relatively lower energy potential

than animal manure because of the typically lower COD.

Step 4: Energy balance

Th e cost and revenues associated with an anaerobic

digester are complex. In considering the input of energy,

the most important components are heating the reactor to

optimal temperatures and the transportation of feedstock

Table 6. Calculation of carbon/nitrogen ratio from waste blending.

Food waste water 1* Food waste water 2* ManureC (g/L) 6.13 61

COD (g/L) – 13.2 111

TN (g/L N) 0.08 0.083 5.6

C/N 77 – 11

COD/N – 159 19.8

Calculation1 L manure add A liters food waste water to reach C/N = 30:1 [(61 g/L × 1L + 6.13 g/L × A L)]/[(5.6 g/L

× 1 L + 0.08 g/L × A L)] = 30

1 L manure add B liters food waste water to reach COD/N = 30:1 [(111 g/L × 1 L + 13.2 g/L × B L)]/

[(5.6 g/L × 1L + 0.083 g/L × B L)] = 30

A = 29 L, 1 L manure added into 29 L food waste water can make fi nal C/N ratio to 30:1.

B = 5.3 L, 1 L manure added into 5.3 L food waste water can make fi nal COD/N ratio to 30:1.

*: Grape waste water and condensate food waste water (CFW) from Table 3 were used in the calculation.

Table 7. Potential energy produced from manure generated from 1000 cows.

Manure (105 L/yr) COD (103 kg/yr) Energy* (103 MJ/yr)

186 2065 7868

*Calculated using Eqn (1).



to the digester. Transportation of feedstock is highly site-

dependent and the energy requirements cannot be genera-

lized. Th e energy to maintain proper digester temperature

can be estimated, as discussed below. Mesophilic bacteria

require a temperature of 30o~35oC for optimum biological

activity. Th e waste stream and ambient temperature is lower

in several parts of the country resulting in the need to heat

the infl uent and to maintain the digester temperature.

Th e Energy (Qi) needed to heat the digester can be

computed by Eqn (2).

Qi = m Cp (To − Tt) (2)

Qi: energy needed to heat the digester to the optimal

temperature (kJ/h)

m: mass fl ow rate (kg/h)

To: effl uent temperature which is equal to the digester

temperature (°C)

Ti: infl uent temperature (°C)

Cp: specifi c heat of feedstock (mixture of fruit/vegetable

waste water and manure), approximated at water’s

specifi c heat, 4.186 kJ/kg °C

An anaerobic digester heat transfer model was used to

calculate the heat transfer through the cover, fl oor, and walls

of a below-ground, lagoon-type digester at diff erent weather

conditions and generate diff erent heat-loss-to-biogas-heat

ratio for various geometrical parameters.17 In this article,

energy leaving the system to the surrounding environment

is assumed at 5% of energy generated from biogas in calcu-

lations of the case study. A more accurate ratio should be

adjusted for real system accordingly. Th e minimal energy

needed to heat the digester (Ead, min

, kJ/h) from biogas can

be calculated using Eqn 3. Combining Eqns 1 to 3, the net

energy (Enet

, kJ/h) and minimal COD concentration (g/L) to

make energy balance can be estimated using Eqns 4 and 5,

respectively.

Ead,min = m Cp (To − Ti) + 0.05 × Ead (3)

Enet = 0.95 Ead − Qi (4)

CODmin = kg

1000 g

L

1kgQi

0.95 × m × c × d× × (5)

For the Michigan case study, the initial temperature of

fruit- and vegetable-processing waste water is estimated at

21°C in summer, 11°C in spring and fall and 5°C in winter.

Th ese temperatures are based on the average temperature

of the Lansing, Michigan region in 2007.18 If the diges-

tion temperature is 35°C, the energy available from the

biomass and required to heat the digester was calculated

(Table 8). Waste water exiting a processing plant may,

however, be warmer than ambient temperature used in this

example because of heating that may have been part of the

processing. Assuming that all of the fruit- and vegetable-

processing waste water is anaerobically digested, the energy

produced is 13 × 103 MJ/h calculated from Eqn 1 (Table 8).

As observed in Table 8, the energy required to heat the

waste water (85 × 103 MJ/h, 146 × 103 MJ/h, 183 × 103 MJ/h in

summer, spring/fall and winter, respectively) is much higher

than the energy generated from the digester. To produce

enough biogas to provide the minimum heat for the digester

requires COD concentrations of the feedstock in summer,

spring/fall and winter of 16 g/L, 28 g/L and 35 g/L, respec-

tively (Table 8). Th is analysis ignores the likely imbalance of

nutrients (not meeting the optimum C/N/P ratio) typical of

Table 8. Energy balance of digester at different seasons.

System Calculation Season

Reactor at 35°CSummer X = 21°C; Fall X = 11°C; Winter X = 5°C

summer fall /spring winter

Ead, energy generated from biogas (103 MJ/h) Energy produced from top 14 crops* (Eqn 1) 13 13 13

Qi, energy to heat digester (103 MJ/h) (Eqn 2)** 85 146 183

Energy balance (103 MJ/h) Net energy generated from biogas (Eqn 4) −73 −134 −171

Minimum COD (g/L)COD concentration to make energy balance, Enet = 0 (Eqn 5) 16 28 35

*: Calculated as the total energy produced from top 14 crops from Table 1.

**: Total waste water mass fl ow was 1458 × 103 kg/h (12,773 × 106 L/yr in Table 1, assuming density at 1 kg/L).



fruit- and vegetable-processing waste water and the resulting

non-optimum biogas production.

In Table 9, calculations were made showing how much

manure is needed to be blended with the fruit- and vegetable-

processing waste water to produce a feedstock with a COD

concentration of 35 g/L COD (the minimum amount needed

to heat the digester in winter). For this analysis, the COD/

BOD the fruit- and vegetable-processing waste water was

assumed to be consistent at 2.1:1 and the percent of COD

that was converted to biogas is 30%. If actual values can be

obtained or measured, a more accurate estimation will result.

Th e above analysis was designed to provide an adequate

COD to provide the energy needed to heat the digester. An

adequate C/N ratio is also needed, however. For the above

analysis, the COD/N ratio was in the range of 20 ~ 22:1

(Table 9), which was satisfi ed the minimal range of C:N ratio

of 25 ~ 32:1 assuming COD is an adequate surrogate for C.

Table 9 shows the amount of manure required to be blended

with specifi c commodities and the total energy produced

from the blended wastes.

Raw manure, with a COD concentration about 111g/L, is

usually diluted to 6–7% solid concentration for digestion.10

Th is water oft en originates from fl ushing barn and storage

lagoons. An interesting scenario is to consider the use of

fruit- and vegetable-processing waste water as cleaning and

dilution water. Although much detail needs to be consid-

ered concerning the feasibility of such a scenario, including

techniques to maintain sanitary conditions, the procedure to

calculate energy-related parameters is interesting. Although

not practical at State level, for convenience these calculations

have been made in Table 10 for the case study that has been

threaded throughout this article. Th e waste water, as tabu-

Table 9. Blending manure with food waste water and its potential energy production.*

CropWaste water

(106 L/yr) Food: Manure Ratio, R (v:v)*

Manure needed** (106 L/yr)

Cows*** COD:N+ Energy++ (106 MJ/yr)

Apples 2 389 2.3:1 1 041 55 804 20.6 456

Cucumbers 2 166 2.4:1 911 48 859 21.1 409

Potatoes 1 550 2.6:1 594 31 843 22.6 285

Squash 1 384 2.2:1 622 33 351 20.2 267

Cherries, Tart 1 365 2.3:1 590 31 635 20.7 260

Beans, Snap 998 2.2:1 448 24 008 20.2 192

Tomatoes 915 2.2:1 412 22 070 20.1 176

Grapes 460 2.3:1 203 10 864 20.4 88

Pumpkins 448 2.4:1 190 10 174 21.0 85

Carrots 447 2.3:1 192 10 307 20.8 85

Blueberries 322 2.3:1 142 7 638 20.4 62

Peppers 134 2.3:1 59 3 144 20.5 26

Cherries, Sweet 116 2.3:1 49 2 644 20.9 22

Peas 80 2.3:1 35 1 868 20.6 15

* COD concentration is fi xed at 35 g/L (the amount needed to close the energy balance in winter), the COD/BOD of the fruit-and vegetable-processing waste water was assumed to be consistent at 2.1:1 and the percent of COD that was converted to biogas was 30%. The ratio of food: manure, R: 1 was calculated as:

(COD Concentration of manure, mg/L × 1) + (COD concentration of crop, mg/L × R) = 35000 mg/L × (1+R).

** Manure needed L/yr to blend food waste water was calculated as: (volume of waste water L/yr)/R.

*** Total cows needed for blending was calculated as: (manure volume, L/yr)/ (18652 L/cow/yr).+ Calculated as: (waste water, L/yr + Manure, L/yr) × 35 g/L COD/(104000 g/cow/yr N × cows needed from blending), ignoring nitrogen from

food waste water.++ Calculated using Eqn (1).



lated in Table 1, can dilute manure generated by about 175%

more cows than are in Michigan (Michigan has 3013 dairy

operation with total number of 300 000 diary cows3). Th is

assumes plug-fl ow digesters with COD loading of 60 g/L will

produce net energy in all seasons (Table 10).

Discussion and conclusions

In this article, a protocol to evaluate the potential energy

production and balance of anaerobic digestion was presented

along with a case study. Energy generated from biogas can

balance the energy required for heating the digester if the

COD concentration of the feedstock is above a critical level.

In regions with low ambient temperature, this critical level

may be high. Because of the importance of temperature in

determining the economics of digestion, direct measure-

ments instead of estimates are important to increase the

accuracy of energy estimates. Process water may be warmer

than ambient temperature if it was heated in processing or

used for cooling. Similarly, the characterization parameters

should be directly measured whenever possible to increase

the accuracy of predictions.

Blending wastes to form a feedstock that optimizes biogas

is only feasible if the animal agricultural and fruit/ vegetable

processing plant is in close proximity. Short distance

between food facilities and farms is essential to minimize

transportation costs of wastes. Th is article focused on fruit

and vegetable waste water. A site evaluation will consider a

wider range of biomass sources in the evaluation of project

feasibility. Th is concept, a centralized digester producing

biogas, is a well-established technological practice in

Denmark. 19

To help locate centralized digesters, several States have

developed mapping systems as a tool to locate waste

biomass. Particularly noteworthy is the Interactive Mapping

Anaerobic Digester resource in Iowa (http://programs.

iowadnr.gov/ims/website/digester/viewer.htmand), main-

tained by its Department of Natural Resources. Michigan is

developing a unique tool that not only identifi es sources of

waste biomass but also non-productive land that can be used

to grow energy-rich biomass. For specifi c locations, the tool

estimates gross energy availability and energy requirements

to process the biomass; it also identifi es constraints that may

substantially impact project feasibility. Policy issues must

be considered as well as energy potential. Of importance is

the legal classifi cation of the food-processing waste water. If

considered industrial waste, regulation concerning its trans-

portation may be more stringent than the transport of agri-

cultural residual. Also the land application of the digestate

may not be permissible.

As the purpose of the protocol is to provide a rough theo-

retical screening tool to determine the feasibility of blending

waste, if the analysis shows promise, the next step is to

conduct simple biogas assay potential assays. Such assays

use actual feedstock, an idealized microbial community, and

all needed nutrients so that the amount of gas produced can

Table 10. Potential energy generated from co-digestion of food waste water and manure.*

System Calculation SeasonSummer Fall/Spring Winter

COD (g/L) 60 60 60

Waste water Mass Flow (103 kg /h) Total food-processing waste water (Table 1) 1 458 1 458 1 458

Raw diluted manure (103 kg /h) Raw manure with COD concentration at 110 g/L10 1 750 1 750 1 750

Total mass fl ow (ton/h) waste water plus manure 3 208 3208 3 208

Cows needed per year (cows/yr)18 560 kg of raw manure produced by a cow annually (Table 4) 8 25 970 8 25 970 8 25 970

Percentage of total cows in Michigan (%) 300 000 cows in Michigan3 275 275 275

Energy heat digester (103 MJ/h) (Eqn 2) 188 322 403

Energy generated from biogas (103 MJ/h) (Eqn 1) 731 731 731

Net energy (103 MJ/h) (Eqn 4) 506 372 292

* Calculations assumed using a plug-fl ow digester with COD loading of 60 g/L and density at 1 kg/L.



be compared to that estimated. Th ese studies are typically

conducted using serum bottles or anaerobic respirometry.20

If low gas production results, this could indicate a potential

toxicity issues. Th e results of the analysis demonstrate

the high energy requirements in northern climates for

heating waste to optimal levels for digestion and the need

for blending waste to achieve economical operation of an

anaerobic digester.

Reference 1. Safferman S, Wright T, Miller S and Higginbotham A, Alternative for food

processors wastewater, http://www.egr.msu.edu/~safferma/Research/

Greeen/greeenprojecttasks.html

2. Matteson GC and Jenkins BM, Food and processing residues in

California: resource assessment and potential for power generation.

Bioresource Technol 98:3098–3105 (2007).

3. EPA, Market opportunities for biogas recovery systems, http://www.epa.

gov/agstar/resources.html

4. VanderHaak Dairy Digester www.harvestcleanenergy.org/conference/

HCE7/PDFs/VanderHaak.pdf

5. USDA, An analysis of energy production costs from anaerobic digestion

systems on U.S. livestock production facilities. USDA Technical Note

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Yan Liu, Ph.D.

Yan Liu, an Assistant Professor at the

Department of Biosystems and Agricultural

Engineering at Michigan State University, has

been working in the bioenergy/bioproducts

area for more than 10 years. Current research

projects Dr Liu is working on are: anaerobic

digestion systems to convert animal and food

wastes to renewable energy and other value-added products;

algal culture to produce renewable energy such as bioethanol,

biodiesel and hydrogen; microbial community dynamics and

biochemistry during agricultural/industrial waste treatment; fungal

fermentation to produce high-value products such as organic

acids, enzymes, and nutraceutical/pharmaceutical products.

Dr Liu received her first doctorate in food science from China

Ocean University, China, and her second doctorate in biosys-

tems engineering from Washington State University. D. Liu was a

research associate at Washington State University before joining

Michigan State University.

screening co-digestion of food waste water with manure for biogas production

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