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Wastewater transformations and fertilizer value whenco-digesting differing ratios of swine manure and usedcooking grease in low-cost digesters

Stephanie Lansing a,*, Jay F. Martin b, Raul Botero Botero c, Tatiana Nogueira da Silva c,Ederson Dias da Silva c

aDepartment of Environmental Science and Technology, University of Maryland, 1445 Animal Sci./Ag. Eng. Bldg., College Park,

MD 20742-2315, United StatesbDepartment of Food, Agricultural, and Biological Engineering, The Ohio State University, 590 Woody Hayes Drive, Columbus,

OH 43210-1057, United StatescEARTH University, Apartado Postal 4442 e 1000, San Jose, Costa Rica

a r t i c l e i n f o

Article history:

Received 2 December 2008

Received in revised form

17 June 2010

Accepted 1 July 2010

Available online 24 July 2010

Keywords:

Anaerobic digestion

Methane

Biogas

Renewable energy

Costa Rica

* Corresponding author. Tel.: þ1 301 405 119E-mail address: slansing@umd.edu (S. La

0961-9534/$ e see front matter ª 2010 Elsevdoi:10.1016/j.biombioe.2010.07.005

a b s t r a c t

A nine-month co-digestion investigation was conducted in Costa Rica to optimize animal

wastewater treatment, renewable energy production, and fertilizer creation using 12

Taiwanese-model, plug-flow digesters (250 L each) constructed of tubular polyethylene and

PVC piping, operating without mechanical or heating components. The experiment tested

three replications of four treatment groups: the control (T0), which contained only swine

manure, and T2.5, T5, and T10, which contained 2.5%, 5%, and 10% used cooking grease (by

volume) combined with swine manure.

T2.5 had the greatest methane production (45 L d�1), a 124% increase from the control. No

adverse effects were observed from co-digesting 2.5% grease in terms of organic matter

removal, pathogen reduction, grease removal, and pH. Chemical oxygen demand (COD)

was reduced 94.7% to 1.96 g L�1, fecal coliforms and Escherichia coli were reduced 99.2 and

97.1%, respectively, and grease removal was 99.9%. The average effluent pH (7.05) and

alkalinity in T2.5 was within the optimal range for methanogens and increased signifi-

cantly during the nine-month experiment, likely due to adaptation of the methanogenic

organisms to the influent grease concentrations. Total nitrogen concentration decreased

34.0%, and NH4-N increased 97.1% during digestion in T2.5, with no significant differences

between T2.5 and T0. There was less phosphorus reduction with co-digestion, with

181 mg g�1 of total phosphorus (TP) in T2.5 and only 90.6 mg g�1 of TP in T0, resulting in

lower N:P ratios in the grease treatment groups due to the greater concentration of

phosphorus in the effluent.

ª 2010 Elsevier Ltd. All rights reserved.

7; fax: þ1 301 314 9023.nsing).ier Ltd. All rights reserved.

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 01712

1. Introduction mineralization, (3) increases N uptake shortly after applica-

Untreated wastewater from animal operations results in

contamination of waterways, noxious odors, and the release

of methane, which is a greenhouse gas with 21 times the

global warming potential of carbon dioxide [1]. When properly

harnessed in a digester, however, animal waste can be

transformed into an environmental and economic benefit. A

digester provides an optimal environment for microorgan-

isms that produce methane by using the wastewater as

a nutrient source. The digestion process results in a number of

benefits: the captured methane becomes a source of renew-

able energy, a liquid fertilizer is created, and wastewater

pollution, greenhouse gas emissions, and noxious odors are

sharply reduced [2e5]. Currently, there are over 35 million

small-scale digesters in India and China alone [6]. Small-scale

agricultural digesters can be inexpensive when located in

a tropical climate, and thus, can serve as an appropriate

technology that can increase household income, while

reducing environmental degradation associated with

improper manure management [7e10].

Research and development in digestion technology has

focused on large-scale, capital-intensive systems [11], which

are appropriate for industrial-scale farms, but are largely

inaccessible to the small and medium-scale farmer. Animal

feeding operations with fewer than 200 cows have been found

to be significant contributors of water pollution [12]. A small-

scaledigestercouldallowthesefarmers toutilizeandtreat their

waste on-site, while co-digesting this waste could increase

methane production and the economic value of the digester.

Small-scale digesters are often plug-flow, the contents are

not mixed, and continuous management or internal heating

are not requiredwhen located in a tropical climate [7,8,13]. The

solids tendtosettleout, resulting inbetterdegradationof solids

in these systems compared to completely mixed reactors [14].

Biogas with a highmethane content (60e75%) is produced and

used for heating, cooking, or electricity generation [9,15e17].

The effluent has a reduced odor due to ammonia gas capture,

a reduced concentration of organic pollutants in the effluent,

and during digestion nutrients are transformed from an

organic state to a dissolved state, which is a more useful form

for plant uptake [3,18,19]. A digester can add value to manure,

which is often viewed as a waste and not a resource [8,20].

Previous studies have shown the digester effluent has been

used successfully to enhance crop production, as a feed for

aquaculture, or as a fertilizer for algae and duckweed, which

can be harvested for animal food [14,19,21]. Ref [22] reported

higher corn yields from digested cattle slurry than raw

manure. Other studies have reported similar yields from

digested and rawmanure fertilized crops [23e25]. The benefits

of using digested manure for fertilizer reaches beyond food

production. When land applied, digested manure emits less

greenhouse gases and results in less N loss from leaching,

denitrifcation, and NH3 emissions than raw manure. Ref [26]

found less greenhouse gas emissions (CH4 and CO2)

following land application of digested manure due to the

decreased C:N ratio in digested swinemanure (10.5) compared

to raw manure (17.0). A decreased C:N ratio has a lower

viscosity, which (1) reduces N immobilization, (2) increases N

tion, (4) decreases N loss through denitrification, and (5)

decreases NH3 emissions due to increased infiltration into the

soil surface compared to raw manure [27e29].

1.1. Co-digestion

In order to increase the use and profitability of small-scale

digestion systems inboth thedevelopedanddevelopingworld,

these systems should be optimized for methane production to

increase energy availability. Co-digestion with carbon-rich

foodwastes, suchasgrease,hasbeenused in industrydue to its

positive effect on biogas production [30e33], but themixture is

usually a function of availability and is not always based on

knowledge of an optimal mixture [34e36].

Small-scale digesters are optimal for co-digestion due to

the dispersed nature of waste production sources [34,36]. By

co-digesting manure with grease, a digester can create a safe

and profitablemethod for disposal of household grease, whey,

or restaurant waste, which will extend the life of septic tanks,

the only waste system available in most rural areas [37]. The

small amount of grease co-digested withmanure ensures that

the most abundant waste source in rural areas, manure, can

be digested with obtainable amounts of seasonal or small-

scale production of grease wastes.

Previous studies have shown that digesting materials with

high-lipid content increases methane yield [38]. It has been

established that manure is the best co-digesting material for

high-fat wastes due to the high alkalinity of manure, which

increases digester resistance to acidification [39e42]. Addi-

tionally, manure has high ammonium levels, which are

important in bacterial growth. Co-digesting olive oil mill

wastewaters and fish oil (5%) with manure doubled the

production of biogas due to the higher concentration of lipids

and the higher biodegradability of the oil/grease containing

wastewaters compared to manure [40]. Co-digestion of olive

mill wastewaters with diluted poultry manure also increased

methane production by 150% without chemical addition [42].

Previous co-digestion studies have largely been conducted on

lab-scale systems or small, un-replicated pilot-scale systems

that are highly specialized, expensive, and energy intensive to

build and maintain [40,42e45]. These systems are largely

inaccessible to small-scale farmers [8,40,46].

1.2. Study objectives

Swine manure was co-digested with small amounts of used

cooking grease (2.5, 5, and 10% by volume of grease) using 12

field-scale digesters over nine months to investigate methane

production and transformation/removal processes of organic

matter, solids, nutrients, indicator microorganisms, and

grease/fats. The objective of the studywas to determinewhich

ratio of manure and grease was optimal both in terms of

methane production and wastewater treatment, and more

specifically, this study sought to determine if the increase in

methane production that could be obtained from the addi-

tional organic loading of the used cooking grease also resulted

in decreased fertilizer value or increased environmental

contamination risk.

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 0 1713

2. Methods

2.1. Site description

The study was conducted in an outdoor digester laboratory at

the Escuela de Agricultura de la Region Tropical Humeda

(EARTH) University in Costa Rica. EARTH University is located

in the humid tropics (10� 110 N, 83� 400 E) at an elevation of

50 m, with average temperatures ranging from 24 �C in

January to 26 �C in May, and an average annual precipitation

of 3e4 m, distributed throughout the calendar year. The

digester laboratory is housed in an open-air, roofed building,

with the digesters operating at ambient temperatures.

Twelve field-scale Taiwanese-model, plug-flow digesters

were constructed in the digester laboratory using 250 L tubular

polyethylene bags. Each tubular bag had a diameter of 0.32 m,

and length of 3.1 m, and the polyethylene material had

a thickness of 0.2 mm (Fig. 1). The digesters had an 80% liquid

phase (200 L), with up to 50 L available for in-vessel biogas

storage. The majority of the produced biogas flowed by pres-

sure to a 250 L biogas storage bag located above each digester.

In the co-digestion experiment, there were three replica-

tions of four treatment groups: the control (T0), which con-

tained only swine manure and no waste grease, and T2.5, T5,

and T10, which contained 2.5%, 5%, and 10% used cooking

grease (by volume) combined with swine manure. The

digesters were fed daily with 5 L of the prescribed mixture.

Each digester had a 40-day retention time. To begin the study,

manurewas added to all 12 digesters for 90 days to ensure that

all digesters were beginning the experiment with an active

microbial community. The methane production stabilized

after 90 days, and the nine-month experiment (May

2007eFebruary 2008) commenced.

The swine manure was collected daily from the EARTH

University 50-pig farm. The pigs were, on average, 50-kg and

were fed campus cafeteria food wastes, sugar cane, whey,

floating aquatic vegetation, and protein feed. The daily prac-

tice at the farm was to wash the concrete stalls daily with

Fig. 1 e Pictured are the 12 Taiwanese-model, plug-flow

digesters used in this experiment housed in an outdoor

digester laboratory at the Escuela de Agricultura de la

Region Tropical Humeda (EARTH) University in Costa Rica.

The digesters consist of tubular polyethylene, PVC piping,

and plastic hosing. There are no mixing components,

mechanical devices, or heating apparatus in the digesters.

a high velocity spray hose. Themanure/wash water flowed by

gravity through a 1-inch mesh screen to remove fibers and

into a full-scale anaerobic digester that supplied cooking fuel

and electricity for the farm [17]. The accumulated manure

fibers collected on the screens were removed daily and com-

posted. There was no bedding in the stalls.

For this experiment, manure was manually collected daily

using a shovel from the concrete pins before the morning

washing, placed in two 5 gallon buckets, and carried to the

outdoor digestion laboratory. Once in the digester laboratory,

themanurewasfiltered througha1-inchsieve to removefibers,

and the liquid fraction was diluted 4:1 with wash water in

accordance with farm practices. The used cooking grease was

collected monthly from the EARTH University cafeteria and

stored in a 250 L opaque plastic drum at ambient temperature.

The water quality characteristics of the manure and used

cooking grease are listed in Table 1. The increase in volatile

solids (VS) fromthecontrolwithgreaseadditions intheT2.5,T5,

andT10treatmentgroupswere113,206,and453%, respectively.

2.2. Water quality analysis

Influent and effluentwater sampleswere collected everyweek

fromMay15, 2007 toAugust 15, 2007 andevery twoweeks from

September 1, 2007 to February 1, 2008. The samples were

analyzedon-site for temperatureandpHusingahand-held556

MPS YSI� probe. Samples were analyzed for chemical oxygen

demand (COD), volatile solids (VS), ammonium (NH4-N), total

kjeldahl nitrogen (TKN), alkalinity, phosphates (PO4-P) and

total phosphorus (TP) using standard methods at the EARTH

University Water and Soil Laboratory [47].

Influent and effluent samples were collected and analyzed

at LAMBDA Laboratory in San Jose, Costa Rica for percentage

of grease, total coliforms, and Escherichia coli (E. coli) within

24 h of collection [47,48]. The total coliforms and E. coli results

are reported as Most Probable Number (M.P.N) per m3.

2.3. Biogas analysis

Biogas production was measured three times a week using 12

American Meter Company gas flow meters (model AC-250) with

IMAC Systems pulse digital counters and a vacuum pump. The

percentage of CH4 in the produced biogas was analyzed

weekly using an IR-30M methane meter (Environmental

Sensors�). Methane production data was calculated by

multiplying the biogas production rate by the percentage of

methane in the produced biogas.

2.4. Statistical analysis

Biogas and water data were analyzed using analysis of vari-

ance (ANOVA) and subsequent TukeyeKramer multiple

comparisons to determine which variables were significantly

different. P values <0.05 were considered significant. The

following variables were log-transformed in order to meet the

ANOVA assumptions: COD, TS, VS, turbidity, TP, total coli-

forms, E. coli, and biogas production. After transformation, the

skewness, kurtosis, and normality improved. All values in the

results section and tables are averages and include standard

error and n-values.

Table 1 e Waste characteristics for the swine manure and used cooking grease used in the experimental digesters. Allvalues are averages ± standard error, with number of samples in ( ). The waste characteristics include temperature, pH,chemical oxygen demand (COD), and volatile solids (VS).

Temperature (�C) pH COD (g L�1) VS (g L�1)

Manure 25.2 � 0.3 (26) 7.5 � 0.1 (26) 20.0 � 1.1 (25) 13.7 � 1.4 (23)

Used Cooking Grease 26.8 � 0.5 (19) 4.6 � 0.2 (18) 811 � 195 (19) 714 � 75 (16)

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 01714

3. Results

3.1. Temperature, pH, and alkalinity

All influent and effluent results are given in Tables 2 and 3,

respectively. The liquid temperature of the digesters averaged

25.5 �C. The influent and effluent temperatures were signifi-

cantly greater in the first six months (26.8 and 25.6 �C,respectively) of the experiment (MayeOctober) than the last

three months (24.2 and 23.5 �C, respectively) (Novem-

bereJanuary) (influent ANOVA P < 0.001; F ¼ 29.2; effluent

ANOVA P < 0.001; F ¼ 277). There were no significant differ-

ences in influent pH amongst the four treatment groups, likely

due to the immiscible nature of the grease/manure mixtures

interfering with pH measurements (Table 2). The average

effluent pH of the last three months (7.17) of the experiment

was significantly greater than the first six months (6.98)

(ANOVA P < 0.001; F ¼ 36.1).

Effluent pH and alkalinity were significantly greater in the

control than T2.5, but only by 1.8 and 2.2%, respectively (Table

3). The influent and effluent alkalinities were significantly

greater in the last three months of the experiment (2.22 and

2.46 g L�1 as CaCO3, respectively) than the first three months

(1.60 and 1.83 g L�1 as CaCO3, respectively) (influent ANOVA

P ¼ 0.008; F ¼ 5.23; effluent ANOVA P < 0.001; F ¼ 34.1).

3.2. Organic matter and solids reductions

Overall, COD concentrations in the 12 digesters were reduced

94.7% from the influent to the effluent (ANOVA P < 0.001;

F ¼ 2720), with all grease treatments having significantly

higher influent COD concentrations than the control (Table 2),

Table 2e Influentwaste characteristics for the four treatment gon co-digesting swinemanure and used cooking grease. Thew(COD), total solids (TS), volatile solids (VS), total kjeldahl nitrogorthophosphate (PO44

L-P). All values are averages ± standard esignificant differences among treatment groups (within each r

Variable Units T0 T2.5

Temperature (�C) 25.2 � 0.3 (26) 26.4 � 0.4 (26

pH 7.49 � 0.10 (26) 7.28 � 0.09 (2

Alkalinity g L�1 as CaCO3 2.44 � 0.16 (19)A 2.18 � 0.12 (1

COD g L�1 20.0 � 1.1 (25)A 37.8 � 3.8 (23

TS g L�1 18.0 � 1.6 (22)A 34.4 � 3.1 (24

VS g L�1 13.7 � 1.4 (23)A 29.2 � 2.8 (24

TKN mg g�1 925 � 40 (23) 864 � 42 (25

NH4þ-N mg g�1 214 � 13 (21) 224 � 14 (24

TP mg g�1 285 � 22 (15) 246 � 22 (15

PO4�eP mg L�1 148 � 21 (13) 164 � 22 (11

Oil/Grease content mg L�1 639 � 98 (3)A 15800 � 3610 (

and T10 having significantly higher effluent COD than all

other treatments (Table 3). Overall, the TS concentrations

were reduced 92.9% from the influent to the effluent (ANOVA

P-value < 0.001; F ¼ 3520) and the VS concentrations were

reduced 96.6% from the influent to the effluent (ANOVA

P< 0.001; F¼ 3730). The influent and effluent VS concentration

in T10 were significantly greater than all other treatments

(Tables 2 and 3).

3.3. Nutrient transformations

On average, TKN was reduced 36.2% from the influent to the

effluent (ANOVA P < 0.001; F ¼ 387), while NH4þ-N increased

87.8% (ANOVA P < 0.001; F ¼ 467). There were no significant

differences amongst the four treatment groups in the influent

TKN, NH4þ-N, TP, or PO4

�-P concentrations (Table 2). The

average TPwas reduced 33.5% from the influent to the effluent

(ANOVA P < 0.001; F ¼ 60.8), and PO4�-P was reduced 33.2%

(ANOVA P < 0.001; F ¼ 37.4). T0 had significantly higher

effluent NH4þ-N concentration than T5 and T10 and signifi-

cantly lower TP and PO4�-P concentrations than all grease

treatments (Table 3).

3.4. Grease removal

The grease concentrations in T2.5 and T5 were reduced 99.9%

(ANOVA P < 0.001; F ¼ 211; ANOVA P < 0.001; F ¼ 177,

respectively), and 99.8% in T10 (ANOVA P < 0.001; F ¼ 49.7)

(Tables 2 and 3). The effluent grease concentrations were not

significantly different between T0, T2.5 and T5 (Table 3). In

T2.5 and T5, the effluent grease concentrations were signifi-

cantly greater in August and October than in December and

January (ANOVA P¼ 0.007; F ¼ 12.2; ANOVA P< 0.001; F¼ 34.8,

roups (T0, T2.5, T5, and T10) froma nine-month experimentaste characteristics given include chemical oxygen demanden (TKN), ammonium (NH4

D-N), total phosphorus (TP), andrror, with number of samples in ( ). Letters representow) from the TukeyeKramer analysis of difference.

T5 T10 ANOVA results

) 26.5 � 0.4 (26) 26.5 � 0.4 (25) P ¼ 0.08; F ¼ 2.3

6) 7.23 � 0.11 (25) 7.08 � 0.14 (25) P ¼ 0.07; F ¼ 2.4

9)AB 1.87 � 0.15 (19)BC 1.65 � 0.11 (19)C P ¼ 0.001; F ¼ 6.4

)B 44.9 � 4.5 (24)BC 56.5 � 5.6 (24)C P < 0.001; F ¼ 27.8

)B 43.6 � 3.9 (24)B 77.6 � 5.1 (20)C P < 0.001; F ¼ 58.0

)B 41.9 � 4.2 (26)C 75.8 � 5.8 (22)D P < 0.001; F ¼ 70.6

) 941 � 40 (25) 900 � 52 (23) P ¼ 0.60; F ¼ 6.2

) 221 � 14 (24) 225 � 11 (24) P ¼ 0.93; F ¼ 0.15

) 253 � 24 (14) 258 � 23 (15) P ¼ 0.61; F ¼ 0.61

) 134 � 19 (13) 138 � 17 (13) P ¼ 0.76; F ¼ 0.49

3)B 25700 � 750 (3)BC 47200 � 11000 (3)C P < 0.001; F ¼ 98.0

Table 3e Effluentwaste characteristics for the four treatment groups (T0, T2.5, T5, and T10) froma nine-month experimenton co-digesting swinemanure and used cooking grease. Thewaste characteristics given include chemical oxygen demand(COD), total solids (TS), volatile solids (VS), total kjeldahl nitrogen (TKN), ammonium (NH4

D-N), total phosphorus (TP), andorthophosphate (PO4

L-P). All values are averages ± standard error, with number of samples in ( ). Letters representsignificant differences among treatment groups (within each row) from the TukeyeKramer analysis of difference.

Variable Units T0 T2.5 T5 T10 ANOVA results

Temperature (�C) 24.7 � 0.1 (78) 25.0 � 0.1 (78) 24.9 � 0.1 (78) 25.0 � 0.1 (77) P ¼ 0.22; F ¼ 1.5

pH 7.18 � 0.01 (76)A 7.05 � 0.02 (78)B 7.02 � 0.02 (78)B 6.88 � 0.02 (75)C P < 0.001; F ¼ 51.6

Alkalinity g L�1 as CaCO3 2.60 � 0.06 (69)A 2.15 � 0.05 (68)B 2.00 � 0.05 (69)B 1.71 � 0.05 (68)C P < 0.001; F ¼ 46.6

COD g L�1 1.82 � 0.12 (73)A 1.96 � 0.12 (72)A 1.96 � 0.12 (73)A 2.62 � 0.16 (67)B P < 0.001; F ¼ 9.5

TS g L�1 2.98 � 0.06 (72) 3.02 � 0.07 (72) 2.95 � 0.05 (68) 3.11 � 0.08 (67) P ¼ 0.37; F ¼ 1.1

VS g L�1 1.30 � 0.05 (75)A 1.33 � 0.05 (74)A 1.28 � 0.04 (71)A 1.54 � 0.06 (69)B P < 0.001; F ¼ 6.1

TKN mg g�1 605 � 13 (67)A 575 � 12 (60)AB 582 � 11 (62)AB 553 � 12 (63)B P ¼ 0.024; F ¼ 3.2

NH4þ-N mg g�1 447 � 10 (74)A 415 � 9 (71)AB 410 � 9 (74)B 387 � 7 (73)B P < 0.001; F ¼ 7.6

TP mg g�1 90.6 � 5.0 (44)A 181 � 10 (43)B 199 � 8 (44)BC 224 � 7 (40)C P < 0.001; F ¼ 7.6

PO4�eP mg L�1 51.5 � 2.1 (38)A 92.2 � 3.6 (39)B 109 � 4 (38)C 139 � 5 (34)D P < 0.001; F ¼ 85.9

Oil/Grease content mg L�1 10.4 � 1.7 (12)A 9.7 � 2.6 (11)A 14.8 � 3.6 (12)AB 89.7 � 36.4 (9)B P < 0.01; F ¼ 4.3

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 0 1715

respectively), but there were no significant differences in

effluent grease concentrations over time in T10 (ANOVA

P ¼ 0.14; F ¼ 2.8).

3.5. Indicator microorganism removal

The average total coliforms were reduced 99.2% (ANOVA

P < 0.001; F ¼ 27.6) and E. coli counts were reduced 97.1%

(ANOVA P< 0.001; F¼ 29.2) (Table 4). Therewere no significant

differences amongst the treatment groups in effluent coli-

forms (ANOVA P ¼ 0.96; F ¼ 0.10) or E. coli (ANOVA P ¼ 0.98;

F ¼ 0.06). The influent variation was greater than the effluent,

but both were highly variable, which is common with micro-

bial analysis of wastewater [49].

3.6. Methane and biogas production

T10 and T2.5 had the highest biogas production rates, which

were significantly greater than T5 and T0 (Table 5). T0 and T2.5

treatment had a greater percentage of CH4 in the produced

biogas than T5 and T10 (ANOVA P < 0.001; F ¼ 171) (Table 5).

Multiplying biogas production by the percentage of methane

revealed total methane production. Methane production was

greatest in T2.5, which was 124% greater than T0. T10 had the

highest biogas production, but due to the lower percentage of

methane in the produced biogas, the overall methane

Table 4 e Average total coliforms and Escherichia coli inthe influent and effluent of all four treatment groups froma nine-month experiment on co-digesting swinemanureand used cooking grease are given. All values areaverages ± standard error, with number of samples in ( ).The values are reported as most probably number(M.P.N.) per m3.

Total Coliforms Escherichia coli

M.P.N. � 1012 per m3 M.P.N. � 109 per m3

Influent 279 � 106 (9) 376 � 47 (9)

Effluent 2.33 � 0.53 (34) 10.9 � 2.7 (34)

% Reduction 99.2% 97.1%

production was lower. No further benefits, in terms of total

methane production, were seen from increasing the amount

of grease above T2.5.

4. Discussion

The co-digestion experiments on Taiwanese-model, plug-flow

digesters operating at the lower end of the mesophilic range

(22e26 �C) revealed a 124% increase in energy production

when swine manure was co-digested with 2.5% used grease

(by volume). The long retention time (40 days) of the plug-flow

digesters in the current study, combined with high VS

reduction (96.6%), and co-digestingwith amaterial with a high

VS load (113% increase in VS in T2.5 compared to T0) led to

methane yields, based onVS added, of 0.31m3 kg�1 d�1 in T2.5,

which is similar or greater than completely mixed manure

digesters operating at 35 �C treating dilute swine manure [50],

dilute poultry manure [51], and dairy manure [52] (Table 6).

Conversely, the methane yields were similar or lower than

completely mixed digesters that co-digested manure with

high-lipid containing compounds, whose methane yields’

ranged from 0.25e0.68 m3 kg�1 d�1 [41,42,52,53] (Table 6). Co-

digesting could increase the profitability of Taiwanese-model

Table 5 e Biogas quantity (biogas production), quality(methane), and total methane production are given foreach treatment group (T0, T2.5, T5, and T10) from a nine-month experiment on co-digesting swine manure andused cooking grease. All values are averages ± standarderror, with number of samples in (). Letters representsignificant differences among treatment groups (withineach column) from the TukeyeKramer analysis ofdifference.

Treatment BiogasProduction

(L d�1)

Methane (%) MethaneProduction

(L d�1)

T0 28.8 � 0.7 (93)A 69.9 � 0.2 (57)A 20.1

T2.5 67.3 � 1.5 (117)B 66.9 � 0.2 (58)B 45.0

T5 57.8 � 1.4 (117)C 65.9 � 0.2 (65)C 38.1

T10 69.7 � 1.8 (114)BD 63.2 � 0.1 (65)D 44.1

Table 6eComparisons are given between the current study, non-mixed digesters, and completelymixed reactorswith andwithout co-digestion. Variables given include reactor size, temperature, retention time, methane yield, and volatile solid(VS) reduction. Average values or ranges of values from various treatment groups are given.

ReactorSize (L)

Temp(�C)

RetentionTime (days)

MethaneYield

on Influent VS(m3 kg�1 d�1)

VS Reduction(%)

Reference

Non-mixed digesters

Swine manure and used cooking grease 250 22e26 40 0.31 96.6 This study

Dilute dairy manure: baffled reactor 2 35 2e15 0.025e0.166 49e77 [51]

Swine manure: attached growth reactor 20 35 10 0.23a 69b [50]

Completely mixed reactors

Dilute poultry manure 40,000 56.6 10 0.155 49.7 [69]

Dilute dairy manure 2 35 2e15 0.024e0.156 54e72 [51]

Dairy manure 3 37 15 0.224 37 [52]

Completely mixed reactors with co-digestion

Dairy manure and fish offal 1 35 N.A. 0.38c 43.7 [53]

Poultry manure and olive mill wastewater 25 35 20 0.25e0.32 69.8d [42]

Dairy manure and glycerol trioleate 3 37 15 0.382 51 [52]

Swine manure, slaughterhouse,

grease trap, restaurant, fruit and vegetable waste

3 35 28e36 0.68 N.A. [41]

a Methane yield based on COD removed in m3 kg�1 d�1.

b Reduction of volatile suspended solids (VSS).

c Methane yield based on VS removed in m3 kg�1 d�1.

d The authors stated the high VS reduction is partially attributed to weekly removal of foam which was not accounted for in calculating VS

removal; N.A.: not available.

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 01716

digestion systems and allow them to compete with more

expensive completely mixed reactors in terms of methane

production, while operating at lower temperatures without

energy-intensive heating components and removing a greater

percentage of solids and organic pollutants than most indus-

trial digesters.

In addition, to the energy production enhancement capa-

bilities of co-digestion, this study sought to understand the

water quality implications of co-digesting grease in low

temperature, non-mixed, plug-flow reactors. Due to decreases

in total methane production when increasing grease concen-

trations beyond 2.5%, the discussion will focus on the effects

of effluentwater quality in T2.5 compared to the control, T0. In

terms of organic matter (COD, TS, VS), pathogens (indicator

organisms) and grease reductions, no adverse effects were

shown from co-digesting with 2.5% used cooking grease.

Additionally, pH and alkalinity levels rose during the experi-

ment in all treatment groups, with the highest levels seen

during the last three months of the nine-month study, likely

due to adaptation of the methanogenic organisms to the

influent grease concentrations [49]. The average effluent pH in

T2.5 was above 7.0 and within the optimal range (6.5e7.5) for

methanogens [54]. While there were no significant differences

in effluent nitrogen (TKN, NH4þ-N) concentrations between

T2.5 and T0, phosphorus levels (TP and PO4�-P) were signifi-

cantly lower in T0, which affects effluent fertilizer application

rates and implies that solid accumulation was occurring, as

discussed below.

4.1. Organic matter and solids reductions

Small-scale, plug-flowdigesters can reduceorganicmatter and

solids by 50e95% [16,55e58]. The current study had a higher

COD loading (20.0e56.5 g L�1) than previous studies on

Taiwanese-model digesters in Vietnam (1.91 g L�1) [55],

Colombia (0.012 g L�1) [56], and Costa Rica (0.96e4.7 g L�1)

[16,57], which likely led to the higher percent reductions in

COD. The high variability of influent organic matter concen-

trations in Taiwanese-model digesters reinforces the need to

co-digest according to increases in COD (or VS) in the co-

digesting greasematerial and not by volume. T2.5 corresponds

to a 113% increase in VS and an 89% increase in COD compared

to T0. The higher loading rate of the current study in compar-

ison to other Taiwanese-model digesters, suggests that the

optimal volumeof used cooking grease could be even less than

2.5% in currently operating Taiwanese-model digesters.

The percent reduction of VS in the current study (96.6%) is

also higher than completely mixed, non-mixed baffled, and

fixed-film digesters, which operated at higher temperatures

and shorter retention times than the current study and aver-

aged VS reductions of 50% (Table 6). The higher solids reten-

tion time of the plug-flow digesters used in this study likely

contributed to the higher organic matter removal efficiencies

and increased methane production due to additional micro-

bial binding sites within the retained solids and additional

time for the methanogens to utilize the substrate at the lower

operating temperatures [14,58e60].

The non-mixing aspect of Taiwanese-model digesters

encourages settling and long-term degradation of the solids.

Long-term (2e10 years) studies, tracer studies, and sampling

along the length of the plug-flow Taiwanese-model digesters

need to be conducted to determine the effect of solid retention

on digester performance over time and the extent to which

the high VS reductions seen in this study is due to conversion

of substrate to methane versus accumulation of solids within

the digestion environment.

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 0 1717

4.2. Grease digestion and retention in Taiwanese-modeldigesters

While thousands of Taiwanese-model digesters are opera-

tional around the world, digester clogging has been sited as

a concern in many countries [61]. Co-digestion with grease

could compound the clogging risk. The digesters were opened

after the experiment was terminated, and clogging was not

observed in T0 and T2.5, but was evident in T10. In addition, it

was determined that after one year of waste addition VS

concentrations were greater inside the digester than in the

influent wastewater for T0 (46.7%), T5 (65.6%), and T10 (572%),

which can be attributed to the large quantity of active biomass

in the digesters and build-up of recalcitrant material. In

addition to T2.5 having a 124% increase in CH4 production,

there was 14.8% less VS inside the digester compared to its

influent wastewaters. This data implies that: (1) solid accre-

tion is occurring in T0, T5, and T10, but not necessarily in T2.5

(2) co-digestion with small amounts of grease (T2.5) increases

overall digestion efficiency resulting in less sold accumulation

inside the digester over time, but (3) increasing the grease

concentration above this threshold could be detrimental and

decrease the lifetime of the system due to clogging.

The reductions in TP and TKN in T0 (68% and 35%,

respectively) and T2.5 (26% and 34%, respectively) imply that

solid retention is occurring. Decreasing TKN concentrations

have been found in a previous study of Costa Rican Taiwa-

nese-model digesters (45.7%) and in attached growth digesters

(8.4%) [11,54]. TP reductions have been found in a sludge bed

reactor (25.5%), attached growth digesters (20%), and a plug-

flow dairy digester with vertical mixing (8.7%), but no signifi-

cant reductions in TP have been found in numerous studies of

completely mixed reactors treating swine manure

[29,50,62,63]. The significantly higher reduction in TP in the

control group implies that more manure solids were accu-

mulating in T0 compared to the grease treatments. The grease

additions did not contain additional influent phosphorus

(Table 2), and thus it is believed that any grease that did

accumulate in the digester resulted in less manure solids

settling in digestion environment and more manure solids

and the phosphorus associated these solids reaching the

effluent of the grease treatments.

4.3. Indicator organism reductions

Livestock fecal material contains large numbers of microor-

ganisms, including bacteria, viruses, and protozoa. The vast

majority of fecal microbes are harmless, but pathogenic

organisms can be present and their release to aquatic envi-

ronments can be a transport route to humans [64]. Only a few

studies have assessed the efficiency of pathogen removal

through anaerobic digestion, and these studies were largely

conducted on lab-scale, mixed systems [65].

The current study showed 99.2% and 97.1% reductions in

coliforms and E. coli, respectively, but there were still

2.33 � 1012 and 10.9 � 109 M.P.N. per m3 respectively, in the

effluent. Due to the relatively low temperature of the digesters

in this study (25 �C), complete organism die-off is unlikely. A

portion of the indicator organisms likely settled in the digester

with the solid material and could be released into the envi-

ronment if the settled solids were disturbed.

Similar effluent reductions/concentrations of indicator

organisms were found in a study of three Taiwanese-model

digesters at small farms in Vietnam (94.1 and 81.7% reductions

in coliforms and E. coli, respectively) [55], and in a mesophilic,

plug-flow digester with vertical mixing in the U.S. (99.5%

reductions in coliforms, which had 39.8 � 109 colony-forming

units (C.F.U.) per m3 in the effluent) [63]. It should be noted

that CFU usually yield lower microbial counts than MPN. Due

to the probabilistic basis for calculating the MPN [66]. Ref [65]

found 97.9e99.9% reductions in indicator organisms in

psychrophilic (20 �C) sequencing batch reactors treating dilute

swine slurries, with pathogens (Salmonella, Cryptosporidium

and Giardia) undetected in all effluent samples. The ability of

a psycrophilic digester to achieve reductions of indicator

organisms to undetectable levels with low COD concentra-

tions (12.9e96.3mg L�1) are added is optimistic for Tawainese-

model digesters, which operate in the lower-portion of the

mesophilic range. If pathogen control is imperative for the

success of the digester, Ref [65,67,68] studies imply that

50e100% reduction of pathogens is achievable at low

temperature (20e35 �C) with long retention times, but to

obtain complete pathogen removal in the Tawainese-model

digestion systems, higher temperatures and/or treatment of

more dilute manure may be necessary.

4.4. Fertilizer value

The percent increase in NH4þ-N during digestion was larger in

T0 (109%) than T2.5 (85.5%), and larger than those reported in

Taiwanese-model digestion studies in Costa Rica (78.5%) and

Columbia (33.1%) [16,56], which implies greater degradation of

the solid material in the current study. It should be noted that

the overall concentration of NH4þ-N in the effluent of T2.5

(415 mg g�1) is similar to other Taiwanese-model studies

[16,19], but significantly less than completely mixed reactor

studies, which have reported effluent NH4þ-N concentrations

near or above 2 g L�1 [53,63,69]. TP was reduced 68.2% during

digestion in the control, but only 26.4e13.2% in the grease

treatments, resulting in effluent TP concentrations in the

grease treatments ranging from 181 to 224 mg g�1, more than

double the control (90.6mg g�1). Effluent PO4�-P concentrations

were also almost double in the grease treatment groups

(92.2e139 mg L�1) compared to the control (51.5 mg L�1).

Thus, the grease treatments had less nitrogen (N) andmore

phosphorus (P) in the effluent compared to the control, which

imply that the fertilizer value of the effluent is affected by

grease addition. The N:P ratio of the control effluent was 6.7:1,

which is close to uptake needs of corn (7.5:1) [70]. The T2.5

treatment had half the N:P ratio of the control group (3.2:1),

which is less than raw manure (3.9:1) and effluent from

psychrophilic anaerobic sequencing batch reactors (5.2:1) [29].

The Taiwanese-model digesters without grease co-digestion,

T0, did have a greater effluent fertilizer compared to raw

manure and completely mixed reactor effluents, which did

not report P retention [14], and thus, application of co-digested

manure in plug-flow systems may not recommended for P-

saturated fields.

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 1 1e1 7 2 01718

5. Conclusions

The results from this study prove that co-digestion with

organic-richsubstances, suchasusedcookinggrease,optimize

low-cost digesters for methane production, while preserving

the waste treatment value of the low-cost digestion system.

Specifically, the study found that co-digesting small amounts

of used cooking grease (2.5% by volume or 113% by VS) with

swine manure more than doubles CH4 production, which will

allow the small to medium-scale farmer to obtain more

renewable energy from their wastes. Co-digesting with grease

does not affect the concentration of organics, solids, indicator

organisms, or grease in the effluent, and thus, does not

increase the environmental impact of digested wastes on the

surrounding environment. The high alkalinity of the manure

allows the digesters to function with low concentrations of

grease without pH control devices. The grease treatments

effluents had lower N:P ratios due to the greater concentration

of phosphorus in the effluent compared to the control, and

thus, the effluent should be used in treatment wetlands, if

nearby crop areas are phosphorus-saturated.

Co-digestion allows low-cost digesters to compete with

moreexpensivecompletelymixedreactors in termsofmethane

production,whilepreserving their affordability,easeofuse,and

high removal of solids andorganicmatter. The results from this

study canbeusedby farmerswithexisting low-cost digesters or

can be used as an incentive for small to medium-scale farmers

to install low-cost digesters. More long-term studies need to be

conducted to assess solid retentionwithin low-cost digesters to

determine if clogging will occur over time.

Acknowledgements

This material is based upon work supported by the National

Science Foundation (project # 60012470), Department of

Energy (award # DE-FG02-04ER63834), and the Ohio State

University’s Targeted Investments in Excellence ‘Carbon-

eWatereClimate’ Project. We would like to thank the labora-

tory and research staff at EARTH University for their

assistance in the research, including Bert Kohlmann, Jane

Yeomens, and Herbert Arrieta. We also wish to thank the

student workers at EARTH University and our Central State

University counterparts, Sritharan Subramania and Bryan

Smith. Additional thanks are also extended to Richard Fort-

ner, David Hansen, Pat Rigby, and Carol Moody for their

administrative guidance.

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