Agricultural Wastes 11 (1984) 157-166

Anaerobic Digestion of Swine Manure at Various Influent Solids Concentrations

J. R. F i scher

us Department of Agriculture, University of Missouri, Bldg. TI2, Columbia, MO 65211, USA

&

E. L. Iannotti & J. H. Porter

Department of Agricultural Engineering, University of Missouri, Columbia, MO 65211, USA

A B S T R A C T

The objective o] the work described in this paper was to operate an anaerobic digester on swine manure at the highest possible influent Volatile Solids concentration. As in[tuent Volatile Solids concentration increased, less methane was produced per gram o f Volatile Solids added to the digester. However, volumetric e[]~ciency (litres o f CH 4 produced per litre o f digester) increased up to an in[tuent solids concentration o f 70 g litre- 1 and then decreased. Swine manure was converted to methane via anaerobic fermentation at in[tuent Volatile Solids concentrations up to 97g of Volatile Solids" per litre. A relationship was evaluated which relates methane production per gram o f Volatile Solids added, and methane production per volume of digester, to hydraulic' retention time and in[tuent Volatile Solids concentration.

I N T R O D U C T I ON

Conversion of animal manures to methane via anaerobic digestion has been shown to be feasible but capital intensive. A digester system has been

157

158 J. R. Fischer, E. L. lannotti, J. H. Porter

developed at Cornell University (Jewell et al. , 1976) for dairy cattle manure and operates at an influent Volatile Solids (VS) concentration (So) of approximately 140 g of VS per litre. The University of Missouri- Columbia (UMC) pilot digester for swine manure had always operated at an influent concentration of 60g of VS per litre. The objective of the research reported in this paper was to define the highest concentration of influent solids that would support stable anaerobic digestion of swine manure.

E Q U I P M E N T AND METHODS

A 0.42 m 3 anaerobic digester (Fischer et al., 1979) was operated for 773 days at 35 °C and a 15-day hydraulic retention time (HRT). The digester was loaded daily with a mixture of manure and tap water. Manure used in the experiment was collected from a concrete feeding floor from finishing hogs fed a corn/milo ration (14 ~o protein). Manure was collected weekly and refrigerated until used. The digester contents were agitated by gas recirculation twice daily for 1 h except for S O above 80 g of VS per litre, when mixing was increased to three times a day.

The experimental plan was to begin at an S O of 60 g of VS per litre and increase this concentration by increments of 12g of VS per litre. The digester was operated at each S O for 4 HRTs and samples and gas data were taken during the next 20 days. Total time for each So was more than 80 days.

Samples of the influent and effluent were analyzed for Total Solids (TS), VS, chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), pH and alkalinity (American Public Health Association, 1975). Ammonia (NH3-N) was measured by an Orion ammonia probe. Volatile Fatty Acids (VFA) were determined by gas chromatography (Iannotti et al., 1973) except that a Chromosorb 101 column was used. Gas composition was determined by using a dual column, dual detector, gas partitioner (Fisher Model 1200). Total microbial numbers were determined using media developed by Iannotti et al. (1982).

RESULTS

The percentage of VS converted to biogas decreased significantly as So increased (Table 1). Of the Total Solids, the percentages of Volatile Solids

In[tuent sol ids and p ig waste digestion 159

TABLE 1 VS Concentration in the Influent and Effluent and the Per Cent Reduction

In[tuent VS Effluent VS Per cent (g l i tre - 1 ) (g l i t r e - 1 ) reduction

60-3* 23.9 60.3

68. I 32.1 53.0

81.9 36.7 55.1

87.9 56.5 35.7

97.1 63.0 35-1

108.0 - - - -

* Data obtained from Fischer et al. (1979).

in the influent and effluent were 85.2 % and 75.5 %, respectively. The VS concentration in the effluent was not obtained for an influent solids content of 108 g per litre.

The efficiency of converting swine manure to methane can be measured as methane yield (B) in litres o f C H 4 per gram of VS added or as methane production rate (~, V) in litres CH 4 (litres of digester volume per day). B is a measure of the extent of VS conversion to methane. B decreased as S O increased (Fig. 1). Approximately 0"43 litre CH4 per gram of VS added

.4

i

"t3 .3 O)

"o c~

.2 03 >

,¢ I .1 (.9 d

m 0.00

Fig. I.

X X

B 3 . ~

DATA FISCHER 1975 T O DATA OBTAINED THIS EXPER >

2 d ~'V t . A ~ I '~" I

i d i

2'0 4'0 6JO 8'0 I 0 0 INFLUENT VS. CONC. (gE')

Effect of influent VS concentration on methane yield (B) and production rate (TV).

160 J. R. Fischer, E. L. lannotti, J. H. Porter

was produced at S O less than 40 g of VS per litre. The increase in B between 60 and 90 g of VS per litre cannot be explained even though a series of six independent samples were obtained in this range of So. 7 V increased until So reached 97 g of VS per litre, then decreased (Fig. 1).

The data from the USA (data from this paper; Fischer et al., 1975: Hashimoto, 1982) indicate significantly higher B than the results from European researchers (van Velsen, 1977; Summers & Bousfield, 1980) (Fig. 2). This result may be because the rations of American hogs consist

.5

-o

o .3 Or) >

-I- 0 I .I v

rn

0.0

DATA FISCHER 1975 x x O DATA OBTAINED THIS EXPERIMENT

HASHIMOTO (USA)

x ~ SUMMERS (SCOTLAND) N

\ \

N \ vanVELSEN (NETHENLAND}

I I I

2 4 6 8 lt0

LOAD RATE (g VS I -I d-')

Effect of loading rate on methane yield. Fig. 2.

12

mainly of corn, but European rations are made of small grains such as barley. All data sets show that, as loading rate increased (or S O increased), the efficiency of the digester in converting VS to methane gradually decreased until, at high loading rates, the efficiency decreased rapidly. The data in Fig. 2 were obtained for digesters operating at 35°C. However, data for digesters operated at 55°C (Hashimoto, 1982) demonstrated the same trend. The maximum efficiency of the digester in converting VS to methane for this manure and operating conditions is in the range of 0.42 to 0"43 litre CH 4 per gram of VS added (Fig. 2).

The concentrations of various measured chemical parameters in the digester effluent at different S O are given in Table 2. As So increased, all parameters increased. The data indicate that the digestion process was stable (with respect to the low VFA concentrations) up to the So of 87.8 g litre- 1 even though the NH 3 N, pH and alkalinity increased up to 4.24g

TA

BL

E 2

T

he

Con

cen

trat

ion

s of

Mea

sure

d

Par

amet

ers

in D

iges

ter

Eff

luen

t at

Six

Inf

luen

t S

olid

s C

once

ntr

atio

ns

So

(gV

Slit

re

l)

NH

3N

pH

Alk

alin

ity

Tota

l A

cetic

P

ropi

onic

ls

obut

yric

B

uty

rtc

lsov

aler

ic

Va

leri

c H

ydro

gen

(mgl

itre

t)

(u

nit

s)

(mgC

aCO

3

VF

A

(mgl

itre

l)

(m

glitr

e l)

(m

glit

re

J)

(mgh

tre

l)

(mgl

itre

-l)

(mgl

itre

1)

%

lit

re

t)

(mgl

itre

-l)

60.3

2

93

6

7.6

1301

4 37

0 0-

009

68.1

2

54

0

7.7

1308

3 12

8 11

5 13

81

.9

2 75

0 7.

6 14

465

15

7 13

9 17

--

0.

005

.~.~

87.8

4

245

7.9

19 4

00

205

190

15

--

0.00

4

97- I

3

672

7.9

20 3

70

1 21

1 27

5 91

4 22

-

--

0-00

5 e~

108.

0"

6 08

3 7.

9 27

433

6

532

2 05

2 2

696

719

109

896

60

0.01

7

* C

hem

ical

dat

a w

ere

tak

en a

fter

on

ly 3

7 d

ays

of

op

erat

ion

at

S o

=

10

8g

htr

e 1.

TA

BL

E 3

V

FA

Con

cen

trat

ion

s D

uri

ng

the

105

Day

s of

Dig

este

r O

per

atio

n a

t S

O =

108

g li

tre-

1

ra.

Day

s fro

m

Tota

l A

ceti

c P

ropi

onic

ls

obut

yric

B

utyr

ic

Isov

aler

ic

Val

eric

st

art

of

VF

A

(mgl

itre

1)

(m

glit

re-l

) (m

glit

re

t)

(mgl

itre

-1)

(mgl

itre

-t)

(mgl

itre

-l)

So =

108

glit

re

t (m

glit

re-1

)

17

1661

78

1 70

4 92

--

84

--

37

65

32

2 05

2 2

696

719

109

896

60

93*

4961

13

4 4

26

3

130

31

273

130

105

1966

80

17

56

14

12

85

19

* A

t 39

day

s, t

he

S O

was

dec

reas

ed t

o 48

g V

S l

itre

- 1

and

gra

du

ally

in

crea

sed

ove

r th

e n

ext

66 d

ays,

bu

t ne

ver

reac

hed

S O

= 10

8.

,-

162 J. R. Fischer, E. L. Iannotti, J. H. Porter

litre- ~, 7.9 and 19.4 g litre- 1, respectively. At S O = 97.1 g litre- 1, the digester began showing signs of instability, as indicated by an increase in VFA concentration and especially the increase in propionic acid (914 mg litre-1), which is a measure of instability (Georgacakis, 1980). At So = 108 g litre- 1, the digester was unstable, as indicated by the very high NH3-N, alkalinity, VFA concentrations and low gas production.

Table 3 shows the VFA concentrations in the digester during 105 days of attempted operation at S O = 108 g litre- 1. The S O of the digester was reduced to 48g litre -1 on the 39th day because of the high VFA concentration and foaming in the digester. The foaming was very intense, and filled the gas condensate trap and flowed through the gas meters. The S O was never increased back to 108 g litre-1 because of the previous difficulty of operation. It is interesting to note the almost twofold increase in propionic acid 54 days after S O was decreased. Apparently, a biochemical pathway for converting propionic acid to methane did not exist, or was inhibited in the system. The acetic acid concentration decreased approximately 15-fold in the same period. In fact, propionic acid was the only acid to increase during the 54 days of operation.

MODEL PROJECTIONS

As discussed previously, the two measures of efficiency for digester operation are B and V V. Using the methane fermentation kinetics model developed by Chen & Hashimoto (1978), the effects of So, HRT and temperature can be extrapolated to show the interactions of these two efficiency measures. Thus, B and 7V can be described by:

B = B o 1 - O # m _ l + K (1)

7 v = B o 1 Ol~m- l + K (2)

where 7V = volumetric CH4 production rate (litres of CH 4 per litre of digester volume per day); So=inf luent VS concentration (glitre-~); Bo = ultimate CH4 yield (litres of CH 4 per gram of VS added) as 0 ~ oe; 0 = HRT (days); #m = maximum specific growth rate of microorganisms per day (for 35 °C, #m = 0.33); K = kinetic parameter (dimensionless). K was calculated using the values of B from this study and from Fischer et

In.fluent solids and pig waste digestion 163

DATA FISCHER 1975 6 0 OATA OBTAINED THIS EXPERIMENT /

J /

, I _ , I

0 210 4 0 6t0 810 1 0 0

INFLUENT VOLATILE SOLIDS (g VS L-' )

The relationship between the dimensionless parameter, K, and influent VS concentration.

Fig. 3.

al. (1975), Bo = 0.49 litre CH 4 per gram of VS added as determined by Hashimoto (1982) and # m = 0 . 3 3 d a y -I at 35°C as reported by Hashimoto et al. ( 1981). Figure 3 is a plot of calculated Kvalues versus So, and an empirical relationship described by:

K = 0"5 + 0.03 exp (0.05'So) (3)

The r 2 for this empirical relationship was 0-86. This equation is of the same form as that proposed by Hashimoto (1982); however, the constants in the equations are different. From data in this experiment, K increased greatly at S o in the range of 80 to 100 g VS litre - 1 instead of the range of 50 to 70 g VS litre-1 from Hashimoto's data. This indicates that swine manure can be digested at a higher influent solids concentration without decreased biological activity, as previously reported. However, the digester operating in the range of 70 to 90 g VS litre- ~ was easily upset. Changes in digester temperature or in loading procedure could upset the system.

Using the Kvalues shown in Fig. 3 and eqn (1), Bcan be calculated for various HRTs (0). Data in this experiment were collected for only one HRT. However, the equation can be used to compare with other data and to indicate trends. A three-dimensional plot of HRT and So versus B is represented in Fig. 4. This plot shows that, as HRT increases, B increases.

164 J. R. Fischer, E. L. Iannotti, J. H. Porter

.4

(LCH4 g VS I)

20 40

So(gVS I~')80 i 0 0 ~ i ~ ~ / IO~H:T(d)

Fig. 4. Theoretical relationship between litres oFCH4 produced per gram VS added and HRT versus influent VS concentration. The shaded area represents the HRT at which data

in this paper were collected.

The theory behind this observation is that the bacteria have more time to break down macronutrients from the manure into usable substrates for methane formation. However, as S O increases, B decreases. The bacteria seem to become overwhelmed by nutrients and toxic byproducts of digestion. These nutrients are flushed out of the digester at short HRTs before the bacteria can utilize the nutrients. This is diagrammatically shown when comparing a 5-day HRT with a 30-day HRT. Thus, a digester can be loaded at an S O of 70 to 80g of VS per litre without seriously reducing B if the HRT is longer.

Figure 5 shows the three-dimensional calculated relationship o f S o and HRT versus 7V. This graph shows that an opt imum 7V exists at an S O = 50 g of VS per litre at a 5-day HRT and S O = 70 g of VS per litre at a 10-day HRT. As HRT increases, ~ Vdecreases. The maximum values of), V for 5-, 10-~ 15-, 20-and 30-day HRTs are 2-1, 1.7~ 1.4~ 1-2 and 1.0 litres of CH 4 per litre per day, respectively.

The two efficiencies vary opposite to each other in relation to both HRT and So. As HRT increases, B increases and ~ V decreases beyond an HRT of ! 0 days. However, B decreases as S O increases at a constant HRT but 7Vincreases until S O approaches 70-80 g of VS per litre (Fig. 1; our data show this to be 90 g of VS per litre).

Influent solids and pig waste digestion 165

y v

(LCH4(LDV) I d -l)

o 2 'I II!E

Fig. 5. The relationship between litres of methane per litre of digester volume per day and HRT versus influent VS concentration added to the digester. The shaded area

represents the HRT at which data in this experiment were collected.

For a livestock producer, the objective is to optimize the quantity of biogas produced from the manure available and accomplish this in the most economical way. Maximum biogas from a given amount of manure dictates a very low S o at a long HRT, requiring a large digester volume. Economics become a factor here because of the large digester size. In order to optimize the production o fCH 4 from a given volume of digester, the producer would select a short HRT and an So of approximately 70 g of VS per litre, but this would severely sacrifice efficiency of B. Thus, a trade- off exists. Since digesters are subject to economy of scale, one might conclude that increasing the volume of the digester at an S o of 70 g of VS per litre in order to increase H R T - - a n d thus improving the B efficiency-- is the most likely alternative for a livestock producer. Another reason to increase retention time is to reduce odors (van Velsen, 1977). The odors associated with swine manure are significantly reduced at a 20-day retention time compared with 10-day retention time.

A C K N O W L E D G E M E N T

Contribution from US Department of Agriculture, Agricultural Research Service, in co-operation with the University of Missouri Agricultural Experiment Station, Journal Series No. 9280.

166 J. R. Fischer, E. L. lannotti, J. H. Porter

R E F E R E N C E S

American Public Health Association (1975). Standard methods .for the examination of water and wastewater (14th edn). New York.

Chen, Y. & Hashimoto, A. G. (1978). Kinetics of methane fermentation. Proceedings of Biotechnology-Bioengineering Symposium, 8, 269-82.

Contois, D. E. (1959). Kinetics of bacterial growth: Relationship between population density and specific growth of continuous cultures. J. Gen. Microbial., 21, 40.

Fischer, J. R., Sievers, D. M. & Fulhage, C. D. (1975)'. Anaerobic digestion in swine waste. In: Energy, agriculture and waste management (Jewell, W. J. (Ed.)). Ann Arbor Science, Ann Arbor, MI, p. 307.

Fischer, J. R., Iannotti, E. L., Porter, J. H. & Garcia, A. (1979). Producing methane gas from swine manure in a pilot-size digester. Transactions of the ASAE, 22, 370.

Georgacakis, Dimitrius (1980). VFA-NH 3 buj]er relationship ht anaerobic digesters. Thesis, University of Missouri, Columbia, MO, Department of Agricultural Engineering.

Hashimoto, A. G. (1982). Thermophilic and mesophilic anaerobic fermentation of swine manure. Agricultural Wastes, 3, 175-91.

Hashimoto, A. G., Chen, Y. R. & Varel, V. H. (1981). Theoretical aspects of methane production: State of the art. In: Livestock waste." ,4 renewable resource. American Society of Agricultural Engineers, St. Joseph, MI, 86-91, 95.

Iannotti, E. L., Kafkewitz, D., Wolin, N. J. & Bryant, M. P. (1973). Glucose fermentation products of Rumincoccus albus grown in continuous culture with Vivico succinogens; changes caused by interspecies transfer of H 2. Bacteriol., 114, 1231~40.

lannotti, E. L., Fischer, J. R. & Sievers, D. M. (1982). Medium for enhanced growth of bacteria from a swine manure digester. Appl. and Environmental Microbiol., 43, 1. 247-9.

Jewell, W. J., Davis, H. R., Gunkei, W. W., Lathweil, D. J., Martin, J. H., Jr., McCarty, T. R., Morris, G. R., Price, D. R. & Williams, D. W. (1976). Bioconversion of agricultural wastes for pollution control and energy conservation. Final Report, ERDA-NSF-741222A01. Cornell University, Ithaca, NY.

Summers, R. & Bousfield, S. (1980). A detailed study of piggery-waste anaerobic digestion. Agricultural Wastes, 2, 61-78.

van Velsen, A. F. M. (1977). Anaerobic digestion of piggery waste. 1. The influence of detention time and manure concentration. Neth. J. Agricultural Science, 25, 15 i -9.