Biological Wastes 20 (1987) 117-131

Anaerobic Digestion of Poultry Manure at High Ammonium Nitrogen Concentrations

Z. Pechan , a O. Knappovf i , b B. Petrovi6ovfi c & O. A d a m e c c

a Institute of Experimental Veterinary Medicine, b Institute of Zoohygiene and Veterinary Technique, c Institute of Animal Physiology of the Slovak Academy of Sciences,

900 28 Ivanka pri Dunaji, Czechoslovakia.

(Received 6 February 1986; accepted 5 August 1986)

A B S T R A C T

Methanogenic fermentation of poultry manure was studied on a laboratory scale at mesophilic temperatures. At average concentrations of Total Solids between 11.3% and 14.1% in the influent (Volatile Solids from 7"8% to 9.7%) and average retention times from 27 to 58 days, no adverse effect on biogas production was observed, despite mean ammonia nitrogen concentrations in the effluent of 4"07 to 5.85g liter -1, with extreme values reached as high as 7.5g liter-~. With only one exception, in five fermentors of different construction no significant negative correlation between ammonia nitrogen concentration and biogas production, or fermentor productivity, was found. Ammonia nitrogen concentration increased with the increased Volatile Solids loading rate, and, in agreement with data in the literature, the direct dependence of productivity on loading rate was confirmed in the range from approximately 1.2 to 4"0 kg of Volatile Solids per cubic metre per day. The digester construction did not seem to be critical.

I N T R O D U C T I O N

From the practical point of view it is desirable that poultry manure scraped from laying hen cages, and of a Total Solids concentration of at least 25%, is diluted with the minimal quantity of water before methanogenic fermentation. It is generally accepted that the limiting factor for the

117 Biological Wastes 0269-7483/87/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

118 Z. Pechan, O. Knappov6, B. Petrovikov6, O. Adamec

dilution ratio is the concentration of ammonia nitrogen (Bousfield et al., 1979; Webb & Hawkes, 1985a,b). The inhibitory effect of ammonia nitrogen has been shown by several authors (e.g. McCarty & McKinney, 1961; Hobson & Shaw, 1976; Koster & Lettinga, 1983, 1984). Contrarywise, it has been repeatedly shown that methanogens can adapt to increased concentrations of ammonia nitrogen: Van Velsen (1979), in discontinuous experiments, found an inhibition limit at 3 to 5 g N H ~ - N liter-1 for digested sewage sludge and pig manure, Hobson & Shaw (1976) estimated for a pure culture of hydrogen-consuming Methanobacterium formicicum the threshold of inhibition at 4 g liter-1

The present paper refers to a successful biogas production from poultry manure diluted to influent Total Solids from 10%o to 15%, at ammonia nitrogen concentrations varying from 5 to 7 g N H ~ - N liter- ~ and pH above 7.5.

METHODS

Material

Poultry manure was collected from laying hens kept in cages on a standard feed mixture. Before diluting to the proper concentration it was kept in a refrigerator at 4°C for up to a month. From the original 26-38% TS it was diluted to 10-14% TS with tap water and supplemented with trace elements (Fe, Co, Ni, Mo, Mn). Then it was homogenized by a laboratory blender for 2.5 min at full speed (blender type 309, Mechanika Precyzyjna, Poland) and passed through a 2 × 2 mm sieve. The influent for the fermentors was prepared once or twice weekly and kept in a refrigerator.

Fermentors

(a) A classical methanogenic fermentor (No. 2) was constructed from a 5- liter cylindrical glass container with a conical bottom. The mixture to be fermented was added through the bottom tube once a day employing an infusion piston syringe of 150ml capacity made of plastic (Janett type, Chirana, t~SSR). On adding the feed an equal volume of fermentor contents was pushed out of a tube which ended just beneath the liquid level.

(b) A fixed-biofilm column (No. 1) (Kennedy & Van den Berg, 1982) was made of an acrylic cylinder, of internal diameter 100mm and height 1000 mm, filled with 900 mm long tubes of fired clay of 20 mm in diameter. The inlet was fixed on a flow disperser on the top of the tubes. The outlet from the bottom of the column kept a constant liquid level by a syphon.

Digestion of poultry manure 119

The total column volume was 9.5liters and the liquid void volume, 6.5 liters.

(c) In a second version (No. 1') the packed column was connected as a second stage to a classical fermentor as described under (a). The volumes of the vessels were 5 + 9.5 liters.

(d) In two other fermentors (Nos 3 and 4) the inner space was divided by an inserted PVC-tube of a smaller diameter. The influent was led by a tube to the bot tom of the inner cylinder, then the contents flowed over to the top of the outer space and the effluent was finally taken from the bot tom of the outer space. These fermentors differed by the ratio of the inner and outer spaces, these being 2.26:5"28 and 1.38: 7.21, respectively. The total volumes of the digesters were 7.8 and 8.88 liters.

Initially, the temperature (between 31 °C and 38°C) was kept constant by a heating wire coiled around the fermentors, later, the fermentors were placed in a thermostatically-controlled cabinet. The volume of biogas produced was measured by pivoted chambers divided into halves placed in a plastic chamber filled with acidified water, which, at each turn, sent an electrical impulse into a counter.

Analytical methods

The determination of Total Solids and Volatile Solids was by Standard Methods (1980). Ammonia nitrogen was determined volumetrically after steam distillation at pH 9"5 (Methods for chemical analysis, 1979), or also by direct photometry using the sodium phenate method (Methods for chemical analysis, 1979) as modified by Petrovi~ov~ & Pechan (1984). Biogas analysis was by gas chromatography on a Carlo Erba Fractovap 4200 equipped with a Hewlett Packard 3390A integrator, using a hot wire detector and a stainless steel column 2 m m inside diameter and 2 m long, packed with Separon AE, 200-300/tm (Lachema N.E., Brno, (~SSR). The isothermic separation was made at 45°C with argon as carrier gas. Individual Volatile Fatty Acids were determined on the same apparatus with a flame ionization detector under the following conditions (Wliner, 1982): glass column, 0 .4x 120cm; packing, 10% SP 1200/1% H3PO 4 oR 80/110 Chromosorb W AW (Supelco). The thermostat temperature was programmed from 90°C to 124°C at a rate of 3-5°C per min. Carrier gas was a mixture of nitrogen, hydrogen and air. The total VFA were measured titrimetrically after steam distillation (Standard Methods, 1980). pH was measured with a glass electrode on a digital pH-meter (Radelkis OP-208, Budapest, Hungary). Factor F42 o was measured spectrofluorimetrically on a Perkin-Elmer MF 3 spectrofluorimeter according to the Grekshk et al. (1981) modification of the method of Binot et al. (1981).

120 Z. Pechan, O. Knappovti, B. Petrovi~ov~, O. Adamec

Anaerobic conditions in the fermentors were monitored by passing the biogas through a flask filled with a colourless resazurin solution.

Fermentor start-up and follow-up

Fermentor No. 1 was started by inoculating it with sludge from a sewage digester, the others were inoculated with effluent from a fermentor already in operation.

The influent was fed to the fermentors daily, the gas production was measured continually. Other analyses were carried out once a week.

Time course of experiments

(a) Fermentor No. 1. The analytical data of the initial 6-week start-up period are not included as they are incomplete. The experimental period of 32 weeks is regarded as one phase as the deviations of hydraulic retention time (HRT) from the average of 54 days seemed not to be substantial. Since the shortening of the HRT to 36 days for 3 weeks induced clogging problems, it was prolonged to 57 days in the following month.

(b) Fermentor No. 1'. This fermentor's time course could be divided into three phases. Each began with a start-up period characterized by a long HRT, which was gradually shortened until an acceptable limit. Phase No. I: 62 to 19 days HRT, 9 months; phase No. II: 107 to 30 days HRT, 5 months; phase No. III: 193 to 28 days, 6 months.

(c) Fermentor No. 2. After two start-up periods a standard HRT of 33 days was kept except the last half year, when no substrate was added on Saturdays and Sundays. Thus the HRT was shifted to 39 days.

(d) Fermentor No. 3. The start-up period lasted approximately 2 months (monthly averages of HRT were 53 and 29 days). During the experimental period (4 months) the HRT varied only slightly around 20 days.

(e) Fermentor No. 4. The analogous values as (d) were 57 and 26 days during the 2 months start-up period and 22 days during the 3 months experimental period.

Statistical methods

The correlations were computed at the Computing Centre of the Slovak Academy of Sciences, Bratislava, on an EC 1045 computer, together with appropriate graphics, which were made by a DIGIGRAF plotter (ZPA Nov~ Bor, (2SSR) combined with an SM 52-11 computer. The significance of correlation coefficients was tested according to Tables (Weber, 1967).

Digestion of poultry manure 121

RESULTS

Characteristics of influent, effluent and fermentor performance

Poultry manure as supplied from the farm had TS 32.6 __ 5.7% (SD, number of analyses, n = 28) and VS 71.5 +_ 44% of TS (n = 18). The average influent composition is summarized in Table 1.

TABLE 1 Composition of Influent

Period 349/420 Whole period

Total Solids (%) 11.9 1-25 (24) 11-9 1-6 (114) Volatile Solids (% of TS) 72.2 3.76 (23) 69.2 4.21 (102) Ammonia N (g liter -1) 2.52 0.98 (21) 3"53 1.09 (100) Total VFA ( as acetic acid g liter-t) 8"57 1.94 (23) 10"90 3-77 (101) pH 7"16 0'36 (22) 7'50 0-50 (74)

The heading: 'Period 349/420' denotes the period of 175 days when fermentors 2, 3, 4 and 1' were operated simultaneously. Means + standard deviations are given followed by a number of measurements averaged (in parentheses),

As shown in Table 2, the values given being averaged throughout the whole period of observation, some characteristics of fermentors ' performance are as expected, such as the methane content of biogas from 59% to 67%, the biogas yield per input Volatile Solids from 0.239 to 0,370 liter g - t and Volatile Solids reduction from 58% to 64%. The concentrat ion of Volatile Fat ty Acids is rather high, the averages for individual fermentors vary from 4.6 to 9.3 g liter-~ (as acetic acid), pH is rather high, around 8.1, but very stable. The concentrations of the factor F,20, all averages being over 1/~mol liter -~ and individual values sometimes reaching as much as 4/zmol liter- 1, are proof of the presence of large numbers of methanogens in the fermentors. The concentrations of individual Volatile Fat ty Acids are as expected; that is, their quantity diminishes with increased number of carbon atoms (Table 3).

In the majority of parameters quoted there were no substantial differences between the fermentors.

Effect of ammonium concentration

The ammonia nitrogen concentra t ion in the effluent did not drop under 6 g liter-1 in certain periods and the highest value found was 7-7 g liter-1

TA

BL

E 2

Pe

rfor

man

ce o

f Fe

rmen

tors

and

Eff

luen

t C

hara

cter

isti

cs.

(Ave

rage

s of

all

valu

es i

nclu

ding

sta

rt-u

p pe

riod

s.)

to

Fer

men

tor

No.

E

xper

imen

tal

peri

od,

days

(w

eeks

) I

1'

2 3

4 26

1 (3

2)

588

(84)

79

6 (1

16)

175

(24)

17

5 (2

4)

Tem

pera

ture

(°C

) 37

.7

(2-0

) 33

.6

(1-8

) 34

-3

(2"0

) 34

"9

(0"6

) 34

.9

(0.6

) L

oadi

ng r

ate

(g V

S lit

er -1

day

-1)

1"13

(0

-46)

2.

33

(1"3

5)

2"26

(0

"50)

3"

60

(0"8

9)

3-01

(1

'01)

R

eten

tion

tim

e (d

ays)

51

-0

(21'

8)

58-7

(8

5-1)

40

.3

(22-

3)

27.3

(1

3-0)

31

"9

(15-

3)

Prod

ucti

vity

(lite

rs o

f bi

ogas

lite

r 1

day-

~)

0"47

(0

-25)

0'

71

(0-4

8)

0"77

(0

"38)

0"

86

(0"3

3)

0"87

(0

"48)

B

ioga

s yi

eld

(lite

rs g

-~ V

S)

0-33

0 (0

-160

) 0"

279

(0"0

96)

0-36

5 (0

'185

) 0"

236

(0-0

87)

0"27

8 (0

"012

) M

etha

ne c

onte

nt (

%)

61.5

(7

-9)

61-5

(1

2.4)

59

"9

(9"5

) 61

"0

(13.

2)

64-3

(9

"9)

Tot

al S

olid

s (%

) 4.

1 (1

.86)

4"

87

(2"1

8)

5"68

(2

"06)

5"

03

(1'8

3)

6-05

(3

"27)

V

olat

ile

Solid

s (%

of

TS)

57

-7

(4-1

) 59

.7

(7.4

) 57

.6

(6.1

0)

60.4

(7

.0)

61'5

(5

"1)

[19]

C

OD

(g

lite

r- 1

) nd

40

.9

(12.

9)

35"1

(1

1.5)

nd

nd

[8

] [1

4]

Am

mon

ia N

(g

liter

~)

4"

32

(0-6

8)

5-37

(1

.35)

5-

69

(1-2

2)

5"88

(1

-19)

5"

41

(1"3

4)

Vol

atile

Fat

ty A

cids

~ (g

lit

er-

~)

6'94

(5

'68)

5"

71

(2"5

9)

7.94

(4

-19)

8"

50

(2"9

1)

7"74

(2

"90)

pH

8-

33

(0-2

6)

8'27

(0

:22)

8"

07

(0-2

5)

8-32

(0

-14)

8.

25

(0-2

2)

Fact

or F

,20

(#m

ol l

iter

-~)

nd

2"07

(1

"03)

2"

91

(1"2

7)

2"91

(0

"78)

3'

90

(1-2

4)

VS

redu

ctio

n (%

of

VS~

n )

73-3

(6

-1)

62"4

(2

2"6)

63

-7

(14-

8)

66"0

(1

7-0)

63

"7

(21-

9)

.N

e~

a.

Mea

n +

stan

dard

dev

iati

on,

the

squa

re b

rack

ets

indi

cate

the

num

ber

of m

easu

rem

ents

bei

ng l

ower

tha

n th

e to

tal

num

ber

of w

eeks

wit

hin

the

expe

rim

enta

l pe

riod

. nd

, N

ot d

eter

min

ed.

As

acet

ic a

cid.

Digestion o f poultry manure 123

(0"55 mol liter- 1). If we assume the inhibitory effect of ammonium ions or free ammonia, there should be a negative correlation between the N H ~ - N concentration and biogas production. Such correlation was actually found in one of four fermentors only, No. 1; in the other four experiments the correlation between ammonia N concentration and biogas production or fermentor productivity, respectively, was significantly positive (Table 4). The relationship of individual measurements is shown in Fig. 1. The anomaly of fermentor No. 1 will be discussed later.

TABLE 3 Concentration of Individual Volatile Fatty Acids in lnfluent and Effluent of Digester No. 2

Acid lnfluen t a (g li ter- 1) Effiuen t ( g l i ter- i )

Acetic 4.65 ~(2.14) 1.54 (1.34) Propionic 2.63 (0.94) 1'10 (0-72) Isobutyric 0"90 (0-64) 0"52 (0-56) Burytic 0-98 (0.54) 0'37 (0"35) lsovaleric 0.58 (0.38) 0.42 (0.25) Valeric 0.24 (0.27) 0.27 (0.14)

Means _+ standard deviations measured during 11 months. a Averaged 37 measurements.

It can be seen from Table 4 that in all cases there is a positive correlation between N H ~ - N and influent volume, that is, the hydraulic loading rate. The relationship between N H 2 - N and organic loading rate is obvious, the correlation is significant four times if expressed as influent Total Solids and three times when expressed as quantity of volatile substances. A negative correlation with the retention time is also in agreement with this.

The dependence of fermentor productivity on the organic loading rate was shown recently in the digestion of poultry manure and its validity is demonstrated in Fig. 2, where our results are presented and in Fig. 3 where data from the literature are shown (Aubart & Fauchille, 1983; Morrison el al., 1981; Sojka, 1982; Webb & Hawkes, 1985a,b).

It is difficult to predict the course of dependence of productivity on loading rates higher than 5 g VS liter-1 day-1. It was shown (Lo et al., 1984) that, for screened dairy manure, the productivity of a fixed-film reactor increased up to a loading rate of 672g VS l i t e r - lday -1 ( l h hydraulic retention time), but this rate is certainly not attainable with a 13% TS concentration due to clogging problems.

124 Z. Pechan, O. Knappovd, B. Petrovi~ovd, O. Adamec

o 1.5

e ~

~ 0.9-

~- 0.6-

0.3-

0.0

A

A 0

0 0 0

0 0

0

(a)

o

o ~o O~ A A O~

oAO

A A~ 0 0~

~ 0 A

°o-to o

'b'~ ~ ~ ~ o,C o ~

A o ° ~ o

aoq~ ~ A A ¢b~o

o ~ ~ o °

A o A a

o~ o

@ 4

NH4+ N (G/L)

'~< 1.5

N

~ 0.9-

~- 0.6-

0.3-

0.0

+

(b)

+ + +

+

+ ÷ ~ + t #

+ t#t l ~ • ~ K- ~

N

N ÷

+

÷ +

NH4+ N (G/L)

Fig. 1. Correlation of N H 2 - N concentration in effluent and fermentor biogas productivity. (a)---In fermentors Nos 1' (O) and 2 (A). (b)---In fermentors Nos 3 (*) and 4 (+).

(BG--biogas, M3--m3).

TA

BL

E 4

C

orre

lati

on o

f E

fflu

ent

Am

mo

nia

Nit

roge

n C

once

ntra

tion

wit

h S

ome

Oth

er P

aram

eter

s of

Ana

erob

ic F

erm

enta

tion

Par

amet

er

Fer

men

tor

r f

F

a ( +

_ SE

) b

( +_ S

E)

No.

Vol

umet

ric

load

= x

1

0.51

4"*

21

7.19

2-

68

(0-6

1)

7.77

(2

-90)

(l

itre

s d

ay-

1)

1'

0.40

3***

* 68

13

.0

4-36

(0

-33)

2.

44

(0"6

8)

2 0.

500*

***

104

34.4

2-

03

(0.6

3)

26-8

(4

.6)

3 0.

861"

***

23

63.0

2.

48

(0.4

5)

10-3

(1

'3)

4 0.

445"

22

5"

18

3.52

(0

-87)

5.

87

(2"5

8)

Pro

duct

ivit

y =

y

1 -0

.509

**

23

7.69

1'

27

(0-3

1)

-0.1

94

(0

"070

) (l

itre

s li

ter

1 d

ay-l

) 1'

0.

541"

***

62

25'2

-0

"298

(0

-210

) 0"

189

(0"0

38)

2 0"

210"

10

4 4"

75

0-39

5 (0

'172

) 0-

064

(0'0

30)

3 0"

717"

***

23

23'2

-0

-31

(0

"25)

0.

198

(0-0

41)

4 0-

461

* 22

5-

67

0"05

1 (0

-372

) 0"

159

(0-0

67)

Tot

al S

olid

s =

y 1

0.71

5"**

* 20

19

.8

- 8"

14

(7.9

7)

8' 1

2 (1

-83)

(i

nflu

ent

(g d

ay-

1)

1'

0.31

6"**

66

7"

28

17-7

(1

2.6)

6.

06

(2.2

6)

2 0.

282*

**

103

8.80

11

.6

(1.7

) 0.

869

(0-2

93)

3 0-

614"

**

23

43"2

-3

.31

(4

-49)

7.

11

(1'0

8)

Loa

ding

rat

e =

y

1'

0.29

3*

57

5.26

0.

795

(0-7

61)

0.31

1 (0

-136

) (V

S g

day-

1)

2 0.

432*

***

93

21.1

1.

32

(0.2

2)

0.17

2 (0

"038

) 3

0.65

8***

* 22

58

'6

-0"0

65

(0.4

89)

0'62

3 (0

"081

)

q

Am

mo

nia

N i

n g

per

lite

r.

r =

coef

fici

ent

of c

orre

lati

on a

nd i

ts s

igni

fica

nce

at p

roba

bili

ty l

evel

P:

* <

0-05

, **

< 0

.02;

***

< 0

-01;

***

* <

0.00

1.

f=

degr

ees

of f

reed

om,

F =

F-v

alue

, a

and

b =

coef

fici

ents

of

line

ar r

egre

ssio

n _+

sta

ndar

d er

ror.

126 Z. Pechan, O. Knappov~, B. Petrovi6ov~, O. .4damec

Fig. 2.

>-

c~ I.~

CD ~n

1.2-

0.9- o

o

o.6-

0.3 ̧

0.0

w w w w

w

ww w w

w

w

w

~ 4 5 LOADING RATE [KG VSIM3 DAY]

Dependence of fermentor No. 3 productivity on loading rate.

~ 3 ~E

~E >- I.- > 2 I-- (.~

r~ 0 r r D-

i i i I

/ A

; j l J

• /

~ y ij i

N "°. .... ~Q/., 0 ~.""

~ , ~

LOADING RATE (KG VSlM3 D}

Fig. 3. Literature data on correlation of fermentor productivity with VS loading rate. Aubart & Fauchille (1983): - - , Total Solids, %: A 3'8; A, 6"0; A , 8'1. Morrison et al. (1981): . . . . , retention time, days: I--l, 70; ill, 25; g~, 15; I , 10. Sojka (1982): x ..- x. Webb &

Hawkes: . . . . . . ; O (1985a), @ (1985b).

Digestion of poultry manure 127

DISCUSSION

Van Velsen (1979) was not the first to revise the view on the degree of toxicity of ammonium ions. In his 1979 paper eight authors of the 1971 to 1977 period are quoted as having achieved a satisfactory methane production at ammonia N concentrations considerably over 1500 mg liter- 1.

There are several possible explanations for the negative correlation between the fermentor productivity or biogas production, respectively, and ammonia N concentration in the effluent of fermentor No. 1. The most probable hypothesis is that the methanogenic microorganisms were not sufficiently selected or adapted. This reactor was put into operation first and was inoculated by non-adapted sewage sludge. A less probable assumption is that this phenomenon is specific just for this fixed-film type of fermentor.

The average retention time related to Table 4 or Fig. 1 is given in Table 2 and the influent solids concentration in Table 1. This varied only slightly around 12% TS.

By a statistical procedure all 3150 correlation coefficients for 36 parameters were computed. In this paper only those are evaluated in which ammonium ions in the effluent are involved. Of the total number of 175 of these correlations 71 appeared to be significant. In Table 4 a selection is presented which was supposed to have enough weight for a generalization when comparable results were obtained in at least four sets of data. There were six parameters that significantly correlated with NH~--N in three digesters: the values for loading rate are presented in Table 4, the correlations of influent VFA and influent TS concentration were inconsistent (twice positive, once negative and twice negative, once positive, respectively), whereas the correlations of HRT with Volatile Solids reduction were negative (degrees of freedom in brackets, significance level denoted by * numberas in Table 4; r = -0.471"*** (104), -0-866**** (23), - 0"464* (22) for HRT and r = - 0"417"*** (91), - 0-356** (45), - 0"706**** (20) for VS reduction) and positive with the Total Solids concentration in the effluent (r = 0"832**** (22), 0-356*** (65), 0"358**** (103). The decrease of the percentage of mineralized organic matter with an increased ammonium ion concentration indicates the inhibitory effect. The observation of positive correlation between biogas yield and effluent N H ~ - N in digester No. 4 (r = 0"562, ~ < 0"01,f= 21) could not be confirmed in other sets of data. For the purpose of correlation analysis, the greater the number of observations the better the chance to reveal a relationship; therefore, also, data from start-up periods were included. This resulted in a broader range of the values of variables. This does not mean that the fermentors did not

128 Z. Pechan, O. Knappovd, B. Petrovi~ovd, O. Adamec

come to a steady state, judged by the variation of the HRT. The only exception was phase III in fermentor 1'; in the others the steady state lasted at least 3 or 4 months (4.5 or 2.6 times the HRT) and up to 12 months in fermentor No. 2, phase I (11 HRT).

It might have been useful to identify the methanogenic species prevailing in our fermentors, but this is rather a difficult task. Sprott et al. (1984), in their recent study of the mechanism of inhibition of methane production by ammonium ions in pure cultures of seven methanogenic bacteria, demonstrated that there exist substantial species differences in the sensitivity towards ammonium ions.

Undissociated ammonia is commonly believed to be the actual toxic agent (McCarty & McKinney, 1961; Webb & Hawkes, 1985a,b). We would like to mention that, in the Henderson-Hasselbach equation, quoted by McCarty & McKinney (1961), activities of [NH~] and [H ÷] should be substituted instead of their concentrations. We could estimate the ionic strength (p = ½ciZ 2) of our fermentor liquor at around 0"6 to 0"8 based on one complete analysis of effluent for inorganic cations and anions. Consequently, the activity coefficient [NH2] may be less than 0.5. Using this correction after substituting it into the equation quoted we obtain, at pH 8.5:

10- 9 0"42 x 0"5 [NH3] = 1"13 x 5 x 10 -5 - 0"0475m°lliter-1

which gives 665mgliter-1 NHs_N, more than four times the value of 150 mg liter-1 supposed to be the toxicity threshold.

Methanogens belong to the group of Archaebacteria (Zehnder et al., 1982), phylogenetically very old, which developed on our planet in ages of an oxygen-free atmosphere exhibiting reducing features. In accord with this they behave as strict anaerobes for which even traces of oxygen are toxic. Taking into account this fact, the sensitivity towards NH 3 seems rather surprising; one would rather presume some inhibitory effect of oxidized nitrogen compounds like nitrates or nitrites.

As a definite symptom of the malfunction of the methanogenic process the slowed degradation of propionic acid (e.g. Van Velsen, 1979), or the accumulation of higher Volatile Fatty Acids can be cited. The inhibition of propionate breakdown could be clearly demonstrated (Van Velsen, 1979) in a series of batch experiments using a mixture of acetate, propionate and butyrate as substrate, when, at approximately 5 g NH 2 - N liter- 1 on day 80 of incubation the ratio of acetate to propionate was 1:6, or, with adapted sludge at 3.1 g, N H 2 - N on day 18, zero to 700 mg liter-1, respectively. In our experiments such symptoms, as illustrated by the example of fermentor No. 2 in Table 3, could not be shown. Acetic acid was always the most

Digestion of poultry manure 129

abundant species among VFA in all fermentors, thus supporting the idea that the microbial population was effectively adapted to the high ammonium ions level.

In continuous or semi-continuous experiments it is impossible to study the lag-phase. Also, we cannot exclude some partial inhibition of methane production that is decreasing the maximal biogas production rate. Van Velsen (1979), in discontinuous experiments with the mixture of acetic, propionic and butyric acids as substrate, found a linear dependence of the diminishment of the specific gas production rate (k) on N H 2 - N concentration in the range from 880 to 3500 mg liter-1 with very similar slopes of regression lines (b = -0.0028) for digested sewage sludge and (b = -0.0023) for digested pig manure. In a discontinuous experiment with poultry manure, TS concentration being 13"8 % and N H 2 - N 4.71 g liter- 1 (unpublished), we estimated the maximal specific gas production rate to be 13-2 ml g- ~ VS day- 1. When Van Velsen's (1979) lines were extrapolated to this N H 2 - N concentration a very good agreement was obtained, because this value falls just between the value k = 10 for non-adapted, and k = 20 ml g- ~ day- 1 for adapted, sludge.

Koster & Lettinga (1984) state that ammonium nitrogen inhibits methane production, the decrease being much more pronounced in the range 680-1700mgNH2-N liter - t than at higher concentrations, 1700-2600 mg N H 2 - N liter- 1. If we extrapolate their regression line of a batch experiment, where ammonium chloride was stepwise added to give higher N H 2 - N concentration we found that, at 3.6 g liter-~ N H 2 - N the gas production would drop to zero, so it might be suggested that in their experimental arrangement the effect of ammonium chloride added was, in fact, summed with some other inhibitory factors, e.g. specific metabolic products of methanogens.

After the completion of our experiments, a paper by Webb & Hawkes (1985b) was published where the problem of methanogenic fermentation of poultry manure is discussed. They worked with substantially lower influent TS concentrations, varying from 1% to 10% , and also N H 2 - N concentrations were correspondingly lower, 465 to 4274 mgliter-1. Their discontinuous experiments show that a mixed culture of methanogens could be selected or adapted to as high as 3-4 g liter- 1 NH2-N. The present results indicate that the level may be even higher.

There was no correlation between ammonia nitrogen and biogas yield, with the exception of fermentor No. 4 where the correlation coefficient was positive (r=0"562, for 21 degrees of freedom significant at the 1% probability level). Despite the fact that the yields obtained by us were below 0 " 3 7 0 m 3 per kg added VS and thus somewhat lower than that obtained by Webb & Hawkes (1985b), we do not think this was caused by the presence

130 Z. Pechan, O. Knappov6, B. Petrovi~ovh, O. Adamec

of ammonia N in higher concentrations. In our opinion the limiting step for the yields might rather be the slower rate of the hydrolytic phase of the methanogenic process. There is no indication in the paper mentioned (Webb & Hawkes, 1985b) of the concentration of methanogenic bacteria, e.g. by determining the factor F42 o.

The philosophy of using fermentor productivity expressed as volume of gas produced per fermentor volume unit per day as a decisive parameter was based on economic considerations. If we have a substrate of very low cash value exhibiting negative environmental features, such as poultry manure, the running economy of a farm digester or a big unit producing biogas would be only slightly affected whether the gas yield was 200 or 300 liters of gas per kilogram of substrate. On the other hand, if a fermentor productivity of 3 (cubic meters of gas per cubic meter of volume per day) instead of the common 1 ( m 3 m - a d a y -1) could be achieved this would reduce capital costs for the construction of a digestor to produce the same quantity of biogas. Of many authors following the same idea Lo et al. (1984) may be mentioned as an example.

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

The authors gratefully acknowledge the skilled technical assistance of Mrs E. Guldfinovfi and Mrs V. Suchfi. The statistical computations, including the drawing of graphs, were carried out at the Computing Center of the Slovak Academy of Sciences, Bratislava, for which the authors wish to express their gratitude, especially to Dr P. Slavkovsk~, and Dr B. Viktorinovfi.

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