effect of liquid—solids separation on biogas production from dairy manure
TRANSCRIPT
Energy in Agriculture, 3 (1984) 61--69 61 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
E F F E C T O F L I Q U I D - - S O L I D S S E P A R A T I O N ON B I O G A S P R O D U C T I O N F R O M D A I R Y M A N U R E
P.H. LIAO, K.V. LO and S.T. CHIENG
Bio-Resource Engineering Department, University of British Columbia, Vancouver, B.C. V6T IW5 (Canada)
(Accepted 24 October 1983)
ABSTRACT
Liao, P.H., Lo, K.V. and Chieng, S.T., 1984. Effect of liquid~solids separation on biogas production from dairy manure. Energy Agric., 3: 61--69.
The effect of liquid--solids separation on methane production from mesophilic anaerobic digestion of dairy cattle manure at 35°C was investigated in the laboratory. Screening out the coarse solids from the manure before digestion had a significant effect on biogas production. It was found that total methane production and methane content of biogas from screened manure were consistently higher than from unscreened manure.
The results support the concept that a liquid--solids separation pretreatment step could significantly reduce the volume of digester required for a dairy farm, without decreasing biogas production.
INTRODUCTION
Energy genera t ion f r o m an imal m a n u r e b y anae rob ic digest ion has re- ceived increased a t t e n t i o n in r ecen t years . With the increasing wor ld popu- la t ion and l imi ted reserves of fossil fuel resources , biogas convers ion is a poss ible a l te rna t ive source available to m e e t f u tu r e energy demands .
Anae rob ic d iges t ion has been wide ly appl ied in munic ipa l and industr ial was t ewa te r t r e a t m e n t p lan ts fo r b iological ly stabil izing organic solids, re- duc ing sludge vo l um es requir ing u l t i m a t e disposal , cont ro l l ing odour , and recover ing m e t h a n e gas for in-plant hea t ing and energy r e q u i r e m e n t s (Sorge, 1970) . However , s tudies c o n c e r n e d wi th the r ecove ry of energy by anaerob ic process ing of da i ry cat t le m a n u r e are few ( H o b s o n and R o b e r t s o n , 1977 ; Varel e t al., 1977; Bousf ie ld et al., 1979 ; Ja in e t al., 1981) .
Most o f the mun ic ipa l sludge used in anae rob ic digest ion con ta ins typ i - cally 3--4% to ta l solids, highly digest ible organic mater ia ls , and a low ni t ro- gen c o n t e n t (Maly and Fa t rus , 1971) . Howeve r , an imal m a n u r e is usual ly high in n i t rogen and t o t a l solids (TS). Dai ry ca t t le m a n u r e in par t i cu la r con ta ins a high p r o p o r t i o n o f n o n b i o d e g r a d a b l e mater ia ls . Accord ing ly , m a n y researchers r e p o r t e d d i f f icu l ty involving mix ing (Bar t l e t t et al., 1977,
0167-5826/84/$03.00 © 1984 Elsevier Science Publishers B.V.
62
1980; Hein et al., 1977) and inhibition by ammonia and volatile organic acids (Jewell et al., 1976; Varel et al., 1977; Converse et al., 1977; Hill and Barth, 1977) when cattle manures were subjected to anaerobic fermentation. To solve these problems additional water is sometimes added to bring down the TS to around 8% level or less (Ecotope Group, 1978). However, the addition of water results in an increase of digester volume requirement and hence higher capital and operating costs.
The purpose of this s tudy was to investigate the feasibility of increasing the biogas output capability of laboratory scale digesters by using liquid-- solids separation pretreatment to remove the less biodegradable port ion of the volatile solids in the dairy manure.
MATERIAL AND METHODS
Feed material
Manure from a confined Holstein dairy herd in Canada (Blair Farms, 7851--184th Street, Aldergrove, B.C.) was used in this study. The cows were fed a ration of dairy concentrate (16% protein), alfalfa hay, grass and/ or corn silage, salt and minerals. No antibiotics were incorporated into the feed. Manure from the free-stall barn is scraped twice daily to the end of the alley; samples of this feces-and-urine mixture were collected in plastic barrels every 4--6 weeks and placed in cold storage at 4°C. Feed materials were prepared weekly by mixing equal volumes of the stored manure with distilled water to give a slurry of about 7.5% total solids (TS) and 6% volatile solids (VS) content. The slurry was then passed through a 2.0-mm (No. 10 mesh) and a 2.44-mm (No. 8 mesh) screen to yield liquid filtrates of about 2.9% VS and 3.4% VS.
Anaerobic digesters
The cylindrical digesters were constructed of acrylic plastic and had a working capacity of 4 1. Complete mixing was achieved in each digester by using a DC motor-driven impeller for 15 rain every hour. The digester temperature was maintained at 35°C using a thermostatically controlled internal electric heater.
Gas storage was accomplished with a fluid displacement system consisting of a graduated collector which was connected to a reservoir filled with saline water. Daily gas yield was measured by equilibrating the gas to atmospheric pressure, as indicated by a mercury manometer , and reading the liquid level in the collector.
Chemical analysis
Chemical analyses were performed weekly for the feeds and the effluents of all digesters. Analyses for TS, VS, pH were carried out according to stan-
TA
BL
E I
Hy
dra
uli
c re
ten
tio
n t
imes
an
d c
hem
ical
ch
arac
teri
stic
s o
f fe
ed m
ater
ials
an
d e
fflu
ent
Dig
este
r H
RT
C
OD
(g/
l)
TS
(%
) V
S (
%)
TK
N--
N (
g/l)
N
H3-
-N (
g/l)
(d
ays)
in
o
ut
% r
edu
ctio
n
in
ou
t %
red
uct
ion
in
o
ut
% c
on
ver
sio
n
in
ou
t in
o
ut
A
10
29
.2
22
.3
23
,5
3.9
2.8
24.9
2.
9 2.
0 3
1.0
8
38.7
31
.9
17.6
3.
9 3.
1 19
.3
2.9
2.1
27
.4
6 36
.3
31
.3
13.8
4.
1 3.
3 18
.8
2.9
2.2
22
.0
10
35
.0
27.3
22
.0
4.1
3.1
24
.4
3.5
2.5
28
.6
8 4
0,3
32
.5
19
.4
4.3
3.3
23.3
3.
6 2.
6 27
.8
6 36
.2
29.2
18
.9
4.0
3.3
19.0
2,
8 2.
2 2
3.3
10
32.6
21
.3
11.2
4.
6 3.
3 2
1.4
3.
5 2.
7 2
2.4
8
36
.6
32
.6
10.9
4.
5 3.
3 18
.6
3.6
2.9
20
.2
6 28
.2
25.4
9.
9 4.
6 4.
0 11
.6
3.8
3.3
14.0
D
10
51.7
4
4.5
14
.0
7.3
5.4
25
.6
6,1
4.6
25.1
8
54
.6
47.5
13
.1
7.1
5.7
20.0
5.
8 4.
6 2
0.6
2.3
6
2.35
0
.40
0
.72
2,
35
2.4
6
0.4
4
0.6
7
2,57
2
.72
0.
91
0.9
6
2.4
8
2.1
3
0.3
9
0.6
7
2.22
2
.08
0
.34
0
.59
2.
62
2.58
0
.83
0
.93
1.96
1.
78
0.2
2
0.5
0
1.85
1
.96
0
.29
0
.48
1.
97
2.1
4
0.6
4
0.8
3
2.43
2.
44
0.47
0
.71
2
.44
2.
55
0.5
1
0.8
0
64
dard methods (APHA, 1975). Total Kjeldahl nitrogen (TKN) and ammonia- N were determined using a block digester and a Technicon Auto Analyser II according to the method of Schumann et al. (1973).
The biogas production rates were monitored daily and the gas was ana- lyzed for methane content every 2 weeks during each phase. Gas samples, collected in gas-tight syringes, were analyzed with a Fisher-Hamilton Gas Partitioner equipped with a Chromosorb W column. All gas measurements were expressed at 0°C and standard pressure (760 mm Hg). No correction was made for water vapor since it was believed to be present in negligible amounts.
Experimental procedure
The manure concentrations and the hydraulic retention times (HRT) applied in the experiments are given in Table I. Digesters A and B were loaded respectively with slurry prepared from filtering a 1 : 1 manure--water mixture through a 2.0-mm (No. 10 mesh) and a 2.44-mm (No. 8 mesh) vibrating screen. The choice of No. 10 mesh was based on preliminary screening trials aimed at obtaining the desired volatile solids levels for feed materials. The No. 8 mesh was chosen to match the aperture of a commer- cially available, rotating screen, l iquid-solids separator (Rotostrainer, Hycor Corporation, Lake Bluff, IL 60044, U.S.A.). Digester C received unscreened slurry which was diluted to obtain a solids content equal to the feed for digester B. Digester D received unscreened one-to-one manure--water slurry and served as the control representing the current on-farm digester operation.
Digesters A and B compared the effect of screen size on filtrate VS content and biogas production rate. Digesters C and D compared two di- lutions of unscreened slurry. Digesters B and C compared screened and unscreened slurries of equal nutrient concentrations.
The effluent from a laboratory fermenter acclimated to dairy waste was used as the inoculum. All digesters were fed daily. The feeding procedure was as follows. After the determination of the biogas production a prescribed volume (determined by HRT) of the mixed digester contents was removed. Then the manure was added according to the organic and hydraulic load. Before and during the feeding procedure the DC motor driven impeller was turned on to provide complete mixing of the digester content. The experi- ments were carried out during the period of April to November 1982.
RESULTS
The conditions and the results of the laboratory experiments are summa- rized in Tables I and II. The data in these tables are the average values of the analytical results obtained during a steady state of the digestion processes in each HRT. Based on the results from previous trials run at 30°C, the di- gesters were operated at relatively short HRTs (10, 8 and 6 days).
TA
BL
E I
I
Dig
este
r o
per
atio
n s
pec
ific
atio
ns
and
met
han
e p
rod
uct
ion
res
ults
Dig
este
r S
cree
n H
RT
H
RT
L
oad
ing
M
eth
ane
mes
h (d
ays)
cy
cles
ra
te
com
po
siti
on
(g VS/
I (%
) pe
r da
y)
met
han
e p
rod
uct
ion
(1 C
H4/
I (R
ang
e)
per
day
) (1
CH
4/g
VS
ad
ded
per
d
ay)
(Ran
ge)
(I
CH
4/g
VS
de-
st
roy
ed
per
day
A
No
10
10
4 2.
92
64.3
(2
mm
) 8
11
3.51
64
.3
6 8
4.81
63
.8
B
No
8
10
4 3.
45
64.3
(2
.44
ram
) 8
11
4.54
64
.3
6 8
4.76
62
.5
C
N/A
10
4
3.45
51
.8
8 11
4.
54
59.4
6
10
6.33
51
.2
D N
/A
10
4 6.
10
51.6
8
11
7.29
49
.1
0.49
0
.62
-0.4
2
0.62
0
.74
-0.4
1
0.61
0
.75
--0
.46
0.51
0
.66
-0.4
0
0.71
0
.93
--0
.36
0.
60
0.8
4-0
.42
0.18
0
.30
--0
.08
0.
24
0.2
8-0
.19
0.
21
0.3
3-0
.15
0.24
0
.43
-0.1
2
0.26
0
.32
-0.1
3
0.1
80
0.
178
0.12
7
0.14
7 0.
157
0.1
26
0.0
53
0
.05
3
0.0
33
0.0
40
0
.03
6
0.2
10
--0
.14
2
0.2
02
--0
.11
2
0.1
55
--O
.09
6
0.1
90
--0
.11
6
0.2
05
--0
.07
9
0.1
70
--0
.08
5
0.0
86
--0
.01
8
0.0
62
--0
.04
2
0.0
52
--0
.02
4
0.0
71
--0
.02
0
0.0
44
--0
.01
7
0.55
0
.60
0.
58
0.5
2
0.56
0.
54
0.27
0
.26
0.
24
0.1
6
0.17
N/A
Sta
nds
for
no
t ap
plic
able
.
66
Methane product ion rates are expressed as 1 CH4/1 digester per day, 1 CH4/g VS added and 1 C H J g VS destroyed per day (Table II). Methane production from the screened dairy manure (digesters A and B) was con- sistently higher than that from the unscreened manure (digesters C and D). For all digesters, the maximum volumetric methane product ion rate (1 CH4/1 per day) was achieved at 8 days HRT.
The methane content of the biogas produced from digesters A and B, fed with screened dairy manure, was higher than that produced from digesters C and D which were fed with unscreened manure. The biogas composit ion of digesters A and B was 63.5 -+ 1% methane with the balance being CO2 and trace impurities. However, the methane content of the biogas from digesters C and D was between 49.1% to 59.4%.
In all digesters mixing was applied to prevent the formation of a floating scum layer. Nevertheless, in digesters C and D which received unscreened manure, a scum layer was formed, which could be disrupted by continuous stirring or shaking for several minutes. None of the digesters had shown signs of instability except in the case of digester D, when it was operated at 6 days HRT.
The quality of the feed material was relatively stable throughout the experiments. The TS, VS and COD reductions presented in Table I are the average values calculated based on the TS, VS and COD data of the influents and the effluents. COD, TS and VS reduction were generally higher in the screened than in the unscreened manures. Differences in total nitrogen content (TKN) between the influent and the effluent showed negligible loss of total nitrogen from all digesters. Ammonia nitrogen increased through treatment in all cases. The pH in all digesters remained within the range of 6.9--7.2 and increased only slightly from that of the feed materials. No alkalinity adjustment was necessary except in digesters C and D operated at 6 days HRT.
DISCUSSION
Effect of liquid--solids separation on biogas production
The results indicate that dairy cattle manure, both screened and un- screened, is a good substrate for anaerobic digestion. Successful digestion could be obtained at loading rates of 2.9--4.8 g VS/1 per day for screened manure and at 3.4--7.3 g VS/1 per day for unscreened manure. At 10, 8 and 6 days HRT, methane production from digesters receiving screened manure (A and B) was much higher than those (C and D) fed with unscreened manure. In all cases, the maximum volumetric methane product ion rate (l CH4/I per day) was obtained at 8 days HRT. While digesters A, B and C received approximately the same VS loading rate, methane gas production from digester C, was only about 33--39% of those from digesters A or B. For example, at 8 days HRT and a loading rate of 4.54 g VS/1 per day, the
67
methane production rate from digesters B and C was 0.71 and 0.24 1 CH4/1 per day, respectively. The higher VS loading rate of unscreened manure (digester D -- approximately double that of digesters A, B and C) resulted in only a slightly greater gas production than that of digester C. It appears that at the relatively short HRT of 6--10 days, anaerobic digestion using liquid filtrate (digesters A and B), has a definite advantage over that of unscreened liquid manure (digesters C and D). The data are comparable to those reported by Chen et al. (1980).
Process efficiency
Digester performance was measured in terms of biogas production and volatile solids destruction. In the experiments the VS reduction for both screened and unscreened manures varied from 20.6 to 31.2%. The reduction efficiencies of volatile solids were greatest at 8--10 days HRT regardless of treatment. The volatile solids conversion of screened manure was higher than that of unscreened manure at the same HRT. For example, digesters B and C were fed with materials of the same VS concentration at 10 days HRT. Volatile solids conversions were 28.2% for the pretreated screened manure (digester B) and 22.4% for the diluted unscreened manure (digester C). Furthermore, the results shown in Table I indicate that the VS and TS reductions tend to decrease at decreasing hydraulic retention time.
Average COD reductions over all HRT runs were high in the screened slurries in comparison with that of the unscreened slurries. The relatively higher variation in COD values were expected as a very large dilution factor was involved in the chemical analysis. The biogas production from screened manure varied from 126 to 180 1 of methane gas per kg VS added. The effect of increasing the organic load (either by increasing the VS concentra- tion of the manure feed or by decreasing the hydraulic retention time) upon biogas methane content was not significant in the HRT range of 10--6 days. The methane content of the biogas produced from the screened manure was 63.5 + 1% in all cases, while from unscreened manure it varied from 49.1 to 57.4%.
Ammonia nitrogen concentration
The relationship of HRT and feed concentration to the level of ammonia nitrogen is shown in Table I. No significant increase in ammonia concen- tration was found at the higher feed concentration or shorter HRT regardless of treatment. The ammonia concentrations obtained in the experiments were far below the toxic level for methane fermentat ion reported by McCarty (1964).
68
Practical application
Assuming that the data obtained in this s tudy could be successfully scaled-up to a farm-sized operation, then the results could have a significant impact on the design of full-scale biogas product ion systems for dairy manure. In the liquid--solids separation step, the total mass of the liquid filtrate was about equal to that of the solid residue. Assuming digester D represents the current on-farm operation, then for a given quanti ty of dairy manure, if liquid--solids separation (L/S) pretreatment is carried out on the feed to an anaerobic digester, the amount of liquid manure entering the digester could be reduced by half and would result in a 50% reduction in the required digester volume even if the same retention times were used. Due to the more rapid breakdown of the filtered manure it is possible to reduce the hydraulic retention from the conventional 16 days down to 10, 8 or even 6 days. The corresponding further reduction of the digester volume for 6, 8 and 10 days HRT would be 63.5%, 50% and 37.5%. Com- bining the L/S pretreatment and the shorter HRT would then result in a digester volume ranging from 18.75% to 25%, and 31.25% (HRT 6, 8 and 10 days) of the conventional anaerobic digester.
This represents more than a 60% reduction in digester volume and would certainly result in a significant decrease in the capital cost of building anaer- obic digesters.
CONCLUSIONS
Total methane product ion from screened manure is consistently higher than that from unscreened manure.
The methane content of the biogas produced from screened manure is consistently slightly higher than that produced from unscreened manure.
The methane product ion (1 CH4/g VS added) decreases as HRT decreases. Successful anaerobic digestion has been attained at organic loads in the
range of 2.9--4.7 g VS/1 per day for screened manure and HRT of 6 days, and in the range of 3.4--7.3 g VS/1 per day for unscreened manure and HRT of 8 days.
Combining the liquid--solids separation pretreatment and the shorter HRT's attainable with the screened manure could result in substantial reduction in the digester volume requirement.
ACKNOWLEDGEMENT
The authors acknowledge the financial support provided by the Natural Sciences and Engineering Research Council of Canada, as well as the technical assistance provided by Alan March, Alan Whitehead, Rober t Stephenson and Adeline Chen.
69
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