Evaluation of physical properties of pig manure

J. agric. Engng Res. (1974) 19, 199-207

Evaluation of Physical Properties of Pig Manure J. R. BACKHURST*;J. H. HARKER* This investigation reports the results obtained in evaluating the physical properties of pig manure during the course of feeding trials with emphasis on density, viscosity and calorific value. In the case of calorific value, values of 17.9 MJ/kg dried faeces have been obtained and this quantity of heat will contribute significantly in any incineration operation. Technical scale tests on drying and filtration of slurries are reported with mean transfer rates of 2.2. 10- ~ kg/m~s and 7-0 ~<10 -5 kg/m~s respectively.

1.

Introduction

In the development of modern agricultural production techniques, it is vital that pollution of the environment should be minimized especially from animal production units, where successful manure disposal units are of prime importance. This is particularly important in intensive pig and poultry units, where hitherto land spreading has been the main method of manure disposal. As units become more intensified, the availability of land is reduced, more so in urban areas close to the consumer, and the resulting increase in costs of disposal and transportation render alternative methods economically attractive. In the case of pig manure, alternative means of disposal include biological treatment and slurry thickening though throughout, one is beset by the very high moisture content of the material coupled with the odour problem. Whilst the situation is such that biological treatment is now a distinct economic proposition, it is obvious that more expensive processing operations such as filtration, drying or even complete incineration, must be considered, particularly in the long term. Essential to the development of manure disposal systems is a clear understanding of the properties of the manure. Data on production rates from pig units together with chemical and biological properties have been reported recently by O'Callaghan, Dodd, O'Donoghue and Pollock 1 though information on physical properties, especially in relation to disposal by drying and incineration is sparse in the extreme. This paper aims at reporting such data, obtained during the course of a series of feeding trials. 2.

Previous work

Properties of pig manure are influenced by many factors including type and level of feed, animal size and environment and indeed, unless this information is available, the results of any trial are somewhat limited. Table I illustrates the wide variation of data quoted in the literature which, in view of changes in pig production techniques over the past decade, may not be strictly applicable at the present time. Taking into account the limited data available and the growing importance of work in this field, O'Callaghan et al. 1 recently carried out trials in which the daily faecal and urinary production from individual pigs were measured over the live weight range 20-90 kg. Three different feeding regimes were employed and it was found that these had an effect on waste production which could be expressed as a percentage of meal and water consumed. The feeding regime also significantly influenced properties of pig manure, including biochemical oxygen demand, chemical oxygen demand, total and volatile solids and composition. The more important results are summarized in Tables II and III. In addition the pH of the manure was found to be 8.2 and no reduction in oxygen demand was observed even after some 18 weeks storage. * Dept. o f Chemical Engineering, University o f Newcastle upon Tyne

199

200

PHYSICAL PROPERTIES OF PIG MANURE

This w o r k is o f considerable significance in relation to disposal o f waste by land spreading and biological t r e a t m e n t and yet o f limited value in considering disposal by drying, filtration or incineration. This p a p e r reports the results o f tests carried o u t d u r i n g the course o f the same trial, to evaluate basic physical properties such as density and viscosity and also the findings o f technical scale trials to evaluate drying and filtration characteristics. TABLE I

Production of pig manure

Size of animal (lb) (kg)

ReJerence Van Slyker 2 Salter 2 Hart z Jeffrey 2 Taiganides 2 Schmid ~ Scheltinga 4 Scheltinga s Baxter 6 Robinson ~

Manure production (/b/day) (kg/Ms)

100

45

I00

45

I00 I00

45 45

96-103-6

43"5 46"9

8"4 9"5 2"8 9'3 5"0 9"0 8"8 6"85 21 "0 9"55

44"2 49"8 14"7 48"7 26"2 47"2 46"1 35'9 I10"0 50'0

TABLE It Mean values of BODs, COD and solids contents I

Sample Crate group 1 Crate group 2 Crate group 3 Pen no. 5 Pen no. 6

BOD~ (mg/l)

COD (mg/l)

Total solids as % wet manure

Volatile solids as % total solids

BOD/meal (mg/l. kg)

COD/meal (mg//.kg)

21482 21992 16697 23300 19000

71949 75153 47134 69000 59000

9'53 9"52 5"59 7"55 6'09

83"24 83"60 83"01 82"29 80"95

0.0429 0-0379 0.0372

0-177 0"150 0.114

TABLE 111 Nitrogen, phosphorus and potassium content of pig manure I

% P w/w [ % K w / w

Sample Crates Crates Pen 1 Pen 2 Pen 5 Pen 6

Urine Faeces Wet manure Wet manure Wet manure Wet manure

3.

0"119 0'422 0"27 0"17 0"14 0"16

0"196 0"396 0"26 0'22 0'19 0'19

% N w/w

Total solids (%)

0"95 0"90 1"06 0'66 0"60 0'63

12"64 8"75 5"62 6"13

E v a l u a t i o n o f p h y s i c a l p r o p e r t i e s o f pig w a s t e

Th e e v a l u a t i o n o f the physical properties o f the pig waste were carried out in c o n j u n c t i o n with the feeding trials m e n t i o n e d in Section 2 o f this paper. Samples o f separated urine and faeces were taken f r o m each o f 12 pigs at fortnightly intervals and which were fed either f r o m a pipeline or were subject to floor feeding. Details o f the trials, together with housing and feed c o n s u m p t i o n have been reported.t

201

J. R. BACKHURST; J. H. HARKER 3.1.

Density measurements

D u r i n g the trials, the density o f samples o f the s e p a r a t e d faeces was measured using a specific gravity bottle a c c o r d i n g to British S t a n d a r d 733 and in a similar way, the densities o f the urine samples were d e t e r m i n e d using a set o f h y d r o m e t e r s in a c c o r d a n c e with B.S. 718. The results for the first f o u r weeks o f the trial are presented in Table IV and for the c o m p l e t e p e r i o d o f the trial, averaged d a t a are s u m m a r i z e d in T a b l e V. TABLE IV Measurements of density Density (kg/m :~) Crate No. Urine Week 2 I

2 3 4 5 6 7 8 9

10 11 12

Week 4

1018 1023 1026 1024 1014 1020 1017 1017 1020 1020 1017 1020 Average 1020

1032 1028 1020 1029 1030 1014 1022 1020 1021 1020 1024 1021 1023

SeparatedJbeces Week 2 Week 4

1215 1133 1128 1176 1204 1181 1134 1156 1212 I110 1213 1116 1165

1163 1190 1201 1176 1220 1154 1125 1206 1135 1070 1189 1122 1163

Feeding method

Floor Floor Floor Floor Floor Floor Floor Pipeline Pipeline Pipeline Pipeline Pipeline

TABLE V

Measurements of density--summary of results obtained Density (kg/m :~) Week No. Urine floor and pipeline feeding

2 4 6 8 10 12 14 Average

1020 1023 1007 1012 1015 1017 1020 1016

Separated faeces floor pipeh'ne Jeeding feeding

1167 1175 1172 1090 1095 1120 1115 1133

1161 1144 1150 1115 1103 1105 1095 1125

3.2. Viscosity m e a s u r e m e n t s Difficulty was experienced in d e t e r m i n i n g the viscosity o f either the s e p a r a t e d faeces or the raw sewage sludge using a S h i r l e y - F e r r a n t i viscometer as the semi-solid r o u g h a g e o f the faecal cake prevented the cone f r o m spinning at c o n s t a n t speed, thus p r o d u c i n g inconsistent data. It was possible, however to o b t a i n the viscosity o f the urine by means o f a previously calibrated U-tube a c c o r d i n g to B.S. 188. The tests were carried out in an electrically heated water bath with accurate t e m p e r a t u r e c o n t r o l a n d the results are presented in Table VI.

202

P H Y S I C A L P R O P E R T I E S OF PIG M A N U R I :

TABLE VI

Viscosity of urine Crate No.

1

Feeding method

Viscosity (mN.s/m ~) Week 2 Week 4

Floor Floor Floor Floor Floor Floor Floor Pipeline Pipeline Pipeline Pipeline Pipeline

2 3 4 5 6 7 8 9 10 I1 12

1"158 1"113 1-179 1"123 1.093 1"087 1"137 1"120 1-096 1"099 1.124 1'108 1"121

1-063

1.079 1.090 1-108

1"116

Average 1.091

3.3. Calorific value of dried faeces A sample of the separated faeces was dried in an oven at 376 K until no further loss in weight was recorded. A sample of the dry material was intimately mixed with excess sodium peroxide (14 g/g faeces) and charged to the combustion chamber of a Rowland-Wild calorimeter according to the standardized procedure. Results for the calorific value of samples from the first 4 weeks of the trial obtained in this way are presented in Table VII, while Table VIII shows averaged data for the entire trial period. 3.4. Drying characteristics The rate of drying samples of separated faeces (approximately 70 % moisture) was evaluated at frequent intervals during the course of the feeding trial using material from different crates. 5-8 g of the samples was weighed and placed in an oven maintained at 368-372 K and at atmospheric pressure. At intervals of some 2 ks, the dish was removed, cooled in a dessicator and

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203

J, R. B A C K H U R S T : J. H, H A R K E R TABLE VII

Calorific value of dried faeces (0 % moisture) Feeding method

C'rate No.

1 2 3 4 5 6 7 8 9 10 I1 12

Calorific wdue (MJ/kg) Week 2 Week 4

Floor Floor Floor Floor Floor Floor Floor Pipeline Pipeline Pipeline Pipeline Pipeline Average

17-6 16.2 15.8 15-3 16.5 15.8 16.8 16.3 15.8 17.2 16.2 16.4 16.3

16-3 15.9 15.6 15.0 15.7 16.8 16.1 15.3 17-3 15.9 17.5 16.8 16-3

Samples taken from each crate during week 2 and 4 of the trial TABLE VIII

Calorific value of dried faeces (0 % moisture)

Week No.

Calorific value ( MJ/kg ) Floor ,feeding

Pipeline feeding

2 4 6 8 10 12 14 Average

16.3 16.1 17'8 18.1 19.0 19.5 18"4 17,9

15'3 16-4 17.2 18"6 19.3 19.1 18.5 17-9

Summary of results obtained over the period of the trial TABLE IX

Specific resistance of raw slurry

Run

1

2 3 4 5 6 7 8 9 10

( t - t l ) / ( V - V1)" (s/cm 6 x 10 2)

ziP (N/m 2 x 10 -4)

1 "93 I "64 1 "60

7.48 6"31 5.97 0-47 4.83 3.29 3.51 1.11 1-19 0.71

0"27

1 "52 1 "28 1"14 1 '90

I "08 I "42

Specij~'c (cm:Vcm :~) 0"154 0"154 0"154 0' 160 0"114 0'131 0"121 0'093 0.080 0.086

resistance, r (1/cm ~ x 10 -1°) 6"89 4"94 4"56 0"58 4"73 2'36 2"43 1 "67 1"18 0"86

204

PHYSICAL

PROPERTIES

Ot: PIG

MANURt

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]nit4olrnolsfure content (kg/kg) 2 49 X 259 o 252

L5

247

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1 18

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22

Moi sfure con fenf(kg/kg dr}' solid)

Fig. 2.

Drying rate curve [br pig faeces (samples from week 6 of trial)

weighed--this cycle being continued until no further loss in weight was observed. Typical results for week 6 of the trial are shown in Fig. 1 as weight of water evaporated against time, for different crate numbers. In this plot, A is the area of the sample dish in cm 2. The linear relation during the constant rate period when water is evaporating from the surface only is observed and this is followed by a rapid decrease in the water evaporated as the water-air interface moves into the material. The spread of data in Fig. 1 is due to different areas being exposed for drying and in order to allow for this, the results have been recalculated as drying rates per unit areas. The results plotted against moisture content (weight of water per unit weight of dry solids) in Fig. 2, resulting in a

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Fig. 3. D r y i n g r a t e curves f o r p i g f a e c e s

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R.

BACKHURST"

.I.H.

205

HARKER

10 8

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--.Ik

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50

75

Volume

--A

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of filtrate ( cm s)

Fig. 4. Filtration o f raw slurry

much better correlation; the spread here being due to different crate numbers and the initial moisture content, which ranged from 2-47-2.81 kg/kg dry solids. Various results for different samples, as a function of crate number and date of sample are shown in Fig. 3. In Fig. 3, the critical moisture content, that at which the drying from the surface is complete and the operation enters the falling rate period, varies from 0.44-0-75 kg/kg dry solid. The initial moisture content of the separated faeces varied from 2.0-2.8 kg/kg dry solid which is equivalent to a moisture content of 67-73.5 % on a wet basis. During the constant rate period, the drying rate varied from 1.8-2.8 × 10 -4 kg/m%; the mean value being 2.2× 10 -4 kg/m2s. This rate applied between the initial moisture content of say 70% and the mean critical value of 42-45 ~ . It is of interest to note that under the same conditions of temperature and atmosphere, pure water was found to evaporate at a rate of 4 × 10 -4 kg/m2s and perhaps of greater significance is the fact that the odour appeared to be driven off when the moisture content was reduced to 50 %. 10 II

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Fig. 5. Compressibility curve f o r raw sludge

I I I IO2

206

PHYSICAL

PROPERTIES

OF

PIG

MANUR[_

3.5. F i l t r a t i o n o f r a w s l u r r y A simple filtration facility was assembled in which the raw slurry was filtered through various cloths, mainly of Terylene and also metal gauzes, supported in a Buchner funnel; the filtrate being collected in a calibrated flask. Vacuum was applied to the flask by means of a water ejector and the absolute pressure was indicated by a mercury in glass manometer. The filter cake thickness was measured by means of a travelling microscope. Typical results are shown in F~. 4, where the time to collect unit volume of filtrate is plotted against the volume of filtrate collected. The slopes of the lines in this figure are: (t--tl)/(V--V1)

2-

r/t v/(2A 2 . AP)

where V, VI are the volumes of filtrate at times t and tl (cm3), r --- specific resistance (l/cm~), /z ----viscosity of filtrate assumed !.1 mNs/m2), v -- volume of cake deposited per unit volume of filtrate (--), A = area of filter (63.5 cm2), ziP -- pressure drop across the cake (N/m 2) and using this equation, values of r, the specific resistance have been calculated as shown in Table IX. The specific resistance is a function of the applied pressure, given by: r - k. AP"

where n is the compressibility coefficient. The data in Table IX are plotted on logarithmic axes in Fig. 5 from which the mean line has a slope, n -- 0.82. The moisture content of the filter cake was determined during these tests and a mean value of 73 % (3 kg water/kg dry solid) was obtained. This corresponds to the value for the separated faeces. The mean filtration rate was 7.0 × 10 -5 kg/m~s of dry solid and the total solids content in the filtrate was less than 0.01%. 4.

Conclusions

The more important conclusions which may be drawn from this investigation may be summarized as follows. (a) Over the 14-week feeding trial, the densities of the urine and separated faeces varied only slightly and mean values of 1016 and 1130 kg/m ~ were obtained respectively. (b) Similarly the mean viscosity of the urine was found to be 1.10 m N s / m 2 and the calorific value of the dried faeces was 17.9 MJ/kg, which will contribute significantly in any incineration operation. (c) The tests on drying of faeces initially containing 70~o moisture showed that the critical moisture content was 42-45 ~ and above this the mean drying rate was 2.2 × l0 -4 kg/m2s. The absence of odour when the moisture is reduced below 50 % suggests that disposal by partial drying followed by land spreading is worthy of further consideration. (d) Raw slurry was filtered to produce a cake containing 73 ~ moisture and a rate of 7.0:~ 10 -5 kg/m2s of dry solid producing a filtrate containing less than 0.01 ~ total solids. (e) On the basis of present profit margins, it may be that incineration of waste could be within the bounds of a viable commercial proposition, especially taking into account the decrease in total operating costs with reduced moisture content resulting from the contribution made by the calorific value of the dried faeces. In the long term, incineration as the only complete disposal method may prove to be the ultimate solution to the problems involved and certainly on the basis of the results reported here, the operation merits further investigation especially on a larger scale. 5. Acknowledgements This research was generously supported by a grant from the Agricultural Research Council. The authors would like to acknowledge contributions from the Departments of Agricultural and Civil

J,

R.

BACKHURST:

J.

H.

207

HARKER

Engineering, The University of Newcastle upon Tyne and in particular to Professor J. R. O'Callaghan under whose guidance the work was carried out and to Mr J. A. Podwojski for assistance with the experimental work. REFERENCES

1 0 ' C a l l a g h a n , J. R.; Dodd, V. A.; O'Donoghue, P. A. J.; Pollock, K. A. J. agric. Engng Res.. 1971 16 (4) 399-419, 1973 18 (1) 1-12 2 Taiganides, E. P.; Hazan, T. E. Trans. Am. Soc. agric. Engrs, 1966 9 374 3 Schmid, L. A.; Lipper, R. I. Animal Waste Management Conference. Cornell University, 1969 Sehelfinga, H. M. J.; Poelma, H . R . Symposium on Farm Wastes. University of Newcastle upon Tyne, 1970 s Seheltinga, H. M. J. J. Inst. Wat. Pollut. Control, 1969 68 403-409 6 Baxter, S. H.; Pontin, R. J. Inst. Wat. Pollut. Control, 1968 67 632-638 7 Robinson, K.; Baxter, S. H.; Saxon, J. R. Symposium on Farm Wastes. University of Newcastle upon Tyne, 1970 8 0 ' C a l l a g h a n , J. R.; Pollock, K. A.; Dodd, V. A. J. agric. Engng Res., 197l 16 280-300