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Effect of pH on the behaviour of volatile compounds in organic manures during dry-matter determination
Bioresource Technology 49 (1994) 41-45 O 1994 Elsevier Science Limited Printed in Great Britain. All fights reserved 0960-8524/94/$7.00 ELSEVIER
EFFECT OF pH ON THE BEHAVIOUR OF VOLATILE COMPOUNDS IN ORGANIC MANURES DURING DRY-MATTER DETERMINATION P. J. L. Derikx,* H. C. Willers & P. J. W. ten Have Department of Buildings and Environmental Technology, Institute of Agricultural and Environmental Engineering (IMA G-DLO), PO Box 43, NL-6700 AA Wageningen, The Netherlands (Received 4 February 1993; revised version received 22 April 1994; accepted 26 April 1994)
Abstract Dry matter determination by drying at 105 °C is one of the most widespread methods used in the characterization of manure. Loss of volatile compounds during drymatter determination may lead to considerable errors in mass balances. Since the behaviour of most volatile compounds depends upon pH, these losses may vary from sample to sample. The loss of ammonia and volatile fatty acids from pig, cattle and poultry manure was studied as a function of pH. It was shown that above pH 8 all ammonia was volatilized and below p H 5 all volatile fatty acids evaporated. Total fixation of ammonia was achieved below p H 4. Above p H 10 all volatile fatty acids were fixed in the residue after drying. The three types of manure showed only slightly different patterns of losses of volatile compounds. The amount of acid or alkali needed to obtain the desired p H varied strongly between the various kinds of manure.
determination are used for comparison only, the omission of a constant part of the volatile substances causes only minor errors. However, although acidification of manure is an effective means of reducing volatilization of ammonia from manure during storage, application or drying (Stevens et al., 1989; Frost et al., 1990; ten Have, 1993), the loss of volatile substances no longer remains a constant factor. Furthermore, the results of the drymatter determination method discussed here are commonly used in mass-balance calculations for large-scale manure treatment plants. This may lead to considerable errors because an unknown part of the volatile substances is included in the results of the dry-matter determination. For these reasons a closer look at the loss of volatile substances during dry-matter determinations is necessary. Because of their relatively large abundance in manure and their contribution to environmental pollution, attention is focused on ammonia and volatile fatty acids. The goal of this investigation was to quantify the loss of ammonia and volatile fatty acids as a function of pH from different kinds of manure during the drymatter determination.
Key words: Ammonia, volatile fatty acids, manure, drying method, pH. INTRODUCTION The dry-matter content of manure is generally measured by drying at 105°C to constant weight. It is one of the most widespread methods used in the characterization of manure due to its low requirements for sophisticated laboratory equipment and the relative ease of its execution. The method is well documented (APHA, AWWA & WPCF, 1985; NEN, 1986). It is generally accepted that in addition to evaporation of water, there is a loss of volatile substances during the dry-matter determination. Recently, Hayward and Pavlicik (1990) recommended a sulphuric acid titration method in order to be able to correct for the loss of acetic acid from samples originating from an anaerobic digester. The loss of volatile substances will be relatively constant as long as the samples are taken from the same type of material and no big changes in pH occur. If the values obtained by the dry-matter
METHODS
*To whom correspondence should be addressed. 41
Manure was obtained from various farms in The Netherlands. Pig manure originated from fattening pigs and cattle manure from dairy cattle, both kept on slatted floors. Poultry manure was produced by laying hens kept in cages. All animals consumed standard commercial feed. From each type of manure, three different batches were collected. The average characterization is given in Table 1. Each batch was stored at 4°C and was well mixed before use. Titration curves were obtained by adding hydrochloric acid (12 tool/ litre) or sodium hydroxide (4 mol/litre) step by step to about 130 g manure (fresh weight) under constant mixing. The dry-matter determination was performed at 105°C to constant weight with subsamples of about 40 g fresh weight according to the standard procedure
42
P. J. L. Derikx, H. C Willers, P. J. W. ten Have
(APHA, AWWA & W P C E 1985; NEN, 1986). Normally, overnight drying was sufficient when no excessive crust formed. Different amounts of hydrochloric acid or sodium hydroxide were added to achieve the desired pH before drying. After mixing, the pH obtained was measured. In order to determine the recovery of ammonia and volatile fatty acids in the residue after drying, water was added to the residue to such an extent that the original dry-matter content was approximately restored. The concentration of ammonia and volatile fatty acids was then determined after one hour of extraction at room temperature. Extraction with other solutions (0.1 mol/ litre hydrochloric acid, 0" 1 mol/litre sodium hydroxide or 2 mol/litre potassium chloride) did not affect the recovery of ammonia or volatile fatty acids, respectively (data not shown). Relative recoveries were calculated with respect to the amount present in the original manure (see Table 1 ). Ammonia was measured spectrophotometrically according to NEN 6472 (NEN, 1983). Volatile fatty acids were measured on a Packard 427 gas chromatograph, equipped with a flame ionization detector. A glass column (2 m, 6 mm outer diameter, 2 mm inner diameter) packed with 10% Fluorad 431 on Supelcoport (100-120 mesh) was used at 130°C isothermally. Nitrogen, fully saturated with formic acid, served as carrier gas at a flow of 30 ml/min. Injector and detector temperatures were 220 and 240°C, respectively, pH measurements were performed at room temperature with a Hanna Instruments glass electrode (model HI 8417) directly submerged in the manure. Curve fitting on the measured relative recovery of ammonia and volatile fatty acids was done with GenStat 5 according to the general logistic model, using the equation: Relative recovery = a + 1 + e-b(pH- d)
RESULTS In Fig. 1 the titration curves of the three types of manure are shown. The addition of alkali is presented as negative addition of acid and is given as mmol H÷/g dry weight. By expressing the amount of acid added as 15
i
A
10
×0
×cJ xco
~< xo
B 10 pH O0~xO ~x 0;,0
5
©
I
p
)
x ×x
~e ,
•
C I I
10
"h
5 Q~ 0
The values of the constants a, b, c and d followed from the fitting process. To enable curve fitting for the recoveries of ammonia with the method mentioned above, the obtained values for the relative recovery of ammonia were subtracted from 120. In this way positive values inclining with pH were obtained.
-5
0
5 mmol H+/g dry weight
10
Fig. 1. Titration curves of manure from (A) pigs, (B) cattle and (C) poultry. Different symbols refer to different batches of the same type of manure.
Table 1. Characterization of pig manure, cattle manure and poultry manure used. Values given are the averages and the standard deviation of three different batches
Dry matter (%) Ash content (% of dry matter) pH Ammonia (mmol/litre) Acetic acid (mmol/litre) Propionic acid (mmol/litre) iso Butyric acid (mmol/litre) Butyric acid (mmol/litre) iso Valeric acid (mmol/litre) Valeric acid (mmol/litre)
Pig manure
Cattle manure
Poultry manure
9"86 + 0.34 31.9 + 1.6 7-88 + 0"05 4395-9.5 82.1 5-6"8 32-4+3-5 4-2 + 0"5 2"25-0.35 6"6 5- 0"6 0"7 + 0"3
8-42 + 0"53 22.5 + 0.5 7"555- 0"23 1665- 1"0 785-33 20+ 13 2.6 5- 1"3 4-95-2"8 2-5 + 0"9 0-6 + 0-3
15"7 5- 3"9 27.9 + 3"0 6"99 5- 0"14 6465-99 4585-68 1145-21 13"1 5- 2.6 75"35- 16 9-2 _+3-9 2"6 5- 1"0
Volatilization losses during dry-matter determination mmol H+/g dry weight, the differences observed between the various batches of the same kind of manure were reduced remarkably. The three types of manure showed a similar response to the addition of alkali. However, the titration curve of pig manure differed from the titration curves of cattle and poultry manure with respect to the addition of acid. Starting with fresh manure, about twice as much acid was needed to lower the pH of pig manure compared to cattle and poultry manure. Using the information from Fig. 1, acid or alkali was added to subsamples of the manures to obtain various pH-values over a pH range of 1-12. After drying to a constant weight, the amount of ammonia remaining in the solids was determined after extraction. The relative recovery is given in Fig. 2 for the three types of
120 Q
•
A
80
40
~enaa
i
,e
e
B
8O
!° u
43
manure, together with the results of curve fitting. No noticeable differences in recovery were observed between the three batches of the various types of manure. Therefore, all results from each type of manure were fitted by one curve. The values of the constants obtained by the curve fitting are summarized in Table 2. Due to the limited number of observations at low pH values for the poultry manure, the fitted curve did not reach the theoretical value of 100. Pig manure showed recoveries as high as 80% for ammonia at pH 6, which is high compared with cattle and poultry manure. For the latter two, the pH must be as low as 4-5 to retain ammonia to the same extent in the solids. At pH 7 or higher, nearly all ammonia was lost by drying the three types of manure tested. The differences between the three types of manure were small, but the recovery of ammonia in pig manure at pH 7-8 seemed to be slightly higher. The recovery of volatile fatty acids, obtained in similar experiments to those described above, is shown in Fig. 3. The recoveries were calculated after summing the concentrations of all the volatile fatty acids mentioned in Table 1, as the relative volatilization behaviours of each of the individual volatile fatty acids were shown to be the same (data not shown). Here too, no differences were observed between the recoveries for the three batches of manure, although the results showed more scatter. For this reason, the calculated curves were less accurate, especially for poultry manure. As a result, large differences between the values of the constants were observed for the three types of manure (see Table 2). The recovery of volatile fatty acids after drying at various pH values did not differ much from one type of manure to the other. At pH 9-10, all the volatile fatty acids were completely fixed in pig and cattle manures. Below pH 6, nearly all volatile fatty acids were lost during the drying process. The results obtained with poultry manure varied too much to justify any firm statement, but roughly the same tendency was observed.
Table 2. Values of the constants obtained by fitting the ammonia and volatile fatty acid recoveries according to the general logistic model
C
Constants
80
o\ \
"\
40
0
"~
0
5
** "~
10
15
pH before drying Fig. 2. The recovery of ammonia in the residue after the dry-matter determination of manure from (A) pigs, (B) cattle and (C) poultry. The symbols represent the observations and the solid line is the result of curve fitting.
Relative recovery of ammonia ~ Pig manure Cattle manure Poultry manure Relative recovery of volatile fatty acids Pig manure Cattle manure Poultry manure
a
b
c
d
18"7 16.0 33-0
1.85 1.15 1.91
104 104 86.8
6-60 5-44 5.27
2.32 4.15 - 8.09
1.07 110 1.53 82"9 0 - 2 5 3 2930
8.50 7-44 25.20
aThe value of the relative recovery obtained with the use of these constants should be subtracted from 120 to obtain the relative ammonia recovery.
44
P. J. L. Derikx, H. C. Willers, P. J. W. ten Have 120
. f
80
40
O
i
L
eO
4O
0
•
vB
8O
/
4O
0o
5
10
15
pH beforedrying
Fig. 3. The recovery of volatile fatty acids in the residue after the dry-matter determination of manure from (A) pigs, (B) cattle and (C) poultry. The symbols represent the observations and the solid line is the result of curve fitting.
DISCUSSION As stated in the results, differences observed in the titration curves of the three batches of the same type of manure correlate with the differences in dry-matter content. For ease of comparison, titration curves should therefore be preferentially given in mmol H+/g dry weight. Compared to cattle and poultry manure, pig manure requires more acid to lower the pH. The amount of acid needed to reach pH 5 in pig and cattle manure, expressed as mmol H+/g dry weight, was approximately the same as the required dosage reported elsewhere (Stevens et al., 1989; Frost et al., 1990). An indication of the identity of the substances responsible for the behaviour of manure during a titration can be obtained from the shape of the titration curves, because pH changes are the smallest around the pKa values of the substances present. The high amount of ammonia present in pig and poultry manure causes a flatness in their titration curves around pH 9.25. The presence of carbonate ions widens this effect to higher pH values and around pH 6.4 in the titration curve of pig manure. The presence of high amounts of
volatile fatty acids in poultry manure (Table 1) results in a slowly decreasing pH at a value of about 4.8. This value coincides with the average pKa value of the volatile fatty acids (Weast, 1977). Complete fixation of ammonia during dry-matter determination is only possible if the pH is as low as 4-5. In this range, less than 0.01% of the ammonia present is in the form of NH3, indicating how extremely the equilibrium must be influenced to avoid the volatilization of ammonia under these conditions. Compared to cattle and poultry manure, the higher recoveries of ammonia observed in pig manure at the same pH values may be caused by the presence of higher amounts of other salts, such as calcium chloride or ferric chloride, as reported earlier (MoUoy & Tunney, 1983). In order to retain volatile fatty acids completely, the same large gap between the actual pH before drying and the pKa of the volatile fatty acids must be created. Here, no significant difference was observed between the different types of manure tested, indicating that there is no effect of salinity on the volatilization of volatile fatty acids. The results reported here show that nearly all of the ammonia is lost during normal dry-matter determination of untreated manure originating from pigs, cattle or poultry. The loss of volatile fatty acids is less complete, but may amount to 75%. It is further confirmed that alteration of pH before drying influences the volatilization of both ammonia and volatile fatty acids. Both ammonia and volatile fatty acids are completely fixed at no one pH value. Therefore, corrections are always necessary when using the results of dry-matter determination in mass-balance calculations. The actual correction factor can be obtained from the fitted curves taking into account the pH before drying. As the emission of ammonia and volatile fatty acids is often related to the emission of odour, these results suggest that changing the pH of manure may also influence the emission of odour during drying processes. However, because many other compounds, such as volatile amines and sulphur compounds, are involved in the odour of manure (Merkel et al., 1969; Miner et a/., 1975; Derikx, 1991), the net effect of a given change in pH on the total emission of odour is uncertain. Although the results presented here will not be applicable for every drying process, they may serve as a general indication of the effect of pH on the volatilization of ammonia and volatile fatty acids during the drying of manure at a temperature of about 100°C. REFERENCES
APHA, AWWA & WPCF (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn. American Public Health Association, Washington, DC, USA, pp. 92-101. Derikx, P. J. L. (1991). Gaseous compounds and microbial processes involved in the preparation of the substrate of Agaricus bisporus. Thesis, Catholic University, Nijmegen, The Netherlands.
Volatilization losses during dry-matter determination Frost, J. P., Stevens, R. J, & Laughlin, R. J. (1990). Effect of separation and acidification of cattle slurry on ammonia volatilization and on the efficiency of slurry nitrogen for herbage production. J. Agric. Sci., 15, 49-56. ten Have, P. J. W. (1993). Nitrogen and the industrial processing of pig manure. In Proc. Symp. Nitrogen Flow in Pig Production and Environmental Consequences, Doorwerth, The Netherlands, 8-11 June 1993, ed. M. W. A. Verstegen, L. A. den Hartog, G. J. M. van Kempen, J. H. M. Metz. EAAP publ. no. 69, pp. 404-9. Hayward, G. & Pavlicik, V. (1990). A corrected method for dry matter determination for use in anaerobic digester control. Biol. Wastes, 34, 101-11. Merkel, J. A., Hazen, T. E. & Miner, J. R. (1969). Identification of gases in a confinement swine building atmosphere. Trans. Am. Soc. Agric. Eng., 12, 310-15. Miner, J. R., Kelly, M. D. & Anderson, A. W. (1975). Identification and measurement of volatile compounds within a swine building and measurement of ammonia evolution
45
rates from manure-covered surfaces. Proc. 3rd Int. Syrup. Livestock Wastes Am. Soc. Agric. Eng., pp. 351-3. Molloy, S. E & Tunney, H. (1983). A laboratory study of ammonia volatilization from cattle and pig slurry. It. J. Agric. Res., 22, 37-45. NEN (1983). Fotometrische bepaling van her gehalte aan ammonium. NEN 6472, Nederlands Normalisatie Instituut, Delft, The Netherlands. NEN (1986). Bepaling van de indamprest en de gloeirest in afvalwater en slib: Gravimetrische methode. NEN 6620, Nederlands Normalisatie Instituut, Delft, The Netherlands. Stevens, R. J., Laughlin, R. J. & Frost, J. E (1989). Effect of acidification with sulfuric acid on the volatilization of ammonia from cow and pig slurries. J. Agric. Sci., 113, 389-95. Weast, R. C. (ed.) (1977). CRC Handbook of Chemistry and Physics, 58th edn. The Chemical Rubber Co., Cleveland, OH, USA.
EFFECT OF pH ON THE BEHAVIOUR OF VOLATILE COMPOUNDS IN ORGANIC MANURES DURING DRY-MATTER DETERMINATION P. J. L. Derikx,* H. C. Willers & P. J. W. ten Have Department of Buildings and Environmental Technology, Institute of Agricultural and Environmental Engineering (IMA G-DLO), PO Box 43, NL-6700 AA Wageningen, The Netherlands (Received 4 February 1993; revised version received 22 April 1994; accepted 26 April 1994)
Abstract Dry matter determination by drying at 105 °C is one of the most widespread methods used in the characterization of manure. Loss of volatile compounds during drymatter determination may lead to considerable errors in mass balances. Since the behaviour of most volatile compounds depends upon pH, these losses may vary from sample to sample. The loss of ammonia and volatile fatty acids from pig, cattle and poultry manure was studied as a function of pH. It was shown that above pH 8 all ammonia was volatilized and below p H 5 all volatile fatty acids evaporated. Total fixation of ammonia was achieved below p H 4. Above p H 10 all volatile fatty acids were fixed in the residue after drying. The three types of manure showed only slightly different patterns of losses of volatile compounds. The amount of acid or alkali needed to obtain the desired p H varied strongly between the various kinds of manure.
determination are used for comparison only, the omission of a constant part of the volatile substances causes only minor errors. However, although acidification of manure is an effective means of reducing volatilization of ammonia from manure during storage, application or drying (Stevens et al., 1989; Frost et al., 1990; ten Have, 1993), the loss of volatile substances no longer remains a constant factor. Furthermore, the results of the drymatter determination method discussed here are commonly used in mass-balance calculations for large-scale manure treatment plants. This may lead to considerable errors because an unknown part of the volatile substances is included in the results of the dry-matter determination. For these reasons a closer look at the loss of volatile substances during dry-matter determinations is necessary. Because of their relatively large abundance in manure and their contribution to environmental pollution, attention is focused on ammonia and volatile fatty acids. The goal of this investigation was to quantify the loss of ammonia and volatile fatty acids as a function of pH from different kinds of manure during the drymatter determination.
Key words: Ammonia, volatile fatty acids, manure, drying method, pH. INTRODUCTION The dry-matter content of manure is generally measured by drying at 105°C to constant weight. It is one of the most widespread methods used in the characterization of manure due to its low requirements for sophisticated laboratory equipment and the relative ease of its execution. The method is well documented (APHA, AWWA & WPCF, 1985; NEN, 1986). It is generally accepted that in addition to evaporation of water, there is a loss of volatile substances during the dry-matter determination. Recently, Hayward and Pavlicik (1990) recommended a sulphuric acid titration method in order to be able to correct for the loss of acetic acid from samples originating from an anaerobic digester. The loss of volatile substances will be relatively constant as long as the samples are taken from the same type of material and no big changes in pH occur. If the values obtained by the dry-matter
METHODS
*To whom correspondence should be addressed. 41
Manure was obtained from various farms in The Netherlands. Pig manure originated from fattening pigs and cattle manure from dairy cattle, both kept on slatted floors. Poultry manure was produced by laying hens kept in cages. All animals consumed standard commercial feed. From each type of manure, three different batches were collected. The average characterization is given in Table 1. Each batch was stored at 4°C and was well mixed before use. Titration curves were obtained by adding hydrochloric acid (12 tool/ litre) or sodium hydroxide (4 mol/litre) step by step to about 130 g manure (fresh weight) under constant mixing. The dry-matter determination was performed at 105°C to constant weight with subsamples of about 40 g fresh weight according to the standard procedure
42
P. J. L. Derikx, H. C Willers, P. J. W. ten Have
(APHA, AWWA & W P C E 1985; NEN, 1986). Normally, overnight drying was sufficient when no excessive crust formed. Different amounts of hydrochloric acid or sodium hydroxide were added to achieve the desired pH before drying. After mixing, the pH obtained was measured. In order to determine the recovery of ammonia and volatile fatty acids in the residue after drying, water was added to the residue to such an extent that the original dry-matter content was approximately restored. The concentration of ammonia and volatile fatty acids was then determined after one hour of extraction at room temperature. Extraction with other solutions (0.1 mol/ litre hydrochloric acid, 0" 1 mol/litre sodium hydroxide or 2 mol/litre potassium chloride) did not affect the recovery of ammonia or volatile fatty acids, respectively (data not shown). Relative recoveries were calculated with respect to the amount present in the original manure (see Table 1 ). Ammonia was measured spectrophotometrically according to NEN 6472 (NEN, 1983). Volatile fatty acids were measured on a Packard 427 gas chromatograph, equipped with a flame ionization detector. A glass column (2 m, 6 mm outer diameter, 2 mm inner diameter) packed with 10% Fluorad 431 on Supelcoport (100-120 mesh) was used at 130°C isothermally. Nitrogen, fully saturated with formic acid, served as carrier gas at a flow of 30 ml/min. Injector and detector temperatures were 220 and 240°C, respectively, pH measurements were performed at room temperature with a Hanna Instruments glass electrode (model HI 8417) directly submerged in the manure. Curve fitting on the measured relative recovery of ammonia and volatile fatty acids was done with GenStat 5 according to the general logistic model, using the equation: Relative recovery = a + 1 + e-b(pH- d)
RESULTS In Fig. 1 the titration curves of the three types of manure are shown. The addition of alkali is presented as negative addition of acid and is given as mmol H÷/g dry weight. By expressing the amount of acid added as 15
i
A
10
×0
×cJ xco
~< xo
B 10 pH O0~xO ~x 0;,0
5
©
I
p
)
x ×x
~e ,
•
C I I
10
"h
5 Q~ 0
The values of the constants a, b, c and d followed from the fitting process. To enable curve fitting for the recoveries of ammonia with the method mentioned above, the obtained values for the relative recovery of ammonia were subtracted from 120. In this way positive values inclining with pH were obtained.
-5
0
5 mmol H+/g dry weight
10
Fig. 1. Titration curves of manure from (A) pigs, (B) cattle and (C) poultry. Different symbols refer to different batches of the same type of manure.
Table 1. Characterization of pig manure, cattle manure and poultry manure used. Values given are the averages and the standard deviation of three different batches
Dry matter (%) Ash content (% of dry matter) pH Ammonia (mmol/litre) Acetic acid (mmol/litre) Propionic acid (mmol/litre) iso Butyric acid (mmol/litre) Butyric acid (mmol/litre) iso Valeric acid (mmol/litre) Valeric acid (mmol/litre)
Pig manure
Cattle manure
Poultry manure
9"86 + 0.34 31.9 + 1.6 7-88 + 0"05 4395-9.5 82.1 5-6"8 32-4+3-5 4-2 + 0"5 2"25-0.35 6"6 5- 0"6 0"7 + 0"3
8-42 + 0"53 22.5 + 0.5 7"555- 0"23 1665- 1"0 785-33 20+ 13 2.6 5- 1"3 4-95-2"8 2-5 + 0"9 0-6 + 0-3
15"7 5- 3"9 27.9 + 3"0 6"99 5- 0"14 6465-99 4585-68 1145-21 13"1 5- 2.6 75"35- 16 9-2 _+3-9 2"6 5- 1"0
Volatilization losses during dry-matter determination mmol H+/g dry weight, the differences observed between the various batches of the same kind of manure were reduced remarkably. The three types of manure showed a similar response to the addition of alkali. However, the titration curve of pig manure differed from the titration curves of cattle and poultry manure with respect to the addition of acid. Starting with fresh manure, about twice as much acid was needed to lower the pH of pig manure compared to cattle and poultry manure. Using the information from Fig. 1, acid or alkali was added to subsamples of the manures to obtain various pH-values over a pH range of 1-12. After drying to a constant weight, the amount of ammonia remaining in the solids was determined after extraction. The relative recovery is given in Fig. 2 for the three types of
120 Q
•
A
80
40
~enaa
i
,e
e
B
8O
!° u
43
manure, together with the results of curve fitting. No noticeable differences in recovery were observed between the three batches of the various types of manure. Therefore, all results from each type of manure were fitted by one curve. The values of the constants obtained by the curve fitting are summarized in Table 2. Due to the limited number of observations at low pH values for the poultry manure, the fitted curve did not reach the theoretical value of 100. Pig manure showed recoveries as high as 80% for ammonia at pH 6, which is high compared with cattle and poultry manure. For the latter two, the pH must be as low as 4-5 to retain ammonia to the same extent in the solids. At pH 7 or higher, nearly all ammonia was lost by drying the three types of manure tested. The differences between the three types of manure were small, but the recovery of ammonia in pig manure at pH 7-8 seemed to be slightly higher. The recovery of volatile fatty acids, obtained in similar experiments to those described above, is shown in Fig. 3. The recoveries were calculated after summing the concentrations of all the volatile fatty acids mentioned in Table 1, as the relative volatilization behaviours of each of the individual volatile fatty acids were shown to be the same (data not shown). Here too, no differences were observed between the recoveries for the three batches of manure, although the results showed more scatter. For this reason, the calculated curves were less accurate, especially for poultry manure. As a result, large differences between the values of the constants were observed for the three types of manure (see Table 2). The recovery of volatile fatty acids after drying at various pH values did not differ much from one type of manure to the other. At pH 9-10, all the volatile fatty acids were completely fixed in pig and cattle manures. Below pH 6, nearly all volatile fatty acids were lost during the drying process. The results obtained with poultry manure varied too much to justify any firm statement, but roughly the same tendency was observed.
Table 2. Values of the constants obtained by fitting the ammonia and volatile fatty acid recoveries according to the general logistic model
C
Constants
80
o\ \
"\
40
0
"~
0
5
** "~
10
15
pH before drying Fig. 2. The recovery of ammonia in the residue after the dry-matter determination of manure from (A) pigs, (B) cattle and (C) poultry. The symbols represent the observations and the solid line is the result of curve fitting.
Relative recovery of ammonia ~ Pig manure Cattle manure Poultry manure Relative recovery of volatile fatty acids Pig manure Cattle manure Poultry manure
a
b
c
d
18"7 16.0 33-0
1.85 1.15 1.91
104 104 86.8
6-60 5-44 5.27
2.32 4.15 - 8.09
1.07 110 1.53 82"9 0 - 2 5 3 2930
8.50 7-44 25.20
aThe value of the relative recovery obtained with the use of these constants should be subtracted from 120 to obtain the relative ammonia recovery.
44
P. J. L. Derikx, H. C. Willers, P. J. W. ten Have 120
. f
80
40
O
i
L
eO
4O
0
•
vB
8O
/
4O
0o
5
10
15
pH beforedrying
Fig. 3. The recovery of volatile fatty acids in the residue after the dry-matter determination of manure from (A) pigs, (B) cattle and (C) poultry. The symbols represent the observations and the solid line is the result of curve fitting.
DISCUSSION As stated in the results, differences observed in the titration curves of the three batches of the same type of manure correlate with the differences in dry-matter content. For ease of comparison, titration curves should therefore be preferentially given in mmol H+/g dry weight. Compared to cattle and poultry manure, pig manure requires more acid to lower the pH. The amount of acid needed to reach pH 5 in pig and cattle manure, expressed as mmol H+/g dry weight, was approximately the same as the required dosage reported elsewhere (Stevens et al., 1989; Frost et al., 1990). An indication of the identity of the substances responsible for the behaviour of manure during a titration can be obtained from the shape of the titration curves, because pH changes are the smallest around the pKa values of the substances present. The high amount of ammonia present in pig and poultry manure causes a flatness in their titration curves around pH 9.25. The presence of carbonate ions widens this effect to higher pH values and around pH 6.4 in the titration curve of pig manure. The presence of high amounts of
volatile fatty acids in poultry manure (Table 1) results in a slowly decreasing pH at a value of about 4.8. This value coincides with the average pKa value of the volatile fatty acids (Weast, 1977). Complete fixation of ammonia during dry-matter determination is only possible if the pH is as low as 4-5. In this range, less than 0.01% of the ammonia present is in the form of NH3, indicating how extremely the equilibrium must be influenced to avoid the volatilization of ammonia under these conditions. Compared to cattle and poultry manure, the higher recoveries of ammonia observed in pig manure at the same pH values may be caused by the presence of higher amounts of other salts, such as calcium chloride or ferric chloride, as reported earlier (MoUoy & Tunney, 1983). In order to retain volatile fatty acids completely, the same large gap between the actual pH before drying and the pKa of the volatile fatty acids must be created. Here, no significant difference was observed between the different types of manure tested, indicating that there is no effect of salinity on the volatilization of volatile fatty acids. The results reported here show that nearly all of the ammonia is lost during normal dry-matter determination of untreated manure originating from pigs, cattle or poultry. The loss of volatile fatty acids is less complete, but may amount to 75%. It is further confirmed that alteration of pH before drying influences the volatilization of both ammonia and volatile fatty acids. Both ammonia and volatile fatty acids are completely fixed at no one pH value. Therefore, corrections are always necessary when using the results of dry-matter determination in mass-balance calculations. The actual correction factor can be obtained from the fitted curves taking into account the pH before drying. As the emission of ammonia and volatile fatty acids is often related to the emission of odour, these results suggest that changing the pH of manure may also influence the emission of odour during drying processes. However, because many other compounds, such as volatile amines and sulphur compounds, are involved in the odour of manure (Merkel et al., 1969; Miner et a/., 1975; Derikx, 1991), the net effect of a given change in pH on the total emission of odour is uncertain. Although the results presented here will not be applicable for every drying process, they may serve as a general indication of the effect of pH on the volatilization of ammonia and volatile fatty acids during the drying of manure at a temperature of about 100°C. REFERENCES
APHA, AWWA & WPCF (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn. American Public Health Association, Washington, DC, USA, pp. 92-101. Derikx, P. J. L. (1991). Gaseous compounds and microbial processes involved in the preparation of the substrate of Agaricus bisporus. Thesis, Catholic University, Nijmegen, The Netherlands.
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