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Buffer stability in manure digesters
Agricultural Wastes411982) 427 441
B U F F E R STABILITY IN M A N U R E DIGESTERS* DIMITRIS GEORGACAKIS
Agrieultural College of Athens, Laboratory of Agricultural Structures, Athens, Greece & D. M. SIEVERS~¢. E. L. IANNOTT1
Department of Agricultural Engineering, Unirersity of Missouri-Columbia, Columbia, Missouri, USA
ABSTRACT
The r()le of buffers in maintaining process stability in a swine manure digester was im'estigated in laboratory digesters. Volatile fatty acids, bicarbonate alkalinity and ammonia were the buffers int,estigated. Buffering theory is presented and compared with digester perJormanee data. Recommendations for maintaining digester stability through control of the buffer system are made.
INTRODUCTION
For anaerobic digesters to be accepted on farm operations, the operational parameters for their stable and efficient operation must be clearly defined. One of the most important parameters affecting digester stability is the buffer system. Little is known about the interactions of buffers in a digester containing high nitrogen waste such as swine manure. A better understanding of these interactions could lead to better process control and greater farm use of anaerobic digestion. The objective of this study was to determine the r61e of buffers in maintaining the stability of methane production in a swine manure digester.
METHODS
Nine Erlenmeyer flasks (l litre working volume each) served as digesters. Each digester was connected to an individual gas collection chamber where gas volumes * Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 8764.
427 A grieultural Wastes 0141-4607/82/0004-0427/$02.75 Printed in Great Britain
~Z Applied Science Publishers Ltd, England, 1982
428
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI TABLE 1 ANALYSIS OF FROZEN RAW MANURE OVER SEVENTEEN-MONTH STUDY
Parameter
,4 verage o f 17 months (mg gram - 1)
COD TS VS NH 3 N TKN Acetate Propionate Butyrate Valerate Total V F A Sodium Potassium Calcium Magnesium
Standard deviation
276'0 28.8% 24-9 % 5-6 15.0 11-8 3-5 1.9 0.0 17.3 1.2 3.8 1.0 1.0
_+28.3 +0.7 _+0.7 _+0-4 _+0.8 _+ 1.4 + I. 1 -+ 0.4 -+ 0.0 _+2.5 _+0.1 _+0-3 _+0.1 _+0.1
were measured by water displacement. All digesters and gas collection chambers were collectively housed in a convection incubator with the temperature maintained at 35 + 1 °C. The physical system has been described in detail by Sievers & Brune (1978). A single batch of swine manure was collected from a concrete feeding floor, thoroughly mixed, and frozen in several 2-1itre containers. The manure was taken from animals fed a 50/50 corn milo ration supplemented with protein to a 14 ~,,,level. Variability in the frozen samples was checked monthly and found to be very slight (Table i). Each digester was loaded daily with 12.6 g litre- 1 wet swine manure (3.1 g VS litre- l d a y - 1) plus precalculated amounts of urea or glucose to vary the concentrations of ammonia or Volatile Fatty Acids (VFA). The digesters were initially started with contents from an operating swine digester. Detention time was 20 days in all digesters. Each digester was acclimated to a given carbon-nitrogen loading for 50-70 days (Table 2). Biogas production, pH and ammonia levels were taken weekly during the TABLE 2 DAYS OF ACCLIMATION OF EACH DIGESTER RECEIVING A DAILY RAW MANURE LOAD OF 1 2 ' 6 g AND THE SCHEDULED GLUCOSE AND UREA
Digester
Urea (g litre- l)
1 0"0
A B C D E
0.00 0.15 0.30 0.45 0.60
70 70 70 70 70
2
3 4 Glucose (g litre- 1) 0"5 4"0 8"0 60 60 60 60
50 50 50 50 50
Fail. Fail. Fail 70 70
5 10"0 ---Fail. Fail.
BUFFER STABILITY IN MANURE DIGESTERS
429
acclimation period. If the three parameters were consistent over the last half of the acclimation period, each digester was considered stable and data were taken over an 8-day period. These data were averaged over the 8 days and represented stable operating conditions for that particular loading of carbon and nitrogen. Digester contents were analysed for acidity, total alkalinity, Total Solids (TS), Volatile Solids (VS), COD, pH, total Kjeldahl nitrogen (TKN), ammonia, VFA, CO 2 and CH 4. Acidity, alkalinity, TS, VS, COD and TKN were determined according to the American Public Health Association (1975). Ammonia was measured with an Orion Model 95-10 electrode. Methane and CO z were quantified by a Fisher Model 25 gas partitioner and VFA were determined, after acidification and centrifugation, by direct injection of the supernant on a Chromosorb 101 column as described by Iannotti et al. (1979). Total titrable alkalinity is routinely used to express buffer changes in anaerobic digesters. Bicarbonate alkalinity may be a more sensitive parameter (Brovko et al., 1977). In this study bicarbonate alkalinity was calculated from total alkalinity and Volatile Fatty Acids concentrations as follows (McCarty, 1964): Bicarbonate Alkalinity = Total Alkalinity -0.71 (Volatile Fatty Acids)
(1)
(mglitre-1 as C a C O 3 ) (mglitre 1 as C a C O 3 ) (mglitre-1 as acetate)
BUFFER THEORY
Anaerobic digestion reportedly proceeds most optimally between pH 6.6 and 7.6 (McCarty, 1964) with pH and process stability largely resulting from chemical equilibria established between the three primary buffers of Volatile Fatty Acids, bicarbonate and ammonia (Pohland, 1968). Volatile Fatty Acids decrease the buffering capacity of the bicarbonate ions according to eqn. (2) (Albertson, 1961): HAc + NH4HCO 3 ~ NH4Ac + H z C O 3
(2)
while the addition of ammonia will increase the bicarbonate by forming an ammonium salt with bicarbonate taken from the CO 2 pool: NH 4 + O H - + H + + H C O f ~-~NH~ + HCO 3 + H 2 0
(NH 3 + H20)
(CO 2 + H20)
(3)
(salt)
The buffering capacity of any buffer can be defined as the amount of strong acid or base required to change the pH by one unit and is known as the buffering capacity index or buffer intensity (B) (Butler, 1964). The buffer intensity of a solution is determined from the concentrations of each buffer, their pK values (location on pH
430
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI
scale) and the pH of the solution as follows (Butler, 1964)"
B-
dC.
dpH
dCb --2.303
dpH
-
[ H + + ,< + V(c,.K,..+ H -7 , / ~ \ ( ~ + H ~ ) 2 ] J
(4)
i=1
where'
B = buffer intensity in Meq litre l p H = concentration of strong acid and base added, respectively in Meq litre
Ca,Q K,a,
H + + H 7 = concentration of hydrogen and hydroxyl ions in the solution representing the water buffering capacity effect
i=1
, = t h e sum of the buffering effects of each buffer in / solution C i = concentration of the ith buffer in Meq litre-1 Ki = dissociation constant of the ith buffer in mollitre
i
A graphical presentation ofeqn. (4) is shown in Fig. 1 for the common buffers found in anaerobic digesters: VFA, bicarbonate and ammonia. Carbonate was added because of its intimate association with bicarbonate. Their combined effect, when all are simultaneously in solution, is shown as the sum of the individual intensities. The concentration of each buffer was assumed to be 1 Meq litre i at 35 °C. Note that the m a x i m u m intensity of an individual buffer is determined from its p K value and relative concentration in solution. Equation (4) is valid only for solutions with ionic strength <0-1 g and hence cannot be used for digesters. The buffer intensity of a solution may be determined experimentally as the slope of the alkalimetric and acidimetric titration curves (Kleijn, 1965). Two samples of the solution are titrated from the initial pH; one with standard acid, the second with standard base. The titration results are plotted on one graph versus pH. The slope of the curve measured at the initial pH represents the buffer intensity, B (Fig. 2(a)). This procedure was used on all digesters in this study. By definition, the slope of a curve is equal to the first derivative of the curve. The experimentally obtained titration curves for all digesters were subjected to regression analysis and expressed mathematically in the general form: Y = aX 3 + where:
bX 2
-+-cX q- d
Y = acid and base used in Meq litre-1 X = p H of the solution a, b, c, d = constants
(5)
431
BUFFER STABILITY IN MANURE DIGESTERS
0.7
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0.6 "
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Fig. 1.
Changes in buffering intensity (B) with pH for various buffers commonly found in anaerobic digesters.
From the mathematically defined titration curves, buffer intensity (B) curves were obtained by differentiating each titration curve at various pH values (Fig. 2(b)). The B curves generated from the titration curves form valleys coincident with that formed between the buffer intensity peaks of Fig. 1. It appears that the pH in a stable digester assumes a position of stability between two buffer intensities at a position of least numerical B value. This behaviour of buffers upon pH can be compared with a ball seeking the position of least potential energy but greatest stability in physics (Model A in Fig. 3). The ball tends always to return to the most stable position (position 1) from any other on the sloped walls (2 and 3). Any force tending to move the ball from stability will meet with increased resistance (increased slope) and will fail unless the force can overcome the resistance or the resistance can be removed. In a stable anaerobic digester, the pH should be safely positioned between the bicarbonate and ammonia buffer intensity peaks (Fig. 1). Addition of Volatile Fatty Acids will cause the pH to attempt to move up the bicarbonate intensity curve according to the chemistry of eqn. (2). If pH overtops the bicarbonate curve, it will be free to slip into the valley formed by the bicarbonate and VFA intensity peaks and digester instability occur. The addition of increasing amounts of ammonia will act as a force tending to push
432
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI
/
400 TITRATION CURVE(a)
/
200 g E
w" o
-2oo~
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DIGESTER
-40C
300
g
BUFFER INTENSITY (b)
200
E
I00
0 6
i
i
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7
8
9
pH
Fig. 2.
Experimentally obtained titration (a) and mathematically derived buffer intensity (b) curves for Digester C2. MODEL A
LEAST [~IE.RO~'(FOTENTIAI.)GREATEST~ L I T Y
Fig. 3.
Model A: pH stability (ball) in terms of buffer intensity curve (energy).
433
BUFFER STABILITY IN MANURE DIGESTERS
t
//
IC]IA~ \\ \ \
./
L,
• ~
'///I/
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Fig. 4.
;
7
i
8
i
9
i
,o
pH Shifts in digester buffer intensity and pH stability due to changes in the primary buffers, ammonia, bicarbonate and VFA.
the pH up along the ammonia buffer intensity curve as NH4HCO 3 is formed (eqn. (3)). However, the pH probably will not top the ammonia buffer peak because of ammonia toxicity to the biological community (Albertson, 1961 ; McCarty, 1964). Maintaining satisfactory anaerobic digestion of livestock manure will entail keeping the system pH centred between the bicarbonate and ammonia buffer intensities. The combined operational sequence of the three primary buffers (VFA, bicarbonate and NH3) in a digester can be summarised schematically in Fig. 4. Digesters with high ammonia concentrations (curve A) and low VFAs (IA) result in high bicarbonate buffer intensities (IIA). In this case, pH is positioned safely between the two buffer peaks (position 1) and the digester operates stably. If VFA concentration increases (increased carbon loading), ammonia bicarbonate will buffer according to eqn. (2) and bicarbonate buffer decreases (curve B). If VFA's continue to increase (IB), the bicarbonate buffer intensity peak drops (IIB) and pH moves to a lower value (position 2) where electroneutrality is re-established. The digester is still operating but with less buffer reserve to maintain stability. If this pattern continues, a point is reached where electroneutrality is satisfied only by ammonia and VFA ions (curve C), all bicarbonate ions have returned to the CO z pool, and bicarbonate buffer intensity is essentially zero (IIC). The pH shifts to position 3 between the volatile acids and ammonia peak buffer intensities. This is the lowest position of biochemical stability at which a digester can be maintained in
434
DIMITRIS G E O R G A C A K I S , D. M. SIEVERS, E. L. I A N N O T T I
terms of pH and electroneutrality. A further increase in Volatile Fatty Acids (position IC) would result in a movement of pH upward along the VFA buffer intensity curve (position 4) and probably result in digester failure due to lack of a proper buffer to maintain pH in a stable position.
RESULTS A N D D I S C U S S I O N
High acid-Low bicarbonate and ammonia
The data of Table 3 illustrate the changes in a digester's buffer system as VFA increase. The data are for digester A which received increases in glucose but no additional nitrogen. Increases in organic carbon produced increases in VFA, resulting in decreases in pH, bicarbonate alkalinity and buffer intensity. Eventually the acids destroyed the bicarbonate buffer and pH dropped quickly, resulting in digester A4 failing. The buffer intensity values in Table 3 indicate that digester A moved successively from position 2, curve B, in Fig. 4 via position 3 to position 4, curve C. The bicarbonate buffer was essentially removed and could no longer maintain pH safely between the bicarbonate and ammonia buffer peak intensities. TABLE 3 CHANGES 1N THE BUFFER SYSTEM DUE TO INCREASES IN CARBON LOADING OF DIGESTER A
Digester
A1 A2 A3 A4
Carbon loading (glitre -~)
pH
38-21 42.41 70.21 102.21
7-40 7.45 7.14 5.95
VIA (mg litre - 1) acetic acid) 181 147 288 13500
Bicarbonate alkalinity (mglitre-1CaC03) 10864 9438 4149 258
Buffer intensity, B (Meqlitre-lpH
1)
69 55 45 58
A digester operating at position 3 (Fig. 4) is in a weakened condition and pH must be controlled externally. This is generally accomplished by dilution, loading reduction or lime addition. Whether the pH will shift to higher values from position 3 will depend on the bacterial population. If they can begin to convert VFA's to methane, bicarbonate buffer intensity will increase, stabilising pH between the bicarbonate and ammonia buffers (positions 1 and 2) and the digester will recover. If bacterial activity fails and pH continues to drop, failure will occur. Thus, the region surrounding position 3 can be called a transition zone between recovery through the establishment of bicarbonate buffer and pH stability and increasing acid build up leading to failure. The relationship of the transition zone to bicarbonate buffer and buffer intensity (B) is shown in Fig. 5. The glucose loading to stable digester D4 was increased from 8 to 10 g litre 1 (D5) on day 4. The digester immediately began to fail, as evidenced by the large decreases in biogas production, bicarbonate alkalinity and pH. On day
BUFFER
STABILITY
IN
MANURE
435
DIGESTERS
uj o_
IE
C ~
810GAS
4
pH
[]
E
7.5
7.0
m
.~
6.5
0
8
m •Co
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70
~_ o~
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t~
8
g'
5O
~.. c.
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2 30
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,;
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DAYS
Fig.
5.
Transition zone changes of B, bicarbonate alkalinity and p H f o r D i g e s t e r s D4 and
D5.
8 all loading ceased and did not resume until day 12 when the daily addition of swine manure (no glucose or urea) was resumed. During the interim 4 days following cessation of loading, digester D5 began to recover, as evidenced by the increasing pH and bicarbonate alkalinity. Buffer intensity was slower to recover but was increasing steadily when the experiment was terminated. Data of digester D5 suggest that the transition zone lies between pH 6-5 and 7-0 with a corresponding range of buffer intensities of 0.030 to 0-040 Meq litre- 1 pH 1
436
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI o STABLE ,x UNSTABLE 150
:z
o
I00
TRANSITION ZONE
E =-
50
~.
.
I
I
I
.
I
6
Fig. 6.
I
c°j ~
txJ
I
7
I
8
pH Relationship of transition zone to digester pH and B.
Other failing digesters exhibited similar large reductions in pH and bicarbonate alkalinity within the same pH and buffer intensity ranges. Buffer intensity and pH data for all digesters (stable and unstable) are plotted in Fig. 6. The regression curve has an R 2 value of 0.799. Unstable digesters included those which were failing (pH and gas production decreasing) and those recovering following modification of loading procedures. The minimum pH is 6-7 with a corresponding buffer intensity of 0.025 Meq 2.5
2.0
E
"
1.5
> ~n I.O O STABLE DIGESTERS
¢:0
/~ DIGESTER D-5
0.5
0
t
O
Fig. 7.
i
i
i
5 IO 15 20 BICARBONATE ALKALINITY, mg/] CaCO S (IOOO's)
Relationship of bicarbonate alkalinity to biogas production.
437
BUFFER STABILITY IN MANURE DIGESTERS
litre- 1 p H - 1. No stability was observed below pH 7-1 (B = 0.030). The portion of the regression curve to the left o f p H 6.7 can be considered to be the increasing slope of the VFA buffer intensity peak of Fig. 1. Digesters exhibiting decreasing pH values below 6.7 are moving towards decreasing stability and eventual failure. Thus, the transition zone could be defined as the pH range 6.6-7.1 (B =0.025 to 0.030). When data from digester D5 is compared to that of the stable units (Fig. 7), it appears that a minimum bicarbonate alkalinity of 6000mglitre -1 CaCO 3 is required to maintain high biogas production in the swine manure digester. This value corresponds to pH's above 7-1 and reflects the beginning of the transition zone. Once pH and bicarbonate buffer fall into the transition zone, instability begins. High bicarbonate and ammonia
Increases in nitrogen loading to an anaerobic digester result in increased production of NH4HCO 3 which, in turn, creates a greater buffer capacity and leads to greater stability. This increase in buffer stability is demonstrated by the increasing buffer intensity (B) and bicarbonate alkalinity values of Table 4 and corresponds to operating the digesters near position 1, curve A of Fig. 4, where pH is secured between increasingly larger ammonia and bicarbonate buffer peaks. TABLE 4 CHANGES IN DIGESTER BUFFER CHEMISTRY DUE TO INCREASESIN AMMONIA
Digester p H
A B C D E
7-40 7.60 7.75 7.90 7.80
NH 3- N (rag litre- 1)
VFA (rng litre- l)
Bicarbonate alkalinity (mglitre -1 CaC03)
1986 3267 4306 5988 6366
181 421 3824 4951 7590
10864 15240 17066 19017 21299
Buffer CH 4 NHs--N intensity, B (ml millilitre- 1) VFA (Meqlitre 1 pH - l ) 62.2 71.4 72.2 84,4 108,7
0.70 0-68 0-57 0-53 0-41
10-97 7.76 1.13 1.21 0.84
The increased stability brought about by increased nitrogen loading exacts a toll, as evidenced by the 42 ~o decrease in methane production and VFA increase 42 times that of the digester receiving the lowest nitrogen loading. The decrease in methane production was accompanied by an 88 ~o increase in free ammonia. Free ammonia toxicity towards methane bacteria is well documented with reported toxic levels varying from 150mglitre- l (McCarty & McKinney, 1961) to 350 mglitre- l (Kroeker et al., 1979). While ammonia does have a negative effect on methane production from high nitrogen wastes (Sievers & Brune, 1978), it is possible to operate stable units at very high free ammonia levels (Table 5). Stable digestion with a methane production of 0.5 ml millitre-1 was achieved at a free ammonia concentration of 663 mglitre-
438
D I M I T R I S G E O R G A C A K 1 S , D. M. SIEVERS, E. L. I A N N O T T I
VARIATIONS 1N BUFFER CHEMISTRY FOR
TABLE 5 A DIGESTERRECEIVINGTHE HIGHESTNITROGENLOADING (9"4 g litre - 1)
Free ammonia (rag litre- 1)
CH 4 (ml millilitre - 1)
VFA (mg litre 1)
Bicarbonate alkalinity (rag litre - l CaC03)
Carbon~Nitrogen* ratio
440 663 409 349
0.41 0.50 1.77 3.06
7590 7673 1739 6876
21299 20099 16390 13103
4.1 4.5 7-5 10.9
* Total organic carbon/Total Kjeldahl nitrogen. Stability at such high free ammonia levels is attributed to the acclimation of the bacteria to high nitrogen levels and the large bicarbonate alkalinity. The large increases in VFA are probably due also to the inhibitive effect of ammonia. Albertson (1961) hypothesised that if an ammonia addition was sufficiently large to cause a significant rise in pH beyond the normal optimum range for methane bacteria, bacterial activity would be slowed, resulting in an accumulation of VFA. The tendency of the VFA to decrease pH would be buffered by the NH4HCO3, allowing the bacteria time to recover and establish stability at a lower gas production level. The data of Table 4 agrees with Albertson's hypothesis and would account in part for the ability of swine and poultry digesters to operate successfully at high ammonia and VFA levels (Converse et a l . , 1977; van Velsen, 1977; Kroeker e t al:, 1979). The same data also gives an indication that ammonia's buffering effect against VFA is limited, as evidenced by the decreasing N H 3 N/VFA ratio. There appears to be a point involving the concentrations of VFA and N H 3 - - N beyond which buffering by N H 3 - - N has no effect and the methane bacteria succumb, leading to digester failure. The exact cause of bacterial inhibition at this point cannot be discerned from this study, but the point at which failure occurs can be estimated from the N H 3 - - N / V F A ratio. Table 6 lists N H 3 - - N / V F A ratios at observed digester failure for various livestock wastes. Inspection of the data would suggest that the N H 3 - - N / V F A ratio could be used as an operational parameter to monitor digester stability for high nitrogen wastes. Values of 1.0 for swine and beef and 0.5 for poultry appear sufficiently conservative to provide for variations in field operations and provide the operator with sufficient time to make the necessary corrective adjustments. Process
operation
The bicarbonate-ammonia buffers in an anaerobic digester are the primary agents controlling pH and maintaining process stability. For wastes low in nitrogen, the bicarbonate alkalinity is the chief source of stability control by buffering against pH drop due to high VFA. In our studies, stability was achieved by maintaining a
BUFFER STABILITY IN MANURE DIGESTERS
439
TABLE 6 NH 3 N / V F A RATIOSAT DIGESTER FAILURE Waste
NH 3
Swine
0"63 0"83 0.64 0.24 0.33 0.46 0.16 0.34
Swine Swine Poultry Beef
N/VFA
ReJerence
(A4)* (B4) (C4) (D5)
This study
van Velsen (1977) Kroeker et al. (1975) Hill & Barth (1977) Hein et al. (1977)
* Digester.
minimum bicarbonate alkalinity of 6000mglitre 1 CaCO 3 with a pH above 7-1. Permitting the bicarbonate alkalinity to drop below 6000 mg litre 1 will place the pH into the stability transition zone and failure could result. Nitrogen additions to a digester will increase stability by maintaining pH between the bicarbonate and ammonia buffers. Digesters receiving swine and poultry manure possess an inherent buffer stability due to the naturally high level of ammonia in the wastes. However, this advantage is offset somewhat by ammonia's inhibition of methane production (Table 4) and poses the question of where to operate the digester to achieve a proper compromise between adequate buffering and minimum methane inhibition. If one considers the r61e of the buffering system in a digester as an interaction between carbon (VFA and HCOa) and nitrogen (NH3) then an answer to the operating question posed above might be achieved by looking at the c a r b o n nitrogen ratio (C/N). Methane production per unit of Volatile Solids loaded for all stable digesters is presented in Fig. 8 versus C/N ratio. Optimum methane 2.0
[5 0 0
/
?
2
o.s
C/N
Fig. 8.
RATIO
Carbon/nitrogen ratio relationship to methane production.
440
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI
20 ®
®O O
I0
6
~doo
Ioboo
®
r-,6~o
~
zr~o
BICARBONATE ALKALINITY
(mgll as CoCo 3) • Fig. 9. Carbon/nitrogen ratios versus bicarbonate alkalinity for all stable digesters. p r o d u c t i o n occurred within a C / N ratio of 10/1 to 15/1. Digesters with ratios less than 10/1 exhibit a m m o n i a i n h i b i t i o n , while digesters with ratios exceeding 18/1 failed due to low b i c a r b o n a t e alkalinity. The 10/1 to 15/1 range of C / N ratio c o r r e s p o n d s to a b i c a r b o n a t e alkalinity range of 6 0 0 0 - 1 3 0 0 0 m g l i t r e -1 C a C O 3 (Fig.9) a n d N H 3 - - N / V F A ratios above 1.0 (Table 4). O p e r a t i n g swine digesters within this buffer range should ensure good stability a n d o p t i m u m m e t h a n e p r o d u c t i o n . REFERENCES ALBERTSON,O. E. (1961). Ammonia nitrogen and the anaerobic environment. Journal Water Pollution Control Federation, 33(9), 978-95. AMERICANPUBLICHEALTHASSOCIATION(1975). Standard methods for the examination of water and wastewater. (13th edn.), Washington, DC. BROVKO,N. et al. (1977). Optimizing gas production, methane content and buffer capacity in digester operation. Water and Sewage Works, 24(7), 54-57. BUTLER,J. N. (1964). Ionic equilibria, a mathematical approach. Addison-Wesley Pub. CO., Menlo Park, California. CONVEKSE,J. C. et al. (1977). Performance of a large size anaerobic digester for poultry manure. American Society of Agricultural Engineers Paper No. 77-0451, St. Joseph, Michigan. HEIN, M. E. et al. (1977). Some mechanical aspects of anaerobic digestion of beef manure. American Society of Agricultural Engineers Paper No. 77-4056, St. Joseph, Michigan. HILL, D. T. & BARTH,C. L. (1977). A dynamic model for simulation of animal waste digestion. Journal Water Pollution Control Federation, 49(10), 2129-43. IANNOa'n, E. L. et al. (1979). Changes in swine manure during anaerobic digestion. Developments in Industrial Microbiology, 20, 519-29. KLEIJN, H. F. W. (1965). Buffer capacity in water chemistry. Journal o f Air and Water Pollution, 9, 401-13. KROEr~R, E. J. et al. (1975). Cold weather energy recovery from anaerobic digestion of swine manure. In: Energy, agriculture and waste management, Ann Arbor, Michigan, pp. 337-52.
BUFFER STABILITY IN MANURE DIGESTERS
441
KROEKER, E. J. et al. (1979). Anaerobic treatment process stability. Journal Water Pollution Control Federation, 51(4), 718 27. MCCARTY, P. L. (1964). Anaerobic waste treatment fundamentals: II, Environmental requirements and control. Public Works. (October), 123-6. MCCAR'rY, P. L. & MCKINNEY, R. E. (1961). Salt toxicity in anaerobic digestion. Journal Water Pollution Control Federation, 33(4), 399-415. POHLAND, F. G. (1968). High-rate digestion control Ill--Acid-base equilibrium and buffer capacity. Proc. 23rd Industrial Waste Conference, Purdue University, Lafayette, Indiana, 275-84. S1EVERS,D. M. & BRUNE,D. E. (1978). Carbon/nitrogen ratio and anaerobic digestion of swine waste. Transactions of American Society of Agricultural Engineers, 21(3), 537-41,549. VAY VELSEN,A. F. M. (1977). Anaerobic digestion of piggery waste. I. The influence of detention time and manure concentration. Netherlands Journal of Agricultural Science, 25, 151 69.
B U F F E R STABILITY IN M A N U R E DIGESTERS* DIMITRIS GEORGACAKIS
Agrieultural College of Athens, Laboratory of Agricultural Structures, Athens, Greece & D. M. SIEVERS~¢. E. L. IANNOTT1
Department of Agricultural Engineering, Unirersity of Missouri-Columbia, Columbia, Missouri, USA
ABSTRACT
The r()le of buffers in maintaining process stability in a swine manure digester was im'estigated in laboratory digesters. Volatile fatty acids, bicarbonate alkalinity and ammonia were the buffers int,estigated. Buffering theory is presented and compared with digester perJormanee data. Recommendations for maintaining digester stability through control of the buffer system are made.
INTRODUCTION
For anaerobic digesters to be accepted on farm operations, the operational parameters for their stable and efficient operation must be clearly defined. One of the most important parameters affecting digester stability is the buffer system. Little is known about the interactions of buffers in a digester containing high nitrogen waste such as swine manure. A better understanding of these interactions could lead to better process control and greater farm use of anaerobic digestion. The objective of this study was to determine the r61e of buffers in maintaining the stability of methane production in a swine manure digester.
METHODS
Nine Erlenmeyer flasks (l litre working volume each) served as digesters. Each digester was connected to an individual gas collection chamber where gas volumes * Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 8764.
427 A grieultural Wastes 0141-4607/82/0004-0427/$02.75 Printed in Great Britain
~Z Applied Science Publishers Ltd, England, 1982
428
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI TABLE 1 ANALYSIS OF FROZEN RAW MANURE OVER SEVENTEEN-MONTH STUDY
Parameter
,4 verage o f 17 months (mg gram - 1)
COD TS VS NH 3 N TKN Acetate Propionate Butyrate Valerate Total V F A Sodium Potassium Calcium Magnesium
Standard deviation
276'0 28.8% 24-9 % 5-6 15.0 11-8 3-5 1.9 0.0 17.3 1.2 3.8 1.0 1.0
_+28.3 +0.7 _+0.7 _+0-4 _+0.8 _+ 1.4 + I. 1 -+ 0.4 -+ 0.0 _+2.5 _+0.1 _+0-3 _+0.1 _+0.1
were measured by water displacement. All digesters and gas collection chambers were collectively housed in a convection incubator with the temperature maintained at 35 + 1 °C. The physical system has been described in detail by Sievers & Brune (1978). A single batch of swine manure was collected from a concrete feeding floor, thoroughly mixed, and frozen in several 2-1itre containers. The manure was taken from animals fed a 50/50 corn milo ration supplemented with protein to a 14 ~,,,level. Variability in the frozen samples was checked monthly and found to be very slight (Table i). Each digester was loaded daily with 12.6 g litre- 1 wet swine manure (3.1 g VS litre- l d a y - 1) plus precalculated amounts of urea or glucose to vary the concentrations of ammonia or Volatile Fatty Acids (VFA). The digesters were initially started with contents from an operating swine digester. Detention time was 20 days in all digesters. Each digester was acclimated to a given carbon-nitrogen loading for 50-70 days (Table 2). Biogas production, pH and ammonia levels were taken weekly during the TABLE 2 DAYS OF ACCLIMATION OF EACH DIGESTER RECEIVING A DAILY RAW MANURE LOAD OF 1 2 ' 6 g AND THE SCHEDULED GLUCOSE AND UREA
Digester
Urea (g litre- l)
1 0"0
A B C D E
0.00 0.15 0.30 0.45 0.60
70 70 70 70 70
2
3 4 Glucose (g litre- 1) 0"5 4"0 8"0 60 60 60 60
50 50 50 50 50
Fail. Fail. Fail 70 70
5 10"0 ---Fail. Fail.
BUFFER STABILITY IN MANURE DIGESTERS
429
acclimation period. If the three parameters were consistent over the last half of the acclimation period, each digester was considered stable and data were taken over an 8-day period. These data were averaged over the 8 days and represented stable operating conditions for that particular loading of carbon and nitrogen. Digester contents were analysed for acidity, total alkalinity, Total Solids (TS), Volatile Solids (VS), COD, pH, total Kjeldahl nitrogen (TKN), ammonia, VFA, CO 2 and CH 4. Acidity, alkalinity, TS, VS, COD and TKN were determined according to the American Public Health Association (1975). Ammonia was measured with an Orion Model 95-10 electrode. Methane and CO z were quantified by a Fisher Model 25 gas partitioner and VFA were determined, after acidification and centrifugation, by direct injection of the supernant on a Chromosorb 101 column as described by Iannotti et al. (1979). Total titrable alkalinity is routinely used to express buffer changes in anaerobic digesters. Bicarbonate alkalinity may be a more sensitive parameter (Brovko et al., 1977). In this study bicarbonate alkalinity was calculated from total alkalinity and Volatile Fatty Acids concentrations as follows (McCarty, 1964): Bicarbonate Alkalinity = Total Alkalinity -0.71 (Volatile Fatty Acids)
(1)
(mglitre-1 as C a C O 3 ) (mglitre 1 as C a C O 3 ) (mglitre-1 as acetate)
BUFFER THEORY
Anaerobic digestion reportedly proceeds most optimally between pH 6.6 and 7.6 (McCarty, 1964) with pH and process stability largely resulting from chemical equilibria established between the three primary buffers of Volatile Fatty Acids, bicarbonate and ammonia (Pohland, 1968). Volatile Fatty Acids decrease the buffering capacity of the bicarbonate ions according to eqn. (2) (Albertson, 1961): HAc + NH4HCO 3 ~ NH4Ac + H z C O 3
(2)
while the addition of ammonia will increase the bicarbonate by forming an ammonium salt with bicarbonate taken from the CO 2 pool: NH 4 + O H - + H + + H C O f ~-~NH~ + HCO 3 + H 2 0
(NH 3 + H20)
(CO 2 + H20)
(3)
(salt)
The buffering capacity of any buffer can be defined as the amount of strong acid or base required to change the pH by one unit and is known as the buffering capacity index or buffer intensity (B) (Butler, 1964). The buffer intensity of a solution is determined from the concentrations of each buffer, their pK values (location on pH
430
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI
scale) and the pH of the solution as follows (Butler, 1964)"
B-
dC.
dpH
dCb --2.303
dpH
-
[ H + + ,< + V(c,.K,..+ H -7 , / ~ \ ( ~ + H ~ ) 2 ] J
(4)
i=1
where'
B = buffer intensity in Meq litre l p H = concentration of strong acid and base added, respectively in Meq litre
Ca,Q K,a,
H + + H 7 = concentration of hydrogen and hydroxyl ions in the solution representing the water buffering capacity effect
i=1
, = t h e sum of the buffering effects of each buffer in / solution C i = concentration of the ith buffer in Meq litre-1 Ki = dissociation constant of the ith buffer in mollitre
i
A graphical presentation ofeqn. (4) is shown in Fig. 1 for the common buffers found in anaerobic digesters: VFA, bicarbonate and ammonia. Carbonate was added because of its intimate association with bicarbonate. Their combined effect, when all are simultaneously in solution, is shown as the sum of the individual intensities. The concentration of each buffer was assumed to be 1 Meq litre i at 35 °C. Note that the m a x i m u m intensity of an individual buffer is determined from its p K value and relative concentration in solution. Equation (4) is valid only for solutions with ionic strength <0-1 g and hence cannot be used for digesters. The buffer intensity of a solution may be determined experimentally as the slope of the alkalimetric and acidimetric titration curves (Kleijn, 1965). Two samples of the solution are titrated from the initial pH; one with standard acid, the second with standard base. The titration results are plotted on one graph versus pH. The slope of the curve measured at the initial pH represents the buffer intensity, B (Fig. 2(a)). This procedure was used on all digesters in this study. By definition, the slope of a curve is equal to the first derivative of the curve. The experimentally obtained titration curves for all digesters were subjected to regression analysis and expressed mathematically in the general form: Y = aX 3 + where:
bX 2
-+-cX q- d
Y = acid and base used in Meq litre-1 X = p H of the solution a, b, c, d = constants
(5)
431
BUFFER STABILITY IN MANURE DIGESTERS
0.7
I//
,
/ ~,, /I I
0.6 "
o,
IIII ~*\\ . j / I f
•
1
/
~"\
~
\\\\
l/ I
~.\\
//
/',
1/
II
\
Y^
II!/
\
\'
',,/~,',"/L tIt ~o;41/tl7'~,
o.,.
~,
0.2
0.1"
O. ?.
3
4
5
6
7
I 8
i 9
I I0
iii
pH
Fig. 1.
Changes in buffering intensity (B) with pH for various buffers commonly found in anaerobic digesters.
From the mathematically defined titration curves, buffer intensity (B) curves were obtained by differentiating each titration curve at various pH values (Fig. 2(b)). The B curves generated from the titration curves form valleys coincident with that formed between the buffer intensity peaks of Fig. 1. It appears that the pH in a stable digester assumes a position of stability between two buffer intensities at a position of least numerical B value. This behaviour of buffers upon pH can be compared with a ball seeking the position of least potential energy but greatest stability in physics (Model A in Fig. 3). The ball tends always to return to the most stable position (position 1) from any other on the sloped walls (2 and 3). Any force tending to move the ball from stability will meet with increased resistance (increased slope) and will fail unless the force can overcome the resistance or the resistance can be removed. In a stable anaerobic digester, the pH should be safely positioned between the bicarbonate and ammonia buffer intensity peaks (Fig. 1). Addition of Volatile Fatty Acids will cause the pH to attempt to move up the bicarbonate intensity curve according to the chemistry of eqn. (2). If pH overtops the bicarbonate curve, it will be free to slip into the valley formed by the bicarbonate and VFA intensity peaks and digester instability occur. The addition of increasing amounts of ammonia will act as a force tending to push
432
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI
/
400 TITRATION CURVE(a)
/
200 g E
w" o
-2oo~
~
DIGESTER
-40C
300
g
BUFFER INTENSITY (b)
200
E
I00
0 6
i
i
J
7
8
9
pH
Fig. 2.
Experimentally obtained titration (a) and mathematically derived buffer intensity (b) curves for Digester C2. MODEL A
LEAST [~IE.RO~'(FOTENTIAI.)GREATEST~ L I T Y
Fig. 3.
Model A: pH stability (ball) in terms of buffer intensity curve (energy).
433
BUFFER STABILITY IN MANURE DIGESTERS
t
//
IC]IA~ \\ \ \
./
L,
• ~
'///I/
~_
E .q
/
/
IA
i
4
Fig. 4.
;
7
i
8
i
9
i
,o
pH Shifts in digester buffer intensity and pH stability due to changes in the primary buffers, ammonia, bicarbonate and VFA.
the pH up along the ammonia buffer intensity curve as NH4HCO 3 is formed (eqn. (3)). However, the pH probably will not top the ammonia buffer peak because of ammonia toxicity to the biological community (Albertson, 1961 ; McCarty, 1964). Maintaining satisfactory anaerobic digestion of livestock manure will entail keeping the system pH centred between the bicarbonate and ammonia buffer intensities. The combined operational sequence of the three primary buffers (VFA, bicarbonate and NH3) in a digester can be summarised schematically in Fig. 4. Digesters with high ammonia concentrations (curve A) and low VFAs (IA) result in high bicarbonate buffer intensities (IIA). In this case, pH is positioned safely between the two buffer peaks (position 1) and the digester operates stably. If VFA concentration increases (increased carbon loading), ammonia bicarbonate will buffer according to eqn. (2) and bicarbonate buffer decreases (curve B). If VFA's continue to increase (IB), the bicarbonate buffer intensity peak drops (IIB) and pH moves to a lower value (position 2) where electroneutrality is re-established. The digester is still operating but with less buffer reserve to maintain stability. If this pattern continues, a point is reached where electroneutrality is satisfied only by ammonia and VFA ions (curve C), all bicarbonate ions have returned to the CO z pool, and bicarbonate buffer intensity is essentially zero (IIC). The pH shifts to position 3 between the volatile acids and ammonia peak buffer intensities. This is the lowest position of biochemical stability at which a digester can be maintained in
434
DIMITRIS G E O R G A C A K I S , D. M. SIEVERS, E. L. I A N N O T T I
terms of pH and electroneutrality. A further increase in Volatile Fatty Acids (position IC) would result in a movement of pH upward along the VFA buffer intensity curve (position 4) and probably result in digester failure due to lack of a proper buffer to maintain pH in a stable position.
RESULTS A N D D I S C U S S I O N
High acid-Low bicarbonate and ammonia
The data of Table 3 illustrate the changes in a digester's buffer system as VFA increase. The data are for digester A which received increases in glucose but no additional nitrogen. Increases in organic carbon produced increases in VFA, resulting in decreases in pH, bicarbonate alkalinity and buffer intensity. Eventually the acids destroyed the bicarbonate buffer and pH dropped quickly, resulting in digester A4 failing. The buffer intensity values in Table 3 indicate that digester A moved successively from position 2, curve B, in Fig. 4 via position 3 to position 4, curve C. The bicarbonate buffer was essentially removed and could no longer maintain pH safely between the bicarbonate and ammonia buffer peak intensities. TABLE 3 CHANGES 1N THE BUFFER SYSTEM DUE TO INCREASES IN CARBON LOADING OF DIGESTER A
Digester
A1 A2 A3 A4
Carbon loading (glitre -~)
pH
38-21 42.41 70.21 102.21
7-40 7.45 7.14 5.95
VIA (mg litre - 1) acetic acid) 181 147 288 13500
Bicarbonate alkalinity (mglitre-1CaC03) 10864 9438 4149 258
Buffer intensity, B (Meqlitre-lpH
1)
69 55 45 58
A digester operating at position 3 (Fig. 4) is in a weakened condition and pH must be controlled externally. This is generally accomplished by dilution, loading reduction or lime addition. Whether the pH will shift to higher values from position 3 will depend on the bacterial population. If they can begin to convert VFA's to methane, bicarbonate buffer intensity will increase, stabilising pH between the bicarbonate and ammonia buffers (positions 1 and 2) and the digester will recover. If bacterial activity fails and pH continues to drop, failure will occur. Thus, the region surrounding position 3 can be called a transition zone between recovery through the establishment of bicarbonate buffer and pH stability and increasing acid build up leading to failure. The relationship of the transition zone to bicarbonate buffer and buffer intensity (B) is shown in Fig. 5. The glucose loading to stable digester D4 was increased from 8 to 10 g litre 1 (D5) on day 4. The digester immediately began to fail, as evidenced by the large decreases in biogas production, bicarbonate alkalinity and pH. On day
BUFFER
STABILITY
IN
MANURE
435
DIGESTERS
uj o_
IE
C ~
810GAS
4
pH
[]
E
7.5
7.0
m
.~
6.5
0
8
m •Co
z
,,:(
o 0
70
~_ o~
60
t~
8
g'
5O
~.. c.
o_ m
E
2 30
0
~
,5
,;
~
2~
3o
DAYS
Fig.
5.
Transition zone changes of B, bicarbonate alkalinity and p H f o r D i g e s t e r s D4 and
D5.
8 all loading ceased and did not resume until day 12 when the daily addition of swine manure (no glucose or urea) was resumed. During the interim 4 days following cessation of loading, digester D5 began to recover, as evidenced by the increasing pH and bicarbonate alkalinity. Buffer intensity was slower to recover but was increasing steadily when the experiment was terminated. Data of digester D5 suggest that the transition zone lies between pH 6-5 and 7-0 with a corresponding range of buffer intensities of 0.030 to 0-040 Meq litre- 1 pH 1
436
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI o STABLE ,x UNSTABLE 150
:z
o
I00
TRANSITION ZONE
E =-
50
~.
.
I
I
I
.
I
6
Fig. 6.
I
c°j ~
txJ
I
7
I
8
pH Relationship of transition zone to digester pH and B.
Other failing digesters exhibited similar large reductions in pH and bicarbonate alkalinity within the same pH and buffer intensity ranges. Buffer intensity and pH data for all digesters (stable and unstable) are plotted in Fig. 6. The regression curve has an R 2 value of 0.799. Unstable digesters included those which were failing (pH and gas production decreasing) and those recovering following modification of loading procedures. The minimum pH is 6-7 with a corresponding buffer intensity of 0.025 Meq 2.5
2.0
E
"
1.5
> ~n I.O O STABLE DIGESTERS
¢:0
/~ DIGESTER D-5
0.5
0
t
O
Fig. 7.
i
i
i
5 IO 15 20 BICARBONATE ALKALINITY, mg/] CaCO S (IOOO's)
Relationship of bicarbonate alkalinity to biogas production.
437
BUFFER STABILITY IN MANURE DIGESTERS
litre- 1 p H - 1. No stability was observed below pH 7-1 (B = 0.030). The portion of the regression curve to the left o f p H 6.7 can be considered to be the increasing slope of the VFA buffer intensity peak of Fig. 1. Digesters exhibiting decreasing pH values below 6.7 are moving towards decreasing stability and eventual failure. Thus, the transition zone could be defined as the pH range 6.6-7.1 (B =0.025 to 0.030). When data from digester D5 is compared to that of the stable units (Fig. 7), it appears that a minimum bicarbonate alkalinity of 6000mglitre -1 CaCO 3 is required to maintain high biogas production in the swine manure digester. This value corresponds to pH's above 7-1 and reflects the beginning of the transition zone. Once pH and bicarbonate buffer fall into the transition zone, instability begins. High bicarbonate and ammonia
Increases in nitrogen loading to an anaerobic digester result in increased production of NH4HCO 3 which, in turn, creates a greater buffer capacity and leads to greater stability. This increase in buffer stability is demonstrated by the increasing buffer intensity (B) and bicarbonate alkalinity values of Table 4 and corresponds to operating the digesters near position 1, curve A of Fig. 4, where pH is secured between increasingly larger ammonia and bicarbonate buffer peaks. TABLE 4 CHANGES IN DIGESTER BUFFER CHEMISTRY DUE TO INCREASESIN AMMONIA
Digester p H
A B C D E
7-40 7.60 7.75 7.90 7.80
NH 3- N (rag litre- 1)
VFA (rng litre- l)
Bicarbonate alkalinity (mglitre -1 CaC03)
1986 3267 4306 5988 6366
181 421 3824 4951 7590
10864 15240 17066 19017 21299
Buffer CH 4 NHs--N intensity, B (ml millilitre- 1) VFA (Meqlitre 1 pH - l ) 62.2 71.4 72.2 84,4 108,7
0.70 0-68 0-57 0-53 0-41
10-97 7.76 1.13 1.21 0.84
The increased stability brought about by increased nitrogen loading exacts a toll, as evidenced by the 42 ~o decrease in methane production and VFA increase 42 times that of the digester receiving the lowest nitrogen loading. The decrease in methane production was accompanied by an 88 ~o increase in free ammonia. Free ammonia toxicity towards methane bacteria is well documented with reported toxic levels varying from 150mglitre- l (McCarty & McKinney, 1961) to 350 mglitre- l (Kroeker et al., 1979). While ammonia does have a negative effect on methane production from high nitrogen wastes (Sievers & Brune, 1978), it is possible to operate stable units at very high free ammonia levels (Table 5). Stable digestion with a methane production of 0.5 ml millitre-1 was achieved at a free ammonia concentration of 663 mglitre-
438
D I M I T R I S G E O R G A C A K 1 S , D. M. SIEVERS, E. L. I A N N O T T I
VARIATIONS 1N BUFFER CHEMISTRY FOR
TABLE 5 A DIGESTERRECEIVINGTHE HIGHESTNITROGENLOADING (9"4 g litre - 1)
Free ammonia (rag litre- 1)
CH 4 (ml millilitre - 1)
VFA (mg litre 1)
Bicarbonate alkalinity (rag litre - l CaC03)
Carbon~Nitrogen* ratio
440 663 409 349
0.41 0.50 1.77 3.06
7590 7673 1739 6876
21299 20099 16390 13103
4.1 4.5 7-5 10.9
* Total organic carbon/Total Kjeldahl nitrogen. Stability at such high free ammonia levels is attributed to the acclimation of the bacteria to high nitrogen levels and the large bicarbonate alkalinity. The large increases in VFA are probably due also to the inhibitive effect of ammonia. Albertson (1961) hypothesised that if an ammonia addition was sufficiently large to cause a significant rise in pH beyond the normal optimum range for methane bacteria, bacterial activity would be slowed, resulting in an accumulation of VFA. The tendency of the VFA to decrease pH would be buffered by the NH4HCO3, allowing the bacteria time to recover and establish stability at a lower gas production level. The data of Table 4 agrees with Albertson's hypothesis and would account in part for the ability of swine and poultry digesters to operate successfully at high ammonia and VFA levels (Converse et a l . , 1977; van Velsen, 1977; Kroeker e t al:, 1979). The same data also gives an indication that ammonia's buffering effect against VFA is limited, as evidenced by the decreasing N H 3 N/VFA ratio. There appears to be a point involving the concentrations of VFA and N H 3 - - N beyond which buffering by N H 3 - - N has no effect and the methane bacteria succumb, leading to digester failure. The exact cause of bacterial inhibition at this point cannot be discerned from this study, but the point at which failure occurs can be estimated from the N H 3 - - N / V F A ratio. Table 6 lists N H 3 - - N / V F A ratios at observed digester failure for various livestock wastes. Inspection of the data would suggest that the N H 3 - - N / V F A ratio could be used as an operational parameter to monitor digester stability for high nitrogen wastes. Values of 1.0 for swine and beef and 0.5 for poultry appear sufficiently conservative to provide for variations in field operations and provide the operator with sufficient time to make the necessary corrective adjustments. Process
operation
The bicarbonate-ammonia buffers in an anaerobic digester are the primary agents controlling pH and maintaining process stability. For wastes low in nitrogen, the bicarbonate alkalinity is the chief source of stability control by buffering against pH drop due to high VFA. In our studies, stability was achieved by maintaining a
BUFFER STABILITY IN MANURE DIGESTERS
439
TABLE 6 NH 3 N / V F A RATIOSAT DIGESTER FAILURE Waste
NH 3
Swine
0"63 0"83 0.64 0.24 0.33 0.46 0.16 0.34
Swine Swine Poultry Beef
N/VFA
ReJerence
(A4)* (B4) (C4) (D5)
This study
van Velsen (1977) Kroeker et al. (1975) Hill & Barth (1977) Hein et al. (1977)
* Digester.
minimum bicarbonate alkalinity of 6000mglitre 1 CaCO 3 with a pH above 7-1. Permitting the bicarbonate alkalinity to drop below 6000 mg litre 1 will place the pH into the stability transition zone and failure could result. Nitrogen additions to a digester will increase stability by maintaining pH between the bicarbonate and ammonia buffers. Digesters receiving swine and poultry manure possess an inherent buffer stability due to the naturally high level of ammonia in the wastes. However, this advantage is offset somewhat by ammonia's inhibition of methane production (Table 4) and poses the question of where to operate the digester to achieve a proper compromise between adequate buffering and minimum methane inhibition. If one considers the r61e of the buffering system in a digester as an interaction between carbon (VFA and HCOa) and nitrogen (NH3) then an answer to the operating question posed above might be achieved by looking at the c a r b o n nitrogen ratio (C/N). Methane production per unit of Volatile Solids loaded for all stable digesters is presented in Fig. 8 versus C/N ratio. Optimum methane 2.0
[5 0 0
/
?
2
o.s
C/N
Fig. 8.
RATIO
Carbon/nitrogen ratio relationship to methane production.
440
DIMITRIS GEORGACAKIS, D. M. SIEVERS, E. L. IANNOTTI
20 ®
®O O
I0
6
~doo
Ioboo
®
r-,6~o
~
zr~o
BICARBONATE ALKALINITY
(mgll as CoCo 3) • Fig. 9. Carbon/nitrogen ratios versus bicarbonate alkalinity for all stable digesters. p r o d u c t i o n occurred within a C / N ratio of 10/1 to 15/1. Digesters with ratios less than 10/1 exhibit a m m o n i a i n h i b i t i o n , while digesters with ratios exceeding 18/1 failed due to low b i c a r b o n a t e alkalinity. The 10/1 to 15/1 range of C / N ratio c o r r e s p o n d s to a b i c a r b o n a t e alkalinity range of 6 0 0 0 - 1 3 0 0 0 m g l i t r e -1 C a C O 3 (Fig.9) a n d N H 3 - - N / V F A ratios above 1.0 (Table 4). O p e r a t i n g swine digesters within this buffer range should ensure good stability a n d o p t i m u m m e t h a n e p r o d u c t i o n . REFERENCES ALBERTSON,O. E. (1961). Ammonia nitrogen and the anaerobic environment. Journal Water Pollution Control Federation, 33(9), 978-95. AMERICANPUBLICHEALTHASSOCIATION(1975). Standard methods for the examination of water and wastewater. (13th edn.), Washington, DC. BROVKO,N. et al. (1977). Optimizing gas production, methane content and buffer capacity in digester operation. Water and Sewage Works, 24(7), 54-57. BUTLER,J. N. (1964). Ionic equilibria, a mathematical approach. Addison-Wesley Pub. CO., Menlo Park, California. CONVEKSE,J. C. et al. (1977). Performance of a large size anaerobic digester for poultry manure. American Society of Agricultural Engineers Paper No. 77-0451, St. Joseph, Michigan. HEIN, M. E. et al. (1977). Some mechanical aspects of anaerobic digestion of beef manure. American Society of Agricultural Engineers Paper No. 77-4056, St. Joseph, Michigan. HILL, D. T. & BARTH,C. L. (1977). A dynamic model for simulation of animal waste digestion. Journal Water Pollution Control Federation, 49(10), 2129-43. IANNOa'n, E. L. et al. (1979). Changes in swine manure during anaerobic digestion. Developments in Industrial Microbiology, 20, 519-29. KLEIJN, H. F. W. (1965). Buffer capacity in water chemistry. Journal o f Air and Water Pollution, 9, 401-13. KROEr~R, E. J. et al. (1975). Cold weather energy recovery from anaerobic digestion of swine manure. In: Energy, agriculture and waste management, Ann Arbor, Michigan, pp. 337-52.
BUFFER STABILITY IN MANURE DIGESTERS
441
KROEKER, E. J. et al. (1979). Anaerobic treatment process stability. Journal Water Pollution Control Federation, 51(4), 718 27. MCCARTY, P. L. (1964). Anaerobic waste treatment fundamentals: II, Environmental requirements and control. Public Works. (October), 123-6. MCCAR'rY, P. L. & MCKINNEY, R. E. (1961). Salt toxicity in anaerobic digestion. Journal Water Pollution Control Federation, 33(4), 399-415. POHLAND, F. G. (1968). High-rate digestion control Ill--Acid-base equilibrium and buffer capacity. Proc. 23rd Industrial Waste Conference, Purdue University, Lafayette, Indiana, 275-84. S1EVERS,D. M. & BRUNE,D. E. (1978). Carbon/nitrogen ratio and anaerobic digestion of swine waste. Transactions of American Society of Agricultural Engineers, 21(3), 537-41,549. VAY VELSEN,A. F. M. (1977). Anaerobic digestion of piggery waste. I. The influence of detention time and manure concentration. Netherlands Journal of Agricultural Science, 25, 151 69.