titration methodologies for monitoring of anaerobic digestion in developing countries a review
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Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 79:1331– 1341 (online: 2004)DOI: 10.1002/jctb.1143
Review
Titration methodologies for monitoring
of anaerobic digestion in developingcountries—a reviewO Lahav1∗ and BE Morgan2
1Faculty of Civil and Environmental Engineering, Technion—Israel Institute of Technology, Haifa, 32000, Israel 2Department of Civil Engineering, University of Cape Town, Rondebosch 7700, South Africa
Abstract: An increase in volatile fatty acids (VFA) concentration (or the proportionaldecrease in carbonate
alkalinity concentration) is the first practical measurable indication that an anaerobic treatment system is
in a state of stress. If the system is not rectified at this early stage, failure is likely. Current methods for VFA
measurement include distillation, colorimetry, gas chromatography and various titration techniques. In
terms of simplicity, speed and cost-effectiveness it is generally accepted that titration methods are superiorfor the purpose of on-site routine monitoring and control, particularly in developing countries. This paper
reviews the methods published in the last four decades concerning on-site titration measurement of VFA
and carbonate alkalinity concentrations. The review encompasses the following: aquatic chemistry related
to the theory on which most of the methods are based, and a detailed description of each of the principal
methods published followed by critical and comparative evaluation.
© 2004 Society of Chemical Industry
Keywords: volatile fatty acids; titration methods; anaerobic digester monitoring; developing countries
INTRODUCTION
Start-up and successful operation of anaerobic treat-ment facilities is a difficult and delicate process,
requiring reasonably accurate and rapid monitoring
techniques. The control strategy is based on maintain-
ing a low concentration of volatile fatty acids (VFA)
and a pH in the range 6.6 < pH < 7.4. Normally in
anaerobic reactors the carbonate system forms the
main weak-acid system responsible for maintaining
the pH around neutral, while the VFAs (mainly acetic,
propionic, and butyric acids) are the major cause for a
decline, in pH.
Under stable operating conditions, the H2 and
acetic acid formed by acidogenic and acetogenicbacterial activity are utilized immediately by the
methanogens and converted to methane. Conse-
quently, the VFA concentration in properly running
anaerobic digesters is typically fairly stable and low
(typically 0.5 – 2.0 mmoldm−3),1 carbonate alkalinity
is not consumed in excess and the pH is stable. In
contrast, under overload conditions or in the presence
of toxins or inhibitory substances, the activity of the
sensitive methanogenic and acetogenic populations
is reduced, causing an accumulation of VFA which
in turn increases the total acidity in the digester, thus
reducing the pH (the term ‘total acidity’ is used here todefine the total proton-donating capacity of a solution,
including the contribution of all weak-acid subsystems
present). The onset of reactor failure can have a spiral-
ing effect on the methanogenic population,2 where the
buffering capacity cannot keep up with the increasing
production of VFAs, causing further, and ultimately,
complete, failure. The extent of the pH drop depends
primarily on the H2CO3∗ alkalinity concentration. The
term H2CO3∗ alkalinity is used here to define the
total proton-accepting capacity of the carbonate weak-
acid subsystem combined with the proton-accepting
capacity of the water system3 (ie H2CO3∗ alkalinity =2[CO3
2−] + [HCO3−] + [OH−] − [H+]). In terms of
routine monitoring, pH measurement cannot form
the sole indication of imminent failure, because in
medium or well-buffered waters high VFA concentra-
tion would have to form in order to cause a detectable
drop in pH, by which time failure would already occur.
Consequently, direct measurement of either (or both)
VFA or H2CO3∗ alkalinity concentration is necessary.
Measurement of H2CO3∗ alkalinity in a mixture
of weak-acid subsystems cannot be carried out via
∗ Correspondence to: O Lahav, Faculty of Civil and Environmental Engineering, Technion—Israel Institute of Technology, Haifa, 32000,Israel
E-mail: [email protected]
( Received 23 June 2004; revised version received 9 July 2004; accepted 19 July 2004 )
Published online 14 September 2004
© 2004 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2004/$30.00 1331
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O Lahav, BE Morgan
the standard titration procedure to the H2CO3∗
equivalence point (pH 4.5) because (i) the point is
not defined sharply and (ii) titration to 4.5 does
not account for all the proton-accepting capacity of
the VFA system (ie the non-protonated forms of
acetate, butyrate and propionate). Characterization of
the carbonate subsystem can be carried out using an
inorganic carbon analyzer; however, this instrument,apart from not being generally available on-site, is
prone to gross inaccuracy due to CO2 loss occurring
between sampling and measurement. Therefore, an
increase in VFA concentration is the first practical
measurable indication that an anaerobic treatment
system is in a state of stress. If the system is not
rectified at this early stage, failure is likely.
The demand for reliable VFA measurement has
increased in recent years due to the introduction
and widespread use of high-rate anaerobic treatment
processes, where more rigorous control is needed.4
In addition to conventional anaerobic digesters, other
treatment systems such as biological sulfate removal
reactors and hydrolysis reactors (prefermenters)
depend on VFA measurement as a principal means
of monitoring reactor performance. Furthermore,
anaerobic treatment of municipal sewage has gained
popularity recently as evidenced by the increasing
introduction of full-scale UASB reactors,5 particularly
in tropical areas—the majority of which are in
developing countries where sophisticated technology
tends to be unsuccessful.
Currently, VFA can be measured using straight dis-
tillation, steam distillation, a colorimetric technique,
gas chromatography, and titration techniques. Someof these methods are time consuming, others require
expensive equipment and a dedicated operator, and
often, in particular in developing countries, the equip-
ment is not available on-site.
Combining such factors as simplicity, speed and
cost-effectiveness it is generally accepted that titrative
methods are superior for the purpose of routine
monitoring and control.6 While difficult to verify,
it would appear that in developing countries the
vast majority of anaerobic digesters are monitored
by various titration techniques. This also holds true
for many treatment plants in the developed world.
During the last four decades a considerable number
of quantitative and semi-quantitative titration methods
have been proposed for the measurement of either
VFA or H2CO3∗ concentrations or both. These
titrative methods can be roughly divided into three
categories of approaches:
1 Approximation of VFA concentration alone or
approximation of both VFA and H2CO3∗ alkalinity,
both by titration techniques.7–11
2 Measurement of H2CO3∗ alkalinity only by direct
titration, with or without an external measurement
of VFA concentration using a different analytical
approach.12–15
3 Accurate measurement of both VFA and H2CO3∗
alkalinity with differing levels of complexity and
accuracy using a titration technique followed by a
mathematical algorithm.16–19
In addition to these, automated, in-line methods
based on one of the above are also found in the
literature.20,21
The multiplicity of methods available in the
literature and the large difference in approach andin the results obtained from each method emphasizes
the need for a comprehensive review of the subject,
extending the review published by Moosbrugger
et al in 1993.6 The current review encompasses the
following: aquatic chemistry theory on which most of
the methods are based, and a detailed description of
each of the principal methods published followed by
critical and comparative evaluation.
It is hoped that the review will provide assistance
to researchers, engineers, and laboratory technicians
in their quest for the most appropriate method for
the control of anaerobic processes. A list of the main
methods covered, including characterization, ease of
execution, and suitability for use as the monitoring
technique is given in Table 1.
THEORY OF WEAK-ACID SYSTEMS AND
CORRESPONDING BUFFERING INTENSITY IN
ANAEROBIC REACTORS
All the titrative procedures proposed for VFA and
H2CO3∗ determination stem from classical aqueous
solution weak-acid equilibrium theory. Thus, in order
to evaluate the various methods using common
grounds, a brief review of fundamental aquaticchemistry principles is given.
Acids or bases that dissociate only partially in
solution are defined as ‘weak’. The principal weak-acid
subsystems commonly found in anaerobic reactors are
the carbonate, ammonium, phosphate, VFAs (namely
acetic, propionic and butyric acids), and sulfide
subsystems. The various species of these subsystems
can be represented as a function of the total species
concentration of a particular weak-acid subsystem
and its apparent equilibrium constant adjusted for
temperature and Debye–Huckel effects. An example
of such representation is given below for the carbonateand VFA subsystems.
The equilibrium and mass balance equations for the
carbonate subsystem are:
(H+) · [HCO3−]/[H2CO3
∗] = K C1 (1)
(H+) · [CO32−]/[HCO3
−] = K C2 (2)
C T = [H2CO3∗] + [HCO3
−] + [CO32−] (3)
Where () denotes activity, [] denotes molarity, K
equals apparent equilibrium constant after adjustment
for Debye– Huckel effects, and C T = total inorganic
carbon concentration (mol dm−3).
For the VFA subsystem (VFAs are commonly
considered to constitute a single weak-acid system
1332 J Chem Technol Biotechnol 79:1331–1341 (online: 2004)
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Monitoring of anaerobic digestion—a review
T a b l e
1 . S
u m m a r y a n d c o m p a r i s o n o f p r i n c i p a l m e t h o d
s ( +
=
l o w , + + + + +
=
v e r y h i g h )
A u t h o r s
P a r a m e t e r
m e a s u r e d
C h a r a c t e r i z a t i o n
E q u i p m e n t r e q u i r e d
E a s e o f
e x e c u t i o n
A c c u r a c y
S u i t a b i l i t y
D i L a l l o a n
d A l b e r t s o n ( 1 9 6 1 ) 7
P a u s s e t a l ( 1 9 9 0 ) 1 0
V F A
R e m o v a l o f C O 2 ( g ) a t p H 3 . 5 a n d b
a c k
t i t r a t i o n t o p H 7
B a s i c l a b o r a t o r y e q u i p m e n t
+ +
+
G i v e s v e r y r o u g h a p p
r o x i m a t i o n ; a p p r o p r i a t e
o n l y f o r w e l l - b u f f e r
e d r e a c t o r s .
M c G h e e ( 1 9 6 8 ) 8
V F A
T i t r a t i o n b e t w e e n p H 5 a n d 4
B a s i c l a b o r a t o r y e q u i p m e n t
+
+ + +
+
G i v e s t h e c h a n g e i n V F A c o n c e n t r a t i o n
r a t h e r t h a n a v a l u e
. C a n b e u s e d i n
c o n j u n c t i o n w i t h a
m o r e a c c u r a t e m e t h o d .
R i p l e y e t a l ( 1 9 8 6 ) 9
J e n k i n s e t a l ( 1 9 8 3 ) 1 3
V F A a n d H 2 C O 3 ∗
a l k a l i n i t y
T i t r a t i o n t o p H 5 . 7 5 a s i n d i c a t i o n o f
H 2 C O 3 ∗
a l k a l i n i t y a n d b e t w e e n p
H
5 . 7 5 a n d 4 . 3 f o r V F A d e t e r m i n a t i o n
B a s i c l a b o r a t o r y e q u i p m e n t
+
+ + +
+ +
C a n g i v e g o o d i n d i c a
t i o n o f t r e n d s ,
e s p e c i a l l y i n t h e c a
s e s w h e n t h e r a t i o
H 2 C O 3 ∗
a l k a l i n i t y t o V F A c o n c e n t r a t i o n s
i s l o w e r t h a n 1 0 t o
1 .
M u n c h a n
d G r e e n fi e l d ( 1 9 9 8 ) 1 1
V F A
D i r e c t l i n k a g e o f r e a c t o r p H a n d V F
A
c o n c e n t r a t i o n
p H m e t e r
+ + + + +
+ + +
S u i t a b l e o n l y f o r p r e f
e r m e n t e r s o p e r a t e d a t
l o w
p H a n d h i g h V
F A c o n c e n t r a t i o n s .
R o z z i a n d
B r u n e t t i ( 1 9 8 1 ) 1 2
D i P i n t o e
t a l ( 1 9 9 0 ) 1 4
H 2 C O 3 ∗
a l k a l i n i t y
T i t r a t i o n t o p H < 4 a n d m e a s u r e m e
n t o f
v o l u m e o r p r e s s u r e o f C O 2 ( g ) e m
i t t e d
G a s c o l l e c t i o n a p p a r a t u s
+ + +
+ + +
S u i t a b l e p a r t i c u l a r l y f o r l o w - b u f f e r e d w a t e r s .
T h e m a j o r d i s a d v a
n t a g e i s t h e c h o i c e o f
H 2 C O 3 ∗
a l k a l i n i t y a s t h e s o l e c o n t r o l
p a r a m e t e r .
K a p p ( 1 9 8 4 ) 1 6
V F A
F o u r - p o i n t t i t r a t i o n . A l g o r i t h m
b a s e
d o n
e m p i r i c a l r e l a t i o n s
B a s i c l a b o r a t o r y e q u i p m e n t
+ + +
+ + + +
A c c u r a t e w h e n a p p l i e d t o h i g h s t r e n g t h
a n a e r o b i c d i g e s t e r s w i t h l o w
c o n c e n t r a t i o n s o f o t h e r w e a k a c i d
s y s t e m s . O t h e r w i s
e , t h e a p p l i c a t i o n
d e p e n d s o n t h e s i m i l a r i t y o f o p e r a t i o n a l
c o n d i t i o n s t o t h o s e u n d e r w h i c h t h e
e m p i r i c r e l a t i o n s w
e r e d e r i v e d .
M o o s b r u g
g e r e t a l ( 1 9 9 3 ) 1 7 , 1
8
V F A a n d H 2 C O 3 ∗
a l k a l i n i t y
F i v e - p o i n t t i t r a t i o n , i n c l u d e s o t h e r
w e a k - a c i d s y s t e m s , E C , a n d
t e m p e r a t u r e i n a l g o r i t h m
B a s i c e q u i p m e n t +
c o m p u t e r
+
p h o s p h a t e , a m m o n i u m
a n d s u l fi d e a n a l y s i s
+ +
+ + + + +
C a n b e a p p l i e d g e n e
r a l l y p r o v i d e d t h a t
C T >
2 A T .
L a h a v e t a l ( 2 0 0 2 ) 1 9
V F A a n d H 2 C O 3 ∗
a l k a l i n i t y
E i g h t - p o i n t t i t r a t i o n , i n c l u d e s o t h e r
w e a k - a c i d s y s t e m s , E C , a n d
t e m p e r a t u r e i n a l g o r i t h m
B a s i c e q u i p m e n t +
c o m p u t e r
+
p h o s p h a t e , a m m o n i u m
a n d s u l fi d e a n a l y s i s
J Chem Technol Biotechnol 79:1331–1341 (online: 2004) 1333
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O Lahav, BE Morgan
with equilibrium constant K a because of the similarity
of their p K values):6
(H+) · [A−]/[HA] = K a (4)
AT = [HA] + [A−] (5)
where: AT = total VFA species concentration (moldm−3), HA represents the acidic, protonated species
and A− the ionized form of each acid.
Representing the individual species of the carbonate
and VFA subsystems as a function of C T, AT, and
equilibrium constants:
[H2CO3∗] = C T/{1 + K C1/(H+)
+ K C1 · K C2/((H+))2} (6)
[HCO3−] = C T/{1 + K C2/(H+) + (H+)/ K C1} (7)
[CO32−] = C T · K C2/{(H+) + K C2
+ ((H+))2/ K C1} (8)
[A−] = AT · K a/{(H+) + K a} (9)
[HA] = AT/{1 + K a/(H+)} (10)
Similar equations can be developed for the phos-
phate, sulfide, and ammonium proton-accepting
species.
Buffer intensity
The buffering contribution of each subsystem can be
calculated through a parameter called buffer intensity,
defined as the slope of a titration curve plotted fromthe cumulative mass of strong acid (or base) added to
a sample vs the change in pH:
β = −d M a/d pH = d M b/d pH (11)
where:
M a, M b = concentration of strong acid or strong base,
respectively, added to 1 dm3 of solution
(mol dm−3 solution).
β = buffer intensity index (mol dm−3 solution/
pH ).
The equation for the calculation of the buffer inten-sity index for monoprotic weak-acid subsystems and
for diprotic weak-acid subsystems with dissociation
constants differing by four pH units or more is given
by the following term:22
β = 2.303 · [ AT K a(H+)]/[ K a + (H+)]2 (12)
For the water subsystem the buffer intensity index is
given by:
β = 2.303{(H+) + K W/(H+)} (13)
The overall buffer intensity index of a solution
composed of a number of weak-acid subsystems is
the sum of the buffer intensities of all the weak-acid
subsystems including the water subsystem.
pH—log species and pH—buffer intensity index
diagrams
Using equations describing individual weak-acid
species concentration (such as eqns (6)–(10)) and
buffer intensities (eqns (12) and (13)), ‘pH—log
species’ and ‘pH— buffer intensity index’ diagrams
can be plotted. In Fig 1 an example of such plot is
given for a typical species concentration distribution
encountered in anaerobic digesters. Using Fig 1 as it
applies to titration methods, a number of points can
be made:
1E-25
1E-23
1E-21
1E-19
1E-17
1E-15
1E-13
1E-11
1E-09
1E-07
1E-05
0.001
0.1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH
L o g S p e c i e s
0
0.005
0.01
0.015
0.02
0.025
0.03
B u f f e r i n t e n s i t y
( m o l / ( l p H ) )
HCO3
−
H3PO4
PO43−
CO3
2−
HAcNH3
H2CO
3
HPO4
2−H2PO4−
Ac−
Actual bufferintensity
Carbonatebuffer intensity
VFA buffer
intensity
Figure 1. pH–log species and buffer intensity index diagrams in a typical anaerobic digestion sample ( CT = 1000mg dm−3 as CaCO3, VFA = 100mgdm−3 as HAc, total phosphate concentration (PT) = 50mgdm−3 as P, total sulfide concentration (ST) = 20mgdm−3 as S, and total
aqueous ammonium concentration (N T) = 50mgdm−3 as N, temperature = 22 ◦C, TDS = 3000mg dm−3 ). Actual buffer intensity is the sum of
buffer intensity curves of all subsystems.
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Monitoring of anaerobic digestion—a review
• The magnitude of the (cumulative) curve at a
particular pH specifies the buffering capacity of
the solution at that pH or in other words, its ability
to minimize a change in pH when strong acid or
strong base is added. For a titration between any
two pH points, the area under the buffer intensity
curve equals the mass of strong acid (or base) that
has to be added to bring about the pH change.• For a particular weak-acid subsystem, the buffering
intensity is maximal at the p K (p K = a pH value
where two species of a weak-acid subsystem are
equal in concentration). On either side of the p K the
buffer intensity decreases sharply, becoming only
1% of its maximal value within two pH units.
• The shape of the actual buffer intensity curve (ie
the cumulative curve) depends on both the con-
centrations and the respective apparent dissociation
constants of the weak-acid subsystems present in the
water. At the normal pH range maintained in anaer-
obic reactors, the carbonate, sulfide and phosphate
subsystems can have a significant contribution to
the cumulative curve as their respective p K values
are close to the range 6.7 > pH <7.4. Typically,
the carbonate subsystem is present at high con-
centrations and thus its component has the most
significant effect on the cumulative buffer intensity
curve. The sulfide and phosphate concentrations are
usually much lower compared with the carbonate
subsystem, but nevertheless neglecting their effect
may lead to an erroneous measurement. The p K of
the ammonium subsystem (p K N ∼ 9.4) is such that
its contribution to the buffer intensity curve at the
relevant pH range is very small up to relatively highconcentrations (around 1000 mg N dm−3).
• The VFA subsystem, which is invariably represented
by the acetic acid subsystem, has a p K of 4.75
and under normal operating conditions is present
at relatively low concentrations (typically AT <
2 mmoldm−3). Accordingly it has only a small effect
on the cumulative buffer intensity curve at 6.7
> pH <7.4 but a much larger effect is apparent
at 4.25 < pH <5.25. On the other hand, given
that the carbonate subsystem is present at high
concentrations relative to the VFA subsystem, its
contribution to the cumulative curve at this pHrange (4.25 < pH <5.25) may also be relatively high.
The overlap between the buffering intensity curves
of the carbonate and VFA subsystems precludes
the use of the standard measurement of carbonate
alkalinity (ie titration to an ‘end point’ near pH
4.5) as a meaningful means of control for anaerobic
reactors. In addition, acid-titration to two pH points
around the p K of the VFA system will produce
the sum of the proton-accepting capacity of both
the VFA and carbonate subsystems between the
two points, reducing the ability of this approach to
serve as means of direct measurement of the VFA
concentration, particularly in cases where the ratio
C T to AT is large (ie where the area under the
buffer intensity curves of the carbonate and VFA
subsystems between two pH values are of the same
order of magnitude).
• From a technical standpoint, obtaining accurate
titrative results depends on the stability of the pH
readings. The stability of a reading at a given
pH, using an appropriate pH probe, depends on
the magnitude of the cumulative buffer intensity
curve at that pH and to a lesser degree on themixing conditions governing the exchange of volatile
species such as CO2 and H2S with the atmosphere
(such volatilization might introduce errors in low-
pH titration point measurements). The magnitude
of the cumulative buffer capacity at any pH depends
on the total species concentration and the proximity
to a relevant p K value. As a rule, the buffer intensity
is highest close to the p K values and lowest in
between them. In a mixture of weak-acid systems of
unknown concentrations it is impossible to predict a
priori the exact shape of the cumulative curve, but it
is safe to assume that titration to pH values close to
the known p K values would increase the reading’s
stability and result in more accurate observations.
• The effect of the ionic strength and temperature of
the tested solution on the p K values of the weak-
acid systems, and thus on the shape of the buffer
intensity curve is also noteworthy. Changes in the
salt composition and concentration (and to a lesser
degree in temperature) may shift the p K values by
up to 0.5 pH units, changing the cumulative curve
significantly. Therefore, neglecting these parameters
can lead to a large error in interpretation of
experimental results.
• There are particular waters (eg agro-industrialwaters, distilleries, paper mills, landfill drainage) in
which not all proton-accepting species can be readily
identified (eg lignin fractions). If these species are
present in significant concentrations, an assessment
is essential for correct interpretation of the titration
data.
PUBLISHED METHODS
Acknowledgment that monitoring of the carbonate
alkalinity or VFA concentration (or preferably both)
is crucial for control of anaerobic reactors has led,since the early 1960s, to the publication of a variety of
practical procedures based upon titration techniques.
In all cases the incentive was to develop a cheap,
simple and rapid method to measure at least one of
the two parameters, H2CO3∗ alkalinity or VFA.
In the following, these methods are grouped
according to the three categories outlined in the
introduction. Within the groups, the methods are by
and large presented in chronological order of their
appearance in the literature.
Approximate measurement of VFA alone or both
VFA and H2CO3∗ alkalinity
The first to propose a titration method for VFA
measurement were DiLallo and Albertson.7 Their goal
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O Lahav, BE Morgan
was to develop a ‘reasonably accurate method’ for
VFA determination using only equipment normally
found in a treatment plant laboratory in order to
decrease the time to obtain results and to gain better
reactor control. The authors developed a technique
aimed at detecting the change in VFA concentrations
rather than measuring accurately their absolute value.
Their fundamental idea was to circumvent the overlapbetween the buffer intensity curves of the carbonate
and VFA systems by removing the inorganic carbon
concentration as CO2, thus isolating the VFA system
so that it can be measured directly through titration.
‘Total alkalinity’ in their method was defined as
the proton-accepting capacity of the solution titrated
down to pH 4, at which pH it was assumed that
all carbonate species are in the form of CO 2. After
recording the amount of standard acid added to pH
4, the pH is lowered to between 3.3 and 3.5 and
the sample is boiled lightly for 3 min to completely
remove CO2
. Thereafter, the amount of standard base
required to elevate the pH from 4 to 7 is recorded.
This value is considered in the method to consist
80% of the VFA alkalinity (irrespective of the VFA
concentration). Because the acid titration at this pH
range also includes a ‘minor’ contribution of what is
termed ‘base alkalinity’ (referring to proton-accepting
capacity of subsystems such as the phosphate and
sulfide subsystems), the VFA concentration is attained
by multiplying the titration results by a factor of
1.5 when the method yields a VFA concentration
above 180 mg dm−3. Below this value a factor of
1.0 is used (ie no correction factor is applied). The
carbonate alkalinity is calculated by subtracting theVFA alkalinity from the ‘total alkalinity’. The method
proposed by DiLallo and Albertson, although having
the credit of being the first titrative method, suffers
from a number of shortcomings. The method requires
the compulsory addition of both standard acid and
base and the boiling of the sample, a step that tends
to be cumbersome. More importantly, several steps
in the procedure are prone to gross inaccuracy: First,
boiling of the sample to remove CO2 can result in a
loss of a fraction of the VFA due to stripping that will
depend on the VFA concentration and composition,
and on the type of boiling. The authors suggest 3-min ‘gentle’ boiling, but such procedure can hardly
be standardized. Also, an unknown volume of water
is vaporized in the boiling procedure. Second, the
back-titration between pH 4 and pH 7 is assumed in
the method to incorporate 80% of the VFA alkalinity.
This value is a not a bad approximation since its
magnitude is relatively insensitive to such factors as
the VFA concentration, the composition of the acids,
and the ionic strength and temperature of the sample
which affect the dissociation equilibrium constant.
However, VFA concentration is calculated in the
method directly from the titration results for values
lower than 180 mg dm−3 VFA as CH3COOH (HAc)
and multiplied by a factor of 1.5 above 180 mg dm−3.
This approach almost invariably results in a large error:
for example, for a VFA concentration of 200 mg dm−3
as HAc with total phosphate of 150 mgdm−3 as
P, the approximation results in 23% error in the
VFA concentration (0.016mgdm−3 instead of 0.013).
Despite its faults, and considering that the procedure
can be modified to include externally measured
weak-acid subsystems (phosphate, sulfide, ammonia,
etc), the method may be used to detect a largeupsurge in VFA concentrations, as intended by the
authors. Indeed, it is the most popular method in
Israel and it is also practiced in many other places.
However, it should be noted that the method is
practical only where relatively large changes in VFA
concentration are not detrimental to the process,
as might sometimes be the case in well-buffered
reactors.
Pauss et al 10 proposed a similar back-titration
method in which bicarbonate alkalinity is the mon-
itored parameter rather than VFA. In their method,
the solution is first titrated from the initial pH to pH4.5–4.0 and CO2(g) is removed by vacuum boiling.
Subsequently, the solution is back-titrated to the ini-
tial pH and the bicarbonate concentration is calculated
by the difference between the acid and base titration.
McGhee8 presented a different approach to approx-
imating VFA concentrations. He suggested determin-
ing the slope of the titration curve between pH 5
and pH 4 as a simple and rapid means of estimating
VFA concentrations. The sample is titrated rapidly
to pH 5.5, a short delay is given to allow CO2 to
reach equilibrium with the atmosphere, and thereafter
the titration is continued drop-wise to a pH slightly
above 5. From this point one records the amount of
additional acid required and pH attained. The val-
ues attained are then plotted, and the reciprocal of
the slope is calculated. The method is based on the
idea that for a given reactor, with a high and there-
fore fairly constant carbonate alkalinity concentration,
the amount of standard acid added to effect a pH
change between 5 and 4 reflects the change in VFA
concentration.
The author intended the approach to supplement
but not replace the more accurate methods for VFA
determination such as chromatographic techniques.
As such it has value, however, as a more generaltool the approach has several faults. First, it cannot
serve as a tool for determining VFA concentration
but simply to detect large changes in concentration.
Second, even within this scope its application is limited
because the titration between pH 5 and 4 accounts
for only about 50% of the VFA alkalinity, and the
effect of the carbonate system on the proton-accepting
capacity in this pH range is not negligible, especially
when the C T to AT ratio is high (above ten, as in
most anaerobic reactors). Furthermore, in anaerobic
reactors any increase in VFA alkalinity is accompanied
by a similar decrease in carbonate alkalinity (and/or
in the alkalinity of other proton-accepting species
such as the phosphate subsystem). As a result, an
increase in VFA concentration will not be represented
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Monitoring of anaerobic digestion—a review
proportionally in the amount of acid added between
pH 5 and 4.
Ripley et al 9 working on poultry manure treatment,
suggested another way to monitor the biological
stability of a high-strength anaerobic digester. They
recommended titration to two end points and the
use of the ratio PA (volume of strong acid required
to titrate the solution down to pH 5.75) to IA(volume of strong acid required to titrate the solution
from pH 5.75 to pH 4.3) as a means of rapid
detection of possible stress (they suggested a value
exceeding 0.3 is indicative of stress). PA relates
roughly to bicarbonate alkalinity and the titration
from 5.75 to 4.3 (IA) approximates VFA alkalinity.
The concept of titration to pH 5.75 as means of
estimating bicarbonate alkalinity was first introduced
by Jenkins et al 13 They claimed that 80% of the
bicarbonate is converted to CO2 at pH 5.75 while
at the same pH only around 20% of the VFA will
have contributed to the alkalinity. Therefore, for the
high alkalinity concentrations encountered in high
strength reactors, the effect of VFAs on the bicarbonate
alkalinity (PA) value would be minor, even if the VFA
reached high concentrations. Ripley et al 9 added that
titration between 5.75 and 4.3 gives roughly the VFA
concentration, and so they assert that the ratio between
the two values is analogous to the ratio of VFA to
carbonate alkalinity.
The clear advantages of this method are simplicity,
cost effectiveness, and rapidity. As the ratio PA
to IA is dimensionless it does not require titrant
standardization nor sample volume measurement. On
the other hand, because only about 65% of the VFA isrepresented in the titration between 5.75 and 4.25 and
in reality less than 70% of the carbonate alkalinity is
titrated at pH 5.75 (value calculated for samples with
TDS higher than 3000 mgdm−3) the method lacks
accuracy and is somewhat insensitive to increase in
VFA concentrations, especially in the case of high C Tto VFA ratios.
A concept for approximation of VFA concentra-
tions in prefermenters (hydrolysis reactors), based
on pH reading only, was developed by Munch and
Greenfield.11 In prefermenters, unlike typical anaer-
obic reactors, the VFA are the desired products of the anaerobic activity and their build-up is a sign of a
healthy process. The typical operational pH range is
5– 6. The authors developed a mathematical func-
tion relating the VFA concentration to pH using
a set of simplified assumptions. The value of this
method is that without any additional work a simple
pH reading from the working reactor can give a good
indication of the VFA concentration. The method
is only suitable for reactors working with high VFA
concentrations and low pH and providing that the
simplifying assumptions on which the model is based
are met. The major disadvantage of this approach
appears to be the possible lack of reliable pH mea-
surements emanating either from unaccounted CO2
supersaturation or from other pH probe inaccuracies.
It is therefore recommended to use the method in
conjunction with (at least) a weekly analytical VFA
measurement.
Measurement of H2CO3∗ alkalinity only by direct
titration, with or without an external
measurement of VFA concentration
To overcome the shortcomings of using the conceptof total alkalinity (ie titration to pH 4.5) for anaerobic
reactor monitoring, a number of modified alkalinity
procedures have been proposed.
Hattingth et al 23 advocated the use of an alkalinity
proton accepting capacity (PAC) value titrated down
to pH 6 and expressed as HCO3− alkalinity as a
realistic measure of the available buffering capacity of
an anaerobic digester.
Rozzi and Brunetti12 proposed a method where
a digester sample is saturated with CO2 (to yield
P CO2 = 1 bar) and subsequently the pH is reduced to
3.7 by the addition of standard acid. Such addition
of CO2 does not alter the bicarbonate alkalinity.
The volume of CO2(g) released from solution at
pH 3.7 is then measured by a gas meter. As the
loss of CO2(g) during titration is negligible and
assuming that the original CO32− concentration
at the operational pH is very low, this measured
volume of CO2(g) is proportional to the mass of
HCO3− converted. Bicarbonate alkalinity (BA) is thus
determined by:
BA(in mg dm−3as CaCO3) = (V CO2 − V acid)/V sample
× 50 000/22.4 × C ) (14)
where:
V CO2 — volume of gaseous CO2 released at pH 3.7
(dm3)
V acid — volume of standard acid from initial pH to
pH 3.7 (dm3)
C —correction factor, which adjusts for temperature
(T) and pressure (P) effects in the vessel:
C =T 0
T ·
P − P V
P 0(T 0 = 273.16 and
P 0 = 1.013 bar) (15)
Results from the proposed method were compared
with results derived from total alkalinity minus the
VFA concentration (calculated via chromatographic
techniques) and found to be very accurate and
reproducible in the presence and absence of VFA
(±3%). It was therefore concluded that the initial
CO2 bubbling does not cause volatile acid stripping.
According to the authors, this method, in addition
to the parallel method in which the CO2(g) pressure
change (rather than the volume released) is measured
following titration to pH <4, is suitable for automated
control of anaerobic reactors.24
It appears that from a theoretical standpoint both
methods are robust and sound. Because the calculation
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O Lahav, BE Morgan
in the procedure is independent of equilibrium
constants and pH measurements, and no problematic
simplifying assumptions are made, the method should
give accurate results. However, on the negative side,
the procedure is relatively complex and specific
equipment (pressure vessel, gas flow meter or pressure
gauge, CO2 bottle) is required in addition to the
standard laboratory equipment. On a more generalnote, concern for the usage of these methods (and
similar procedures) may stem from the very choice
of bicarbonate alkalinity as the principal control
parameter in anaerobic reactors, especially where the
total inorganic carbon concentration is much higher
than VFA concentration. In such cases a small change
in the carbonate alkalinity could mean a large increase
(percentage wise) in VFA concentration. Such small
change may fall within the accuracy of the method and
go unnoticed, detracting from the effectiveness of the
concept.
Accurate measurement of both VFA and H2CO3∗
alkalinity with differing levels of complexity and
accuracy
This group of methods is composed of more complex
titrative methods typically requiring several titration
points and computerized data interpretation.
Colin15 suggested an automated method based on
acid and base titration to three endpoints. In this
method, after measurement of the initial pH, the
sample is divided in two. One part is acid-titrated
to pH 2 and the other is base-titrated to pH >10.
The sample that was acid-titrated to pH 2 was further
base-titrated up to pH >10. This procedure yieldsthree pairs of pH and V x (acid dosage). Using these
three pairs and the initial pH, C T, AT and N T(total ammonium concentration) are obtained using
equilibrium equations and a computer program. The
author reports good accuracy as compared with VFA
and C T values determined by analytical methods.
However, proper evaluation of the advantages and
disadvantages of the method was not possible because
the method lacks a sufficiently detailed description.
A different empirical-theoretical approach extend-
ing the method suggested by McGhee was developed
by Kapp.16,25
McGhee originally proposed that titra-tion from pH 5 to pH 4 can be considered propor-
tional to the VFA concentration. Kapp accepted this
approach but considered the carbonate subsystem to
also have PAC in the pH range between pH 5 and
4, neglecting the sulfide, phosphate, and ammonium
subsystems. Accordingly, the following equality holds:
VA5 – 4, VFA = VA5– 4, measured − VA5 – 4, H2CO3∗ (16)
where:
VA5 – 4, VFA = volume of acid required to titrate
from pH 5 to pH 4 due to the VFA
PAC
VA5 – 4, measured = volume of acid required to titrate
from pH 5 to 4
VA5 – 4, H2CO3∗ = volume of acid required to titrate
from pH 5 to pH 4 due to the
carbonate PAC
To develop an explicit and simple mathe-
matical expression linking VFA concentration to
VA5 – 4, measured Kapp conducted titration experiments
for a broad VFA and C T concentration range and
derived the following empiric equation linking volume
of titrant to VFA and H2CO3∗ alkalinity concentra-
tions:
VA5 – 4, VFA = 0.1/ N · (−0.0283 + 0.09418 VFA/60)
× V s/20 (17)
VA5 – 4, H2CO3∗ = 0.005 · (0.044875 + 0.00469
× [Alkmeasured]) · V s/ N (18)
where:
N = titrant concentration (eq dm−3)V s = volume of sample (cm3)
[Alkmeasured] = total PAC as titrated to pH = 4.3 (eq
dm−3)
Insertion of eqns (17) and (18) into eqn (16) and
rearranging yields Kapp’s first approximation:
[VFA] = 127 416 · N · VA5 – 4, measured/V s
− 2.99 · [Alkmeasured] − 10.6 (19)
where:
[VFA] = VFA concentration (mg dm
−3
as HAc)Using a further two assumptions, eqn (19) trans-
forms slightly yielding:
[VFA] = 131 340 · N · VA5 – 4, measured/V s
− 3.08 · [Alkmeasured] − 10.9 (20)
Kapp’s approach involves three pH titration set
points (pH 5.0, 4.3 and 4.0), in addition to the initial
pH. Working on samples of digested sludge, Kapp
reported an accuracy of ±10% for VFA concentrations
above 20 mg dm−3 as HAc. Baucher reported a similar
accuracy for samples of raw wastewater, primarysludge, and high- and low-load activated sludge.25
The major drawback of Kapp’s approach is that
it is based on empirical mathematical relationships
that were developed under unique conditions (ionic
strength, temperature, absence of other weak-acid
systems) that are not necessarily generally applicable.
Its advantage stems from its relative simplicity—the
VFA concentration is calculated using a single
equation, the apparatus needed is simple and
laboratory execution is easy and quick. The method is
also suitable for automated execution.
The most general method to date was presented
by Moosbrugger et al 17,18 The authors developed
a model based almost solely on aquatic chemistry
considerations, with very few simplifying assumptions.
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Monitoring of anaerobic digestion—a review
Because of its importance the method is described in
detail.
The authors developed a five-point method involv-
ing equating a mass balance for alkalinity in terms of
volume of titrant added (eqn (21)) to a mass balance of
alkalinity in terms of species concentration (eqn (22)).
M total alk(x) = V e · C a − V x · C a (21)
where: M total alk(x) = total mass of alkalinity after the
addition of V x cm3 of standard strong acid (mol),
V e = the unknown volume of standard strong acid to
be added to the alkalimetric end point (dm3), V x = the
volume of standard strong acid added to a point x with
pH equal to pHx (dm3), and C a = concentration of
standard strong acid titrant (mol dm−3).
M total alk(x) = {[HCO3−]x + 2[CO3
2−]x + [A−]x
+ [HS−]x + 2[S2−]x + [NH3]x + 3[PO43−]x
+ 2[HPO42−
]x + [H2PO4−
]x
+ [OH−]x − [H+]x} · (V x + V s) (22)
Where [ y]x indicates molar concentration of species y
after addition of x cm3 of standard acid (mol dm−3),
[A−] = dissociated short chain VFA species concen-
tration (mol dm−3) and V s = volume of sample (dm3).
Equation (22) can be reformulated in terms of total
weak-acid species concentrations using equilibrium
equations for the weak-acid systems and mass balance
equations for each of the weak-acid systems as
represented in eqns (23)–(25) (For brevity only the
VFA and carbonate subsystems are given. The othersubsystems follow the same approach.)
[HCO3−]x = C T · V s/(V x + V s)/{1 + K C2/(H+)x
+ (H+)x/ K C1} (23)
[CO32−]x = C T · V s/(V x + V s) · K C2/{(H+)x + K C2
+ ((H+)x)2/ K C1} (24)
[A−]x = AT · V s/(V x + V s) · K a/{(H+)x
+ K a} (25)
Similar equations can be developed for thephosphate, sulfide and ammonium proton-accepting
species. Substituting the equations for each species
concentration into eqn (22) (for example as given in
eqns (23)– (25) for the carbonate and VFA subsys-
tems) gives an equation for total mass of alkalinity in
terms of AT, C T, P T, N T, S T and pH:
M total alk(x) = {C T · V s/(V s + V x) · F n1(pH)x
+ AT · V s/(V s + V x) · F n2(pH)x
+ P T · V s/(V s + V x) · F n3(pH)x
+ S T · V s/(V s + V x) · F n4(pH)x
+ N T · V s/(V s + V x) · F n5(pH)x + 10−(14−pHx)/ f m
− 10−pHx / f m} · (V s + V x) (26)
where: P T, S T, and N T represent the total phos-
phate, sulfide and ammonium concentrations, f m =
monovalent activity coefficient, and F n1 to F n5 are
functions of pHx and equilibrium constants for the
carbonate, acetate, phosphate, sulfide and ammonium
subsystems respectively.
Equating eqns (21) and (26) gives the desired
equation linking the mass of alkalinity based on acidadded to the mass of alkalinity based on species
concentrations:
(V e − V x) · C a = {C T · V s/(V s + V x) · F n1(pH)x
+ AT · V s/(V s + V x) · F n2(pH)x
+ P T · V s/(V s + V x) · F n3(pH)x
+ S T · V s/(V s + V x) · F n4(pH)x
+ N T · V s/(V s + V x) · F n5(pH)x
+ 10−(14−pHx)/ f m − 10−pHx / f m} × ·(V s + V x) (27)
At each point in the titration (ie for each V xand corresponding pHx), eqn (27) includes three
unknowns: V e, AT and C T. Thus, to solve for V e,
AT and C T only three data pairs (ie three values
for corresponding V x and pHx pairs) need to be
known. This was found to lead to poor prediction.
Moosbrugger et al found that the best first estimate
for AT and C T and can be obtained from four titration
data points (ie two pairs of data points), each pair
symmetrical about p K C1 and p K a (they suggested
approximately half a pH unit to either side of the
respective p K values).17
When inserted into eqn (27), the data from the
four titration points give four equations. The pair of
observations around p K a (ie the third and the fourth
points) is in a region where the buffer capacity of the
VFA system dominates that of the carbonate system
and vice versa for the first and second titration points.
Consequently, subtracting the equation formed from
the fourth data point from that derived from the
third, gives an equation in terms of C T and AT in
which the VFA alkalinity term, ie the species [A−]x,
dominates. Similarly, subtracting the equation formed
from the second data point from that derived from
the first, gives an equation in terms of C T and AT inwhich the H2CO3
∗ alkalinity term, namely [HCO3−]x,
significantly dominates. This technique enables a
relative separation between the two subsystems in
which the third and the fourth point are mainly
responsible for the VFA derivation. Therefore, an
error in the first two pH observations would be largely
‘absorbed’ by the carbonate subsystem, minimizing the
effect on the VFA calculation. The two new equations
are solved to produce the first estimate of AT and C T.
Modification of the first estimate in this approach
is carried out as follows: a second estimate of AT
and C T is calculated by again taking two pH pairs:
one symmetrical about p K a (ie pH3; pH4) but
the other asymmetrical about p K C1 (ie pH1; pH4).
Subsequently, these two C T values (calculated in
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O Lahav, BE Morgan
the first and second estimate) are compared, and if
different, all pH observations are adjusted by pH
and the calculation procedure is repeated (by changing
pH ) until the difference between the two C T values
is negligible. The DOS computer program required
for this method can be purchased from the Water
Research Commission of South Africa at a cost of
approximately $50.The necessity for correcting observed pH values was
attributed by the authors to either a residual liquid
junction potential (error in pH measurements caused
by differences in the dissolved solids concentration
between the buffer solution used to calibrate the
probe and the test solution), or from poor pH meter
calibration.18 Execution of the five-point method is
facilitated by a computer program supplied by the
authors. The method was tested with results typically
within 2% accuracy.18,26,27 Application of the method
is restricted to cases where the C T is greater than twice
AT
, otherwise the systematic pH correction does not
converge.
As mentioned above, the five-point method is the
most ambitious effort thus far to include all system
components in a single sophisticated, yet simple model
(titration to five points is carried out rapidly and with
ease, because the selected pH points do not have to
be reached accurately and the selected pH areas are
very stable). However, the final step (correction of pH
observations) tends to detract from the theoretical
justification of the method, and from its general
applicability. With regard to errors emanating from
residual liquid junction potential: the total dissolved
solids concentrations in the samples tested in the five-point approach varied between 500 and 1000 mg dm−3
(after dilution), which is very close to the dissolved
solids concentration in standard buffer solutions. For
this reason and others it is very difficult to ascribe
a ‘systematic pH error correction’ to liquid junction
effects.27
Lahav et al proposed a new eight-point titration
method for measuring both VFA and H2CO3∗
alkalinity.19 As in the five-point method, execution
is facilitated by a computer program (available free
of charge from the author). The model extends the
‘five-point method’ by resolving the mathematical andanalytical problems which gave rise to the ‘systematic
pH error’ of the Moosbrugger approach. In this
modification, total alkalinity (PAC of all species in
the sample) is measured accurately using the Gran
titration technique,28 and this value is used, in addition
to the first estimate of AT and C T, to give the final
result. The Gran procedure requires a further three
pairs of (V x, pH) points taken in the pH range of
2.4 < pH <2.7. The assessment of the first estimate
of AT and C T is effected as follows: AT and C T,
determined from the first estimate, and V e, determined
from the Gran function analysis, are inserted in
eqn (27) together with the initial pH value (ie where
V x = 0). Both AT and C T are now multiplied by a
proportional term ‘x’, to account for inconsistencies in
pH observations yielding:
V e · C a = {x · C T · F n1(pH0) + x · AT · F n2(pH0)
+ Const} · V s (28)
where: Const = a constant representing the proton-
accepting term for the phosphate, sulfide, ammonium
and water subsystems at the initial pH (pH0).
Solving for x gives an assessment of the first estimate
for AT and C T. The closer x is to unity, the better the
first estimate conforms to the accurately measured
V e. An acceptable value for x is a relative error
(|(x − 1)| · 100) of less than 5%. The improved values
for AT and C T are then obtained by multiplying each
of the two parameters by x to conform to the accurately
measured V e using the initial pH. For the final output
of the algorithm, the improved AT gives the final
value for VFA concentration and the improved C T is
used to calculate the final value for H2CO3∗ alkalinity
using the initial pH. Typical results obtained usingthe method on simulative and industrial effluents are
similar to the five-point method, ie less than 2% error,
but in contrast to the Moosbrugger approach, this
method can be applied for any C T to AT ratio.19
It should be noted that in both the five-point and the
eight-point methods, when the initial pH is lower than
about 6.85, a known volume of standard base should
be added to the sample to allow acid titration to the
prescribed pH points. In such cases the algorithm is
changed as follows: (i) V s is modified by the volume of
NaOH addition; (ii) V e is derived as before and then
modified giving:
V e (final) = (V e (Gran function) · C a − V NaOH
× C NaOH)/C a (29)
where: V NaOH = volume of standard NaOH solu-
tion added to lift pH above 6.85dm3 and
C NaOH = concentration of standard NaOH solution
(mol dm−3).
The disadvantages of Lahav et al ’s method include
a more tedious titration procedure, the need for
measuring all weak-acid subsystems in addition to EC
and temperature, andthe need for using base titrant forsamples with initial pH lower than 6.85. Advantages
include high accuracy, and general applicability.
SUMMARY
This review examines multiple on-site, titrative mea-
surement approaches to monitor anaerobic processes.
The main objective is to clarify the theoretical basis on
which the methods were developed, to discuss their
advantages and disadvantages, and to allow the oper-
ators to choose a method that is suitable for their
process needs.
Multipoint titration methods that take into account
the various weak-acid subsystems and the important
parameters of EC and temperature are generic and
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Monitoring of anaerobic digestion—a review
highly accurate. It appears that there is little reason to
choose a more rudimentary approach with the current
availability of relatively inexpensive computerized
and programmable titration equipment. However,
even the more simple methods can be applied
successfully provided that the operators understand
the assumptions and simplifications behind the
method, and provided that the accuracy level is suchthat it allows for proper detection of changes in VFA
concentrations and rapid intervention.
REFERENCES1 Van Haandel AC and Lettinga G, Anaerobic Sewage Treatment .
John Wiley & Sons Ltd, London (1994 ).
2 Florencio L, Field JA and Lettinga G, Substrate competition
between methanogens and acetogens during the degradation
of methanol in UASB. Wat Res 29:915–922 (1995).
3 Loewenthal RE, Wentzel MC, Ekama GA and Marais GvR,
Mixed weak acid systems. Part 2: Dosing estimation, aqueous
phase. Water SA 17:107–122 (1991).
4 Pretorious WA, pH controlled feed-on-demand for high rateanaerobic systems. Wat Sci Tech 30:1–8 (1994).
5 Seghezzo L, Zeeamn B, van Lier JB, Hamelers HVM and
Lettinga G, A review: the anaerobic treatment of sewage in
UASB andEGSB reactors. Bioresource Technology 65:175–190
(1998).
6 Moosbrugger RE, Wentzel MC, Ekama GA and Marais GvR,
Weak acid/bases and pH control in anaerobic systems— a
review. Water SA 19:1–10 (1993).
7 DiLallo R and Albertson OE, Volatile acids by direct titration.
Journal WPCF 33:356–365 (1961).
8 McGhee J, A method for approximation of volatile acid
concentrations in anaerobic digesters. Water & Sewage Works
115:162–166 (1968).
9 Ripley LE, Boyle WC and Converse JC, Improved alkalimetric
monitoring for anaerobic digestion of high strength wastes. Journal WPCF 58:406–411 (1986).
10 Pauss A, Rozzi A, Ledrut MJ, Naveau H and Nyns EJ, Bicar-
bonate determination in complex acid– base solutions by a
back-titration method. Environmental Technology 11:469–476
(1990).
11 Munch E and Greenfield PF, Estimating VFA concentrations
in prefermenters by measuring pH. Wat Res 32:2431–2441
(1998).
12 Rozzi A and Brunetti A, Direct bicarbonate determination in
anaerobic digester liquors. Environmental Technology Letters
2:385–392 (1981).
13 Jenkins SR, Morgan JM and Sawyer CL, Measuring anaerobic
sludge digestion and growth by a simple alkalimetric titration.
Journal WPCF 55:448–453 (1983).
14 Di Pinto AC, Limoni N, Passino R, Rozzi A and Tomei MC,Anaerobic process control by bicarbonate monitoring.
Abstract, 5 th IAWPRC workshop on Instrumentation, Control,
and Automation of Water and Wastewater Treatment . Yoka-
homa, Kyoto, August (1990).
15 Colin F, Development of an automatic equipment for the study
of acid-base equilibria for the control of anaerobic digestion,
in Anaerobic Digestion and Carbohydrate Hydrolysis of Waste, ed
by Ferrero GL, Ferraut MP and Naveau H. Elsevier Science,
London, pp 381–393 (1984).
16 Kapp H, Schlammfaulung mit hohem Feststoffgehalt. Stuttgarter
Berichte zur Siedlungswasserwirtscaft, Band 86 , Oldenbourg
Verlag, Munchen, 300 pp (1984).
17 Moosbrugger RE, Wentzel MC, Loewenthal RE, Ekama GA
and Marais GvR, A 5-point titration method to determine
the carbonate and SCFA weak acid/bases in aqueous
solution containing also known concentrations of other weak
acid/bases. Water SA 19:29–39 (1993).
18 Moosbrugger RE, Wentzel MC, Ekama GA and Marais GvR,
A 5-point titration method for determining the carbonate and
SCFA weak acid/bases in anaerobic systems. Wat Sci Tech
28:237–245 (1993).
19 Lahav O, Morgan BE and Loewenthal RE, A rapid, simple and
accurate method for measurement of VFA and carbonate
alkalinity in anaerobic reactors. Env Sci & Tech 36:2736–2741
(2002).20 Powell GE and Archer DB, On-line titration method for
monitoring buffer capacity and total volatile fatty acids
levels in anaerobic digesters. Biotechnology and Bioengineering
33:570–577 (1988).
21 Feitkenhauer H, von Sachs J and Meyer U, On-line titration
of volatile fatty acids for the process control of anaerobic
digestion plants. Wat Res 36:212–218 (2002).
22 Loewenthal RE and Marais GvR, Carbonate chemistry of high
salinity waters. Water Research Commission SA, Research
Report No W46 (1983).
23 Hattingth WHJ, Kotze JP, Thiel PG, Toerien DF and
Siebert ML, Biological changes during the adaptation of an
anaerobic digester to a synthetic substrate. Wat Res 1:255–277
(1967).
24 Rozzi A, Di Pinto AC andBrunetti A, Anaerobic processcontrolby bicarbonate monitoring. Environmental Technology Letters
6:594–601 (1985).
25 Baucher K, A comparison of two simple titration procedures to
determine volatile fatty acids in influents to wastewater and
sludge treatment processes. Water SA 24:49–56 (1998).
26 De Hass DW and Asam N, Use of simple titration procedure
to determine H2CO3∗ alkalinity and volatile fatty acids
for process control in wastewater treatment. Water SA
21:307–318 (1995).
27 Lahav O and Loewenthal RE, Measurement of VFA in
anaerobic digestion: the five point titration method revisited.
Water SA 26:389–391 (2000).
28 Gran G, Determination of the equivalence point in potentio-
metric titrations. The Analyst 77:661–671 (1952).
J Chem Technol Biotechnol 79:1331–1341 (online: 2004) 1341