titration methodologies for monitoring of anaerobic digestion in developing countries a review

11
7/16/2019 Titration Methodologies for Monitoring of Anaerobic Digestion in Developing Countries a Review http://slidepdf.com/reader/full/titration-methodologies-for-monitoring-of-anaerobic-digestion-in-developing 1/11  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 developing countries—a review O Lahav 1and BE Morgan 2 1 Faculty of Civil and Environmental Engineering, Technion—Israel Institute of Technology, Haifa, 32000, Israel 2 Department of Civil Engineering, University of Cape Town, Rondebosch 7700, South Africa Abstract:Anincreaseinvolatilefattyacids(VFA)concentration (ortheproportionaldecreaseincarbonate alkalinity concentration) is the first practical measurable indication that an anaerobic treatment system is in a state of stress. If the system is notrectified at this early stage, failureis likely. Current methods forVFA 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 superior for 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 H 2  and acetic acid formed by acidogenic and acetogenic bacterial 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 0.5–2.0mmoldm 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 to define 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 H 2 CO 3 alkalinity concentration. The term H 2 CO 3 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 system 3 (ie H 2 CO 3alkalinity = 2[CO 3 2] + [HCO 3 ] + [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 H 2 CO 3 alkalinity concentration is necessary. Measurement of H 2 CO 3 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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7/16/2019 Titration Methodologies for Monitoring of Anaerobic Digestion in Developing Countries a Review

http://slidepdf.com/reader/full/titration-methodologies-for-monitoring-of-anaerobic-digestion-in-developing 1/11

 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

7/16/2019 Titration Methodologies for Monitoring of Anaerobic Digestion in Developing Countries a Review

http://slidepdf.com/reader/full/titration-methodologies-for-monitoring-of-anaerobic-digestion-in-developing 2/11

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)

7/16/2019 Titration Methodologies for Monitoring of Anaerobic Digestion in Developing Countries a Review

http://slidepdf.com/reader/full/titration-methodologies-for-monitoring-of-anaerobic-digestion-in-developing 3/11

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     )

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     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

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    a    c     k

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     B    a    s     i    c     l    a     b    o    r    a     t    o    r    y    e    q    u     i    p    m    e    n     t

     +     +

     +

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    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

     +

     +     +     +

     +     +

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     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

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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.

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