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 79:13311341 (online: 2004)DOI: 10.1002/jctb.1143

    ReviewTitration methodologies for monitoringof anaerobic digestion in developingcountriesa reviewO Lahav1 and BE Morgan21Faculty of Civil and Environmental Engineering, TechnionIsrael Institute of Technology, Haifa, 32000, Israel2Department of Civil Engineering, University of Cape Town, Rondebosch 7700, South Africa

    Abstract: An increase in volatile fatty acids (VFA) concentration (or the proportional decrease in carbonatealkalinity concentration) is the first practical measurable indication that an anaerobic treatment system isin a state of stress. If the system is not rectified at this early stage, failure is likely. Currentmethods for VFAmeasurement include distillation, colorimetry, gas chromatography and various titration techniques. Interms of simplicity, speed and cost-effectiveness it is generally accepted that titrationmethods are superiorfor the purpose of on-site routine monitoring and control, particularly in developing countries. This paperreviews the methods published in the last four decades concerning on-site titration measurement of VFAand carbonate alkalinity concentrations. The review encompasses the following: aquatic chemistry relatedto the theory on which most of the methods are based, and a detailed description of each of the principalmethods published followed by critical and comparative evaluation. 2004 Society of Chemical Industry

    Keywords: volatile fatty acids; titration methods; anaerobic digester monitoring; developing countries

    INTRODUCTIONStart-up and successful operation of anaerobic treat-ment facilities is a difficult and delicate process,requiring reasonably accurate and rapid monitoringtechniques. 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 inanaerobic reactors the carbonate system forms themain weak-acid system responsible for maintainingthe pH around neutral, while the VFAs (mainly acetic,propionic, and butyric acids) are the major cause for adecline, in pH.

    Under stable operating conditions, the H2 andacetic acid formed by acidogenic and acetogenicbacterial activity are utilized immediately by themethanogens and converted to methane. Conse-quently, the VFA concentration in properly runninganaerobic digesters is typically fairly stable and low(typically 0.52.0 mmol dm3),1 carbonate alkalinityis not consumed in excess and the pH is stable. Incontrast, under overload conditions or in the presenceof toxins or inhibitory substances, the activity of thesensitive methanogenic and acetogenic populationsis reduced, causing an accumulation of VFA which

    in turn increases the total acidity in the digester, thusreducing 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 subsystemspresent). The onset of reactor failure can have a spiral-ing effect on the methanogenic population,2 where thebuffering capacity cannot keep up with the increasingproduction of VFAs, causing further, and ultimately,complete, failure. The extent of the pH drop dependsprimarily on the H2CO3 alkalinity concentration. Theterm H2CO3 alkalinity is used here to define thetotal proton-accepting capacity of the carbonate weak-acid subsystem combined with the proton-acceptingcapacity of the water system3 (ie H2CO3 alkalinity =2[CO32] + [HCO3] + [OH] [H+]). In terms ofroutine monitoring, pH measurement cannot formthe sole indication of imminent failure, because inmedium or well-buffered waters high VFA concentra-tion would have to form in order to cause a detectabledrop 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 mixtureof weak-acid subsystems cannot be carried out via

    Correspondence to: O Lahav, Faculty of Civil and Environmental Engineering, TechnionIsrael Institute of Technology, Haifa, 32000,IsraelE-mail: agori@techunix.technion.ac.il(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 02682575/2004/$30.00 1331

  • O Lahav, BE Morgan

    the standard titration procedure to the H2CO3equivalence point (pH 4.5) because (i) the point isnot defined sharply and (ii) titration to 4.5 doesnot account for all the proton-accepting capacity ofthe VFA system (ie the non-protonated forms ofacetate, butyrate and propionate). Characterization ofthe carbonate subsystem can be carried out using aninorganic carbon analyzer; however, this instrument,apart from not being generally available on-site, isprone to gross inaccuracy due to CO2 loss occurringbetween sampling and measurement. Therefore, anincrease in VFA concentration is the first practicalmeasurable indication that an anaerobic treatmentsystem is in a state of stress. If the system is notrectified at this early stage, failure is likely.

    The demand for reliable VFA measurement hasincreased in recent years due to the introductionand widespread use of high-rate anaerobic treatmentprocesses, where more rigorous control is needed.4

    In addition to conventional anaerobic digesters, othertreatment systems such as biological sulfate removalreactors and hydrolysis reactors (prefermenters)depend on VFA measurement as a principal meansof monitoring reactor performance. Furthermore,anaerobic treatment of municipal sewage has gainedpopularity recently as evidenced by the increasingintroduction of full-scale UASB reactors,5 particularlyin tropical areasthe majority of which are indeveloping countries where sophisticated technologytends 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 requireexpensive equipment and a dedicated operator, andoften, in particular in developing countries, the equip-ment is not available on-site.

    Combining such factors as simplicity, speed andcost-effectiveness it is generally accepted that titrativemethods are superior for the purpose of routinemonitoring and control.6 While difficult to verify,it would appear that in developing countries thevast majority of anaerobic digesters are monitoredby various titration techniques. This also holds truefor many treatment plants in the developed world.

    During the last four decades a considerable numberof quantitative and semi-quantitative titration methodshave been proposed for the measurement of eitherVFA or H2CO3 concentrations or both. Thesetitrative methods can be roughly divided into threecategories of approaches:

    1 Approximation of VFA concentration alone orapproximation of both VFA and H2CO3 alkalinity,both by titration techniques.711

    2 Measurement of H2CO3 alkalinity only by directtitration, with or without an external measurementof VFA concentration using a different analyticalapproach.1215

    3 Accurate measurement of both VFA and H2CO3alkalinity with differing levels of complexity and

    accuracy using a titration technique followed by amathematical algorithm.1619

    In addition to these, automated, in-line methodsbased on one of the above are also found in theliterature.20,21

    The multiplicity of methods available in theliterature and the large difference in approach andin the results obtained from each method emphasizesthe need for a comprehensive review of the subject,extending the review published by Moosbruggeret al in 1993.6 The current review encompasses thefollowing: aquatic chemistry theory on which most ofthe methods are based, and a detailed description ofeach of the principal methods published followed bycritical and comparative evaluation.

    It is hoped that the review will provide assistanceto researchers, engineers, and laboratory techniciansin their quest for the most appropriate method forthe control of anaerobic processes. A list of the mainmethods covered, including characterization, ease ofexecution, and suitability for use as the monitoringtechnique is given in Table 1.

    THEORY OF WEAK-ACID SYSTEMS ANDCORRESPONDING BUFFERING INTENSITY INANAEROBIC REACTORSAll the titrative procedures proposed for VFA andH2CO3 determination stem from classical aqueoussolution weak-acid equilibrium theory. Thus, in orderto evaluate the various methods using commongrounds, a brief review of fundamental aquaticchemistry principles is given.

    Acids or bases that dissociate only partially insolution are defined as weak. The principal weak-acidsubsystems commonly found in anaerobic reactors arethe carbonate, ammonium, phosphate, VFAs (namelyacetic, propionic and butyric acids), and sulfidesubsystems. The various species of these subsystemscan be represented as a function of the total speciesconcentration of a particular weak-acid subsystemand its apparent equilibrium constant adjusted fortemperature and DebyeHuckel effects. An exampleof such representation is given below for the carbonateand VFA subsystems.

    The equilibrium and mass balance equations for thecarbonate subsystem are:

    (H+) [HCO3]/[H2CO3] = K C1 (1)(H+) [CO32]/[HCO3] = K C2 (2)CT = [H2CO3] + [HCO3] + [CO32] (3)

    Where () denotes activity, [] denotes molarity, K equals apparent equilibrium constant after adjustmentfor DebyeHuckel effects, and CT = total inorganiccarbon concentration (mol dm3).

    For the VFA subsystem (VFAs are commonlyconsidered to constitute a single weak-acid system

    1332 J Chem Technol Biotechnol 79:13311341 (online: 2004)

  • Monitoring of anaerobic digestiona review

    Tab

    le1.

    Sum

    mar

    yan

    dco

    mp

    aris

    onof

    prin

    cip

    alm

    etho

    ds

    (+=

    low

    ,+++

    ++=

    very

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    )

    Autho

    rsParam

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    mea

    sured

    Cha

    racterization

    Equ

    ipmen

    treq

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    Eas

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    urac

    ySuitability

    DiLallo

    andAlbertson

    (196

    1)7

    Pau

    sset

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    0)10

    VFA

    Rem

    oval

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    2(g)

    atpH

    3.5an

    dba

    cktitratio

    nto

    pH7

    Bas

    iclabo

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    equipm

    ent

    +++

    Gives

    very

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    imation;

    approp

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    onlyforw

    ell-b

    ufferedreac

    tors.

    McG

    hee(196

    8)8

    VFA

    Titrationbe

    twee

    npH

    5an

    d4

    Bas

    iclabo

    ratory

    equipm

    ent

    ++++

    +Gives

    thech

    ange

    inVFA

    conc

    entration

    rather

    than

    ava

    lue.

    Can

    beus

    edin

    conjun

    ctionwith

    amoreac

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    metho

    d.

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    al(198

    6)9

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    inset

    al(198

    3)13

    VFA

    andH2CO

    3

    alka

    linity

    Titrationto

    pH5.75

    asindica

    tionof

    H2CO

    3alka

    linity

    andbe

    twee

    npH

    5.75

    and4.3forVFA

    determ

    ination

    Bas

    iclabo

    ratory

    equipm

    ent

    ++++

    ++Can

    give

    good

    indica

    tionof

    tren

    ds,

    espe

    ciallyin

    theca

    seswhe

    ntheratio

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    3alka

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    toVFA

    conc

    entrations

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    than

    10to

    1.

    Mun

    chan

    dGreen

    field

    (199

    8)11

    VFA

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    torpH

    andVFA

    conc

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    pHmeter

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    +++

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    ters

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    dhigh

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    .Roz

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    andsu

    lfide

    analysis

    J Chem Technol Biotechnol 79:13311341 (online: 2004) 1333

  • O Lahav, BE Morgan

    with equilibrium constant K a because of the similarityof their pK values):6

    (H+) [A]/[HA] = K a (4)AT = [HA] + [A] (5)

    where: AT = total VFA species concentration (moldm3), HA represents the acidic, protonated speciesand A the ionized form of each acid.

    Representing the individual species of the carbonateand VFA subsystems as a function of CT, AT, andequilibrium constants:

    [H2CO3] = CT/{1 +K C1/(H+)+K C1 K C2/((H+))2} (6)

    [HCO3] = CT/{1 +K C2/(H+) + (H+)/K C1} (7)[CO32] = CT K C2/{(H+) + K C2

    + ((H+))2/K C1} (8)[A] = AT K a/{(H+) +Ka} (9)[HA] = AT/{1 + Ka/(H+)} (10)

    Similar equations can be developed for the phos-phate, sulfide, and ammonium proton-acceptingspecies.

    Buffer intensityThe buffering contribution of each subsystem can becalculated through a parameter called buffer intensity,defined as the slope of a titration curve plotted fromthe cumulative mass of strong acid (or base) added toa sample vs the change in pH:

    = dMa/dpH = dMb/dpH (11)

    where:

    Ma,Mb = concentration of strong acid or strong base,respectively, added to 1 dm3 of solution(mol dm3 solution).

    = buffer intensity index (mol dm3 solution/pH).

    The equation for the calculation of the buffer inten-sity index for monoprotic weak-acid subsystems andfor diprotic weak-acid subsystems with dissociationconstants differing by four pH units or more is givenby the following term:22

    = 2.303 [ATK a(H+)]/[K a + (H+)]2 (12)

    For the water subsystem the buffer intensity index isgiven by:

    = 2.303{(H+) + K W/(H+)} (13)

    The overall buffer intensity index of a solutioncomposed of a number of weak-acid subsystems isthe sum of the buffer intensities of all the weak-acidsubsystems including the water subsystem.

    pHlog species and pHbuffer intensity indexdiagramsUsing equations describing individual weak-acidspecies concentration (such as eqns (6)(10)) andbuffer intensities (eqns (12) and (13)), pHlogspecies and pHbuffer intensity index diagramscan be plotted. In Fig 1 an example of such plot isgiven for a typical species concentration distributionencountered in anaerobic digesters. Using Fig 1 as itapplies to titration methods, a number of points canbe made:

    1E-251E-231E-211E-191E-171E-151E-131E-111E-091E-071E-05

    0.0010.1

    0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

    pH

    Log

    Spec

    ies

    0

    0.005

    0.01

    0.015

    0.02

    0.025

    0.03

    Buffe

    r int

    ensit

    y(m

    ol/(l p

    H))

    HCO3

    H3PO4

    PO43

    CO32

    HAcNH3

    H2CO3HPO4

    2H2PO4

    Ac

    Actual bufferintensity

    Carbonatebuffer intensity

    VFA bufferintensity

    Figure 1. pHlog species and buffer intensity index diagrams in a typical anaerobic digestion sample (CT = 1000 mg dm3 as CaCO3,VFA = 100 mg dm3 as HAc, total phosphate concentration (PT) = 50 mg dm3 as P, total sulfide concentration (ST) = 20 mg dm3 as S, and totalaqueous ammonium concentration (NT) = 50 mg dm3 as N, temperature = 22 C, TDS = 3000 mg dm3). Actual buffer intensity is the sum ofbuffer intensity curves of all subsystems.

    1334 J Chem Technol Biotechnol 79:13311341 (online: 2004)

  • Monitoring of anaerobic digestiona review

    The magnitude of the (cumulative) curve at aparticular pH specifies the buffering capacity ofthe solution at that pH or in other words, its abilityto minimize a change in pH when strong acid orstrong base is added. For a titration between anytwo pH points, the area under the buffer intensitycurve equals the mass of strong acid (or base) thathas to be added to bring about the pH change.

    For a particular weak-acid subsystem, the bufferingintensity is maximal at the pK (pK = a pH valuewhere two species of a weak-acid subsystem areequal in concentration). On either side of the pK thebuffer intensity decreases sharply, becoming only1% of its maximal value within two pH units.

    The shape of the actual buffer intensity curve (iethe cumulative curve) depends on both the con-centrations and the respective apparent dissociationconstants of the weak-acid subsystems present in thewater. At the normal pH range maintained in anaer-obic reactors, the carbonate, sulfide and phosphatesubsystems can have a significant contribution tothe cumulative curve as their respective pK valuesare close to the range 6.7 > pH pH

  • O Lahav, BE Morgan

    was to develop a reasonably accurate method forVFA determination using only equipment normallyfound in a treatment plant laboratory in order todecrease the time to obtain results and to gain betterreactor control. The authors developed a techniqueaimed at detecting the change in VFA concentrationsrather than measuring accurately their absolute value.Their fundamental idea was to circumvent the overlapbetween the buffer intensity curves of the carbonateand VFA systems by removing the inorganic carbonconcentration as CO2, thus isolating the VFA systemso that it can be measured directly through titration.Total alkalinity in their method was defined asthe proton-accepting capacity of the solution titrateddown to pH 4, at which pH it was assumed thatall carbonate species are in the form of CO2. Afterrecording the amount of standard acid added to pH4, the pH is lowered to between 3.3 and 3.5 andthe sample is boiled lightly for 3 min to completelyremove CO2. Thereafter, the amount of standard baserequired to elevate the pH from 4 to 7 is recorded.This value is considered in the method to consist80% of the VFA alkalinity (irrespective of the VFAconcentration). Because the acid titration at this pHrange also includes a minor contribution of what istermed base alkalinity (referring to proton-acceptingcapacity of subsystems such as the phosphate andsulfide subsystems), the VFA concentration is attainedby multiplying the titration results by a factor of1.5 when the method yields a VFA concentrationabove 180 mg dm3. Below this value a factor of1.0 is used (ie no correction factor is applied). Thecarbonate alkalinity is calculated by subtracting theVFA alkalinity from the total alkalinity. The methodproposed by DiLallo and Albertson, although havingthe credit of being the first titrative method, suffersfrom a number of shortcomings. The method requiresthe compulsory addition of both standard acid andbase and the boiling of the sample, a step that tendsto be cumbersome. More importantly, several stepsin the procedure are prone to gross inaccuracy: First,boiling of the sample to remove CO2 can result in aloss of a fraction of the VFA due to stripping that willdepend on the VFA concentration and composition,and on the type of boiling. The authors suggest 3-min gentle boiling, but such procedure can hardlybe standardized. Also, an unknown volume of wateris vaporized in the boiling procedure. Second, theback-titration between pH 4 and pH 7 is assumed inthe method to incorporate 80% of the VFA alkalinity.This value is a not a bad approximation since itsmagnitude is relatively insensitive to such factors asthe VFA concentration, the composition of the acids,and the ionic strength and temperature of the samplewhich affect the dissociation equilibrium constant.However, VFA concentration is calculated in themethod directly from the titration results for valueslower than 180 mg dm3 VFA as CH3COOH (HAc)and multiplied by a factor of 1.5 above 180 mg dm3.This approach almost invariably results in a large error:

    for example, for a VFA concentration of 200 mg dm3

    as HAc with total phosphate of 150 mg dm3 asP, the approximation results in 23% error in theVFA concentration (0.016 mg dm3 instead of 0.013).Despite its faults, and considering that the procedurecan be modified to include externally measuredweak-acid subsystems (phosphate, sulfide, ammonia,etc), the method may be used to detect a largeupsurge in VFA concentrations, as intended by theauthors. Indeed, it is the most popular method inIsrael and it is also practiced in many other places.However, it should be noted that the method ispractical only where relatively large changes in VFAconcentration are not detrimental to the process,as might sometimes be the case in well-bufferedreactors.

    Pauss et al10 proposed a similar back-titrationmethod 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 pH4.54.0 and CO2(g) is removed by vacuum boiling.Subsequently, the solution is back-titrated to the ini-tial pH and the bicarbonate concentration is calculatedby 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 5and pH 4 as a simple and rapid means of estimatingVFA concentrations. The sample is titrated rapidlyto pH 5.5, a short delay is given to allow CO2 toreach equilibrium with the atmosphere, and thereafterthe titration is continued drop-wise to a pH slightlyabove 5. From this point one records the amount ofadditional acid required and pH attained. The val-ues attained are then plotted, and the reciprocal ofthe slope is calculated. The method is based on theidea 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 pHchange between 5 and 4 reflects the change in VFAconcentration.

    The author intended the approach to supplementbut not replace the more accurate methods for VFAdetermination such as chromatographic techniques.As such it has value, however, as a more generaltool the approach has several faults. First, it cannotserve as a tool for determining VFA concentrationbut simply to detect large changes in concentration.Second, even within this scope its application is limitedbecause the titration between pH 5 and 4 accountsfor only about 50% of the VFA alkalinity, and theeffect of the carbonate system on the proton-acceptingcapacity in this pH range is not negligible, especiallywhen the CT to AT ratio is high (above ten, as inmost anaerobic reactors). Furthermore, in anaerobicreactors any increase in VFA alkalinity is accompaniedby a similar decrease in carbonate alkalinity (and/orin the alkalinity of other proton-accepting speciessuch as the phosphate subsystem). As a result, anincrease in VFA concentration will not be represented

    1336 J Chem Technol Biotechnol 79:13311341 (online: 2004)

  • Monitoring of anaerobic digestiona review

    proportionally in the amount of acid added betweenpH 5 and 4.

    Ripley et al9 working on poultry manure treatment,suggested another way to monitor the biologicalstability of a high-strength anaerobic digester. Theyrecommended titration to two end points and theuse of the ratio PA (volume of strong acid requiredto titrate the solution down to pH 5.75) to IA(volume of strong acid required to titrate the solutionfrom pH 5.75 to pH 4.3) as a means of rapiddetection of possible stress (they suggested a valueexceeding 0.3 is indicative of stress). PA relatesroughly to bicarbonate alkalinity and the titrationfrom 5.75 to 4.3 (IA) approximates VFA alkalinity.The concept of titration to pH 5.75 as means ofestimating bicarbonate alkalinity was first introducedby Jenkins et al13 They claimed that 80% of thebicarbonate is converted to CO2 at pH 5.75 whileat the same pH only around 20% of the VFA willhave contributed to the alkalinity. Therefore, for thehigh alkalinity concentrations encountered in highstrength reactors, the effect of VFAs on the bicarbonatealkalinity (PA) value would be minor, even if the VFAreached high concentrations. Ripley et al9 added thattitration between 5.75 and 4.3 gives roughly the VFAconcentration, and so they assert that the ratio betweenthe two values is analogous to the ratio of VFA tocarbonate alkalinity.

    The clear advantages of this method are simplicity,cost effectiveness, and rapidity. As the ratio PAto IA is dimensionless it does not require titrantstandardization nor sample volume measurement. Onthe other hand, because only about 65% of the VFA isrepresented in the titration between 5.75 and 4.25 andin reality less than 70% of the carbonate alkalinity istitrated at pH 5.75 (value calculated for samples withTDS higher than 3000 mg dm3) the method lacksaccuracy and is somewhat insensitive to increase inVFA concentrations, especially in the case of high CTto VFA ratios.

    A concept for approximation of VFA concentra-tions in prefermenters (hydrolysis reactors), basedon pH reading only, was developed by Munch andGreenfield.11 In prefermenters, unlike typical anaer-obic reactors, the VFA are the desired products ofthe anaerobic activity and their build-up is a sign of ahealthy process. The typical operational pH range is56. The authors developed a mathematical func-tion relating the VFA concentration to pH usinga set of simplified assumptions. The value of thismethod is that without any additional work a simplepH reading from the working reactor can give a goodindication of the VFA concentration. The methodis only suitable for reactors working with high VFAconcentrations and low pH and providing that thesimplifying assumptions on which the model is basedare met. The major disadvantage of this approachappears to be the possible lack of reliable pH mea-surements emanating either from unaccounted CO2supersaturation or from other pH probe inaccuracies.

    It is therefore recommended to use the method inconjunction with (at least) a weekly analytical VFAmeasurement.

    Measurement of H2CO3 alkalinity only by directtitration, with or without an externalmeasurement of VFA concentrationTo overcome the shortcomings of using the conceptof total alkalinity (ie titration to pH 4.5) for anaerobicreactor monitoring, a number of modified alkalinityprocedures have been proposed.

    Hattingth et al23 advocated the use of an alkalinityproton accepting capacity (PAC) value titrated downto pH 6 and expressed as HCO3 alkalinity as arealistic measure of the available buffering capacity ofan anaerobic digester.

    Rozzi and Brunetti12 proposed a method wherea digester sample is saturated with CO2 (to yieldPCO2 = 1 bar) and subsequently the pH is reduced to3.7 by the addition of standard acid. Such additionof CO2 does not alter the bicarbonate alkalinity.The volume of CO2(g) released from solution atpH 3.7 is then measured by a gas meter. As theloss of CO2(g) during titration is negligible andassuming that the original CO32 concentrationat the operational pH is very low, this measuredvolume of CO2(g) is proportional to the mass ofHCO3 converted. Bicarbonate alkalinity (BA) is thusdetermined by:

    BA(in mg dm3as CaCO3) = (VCO2 Vacid)/Vsample 50 000/22.4 C) (14)

    where:

    VCO2 volume of gaseous CO2 released at pH 3.7(dm3)

    Vacid volume of standard acid from initial pH topH 3.7 (dm3)

    Ccorrection factor, which adjusts for temperature(T) and pressure (P) effects in the vessel:

    C = T0T

    P PVP0

    (T0 = 273.16 andP0 = 1.013 bar) (15)

    Results from the proposed method were comparedwith results derived from total alkalinity minus theVFA concentration (calculated via chromatographictechniques) and found to be very accurate andreproducible in the presence and absence of VFA(3%). It was therefore concluded that the initialCO2 bubbling does not cause volatile acid stripping.According to the authors, this method, in additionto the parallel method in which the CO2(g) pressurechange (rather than the volume released) is measuredfollowing titration to pH

  • O Lahav, BE Morgan

    in the procedure is independent of equilibriumconstants and pH measurements, and no problematicsimplifying assumptions are made, the method shouldgive accurate results. However, on the negative side,the procedure is relatively complex and specificequipment (pressure vessel, gas flow meter or pressuregauge, CO2 bottle) is required in addition to thestandard laboratory equipment. On a more generalnote, concern for the usage of these methods (andsimilar procedures) may stem from the very choiceof bicarbonate alkalinity as the principal controlparameter in anaerobic reactors, especially where thetotal inorganic carbon concentration is much higherthan VFA concentration. In such cases a small changein the carbonate alkalinity could mean a large increase(percentage wise) in VFA concentration. Such smallchange may fall within the accuracy of the method andgo unnoticed, detracting from the effectiveness of theconcept.

    Accurate measurement of both VFA and H2CO3

    alkalinity with differing levels of complexity andaccuracyThis group of methods is composed of more complextitrative methods typically requiring several titrationpoints and computerized data interpretation.

    Colin15 suggested an automated method based onacid and base titration to three endpoints. In thismethod, after measurement of the initial pH, thesample is divided in two. One part is acid-titratedto pH 2 and the other is base-titrated to pH >10.The sample that was acid-titrated to pH 2 was furtherbase-titrated up to pH >10. This procedure yieldsthree pairs of pH and Vx (acid dosage). Using thesethree pairs and the initial pH, CT, AT and NT(total ammonium concentration) are obtained usingequilibrium equations and a computer program. Theauthor reports good accuracy as compared with VFAand CT values determined by analytical methods.However, proper evaluation of the advantages anddisadvantages of the method was not possible becausethe method lacks a sufficiently detailed description.

    A different empirical-theoretical approach extend-ing the method suggested by McGhee was developedby 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 thisapproach but considered the carbonate subsystem toalso have PAC in the pH range between pH 5 and4, neglecting the sulfide, phosphate, and ammoniumsubsystems. Accordingly, the following equality holds:

    VA54, VFA = VA54, measured VA54, H2CO3 (16)where:

    VA54, VFA = volume of acid required to titratefrom pH 5 to pH 4 due to the VFAPAC

    VA54, measured = volume of acid required to titratefrom pH 5 to 4

    VA54, H2CO3 = volume of acid required to titratefrom pH 5 to pH 4 due to thecarbonate PAC

    To develop an explicit and simple mathe-matical expression linking VFA concentration toVA54, measured Kapp conducted titration experimentsfor a broad VFA and CT concentration range andderived the following empiric equation linking volumeof titrant to VFA and H2CO3 alkalinity concentra-tions:

    VA54, VFA = 0.1/N (0.0283 + 0.09418 VFA/60) Vs/20 (17)

    VA54, H2CO3 = 0.005 (0.044875 + 0.00469 [Alkmeasured]) Vs/N (18)

    where:

    N = titrant concentration (eq dm3)Vs = volume of sample (cm3)

    [Alkmeasured] = total PAC as titrated to pH = 4.3 (eqdm3)

    Insertion of eqns (17) and (18) into eqn (16) andrearranging yields Kapps first approximation:

    [VFA] = 127 416 N VA54, measured/Vs 2.99 [Alkmeasured] 10.6 (19)

    where:[VFA] = VFA concentration (mg dm3 as HAc)Using a further two assumptions, eqn (19) trans-

    forms slightly yielding:

    [VFA] = 131 340 N VA54, measured/Vs 3.08 [Alkmeasured] 10.9 (20)

    Kapps approach involves three pH titration setpoints (pH 5.0, 4.3 and 4.0), in addition to the initialpH. Working on samples of digested sludge, Kappreported an accuracy of 10% for VFA concentrationsabove 20 mg dm3 as HAc. Baucher reported a similaraccuracy for samples of raw wastewater, primarysludge, and high- and low-load activated sludge.25

    The major drawback of Kapps approach is thatit is based on empirical mathematical relationshipsthat were developed under unique conditions (ionicstrength, temperature, absence of other weak-acidsystems) that are not necessarily generally applicable.Its advantage stems from its relative simplicitytheVFA concentration is calculated using a singleequation, the apparatus needed is simple andlaboratory execution is easy and quick. The method isalso suitable for automated execution.

    The most general method to date was presentedby Moosbrugger et al17,18 The authors developeda model based almost solely on aquatic chemistryconsiderations, with very few simplifying assumptions.

    1338 J Chem Technol Biotechnol 79:13311341 (online: 2004)

  • Monitoring of anaerobic digestiona review

    Because of its importance the method is described indetail.

    The authors developed a five-point method involv-ing equating a mass balance for alkalinity in terms ofvolume of titrant added (eqn (21)) to a mass balance ofalkalinity in terms of species concentration (eqn (22)).

    Mtotal alk(x) = Ve Ca Vx Ca (21)where: Mtotal alk(x) = total mass of alkalinity after theaddition of Vx cm3 of standard strong acid (mol),Ve = the unknown volume of standard strong acid tobe added to the alkalimetric end point (dm3), Vx = thevolume of standard strong acid added to a point x withpH equal to pHx (dm

    3), and Ca = concentration ofstandard strong acid titrant (mol dm3).

    Mtotal alk(x) = {[HCO3]x + 2[CO32]x + [A]x+ [HS]x + 2[S2]x + [NH3]x + 3[PO43]x+ 2[HPO42]x + [H2PO4]x+ [OH]x [H+]x} (Vx + Vs) (22)

    Where [y]x indicates molar concentration of species yafter addition of x cm3 of standard acid (mol dm3),[A] = dissociated short chain VFA species concen-tration (mol dm3) and Vs = volume of sample (dm3).

    Equation (22) can be reformulated in terms of totalweak-acid species concentrations using equilibriumequations for the weak-acid systems and mass balanceequations for each of the weak-acid systems asrepresented in eqns (23)(25) (For brevity only theVFA and carbonate subsystems are given. The othersubsystems follow the same approach.)

    [HCO3]x = CT Vs/(Vx + Vs)/{1 +K C2/(H+)x+ (H+)x/K C1} (23)

    [CO32]x = CT Vs/(Vx + Vs) K C2/{(H+)x +K C2+ ((H+)x)2/K C1} (24)

    [A]x = AT Vs/(Vx + Vs) K a/{(H+)x+ K a} (25)

    Similar equations can be developed for thephosphate, sulfide and ammonium proton-acceptingspecies. Substituting the equations for each speciesconcentration into eqn (22) (for example as given ineqns (23)(25) for the carbonate and VFA subsys-tems) gives an equation for total mass of alkalinity interms of AT, CT, PT, NT, ST and pH:

    Mtotal alk(x) = {CT Vs/(Vs + Vx) Fn1(pH)x+ AT Vs/(Vs + Vx) Fn2(pH)x+ PT Vs/(Vs + Vx) Fn3(pH)x+ ST Vs/(Vs + Vx) Fn4(pH)x+NT Vs/(Vs + Vx) Fn5(pH)x + 10(14pHx)/fm 10pHx/fm} (Vs + Vx) (26)

    where: PT, ST, and NT represent the total phos-phate, sulfide and ammonium concentrations, fm =monovalent activity coefficient, and Fn1 to Fn5 arefunctions of pHx and equilibrium constants for thecarbonate, acetate, phosphate, sulfide and ammoniumsubsystems respectively.

    Equating eqns (21) and (26) gives the desiredequation linking the mass of alkalinity based on acidadded to the mass of alkalinity based on speciesconcentrations:

    (Ve Vx) Ca = {CT Vs/(Vs + Vx) Fn1(pH)x+ AT Vs/(Vs + Vx) Fn2(pH)x+ PT Vs/(Vs + Vx) Fn3(pH)x+ ST Vs/(Vs + Vx) Fn4(pH)x+NT Vs/(Vs + Vx) Fn5(pH)x+ 10(14pHx)/fm 10pHx/fm} (Vs + Vx) (27)

    At each point in the titration (ie for each Vxand corresponding pHx), eqn (27) includes threeunknowns: Ve, AT and CT. Thus, to solve for Ve,AT and CT only three data pairs (ie three valuesfor corresponding Vx and pHx pairs) need to beknown. This was found to lead to poor prediction.Moosbrugger et al found that the best first estimatefor AT and CT and can be obtained from four titrationdata points (ie two pairs of data points), each pairsymmetrical about pKC1 and pKa (they suggestedapproximately half a pH unit to either side of therespective pK values).17

    When inserted into eqn (27), the data from thefour titration points give four equations. The pair ofobservations around pKa (ie the third and the fourthpoints) is in a region where the buffer capacity of theVFA system dominates that of the carbonate systemand vice versa for the first and second titration points.Consequently, subtracting the equation formed fromthe fourth data point from that derived from thethird, gives an equation in terms of CT and AT inwhich the VFA alkalinity term, ie the species [A]x,dominates. Similarly, subtracting the equation formedfrom the second data point from that derived fromthe first, gives an equation in terms of CT and AT inwhich the H2CO3 alkalinity term, namely [HCO3]x,significantly dominates. This technique enables arelative separation between the two subsystems inwhich the third and the fourth point are mainlyresponsible for the VFA derivation. Therefore, anerror in the first two pH observations would be largelyabsorbed by the carbonate subsystem, minimizing theeffect on the VFA calculation. The two new equationsare solved to produce the first estimate of AT and CT.

    Modification of the first estimate in this approachis carried out as follows: a second estimate of ATand CT is calculated by again taking two pH pairs:one symmetrical about pKa (ie pH3; pH4) butthe other asymmetrical about pKC1 (ie pH1; pH4).Subsequently, these two CT values (calculated in

    J Chem Technol Biotechnol 79:13311341 (online: 2004) 1339

  • O Lahav, BE Morgan

    the first and second estimate) are compared, and ifdifferent, all pH observations are adjusted by pHand the calculation procedure is repeated (by changingpH) until the difference between the two CT valuesis negligible. The DOS computer program requiredfor this method can be purchased from the WaterResearch Commission of South Africa at a cost ofapproximately $50.

    The necessity for correcting observed pH values wasattributed by the authors to either a residual liquidjunction potential (error in pH measurements causedby differences in the dissolved solids concentrationbetween the buffer solution used to calibrate theprobe and the test solution), or from poor pH metercalibration.18 Execution of the five-point method isfacilitated by a computer program supplied by theauthors. The method was tested with results typicallywithin 2% accuracy.18,26,27 Application of the methodis restricted to cases where the CT is greater than twiceAT, otherwise the systematic pH correction does notconverge.

    As mentioned above, the five-point method is themost ambitious effort thus far to include all systemcomponents in a single sophisticated, yet simple model(titration to five points is carried out rapidly and withease, because the selected pH points do not have tobe reached accurately and the selected pH areas arevery stable). However, the final step (correction of pHobservations) tends to detract from the theoreticaljustification of the method, and from its generalapplicability. With regard to errors emanating fromresidual liquid junction potential: the total dissolvedsolids concentrations in the samples tested in the five-point approach varied between 500 and 1000 mg dm3

    (after dilution), which is very close to the dissolvedsolids concentration in standard buffer solutions. Forthis reason and others it is very difficult to ascribea systematic pH error correction to liquid junctioneffects.27

    Lahav et al proposed a new eight-point titrationmethod for measuring both VFA and H2CO3

    alkalinity.19 As in the five-point method, executionis facilitated by a computer program (available freeof charge from the author). The model extends thefive-point method by resolving the mathematical andanalytical problems which gave rise to the systematicpH error of the Moosbrugger approach. In thismodification, total alkalinity (PAC of all species inthe sample) is measured accurately using the Grantitration technique,28 and this value is used, in additionto the first estimate of AT and CT, to give the finalresult. The Gran procedure requires a further threepairs of (Vx, pH) points taken in the pH range of2.4 < pH

  • Monitoring of anaerobic digestiona review

    highly accurate. It appears that there is little reason tochoose a more rudimentary approach with the currentavailability of relatively inexpensive computerizedand programmable titration equipment. However,even the more simple methods can be appliedsuccessfully provided that the operators understandthe assumptions and simplifications behind themethod, and provided that the accuracy level is suchthat it allows for proper detection of changes in VFAconcentrations and rapid intervention.

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    John Wiley & Sons Ltd, London (1994).2 Florencio L, Field JA and Lettinga G, Substrate competition

    between methanogens and acetogens during the degradationof methanol in UASB. Wat Res 29:915922 (1995).

    3 Loewenthal RE, Wentzel MC, Ekama GA and Marais GvR,Mixed weak acid systems. Part 2: Dosing estimation, aqueousphase. Water SA 17:107122 (1991).

    4 Pretorious WA, pH controlled feed-on-demand for high rateanaerobic systems. Wat Sci Tech 30:18 (1994).

    5 Seghezzo L, Zeeamn B, van Lier JB, Hamelers HVM andLettinga G, A review: the anaerobic treatment of sewage inUASB and EGSB reactors.Bioresource Technology 65:175190(1998).

    6 Moosbrugger RE, Wentzel MC, Ekama GA and Marais GvR,Weak acid/bases and pH control in anaerobic systemsareview. Water SA 19:110 (1993).

    7 DiLallo R and Albertson OE, Volatile acids by direct titration.Journal WPCF 33:356365 (1961).

    8 McGhee J, A method for approximation of volatile acidconcentrations in anaerobic digesters. Water & Sewage Works115:162166 (1968).

    9 Ripley LE, Boyle WC and Converse JC, Improved alkalimetricmonitoring for anaerobic digestion of high strength wastes.Journal WPCF 58:406411 (1986).

    10 Pauss A, Rozzi A, Ledrut MJ, Naveau H and Nyns EJ, Bicar-bonate determination in complex acidbase solutions by aback-titration method. Environmental Technology 11:469476(1990).

    11 Munch E and Greenfield PF, Estimating VFA concentrationsin prefermenters by measuring pH. Wat Res 32:24312441(1998).

    12 Rozzi A and Brunetti A, Direct bicarbonate determination inanaerobic digester liquors. Environmental Technology Letters2:385392 (1981).

    13 Jenkins SR, Morgan JM and Sawyer CL, Measuring anaerobicsludge digestion and growth by a simple alkalimetric titration.Journal WPCF 55:448453 (1983).

    14 Di Pinto AC, Limoni N, Passino R, Rozzi A and Tomei MC,Anaerobic process control by bicarbonate monitoring.

    Abstract, 5th 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 studyof acid-base equilibria for the control of anaerobic digestion,in Anaerobic Digestion and Carbohydrate Hydrolysis of Waste, edby Ferrero GL, Ferraut MP and Naveau H. Elsevier Science,London, pp 381393 (1984).

    16 Kapp H, Schlammfaulung mit hohem Feststoffgehalt. StuttgarterBerichte zur Siedlungswasserwirtscaft, Band 86, OldenbourgVerlag, Munchen, 300 pp (1984).

    17 Moosbrugger RE, Wentzel MC, Loewenthal RE, Ekama GAand Marais GvR, A 5-point titration method to determinethe carbonate and SCFA weak acid/bases in aqueoussolution containing also known concentrations of other weakacid/bases. Water SA 19:2939 (1993).

    18 Moosbrugger RE, Wentzel MC, Ekama GA and Marais GvR,A 5-point titration method for determining the carbonate andSCFA weak acid/bases in anaerobic systems. Wat Sci Tech28:237245 (1993).

    19 Lahav O, Morgan BE and Loewenthal RE, A rapid, simple andaccurate method for measurement of VFA and carbonatealkalinity in anaerobic reactors.Env Sci&Tech 36:27362741(2002).

    20 Powell GE and Archer DB, On-line titration method formonitoring buffer capacity and total volatile fatty acidslevels in anaerobic digesters. Biotechnology and Bioengineering33:570577 (1988).

    21 Feitkenhauer H, von Sachs J and Meyer U, On-line titrationof volatile fatty acids for the process control of anaerobicdigestion plants. Wat Res 36:212218 (2002).

    22 Loewenthal RE and Marais GvR, Carbonate chemistry of highsalinity waters. Water Research Commission SA, ResearchReport No W46 (1983).

    23 Hattingth WHJ, Kotze JP, Thiel PG, Toerien DF andSiebert ML, Biological changes during the adaptation of ananaerobic digester to a synthetic substrate.WatRes 1:255277(1967).

    24 Rozzi A, Di Pinto AC and Brunetti A, Anaerobic process controlby bicarbonate monitoring. Environmental Technology Letters6:594601 (1985).

    25 Baucher K, A comparison of two simple titration procedures todetermine volatile fatty acids in influents to wastewater andsludge treatment processes. Water SA 24:4956 (1998).

    26 De Hass DW and Asam N, Use of simple titration procedureto determine H2CO3 alkalinity and volatile fatty acidsfor process control in wastewater treatment. Water SA21:307318 (1995).

    27 Lahav O and Loewenthal RE, Measurement of VFA inanaerobic digestion: the five point titration method revisited.Water SA 26:389391 (2000).

    28 Gran G, Determination of the equivalence point in potentio-metric titrations. The Analyst 77:661671 (1952).

    J Chem Technol Biotechnol 79:13311341 (online: 2004) 1341

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