An automated spectrophotometric system for monitoring buffer capacity in anaerobic digestion processes

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  • Water Research 38 (2004) 364

    mro

    sch,

    Engin

    ised f

    Anaerobic biological treatment of wastewater has

    degradation chain. This implies that unfavourable

    conditions such as organic overload, unfavourable pH

    ity problem. Upon instability, the loading to the process

    ARTICLE IN PRESSmay have to be stopped for prolonged periods of time to

    facilitate degradation of the VFAs and recovery of the

    metabolic balance in the reactor. In severe cases the

    instability may cause acidication of the reactor with

    complete inhibition of the methanogenic activity. Such a

    *Corresponding author. Tel.: +46-46-222-8264; fax: +46-

    46-222-4713.

    E-mail address: bo.mattiasson@biotek.lu.se

    (B. Mattiasson).

    0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.wseveral advantages over aerobic treatment processes

    such as lower sludge production and a valuable by-

    product; methane. Despite these obvious advantages,

    the use of anaerobic treatment is not as widespread as it

    could be and one reason for this is its reputation of

    instability during start-up and operation.

    The anaerobic biodegradation of organic matter to

    biogas is conducted by a consortium of microbial groups

    with a high degree of interdependence. The methano-

    genic organisms (pH optimum: 6.5 to 7.5) are considered

    to be the most sensitive organisms in this consortium

    or presence of toxic compounds will cause instability

    and be observed as elevated concentrations of metabo-

    lites in the metabolic chain preceding the action of the

    methanogens. One group of metabolic intermediates

    that is especially of high signicance is the volatile

    fatty acids (VFAs). Increased concentration of VFAs

    may lead to a decrease in the buffering bicarbonate

    concentration, as the bicarbonate becomes protonated

    and is released as carbondioxide gas. The pH of the

    reactor may decrease to levels below the optimal for the

    methanogenic organisms, thereby escalating the instabil-useful by-product. A common way of preventing instability problems and avoiding acidication in anaerobic digesters

    is to keep the organic load to the digester far below its maximum capacity. An improved way of operating digesters

    would be to use monitoring and control systems for increased organic load under controlled conditions such that the

    digester performance is improved. The partial alkalinity, which indicates the bicarbonate concentration, has in many

    cases been found to be a suitable parameter to monitor. Here, an automated monitoring system for alkalinity

    measurements is described. It is shown to be applicable for measuring a wide range of bicarbonate concentrations. The

    system shows potential for monitoring anaerobic digesters as it responds to the alkalinity of digester efuent, as well as

    being stable over a relatively long time span with few maintenance requirements.

    r 2004 Elsevier Ltd. All rights reserved.

    Keywords: Automated monitoring; Buffer capacity; Alkalinity; Spectrophotometric; Anaerobic digestion

    1. Introduction and are also the slowest growing organisms in theAnaerobic digestion is a suitable method for the treatment of wastewater and organic wastes, yielding biogas as aAn automated spectrophotobuffer capacity in anae

    Tor Gunnar Jant

    Department of Biotechnology, Center for Chemistry and Chemical

    Received 7 October 2003; received in rev

    Abstractatres.2004.05.01053650

    etric system for monitoringbic digestion processes

    Bo Mattiasson*

    eering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

    orm 7 May 2004; accepted 12 May 2004

  • and the acid is compared to mixing ratios of the

    ARTICLE IN PRESST.G. Jantsch, B. Mattiasson / Water Research 38 (2004) 364536503646situation makes it necessary to remove the reactor

    content and to start up the reactor again.

    A common way of avoiding instability problems is to

    operate the anaerobic digestion process at an organic

    loading rate which is far below the maximum capacity of

    the system. A more economically favourable way of

    operating a digester and avoiding instability problems

    would be to monitor and control the process. An ideal

    monitoring and control system would detect instability

    and imply countermeasures to compensate for the

    instability. The monitoring system should be online,

    automated, robust and give early indications of in-

    stability in the process. Some process indicators, which

    traditionally have been used for monitoring, are the gas

    production rate, gas composition, pH, alkalinity and

    concentrations of VFAs. Liquid-phase parameters (pH,

    alkalinity, VFA) reect the environment of the micro-

    organisms and therefore, often give a faster response

    than gas-phase parameters (gas composition and pro-

    duction rate). pH is commonly used as a process

    indicator, but the effectiveness of using this as a control

    parameter is strongly dependent on the alkalinity, i.e.

    buffering capacity of the process (Ahring et al., 1995).

    The alkalinity is mainly dependent on the bicarbonate

    and VFA-concentrations (log[acid dissociation con-stant] (pKa) values of 6.35 and 4.75, respectively), and in

    some processes the ammonium concentration (pKa-

    value of 9.4). The most important buffering species

    within the optimal pH for the methanogenic organisms

    is the bicarbonate and therefore it is of interest to

    develop methods to monitor it.

    The total alkalinity (TA) (alkalinity to a pH of 4.3)

    has been used for monitoring anaerobic processes but is

    considered as an insensitive indicator of process

    instability. As shown by the pKa-values of bicarbonate

    and VFAs, measurements of TA will reect both the

    levels of VFAs and bicarbonate. Upon instability, the

    increase in VFA concentrations will cause a decrease in

    bicarbonate concentration resulting in a constant TA-

    value (Jenkins et al., 1991). Partial alkalinity (PA)

    (alkalinity to a pH of 5.75) reects mostly the

    bicarbonate concentration and has been found to be a

    valuable tool for process monitoring (Bjornsson, 2000).

    More sophisticated titration techniques have been

    Nomenclature

    Abbreviations

    VFA volatile fatty acidsdeveloped for the determination of VFA and bicarbo-

    nate concentrations (Moosbrugger et al., 1993). On-line

    methods for monitoring of bicarbonate concentrations

    have been developed based on sample saturation with

    carbon dioxide and acidication to release bicarbonate

    as carbon dioxide with subsequent gas-pressure orstandard samples and used to determine the PA of

    the sample. Upon changes in bicarbonate concentrations

    of the samples, this is reected in a change in the mixing

    ratio between the sample and the acid.

    The monitoring system was based on a method

    described by Jantsch and Mattiasson (2003) and is

    schematically presented in Fig. 1. The spectrophot-

    ometer and the pumps were controlled and data were

    collected by Fieldpoint modules (National Instruments,

    USA) connected via ethernet to a PC. The PC program

    for control, data collection and data handling was

    developed in the Labview program (National Instru-

    ments, USA).gas-ow determination (Hawkes et al., 1993; Rozzi

    and DiPinto, 1994). Also online methods based on other

    principles such as titration have been developed (Powell

    and Archer, 1989; Almeida et al., 2001).

    Previously, a new spectrophotometric method for

    alkalinity measurements based on pH indicators has

    been used for process monitoring of different anaerobic

    digester systems (Bjornsson et al., 2001; Jantsch and

    Mattiasson, 2003). The aim of this investigation was to

    construct and characterise a fully automated system for

    monitoring of anaerobic digestion processes based on

    the new spectrophotometric method.

    2. Materials and methods

    2.1. Titrimetric alkalinity

    The titrimetric alkalinity was measured as PA

    by titration to pH 5.75 with standardised 0.1M HCl

    by using a TitraLabTM 80 titrator (Radiometer,

    Copenhagen, Denmark). The results are reported as g

    CaCO3 11.

    2.2. Automated alkalinity monitoring system

    The principle for the monitoring system is that a pH

    indicator is used to spectrometrically determine the pH

    of a mixture of the sample and an acid. When the pH of

    the mixture is 5.75, the mixing ratio between the sample

    pKa log[acid dissociation constant]TA total alkalinity

    PA partial alkalinityThe solutions of sample and acid were delivered by

    peristaltic pumps (Alitea, Sweden) tted with Tygon

    tubings. The peristaltic pump caused uctuations of the

    ow and these uctuations were dampened by an air

    lled syringe tted into the line just after the pump.

    Teon tubing (outer diameter lmm, inner diameter

  • The response of the monitoring system towards

    ARTICLE IN PRESST.G. Jantsch, B. Mattiasson / Water Research 38 (2004) 36453650 36470.75mm) was used for connecting tubes and mixing coil.

    The conuence point was constructed of a Teon

    polymer Tee (Scantech, Sweden). The tube connections

    were made of silicone rubber tubing. An Ultrospec 1000

    spectrophotometer (Pharmacia Biotech, Sweden)

    equipped with a SOG 1.0 ow cell (Starna) was used

    for detection.

    A ow of the sample was continuously mixed with a

    ow of degassed acid (20 or 25mM HCl) containing

    methylred pH-indicator (5mg l1). The mixture was

    pumped continuously through the ow cell in the

    spectrophotometer.

    Determination of PA by the system was based on the

    measurement of the spectral properties of the indicator,

    that is, the absorbance of the yellow and red colours of

    the pH indicator Methylred. The absorbance maxima

    for the protonated (red) and unprotonated (yellow) were

    438 and 516 nm, respectively, and these were used as the

    wavelengths for measurement.

    For each wavelength the absorbance was monitored

    for l50 s (ve measurements per second) as the mixture

    owed through the ow cell. The absorbance value was

    averaged for the 150 s. The total measurement time for

    each sample was two times 150 s; 5min.

    The ratio between the absorbances at the two

    wavelengths was used to indicate the pH of the mixture.

    A ratio of 2.30 indicated a pH of 5.75 in the mixture.

    The ratios 2.20 and 2.40 indicated pHs of 5.7 and 5.9,

    respectively. For a measurement to be valid, a pH range

    of the mixture of 5.7 to 5.9 was set. If the pH of the

    mixture went outside this range, the measurement was

    considered to be invalid because it deviated too far from

    WasteMixing coil Spectro-

    photometerSample

    Acid

    Pump

    Pump

    PC

    Fig. 1. Automated system for monitoring buffering capacity.the desired value of 5.75. The mixing ratio was recorded

    as sensor response.

    To make the pH-value approach the desired pH, the

    mixing ratio for the subsequent sample was adjusted by

    controlling the speed of the pumps. The size of

    adjustment depended on the deviation from the desired

    pH-value.

    pH pKaHCO3 =H2CO3: 1

    From Eq. (1), the ratio HCO3/H2CO3 can be calculated.

    For the ideal mixture of sample and acid, the pH is

    5.75 which gives a HCO3/H2CO3-ratio of 0.251. At thesamples of Na2CO3 (5, 10, 15, 20 and 25mM) was

    tested.

    Anaerobic digester samples were ltered online from

    a full-scale anaerobic digester treating municipal

    sludge. The lter was a nylon cloth (gap size: 20 mm)inserted into a pipeline where the reactor content were

    circulated from the bottom of the reactor, through a

    heat exchanger and to the top of the reactor. The

    samples were diluted with distilled water or spiked with

    Na2CO3-solution and measured using the monitoring

    system.

    The stability of the system during a 5-day period was

    investigated. Different standard samples were monitored

    continuously in the system during the period.

    The system was applied to monitor a full-scale

    anaerobic digester. The feed to the digester consisted

    of a mixture of municipal wastewater sludge and potato-

    processing waste. An overload situation was induced by

    overloading the digester with a pulse of potato-proces-

    sing waste that corresponded to 20% of the normal daily

    load of the digester (Jantsch, 2003).

    3. Results and discussion

    The linear relationship between the concentration of

    Na2CO3 in standard solutions and the response of the

    monitoring system is illustrated in Fig. 2. In the

    concentration range 525mM Na2CO3 the monitoring

    system response had a regression analysis correlation

    coefcient (R2) of 0.9965. The resolution of the system

    was dened as two times the standard deviation for the

    sample series that had the highest standard deviation,

    and when inserted in the regression analysis linearity

    formulae, this gave a resolution of 1.44mM Na2CO3.

    Guwy et al. (1994) found the range 5 to 50mM

    NaHCO3 with an accuracy of 7.5% (0.383.8mM) in

    a response time of 30min in an automated system based

    on gas ow measurements from an acidied sample.same mixing ratio, the pH of the mixture will decrease to

    5.7 if the bicarbonate concentration of the sample

    decreases by 2.5%. Likewise, at the same mixing ratio

    the pH will increase to 5.9 if the bicarbonate concentra-

    tion of the sample increases by 8.3%. This implies that if

    the bicarbonate concentration in the sample is decreas-

    ing by 2.5% or increasing by 8.3% before the next

    sample is taken, the next measurement will be valid, that

    is, within the range. If the changes are higher than the

    limits, then the measurement will be out of range and the

    system will require one or more measurements with

    adjustment of the mixing ratio for the pH of the mixture

    to be within the range again.

    2.3. Characterisation of the system

  • ARTICLE IN PRESS

    0

    2

    4

    6

    Titrimetric partial alkalinity [g CaCO3 l-1]

    Mon

    itorin

    g

    0 1 2 3 4

    Fig. 3. Monitoring system response as a function of titrimetric

    partial alkalinity of diluted and spiked digester efuent sample

    with linear regression analysis (y 5:4068x 1:0508;R2 0:9863). X-error bars depict 95% condence intervalsfor titrimetric measurements in duplicates; y-error bars depict

    95% condence interval for system response in triplicates.

    6

    8

    10

    12

    ing

    syst

    em re

    spon

    se

    10 15

    20 25

    30

    T.G. Jantsch, B. Mattiasson / Water Research 38 (2004) 364536503648In this system the monitored range of alkalinity of the

    sample was dependent on the concentration of the acid

    titrant. By changing the concentration of the acid

    titrant, the range of sample alkalinity could be shifted,

    thereby expanding the range where the system is useful.

    This can also be used to make the concentration of the

    digester sample more or less diluted in the mixture. By

    diluting the sample in acid and using the ratio between

    the absorbance at two wavelengths, the inuence of the

    intrinsic absorbance of the digester sample is minimised.

    0

    2

    4

    6

    8

    10

    12

    14

    0 5 10 15 20 25 30

    Na2CO3 [mM]

    Mon

    itorin

    g sy

    stem

    resp

    onse

    Fig. 2. Monitoring system response as a function of Na2CO3concentration in standard solutions with linear regression

    analysis (y 05008x 0:3213; R2 0:9863). Error bars depict95% condence interval for the system response (3 to 7

    measurements).The linear relationship between the response of

    the monitoring system and the titrimetric PA of diluted

    or spiked samples of anaerobic digester efuent, in

    the concentration range of 1.12.8 g CaCO3 11 (11

    28mM), is illustrated in Fig. 3. A correlation study

    showed good agreement between the response of the

    monitoring system and the traditional method for

    determination of PA (R2 0:9863). Using the pre-viously dened denition of the resolution, this was

    found to be 0.290 g CaCO3 l1 (2.9mM) for the

    monitoring system (as measured by the titrimetric

    method) when inserted in the regression analysis

    linearity formulae. The resolution for the titrimetric

    method was 0.152 g CaCO3 l1 (1.52mM) (dened as

    previously).

    In a system based on acidication and gas ow

    measurements, when measuring diluted or spiked

    samples of reactor efuent, the response time was 1 to

    1.5 h to changes in sample bicarbonate concentration

    with an accuracy of 5% (0.252.5mM) in the concen-

    tration range 550mM CaCO3 (Hawkes et al., 1993).

    The long-term stability of the monitoring system is

    illustrated in Fig. 4. The system was run for 5 days8

    10

    12

    14

    16

    18

    syst

    em re

    spon

    seconstantly, sampling standard solutions of Na2CO3. The

    sensor response increased throughout the period because

    of precipitation of pH-indicator in the acid. This drift in

    the system did not affect the sensitivity of the system as

    shown by the response of the system to different

    standards throughout the period. When the standard

    solution was changed by 5mM Na2CO3, the system

    equilibrated to give a pH in the mixture within the range

    0

    2

    4

    -1Time [days]

    Mon

    itor

    30 1 2 4 5 6

    Fig. 4. Monitoring system response when running continuously

    with different standard solutions (10, 15, 20, 25 and 30mM

    Na2CO3) over a 5-day period. Order of the samples with

    number of measurements of each sample in brackets: 10mM

    (269), 15mM (355), 20mM (233), 15mM (6), 10mM (8),

    25mM (182), 20mM (3), 15mM (4), 10mM (3), 30mM (308),

    25mM (4), 20mM (3), 15mM (3), 10mM (3) and 15mM (3).

  • there was no need for any maintenance operations. This

    bicarbonate concentrations. The system is reasonably

    Acknowledgements

    The authors gratefully acknowledge the nancial

    support of the Nordic Energy Research Programme,

    The Swedish National Energy Administration, and

    Borregaard Ind. Ltd.

    References

    Ahring, B.K., Sandberg, M., Angelidaki, I., 1995. Volatile fatty

    acids as indicators of process imbalance in anaerobic

    digesters. Appl. Microbiol. Biotechnol. 43 (3), 559565.

    Almeida, C.M.N.V., Lapa, R.A.S., Lima, J.L.F.C., 2001.

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    Bjornsson, L., 2000. Intensication of the biogas process by

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    15.10. 19:12 16.10. 19:12 17.10. 19:12 18.10. 19:12

    T.G. Jantsch, B. Mattiasson / Water Research 38 (2004) 36453650 3649benet as a 5-day period without maintenance operation

    is within the limits that staff at digester plants considers

    being satisfactory (Embrandt, 2000).

    The fact that the system is not using a pH-electrode is

    regarded as advantageous. A pH-electrode may be

    subject to fouling by components in the anaerobic

    digester liquor, thereby reducing the reliability of the

    pH-readings. For a titration method with a pH probe,

    the problem of recalibration of the pH probe has been

    solved in a newly described system where the electrode is

    recalibrated periodically (Almeida et al., 2001). How-

    ever, no details regarding fouling of the pH probe was

    given. The repeatability and reproducibility of the

    method in terms of the relative standard deviance were

    2.4% and 5.1%, respectively, and the analysis time

    13min. An approach by VanVooren et al., based on an

    online wide-range titrimetric sensor, provides data in the

    ppm range of the concentration of ammonia, bicarbo-

    nate and VFAs with a response time of 30min

    (Vanrolleghem and Lee, 2003).

    Several systems have been developed for measure-

    ments of bicarbonate concentration (Hawkes et al.,

    1993; Rozzi et al., 1985), but none of these are, to the

    authors knowledge, commercially available or in

    operation at full-scale anaerobic digesters. However,

    the bicarbonate concentration can be estimated from the

    PA and this has been found to be a valuable parameter

    for anaerobic digester operation (Bjornsson, 2000;

    Jenkins et al., 1991; Ripley et al., 1986).

    When applied to the full-scale anaerobic digester, the

    monitoring system showed a stable baseline in the period

    preceding the organic pulse. Upon the pulse load, the

    system gave a clearly detectable response as shown in

    Fig. 5 (Jantsch, 2003). The full-scale experiment

    illustrates the potential for implementing the monitoring

    system in practice.

    The monitoring system described here can be applied

    to monitor buffer capacities at different pH endpoints

    by selecting a suitable indicator. The results show that

    the system can be used for monitoring the PA of

    anaerobic digester efuent samples. The system could

    be suitable for online application to any process where

    an ofine titration/titrimetric method is applicable,

    provided that a pH colour indicator with suitable

    equilibration points exists.

    4. Conclusions

    The automated monitoring system for buffering

    capacity can be used for measuring a wide range offor valid measurements in less than 40min (eight

    measurements). Upon changes in the analytealkalinity

    smaller than this, the equilibration of the system was

    faster. During the period the system was in operation,stable with time and drift in the system response can be

    accounted for by periodic measurements of standards.

    The system show potential for monitoring anaerobic

    digesters as it responds to the alkalinity of a large scale

    anaerobic process, as well as being stable over a

    relatively long time span without any maintenance

    requirements.Time (date, hours)Fig. 5. Alkalinity system response upon overload in anaerobic

    digestion system over a 3-day period. The monitoring system

    was submitted to calibration procedures during the experiment

    (Jantsch, 2003).4

    5

    6

    7

    8

    9

    10

    11

    12

    13

    Mo

    nito

    ring

    syst

    em re

    spo

    nse

    Overload

    Calibration series

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    Powell, G.E., Archer, D.B., 1989. On-line titration method

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    Ripley, L.E., Boyle, W.C., Converse, J.C., 1986. Improved

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    ARTICLE IN PRESST.G. Jantsch, B. Mattiasson / Water Research 38 (2004) 364536503650

    An automated spectrophotometric system for monitoring buffer capacity in anaerobic digestion processesIntroductionMaterials and methodsTitrimetric alkalinityAutomated alkalinity monitoring systemCharacterisation of the system

    Results and discussionConclusionsAcknowledgementsReferences

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