a simple spectrophotometric method based on ph‐indicators for monitoring partial and total...

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This article was downloaded by: [UQ Library] On: 06 November 2014, At: 05:31 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20 A simple spectrophotometric method based on pHindicators for monitoring partial and total alkalinity in anaerobic processes T. G. Jantsch a & B. Mattiasson a a Department of Biotechnology, Center for Chemistry and Chemical Engineering , Lund University , P.O. Box 124, Lund, S22100, Sweden Published online: 17 Dec 2008. To cite this article: T. G. Jantsch & B. Mattiasson (2003) A simple spectrophotometric method based on pHindicators for monitoring partial and total alkalinity in anaerobic processes, Environmental Technology, 24:9, 1061-1067, DOI: 10.1080/09593330309385646 To link to this article: http://dx.doi.org/10.1080/09593330309385646 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

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Page 1: A simple spectrophotometric method based on pH‐indicators for monitoring partial and total alkalinity in anaerobic processes

This article was downloaded by: [UQ Library]On: 06 November 2014, At: 05:31Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Environmental TechnologyPublication details, including instructions for authors and subscriptioninformation:http://www.tandfonline.com/loi/tent20

A simple spectrophotometric method basedon pH‐indicators for monitoring partial andtotal alkalinity in anaerobic processesT. G. Jantsch a & B. Mattiasson aa Department of Biotechnology, Center for Chemistry and ChemicalEngineering , Lund University , P.O. Box 124, Lund, S‐22100, SwedenPublished online: 17 Dec 2008.

To cite this article: T. G. Jantsch & B. Mattiasson (2003) A simple spectrophotometric method based onpH‐indicators for monitoring partial and total alkalinity in anaerobic processes, Environmental Technology,24:9, 1061-1067, DOI: 10.1080/09593330309385646

To link to this article: http://dx.doi.org/10.1080/09593330309385646

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and ourlicensors make no representations or warranties whatsoever as to the accuracy, completeness, orsuitability for any purpose of the Content. Any opinions and views expressed in this publicationare the opinions and views of the authors, and are not the views of or endorsed by Taylor &Francis. The accuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liable for anylosses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilitieswhatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access and usecan be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: A simple spectrophotometric method based on pH‐indicators for monitoring partial and total alkalinity in anaerobic processes

Environmental Technology, Vol. 24. pp 1061-1067©Selper Ltd, 2003

A SIMPLE SPECTROPHOTOMETRIC METHOD BASED ONpH-INDICATORS FOR MONITORING PARTIAL AND

TOTAL ALKALINITY IN ANAEROBIC PROCESSES

T. G. JANTSCH AND B. MATTIASSON*

1Department of Biotechnology, Center for Chemistry and Chemical Engineering, Lund University,P.O. Box 124, S-22100 Lund, Sweden

(Receicroed 23 May 2002; Accepted 20 February 2003)

ABSTRACT

Partial alkalinity, as a measure of bicarbonate concentration, and total alkalinity, as a measure of the concentration ofbicarbonate and volatile fatty adds, are useful parameters for monitoring anaerobic digestion processes. This paper reports anew method based on pH-indicators and spectrophotometric measurements for the monitoring of partial and totalalkalinity. The method was used in an off-line procedure for monitoring of an anaerobic process and displayed less than 5%deviation from the traditional method of measuring partial and total alkalinity, as well as being rapid and reproducible. Aflow injection analysis system based on the method was used on-line for monitoring overload in a UASB reactor, whichdemonstrated changes in alkalinity not easily registered using traditional methods.

Keywords: Anaerobic digestion, alkalinity, monitoring, pH-indicators, spectrophotometric

INTRODUCTION

In anaerobic wastewater treatment, organic matter isdegraded via a complex pathway to CH4 and CO2. In manyanaerobic processes, bicarbonate is the main compounddetermining the alkalinity of the process [1]. Alkalinity is ameans of expressing the capacity of a liquid to withstand pHchanges upon the addition of acids. The buffering capacity, oralkalinity, of an anaerobic digester is a measure of its ability towithstand a decrease in pH that may be caused by increasingconcentrations of volatile fatty acids (VFAs) or by variationsin the pH of the influent. Alkalinity has long been consideredto be an important parameter in the monitoring of anaerobicprocesses [2].

The standard method for the determination of thealkalinity of an anaerobic process is by direct titration with anacid and either the simultaneous measurement of the pH orobserving the colour change of an indicator [3]. pH indicatorsare weak acids or bases, which change or acquire colourationupon conversion from the protonated to the unprotonatedform (or vice versa). The visually detectable colour change ofan indicator lies normally within the pKa-value of theindicator +/-1 pH unit [4].

A result of organic overload of the anaerobic processcan be an increase in acidogenic activity, leading toenrichment of VF As since the rate of VFA formation is higher

than the rate at which acetogenic and methanogenicorganisms can convert the VF As to biogas. This may result ina decrease in the pH of the digester, which can causeinhibition of metabolic activity and growth of themethanogenic organisms, thus amplifying the effects of theacid overload [5, 6]. This type of overload may not bereflected by total alkalinity measurements in the earlier stagesbecause the total alkalinity, as measured by titration to pH4.3, will reflect both the bicarbonate and VFA alkalinity [7]. Amethod has been described where the sample is first titratedto pH 5.75 (Partial Alkalinity, PA) and then to 4.3 (TotalAlkalinity, TA). The ratio (PA/TA) is then used as anindicator of reactor stability [8]. Bicarbonate has a pK,-valueof 6.3 at 25°C [9] and at pH 5.75 78% will be protonated, whileat pH 4.3 99% will be present as CO2 (according to acid-baseequilibrium). VFAs have pKa-values between 4.7 and 4.9 [10]and at pH 5.75 only 10 % of the VFAs will be in theprotonated form whilst at pH 4.3 76% of the VFAs will be inthe protonated form (as calculated from acid-baseequilibrium). Therefore, changes in organic acid concentrationhave a strong effect on total alkalinity, but very little effect onpartial alkalinity when disregarding the effect of bicarbonate.However, upon increases in VFA concentrations some of thebicarbonate will become protonated and equilibrate todissolved and gaseous carbon dioxide. This implies thatpartial alkalinity is more useful than total alkalinity for

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monitoring anaerobic digestion processes.Several methods based on titration have been

developed in order to discriminate between bicarbonatealkalinity and VFA alkalinity for anaerobic processmonitoring [11, 12, 13] as well as other methods based onother principles [14]. Several on-line methods based ontitration and other principles have also been developed forprocess monitoring [15, 16, 17]. Indirect methods can be usedto measure analytes in complex samples, exemplified by abiosensor for nitrate reduction that has been used formeasuring VFA concentrations in wastewater [18]. Themethod described by Ripley et al. [8] has been shown to be avaluable tool for the monitoring of anaerobic digestionprocesses [19-22]. Limitations are that the method is highlyempirical and is only useful if the history of the reactor isknown and the influent is of a relatively stable composition.There are also limitations if high concentrations of other weakacid/base-systems are present in the solution; in addition, theprecision is poor if the bicarbonate/ VFA ratio is low [20].

Flow Injection Analysis (FIA) is a reliable andreproducible sampling technique [23], which in combinationwith the proper analysis technique can be suitable for on-linemonitoring of biotechnological processes [24].

This paper presents a simple system for themeasurement of the PA and TA in anaerobic digestionprocesses by the use of pH colour-indicators, read by aspectrophotometer. The system was first evaluated off-line inmanual batch tests on samples taken from a CSTR anaerobicdigester before it was automated in a FIA-set-up and used foron-line measurement of PA during overload in a UASBreactor.

MATERIALS AND METHODS

All chemicals used were purchased from Merck,Germany. Standards were made of sodium bicarbonateand/or sodium acetate in purified water (MilliQ). Indicatorsolutions were prepared as previously described (0.001 mgmethyl red and 0.008 mg bromophenol blue were added per 1acid) [25].

Measurement of PA and TA by Titration

The partial and total alkalinity was measured bytitration to pH 5.75 and 4.3, respectively. The titrant used was0.1 M HC1 and the results were given as mg CaCO31'1 sampleequivalents. A TitraLab™ titrator (Radiometer, Copenhagen)was used for the titration procedure.

Principle for Measurement of PA and TA by Photometry

The principle of the method is to mix a dilute acidcontaining an indicator with a sample from the anaerobicdigester to a predetermined pH-value. This pH-value is theendpoint for titration to either PA (5.75) or TA (4.3). The PA orTA is calculated from the PA or TA as determined by the

mixing ratio for standards. If the pH of the mixture is not at5.75 (when measuring PA) or 4.3 (when measuring TA) thenthe mixing ratio for the subsequent sample is adjusted tomake the pH approach the end-point. The indicators methylred (MR) and bromophenol blue (BPB) have pKa-values closeto 5.75 and 4.3 respectively, with the colour transformationoccurring within the pH-range 4.2-6.2 and 3.0-4.6 respectively.The absorbance maxima for the protonated forms are 516 runand 441 nm and for the unprotonated forms 438 run and 591ran for the MR and the BPB indicators, respectively.

Off-line Measurement of PA and TA by Photometry inManual Batch Tests

Off-line measurements of PA and TA by photometry inmanual batch tests were made on samples taken from a 3 1anaerobic digester treating waste with a varying composition.The reactor was subjected to organic overload, resulting invariations in the partial and total alkalinity. All samples werecentrifuged (6000 g, 4 min) to remove particulates whichblocked the pipette tips and thereby caused inaccuracies inthe sample volumes when the samples were transferred bypipette. Analyses of PA and TA by titration and byphotometry were performed on the supernatants. Formeasurement of the PA and TA by photometry, a specificamount of sample (0.01 ml - 0.03 ml) was added to a specificamount of HC1 (1 ml) of a known concentration (0.5 mM)containing the indicator; the absorbances were thenmeasured. The response of the indicators was measured asthe ratio of the absorbances at the maxima for the protonatedform and the unprotonated form of the indicator (qPA andqTA). Ratios of HCl-volume to sample-volume were in therange of 30 to 100. Absorbance measurements wereperformed in 1 ml plastic cuvettes (1 cm path length) on aPharmacia Biotech UltrospeclOOO spectrophotometer. Forevery measured sample a three-point standard curve wasprepared by plotting the ratio between the absorbances at themaxima for the protonated and unprotonated form of theindicator against the alkalinity (as measured by titration) ofstandard samples. Standard samples were made with PA, asmeasured by titration, in the range of 1000-2400 mg CaCO3 T

1

and TA in the range of 1700-2900 mg CaCO3 r1.

On-line Measurement of PA by Photometry in a FIA System

On-line measurements of PA by photometry with theFIA system were based on the same principle as off-linemeasurement of PA by photometry in manual batch tests withthe modification that the absorbance was measured at onewavelength.

A 0.7 1 UASB reactor was fitted with a recycle line andfed a nutrient solution with 1 g I"1 glucose. The load was 2.3 gI'1 d'1 and the temperature was kept at 37 CC by circulatingheated water through a jacket fitted around the reactor. Therecycle to feed ratio was 4.8 and the upflow velocity was 0.2m h'1. The reactor was subjected to overload by injecting 1 g of

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glucose dissolved in 10 ml of water into the feed line. Samplesfor measurement of alkalinity by titration and on-linesampling with the FIA system were taken from the recycleline. Prior to entering the FLA sample-loop the liquid passedthrough a filter (Sartorius, 0.45 urn). The filter was back-flushed with water (37 CC) every 20 minutes.

Generally, in a FIA-system a segment of sample isdriven by a carrier flow (without any mixing between thetwo) and is injected into and mixed with a flow of reagent.The FIA system described here is illustrated in Figure 1.Pumps from Alitea (Sweden) were used fitted with tygontubing. Teflon tubing (outer diameter 1 mm, inner diameter0.75 mm) was used for connection-, mixing- and sample-loop-tubing. The carrier was degassed with 0.2 M sodiumphosphate-buffer and the reagent was HC1 with MR-indicator.Segments of reactor samples or standard samples wereincorporated in the carrier stream through a six-way valvefitted with a 100 microlitres sample-loop. When the carrierstream or sample segment mixed with the reagent stream thepH of the mixture could be determined by measuring theabsorbance of the indicator. The absorbance of the indicator at438 nm was measured with a Pharmacia BiotechUltrospeclOOO spectrophotometer equipped with a flow cell.Data acquisition was conducted with a chart recorder.

The pH of the mixture of the reagent and the carrierwas 5.75 and this was also the pH of the mixture of thereagent and the samples from the reactor prior to organicoverload. This pH represented baseline readings and nochange in absorbance was registered when measuring thedifferent mixtures resulting in a stable baseline. Standardsamples were injected to evaluate the response of the system.Samples with higher or lower PA than the initial reactorsamples gave a positive or negative peak, respectively. As thePA of the reactor changed during the overload experiment the

pH of the mixture of the reagent and the sample changed. Tomaintain the pH of the mixture at 5.75, the mixing ratiobetween the carrier and reagent was changed. The mixingratio could be controlled and monitored by adjustments ofpump speed. A new sample was injected into the FIA systemevery 8 minutes and the sample residence time in the systemwas 3 minutes.

RESULTS

The indicators were chosen because in the case of thePA measurements, the desired pH of the mixture of sampleand acid/indicator was 5.75. For the TA measurements, thedesired pH was 4.3. Wavelength scans of the indicators inbuffers with different pH-values were performed in order todetermine the wavelengths where the absorbance wasmaximal for the indicator in the protonated and theunprotonated form. The absorbance maxima for theprotonated form were 516 nm and 441 nm and for theunprotonated forms 438 nm and 591 nm for the MR and theBPB indicators, respectively.

The effect of varying the concentrations of bicarbonateand acetate upon the ratio between the absorbances at themaxima for the protonated form and the unprotonated formof the indicator (qPA and qTA) was examined. For qPA, therewas a response to an increase in the bicarbonate concentrationas indicated by a steep slope, whereas the response to anincrease in acetate concentration had a low slope. For qTA, asimilar response was detected to increases in both the acetateconcentration and the bicarbonate concentration. For samplesmade of bicarbonate and acetate the PA and TA byphotometry showed better reproducibility than the PA andTA by titration as depicted by the standard deviations forthree replicated samples.

UASB reactor

Pump

Carrier

Reagent

Filter

Injectionvalve

Spectro-photometer

Mixing coil Waste

PumpFigure 1. Schematic presentation of Flow Injection Analysis system for monitoring partial alkalinity of a UASB reactor.

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Off-line Measurement of PA and TA by Photometry inManual Batch Ttests

Verification of the PA and TA by the photometricmethod was made on samples from an anaerobic reactor(CSTR) over a period of several months. The PA and TAvalues, which were measured in the CSTR system, spannedover the range of 1460-2280 mg CaCO31'1 and 1930-2888 mg

CaCO31'1 for the PA and the TA, respectively. A comparisonwas made between the PA as measured by titration and thePA as measured by photometry, the correlation coefficient forthe regression line was 0.9263 for 82 samples (Figure 2.). Wealso compared the TA as measured by titration and the TA asmeasured by photometry, for this the regression coefficientwas 0.9749 for 85 samples (Figure 3.). All the measurementsfor PA and TA by photometry fall within a limit of ± 5% of PAand TA as measured by titration.

2300

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ât

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E1

CaC

(

2100-

1900-

1700-

1500 •

13001300 1500 1700 1900 2100 2300

PA by photometry (mg CaCO31 sample"1)

Figure 2. A comparison of the Partial Alkalinity results from the titration and photometric methods, regression analysisundertaken (PA: y = 0.9498x +104.74, R2 = 0.9263) for samples from a CSTR.

2900

f- 2700

I I 2500

>, O 2300

H en 2100

1900

17001700 1900 2100 2300 2500 2700

TA by photometry (mg CaCO31 sample'1)

2900

Figure 3. A comparison of the Total Alkalinity results from the titration and photometric methods, regression analysisundertaken (TA: y = 1.0565x -151.31, R2 = 0.9749) for samples from a CSTR.

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On-line Measurement of PA by Photometry in a FIA System within 2.0 h as well as a reduction in the reactor pH (Figure4.). The FIA system was used for on-line measurements of PA

The overload in this experimental set-up gave a typical in the UASB reactor and gave a response corresponding wellresponse of a 20% decrease in the PA as measured by titration to the PA measured by titration (Figure 5.).

§ 1

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1.4

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T 7 . 3

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••7 i.

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46.710

Figure 4. The response in pH (A) and partial alkalinity by titration (•) for a UASB reactor at a constant organic loading ratewhen subjected to a pulse of glucose at time zero (error bars depict standard deviance for samples in duplicate).

1.3

1.2

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Figure 5. The response in partial alkalinity by titration (•) and partial alkalinity by FIA-photometry on-line (•) for a UASBreactor at a constant organic loading rate when subjected to a pulse of glucose at time zero.

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DISCUSSION

Titration based systems for measuring the bicarbonatealkalinity are dependent on pH probes, which may not bereliable for an extended period of time because of factors suchas fouling; a factor which may be more readily detectable in aspectrophotometric system. This might have a significanteffect on the practicality on-line systems based on the methodwith regard to the time span between maintenance operations.Other systems for measuring bicarbonate are to the authors'knowledge not commonly applied in industry, althoughindustrial prototypes have been described [1, 17].

Off-line Measurement of PA and TA by Photometry inManual Batch Tests

Samples from the reactor were centrifuged prior tomeasurement and small volumes of sample were added to theacid/indicator solution. The pre-treatment step proved to benecessary due to problems with the delivery of the correctamount of particulate sample. Interference of themeasurements due to the absorbance of the sample was not aproblem because of the small ratio of the sample toacid/indicator volume and the pre-treatment step. Analysisresults were obtained within 15 minutes after sampling thereactor. The comparison between the titration method and thephotometric method shows that there was a good correlationbetween the methods. The measured values for thephotometric method deviated less than 5% from the titrationmethod. The titration method has been evaluated to anaccuracy of ± 5% [16].

On-line Measurement of PA by Photometry in a FIA System

The FIA system could be used for monitoring the PAdecrease following an overload of a UASB reactor. Results ofthe FIA measurements were obtained 5 minutes aftersampling the reactor without optimisation of the samplingprocedure to reduce this time interval. The sample waspumped directly from the sampling point and injected intothe carrier stream with filtering as a pretreatment method.The pretreatment was necessary because of variations in theabsorbance of the reactor medium caused by the organic

overload. Backflushing of the filter with water was necessaryto prevent total clogging of the filter, but this did not interferewith the function of the FIA system. For long-term operationthe filtering procedure applied here may be disadvantageousand more sophisticated sample treatment techniques mayhave to be developed. The minimum time interval betweenmeasurements was 5 minutes. The FIA system requiredcontinuous monitoring and adjustment during themeasurements to maintain the desired pH of the mixture ofthe acid, buffer and samples. However, the system asdesigned here could be automated with appropriate controlsystems for spectrophotometer and pumps.

CONCLUSIONS

The development of a simple and rapid method for themeasurement of the partial and total alkalinity in anaerobicsystems has been described. A comparison between thephotometric method and the traditional method formeasuring samples from an anaerobic reactor subjected toprocess disturbances exhibited a good correlation. Within thelimitations of the PA and TA titration method, this methodhas proven to be useful for the monitoring of anaerobicdigestion processes. It has been shown to work on samplesfrom an anaerobic digestion process and proven to beamenable for conversion to an on-line system. A relativelysimple system can be built that can produce manymeasurements within a short period of time, therebyminimising errors. With diode arrays becoming lessexpensive and more robust, a method such as this one will, inthe future, be very convenient to perform. A FIA system canbe easily automated and used for process control.

ACKNOWLEDGEMENTS

The authors wish to thank The Nordic Energy ResearchProgramme, Borregaard Industries Ltd., The SwedishNational Energy Administration and The Delegation forEnergy Supply in Southern Sweden, DESS, for their financialsupport and Lovisa Björnsson, Marika Murto, GunnarHörnsten and Damien Batstone for their assistance andsuggestions.

REFERENCES

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