spectroscopic techniques

Application of Ultraviolet (UV) Spectrophotometry in theAssessment of Membrane Bioreactor Performance forMonitoring Water and Wastewater Treatment

RUPAK ARYAL, SARVANAMUTHU VIGNESWARAN,* and JAYA KANDASAMYSchool of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney,

Broadway, NSW 2007, Australia

Ultraviolet (UV) spectroscopy has been widely used in monitoring water

and wastewater treatment. In this study UV spectroscopy was used to

investigate fouling development on the membrane surface of membrane

bioreactors. The chemistry of mixed liquor present in the membrane

bioreactor and the foulant deposited on the membrane surface was

compared by analyzing the UV spectra. The mixed liquor showed

different spectra than did the foulant. The foulant spectra showed a shift

in absorbance peaks with operation time. The particle size distribution

(,450 nm) was also examined to explain the UV fingerprints.

Index Headings: Ultraviolet spectroscopy; UV spectroscopy; Absorbance;

Membrane bioreactor; Foulant; Wastewater treatment; Water monitoring.

INTRODUCTION

A broad range of chemical and physical techniques areavailable to assess the different properties of dissolved organiccarbon. The techniques include measuring turbidity, colorim-etry, and chemical oxygen demand; ultraviolet (UV), fluores-cent, and infrared (IR) spectrophotometry; and gaschromatography and nuclear magnetic resonance (NMR).Among the various techniques, UV spectroscopy is wellknown for the quantitative study of water and wastewaterquality. Several studies have shown the importance of UVabsorption spectra as a qualitative tool for identifying thenature and quantity of organic substances, their functionality,and at times their molecular weight.1–4

The difference between peaks in spectra over the broadrange (190–1000 nm) is mainly due to the presence of weakdouble bonds present in various types of molecules. The choiceto use UV spectra is due to the operational simplicity of UVspectroscopy, the small samples required, minimal samplepreparation, and the fact that it is less time consuming thanother techniques.

The UV absorbance recorded at various wavelengths can beused to correlate several types of parameters. This explains

why these correlation methods can be used for quality controlin water and wastewater treatment plants.

There has been concern regarding the presence of organicsubstances in wastewater and treated wastewater. For theoperation of wastewater treatment plants the early detection ofchanges in the quality of wastewater influent is necessary toprevent a possible failure of the treatment plant performance.Quality changes include both changes in concentration andcomposition of the wastewater. Changes of the wastewatercomposition, especially changes at industrial inlets, can lead tooperational problems and/or possible inhibition of the biochem-ical degradation processes. The level of organic matter is usuallydetermined using standard methods that measure parameterssuch as chemical oxygen demand (COD), biochemical oxygendemand (BOD), total organic carbon (TOC), dissolved organicmatter (DOM), etc. These methods are expensive, timeconsuming, and are not reliable for accurate prediction. As analternative tool, UV spectroscopy has long been used to monitorthe pollutants load in wastewater treatment plants as well as tomonitor effluent water quality.5–7 The spectral informationobtained within the UV range can explain the nature and possiblechanges in the composition of the organic matter. Thisinformation cannot be obtained from conventional surrogateparameters such as TOC, DOC, or total suspended solids (TSS).

A number of authors have discussed the detection of organicsubstances using UV spectra and have developed correlationsbetween the UV absorbance and conventional parameters suchas TOC, DOM, COD, and BOD.3,8 UV absorbance at 260 nmand TOC was used to evaluate the treatability of the variousorganic materials by biological treatment plants.9 James et al.10

suggested UV absorbance at 254 nm as an excellent surrogateparameter for total organic carbon (TOC) and total trihalo-methane (THM) formation potential. The absorbance value at254 nm has been used for the estimation of non-specificparameters such as BOD and COD in water and wastewater.11

The choice of this wavelength is not due to scientific reasonsbut due to the widespread availability of cheap UV sources(low-pressure Hg lamps).12 Kalbitz et al.13 characterizeddissolved organic matter, particularly fulvic acid, using UVspectroscopy. They found a strong correlation between fulvicacid and UV absorbance 285 nm. There is also a strong linearrelationship between the molar absorptivity at 280 nm and the

Received 22 December 2009; accepted 15 October 2010.* Author to whom correspondence should be sent.E-mail: s.vigneswaran@eng.uts.edu.au.DOI: 10.1366/10-05848

Volume 65, Number 2, 2011 APPLIED SPECTROSCOPY 2270003-7028/11/6502-0227$2.00/0

� 2011 Society for Applied Spectroscopy

aromatic content of water-soluble fulvic acids.14 A similarrelationship was shown for 300 nm.15 Chevalier et al. 16

compared the use of the conventional standard method and theUV absorbance method to measure BOD5 (5–115 mg/L) andTOC. Their results indicated that the BOD5 measurementperformed well, with a correlation coefficient (R2) of 0.669,whereas TOC did not show a good correlation.

Although applications of UV spectroscopy in monitoringwastewater parameters such as BOD, COD, nitrate, or humicshave been done for a long time, its application to monitoringmembrane fouling has so far not been investigated extensively.In this study, UV spectroscopy was used to investigate thefoulant and its growth on the membrane surface.

THEORY BEHIND ULTRAVIOLETSPECTROSCOPY

Because UV spectroscopy is based on the interaction ofelectromagnetic radiation (EMR) with a molecule, an under-standing of EMR is necessary. Figure 1 is the EMR spectrumover a broad range. It is important to understand wavelengthand frequency and how they are related to one another.

Ultraviolet radiation having wavelengths of less than 190 nmis difficult to handle and is seldom used as a routine tool forquantitative analysis. The near ultraviolet region (200 nm)comprises photon energies of 143 kcal/mole, whereas for thevisible region the energy of photons is 36–72 kcal/mole.

The energies noted above in the UV region are sufficient topromote or excite a molecular electron to a higher energyorbital. Figure 2 shows the various kinds of electronicexcitation that may occur in organic molecules. Of the sixtransitions outlined, only the two lowest energy ones (left-most, solid lines) are achieved by the energies available in the190 to 400 nm spectrum. As a rule, energetically favoredelectron promotion will be from the highest occupiedmolecular orbital (HOMO) to the lowest unoccupied molecularorbital (LUMO), and the resulting species is called an excitedstate.

Ultraviolet spectroscopy (190–380 nm) corresponds toelectronic excitations between the energy levels that corre-

spond to the molecular orbitals of the systems, in particulartransitions involving p orbitals and (n ! p* and p ! p*).Organic molecules that are aromatic or have conjugated doublebonds can absorb light in the UV region and electronictransition occurs within the molecule, promoting electrons to ahigher-energy orbital. Different organic molecules absorbradiation at different wavelengths. This is due to the presenceof certain functional groups (chromophores) that containvalence electrons of low excitation energy. The UV spectrom-eter records the wavelengths at which absorption occurs,together with the degree of absorption at each wavelength. Theresulting spectrum is presented as a graph of absorbance (A)versus wavelength. Absorbance usually ranges from 0 (noabsorption) to 3 (99% absorption), and is precisely defined incontext with spectrometer operation.

Many organic molecules absorb ultraviolet light. In UVspectroscopy the absorbance of a solution increases asattenuation of the beam increases. Absorbance is directlyproportional to the path length and the concentration of theabsorbing species. The method is most often used in aquantitative way to determine concentrations of an absorbingspecies in solution, using the Beer–Lambert law:

A ¼ �logðI=I0Þ ¼ e:c:L ð1Þ

where, A is the measured absorbance, I0 is the intensity of theincident light at a given wavelength, I is the transmitted intensity,L is the path length through the sample, and c is the concentrationof the absorbing species. For each species and wavelength, e is aconstant known as the molar absorptivity or extinctioncoefficient. This constant is a fundamental molecular propertyin a given solvent at a particular temperature and pressure.

As the absorbance of a sample is proportional to the numberof absorbing molecules in the spectrometer light beam (e.g.,their molar concentration in the sample tube), it is necessary tocorrect the absorbance value for this and other operational

FIG. 1. Electromagnetic radiation spectrum.

FIG. 2. Possible electronic transitions.

TABLE I. Organic compounds and key wavelength in UV spectroscopy.

Wavelength (nm) Property Reference

195 Proteins Yabushita et al.19

210 Amino acids Aitken and Learmonth20

214 Peptides Kuipper and Gruppen21

230 Proteins Liu et al.22

254 Aromaticity Her et al.23

260 COD Chevakidagaran24

265 Relative abundance offunctional groups

Chen et al.25

280 Proteins Aitken and Learmonth20

285 Humification index Kalbitz et al.13

300 Characterization ofhumic substances

Artinger et al.26

320 PAHs, phenolics Khorassani et al.12

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factors if the spectra of different compounds are to becompared in a meaningful way.5,17,18 Table I is a collectionof key wavelengths from the literature used for water andwastewater quality assessment.

MATERIALS AND METHODS

Two laboratory-scale membrane bioreactors (12 L) seededwith domestic wastewater, collected from a secondarytreatment plant in Sydney, were set up in the laboratory. Ahollow fiber membrane module with a pore size of 0.1 lm andarea of 0.2 m2 and a flat sheet membrane module with a poresize of 0.14 lm and area of 0.2 m2, containing eight flat sheets,were submerged in the reactors separately. Both membranemodules were made up of polyvinylidene fluoride (PVDF). Themembrane filtration was carried out without relaxation andbackwash. After seeding with domestic wastewater, the reactorwas continuously fed with a synthetic wastewater consisting ofethanol as an organic source, ammonium chloride, andpotassium dihydrogen phospate as nitrogen and phosphorusat an organic load of 1.5 kg COD�m�3�d�1 (COD : N : P ¼150 : 5 : 1), which corresponds to a typically moderate organicloading rate in domestic wastewater. Mixed liquor suspendedsolid (MLSS) concentration was maintained between 4 and 5

g/L with dissolved oxygen concentration greater than 2 mg/Lto make it similar to the wastewater used to seed the membranebioreactor. Mixed liquor was continuously agitated by passingfine air bubbles through the mixture to maximize oxygenmixing. The sludge retention time (SRT) was controlled at 45days for both membranes. The MBR was operated over 45days at room temperature (20 6 2 8C) for stabilization beforethe start of experiments. The experiments were carried out atdifferent permeate flux (5–20 L�m�2�h�1) but kept constantduring the experiment. The influent and effluent flow rateswere made equal to maintain the reactor volume. The hollowfiber was operated up to 49 days (number of samples¼ 29) andthe flat sheet was operated up to 30 days (number of samples¼6) without chemical cleaning. Beyond the mentioned days,severe fouling occurred and operation was halted.

The mixed liquor, sludge cake deposit (on the membrane),and the foulant were collected during each run. The mixedliquor thus collected was treated with 0.1% NaOH solution andfiltered through a 0.45 lm filter. NaOH was selected because ofits ability to solubilize biopolymers (proteins, polysaccharides,lipids, etc.) as well as humic and fulvic substances. The sludgecake was collected from the membrane by gently shaking themembrane in deionized water. The cake thus removed from the

FIG. 3. UV spectra of mixed liquor and foulant (MBR coupled with hollow fiber membrane). (ML: mixed liquor; SC: sludge cake; F: foulant. The first twonumerals of the suffix following ML, SC, and F are the experiment number and the last two numerals are the sample collection day for that experiment.)

APPLIED SPECTROSCOPY 229

membrane was also dissolved in 0.1% NaOH solution andfinally filtered through a 0.45 lm filter. The fouling depositedon the membrane surface was collected by extracting it with0.1 M NaOH solution. TOC of each extract was measuredusing a total organic carbon analyzer (Analytica Jena multi N/C2000). The extract filtrate with similar TOC (5 mg/L) wasanalyzed using a UV spectrometer (Shimadzu, 1700). The UVinstrument was operated at a bandwidth of 1 nm and a quartzcell with a path length of 10 mm, from 190 to 400 nm. Thescanning speed was 190 nm/min (slow). The photometricaccuracy was 0.004 Abs at 1.0 Abs. The colloidal particles inthe sample were analyzed in a Zeta Sizer Nano Series (MalvernZen 3600) using the quartz cuvette with a square aperture, 10mm in optical path length, using water as the dispersant at 258C. The instrument could measure particle size from 0.1 nm to1000 nm and the molecular weight from 1000 Da to 2 3 108

Da. Measurement was done manually with three repeatedmeasurements averaged for each sample.

RESULTS AND DISCUSSION

Seven experiments (2, 5, 10, 20, 26, 35, and 49 days) wereconducted in the hollow fiber membrane to investigate themixed liquor (ML), sludge cake (SC), and foulant (F). Mixedliquor was collected at regular intervals in each experiment (n¼16), whereas sludge cake (n ¼ 6) and foulant (n ¼ 7) werecollected at the end of each experiment by dissolving in 5%NaOH solution. Here mixed liquor refers to the content in the

membrane reactor, sludge cake is the deposit of sludge duringthe filtration process, and foulant is the organic layer deposit onthe membrane surface. No sludge cake was obtained for the 2-day experiments. Figure 3 (top and bottom panels) shows theUV spectra of ML, SC, and F solution after normalization withdissolved organic carbon (TOC ¼ 5 mg/L). The lines in thefigure represent the UV spectrum of ML, SC, and F observedduring different experiments at different times. Because theML spectra are very close to each other, only six representativeML spectra are shown in the bottom panel of Fig. 3. For eachspectrum, the first two numerals following ‘‘ML’’, ‘‘SC’’, and‘‘F’’ are the experiment number and the last two numeralsrepresent the sample collection day for that experiment. Basedon the peaks, the spectra were categorized into three groups: F,SC, and ML. The group F spectra had a peak in the rangebetween 210 and 224 nm and were for the foulant (Fig. 3, top).The group SC spectra were in the range between 193 and 203nm and were only for the sludge cake (Fig. 3, bottom). Thegroup ML spectra were in the range between 190 and 195 nmand were for the mixed liquor (Fig. 3, bottom).

Multivariate statistics (cluster analysis) using PASW Statis-tics 18 were applied to support the categories using all UV data(n¼ 29). Figure 4 shows the dendrogram obtained from clusteranalysis using hierarchical cluster analysis with the Wardmethod using square Euclidian distance. Similar to the UVspectra, three distinct groups appeared in the dendrogram forML, SC, and F. The dendrogram also confirmed the differencein the nature of the fingerprints for mixed liquor, sludge cake,and foulant. The differing natures of the fingerprints (spectra)of the mixed liquor, sludge cake, and foulant indicate that theyare different in terms of the nature of the organic substancesthey contain. The appearance of the spectra as well as the shiftof chemical peaks towards higher wavelengths with timeindicated a change in the physico-chemical process on themembrane with evolution of time.

Figure 5 shows the spectra of mixed liquor and foulant froma flat sheet membrane bioreactor that was operated for differentdurations (3 to 30 days). The UV spectra were normalized withdissolved organic carbon (5 mg/L). The spectra obtained werecompared with those in the literature. The arrows for eachspectra show key wavelengths of chemical moieties (Table I).Humic acid, amino acids, peptides, and phenolic peaks wereobserved in the spectra.

Spectrum 1 shows the mixed liquor sample, whereas spectra2, 3, 4, 5, and 6 show the foulant collected after 3, 5, 7, 9, and

FIG. 4. Dendrogram of UV spectra of mixed liquor (ML), sludge cake (SC),and foulants (F).

FIG. 5. UV spectra of mixed liquor and foulant (MBR coupled with flat sheetmembrane).

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30 days of operation, respectively. The mixed liquor samplesdid not show peak spectra but showed a continuous decrease ofabsorbance with increase of wavelength. This is possibly due tothe presence of excess humic substances.12,27,28 After threedays, a dominant peak at 210 nm was observed, which ispossibly due to amino acid type substances that were producedwith bacterial growth on the membrane surface. After five daysof the experimental run (spectra 3 in Fig. 4), the peak shiftedfrom 210 to 214 nm. According to Kuipers and Gruppen,21 thespectra in the 214 nm region are due to peptides. Since peptidesare the polymers of amino acids, conjugation of amino acidsshifted the absorbance peak to a higher region, which iscommon in the UV spectra. In runs 4 and 5, a broad peak from210 to 230 nm containing a number of small peaks appeared. Asmall but sharp peak at 230 nm indicated the presence ofprotein-type substances.22 This is further confirmed by the peakin the 320 nm region in run 5, which is mainly due to phenoliccompounds (a functional moiety present in protein). This led usto assume that protein-type moieties were slowly formed on themembrane surface with evolution of time. This is supported bythe observation made by Aryal et al.29 They observed proteinformation on the membrane surface with time using fluores-cence spectroscopy.

Absorption of UV light in the range of 215 to 235 nm hasbeen used to measure proteins.20 The amount of UV lightabsorption was directly related to the number of peptidebonds.30 Marshak31 reported the absorption of protein in thewavelength range of 200–300 nm and Dickinson andMcClements32 reported peptide bond absorption in the rangeof 185–230 nm.

The foulant extract sample obtained from the flat sheetmembrane was also investigated with a nano-sizer in order tounderstand the temporal changes in organic compounds ofdifferent molecular size on the membrane surface. Figure 6 andTable II show the particle size distribution (0.1–1000 nm) inmixed liquor and foulant extract (filtered at a level of 0.45 lm).The figure shows that for wastewater there was a very small

peak at around 20 nm particle size. Foulant 2 had a small peakaround sizes of 40 nm and 120 nm. In foulants 3, 4, 5, and 6not only are the peak intensities and the peak height increased,but also the peak shifted to larger molecular weights. Thisindicated that the larger molecular weight substances appearedin the membrane foulant layer.

In all cases, except for the 30-day run, two peaks (peak 1 andpeak 2) were observed. Peak 1 was within the range of 20 to 90nm and peak 2 was within the range of 100 to 300 nm. In thecase of foulant for 30 days, a third peak also appeared in therange of 820 nm. Based on the particle size, the molecularweight of the substance was calculated using the zeta-nanosizer instrument.

Among the samples, the mixed liquor showed peaks at alower size range compared to the foulant. This shows that themixed liquor has relatively low molecular weight substancescompared to the foulant. This is obvious since the mixed liquorcontains low molecular weight biodegradable organics. In thefoulant, both peaks (peak 1 and peak 2) gradually shifted tohigher sizes (molecular weight) with time. This indicates thatthe composition (chemical structure) of the foulant is changingwith time. Peak 1 was in the range of 24–91 nm in particle sizeand corresponded to a molecular weight of 1146–25 590 Da.The molecular weight resembles that of peptides.31,33 Peak 2was in the range of 100–300 nm in particle size. This molecularweight corresponds to 53 623–412 436 Da. Peak 3 wasapproximately 820 nm, which corresponds to a molecular sizeof 4 614 261 Da. The particle size range reflects the presence ofa wide range of biopolymers (proteins).34 The development ofbiopolymers and their sizes on the membrane surface withevolution of time was reported earlier.29

CONCLUSION

Ultraviolet spectroscopy carried out at between 190 and 380nm was applied to monitor the fouling on two membranebioreactors coupled with a hollow fiber membrane and a flatsheet membrane seeded with effluent from a domesticwastewater plant collected at two different occasions. Themixed liquor, sludge cake, and foulant extracted followingdifferent periods of operation showed different UV finger-prints. This showed that the mixed liquor, sludge cake, andfoulant have different chemical compositions and the compo-sition of the fouling changes with time. The spectralinformation and analysis of the nanoparticles showed that inthe first few days, the foulant contained substances of lowmolecular weight, whereas with the evolution of time, thefoulant contained larger molecular weight biopolymer sub-stances. Thus, UV spectroscopy can be used as one of the toolsto monitor MBR performance in wastewater treatmentfacilities.

FIG. 6. Particle size distribution in mixed liquor and foulant attached onmembrane surface.

TABLE II. Peak sizes and approximate molecular weight associated with peaks.

Peak 1 Peak 2 Peak 3 MW of Peak 1 MW of Peak 2 MW of Peak 3

Mixed liquor 24 125 - 1146 53623 -Foulant 3 days 43 164 - 4460 100965 -Foulant 5 days 44 220 - 4706 200202 -Foulant 7 days 78 296 - 17862 399735 -Foulant 9 days 91 301 - 25590 412436 -Foulant 30 days 91 190 824 23681 129458 4614261

APPLIED SPECTROSCOPY 231

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