UNDERSTANDING THE EFFECT OF WASTEWATER ......Abstract Wastewater microbial flocs can protect...
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UNDERSTANDING THE EFFECT OF WASTEWATER FLOCS PROPERTIES ON UV DISINFECTION KINETICS By Shaghayegh Armioun A thesis submitted in conformity with the requirements for the degree of Master of Applied Science, Department of Chemical Engineering and Applied Chemistry University of Toronto © Copyright by Shaghayegh Armioun (2013)
Transcript of UNDERSTANDING THE EFFECT OF WASTEWATER ......Abstract Wastewater microbial flocs can protect...
UNDERSTANDING THE EFFECT OF WASTEWATER FLOCS PROPERTIES ON UV DISINFECTION KINETICS
By
Shaghayegh Armioun
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science,
Department of Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Shaghayegh Armioun (2013)
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Understanding the Effect of Wastewater Flocs Properties on UV Disinfection Kinetics
Master of Applied Science 2013
Shaghayegh Armioun
Department of Chemical Engineering and Applied Chemistry
University of Toronto
Abstract
Wastewater microbial flocs can protect microorganisms from inactivation by UV light.
This effect is detected as tailing at high UV doses in the UV dose response curve. A
double-layer structure composed of an inner compact core surrounded by a loose outer
layer was proposed by earlier studies to describe UV resistance of microbial flocs. Due
to limited oxygen diffusion into the compact cores, the UV inactivation of compact cores
and microbial flocs under anaerobic conditions needed to be addressed. The UV
disinfection kinetics under anaerobic culturing condition was nearly identical to that of
the aerobic study. Moreover, the role of iron concentration on the differences in the UV
inactivation kinetics of flocs and cores was assessed. The increase in UV absorbance
of floc material due to iron addition could dominate the UV disinfection kinetics of flocs
and cores such that they exhibited similar UV disinfection kinetics.
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Acknowledgement
My greatest gratitude goes to Professor Ramin Farnood and Professor Gideon
Wolfaardt for giving me this research opportunity. It was a great learning experience
under their supervision and guidance. I am thankful for their confidence in my work and
their commitment to my education and professional development.
Special acknowledgment goes to Trojan Technologies Inc. for funding this project.
I would like to show my sincerest appreciation for Dr William Cairns and Dr Ted Mao for
their constructive feedback.
Special thanks to Yaldah Azimi for being a great friend, teacher, and co-worker at the
same time. My work would not have been accomplished without her help and support.
My sincere appreciation goes to Dr Otini kroukamp, Dr Elana Bester, Alex Dumitrache,
and Romeo Dumitrache for their guidance and support in completion of this thesis.
I would like to thank Azad Kavoosi and Rasmey Try for helping me throughout my
experiments.
To my husband, Navid, this work would not have been accomplished without his
patience, support, encouragement and love
To my beloved parents, for their endless love and support all the time
To my sister, Arghavan
Thank you all….
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Table of Content
Abstract ........................................................................................................................................................ II
Acknowledgement ......................................................................................................................................III
Table of Content ......................................................................................................................................... IV
List of Tables ............................................................................................................................................... VI
List of Figures ............................................................................................................................................ VII
Nomenclature ............................................................................................................................................ IX
Chapter 1 Introduction: Overview of Research ...........................................................................................1
1.1 Background ..................................................................................................................................1
1.2 Hypothesis and Objectives ...........................................................................................................2
1.3 Thesis outline ...............................................................................................................................3
1.4 Reference .....................................................................................................................................5
Chapter 2 Literature Review ........................................................................................................................7
2.1 Introduction to Wastewater Treatment ......................................................................................7
2.2 Activated Sludge Process in Wastewater Treatment ...................................................................8
2.2.1 Microbial Floc Formation .....................................................................................................8
2.2.2 Oxygen Diffusion in Microbial Flocs .................................................................................. 11
2.3 UV Disinfection of Wastewater ................................................................................................. 12
2.3.1 Impact of Particles on UV Disinfection .............................................................................. 13
2.3.2 Model Fitting of UV Disinfection Performance ................................................................. 20
2.4 Potential Effect of Iron on UV Absorbance and Disinfection .................................................... 22
Chapter 3 Experimental Methods ............................................................................................................. 26
3.1 Experimental Approach ............................................................................................................. 26
3.2 Materials and Methods ............................................................................................................. 26
3.2.1 Sample Collection .............................................................................................................. 26
3.2.2 Sieving ............................................................................................................................... 26
3.2.3 Particle Size Distribution Analysis ..................................................................................... 27
3.2.4 Shearing ............................................................................................................................. 27
3.2.5 UV Bioassay ....................................................................................................................... 29
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3.2.6 Culturability of Flocs and Cores ........................................................................................ 30
3.2.7 Double Exponential Model Fitting .................................................................................... 31
3.2.8 Experimental Setup of the Sequencing Batch Reactors .................................................... 31
3.2.9 Mechanical Strength Test (Breakage Test)........................................................................ 34
3.2.10 Microscopy and Image Analysis ........................................................................................ 35
Chapter 4 Results and Discussion ............................................................................................................. 36
4.1 Particle Size Distribution of Flocs and Cores ............................................................................. 36
4.2 UV Dose-response curves and Model Fitting ............................................................................ 38
4.2.1 UV DRC of Microbial Flocs under Anaerobic and Aerobic Growth Conditions ................. 38
4.2.2 UV DRC of Compact Cores under Anaerobic and Aerobic Growth Conditions ................. 39
4.2.3 Comparison of UV DRC of Microbial Flocs and Compact Cores ........................................ 41
4.2.4 Culturability of Floc and Core............................................................................................ 46
4.2.5 UV DRC of Final Effluent under Anaerobic and Aerobic Growth Conditions .................... 47
4.2.6 Model Fitting of UV DRC for Flocs and Cores .................................................................... 49
4.2.7 Comparison of Model Parameters .................................................................................... 49
4.3 Impact of Iron on Flocs’ Characteristics and UV disinfection Kinetics ...................................... 53
4.3.1 Effect of Iron on PSD ......................................................................................................... 53
4.3.2 Microscopy and Image Analysis ......................................................................................... 55
4.3.3 Effect of Iron on Mechanical Strength of Flocs .................................................................. 56
4.3.4 Effect of Iron on UV DRC .................................................................................................... 57
4.3.5 Effect of Iron Dosing on UV DRCs of Flocs and Cores ........................................................ 59
4.3.6 Model Prediction to Examine the Effect of Iron on UV DRCs of Flocs ............................... 59
4.3.7. Practical Significance ......................................................................................................... 62
Chapter 5 Conclusions and Future Work .................................................................................................. 64
5.1 Conclusions ............................................................................................................................... 64
5.2 Future Work ............................................................................................................................. 65
Reference .................................................................................................................................................. 66
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List of Tables
Table 1 Major Parameters Affecting UV Disinfection and Their Acceptable Values (Das, 2001) ............. 14 Table 2 Increases in Particle Size Directly Affect the UV Dose Demand (Qualls et al. (1983), Emerik et al (1999), Sakamoto and Zimmer (1997)) ..................................................................................................... 19 Table 3 The turbulent Shear Stress as a Function of the Rotational Speed (rpm), Yuan et al., 2010 ....... 29 Table 4 Chemical Composition of the Synthetic wastewater (Liao et al, 2002) ....................................... 33 Table 5 Changes in FeCl2 Concentration as Coagulant in Primary Treatment .......................................... 42 Table 6 Culturability Values for Flocs (Untreated and UV Irradiated) and Compact Cores Extracted from UV Irradiated Flocs .................................................................................................................................... 46 Table 7 Culturability Values for Inner Region of Compact Cores Extracted from Untreated flocs under Aerobic and Anaerobic Growth Condition ................................................................................................ 47 Table 8 Inactivation Rate Constants (k1 and k2) and UV-resistant Fraction (β) of Floc and Core Samples under Anaerobic Growth Condition .......................................................................................................... 50 Table 9 Inactivation Rate Constants (k1 and k2 ) and UV-resistant Fraction (β) of Floc and Core Samples under Aerobic Growth Condition .............................................................................................................. 50 Table 10 Median Shear Stress Values for Floc Breakage for Fe-dosed, and Fe-free Flocs (45-90 µm Size Fraction) .................................................................................................................................................... 57 Table 11 Inactivation Rate Constants (k1 and k2) and UV-resistant Fraction (β) of Iron-free and Iron-dosed Floc of Size fraction 45-63 μm ........................................................................................................ 58
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List of Figures Figure 1 General Schematic of Wastewater Treatment Process Using Activated Sludge Process in Secondary Treatment, Image modified from Metcalf &Eddy ....................................................................... 7 Figure 2 Schematic Model of A) Double-layer by Liao et al. (2002), b) Multi-layer Structure for .............. 10 Figure 3 Range of Electromagnetic Wavelengths (EPA, 2006) .................................................................... 13 Figure 4 Possible Interactions between UV Light and Wastewater Particles (Loge at al., 1996) ............... 15 Figure 5 Typical UV Dose-response Curve for Free-swimming Microorganisms (Zimmer, 1997) .............. 16 Figure 6 Typical UV Dose-response Curve for Wastewater Secondary Effluent that Contains Particles (Azimi et al., 2012) ...................................................................................................................................... 17 Figure 7 Typical UV Dose-response for Filtered and Unfiltered Wastewater (Das, 2001) Similar Results Established by Quals et al. (1983) and Tan (2007) ...................................................................................... 18 Figure 8 Average DRC for Flocs of Different Size Fractions and Free Microbes (Tan, 2007) ...................... 20 Figure 9 Typical UV DRC for Particle-associated Microorganism, Where k1 and k2 Represent Inactivation Rate Constants for the Initial Slope and Tailing Region (Azimi et al., 2012) ............................................... 22 Figure 10 Couette Flow Apparatus, (a) Actual Lab Equipment, (b) Schematic of the Couette Flow Device (Yuan et al., 2007; Azimi et al, 2012) .......................................................................................................... 28 Figure 11 Experimental Setup of the Sequencing Batch Reactors .............................................................. 32 Figure 12 Particle Size Distributions for Flocs and Cores of Size 45-63 µm ............................................... 36 Figure 13 Particle Size Distributions for Flocs and Cores of Size 75-90 µm ................................................ 37 Figure 14 Particle Size Distributions of Final Effluent ................................................................................. 37 Figure 15 Normalized UV DRCs for Flocs of Two Size Fractions under Anaerobic Growth Condition ........ 38 Figure 16 Normalized UV DRCs for Flocs of Two Size Fractions under Aerobic Growth Condition ............ 39 Figure 17 Normalized UV DRCs for Cores of Two Size Fractions under Anaerobic Growth Condition ....... 40 Figure 18 Normalized UV DRCs for Cores of Two Size Fractions under Aerobic Growth Condition ........... 40 Figure 19 Normalized UV DRCs of Cores and Flocs of Size 45-63 µm under Anaerobic Growth Condition43 Figure 20 Normalized UV DRCs of Cores and Flocs of Size 75-90 µm under Anaerobic Growth Condition44 Figure 21 Normalized UV DRCs of Cores and Flocs of Size 45-63 µm under Aerobic Growth Condition ... 44 Figure 22 Normalized UV DRCs of Cores and Flocs of size 75-90 µm under Aerobic Growth Condition ... 45 Figure 23 Normalized UV DRCs of Flocs of Two Size Fractions under Aerobic and Anaerobic Growth Condition ..................................................................................................................................................... 45 Figure 24 Normalized UV DRCs of Cores of Two Size Fractions under Aerobic and Anaerobic Growth Condition ..................................................................................................................................................... 46 Figure 25 Normalized UV DRCs of Final Effluent under Aerobic and Anaerobic Growth Condition .......... 48 Figure 25.b UV DRCs of Final Effluent under Aerobic and Anaerobic Growth Condition……………………... 49 Figure 26 Comparisons of Experimental and Model Predicted Results for Flocs under Anaerobic Growth Condition ..................................................................................................................................................... 51 Figure 27 Comparisons of Experimental and Model Predicted Results for Cores under Anaerobic Growth Condition ..................................................................................................................................................... 52
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Figure 28 Comparisons of Experimental and Model Predicted Results for Flocs under Aerobic Growth Condition ..................................................................................................................................................... 52 Figure 29 Comparisons of Experimental and Model Predicted Results for Cores under Aerobic Growth Condition ..................................................................................................................................................... 53 Figure 30 Effect of Iron Dosing on Particle Size Distribution of Activated Sludge Flocs (Number) ............ 54 Figure 30.b Effect of Iron Dosing on Particle Size Distribution of Activated Sludge Flocs (Volume %)…….55 Figure 31 Microscopy Images for Iron-free and Iron-dosed Flocs .............................................................. 56 Figure 32 Mechanical Strength Tests for Iron-free and Iron-dosed Flocs ................................................... 57 Figure 33 Effect of Iron Dosing (15 mg/L) on UV DRC of floc (45-63µm) .................................................... 58 Figure 34 Effect of Iron Dosing on UV DRC of Flocs and Cores of Size Fraction of 45-63 µm ..................... 59 Figure 35 Model Predictions for Various Levels of Iron Dosing on the Behavior of Flocs (floc/core size is 48µm) .......................................................................................................................................................... 61 Figure 36 Model Predictions for Various Levels of Iron Dosing on the Behavior of Flocs (floc/core size is 80µm) .......................................................................................................................................................... 61 Figure 37 Effect of Iron Dosing on Simulated Final Effluent ....................................................................... 63
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Nomenclature
EPS: Extracellular polymeric substances
CFU: Colony forming unit
N(R, δ): Final concentration (i.e. after UV irradiation) of viable flocs
N0 (R, δ): Initial concentration of viable flocs
I (mW/cm2): Average UV intensity at the surface of a floc
D (mJ/cm²): UV dose delivered to the wastewater. The product of UV intensity (mW/cm²) and time (s)
DRC: Dose-response curve
k (cm²/mJ): The inactivation rate constant of free indicator microorganism
k1 (cm²/mJ): Inactivation rate constant for the UV-susceptible flocs
k2 (cm²/mJ): Inactivation rate constant for the UV-resistant flocs
β: The fraction of UV-resistant flocs, or the ratio of the number of viable UV-resistant flocs to the total number of viable flocs
A (cm-1): The UV absorption coefficient of the floc material
R (µm): The spherical radius of a floc
δ (μm): The distance of the point of interest in the floc from the flocs surface that varies between 0 and R
keff (cm²/mJ): The effective inactivation rate constant at the distance δ from the surface of the floc inwards
Deff (mJ/cm²): The effective dose delivered to a certain point inside a floc
f (δ,R): The probability of finding culturable indicator microorganism at the depth of δ
SS (mg/L): Suspended solids
TSS (mg/L): Total suspended solids
MLSS (mg/L): Mixed liquor suspended solids
COD (mg/L): Chemical oxygen demand
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Chapter 1 Introduction: Overview of Research
1.1 Background
Disinfection of wastewater before discharge reduces the number of pathogenic
organisms and decreases the risk of disease outbreaks. Proper disinfection is vital for
reducing microbial contamination present in wastewater if risks to public health are to
be avoided. The disinfection must inactivate a wide range of bacteria and viruses in a
variety of water and wastewater types. However, many of the existing disinfection
techniques such as ozone and chlorine are incapable of meeting the stringent
disinfection requirements for water reuse (Caretti et al., 2003; Gehr et al., 2003;
Koivunen et al., 2005). Ultraviolet (UV) irradiation, which has long been recognized as
an effective method for wastewater disinfection, is a comparatively cleaner technology
that achieves microbial inactivation without any noticeable adverse toxicological effects
(Blatchley et al., 1997). UV disinfection is a physical method, which does not cause the
production of hazardous by-products associated with chemical disinfectants (Blatchley
et al., 1997). Around the wavelength of 254 nm, UV light can penetrate into the cell wall
and alter the DNA of microorganisms and prohibit their reproducibility (Pfeifer et al.,
2005). However, the presence of suspended particles including microbial flocs in
wastewater adversely affects the efficiency of UV disinfection. Microbial flocs,
generated in the secondary treatment process, are known to harbor microbes and
shield them from UV light (Emerick et al., 1999). This is particularly a limiting factor for
water reuse applications. The microbial inactivation rate of secondary effluent typically
follows a first order kinetics at low UV doses. However, the rate decreases
considerably at higher UV doses, resulting in a near plateau region that is called tailing
phenomenon (Jagger, 1977; Farnood, 2005). Tailing implies the presence of the
residue of surviving microorganisms at high UV doses. It is widely accepted that tailing
is caused by the presence of particle-associated bacteria in wastewater effluents
(Qualls et al., 1983; Loge et al., 1996; Emerick et al, 1999). Unfortunately, tailing limits
the economic viability of UV disinfection if pretreatment is inadequate to control particle
physical properties and quantity.
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1.2 Hypothesis and Objectives
The overall objective of this project is to develop a fundamental understanding of the
effect of microbial flocs' structure on the tailing phenomenon and UV disinfection of
wastewater. It is hypothesized that:
1. Fecal coliform bacteria are only capable of reproducing (culturable) when they
are located in the vicinity of the surface of compact cores, and these bacteria are
partially responsible for the tailing phenomenon.
Earlier studies suggest that microbial flocs are composed of a compact core
surrounded by a loose outer layer (Liao et al, 2002; Tan 2007, Azimi et al., 2012). This
conceptual model has been applied to describe the tailing effect, and it was suggested
that the compact cores mainly contribute to the tailing phenomenon (Azimi et al. 2012).
The focus in the literature has been on the disinfection kinetics of microbial flocs and
compact cores under aerobic growth condition. However, oxygen availability may be a
limiting factor for the bacteria living in the inner regions of microbial flocs. Limited
diffusion of oxygen into the inner regions of the microbial flocs may cause the centre of
the flocs to become oxygen depleted, and incapable of reproduction by aerobic
microbes (De Beer et al., 1998; Glud, R.N., 1998; Davies, 2005; Tsai, D.G., 2008; Han,
Y., 2012). Due to the possibility of the adaptation of embedded bacteria to local
environmental conditions and microscale chemical gradient (Stewart et al., 2008),
including oxygen depletion, there is a need to assess the UV disinfection of embedded
bacteria under anaerobic growth condition.
2. The presence of iron in the microbial flocs increases their mechanical strength
and resistance to UV disinfection
There is evidence in the literature that iron can bridge negatively charged functional
groups within the EPS, which also promotes an increase in floc density, and resistance
to shear (Zhang et al., 2008; Li, 2005). It is also reported in the literature that a higher
UV dose is required to achieve the desired log reduction for effluents with higher iron
content (Cairns et al, 1993; Mamane, 2008; Kozak et al., 2011).
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Accordingly, specific objectives of this project include:
1. Investigate the disinfection kinetics of compact cores and microbial flocs under
aerobic and anaerobic growth conditions and assess the role of compact cores
in the UV disinfection of microbial flocs.
2. Examine and compare the effect of ferrous chloride as coagulants on the UV
absorbance and disinfection kinetics, particle size distribution, morphology, and
mechanical strength of the activated sludge flocs.
The outcome of this project would be applied to develop improved treatment processes
for water reuse applications. Its focus is investigating the tailing phenomenon in UV
dose-response curves, providing deeper insight into the effect of wastewater flocs on
the UV disinfection of effluents, and finding an explanation for the disinfectability trend
described earlier. The main purpose is to understand the formation of UV-resistant
flocs and their properties, and try to find ways to control their formation in upstream
processes. Further understanding of the characteristics and formation of UV-resistant
constituents may assist to establish treatment plant operating conditions to minimize
their formation and adverse effects.
The overall purpose of this study is to recognize the properties of UV-resistant flocs,
and propose feasible ways to change their properties in the favour of UV disinfection. It
may identify treatment plant operating conditions to minimize formation of the UV-
resistant constituent. Accordingly, higher degrees of disinfection or similar degrees but
at lower energy consumption may be achieved. Further understanding of the
mechanism of microbial resistance against UV light by wastewater flocs may also
assist in designing new upstream processes, which will be more effective in eliminating
the UV-resistant constituents.
1.3 Thesis outline
The thesis consists of five chapters as follows:
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• Chapter 1 will discuss a brief introduction on UV disinfection, and the hypothesis
and objectives of the present study.
• Chapter 2 will first present a review on wastewater treatment processes and
activated sludge floc formation. This will be followed by an in-depth discussion
on the fundamentals of UV disinfection kinetics, the effect of wastewater flocs,
and the impact of particle size on UV disinfection kinetics. The discussion will be
followed by the definition of tailing and its influence on the efficiency of UV
treatment. In addition, the existing models for the prediction of UV disinfection
performance will be provided. Finally, this chapter will summarize the potential
effects of iron on UV disinfection kinetics.
• Chapter 3 will discuss all the experimental methods used in this study.
• Chapter 4 will present the results of the present study as well as the discussion
of the results.
• Chapter 5 will present the main conclusions of this study along with some
recommendations for future works.
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1.4 Reference
Azimi, Y., Allen, D. G., & Farnood, R. R. (2012). Kinetics of UV inactivation of wastewater bioflocs. Water Research, 46(12), 3827-3836.
Blatchley III, E. R. (1997). Numerical modeling of UV intensity: Application to collimated-beam reactors and continuous-flow systems. Water Research, 31(9), 2205-2218.
Cairns, W., Sakamoto, G., Comair, C., & Gehr, R. (1993) Assessing UV disinfection of a physico-chemical effluent by medium pressure lamps using a collimated beam and pilot plant. Proc. "Planning, design & operation of effluent disinfection systems", Water Environment Federation Specialty Conf. Whippany, NJ.
Caretti, C., & Lubello, C. (2003). Wastewater disinfection with PAA and UV combined treatment: A pilot plant study. Water Research, 37(10), 2365-2371.
Davies, P. S. (2005). The biological basis of wastewater treatment. Glasgow, UK: Strathkelvin Instruments Ltd.
De Beer, D., Schramm, A., Santegoeds, C. M., & Nielsen, H. K. (1998). Anaerobic processes in activated sludge. Water Science and Technology, 37(4-5), 605-608.
Emerick, R. W., Loge, F. J., Ginn, T., & Darby, J. L. (2000). Modeling the inactivation of particle-associated coliform bacteria. Water Environment Research, 72(4), 432-438.
Emerick, R. W., Loge, F. J., Thompson, D., & Darby, J. L. (1999). Factors influencing ultraviolet disinfection performance part II: Association of coliform bacteria with wastewater particles. Water Environment Research, 71(6), 1178-1187.
Farnood, R., (2005). Flocs and Ultraviolet Disinfection. Chapter 18 in Flocculation in Natural and Engineering Systems. Droppo, I.G. et al. Eds., pp 385-395. CRC Press, Boca Raton, FL.
Gehr, R., Wagner, M., Veerasubramanian, P., & Payment, P. (2003). Disinfection efficiency of peracetic acid, UV and ozone after enhanced primary treatment of municipal wastewater. Water Research, 37(19), 4573-4586.
Glud, R. N., Santegoeds, C. M., De Beer, D., Kohls, O., & Ramsing, N. B. (1998). Oxygen dynamics at the base of a biofilm studied with planar optodes. Aquatic Microbial Ecology, 14(3), 223-233.
Han, Y., Liu, J., Guo, X., & Li, L. (2012). Micro-environment characteristics and microbial communities in activated sludge flocs of different particle size. Bioresource Technology, 124, 252-258.
Huang, L., Zhang, B., Sun, G., & Gao, B. (2011). Role of fe(III) in microbial activity and extracellular polymeric substances. Proceedings - International Conference on Computer Distributed Control and Intelligent Environmental Monitoring, CDCIEM 2011, 994-997.
Jagger, J. (1967). Introduction to Research in Ultraviolet photobiology. Prentice-Hall, Englewood Cliffs, New-Jersey.
Jorand, F., Zartarian, F., Thomas, F., Block, J. C., Bottero, J. Y., Villemin, G., . . . Manem, J. (1995). Chemical and structural (2D) linkage between bacteria within activated sludge flocs. Water Research, 29(7), 1639-1647.
Koivunen, J., & Heinonen-Tanski, H. (2005). Inactivation of enteric microorganisms with chemical disinfectants, UV irradiation and combined chemical/UV treatments. Water Research, 39(8), 1519-1526.
Kozak, J. A., Lordi, D. T., Abedin, Z., O'Connor, C., Granato, T., & Kollias, L. (2010). The effect of ferric chloride addition for phosphorus removal on ultraviolet radiation disinfection of wastewater. Environmental Practice, 12(4), 275-284.
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Li, J. (2005). Effects of fe(III) on floc characteristics of activated sludge. Journal of Chemical Technology and Biotechnology, 80(3), 313-319.
Liao, B. Q., Allen, D. G., Leppard, G. G., Droppo, I. G., & Liss, S. N. (2002). Interparticle interactions affecting the stability of sludge flocs. Journal of Colloid and Interface Science, 249(2), 372-380.
Loge, F. J., Darby, J. L., & Tchobanoglous, G. (1996). UV disinfection of wastewater: Probabilistic approach to design. Journal of Environmental Engineering - ASCE, 122(12), 1078-1084.
Loge, F. J., Emerick, R. W., Heath, M., Jacangelo, J., Tchobanoglous, G., & Darby, J. L. (1996). Ultraviolet disinfection of secondary wastewater effluents: Prediction of performance and design. Water Environment Research, 68(5), 900-916.
Loge, F. J., Emerick, R. W., Thompson, D. E., Nelson, D. C., & Darby, J. L. (1999). Factors influencing ultraviolet disinfection performance part I: Light penetration to wastewater particles. Water Environment Research, 71(3), 377-381.
Mamane, H. (2008). Impact of particles on UV disinfection of water and wastewater effluents: A review. Reviews in Chemical Engineering, 24(2-3), 67-157.
Pfeifer, G. P., You, Y., & Besaratinia, A. (2005). Mutations induced by ultraviolet light. Mutation Research - Fundamental and Molecular Mechanisms of Mutagenesis, 571(1-2 SPEC. ISS.), 19-31.
Qualls, R. G., & Donald Johnson, J. (1983). Bioassay and dose measurements in UV disinfection. Applied and Environmental Microbiology, 45(3), 872-877.
Qualls, R. G., Flynn, M. P., & Johnson, J. D. (1983). The role of suspended particles in ultraviolet disinfection. Journal of the Water Pollution Control Federation, 55(10), 1280-1285.
Qualls, R. G., Ossoff, S. F., & Chang, J. C. H. (1985). Factors controlling sensitivity in ultraviolet disinfection of secondary effluents. Journal of the Water Pollution Control Federation, 57(10), 1006-1011.
Sheng, G., Yu, H. -., & Li, X. -. (2010). Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: A review. Biotechnology Advances, 28(6), 882-894.
Stewart, P. S., & Franklin, M. J. (2008). Physiological heterogeneity in biofilms. Nature Reviews Microbiology, 6(3), 199-210.
Tan, T. (2007). Understanding the effect of particle-size on UV disinfection: Kinetics, mechanism and modeling. (Order No. MR40067, University of Toronto (Canada)). ProQuest Dissertations and Theses, 134.
Tsai, D. G., Lee, D. J., & Lai, J. Y. (2008). Oxygen diffusion in single sludge floc. Advanced Powder Technology, 19(5), 475-481.
Wastewater engineering : Treatment, disposal, reuse (1979). In Tchobanoglous G. (Ed.), (2nd ed. -- ed.). New York: McGraw-Hill.
Zhang, H., Sun, B., Zhao, X., & Gao, Z. (2008). Effect of ferric chloride on fouling in membrane bioreactor. Separation and Purification Technology, 63(2), 341-347.
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Chapter 2 Literature Review
2.1 Introduction to Wastewater Treatment
Wastewater treatment is generally conducted in four stages: preliminary, primary,
secondary, and tertiary treatment. In preliminary treatment, large substances are
removed by screening, sedimentation, flocculation, and flotation. In primary treatment,
suspended and insoluble materials are removed by the means of screening, or settling
tanks. The effluent from the primary stage contains soluble organic materials and fine
particles. Secondary treatment includes the biological treatment, which is considered
as one of the most efficient methods for removing organic materials existing in
wastewater. In this stage, the organic substances are degraded aerobically by certain
microorganisms. This process either has microorganisms in the suspension
(suspended growth system), or growing as films (fixed film systems). In suspended
growth system, organics and microorganism are in a mixed suspension, in which
microorganisms consume organic matter, and microbial flocs are formed. Then, the
solid materials are separated in a clarifier. The clarifier effluent has low organic content,
and discharged to the next stage for disinfection. In fixed film systems, microorganisms
consume organics and grow on a support medium producing biofilms. Secondary
treatment effluent contains pathogens and should be disinfected to regulatory
requirements for discharge into rivers and lakes (Metcalf & Eddy, 1979).
Figure 1 General Schematic of Wastewater Treatment Process Using Activated Sludge Process in Secondary Treatment, Image modified from Metcalf &Eddy
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Disinfection is the final stage in wastewater treatment, for killing or inactivating the
existing pathogens in the water. Lack of proper water disinfection methods may result
in widespread waterborne diseases. A summary of a typical secondary treatment
process is shown in Figure 1.
2.2 Activated Sludge Process in Wastewater Treatment
Activated sludge process is a secondary treatment, and was first applied in England
around one century ago (EPA, 2006). It is a biological treatment (suspended growth
process) in which microorganisms consume the organic materials in wastewater. In the
process, microorganisms oxidize organic matters in the aeration tank and produce
CO2, H2O, NH4, and a new biomass called microbial flocs (Metcalf & Eddy, 1979).
There are two main purposes to be achieved from this process:
1. oxidation of biodegradable organics in wastewater
2. flocculation and aggregation of new biomass for easier separation in the clarifier
Microbial flocs are defined as particle aggregations produced during removing organic
materials process in wastewater.
2.2.1 Microbial Floc Formation
Suspended microorganisms can exist either as free-living cells or attached to flocs in
the effluent. Microbial flocs are referred to particulate aggregates generated in the
activated sludge process as a result of flocculation; these flocs contain
microorganisms, extracellular polymeric substances (EPS), and organic and inorganic
colloidal particles (Urbain et al., 1993; Sanin et al., 1994). Microbial flocs are porous,
heterogonous and irregular in shape. They have a wide range of particle size
distribution, and vary in size from a single bacterial size (1-3 microns) to a large
aggregate size (up to 1mm) (Jorand et al., 1995; Droppo et al., 2005; Yuan, 2007). Li
and Ganczarczyk (1987) reported that floc porosity can vary between 45-90% based on
its size; for flocs smaller than 200 µm, the porosity increases as the floc size increases.
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Microorganisms are bound to each other and other substances by EPS, salt bridges,
polymer bridging, filamentous backbones, hydrophobic interactions, or a combination of
all (Pavoni et al., 1972; Forster et al., 1985; Liss et al., 1996, Liao et al. 2006). Liao et
al. (2002) proposed that in the initial stages of bioflocculation, van der Waals and/or
hydrophobic interactions play an important role. In these early stages, the
microorganisms become close enough to form primary interactions. Then flocs are
attached together by salt bridging/ionic interactions, and further stabilized by hydrogen
bonds between floc surfaces.
The EPS is produced from cell lysis and adsorbed organic matters. It plays a critical
role in activated sludge bio-flocculation, and has a significant impact on the
physicochemical properties of microbial aggregates, including structure, surface
charge, dewatering, and adsorption ability (Huang et al., 2011; Sheng et al., 2010). The
EPS is a complex mixture of carbohydrates, proteins, humic compounds, and nucleic
acids excreted by microorganism (Bura et al., 1998; Liao et al., 2001; Sheng et al.,
2010), while protein is the main component in the composition of EPS in activated
sludge flocs (Bura et al, 1998; Comte et al., 2006).
Pelczar (1977) stated that EPS can be divided into two distinct categories, including the
slime layer and the capsule layer. The capsular material forms outside the cell
membrane whereas the slime is either loosely bound to the cell or is totally free from it.
Sheng et al. (2010) stated that the EPS outside of cells can be commonly divided into
either bound EPS (sheaths, capsular polymers, condensed gels, loosely bound
polymers, and attached organic materials) or soluble EPS (soluble macromolecules,
colloids, and slimes) (Nielsen et al., 1996; Nielsen and Jahn, 1999; Laspidou et al.,
2002). However, the study on the soluble EPS is inadequate, and the EPS mentioned
in the literature without being specified are considered as bound EPS. Bound EPS
commonly consists of two layers (Nielsen and Jahn, 1999). The inner layer consists of
tightly bound EPS (TB-EPS), which has stronger bounds with cells. The outer layer
consists of loosely bound EPS (LB-EPS), which is a loose and dispersible slime layer
and has weaker attachment to the cells (Li et al., 2007; Sheng et al., 2010).
10
A mild method, including high rate shear, heating at low temperature, and high speed
centrifugation can be used to extract the LB-EPS. However, severer methods, including
heating at high temperature, sonication, and chemical extraction method can be
applied for detachment of bound TB-EPS (Sheng et al., 2010).
It has been proposed that microbial flocs have a double-layer structure consisting of a
compact core surrounded by a loose shell (Eriksson et al. 1992, Liao et al. 2002,
Sheng et al. 2006, Tan, 2007, Yuan et al., 2010, Azimi et al., 2012). Liao et al. (2002)
suggested that at higher SRTs the interior region of the flocs, i.e., cores are more
compact and hydrophobic. Yuan et al. (2010) reported that under applying
hydrodynamic shear stress to the flocs in a Couette flow cell, the required shear stress
to break cores could be several times greater than the outer loose shell. It has been
suggested that the higher mechanical strength of compact cores may be due to the
compactly packed structure of the core (Yuan et. al, 2010). Sheng et al. (2006) also
studied the stability of flocs under shear stress and proposed that microbial flocs have
a multilayer structure with two distinct regions composed of stability and dispersible
parts. It was suggested that the outer part is a loosely entangled region in which the
EPS can be dispersed entirely under shearing, and the inner part is more stable that
cannot be dispersed easily except under high shear stress. The proposed floc
structures are demonstrated in Figure 2.
a) b)
Figure 2 Schematic Model of A) Double-layer by Liao et al. (2002), b) Multi-layer Structure for Activated Sludge Floc by Sheng et al. (2006)
11
2.2.2 Oxygen Diffusion in Microbial Flocs
Oxygen availability is a limiting factor for aerobic bacteria living in the inner regions of
microbial flocs. Limited diffusion of oxygen causes oxygen depletion in the centre of the
flocs, and this region may be colonized by facultative anaerobic bacteria (Davies,
2005). Many studies noted that the interior layer of activated sludge flocs has a lower
growth rate than the outer layer, due to the limitation of diffusion of oxygen and
nutrients into the core from the bulk solution (Eriksson et al., 1992; Atkinson et al.,
1976; Liao et al., 2002). Oxygen diffusion takes place through a concentration gradient
along the inner regions of flocs. The free-swimming bacteria are able to grow when the
oxygen concentration in the aeration tank is around 0.6 mg/L. The outer region of the
flocs is mostly colonized by microorganisms of a higher trophic level, including protozoa
and rotifers (Davies, 2005).
Stewart et al. (2008) also reported that the characteristics and composition of free-
swimming bacteria are significantly different from those that are embedded inside
microbial aggregate. They recognized that the diffusion of oxygen and substrate into
the inner layers of a nascent biofilm is rapid and the entire microorganisms in the
biofilm are provided with oxygen and substrate. However, in a mature biofilm, only
microorganisms that are located near the surface of biofilm are provided with oxygen
and substrate. Since oxygen is sparingly soluble, it usually depletes faster and an
anoxic region is developed below the oxic region in the layers of biofilm. Facultative
anaerobes such as fecal coliforms grow by aerobic metabolism in the presence of
oxygen, and fermentatively in the absence of oxygen (Stewart et al., 2008).
Li et al. (2004) studied the heterogeneity of microbial processes and the impacts of
oxygen and substrate concentrations on the microenvironment of activated sludge
flocs. They observed that the aerobic region in the activated sludge flocs is limited to
the vicinity of their surface layer. Anoxic zones below the oxic zones were found under
aerobic operating conditions (Li et al., 2004). Liao et al. (2001) reported that there is a
greater likelihood for larger particles to encounter limitations in the diffusion of oxygen
and nutrients from the bulk solution to the inner regions of the flocs, resulting in a lower
12
growth rate. Consecutively, lower growth can lead to a more hydrophobic region, which
leads to formation of compact cores in the centre of the flocs.
2.3 UV Disinfection of Wastewater
As mentioned earlier, disinfection may be performed by chemical or physical methods.
However, an increased awareness of the several disadvantages of chemical
disinfectants, especially chlorine, has resulted in the selection of UV as an alternative
with valuable features and benefits (Blatchley et al., 1997). UV disinfection is a physical
method, which does not result in the production of hazardous by-products associated
with chemical disinfectants and affect a wide range of pathogens (Blatchley et al.,
1997).
Regarding to the spectrum of electromagnetic radiation, ultraviolet refers to the
wavelengths ranging from 100-400 nm. However, the region between 200-300 nm has
the most efficient germicidal capability in which microorganisms are inactivated by UV
light as a result of photochemical damage to their nucleic acids (Pfeifer et al., 2005).
Around the wavelength of 254 nm, the high energy associated with short wavelength
UV radiation is absorbed by cellular RNA and DNA and prevents replication by altering
DNA structure, and ultimately causes cell death (Pfeifer et al., 2005). The range of
electromagnetic wavelengths is shown in Figure 3.
13
Figure 3 Range of Electromagnetic Wavelengths (EPA, 2006)
According to EPA (2006), the UV dose is the product of UV intensity and exposure time
with the unit of mJ/cm2. Degree of microbial inactivation depends on the amount of UV
dose that microorganisms receive:
UV Dose (mJ/cm2) = I × t
Where, I is the average UV intensity (mW/cm2) and t is the UV irradiation exposure
time (s).
The intensity of UV light decreases as it passes through the media, including
wastewater and has to be corrected accordingly for its UV transmittance. UV
transmittance refers to UV absorbing tendency, and indicates the ease of UV light
passing through water.
2.3.1 Impact of Particles on UV Disinfection
The UV disinfection efficiency strongly depends on the effluent quality characteristics
(Das, 2001). One of the essential characteristics is the UV transmittance (UVT) of the
effluents, which indicates the ease of UV light passing through the effluent, and also
the UV demand for microbial inactivation (Das, 2001). The UV transmittance is
measured by a spectrometer operating at the wavelength of 254 nm and reported as a
14
percentage relative to distilled water transmittance. A low UV transmittance implies that
a smaller amount of the UV light reaches the target microorganisms, resulting in lower
disinfection efficiency (Scheible, 1987). Das (2001) reported that UV transmittance for
effluents from suspended growth treatment processes is generally in the range of 60-
65%, while for fixed film processes it varies between 50-55%.
Table 1 Major Parameters Affecting UV Disinfection and Their Acceptable Values (Das, 2001)
Suspended particles and dissolved chemical compounds absorb UV light and affect the
UV transmittance adversely (Loge et al., 1996). Suspended particles decrease the UV
transmittance in the final effluent. Within the flocs, the UV light is absorbed by the EPS
and cannot reach the particle-associated target organisms (Farnood et al., 2005).
These particles can decrease the efficiency of UV disinfection by protecting the
microorganisms from exposure to UV light. As a result, particle-associated
microorganisms may remain active in wastewater even after the application of high UV
doses (Loge et al., 1996).
Aggregation of microorganisms and solid particles in wastewater reduces the
effectiveness of UV disinfection and shield the microorganisms by decreasing light
transmittance due to absorption and scattering (Loge at al., 1996). Table 1 indicates
the major parameters affecting UV disinfection and their acceptable values. Figure 4
(Loge at al., 1996) represents the possible incomplete penetration of UV light into
wastewater particles.
Parameters Typical values
Percent transmittance (T) or absorbance 35-65
Total suspended solids (TSS) (mg/l) 5-10
Particle size (μm) 10-40
Iron (mg/l) Less than 0.3
Hardness (mg/l) Less than 300
Flow rate or hydraulics -
15
Figure 4 Possible Interactions between UV Light and Wastewater Particles (Loge at al., 1996)
Loge et al. (1999) has reported that UV light is highly absorbed by wastewater flocs,
but it can still penetrate to some extent causing inactivation of microorganisms. They
also found that UV light attenuation within wastewater flocs is mainly due to
absorbance of particles and not due to their scattering. Due to the high porosity of
activated sludge flocs, UV light can penetrate to deeper layers; otherwise, UV light can
only penetrate as far as a few microns into the solid floc material (Loge et al., 1999).
Farnood et al. (2005) stated that EPS is a strong absorber of UV light and the reduction
in UV light intensity within the flocs is due to the presence of EPS. The UV intensity
attenuation of a floc could vary from 1% up to 55%, depending on the positioning of
EPS. UV absorbance of EPS is 400 cm-1 for a 100 µm floc with a density of
approximately 1 g/cm3 and porosity of 90%, when the EPS concentration is 50 mg/g
MLSS(mixed liquor suspended solid)(Farnood et al., 2005).
2.3.1.1 Tailing phenomenon
A semi-logarithmic plot of surviving colony forming units (CFUs) versus UV dose is
termed UV dose-response curve (UV DRC). A typical UV dose-response curve for free-
swimming microorganisms is presented in Figure 5. This curve shows log-linear
inactivation for free-swimming microorganisms. Figure 6 represents a typical response
of coliform bacteria to UV light in wastewater secondary effluent that contains particles.
The near plateau region in this UV DRC that shows the departure from the normal
inactivation kinetics is called tailing, in which a higher UV dose has to be delivered to
16
inactivate the interior indicator microorganism. The presence of two distinct regions
including a steeper initial slope which is followed by a tailing indicates the presence of
UV-resistant particles. As mentioned earlier, the tailing phenomenon occurs at high
doses due to the presence of microbial flocs, which may absorb UV light, or provide
shielding to the microorganisms and prevent UV light from reaching them. The tailing
phenomenon is illustrated in Figure 6, where a decrease in the rate of inactivation can
be observed at higher doses.Tailing implies the presence of residual microorganisms in
water systems even at high UV doses (Qualls, 1983; Loge, 1999; Das, 2001; Tan,
2007), and hence limits the economic viability of UV disinfection. However, tailing also
occurs in chemical disinfection of wastewater where a fraction of coliform bacteria can
survive due to the incomplete penetration of chemical oxidants into the suspended
particles (Berman et al., 1988; Dietrich et al., 2007). The tailing mechanism and the role
of biofloc properties in the UV treatment are not widely understood.
Figure 5 Typical UV Dose-response Curve for Free-swimming Microorganisms (Zimmer, 1997)
17
Figure 6 Typical UV Dose-response Curve for Wastewater Secondary Effluent that Contains Particles (Azimi et al., 2012)
Cerf (1997) suggested that the tailing phenomenon occurs due to the existence of a
diverse population of microorganisms with different degrees of resistance to UV
inactivation. He also specified the association of microorganisms with particles as one
of the possible origins of the tailing effect. Other studies suggest that tailing occurs
when flocs shield the microorganisms from exposure to UV light (Loge et al., 1996;
Loge et al., 1999; Tan, 2007). Loge et al. (1999) discussed that the spatial distribution
of microorganisms significantly affects the degree of inactivation. They hypothesized
that the target microorganisms is located either on a solid-liquid interface with an
unobstructed pathway, or deep within the solid material without a direct pathway to the
liquid bulk. Microorganisms located in regions with an unobstructed light pathway
receive higher doses of UV light compared to the deeply embedded microorganisms
that may escape inactivation. In general, inactivation of pathogens depends on
accessible UV light pathway to defuse or penetrate the aggregates (Emerick et al.,
1999; Dietrich et al., 2007). According to this explanation, floc density would affect the
UV light pathways in such a way that dense particles prohibit UV light from penetrating
and reaching the embedded microorganisms (Emerick et al., 1999). Previously, Jagger
(1967) suggested the presence of two subgroups to explain the two distinct regions of a
dose-response curve, in which one group is more susceptible to UV inactivation
representing 90% of the total particles, and the other group is more resistant to UV
inactivation representing 10% of the total particles. Later, a double-layer structure
18
composed of a compact core surrounded by a loose outer layer was proposed (Liao et
al, 2002; Tan, 2007; Azimi et al., 2012) to describe the tailing effect, and it was
suggested that the compact cores are the main factor contributing to the tailing
phenomenon (Azimi et al. 2012). Azimi et al., (2012) found that between flocs and
cores of similar size, the cores were harder to disinfect with UV light (i.e. higher tailing
level). Moreover, they reported that for cores larger than 45μm, the UV DRCs overlap
and show nearly identical inactivation kinetics.
2.3.1.2 Effect of Particle Size on UV Disinfection
It has been reported in many studies that particle size affects the degree of tailing and
subsequently UV disinfection effectiveness (Quall et al., 1985; Emerick et al., 1999;
Farnood, 2005; Madge et al., 2006). Figure 7 (Das, 2001) shows the effect of removing
particles larger than 8 microns on the UV dose-response curve of final effluents. By
removing the particles larger than 8 microns, the plateau effect is eliminated and the
UV DRC becomes linear. Qualls et al. (1983) obtained similar results, also indicating
that removing larger particles improves the level of disinfection. The limited effect of UV
on the disinfection of larger particles may occur due to the presence of more resistant
coliforms in larger particles.
Figure 7 Typical UV Dose-response for Filtered and Unfiltered Wastewater (Das, 2001) Similar Results Established by Quals et al. (1983) and Tan (2007)
19
Sakamoto and Zimmer (1997) concluded that UV light fully penetrates through particles
smaller than 10 µm, but not through particles larger than 40 µm, even at high UV doses
(Table 2). Thus, 10 µm was a critical size or the lower limit size for shielding
microorganisms (Qualls et al., 1983; Emerick et al., 1999).
Table 2 Increases in Particle Size Directly Affect the UV Dose Demand (Qualls et al. (1983), Emerik et al (1999), Sakamoto and Zimmer (1997))
Particle size(µm) UV penetration/demand <10 Easily penetrated/Low UV demand
10-40 Penetrated to some extent/ Increased UV demand
>40 Will not be completely penetrated/ High UV demand
Madge et al. (2006) showed that the degree of inactivation depends on the particle
size; they concluded that effluents containing smaller particles tend to disinfect more
effectively than the ones with larger particles. However, in their study the particle size
did not exceed 20 μm. Tan (2007) studied the effect of particle size on UV disinfection
of microbial flocs in activated sludge process. He reported that larger particles have
greater tailing propensity. It was concluded that particles larger than 45-53 µm induce
the tailing effect in the UV dose-response curve and as the particle size increases, the
fraction of UV-resistant particles also increases, and the tailing level elevates. The
presence of two distinct regions including a steeper initial slope followed by a tailing
indicates variability in the UV dose response of individual particles within each size
fraction (Tan, 2007). The effect of particle size on UV disinfection kinetics is presented
in Figure 8.
20
Figure 8 Average DRC for Flocs of Different Size Fractions and Free Microbes (Tan, 2007)
The level of pretreatment prior to the disinfection stage affects the particle size
distribution of the effluents. There have been a number of methods suggested to
reduce the degree of tailing by reducing the size of particles. Qualls et al. (1985) and
Das (2001) reported that using filtration within 8 to 10 micron filter as an upstream
process decreases the tailing level. However, Tan (2007) stated that even filtrating of
secondary effluent by 32 µm and 75 µm, led to decreasing of tailing level in UV DRC by
as much as 95% and 80%, respectively. It was suggested the use of ultrasound as an
upstream process for improving the efficiency of UV disinfection (Blume, 2004; Yong et
al. 2009).
2.3.2 Model Fitting of UV Disinfection Performance
In general, a typical UV DRC represents a log-linear inactivation for free-swimming
microorganisms. The UV disinfection of free-swimming (non-particle associated)
microbes corresponds to a single exponential model that follows the Chick-Watson
kinetic model. This model assumes that a single hit is sufficient to inactivate a
microorganism (Zimmer, 1961; Sommer et al., 2001). In this model, the probability of
survival corresponds to first-order kinetics:
21
DkeNN
0
0
−=
Where N is the number of surviving bacteria after UV irradiation, in CFU/100mL, N0 is
the initial number of bacteria before UV light irradiation, in CFU/100mL, D is the
delivered UV dose in mJ/cm2, and k0 is the first-order inactivation rate constant. This
model is applied in the absence of particulate matter and cannot explain the effect of
particulate matter attached to the microorganisms. As mentioned earlier, when a
microorganism is particle associated, the received UV dose by the microorganism is
attenuated. The double exponential model represents the inactivation kinetics of
particle associated microbes for particle-containing effluents, wherein two groups of
particles are assumed to be present within each size fractions: 1) a fraction of UV-
resistant particles mainly contribute to tailing, 2) UV-susceptible particles which their
inactivation kinetics were introduced by the initial slope of the UV DRC (Azimi et al.,
2012).
( ) DkDko eeNN 211 −− +−= ββ
Where N is the number of surviving bacteria after UV irradiation, in CFU/100mL, N0 is
the initial number of bacteria before UV light irradiation, in CFU/100mL, and D is the
delivered UV dose in mJ/cm2. In this equation, k1 and k2 represent the first-order
inactivation rate constants for the initial and the tailing region of UV DRC, which
correspond to UV-susceptible and UV-resistant particles, respectively, and β
represents the fraction UV-resistant particles.
22
Figure 9 Typical UV DRC for Particle-associated Microorganism, Where k1 and k2 Represent Inactivation Rate Constants for the Initial Slope and Tailing Region (Azimi et al., 2012)
2.4 Potential Effect of Iron on UV Absorbance and Disinfection
UV transmittance can be adversely affected by the presence of UV absorbing soluble
compounds, including phenolic compounds, humic acids, lignin sulfonates, iron, and
coloring agents (Mamane, 2008). Lu et al. (2012) investigated the effect of different
coagulants in deionized water on UV light intensity at the wavelength of 253.7 nm. It
was reported that iron salts reduced the UV transmittance of solutions to a higher
degree compared to aluminum salts. While among the aluminum salts, polyaluminium
chloride (PAC) had the greatest worsening effect. Polyferric sulfate (PFS) had the least
UVT compared to other coagulants. Lu et al. (2012) compared UV absorption
coefficients for different coagulants as follows: PFS > FeCl3> Fe2 (SO4)3 > FeSO4 >
PAC > AlCl3 > Al2 (SO4)3. They reported that the UVT of ferric ion (Fe3+) is lower than
that of ferrous ion (Fe2+) in the solution. Asano et al. (2007) also reported that the UV
light molar absorption coefficients at a wavelength of 254 nm are 466 L/mol.cm and
3069 L/mole.cm for Fe2+ and Fe3+, respectively.
The presence of dissolved iron compounds is of certain concern given that these
compounds are commonly added as coagulant agent during primary treatment for
23
odour control and chemical phosphorous removal (Oikonomidis et al., 2010). However,
adding iron decreases suspended solids and turbidity in the effluent (Li, 2005).
UV irradiation causes photochemical alterations in metal-containing particles. Upon
exposure to UV light, Fe (II) and Fe (III) complexes are induced to photolysis
(Schwarzenbach et al., 2003), and these reactions may affect the microorganisms
embedded within or attached to the flocs (Mamane, 2008). Dissolved iron can affect UV
disinfection in a number of ways: 1) it can significantly absorb UV light and lower UV
transmittance; 2) it can also be adsorbed onto wastewater particles and increase the
UV shielding on particle-associated coliform bacteria; 3) Iron can decrease the porosity
of wastewater floc, thereby reducing the availability of unobstructed pathways for UV
light to reach embedded coliform bacteria (Emerick et al., 1999); 4) Iron affects the
effluent particle size distribution (PSD) by increasing the number of small particles, and
consequently the total number of particle aggregates in the effluent (Cairns et al, 1993;
Li, 2005). Kozak et al. (2011) also indicated that a higher UV dose is required to
achieve the desired log reduction when FeCl3 is added to the effluent.
Cairns et al. (1993) established a correlation between the ratio of Fe (III) to suspended
solids (Fe/SS) and UV disinfectability of the effluent. In their study, the reported iron
concentration in SS ranged between 3.7-13.6%. They observed that a higher ratio of
Fe/SS significantly reduces the UV disinfectability of the effluent (Cairns et al, 1993;
Mamane, 2008). Cairns et al. (1993) also reported a high ratio of Fe/SS would create
more nucleation sites for coagulation, and lead to a shift in the size distribution from
larger to small flocs. Moreover, iron reduces the number of larger flocs and produces a
larger fraction of smaller flocs (Li, 2005). Therefore, an increase in the number of
surviving bacteria at higher Fe/SS ratios might be due to: 1) formation of smaller but
more numerous flocs during coagulation; 2) high UV absorbance of the iron; 3) creation
of compressed packing of the iron-rich particles, or a combination of these factors
(Cairns et al., 1993).
Fe (III) can bridge negatively charged functional groups of the EPS, which also leads to
bioflocculation, greater floc density, and more resistance to shear (Zhang et al., 2008;
24
Li, 2005). Moreover, Li (2005) observed that increasing of FeCl3 dosing leads to a
higher concentration of iron in the EPS. In addition, Huang et al. (2011) concluded that
iron concentration could significantly affect the composition and microbial activity of
EPS in sludge biofilms. They showed that the biofilms growing on substrates with
higher concentrations of Fe (III) has a higher EPS content. It is recognized that EPS
are responsible for developing particle aggregates (microbial flocs) and binding cells
and other particulate maters, leading to various activated sludge floc characteristics
(structure and size of flocs) (Cairns et al., 1993; Li, 2005; Oikonomidis et al., 2010).
Li (2005) reported that concentrations of iron up to 23.8 mg/dm-3 can significantly affect
floc morphology and reduce filamentous microorganisms available for the formation of
larger microbial aggregates. In a study by Oikonomidis et al. (2010), light microscopy
observations showed that addition of iron has a noticeable effect on floc morphology.
The rich Fe-dosed flocs appear to be compact, with yellowish-orange colour and
distinct boundaries compared to the flocs without Fe-dosing. They compared the effect
of Fe (III) and Fe (II) salts on the physical and biological properties of activated sludge.
They reported that the flocs’ morphological characteristics significantly vary based on
the type of ion used. Flocs that were dosed by Fe (II) were more compact, and smaller
in size compared to those that were dosed by Fe (III), and contain less filamentous
microorganisms. However, the settleability index for the Fe (II) dosed sludge was lower
than that for the Fe (III) dosed sludge. They reported that Fe (II) is more effective for
flocculation than Fe (III), due to its ability to form stronger ionic bonds with the flocs
prior to its oxidation to Fe (III).
Conventional light and scanning electron microscopy (SEM) images showed that the
surface of the floc dosed with Fe (II) was largely covered by a smooth layer
(presumably EPS), which led to the formation of a well-flocculated floc (Oikonomidis et
al., 2010). In contrary, flocs dosed with Fe (III) were diffuse and irregular. Floc size
decreased with dosing with both units. The destruction of the network of backbone
filaments by Fe was responsible for the flocs size reduction (Oikonomidis et al., 2010).
They observed that iron provides a coating surface by a “cobweb”- like material which
is supposedly responsible to trap and entangle free swimming cells within the floc
25
matrix and enhance the flocculation. The presence of this coating and its thickness
were dependent dramatically on the iron dosing period. When the period of iron dosing
is longer, this surface was thicker and more integrated (Oikonomidis et al., 2010).
26
Chapter 3 Experimental Methods
3.1 Experimental Approach
A double-layer structure composed of a compact core surrounded by a loose outer
layer was proposed by earlier studies (Liao et al, 2002; Azimi et al., 2012) to describe
the behaviour of microbial flocs in UV disinfection. This study investigates the
disinfection kinetics of the aerobic and anaerobic bacteria in compact cores and
microbial flocs. Hydrodynamic shear stress was applied to extract compact cores from
microbial flocs using a Couette flow device. The Couette flow shears the flocs and
peels off their loose outer shell. Then, microbial floc and compact core samples were
fractionated into narrow sizes and their UV disinfection kinetics were determined.
To study the effect of iron on the mechanical strength and UV disinfectability of
activated sludge flocs, a simplified laboratory scale sequencing batch reactor (SBR)
setup was used.
3.2 Materials and Methods
3.2.1 Sample Collection
Wastewater samples were collected from Ashbridges Bay municipal wastewater
treatment plant, located at the eastern end of Toronto, Canada. The plant has a
capacity of 818000 m3/day, and the secondary treatment includes a conventional
activated sludge biological treatment. Treated effluent is disinfected with chlorine
before discharging into the Lake Ontario. Mixed liquor samples were collected from the
aeration tank prior to discharge into the secondary clarifier. Secondary effluents were
collected at the end of the secondary clarifier, right before the point that effluent is
channelled to be disinfected. The collected samples were stored at 4°C and analyzed
within 12 hours to ensure that storing did not change sample characteristics.
3.2.2 Sieving
In order to investigate the effect of particle size, the collected mixed liquor sample was
fractionized into narrow size fractions using a sieve (U.S.A. Standard Testing Sieve).
27
The opening sizes of the sieves used from bottom to top were: 45, 63, 75, 90 microns.
The fractions of interest were: 45-63, 75-90 microns. These particle fractions were
gently washed with deionized distilled water (Milli-Q Water Systems, Millipore
Corporation, MA) for at least 15 minutes to ensure smaller particles were washed
away. Then, the remaining particles were collected off the sieve and used for particle
size distribution analysis and UV bioassay.
3.2.3 Particle Size Distribution Analysis
Particle size distribution analysis was carried out using a Multisizer 3.0 particle size
analyzer (Beckman Coulter Canada, Mississauga, Ontario, Canada). Fractioned
samples were diluted with NaCl solution with a concentration of 9.7 g/L to obtain proper
concentration and then analyzed to evaluate the particles’ size distribution in the
solutions. The Multisizer operates based on Coulter principal, which underestimates the
actual size of porous particles, such as microbial flocs. The Multisizer indicates the
volume of solid fraction in porous particles (solid volume). In this case, the actual
particle sizes are greater than the reported values (Yuan et al., 2009). In this study, the
various particle sizes are referred to those of the sieve openings.
Yuan (2007) reported a correlation between the actual size of particles and their solid
volume size (Coulter) for the same equipment,
D = 0.82 d1.24
where D is the actual particle size according to the sieve opening and d is the Coulter
particle size measurement which is determined by the Multisizer.
3.2.4 Shearing
To extract the compact cores from the microbial flocs, the microbial flocs were
subjected to hydrodynamic shear stress in a Taylor–Couette flow cell, a custom-made
device consisting of a stainless steel spindle (4.0 cm in diameter and 4.0 cm in length)
and a transparent acrylic cup (4.4 cm in diameter and 6.5 cm in length). There is a 0.2
cm gap between the spindle and the cup, which is filled with 10 mL of activated sludge
28
flocs in each experimental run. The spindle is rotated by an adjustable speed motor
(Servodyne 50003-04, Cole-Parmer Instruments Co.), with rotational speeds ranging
from 500 to 6000 rpm set by a digital controller. The turbulent shear stress as a
function of the rotational speed is provided in Table 3. The shear stress generated by
the rotational speed is applied to the activated sludge microbial flocs at a rotational
speed of 3500 rpm for 15 min, causing the loose outer region to peel off. The
suspension is then collected and sieved to separate various size fractions, which are
kept for particle size analysis and UV disinfection assay. Microbial floc and compact
core samples were fractionated into narrow sizes using mechanical sieving and the
corresponding UV disinfection kinetics were determined (Yuan et al., 2007; Yuan et al.,
2010; Azimi et al., 2012).
(a) (b)
Figure 10 Couette Flow Apparatus, (a) Actual Lab Equipment, (b) Schematic of the Couette Flow Device (Yuan et al., 2007; Azimi et al, 2012)
Cup
Spindle
Motor
4.4 cm
4 cm
0.2 cm
Cup
Spindle
Motor
4.4 cm
4 cm
0.2 cm
Cup
Spindle
Motor
4.4 cm
4 cm
0.2 cm
29
Table 3 The turbulent Shear Stress as a Function of the Rotational Speed (rpm), Yuan et al., 2010
N (rpm) Turbulent Shear Stress (Pa)
1000 2.1
2000 5
3000 8.6
4000 12.6
5000 17.1
6000 21.9
3.2.5 UV Bioassay
In this study, a low-pressure mercury vapor UV lamp (Trojan Technologies, London,
Ontario, Canada) was used; approximately 85% of the UV light it produces has a
wavelength of 254 nm (EPA, 2006). The system consists of two lamps kept in a
horizontal stainless steel cover, and a black-painted collimated tube (22cm in length
and 9cm in diameter) that is located vertically downwards to achieve a region of
uniform UV irradiation. The UV incident intensity (I) was measured at the surface centre
of the wastewater sample by means of a calibrated IL radiometer with an SED240
sensor and an NS254 filter (International Light, Newburyport, MA, USA). For each UV
dose, the correlated UV exposure time is calculated by a spreadsheet developed by
Bolton et al.(2003), based on the sample UV intensity and absorption at a wavelength
of 254 nm. The samples’ UV absorption was measured using a Lambda 35 UV/Vis
spectrometer (Perkin Elmer, Wellesley, MA, USA) at a wavelength of 254 nm. In this
experiment, 20 mL of activated sludge was placed in a petri dish with a diameter of 4.8
cm and constantly stirred with a magnetic stirrer during UV irradiation. Then, the
different size fractions were exposed to UV irradiation at various doses (10, 20, 30, 45,
and 60 mJ/cm2), and the corresponding UV dose-response curves were obtained for
three replicates of each UV dose.
30
For each UV dose, the membrane filtration method (Millipore sterile 0.45μm) was used
to determine the number of surviving fecal coliforms after UV irradiation. The fecal
coliforms that remained on the sterile filter were cultured on m FC agar media (VWR,
Mississauga, Ontario), which was then incubated at 44ºC for approximately 24± 2 hr.
After incubation, the colony forming units (CFUs) were enumerated.
For the anaerobic set of experiments, in order to reduce the oxygen in the growth
media for anaerobic culturing, cysteine (i.e. a reducing agent) was added to m FC agar
solution to a concentration of 0.5g/L; resazurin was added as an indicator for reducing
process to a concentration of 0.002 g/L. Resazurin is a blue dye, which its colour
changes to pink in micro oxygen containing environment, and in the absence of oxygen
becomes transparent. The m FC agar prepared for anaerobic culturing was kept in the
anaerobic cabinet (type B vinyl anaerobic chamber; Coy Laboratory Products, MI) to
prevent oxygen diffusion in the culturing media. The anaerobic incubator was located in
the anaerobic cabinet and its temperature was set at 44°C.
3.2.6 Culturability of Flocs and Cores
To investigate the viability of flocs and cores before and after UV irradiation,
wastewater flocs were fractioned into size ranges of 75-90 µm. A portion of the
fractionated floc sample was exposed to UV irradiation at a dose of 100 mJ/cm2.
Subsequently, part of the UV treated floc sample was subjected to hydrodynamic shear
stress using the Couette flow cell at shearing rates of 3500 and 5000 rpm. The sheared
samples were sieved to collect cores larger than 45 µm. The concentration of culturable
flocs and cores was obtained by culturing and enumerating the CFUs, and the total
concentration of flocs and cores was measured by the Multisizer particle size analyzer.
The culturability values were determined by dividing CFU number by the total particle
concentration in the sample.
In order to examine the culturability of the inner regions of the compact cores, i.e., the
region deeper than the thin culturable layer suggested by Azimi et al. (2012), the inner
region of the compact core was extracted by shearing. First, to isolate cores, shear
stress was applied to the flocs of size fraction 75-90 µm at 3500 rpm. Then the sheared
31
samples were sieved to collect cores larger than 45 microns. Subsequently, a higher
shear rate of 6000 rpm was applied to the compact cores and the sheared samples
were sieved to isolate sizes larger than 25 microns. The inner layers of the compact
core were cultured under aerobic and anaerobic conditions, as discussed in Section
3.2.5, and the culturability values were determined.
3.2.7 Double Exponential Model Fitting
As mentioned earlier, flocs and cores were isolated into narrow size fractions and UV
DRCs were determined under aerobic and anaerobic growth conditions to study their
disinfection kinetics. Data was collected over a 6-month period; the averaged UV DRCs
are presented in the Result Section of this document. In all of the UV DRCs from the
experimental data, two distinct regions appeared consisting of a steeper initial slope,
and the tailing region. Earlier studies (Jagger, 1967; Tan 2007, Azimi et al, 2012)
suggested the presence of two subgroups to explain the two distinct regions of a dose-
response curve. The experimental data were fit into the double exponential equation
(section 2.3.3) using non-linear regression by means of Mathematica software 7. The
double exponential equation was employed to represent the inactivation kinetics of the
microorganisms in the activated sludge solution with UV light. In the model, the UV-
resistant particles have inactivation kinetics represented by the slope of the tailing; and
the UV-susceptible particles have inactivation kinetics represented by the initial slope of
the UV dose-response curves. In this model, k1 and k2 represent the initial slope and
the slope of tailing, respectively, and β represents the fraction of UV-resistant particles.
3.2.8 Experimental Setup of the Sequencing Batch Reactors
To study the effect of iron on the mechanical strength and UV disinfectability of
activated sludge flocs, a simplified laboratory scale sequencing batch reactor (SBR)
setup fed with a synthetic feed was used as a short-term critical test. The set up
consisted of three 1400 mL reactors operated in parallel in batch mode (control, iron-
dosed (6 mg/L), and iron-dosed (15 mg/L)). There are several benefits achieved by
using SBRs, which are dealing with a smaller reactor volume, smaller amount of feed
requirements, and proper foaming control. Since sedimentation and reactions occur in
32
the same reactor, the need for a separate settling tank as a clarifier is eliminated in this
system. A synthetic feed with a desired concentration was used so that the daily
fluctuations from industrial or municipal wastewater feed were eliminated. The
simplified experimental setup of the sequencing batch reactors are presented in Figure
11.
Figure 11 Experimental Setup of the Sequencing Batch Reactors
During start-up, the SBRs were inoculated with concentrated mixed liquor without iron
supplementation. The inoculum was obtained from the aeration tank of a wastewater
treatment pilot plant in the National Water Research Institute located at Burlington,
Canada. The inoculum was introduced in the reactors, and fed with a synthetic feed
composed of glucose (Bioreagent grade, Sigma Aldrich) and inorganic salts with a
desired COD (chemical oxygen demand): N: P ratio which was 100:5:1. The synthetic
feed was kept in the refrigerator at 4 °C. All chemicals used were of analytical grade,
and all dilutions were performed using deionized distilled water (Milli-Q Water Systems,
Millipore Corporation, MA). The composition of the synthetic wastewater feed is
presented in Table 4.
33
Table 4 Chemical Composition of the Synthetic wastewater (Liao et al, 2002) (COD=1100 mg/L, COD: N: P = 100:5:1; COD from glucose, N from NH4Cl and P from KH2PO4)
Chemicals
Concentration (mg/L) Grade and Source of Chemicals
MgS04.7H20
5.07 (0.5 Mg2+) Analytical (Sigma Aldrich)
FeSO4.7H20 2.49 (0.5 Fe2+) Analytical (BDH)
Na2Mo04.2H2O
1.26 (0.5 Mo6+) Analytical (Sigma Aldrich)
MnS04.4H20
0.31 (0.1 Mn2+) Analytical (Sigma Aldrich)
CuSO4
0.25 (0.1 Cu2+) Analytical (Sigma Aldrich)
ZnS04.7H20
0.44 (0.1 Zn2+) Analytical (VWR)
NaCl
0.25 (O. 1 Na+) Analytical (Sigma Aldrich)
CaS04.2H20
0.43 (0.1 Ca2+) Analytical (BDH)
CoCl2.6H20
0.41 (0.1 Co2+) Analytical(Sigma Aldrich)
The SBRs were operated for a three-month period and a five-week stabilization period
was required to reach stable conditions. Stable operating conditions were determined
by monitoring the mixed liquor suspended solids (MLSS), and COD removal. When the
data for these parameters reached relatively constant levels the reactors were
considered at stable operating conditions. All three SBRs were operated at a sludge
retention time (SRT) of 7 days. Aeration and mixing were sustained to ensure aerobic
operating conditions inside the batch reactors. The pH was kept at 7-8 by adding
sodium bicarbonate (Bioreagent grade, Sigma Aldrich).The SBRs operated in a fill and
draw cyclic mode with 24 hours per cycle, which consisted of one cycle per day. In each
cycle there were five stages: filling, aeration (reaction), waste, settling, and decanting. The
filling, waste, and decanting were carried out manually once a day for these reactors.
In the filling mode, the synthetic feed was introduced manually to the reactors. When the
reactor volume reached 1400 mL level, the aeration started mixing the contents. In the
waste mode, a volume of 200 mL of the mixed liquor were removed. Then, the air pumps
34
were turned off and the mixed liquor were allowed to settle for approximately 2 hours
(settling). A volume of approximately 1000 mL of the treated effluent, which roughly
represents 75% of the total operating volume, was decanted from the reactors in the
decanting stage. The sample collected from waste stage is mixed liquor sample that
were fractioned into narrow sizes and analyzed for mechanical strength and UV
disinfectability. The sample from decanting stage is the treated final effluent.
3.2.9 Mechanical Strength Test (Breakage Test)
In order to investigate the effect of iron on the mechanical strength of flocs from the
three SBRs, the breakage percentage of the flocs was compared under similar
hydrodynamic shear stress. Floc sample solutions consisting of flocs with size of 45-90
μm with a similar solid content were prepared. Then, 10 mL of each floc sample was
placed in the gap between the spindle and the cup in a Couette flow cell (schematic
provided in Figure 10). The spindle speed was adjusted to create the target shear rate
and the samples were sheared for 8 minutes. During shearing, flocs with a higher
mechanical strength than the applied stress survive the breakage.
After shearing at various rates (ranging from 1500-4500 rpm), the particle size
distribution of the sheared samples was measured using a Multisizer 3 (Beckman
Coulter, Miami, USA). Breakage percentage was calculated based on the number of
flocs that survive the shearing stress (Yuan et al., 2010; Yong et al., 2009; Gibson et
al., 2009):
% Breakage = (1 –0P
Ps
NN
) x 100
where NP0 and NPs represent the number of flocs larger than the mode size before and
after shearing, respectively. From the above equation, the breakage percentage was
calculated as a function of the turbulent shear stress applied by Couette flow.
35
3.2.10 Microscopy and Image Analysis
Optical microscopy images were taken using an Olympus BX51 Microscope (LECO
Instruments, Mississauga, Ontario, Canada) and the Olympus Cellsens 1.4 software
using a 40X oil immersion objective lens, at a resolution of 0.16 µm. The microscopy
images were taken in various depths of the sample, and Combine ZP software
(Combine Z-Pyramid) was used to overlay these images. Combine ZP is image
processing software, which is capable of processing a stack of frames or images and
blending the focused areas of all those partially focused frames. The software
generates an image that is in focus for each pixel, in which the depth of field (DOF) is
extended for all layers.
36
Chapter 4 Results and Discussion
4.1 Particle Size Distribution of Flocs and Cores
Figure 12 and Figure 13 represent the particle size distribution for flocs and cores of
size fractions 45-63, and 75-90 µm. As mentioned earlier, the size detected by the
Multisizer is smaller than the actual dimensions of the particles because this device
ignores the porosity of the particles. For comparison, particle size distribution of final
effluent (collected from the secondary clarifier) is given in Figure 14. These results
show that the mechanical sieving was effective in separating particles into narrow size
fractions.
Figure 12 Particle Size Distributions for Flocs and Cores of Size 45-63 µm
37
Figure 13 Particle Size Distributions for Flocs and Cores of Size 75-90 µm
Figure 14 Particle Size Distributions of Final Effluent
38
4.2 UV Dose-response curves and Model Fitting
Dose-response experiments performed in this study were divided in two sets: 1- UV
DRC of flocs and compact cores under aerobic growth condition; and 2- UV DRC of
flocs and compact cores under anaerobic growth condition. Results of the UV DRC for
flocs and cores of different size fractions are provided and discussed in the following
sections. Followed by the experimental results, the model fitting results are presented,
using the double exponential model discussed in section 3.2.6.
4.2.1 UV DRC of Microbial Flocs under Anaerobic and Aerobic Growth Conditions
The normalized UV DRCs for flocs of two size fractions under anaerobic and aerobic
growth conditions are presented in Figure 15 and Figure 16, respectively.
Figure 15 Normalized UV DRCs for Flocs of Two Size Fractions under Anaerobic Growth Condition
39
Figure 16 Normalized UV DRCs for Flocs of Two Size Fractions under Aerobic Growth Condition
Similar to earlier works, the UV DRCs of all size fractions had two distinct regions: a
steep initial slope followed by a tailing region. It has been suggested that the initial
slope of the UV DRC of flocs corresponds to the disinfection of UV-susceptible
particles and the tailing region represents the disinfection of the UV-resistant particles
(Azimi et al., 2012). Moreover, the degree of susceptibility of particles to the UV
irradiation is proposed to depend on the particle structure and the location of most
shielded microorganism within the particles (Azimi et al., 2012).
Figure 15 and Figure 16 also show that larger particles had higher level of tailing;
similar results were obtained by Tan (2007) and Azimi et al. (2012). Larger particles
can attenuate UV light more effectively hence better shield the embedded
microorganisms from UV disinfection (Loge et al. 1999; Tan 2007; Azimi et al., 2012).
Moreover, larger particles are more likely to contain UV-resistant cores, leading to
higher resistance to UV disinfection (Azimi et al, 2012).
4.2.2 UV DRC of Compact Cores under Anaerobic and Aerobic Growth Conditions
The normalized UV DRCs for core samples under anaerobic and aerobic growth
conditions are presented in Figure 17 and Figure 18, respectively. Similar to the floc
40
samples, the UV DRC of cores exhibited an initial steep slope followed by a tailing
region. This behaviour represents the presence of UV-resistant and UV-susceptible
cores in the sample.
Figure 17 Normalized UV DRCs for Cores of Two Size Fractions under Anaerobic Growth Condition
Figure 18 Normalized UV DRCs for Cores of Two Size Fractions under Aerobic Growth Condition
41
Figure 17 and Figure 18 indicate that under both sets of growth conditions, the tailing
level elevated as the size increased, indicating that larger cores were more resistant to
disinfection. This is contrary to the findings of Azimi et al. (2012), who reported that
when the compact cores were larger than 45μm, the UV DRCs for cores of various size
fractions were nearly identical under aerobic growth conditions.
4.2.3 Comparison of UV DRC of Microbial Flocs and Compact Cores
The normalized UV DRCs for flocs and cores of similar size fractions under anaerobic
and aerobic growth conditions are presented in Figure 19 to Figure 22. No significant
difference in UV DRCs of cores and flocs of the same size fractions was observed
under both sets of conditions. This is contrary to the findings of Azimi et al. (2012), who
reported that compact cores had a higher tailing level compared to flocs of the same
size fractions under aerobic growth condition.
These discrepancies might be due to changes in the FeCl2 addition as coagulant in the
primary stage at the wastewater treatment plant. Table 5 shows the FeCl2
concentration at the Ashbridges Bay treatment plant, where the mixed liquor samples
for this study and the previous study by Azimi et al. (2012) were collected. Following
the study of Azimi et al. that was conducted in 2009, the level of ferrous chloride
steadily increased over the past three years (not including 2010). Increasing the dose
of FeCl2 could increase in the residual iron carried over to the aeration tank. This could
justify the overlapping of the DRC of flocs and cores through one or a combination of
the following possible mechanisms:
1- Given that iron is a strong UV absorber, this will increase the UV absorbance of
floc material and hence floc resistance to UV disinfection. A highly UV-absorbing
floc material could mask variations in the internal structures of the floc and result
in similar UV DRC.
2- Increased iron addition could cause the removal of a larger fraction of the
particles in the primary treatment process. The primary particles that are carried
to the aeration tank are likely the main constituent of the compact cores. If these
particles are removed, the flocs formed during the aeration stage will less likely
42
contain a compact core. Therefore, the material isolated in this study by the
Couette flow device could have a similar structure to that of original flocs, and
hence a similar UV DRC.
3- Increase in the iron concentration can enhance the mechanical stability of the
flocs. Therefore, floc samples in this study were likely more resistant to shear
forces. Hence, the isolated core samples obtained by the shearing in the
Couette flow cell were similar in structure to the original flocs and hence had a
similar UV DRC.
Table 5 Changes in FeCl2 Concentration as Coagulant in Primary Treatment (Data collected from Ashbridges Bay annual reports, provided in
http://www1.toronto.ca/wps/portal/)
Year FeCl2(mg/L)
2009 5.67
2010 5.31
2011 6.01
2012 6.92
In Figure 23 and Figure 24, the normalized UV DRCs for flocs and cores of two size
fractions under both sets of growth contains are compared. The results show that under
both aerobic and anaerobic growth conditions, UV DRCs followed a similar pattern, and
for the same size fraction they nearly overlapped. It should be noted that fecal coliforms
(the indicator microorganisms used in the UV bioassay) are facultative anaerobes and
could grow under both aerobic and anaerobic conditions. Hence, this result suggests
that the fecal coliforms cultured under both aerobic and anaerobic conditions have
similar response to UV irradiation. These figures also show that under aerobic and
anaerobic growth conditions, the tailing level for flocs and cores elevated as the size
increased, showing larger particles were more resistant to UV inactivation.
43
It is worthwhile to note here that in the standard membrane filtration technique (APHA
2001, # 9222 A), one floc/core leads to maximum one CFU count, regardless of
whether there is one or many surviving coliform bacteria embedded within that floc.
This is a critical limitation of this technique; however, it is still the current
industry/regulatory standard for examination of water and wastewater disinfection
technologies (Cairns et al., 1993; Azimi et al., 2012).
Figure 19 Normalized UV DRCs of Cores and Flocs of Size 45-63 µm under Anaerobic Growth Condition
44
Figure 20 Normalized UV DRCs of Cores and Flocs of Size 75-90 µm under Anaerobic Growth Condition
Figure 21 Normalized UV DRCs of Cores and Flocs of Size 45-63 µm under Aerobic Growth Condition
45
Figure 22 Normalized UV DRCs of Cores and Flocs of size 75-90 µm under Aerobic Growth Condition
Figure 23 Normalized UV DRCs of Flocs of Two Size Fractions under Aerobic and Anaerobic Growth Condition
46
Figure 24 Normalized UV DRCs of Cores of Two Size Fractions under Aerobic and Anaerobic Growth Condition
4.2.4 Culturability of Floc and Core
The culturability values for untreated and UV irradiated flocs of size fraction 75-90 µm
were 17% and 0.2%, respectively. The viability values for compact cores extracted from
the UV irradiated flocs (viability=0.2%) at 3500, and 5000 rpm shearing rates were
0.5% and 0.54%, respectively (Table 6). The results showed that the culturability of
cores extracted from UV irradiated flocs increased by a factor of more than two
compared to culturability of UV irradiated flocs, showing that UV light could penetrate
the compact cores to some extent. However, UV light can be attenuated more severely
within the compact cores and hence better shield the embedded microorganisms from
UV disinfection.
Table 6 Culturability Values for Flocs (Untreated and UV Irradiated) and Compact Cores Extracted from UV Irradiated Flocs
Sample Culturability (%)
Untreated Wastewater Flocs (75-90 μm) 17
UV Irradiated Flocs (75-90 μm) 0.2
47
Cores Extracted from UV Irradiated Flocs (>45 μm , @ 3500 rpm) 0.5
Cores Extracted from UV Irradiated Flocs (>45 μm , @ 5000 rpm) 0.54
The results listed in Table 6 also show that cores (>45 μm) isolated at 3500 rpm and
5000 rpm had similar culturability values. Higher rotational speeds are expected to
further reduce the core size by peeling and fragmenting the cores. Therefore, this
finding is consistent with the proposed model by Azimi et al. (2012) that indicator
organisms are distributed evenly within the core material.
Table 7 Culturability Values for Inner Region of Compact Cores Extracted from Untreated flocs under Aerobic and Anaerobic Growth Condition
Inner Region of Compact Core(>25 μm, untreated) Culturability (%)
Aerobic Growth Condition 3
Anaerobic Growth Condition 10
The average culturability values for inner regions of compact cores (>25 μm) were 3%
and 10 % under aerobic and anaerobic growth conditions, respectively (Table 7).
Formation of colony forming units (CFUs) in the plate count method showed that the
inner region of the compact cores contained culturable microorganisms. The
culturability values for the inner region of the cores are higher under anaerobic growth
condition. However, this variation is not statistically significant (p=0.115).
4.2.5 UV DRC of Final Effluent under Anaerobic and Aerobic Growth Conditions
Figure 25 shows the normalized UV DRC of final effluent under aerobic and anaerobic
growth conditions. Similar to the result obtained for cores and flocs, the normalized UV
DRCs of effluents cultured under aerobic and anaerobic conditions followed a similar
pattern and nearly overlapped. The UV DRC of final effluents had steeper slopes and
lower tailing levels compared to those of floc and core samples. The inactivation level
48
reached to approximately one log reduction for flocs of size fraction 75-90 µm, about
two log reductions for flocs of size fraction 45-63 µm, and three log reductions for the
final effluent at a dose of 60 mJ/cm2. This is expected since cores and flocs samples
were consisted of isolated large particles that are inherently more resistant to UV
disinfection while the final effluent contained a small fraction of large particles (See
Figures 13 & 14). Figure 25b shows the UV DRC of final effluent under aerobic and
anaerobic growth conditions. It shows that the initial CFU (N0) under anaerobic growth
conditions was higher than of aerobic ones, However, this variation is not statistically
significant (p=0.22).
Figure 25 Normalized UV DRCs of Final Effluent under Aerobic and Anaerobic Growth Condition
49
Figure 25.b UV DRCs of Final Effluent under Aerobic and Anaerobic Growth Condition
4.2.6 Model Fitting of UV DRC for Flocs and Cores
In this section, the effect of floc and core size on the UV disinfection kinetics under
aerobic and anaerobic growth conditions is reported. Moreover, the double exponential
parameters for each condition are presented and compared.
4.2.7 Comparison of Model Parameters
The double exponential model parameters for floc and core samples; i.e. k1, k2, and β,
for anaerobic and aerobic growth conditions are presented in Table 8 and Table 9,
respectively. Comparison between the model predictions and the experimental results
are given in Figure 26 to Figure 29. The double exponential model fits the
experimental data with an average relative error of 10 %.
50
Table 8 Inactivation Rate Constants (k1 and k2) and UV-resistant Fraction (β) of Floc and Core Samples under Anaerobic Growth Condition
Particle Size(µm) k1(cm2/mJ) k2(cm2/mJ) β
Floc
45-63 0.13±0.02 0.011±0.002 0.41±0.05
75-90 0.10±0.01 0.010±0.002 0.51±0.04
Cores
45-63 0.11±0.02 0.010±0.003 0.39±0.07
75-90 0.10±0.01 0.010±0.002 0.53±0.05
Table 9 Inactivation Rate Constants (k1 and k2 ) and UV-resistant Fraction (β) of Floc and Core Samples under Aerobic Growth Condition
Particle Size(µm) k1(cm2/mJ) k2(cm2/mJ) β
Floc
45-63 0.13±0.02 0.011±0.003 0.40±0.05
75-90 0.11±0.01 0.010±0.001 0.54±0.05
Core
45-63 0.11±0.03 0.011±0.001 0.39±0.06
75-90 0.10±0.01 0.011±0.002 0.56±0.03
The initial inactivation rate constant, k1, for flocs was significantly lower than that of the
free swimming coliform bacteria that has been reported to be 0.67 cm2/mJ (Wright &
Cairns, 1999). This result is in agreement with the findings of Azimi et al. (2012) that
proposed the initial slope of the UV DRC was due to the presence of UV-susceptible
particles and not the free swimming microorganisms.
Table 8 and Table 9 show that k1 decreased with increasing particle size for both the
floc and core samples, emphasizing that larger particles were more resistant to UV
inactivation. However, the effect of size was more pronounced in the case of flocs. In
other words, the initial inactivation rate for the core samples was less sensitive to size
compared to that of floc samples. This may suggest that cores contained a smaller
51
fraction of the UV-susceptible constituent. The reason for this may be that during the
shearing process, loose UV-susceptible flocs were disintegrated and subsequently
removed by the sieving process. This result is in agreement with the previous study by
Azimi et al. (2012) that reported k1 did not vary significantly by changing the cores size
above 45 μm. Contrary to k1, the inactivation rate constant for the tailing region, k2, was
independent of the particle size and did not show any significant variation with the
particle size.
The results also show that larger particles (flocs and cores) had higher β values, and
higher β translates into higher UV resistance and higher tailing level. Therefore, particle
size had a strong effect on the tailing level of UV DRCs, and by increasing particle size,
the UV dose demand increased significantly. Moreover, the results show that flocs and
cores of the same size fraction had similar β values. This result is contrary to the
findings of Azimi et al. (2012), who reported that cores had higher β values than flocs of
the same size fraction. The possible causes for this discrepancy have been discussed
in an earlier section.
Figure 26 Comparisons of Experimental and Model Predicted Results for Flocs under Anaerobic Growth Condition
52
Figure 27 Comparisons of Experimental and Model Predicted Results for Cores under Anaerobic Growth Condition
Figure 28 Comparisons of Experimental and Model Predicted Results for Flocs under Aerobic Growth Condition
53
Figure 29 Comparisons of Experimental and Model Predicted Results for Cores under Aerobic Growth Condition
4.3 Impact of Iron on Flocs’ Characteristics and UV disinfection Kinetics
4.3.1 Effect of Iron on PSD
Iron addition altered the particle size distribution of the activated sludge flocs. Figure 30
indicates that the addition of iron increased the number of smaller flocs. This result is in
agreement with an earlier study by Oikonomidis et al. (2010), who observed that flocs
that were dosed by Fe (II) were smaller in size. It was suggested that the reduction in
filamentous bacteria (essential for large floc formation) by iron was responsible for
changes in the particle size distribution. A moderate number of filamentous
microorganisms are generally required to provide an integrated structure for flocs to
settle efficiently. Cairns et al. (1993) also reported a correlation between iron to
suspended solids (Fe/SS) ratio and particle size distribution of activated sludge flocs,
and found that a higher ratio of Fe/SS led to the formation of a larger number of smaller
flocs and subsequently total flocs, and attributed this finding to the reduction in the
amount of filamentous microorganisms at higher iron levels.
54
Figure 30 Effect of Iron Dosing on Particle Size Distribution of Activated Sludge Flocs (Number)
The peak observed in Figure 30b show an increase in the particle size distribution
between size fractions of 20~40 μm by iron addition. This increase can result in a
greater number of larger flocs and likely lead to a higher tailing level.
55
Figure 30.b Effect of Iron Dosing on Particle Size Distribution of Activated Sludge Flocs (Volume %)
4.3.2 Microscopy and Image Analysis
Figure 31 illustrates the morphological differences between iron-dosed and iron-free
flocs. It is shown that dosing with Fe+2 had a significant effect on the floc morphological
characteristics in a laboratory-based activated sludge system. The iron-dosed flocs
were generally more optically opaque and more compact; they also appeared to have a
more regular shape that suggests the relative absence of filamentous microorganisms.
In addition, the iron-dosed flocs had well-defined boundaries and orange/yellowish
colour, possibly implying the diffusion of iron in the floc structure. Figure 31 may also
addressed that iron could result in the formation of more homogeneous, but more
compact flocs.
56
a) iron-dosed floc b) iron-free floc
Figure 31 Microscopy Images for Iron-free and Iron-dosed Flocs
Oikonomidis et al. (2010) suggested that Fe (II) provides a coating surface for the flocs,
which enhances the floc compactness and integrates filamentous microorganisms
within the floc structure. Subsequently, forming links between particles would be
prohibited to some extent. Filamentous bacteria play a crucial role in the integration of
floc structure since they performs as a rigid backbone that gives the flocs the main
structure. The filamentous microorganism could be significantly enclosed by the
coating surface created by Fe and ultimately buried within the Fe-dosed flocs. This may
result in a decrease in the number of filamentous microorganisms due to substrate
diffusion limitation.
4.3.3 Effect of Iron on Mechanical Strength of Flocs
Figure 32 shows that the floc samples dosed with iron were more resistant to breaking
under hydrodynamic shear stress. Table 10 provides the median shear stress (τ 50), i.e.
the shear stress at which the number of flocs larger than the median floc size
decreased by 50%, for flocs formed with and without iron addition. As shown in Table
10, the iron-dosed flocs had a higher τ 50, suggesting that they were more resistant to
breakage.
The results are in agreement with Oikonomidis et al. (2010) observation who suggested
that Fe (II) can bridge negatively charged functional groups within the EPS, and leads
57
to an increase in floc resistance to shear. They also proposed that iron provides a
coating surface for flocs which could enhance their resistance to shear stress. As
pointed out earlier, an increase in the mechanical strength of flocs could lead to core
samples that have similar UV DRC to that of flocs for the same size fraction.
Table 10 Median Shear Stress Values for Floc Breakage for Fe-dosed, and Fe-free Flocs (45-90 µm
Size Fraction)
Sample τ50(Pa) Floc dosed with 0 mg/L iron 6.7 Floc dosed with 6 mg/L iron 7.7
Floc dosed with 15 mg/L iron 10.7
Figure 32 Mechanical Strength Tests for Iron-free and Iron-dosed Flocs
4.3.4 Effect of Iron on UV DRC
Figure 33 illustrates the effect of iron dosing on UV DRCs of the 45-63 μm flocs. As
shown in the Figure 33, the iron-dosed samples had a higher tailing level which showed
58
higher percentage of UV-resistance of flocs; i.e. higher β. The initial slope of UV DRC
was less steep for iron-dosed flocs, while the slope of the tailing region was not
significantly affected by iron dosing. In conclusion, iron can increase the resistance of
flocs to UV disinfection.
Table 11 Inactivation Rate Constants (k1 and k2) and UV-resistant Fraction (β) of Iron-free and Iron-dosed Floc of Size fraction 45-63 μm
Sample k1 k2 β
Iron-free flocs 0.19±0.03 0.004±0.001 0.18±0.05
Iron-dosed flocs 0.14±0.01 0.005±0.001 0.42±0.03
Figure 33 Effect of Iron Dosing (15 mg/L) on UV DRC of floc (45-63µm)
59
4.3.5 Effect of Iron Dosing on UV DRCs of Flocs and Cores
Figure 34 illustrates the effect of iron dosing on UV DRCs of flocs and cores of size
fraction 45-63 μm. The results show that in case of iron-free flocs, cores had a higher
level of tailing compared to the flocs of the same size fractions, which this result is in
agreement with the previous study by Azimi et al. (2012). More importantly, the results
show that upon iron addition, the difference between the UV DRC of cores and flocs
vanished, which is consistent with the findings of this study. This finding confirms that
the increase in UV absorbance of floc material due to iron addition could dominate the
UV disinfection kinetics of the flocs and cores such that they exhibit similar UV
disinfection kinetics.
Figure 34 Effect of Iron Dosing on UV DRC of Flocs and Cores of Size Fraction of 45-63 µm
4.3.6 Model Prediction to Examine the Effect of Iron on UV DRCs of Flocs
In this section, a simplified model developed by Azimi (2013) is presented to
mathematically describe the expected effect of iron on the UV disinfection kinetics of
60
flocs and cores. In this model, a double-layer structure composed of a compact core
surrounded by a loose outer shell is considered for the flocs. Azimi et al. (2012) also
assumed that
• Flocs can be regarded as spherical particles
• The UV disinfection of a floc is controlled by the most embedded indicator
microorganism that receives the least amount of UV dose
• The loose outer shell is replaced by a spherical shell 20µm in thickness
• The compact core of flocs is assumed to be spherical in shape and with a higher
density
• The UV light penetration within the flocs is expressed by the Beer-Lambert’s law
• Floc is exposed to a diffuse irradiance filed such that its surface is uniformly
exposed to UV light
Since EPS absorbs UV light, the UV dose received by embedded microorganisms is
less than the dose delivered to a free-swimming microorganism in the sample. Details
of the model can be found in the Appendix I (Azimi, 2013). The predicted results from
the model for flocs and cores of size fractions of 48 and 80 µm are presented in Figure
35 and Figure 36, respectively. Here, it is assumed that the UV absorbance of floc to
be approximately 1000 cm−1 (based on Jagger 1967, that approximately 90% of UV
light absorbed in 10 μm of cells), and the the UV absorbance of cores are twice of that
of the floc. In addition, the UV absorbance of iron is assumed ot be 466L/mol.cm
(Metcalf and Eddy, 2007).
61
Figure 35 Model Predictions for Various Levels of Iron Dosing on the Behavior of Flocs (floc/core size is 48µm)
Figure 36 Model Predictions for Various Levels of Iron Dosing on the Behavior of Flocs (floc/core size is 80µm)
The model predicted results are in agreement with the experimental results in Figure
34. The figures showed that when the flocs were not dosed with iron, cores had a
62
higher level of tailing compared to the flocs of the same size fractions. However, upon
iron addition, the difference between the UV DRC of cores and flocs vanished and they
showed similar behaviours. As discussed earlier, iron increases the UV absorbance of
floc material and hence floc resistance to UV disinfection. A highly UV-absorbing floc
material could mask variations in the internal structure of the floc and result in similar
UV DRC.
4.3.7. Practical Significance
In this study, the UV DRCs were constructed for flocs and cores of different size
fractions, which are isolated UV-resistant particles. In order to put the practical
significance of these findings in context, and assess their effect on the UV DRC of a
whole effluent, an effluent UV DRC is constructed (Figure 37). In Figure 37, the total
particle number of final effluent is the sum of free microorganisms and some residual
microbial flocs (N free microorganism +N residual microbial flocs), in which the inactivation rate
constant of free microorganisms is 0.67 cm2/mJ (Wright & Cairns, 1999) and the
inactivation rate constants of iron-dosed and iron-free microbial flocs (k1 and k2), and
the fraction of UV-resistant particles (β) are extracted from Table 11 . In this figure, the
effect of iron addition on the tailing level of the final effluent is shown. The log reduction
in the final effluent UV DRC is higher than that of the floc and core samples due to the
smaller number of UV-resistant particles in the final effluent. However, as expected,
higher tailing level is observed in the case of iron containing effluent.
63
Figure 37 Effect of Iron Dosing on Simulated Final Effluent
64
Chapter 5 Conclusions and Future Work
5.1 Conclusions
The main conclusions arising from this study are as follows:
1) Under both aerobic and anaerobic growth conditions, UV dose-response curves
follow a similar pattern, and even overlap in most of the cases. Fecal coliforms
(the indicator microorganisms used in the UV bioassay) are facultative
anaerobes and could grow under both aerobic and anaerobic growth conditions.
Hence, the fecal coliforms cultured under both sets of conditions have similar
response to UV irradiation.
2) For microbial flocs and compact cores (larger than 45μm), under aerobic and
anaerobic growth conditions, the tailing level elevates as the size increases,
showing that larger particles are more resistant to UV inactivation.
3) The iron-dosed flocs have a higher tailing level compared to that of iron-free
flocs, showing that iron significantly increases the resistance of flocs to UV
inactivation.
4) Upon iron dosing, the difference between the UV DRC of cores and flocs
disappears. The increase in UV absorbance of floc material due to iron addition
can dominate the UV disinfection kinetics of the flocs and cores and cover
variations in the internal structure of the floc such that they exhibit similar UV
disinfection kinetics.
5) Addition of iron affects particle size distribution, and increases the number of
smaller flocs and subsequently total flocs. The iron-dosed flocs are generally
more optically opaque and more compact. Moreover, the iron-dosed flocs have
more regular shape with well-defined boundaries and orange/yellowish colour.
6) Iron enhances the mechanical stability of the flocs and leads to an increase in
floc resistance to shear forces. Hence, the core samples extracted from iron-
dosed flocs can have similar structure to the original flocs.
65
5.2 Future Work
1) As discussed earlier, the indicator microorganism for aerobic and anaerobic
study was fecal coliform in this study, which is capable of reproducing under
both conditions. This indicator microorganism is currently the standard
indicator for industry/regulatory standard in the membrane filtration technique
(APHA 2001, # 9222 A). In this technique, a single floc can give maximum
one CFU count, regardless of whether there are one or many surviving
coliform bacteria embedded within that floc, and this is a critical limitation of
this technique. Using different indicators could potentially shed some lights
on the behaviour of embedded microorganism in the inner regions of the
flocs, which are more likely deprived of oxygen and nutrients. Therefore, an
investigation of more various indicators is desirable to examine the first
hypothesis of this study.
2) Hydrodynamic shear stress was used as a mechanical method for isolating
cores from the flocs in this study. However, some other physical or chemical
methods can also be suitable for EPS extraction and isolating the compact
inner region of the flocs. The extraction methods such as ion-exchange resin
and EDTA could be more efficient in this case, but it should be noted these
chemicals also have the potential to kill some of the microorganisms.
3) Running long-term SBRs is beneficial for a better investigation of the effect of
iron on activated sludge flocs’ properties. The simplified SBRs in this study
were designed for a short-term critical study (three months). In such a short
period, the SBRs may not reach stable operating conditions properly. Even
though addition of iron led to significant changes in a short period, but a long-
term study would assist to examine in depth how iron results in formation of
more resistant and stable flocs during the time.
66
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Appendix I
In this study, Assuming that the most shielded indicator microorganism is located at a
depth δ from the surface of floc, the effective dose, Deff (δ), received by this
microorganism can be estimated from the Beer-Lambert law:
Deff (δ) = D e –A(δ) × δ (4.1)
where D is the actual UV dose delivered to the sample in mJ/cm2, and A(δ) is the UV
absorption coefficient of the floc material.
The inactivation kinetics of a floc with radius R can be expressed by:
N(R, δ) = No(R, δ) e – k Deff
(δ) (4.2)
where, No(R, δ) and N(R, δ) are the initial and the final concentration (after UV
irradiation) of viable flocs in CFU/100 mL; a viable floc is a floc that contains at least
one culturable embedded indicator microorganism, δ is the distance of the indicator
microorganism from the surface of the floc, and Deff(δ) is the effective UV dose in the
floc at the distance of δ from surface. In practice, the average UV dose at the floc
surface, D, is generally reported. Therefore, equations 4.1 and 4.2 can be rearranged
to obtain equation 4-3:
N(δ, R) = No(δ, R) e – keff
(δ) D (4.3)
where keff (δ) is the effective inactivation rate constant inside the floc at the distance δ
from the surface(based on the Beer-Lambert law) defined by:
keff (δ) = k e −A(δ) × δ (4.4)
Since δ varies from 0 to R, the UV DRC of flocs with radius R would be:
N(R) /No(R) = ∫o
δ
f (R, δ) e – keff
(δ) D dδ (4.5)
72
where, N(R) and No(R) are the initial and final concentration of viable flocs with radius
R in CFU/100 ml, and f (δ,R) is the probability of finding a culturable microorganism at
the depth of δ. The f (R, δ) is defined as:
f (R, δ)=((R- δ)3-(R- δ+dδ)3)/R3 (4.6)
It is assumed that the UV absorbance of floc to be approximately 1000 cm−1 (based on
Jagger 1967, that approximately 90% of UV light absorbed in 10 μm of cells), and the
the UV absorbance of cores are twice of the flocs’ one. The UV absorbance of ferrous
ions is 466L/mol.cm (Metcalf and Eddy, 2007).
73
