Griffith School of Engineering 7605ENG Industrial Affiliates Program Final Project Report The Flocculating Effect of Magnesium Chloride in a Magnapool™ System Nonso Okafor 2709055 Submitted: June 25 th Semester 1, 2011 Industry Partner: Poolrite Research Industry Supervisor: Wayne Taylor Academic Advisor: Jimmy Yu A report submitted in partial fulfilment of the requirements for the Master of Engineering degree in Environmental Engineering
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Griffith School of Engineering
7605ENG Industrial Affiliates Program
Final Project Report The Flocculating Effect of Magnesium Chloride in a Magnapool™ System
Nonso Okafor
2709055
Submitted: June 25th
Semester 1, 2011
Industry Partner: Poolrite Research
Industry Supervisor: Wayne Taylor
Academic Advisor: Jimmy Yu
A report submitted in partial fulfilment of the requirements for the Master of
Engineering degree in Environmental Engineering
IAP-FINAL PROJECT REPORT The Flocculating Effect of MgCl2
Nonso Okafor-2709055 Page 1
Executive Summary
In the wake of increasing stringency in the operating guidelines of the swimming pool
industry, due to recent disease outbreaks and infections being linked with swimming pool water
(Croll et al, 200; Glauner et al, 2005; Perkins, 200; Zwiener et al, 2007; WHO, 2006),
Poolrite Research Pty Ltd. has patented a salt blend rich in magnesium chloride, which they
believe, improves pool water quality by clarification. In the absence of hard facts to
substantiate their marketing claim and to promote public acceptance of this hybrid system,
Poolrite requested for independent research to be conducted by Griffith University to
investigate the flocculating effect of magnesium chloride (MgCl2) in a Magnapool™ System.
This report outlines the concepts behind this study with details on all the investigative
approaches (research and experimental), materials, methods and performance evaluation
criteria used in assessing flocculation performance.
The process of ensuring water clarity as well as controlling the presence of pathogens in a
swimming pool is crucial (Perkins, 2000). This can be achieved by the removal of suspended
and colloidal matter in the pool water body so as to ensure bather safety from diseases by the
removal of particles that shield micro-organisms from the action of disinfectants (WHO,
2006).
Flocculation has been defined as a process whereby destabilised or dispersed particles are
brought together to form aggregate flocs of size, large enough to cause their settling and bring
about clarification of the system. This process occurs by various mechanisms namely;
adsorption and surface charge neutralization, sweep flocculation, electrical double layer
compression and inter-particle bridging. The adsorption and surface charge neutralisation
mechanism was found to be the mechanism by which most hydrolysing inorganic metallic
salts, such as MgCl2, flocculate. Thus, the two stages of the flocculation process according to
Bratby, (2006) are the Perikinetic flocculation stage which ensues from thermal agitation,
usually referred to as Brownian movement and is a naturally random process, and the
Orthokinetic flocculation stage that starts immediately after flash mixing, due to induced
velocity gradients that arise in the slow mixing regime, thereby causing increased particle
contraction and consequent agglomeration of these particles.
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Flocculation in water treatment however, has been carried-out from time immemorial using
aluminium sulphate (generally known as alum) and other chloride salts of aluminium and
Iron. Following several concerns raised by experts in water treatment on the use of these
salts, there is now the need for the use of more environmentally sustainable chemicals in
water treatment. Turbidity reduction was the utmost criteria on which flocculation
performance was based in this study.
A review of previous studies in flocculation showed that MgCl2 had been used in some
industrial processes such as in the dye industry, for colour removal from the waste water. A
Capsule Reports from the USEPA (2010) claimed the use of recycled Magnesium for
coagulation purposes. A study by Karami (2009) showed that magnesium ion can be used to
effect modification on the shape of colloidal particles
In a view to substantiate the numerous literature evidence, experimental investigation became
imperative. The testing approach was aimed at establishing whether the Magnapool™ salt
blend does act as a flocculant in water treatment. Experimental investigations were performed
to comparatively asses the flocculation performance obtainable in the Magnapool™ System
with respect to a traditional salt water pool that uses NaCl as its flocculant. The optimum
dosing rate for the Magnapool™ mineral blend was also investigated and the actual
concentration/effect of MgCl2 in the salt blend determined.
This experimental investigation employed the Jar Tester as the main apparatus.
Contaminated water was simulated by dissolving 0.4grams of ISO test dust in 2000ml of tap
water in each jar. Specific amounts of flocculant salts were added while other water
parameters adjusted (pH, system temperature and conductivity) so as to simulate a typical
Magnapool™ operating condition, before mixing stages ensued on the four separate stirring
points of the Jar Tester having pre-programmed the stirrers to run at 120rpm for two mins
(Flash mixing stage) and at 20rpm for the slow mixing stage. Variable settling time, number
of reading, and at times, duration of mixing was varied across some of the test runs as
required.
Results from the experiments showed that the Magnapool™ salt blend is a highly effective
flocculant, as it reduced water turbidity from 175FTU to 40 FTU within a one hour settling
time. The optimum performing mineral concentration was found to be 3350ppm, although the
margin with which it out-performed other concentrations within the range of 3000-3500FTU
IAP-FINAL PROJECT REPORT The Flocculating Effect of MgCl2
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was marginal. Experimental results also showed that the active floccing agent ingredient in
the Magnapool™ mineral blend is the divalent inorganic hydrolysing salt of MgCl2 at a
concentration of 500ppm, whose variation in turbidity reduction potential, when used in
isolation from other salts in the mineral blend compared to the reduction in turbidity achieved
with a complete Magnapool™ mineral blend was marginal (48FTU and 40 FTU
respectively). Its flocculating mechanism is by adsorption and charge neutralization, using
the positive magnesium ion Mg2+
to neutralise the surface charge of the negative colloidal
particles and subsequently precipitates these contaminants in water by formation of Mg
(OH)2. However, water properties such as pH and alkalinity were found to be highly
influential in dictating flocculation performance. The optimum pH of the Magnapool™
mineral was determined to be 7.5
Overall, the Magnapool™ system performed better than a traditional Salt-water pool in terms
of turbidity reduction, as the tests were performed under the same conditions and consistent
initial water quality characteristics. Based on the literature research and experimental
outcomes, recommendations were made for a continued use of the Magnapool™ mineral as a
flocculant in swimming pool water treatment and also on the possible exclusion of NaCl in
the mineral blend since it showed no flocculation tendencies. Again, further research into this
study area was recommended so as to establish the combined effect of Poolrite‟s
Magnapool™ mineral blend and DiamondKleen™ on the final turbidity residual of in the
swimming pool water recirculation cycle and also to determine the extent to which the
Magnapool™ mineral constributeses to bio-fouling of the swimming pool filter media.
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Acknowledgements
The successful completion of this project relied on the unreserved support, motivation,
encouragement and technical assistance rendered to me by a number of people, to whom I
wish to express my sincere appreciation.
The Quality Engineering Group of Poolrite Research Pty Ltd was simply sensational in the
way they harboured and made me part of the team that engineers the company‟s products and
services. I‟m extremely grateful to Wayne Taylor my industry Supervisor, whose effort;
expert and technical initiatives stood out and ultimately provided guidance from inception
though to the completion of this project. Your unflinching support was key to the success of
this project, thank you Wayne. Special thanks to the Boss of the department, Aaron Kelly for
ensuring that work tools and materials were never an issue all through the project cycle. And
to Stuart Anderson, who dedicated his time immensely to ensure that I was always making
progress in my work, I say a huge thank you. Also thanking Jennifer Campbell, a co-student
who ran another project alongside mine, for her co-operation and comradeship all through the
project duration.
I humbly express my profound gratitude to my academic advisor Dr Jimmy Yu, for his
scholarly advice and guidance which helped me maintain track in the project. Thanking you
especially for your patience and understanding while I completed my designated project
tasks. My sincere appreciation also goes to the IAP convenor Dr Graham Jenkins for his
unreserved support and encouragement.
I am grateful to my Uni mates and friends for supporting, encouraging and being there for me
in one way or the other. Special thanks to Matilda Ofosu for her unflinching support and
assistance in reviewing and proof-reading all my assessment item drafts, also to Bahar Nader,
Jay-Jay Okocha, Akin Ajayi, Connie and Sharoo Mkandawire for their assistance.
Finally, I acknowledge the Almighty God, for giving me the wisdom, knowledge and
understanding to see this project to a successful end.
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Nomenclature
FTU: Formazin Turbidity Unit
Ksp: Solubility product constant
mg/l: Milligrams per litre
rpm: Revolutions per minute
ppm: Parts per million
conc: Concentration
THMs: Trihalomethanes
β: Collision frequency
∝: Collision efficiency
G: Velocity gradient
ζp: Zeta potential of particles
IAP-FINAL PROJECT REPORT The Flocculating Effect of MgCl2
21.1 The Magnapool™ salt blend “An Effective Flocculant” ............................................................. 56
21.2 MgCl2 is” Key” to the Efficacy of Magnapool™ Mineral as a Flocculant ................................... 56
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21.3 The Magnapool™ provides a safer water environment compared to a Traditional Salt Water
Pool ................................................................................................................................................... 56
21.4 Flocculation Efficiency is highly influenced by Water Chemistry ............................................. 57
Table 1: Project Team Members....................................................................................................11
Table 2: An Outline of the final report..........................................................................................12
Table 3: Operational Guidelines in Swimming Pool water treatment in Australia..........................15
Table 4: Properties of common metallic ions................................................................................24
Table 5: Summary of flocculation previous experiments...............................................................38
Table 6: Blend Information of the Magnapool™ mineral ..............................................................44
(Adapted from Magnapool blend Worksheet)
List of figures
Fig 1: A typical pool water treatment process.........................................................................................................13
Fig 2: Size range of colloidal particles of concern in water treatment .........................................................17
(Adapted from Labreche & Aiyagari, 1997).
Fig 3: The DLVO theory representation........................................................................................................................18
(Adapted from Sincero and Sincero 2003)
Fig 4: A schematic representation of the electrical double layer concept ...................................................19
(Malvern Instruments Ltd, 2011)
Fig 5: A Schematic of the mechanism of colloidal silica modification by Mg+2...........................................25
Fig 6: Deposition of metal hydroxide species on oppositely charged particles...........................................28
(Adapted from Duan & Gregory 2002)
Fig 7: A negatively charged particle surrounded by a charged double layer..............................................29
Fig 8: A diagrammatic representation of a Jar Test Set-Up................................................................................42
Fig 9 (a-d): Experimental Wares and Flocculant Salt samples .........................................................................42
Fig 10: A Pictorial of Jar Testing in Progress ...........................................................................................................43
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Fig 11: Total Alkalinity Vs Magnapool™ Mineral Dose..........................................................................................46
Fig 12: A Plot of Turbidity Residual Vs MgCl2 Dose.................................................................................................46
Fig 13: Total Alkalinity Reduction Vs NaCl Dose......................................................................................................48
Fig 14: A Plot of Turbidity Residual Vs NaCl Dosage..............................................................................................48 Fig 15: A Plot of Turbidity Reduction over Time......................................................................................................50
Fig 16: A Plot of pH Effect on Turbidity Reduction.................................................................................................51
Fig 17: Plot of Turbidity Residual Vs Concentrations of MgCl2.....................................................................................................................52
Fig 18: Turbidity reduction Vs Time (Varying Initial sample water turbidity)..........................................53
Fig 19: A Plot of Turbidity Reduction Vs Time...........................................................................................................54
(At 3000-3500ppm Magnapool™ Mineral Range)
Fig 20: Turbidity Reduction over Time (Magnapool™ Vs Normal Salt Water Pool)................................55
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Part I: Introduction
This report has been compiled in accordance with the requirements of the Industrial Affiliates
Program, ran by Griffith University with a project offering by the industry partner, Poolrite
Research Pty Ltd. The determination of “The Flocculating Effect of Magnesium Chloride in
a Magnapool™ System” is the project aim, set-out by the industry partner. Thus, this
document reports on all the investigative steps and approaches adopted towards completion
of the project, the conclusions drawn from the outcomes of the investigations and some
recommendations based on the project findings.
1 Project Description
The Magnapool™ system, a brand name in the swimming pool industry has been patented by
Poolrite Pty Ltd. This system has been claimed by the patents to employ a technology that
uses a hybrid salt blend, rich in magnesium ion to generate swimming pool water
disinfectants, while maintaining very high water clarity (minimal turbidity) in the pool. In the
absence of prior exhaustive research and technical data to support their claims, Poolrite
Research has requested that the flocculating effect of magnesium chloride as in water
treatment be researched and experimentally tested.
1.1 Project Scope
In line with the requirements and specifications outlined by the industry partner, the project
completion will be achieved by;
Extensive research into the science and mechanism of flocculation
Standard experimental testing and reporting
o Comparatively analyse the potency of the Magnapool™ mineral against
conventional pool salt.
o Determine the optimum dosage of the Magnapool™ mineral
o Quantify the correct amount of MgCl2 that yields the best floccing effect in
the Magnapool™ mineral blend.
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A draft of a final project report with conclusions and recommendations based on the
project findings, for chemical optimization in the overall swimming pool water
sanitation process
1.2 Project Team
The completion of this project involved active interaction between the three keys members
that make up the project team is presented in the table below;
Table 1: Project Team Members
Team Member Role
Nonso Okafor
Department of Environmental Engineering
Griffith University
Project Facilitator
Wayne Taylor
Chief Engineer
Poolrite Research
Industry Project Supervisor
Dr Jimmy Yu
Senior Lecturer
Griffith University
Academic Advisor
1.3 Project Plan
A planning report was developed in the earlier stages of the project for an efficient
management and timely achievement of the project deliverables. The project however, was
broken down into several milestones, which had to be accomplished towards achieving the
required project outcomes.
This planning report therefore detail the stages, tasks, approaches and methodology to be
adopted in achieving the project deliverables together with a review of some risk factors that
might affect the progress and completion this project. These can be found in the project
planning report document in appendix A.
1.4 Project Report Outline
The structure in which the entire project work was carried towards achieving the expected
deliverables of has been outlined in the table below. As it bears the summary of the major
tasks completed and their relevance towards achieving the milestones set-out in the planning
phase of the project.
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Table 2: An Outline of the Final Report
Structure of the Final Report
Section Report Project Deliverable
Part I: Project Introduction Project Planning Report
Part II: Swimming Pool
Water Treatment Concepts
Milestone 1 Literature Review
Part III: Flocculation
Theories and Kinetics
Milestone 1 Literature Review
Part IV: Experimental
Design and Set-Up
Milestone 2 Test Set-up
Part V: Experimentation and
Data Collation
Milestone 3 Experimental Results and
Outcomes
Part VI: Conclusions and
Recommendations
Milestone 4 Data Analysis and Technical
Report
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Part II: Swimming Pool Water Treatment Concept
2 Swimming Pool Water Treatment Process
Swimming over the years has gained a tremendous popularity as one of the most enjoyable
and satisfying physical activity to indulge in (PWTAG, 2009). In Australia for instance, and
particularly Queensland, there is a big history of recreational activities involving water due to
its climatic conditions (QLD Health, 2003). Zwiener et al (2007) also suggested there are
health benefits associated with swimming as compared to land based recreational activities.
However, several bacterial, fungal and other infectious disease outbreaks have been linked to
the use of swimming pools in recent times (Croll et al, 200; Glauner et al, 2005; Perkins,
2000; Zwiener et al, 2007; WHO, 2006) and this has triggered the regard for pool and
recreational waters as a health priority all round the globe (Zwiener et al, 2007). Following
this trend, there has been a rise in the standard at which these pools are run as regulated by
bodies such as the World Health Organisation, and these guidelines tend to enshrine the core
principles involved in managing pool water (PWTAG, 2009, WHO, 2006).
2.1 Nature and Categories of Swimming Pool Water Contaminants
Bathers in swimming pool may be at risk of contracting infections caused by a number of
micro-organisms in contaminated pool water. The nature of these contaminants (Croll et al,
2007; QLD Health, 2003; Li et al, 2007; WHO, 2006) may be;
Organic: These are usually transmitted into the water body by bathers in the form of
faeces, dead skin, hair, mucus from nose, saliva releases from mouth, accidental
vomit and organic nitrogen compounds of sweat and urine.
Inorganic: These sorts of contaminants could be in the form of sunscreens applied by
pool patrons on their skin and their hair lotions
The nature of pool contaminants highlighted above is conventional with all kinds of
swimming pools, be it an indoor or outdoor set-up. However, another category of
contaminants exist, which are predominantly found in outdoor pools. They are not necessarily
introduced into the pool by the activities of the bathers; rather they originate from
environmental sources (Zwiener et al). Some examples of this category of contaminants
include; silt, sand, grasses and leaves.
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2.2 Swimming Pool Water Treatment System
Ensuring water clarity as well as controlling the presence of pathogens in a swimming pool is
crucial (Perkins, 2000). These could be achieved by the removal of suspended and colloidal
matter in the pool water body so as to ensure bather safety from diseases by removal of
particles that shield micro-organisms from the action of disinfectants (WHO, 2006). An
effective swimming pool water treatment system provides an attractive appearance of the
pool and makes it appealing to swim in (SAHCC, 1992). The figure below shows the layout
of a typical pool treatment system (WHO, 2006).
Fig 1: A typical pool water treatment process
The swimming pool water treatment system above, integrates most of the key
water/wastewater treatment processes of coagulation, filtration and disinfection in a
recirculating loop (PWTAG, 2009). Considering the high level of contamination usually
generated by bathers, swimming pool water can be characterised as a wastewater. Therefore,
the treatment process is designed to comply with the standards of wastewater as well as
drinking water.
The removal of dissolved colloidal particles or suspended material from the pool water is the
main target of every pool water treatment system, as it improves the overall efficiency of the
Strainer Pump
Coagulant/Flocculants Dosing
Filtration
Water disinfection
pH correction dosing
Balance Tank Swimming Pool
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entire treatment process. By way of clumping these dissolved colloidal materials together,
they are more easily trapped in the filtration system (PWTAG, 2009; WHO, 2006). This is
enhanced by the process of Flocculation/ Coagulation.
3 Existing Legislation
There are some regulations and guidelines that govern swimming pool water treatment and
management practices both internationally and in Australia. Some of these have been
outlined below according to what each regulation/standard tries to achieve.
Table 3: Operational Guidelines in Swimming Pool water treatment in Australia
(adapted from Poolrite Research (2010), Technical Manual, The Magnapool™ System)
Standards/Regulations/Guidelines Targets/Requirements Health Protection Queensland Public Health, Swimming and Spa pool Water Quality and Operational Guidelines
Filtration criteria Chemical parameters Testing & recording requirements
Queensland Development Code 2008- NMP 1.9, Swimming pool and Spa Equipment
Disinfection system Water Chemistry
Australian Pesticides and Veterinary Medicines Authority (APVMA)
Efficacy criteria for pool and Spa sanitizers
South Australian Health Commission, Department of Human Services-Standard for the Operation of Swimming Pools and Spa Pools in South Australia
Water Clarity Disinfection and treatment of water Breakpoint chlorination Pool pollution
Standards Australia, AS 3633-1989, Private Swimming Pools – Water Quality
Chemical and Sanitizer concentrations
Pool water maintenance World Health Organisation, Guidelines for Safe Recreational Water Environment, Vol 2, Swimming Pools and Similar Environments, 2006
Good practice for by-product formation minimisation e.g. disinfection systems with less chlorine use
Chemical hazards Exposure to disinfection byproduct
US EPA, SWIMODEL Exposure Estimation Prediction of life time swimmer cancer exposure risk to disinfection by products e.g. Trihalomethanes
Pool Water Treatment Advisory Group (PWTAG), UK-Swimming Pool Water, 2009
Operation and maintenance Hydraulics and circulation
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Pool water chemistry Northern Territory Government, Department of Health and Families- Public Health Guidelines for Aquatic Facilities, August 2006
Design and Construction Circulation and Water Treatment
systems Water quality testing Sanitation and operational
Sect 32- Total water cycle management method and planning
Australian Pesticides and Veterinary Medicines Authority
Exemption of chemical registration of salt water chlorinators.
Brisbane City Council, Urban Management Division, Subdivision and Development Guidelines, Part C. Water Quality Management Guidelines-Section 11 Discharges from Swimming Pools, 2000
Minimise discharges of pool water to storm water
Encourage development of filters and chemical regimes that protect human health
Environmental Sustainability
Royal Life Saving Society Australia- Best Practice. Profile Swimming Pools Maximising Reclamation and Reuse, 2006
Implementation of water saving strategies
Installation of ultra fine filtration systems to reduce backwash frequency and cycle time
Include technical water saving and reuse strategies in pool planning and design
Queensland Government, Department of Natural Resources and Water
Typical Salinity limits for waters, including salt water swimming pools
Queensland Water Commission –WG-20, Water Efficiency Management Plan (WEMP) for Outdoor Water use at Public Pools that use less than 10MegaLitres per year
Water efficiency benchmark (L/visitor/day)
Decrease backwash frequency Measure filter loading with
pressure gauges Government of Western Australia, Department of Water- Water Quality Protection Note WQPN 55, Swimming Pools, February 2009
Wastewater disposal- recycling to pool
Wastewater disposal- garden irrigation
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4 Overview of Coagulation/ Flocculation in water treatment
In water treatment, sedimentation of particles may take place naturally, due to the action of
the force of gravity, in situations of large particles. However, some particles will not settle,
due to their chemical interaction with water. An example of such a case as suggested by Faust
and Aly (1983) are the hydrophilic compounds in water.
The process in which destabilisation of particles takes place by the reduction of the repulsive
potential of the electrical double layer, which in turn forces agglomeration and clumping
together of the suspended material, and bringing them out of the solution is called
coagulation/flocculation (Fasemore, 2004; PWTAG, 2009; Zweiner et al, 2007). These
destabilized particles are brought together to form aggregates normally referred to as „flocs‟,
and these are large enough in size to sediment and are eventually separated from the water.
4.1 A brief history of Coagulation/Flocculation in water treatment
Clarification and particle removal during water treatment has been practiced from time
immemorial, using various substances as agents of coagulation. At about 2000BC, the
Romans used chemical coagulants such as alum Al2(SO4)3) for particle removal in water
while the ancient Egyptians used fine crushed smeared almond to clarify water that had been
fetched from the river, by dipping an arm into the water to properly disperse the crushed
almonds for clarification to occur (Faust & Aly, 1983; Bratby, 2006; Fasemore 2004).
In England as suggested by Bratby (2006), alum was more widely used in the treatment of
municipal water supplies. However, iron coagulants were more frequently used as flocculants
in the Americas, as Isiah Smith Hyatt in 1884 patented the use of ferric chloride for water
treatment for the New Orleans water company (Fasemore, 2004).
In modern water treatment practices however, coagulation and flocculation continues to play
a vital role in the overall success of the entire water treatment process. A recent engineering
survey on the quality of water treatment from over 20 water treatment plants have concluded
that chemical pre-treatment of water before the filtration process is the most crucial step, on
which the success of the treatment plant relies (Bratby, 2006).
Hence, the critical nature of the flocculation process in water treatment has now called for a
better understanding of the dynamics and mechanism of the process, considering the
increasingly stringent requirements on particulate removal in water treatment.
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5 Colloidal Stability and Destabilisation in Water Solution
A colloidal system is a medium in which particulates of different size interact but with the
highest particle diameter no greater than 10µm. In this system, the particles that settle due to
gravity have a maximum settling velocity of 0.01cm/sec while remaining particles are held in
suspension (Faust & Aly, 1983).
Molecular Ultra-fine Fine Coarse
Fig 2: Size range of colloidal particles of concern in water treatment (Adapted from
Labreche & Aiyagari, 1997).
In water systems, most solids are usually present in the form of suspended particles, colloids
dissolved solids and molecules. These particles range in size from very large to typically
small particles. Sand particles in the water system have the largest particle size, followed by
microbes in water such as viruses, algae and bacteria (Fasemore, 2004). However, colloids
are very fine particles with diameters between 10nm and 10µm. (Binnie et al, 2002). Coarse
or fine particles are easy to remove by settlement or filtration. Molecules cannot be removed
by these physical processes, unless after precipitation. Therefore, the removal of colloids is
usually the main focus and the most challenging in water treatment processes (Binnie et al,
2002).
5.1 Colloidal Stability
Stability in a colloidal system simply refers to the ability of the particles to remain
independently within a given dispersion (Bratby, 2006). Kovalchuk et al (2009) described
the stability of a colloidal system as an important characteristic since it is determined by the
net balance of the attractive and repulsive forces that exists between the particles in a
colloidal system. The DLVO theory considers colloidal interactions, taking into account their
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dispersion and electrostatic forces. This theory suggests that coagulation occurs when
identical particles make up the composition of a suspension, where the resultant effect of the
dispersion forces is attraction (Kovalchuk et al, 2009).
Fig 3: The DLVO theory representation (Adapted from Sincero and Sincero 2003)
Ideally, the stability of a colloidal dispersion is enhanced by the interfacial forces (Bratby,
2006) due to;
The presence of a surface charge at the colloid-liquid interface
Hydration of the surface layers of the colloid.
In most water treatment conditions, colloidal particles usually possess a negative surface
charge while exhibiting a dipolar characteristic of hydrophilicity (water-loving tendency) and
hydrophobicity (water repelling tendency).
5.2 Destabilisation of Colloidal systems
Destabilisation of a colloidal system may occur as a result of;
a reduction in the effective surface charge of a particle
a reduction in the number of adsorbed water molecules
a reduction in the zone where the surface charge acts.
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These factors cause the particles to approach close enough to each other and are consequently
held together by the Van der Waals force of attraction (Bratby, 2006). In an electrostatically
stable suspension, destabilisation can be instigated by an adjustment in the system pH, which
causes a reduction in the surface potential (Kovalchuk et al, 2009),and also by increasing the
salt concentration in the dispersion medium, which then lowers the thickness of the
electrical double layer. This is shown in the figure 3 below.
Fig 4: A schematic representation of the electrical double layer concept (Malvern
Instruments Ltd, 2011)
The treatment of the diffused part of the double layer has been recognized by Stern (Bratby,
2006), stating that the finite size of the ions will limit the inner boundary of the diffuse part of
the double layer. According to Bratby (2006), a model has been proposed, in which the
double layer is divided into two, separated by a plane called the Stern plane at a hydrated
ionic radius from the surface. The adsorbed ions may be dehydrated in the direction of the
solid surface, so their centres will lie between the solid surface and the Stern plane. However,
when specific adsorption takes place due to electrostatic and Van der waals forces, counter-
ion adsorption generally predominates over co-ion adsorption.
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6 Nature and Categories of Flocculants
Fasemore, (2004), Sharma et al. (2006) and Renault et al, (2008) generally classify the
material used in coagulation/flocculant processes to be of either inorganic or organic nature.
The use of polymeric additives has been used recently to cause agglomeration of flocs. These
are normally referred to as coagulant aids (Rawlings et al. 2006; Renault, 2009)
6.1 Inorganic flocculants
Inorganic flocculants are usually salts of multivalent metals and have been in use since time
immemorial. This category includes: aluminium sulphate, calcium chloride, ferric chloride
etc. They are known to be highly dependent on the pH of the solution (Fasemore 2004) as
each particular inorganic flocculant performs better over a given Ph range. The sludge deposit
formed by these sorts of flocculants normally bears their colour and this, in some cases
replicates in the colour of the water e.g. the brownish colour of ferric chloride appears on the
hydroxides which forms the flocs. Hence, the sludge picks up the colour of the flocculant
used.
However, metallic flocculants have been found to have numerous disadvantages (Fasemore
2004, Sharma et al. 2006 and Renault et al. 2008) as highlighted below:
(i) They are highly sensitive to pH
(ii) Large amounts are required for efficient flocculation which in-turn produces large sludge
volumes
(iii) They can only be applied to a few disperse systems.
(iv) They are usually inefficient towards fine particles.
Recently, inorganic polymeric flocculants have been proposed. These types of flocculants
contain complex poly-nuclear ions, formed by having high molecular weight and high
cationic charge. An example of this is the pre-hydrolysed polyferric chloride (PFC). They are
relatively more effective at a lower dose than conventional flocculants and can be used over a
wide pH range (Renault et al. 2008).
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6.2 Organic flocculants
Technological developments in recent years have ensured a major advancement in
flocculation technology by the development of organic polymers with remarkable purification
efficiency (Sharma et al. 2006, Renault et al. 2008). They are basically of two types:
Natural organic flocculants: normally based on natural polymers like starch, natural
gums, cellulose and their derivatives
Synthetic organic flocculants; based on various monomers like acrylic acid, acryl
amide, diallylmethyl ammonium chloride.
One of the major advantages of polymeric flocculants is their ability to produce thick and
compact flocs with good settling characteristics. They are readily soluble in aqueous systems
and produce less sludge volumes.
In swimming pool water treatment, flocculants function to gather up bacteria, but are
particularly crucial in helping filter three classes of material which otherwise would pass
through the filter (PWTAG 2009):
The cysts of Cryptosporidium and Giardia- usually small and resistant to disinfection
Humic acid- naturally found in some mains water and a significant precursor of
THMs
Phosphates in mains water and a component of some swimming pool water
chemicals.
The review of various literatures has shown that aluminium and ferric compounds, have
found wider industrial application as coagulants in water treatment (Ahiog 2008; Antunes et
al. 2008; Desjardins et al. 2002; Duan and Gregory 2003; Lee et al. 2005; Rodrigues et al.
2008).
However, the increasing awareness in environmental implications of most of these chemicals
has prompted the demand from stakeholders for the use of more environmentally friendly
chemicals in carrying out water treatment so as to minimise the footprint of these processes in
the ecosystem. The metallic salts of aluminium and iron generate large amounts of sludge by
chemical precipitation (Snurer 2003), which creates sludge handling problems and ultimately
increases the cost of the treatment process by carrying out sludge treatment, dewatering and
disposal. Despite the increasing application of synthetic polymers as flocculants, their
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inability to biodegrade coupled with their relatively high cost has been a major issue (Sharma
et al. 2006)
7 MgCl2 as a Flocculant for Swimming Pool Water Treatment
Poolrite Research Pty Ltd has patented this swimming pool mineral salt, which has a
composition with which chlorine generated in the form of hypochlorous acid and
hypochlorite ion, which sanitizes the swimming pool water and checkmates outbreak of
disease and infections in the pool (QLD Health 2003; PWTAG 2009). This mineral is a blend
of Magnesium chloride, potassium chloride and sodium chloride in the ratio of 33%, 55% and
15% respectively.
This sanitisation process produces the magnesium ion in form of Mg2+
which the patents
suggest functions to bind dissolved solids and other impurities in the water and makes them
available for capture during filtration. This flocculating role has been predominantly
performed in the swimming pool industry by aluminium salts (Zhidong et al. 2009; Perkins
2000; Duan & Gregory 2003; UWRAA 1992). However, current practices have seen its
limitations, in the wake of the campaign for more eco-friendly approaches in the industry
(Sharma et al. 2006; QLD Health 2003; WHO 2006).
7.1 Magnesium Chemistry
Magnesium is a metal usually occurring in a mineral form. It‟s common forms are dolomite
[MgCa(CO3)2] and Epsomite (MgSO4.7H2O). Some other minerals that contain reasonable
amounts of magnesium include; magnesium calcite (MgSO4) and chrysolite [asbestos,
Mg3Si2O5(OH)4] (Maguire & Cowan 2002).
In its pure state, magnesium appears silvery in colour with a white shade. It is a highly
reactive metal and therefore exists in a free form as a cation Mg2+ in an aqueous solution or
remains in the combined mineral forms listed above (Hai Tan et al. 1999; Maguire & Cowan
2002). Magnesium ion belongs to group 2 of the periodic table, having two valence electrons
in its outermost shell (+2). This happens to be two thirds of the charge of aluminium with a
valency of +3. However, magnesium and aluminium posses the same ionic radius which
results in the same surface area of the ion (Weiner 2000).The table below compares the size
of the magnesium ion and its other derivatives with metallic ions such as potassium (K+),
sodium (Na+) and calcium (Ca
+).
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Table 4: Properties of Common Metallic Ions
Ion Ionic
Radius
(Å)a
Hydrated
Radius
(Å)
Ratio of
radii
Ionic
Volume
(Å3)
Hydrated
Volume
(Å3)
Ratio of
volumes
Water
Exchange
rate
(sec-1
)
Transport
number
Na+ 0.95 2.75 2.9 3.6 88.3 88.3 24.5 8 x 10
8 7-13
K+
1.38 2.32 1.7 11.0 52.5 4.8 109 4-16
Ca2+
0.99 2.95 3.0 4.1 108 26.3 3 x 108 8-12
Mg2+
0.65 4.76 7.3 1.2 453 394 105 12-14
Source: Maguire & Cowan (2002)
As outlined in the above table, the ionic radius of Mg2+
is relatively smaller in comparison
with the other ions while it‟s hydrated radius is substantially bigger that of the other three
cations. Considering the fact that volume is radius raised to the third power, therefore it
becomes very obvious when a comparison of the hydrated volume and the ionic volume of
each cation is made. Mg2+
ion in its hydrated form is 400times bigger than it‟s dehydrated
ionic from. This occurrence is not consistent for the other cations, as there is only a marginal
increase in thier volumes (Maguire & Cowan 2002). The transport number is another striking
property of these cations as it depicts the average number of solvent molecules that
effectively contacts with the ion and as they move through the solution as the cation diffuses.
Thus, higher transport number means the presence of larger macromolecular complexes
(Weiner 2000), resulting in better flocculation potential.
7.2 The Solubility Product of MgCl2
The solubility product constant is an equilibrium constant denoted as KSP as it defines the
equilibrium between a solid and its respective ion in a solution (Weiner E.R., 2000). In this
case, it shows the degree to which the magnesium chloride salt dissociates in a water
solution. The KSP expression for a salt is the product of the concentration of the ions, with
each concentration raised to a power equal to the coefficient of that ion in the balanced
equation for the solubility equilibrium. i.e.
MgCl2
Mg
2+ +
Thus, the solubility product constant KSP is written as;
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KSP = [Mg2+
] [
Where [Mg2+
] and [ represents the concentrations of the ions of Mg2+
and
However, the solubility constant value for Mg (OH)2 at 25°C and 1000kPa is 5.61 x 10-12.
The
higher the KSP of a compound, the more soluble it is in water (Manahan S.E, 2009). This is
well proven by the high solubility of alum in water, as Al (OH)3 has a solubility constant of 3
x 10-34
. When these molecules are dissolved in water these molecules are inserted into a
solvent and surrounded by its molecules. But in order for this process to occur, the molecular
bonds between the solute molecules and the solvent molecules need to be broken and
disrupted. Thus, the amount of energy given off when a solute is dissolved in a solvent should
be sufficient to break the bonds between the molecules of the solute and the solvent for
dissolution to occur (Manahan S.E, 2009).
7.3 Practical Evidence of MgCl2 as a Flocculant
According to Ayoub & Semerijan (2002), the efficacy of water treatment using magnesium
compounds dates back to the late 1920s. The magnesium ions used for these coagulation and
precipitation purposes are mainly from magnesium chloride, magnesium carbonate,
magnesium hydroxide, and seawater. They also quoted a study, which they claimed, achieved
significant reduction in the Total Organic Content (TOC) and also reduced the degree of light
absorbance in water caused by the presence of suspended solids.
Hai Tan et al. (1999) has reported a coagulation technique which uses MgCl2 to produce flocs
with dye materials which are then separated from the aqueous dye solution by sedimentation.
During lime treatment, good coagulation has been achieved in the presence of sufficient
magnesium ions while magnesium rich compounds of dolomite and bittern have proved very
effective in achieving turbidity and colour removal (Hai Tan et al. 1999)
US EPA (2010) have published in their technology transfer capsule report that there is a new
magnesium recycle coagulation system which is based on a combination of water softening
and conventional coagulation techniques which can be applied to all types of water.
Magnesium hydroxide is the active coagulant in this system, which offers an alternative
approach to chemical sludge handling as well as reduction in turbidity of raw water. As the
USEPA Capsule report outlines, this technology can be significantly applied to achieve
positive outcomes in the following areas;
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Reduction or complete elimination of sludge water, as it is recovered as treated water,
which is a good source of saving for the plants
The nature of the flocs formed in this system causes a significant increase in the
clarifier loading rates, which in-turn increases the clarifier capacity.
This system causes water softening and chemically stabilizes soft water.
Modification of colloidal silica surfaces has been achieved using magnesium ions. In a study
by Karami (2009), an addition of increased amount of Mg2+
on the surface growth process of
colloidal silica has caused a corresponding decrease in the mean particle size of the colloidal
silica.
Low concentration of
Seed and Mg2+
ions
High concentration of
Seed and Mg2+
ions
Silicic acid; Mg2+
Colloidal silica Modified colloid
Fig 5: A Schematic of the mechanism of colloidal silica modification by Mg2+
As the Mg2+
are adsorbed on the face of the colloidal silica, it becomes modified at the same
time. This can be related with the zeta potential concept, where an increase in the
concentration of the salt in a system lowers the zeta potential and as a result, decreases the
stability of the colloidal silica. Thus, when this is combined with an increasing number of
seeds, gelling and instability occurs in the system (Karami 2009).
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Part III: Flocculation Theory and Process Kinetics
8 The Science and Process of Flocculation
In water and wastewater treatment, the primary aim is usually to make a slowly aggregating
suspension aggregate faster to cause settling. As has been discussed in the earlier chapter, the
distribution of charges in the colloidal system is the main factor that contributes to instability
of the suspension as a build up of these charges on the surface of the particle may lead to an
alteration of the arrangement of molecules in the lattice (Peavy et al. 1985).
Flocculation is therefore a process whereby destabilised or dispersed particles are brought
together to form aggregate flocs of such size, large enough to cause their settling and bring
about clarification of the system. I this way, we can then easily separate the water and the
floc formed (Faust & Aly 1998, Sharma et al. 2006). Naturally, we can have sedimentation of
particles in water, especially where particles in water is large. Faust and Aly (1983) suggested
that hydrophilic compounds in water will have their particles unable to settle as a result of its
chemical interaction with water. However, when particles are too small to be coagulated, the
use of chemical flocculants becomes imperative.
Flocculating agents act on a molecular level, on the surfaces of the particles to reduce the
repulsive forces and systematically increase the forces of attraction between these particles
(Sharma et al. 2006). A negatively charged particle will have an oppositely charged water ion
circling around it, while the negatively charged water ions are not attracted to the particulate
of the colloidal system (Peavy et al. 1985). Hence, two colloids of the same charge will
hardly aggregate together to cause settling as a result of the prevalent like charges in the
water solution. As a result, an electrical potential is created, which increases as distance
between particles decreases (Peavy et al. 1985). To overcome the electrostatic potential or
repulsive force acting between these particles, the Van der Waals force of attraction is
required. This force of attraction decreases exponentially as the distance between particles
increases and happens to be at a maximum when the distance between particles is at a
minimum (Bratby 2006, Peavy et al. 1985).
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8.1 Mechanisms of Flocculation
Flocculation has been said to occur by a number of mechanisms such as an increase in ionic
strength (reduction of the zeta potential) of a system and by adsorption of counter ions to
cause particle neutralization (Duan & Gregory 2002). However there are four generally
accepted destabilization mechanisms in colloidal systems (Binnie et al. 2002). They are as
follows;
Surface charge neutralisation
Double layer compression
Inter-particle bridging
Sweep flocculation
8.1.1 Surface Charge Neutralization
Destabilization can occur in a suspension when the net surface charge of the particles is
reduced (AWWA 1999). This can be readily achieved by the addition of oppositely charged
ions on the colloidal particles, and this leads to the adsorption of the ions on to the colloidal
materials to effect surface charge reduction. This process then promotes agglomeration, due
to the reduction in the electrical forces that separates the particles (Binnie et al. 2002).
Fig 6: Deposition of metal hydroxide species on oppositely charged particles (Adapted from Duan & Gregory 2002)
AWWA (1999) and Duan & Gregory (2002) suggest that organic and synthetic poly-
electrolytes and some of the hydrolysis products formed from hydrolysing metal such as
Mg(OH)2
are more strongly adsorbed on negative surfaces. This adsorption tendency is
usually as a result of poor coagulant-solvent interaction and the chemical affinity of the
flocculant, as they can adsorb on the surface to the extent that a reversal of the net surface
charge occurs and possibly goes on to effect a restabilization of the suspension (AWWA
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1999). These hydrolysing coagulants can neutralize the negative surface charge of many
types of particles including bacteria and clays. Duan and Gregory (2002) went ahead to infer
that the effective charge neutralizing species may be the positively charged colloidal particles
at about pH of not more than 8.
However, restabilization can be controlled in a system by simple pH adjustments with acid or
base. In surfaces with positively charged oxides and hydroxides, the use of simple
multivalent anions such as sulphates will achieve a destabilization of the system by a
reduction of the positive ions (AWWA 1999; Bratby 2006, Sincero &Sincero 2003).
As much as charge neutralization might be the most preferred mechanism of flocculation
(both economically and environmentally) because it enables the coagulant dosage to be
minimized and reduces the residual metal in water, it must be noted that pH control is not
always easy to achieve during water treatment processes (Byun et al. 2005).
8.1.2 Double-Layer Compression
This method of destabilization has been long-existent. The process involves effecting a
compression of the double-layer by the addition of an electrolyte to the solution to increase
its ionic concentration (AWWA 1999; Binnie et al. 2002, Bratby 2006; Sincero & Sincero
2003). This in turn reduces the thickness of the electrical double layer surrounding each
colloidal particle and slows particles to move closer to each other.
Fig 7: A negatively charged particle surrounded by a charged double layer
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A simple electrolyte such as NaCl is added to cause double layer compression. However, the
effectiveness of any electrolyte used, depends on the change in ionic concentration. Thus,
ions of +3 charges are 1000times more efficient than an ions with +1 charge (Binnie et al
2002) but only 20 times more efficient than those with +2 charge (Fasemore 2004; Benefield
et al 1982). This therefore implies that Mg2+
will be 50 times more efficient than Na+
AWWA (1999) maintained that destabilization by double layer compression is not
practicable in most water treatment processes because of its huge salt concentration
requirements and relatively slow rate of floc formation. Binnie et al (2002) also holds that the
effect of the process is only noticed before the formation of insoluble hydroxides and is not a
function of the colloidal material concentration.
8.1.3 Inter-particle Bridging
Destabilization can occur when large organic molecules with multiple electrical charges are
used as flocculants in water treatment. These types of molecules are usually referred to as
anionic or cationic polymers. They are usually of high molecular weight polymers and tend to
form a linkage between the particles by adsorbing on to one or more particles (Sincero &
Sincero 2003). When the polymers come in contact with colloidal particles, some of the
reactive groups on the polymer adsorb on the particle surface, while the remaining extends
into the solution. If the extended groups in the solution become adsorbed to another surface
of a particle, then inter-particle bridging has occurred. However, the suspension might
restabilise when excess polymer has been adsorbed.
8.1.4 Sweep Flocculation
During flocculation, rapid and extensive hydroxide precipitation can achieve optimal particle
removal from the water. Once the hydroxides are precipitated, it causes a rapid aggregation of
the colloidal precipitate particles and an eventual “sweeping out” of these aggregates from
water by an amorphous hydroxide precipitate (Duan and Gregory 2002).
When soluble metallic salts of aluminium or magnesium are added to water at a suitable pH,
hydroxide flocs are precipitated. In the presence of colloids, hydroxides are precipitated using
the particles of the colloid as its nuclei, with floc formation around the colloid particle (Binne
et al. 2002). Thus, contaminants in water are enmeshed in growing hydroxide precipitate and
the effectively swept-out of the suspension.
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The sweep flocculation process tends to achieve better particle removal than in destabilisation
processes due to charge neutralisation. Amitharajah et al. (1991) therefore suggested that the
relationship between the optimum coagulation dose and the concentration of colloidal particle
in a flocculation process is an inverse. This claim was supported by Binnie et al (2002) with
the suggestion that colloidal particles act as a nuclei on which the coagulant precipitate, in a
high colloidally concentrated system while at low concentration of colloids, more precipitated
coagulant is required to entrap the particles of the colloid. Hence, the optimum pH value of a
coagulation process is dependent on the solubility and actual pH of the coagulant
8.2 Flocculation Kinetics
According to Peavy et al. (1985), mixing is a very important aspect of the flocculation
process. He further stressed that for destabilisation to be achieved in a colloidal system, the
Brownian motion operating in that particular system should exceed the system‟s electrostatic
potential. Apart from Brownian motion, a number of other mechanisms which can cause
relative motion and collision between particles in a destabilized suspension include; velocity
gradients in laminar flow, unequal settling velocities and turbulent diffusion (AWWA 1999).
However, when the Van der Waal forces of attraction between particles of the colloidal
system is low while the distance between them is still high, Peavy et al, (1985) proposes the
use of mechanical agitation to increase the collision rate in order to force agglomeration of
the particles, for easy settling out from the system. It is therefore important to utilise
flocculants to achieve agglomeration in systems where mechanical means alone does not
bring about agglomeration of colloidal particles. The two stages of flocculation according to
Bratby, (2006) are the Perikinetic flocculation stage and the Orthokinetic flocculation stage.
8.2.1 Perikinetic Flocculation
This stage of the flocculation process arises from thermal agitation, usually referred to as
Brownian movement and is a naturally random process (Armenante 2007; Bratby 2006). At
this stage, destabilisation is immediate followed by flocculation and is complete within
seconds. This is as a result of the limiting floc size beyond which Brownian motion has little
or no effect. During this stage which entails rapid mixing, hydrolysis, adsorption and
destabilisation all occur (Fasemore 2004; Jiang and Graham, 1998). There is also a reduction
in the potential energy between particles and a significant increase in Brownian movement,
which then leads to collisions between small-sized particles. A reduction in surface potential
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of the colloids occurs and this causes the adsorption of counter-ion s by colloidal particles
during the perikinetic flocculation stage. (Fasemore2004).
Although the potential energy barrier existing between colloidal particles may be overcome
by the thermal kinetic energy of the Brownian motion, as the particles coalesce, the
magnitude of the energy barrier increases approximately proportional to the area of the floc,
so that eventually perikinetic flocculation of such potentially repellent particles must cease
(Bratby 2006).
8.2.2 Ortho-Kinetic Flocculation
At the end of the rapid mixing period of flocculation, there is a stage of slow mixing
(Fasemore, 2004), this stage is called the orthokinetic flocculation stage and it arises from
induced velocity gradients due to mixing of the liquid (Thomas et al. 1999). More particle
contraction is achieved at a higher induced velocity gradient in the liquid and within a given
time, however this high velocity gradient causes floc breakage in the system and eventually
results in smaller floc size formation (Bratby 2006). Thus, low velocity gradients delays the
time taken for flocs to form, but the result is usually a large floc size formation. This
therefore follows that velocity gradient and time are the two key parameters that determine
the rate and extent of particle aggregation and the rate of particle breakup (Bratby, 2006).
Velocity gradients may be induced in flocculation system by various approaches such as;
Passing around baffles or mechanical agitation within a flocculator reactor
Passing through interstices of a granular bed
Differential settlement velocities within the settling basin.
This process sees application in the swimming pool industry in the use of suction and return
lines connected to the pool circulation system to provide effective mixing.
Faust and Aly, (1983) have described the orthokinetic stage as a period when the particles or
flocs formed are large enough that the relative motion due to velocity gradient of the particles
causes a high shear rate in the liquid phase, compared to the initial perikinetic flocculation
stage. The large particles seem to impart their own velocity to the nearby particles.
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9 Flocculation Models
The mathematical representations of the processes of suspended particles destabilization has
been developed from the mechanisms of transport and attachment.
9.1 The Smoluchowski Model
This is a classical expression in the area of flocculation developed as early as 1917 by
Smoluchowski (Amirtharajah et Al. 1991; Bratby 2006; Brostow et al. 2007; Thomas et el.
1999). Von Smoluchowski modelled transport and attachment mechanisms as a rate of
successful collision between two particles of size i and j (Thomas et al. 1999)
rate of flocculation= ∝β(i,j)ninj..............................................................................................................(1)
Where β (i,j) is the collision frequency, ∝= Collision efficiency
Smoluchowski developed a classical analytical expression for the collision frequency for both
Perikinetic and Orthokinetic flocculation based on the following assumptions,
The collision efficiency factor α, is unity for all collisions
Fluid undergoes laminar flow
The particles are mono-dispersed (all of the same size)
No breakage of flocs occur
All particles are spherical in shape
Collision involves two particles
Now based on these assumptions;
In equation (2) above, the subscripts i,j and k stands for the particle sizes. The first term on
the right had side of the equation represents the increase in particle of size k by flocculation
of two separate particles whose total size of particle equals the size of k, and for both
perikinetic and orthokinetic flocculation, the expressions apply;
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In the equations above, = Boltzmann‟s constant, T is the absolute temperature and is the
fluid viscosity, where is the velocity gradient.
An extension of the above equation was made for orthokinetic flocculation by replacing the
shear velocity, du/dy, with the definition of the root-mean-square velocity gradient, G:
Thus, the collision frequency for differential sedimentation is given by (Thomas et al, 1998);
Where - gravity constant,
are the fluid and particle densities.
The American Water Works Association (1999) stressed the need of recognising Differential
Settling as a situation that occurs when particles have unequal settling velocities and their
alignment in the vertical direction causes collision. Gravity is the driving force here and the