Fouling rate of water

2. Literature review

2.1 Introduction

Calcium Carbonate, Calcium Sulfates, other material, and impurities are contributed to the formation of scale. Scale of depositions is formed by the precipitation of water-insoluble components on a metal surface. Cause of is Chlorides. Iron might be deposits on a surface through precipitation and components. All these things increase the fouling rate of water.

In addition to dissolved solids, unrefined water might be containing suspended solids, (e.g. organic or inorganic). Organic components might be in the form of colloidal solutions. At high velocity water, hard suspended and dissolved solids may abrade pipes and equipments. Particles are settle at the bottom might be accelerating corrosion.

2.2 Fouling

2.2.1 Definition of fouling

The collection of unwanted deposit on the heat transfer surfaces is fouling. Fouling is a familiar term that includes of external material that coming to heat exchanger life time. The cause of the deposition, an additional fouling on heat exchanger surface is correspondingly reducing the heat transfer rate and operational capacity (K.J. Bell, 2001).

These deposit are mineral substance (e.g. CaCO3, CaSO4, SiO2), biological material (e.g. bacteria), and some other material (e.g. clay). These deposits on the surface will blockage membranes and reduce the efficiency, increase pressure drop, and increase the cost for maintenance, shut down, and cleaning time. The designer must consider the effect of fouling heat exchanger design.

They are more parameters influence on the fouling factor:

1- Velocity: increasing velocity will decrease fouling rate

2- Bulk temperature of fluid: precipitation and chemical reaction of fouling can be influence on bulk temperature.

3- Temperature of the heat transfer surface: this temperature might be influence of solidification or even precipitation fouling.

4- Surface material: corrosion and biological fouling may also strongly dependent on the choice of the fouling surface material

5- Surface structure and roughness: this also means more area for corrosion or chemical reaction

6- Heat exchanger configuration: this is also depend on the type of heat exchanger and the rate of fouling is deferent

2.2.2 Type of fouling

It is accept that fouling can be classified into five groups (T.R. Bott, 1995):

1- Precipitation fouling (Sedimentation fouling)

This kind of fouling is also called crystallization fouling. A fluid has been used in heat exchanger can be contain dissolved inorganic salts. There is a maximum amount of salt which can be dissolved in this fluid. When the process condition inside the heat exchanger is different from condition at the entrance, super saturation maybe occurs. This thing means that the part of the dissolved salt will be crystallized on the heat transfer surface. This may increase of bulk solution concentration and concentration polarization (T.R. Bott, 1995). Deposits do not adhere strongly to the surface. This type of fouling is strongly affected by velocity, and less so by wall temperature. In addition this type of deposit can "bake on" to hot wall, and very difficult to remove (K.J. Bell, 2001).

2- Inverse solubility fouling

Some of the salts such as calcium sulfate have less solubility in warm water than in cold water. A stream on a wall at a temperature above that corresponding to saturation for the dissolved salt will be crystallizing on the surface. Normally crystallization will be start at especially active points - nucleation sites - such as scratches and pits, and often after induction period, and spread to cover the entire surface. This type of fouling is strong and adherent, and requires vigorous mechanical or chemical treatment to remove it (K.J. Bell, 2001).

3- Chemical reaction fouling

This type of fouling is occurring when fouling considers the deposits that are formed as a result of chemical reaction that result in producing a solid phase at or neat the surface. In this case, hot heat transfer surface has carbonaceous deposit on the surface because of thermal gradation of the components of a process stream. This type of fouling is often extremely tenacious, and need extreme measure as burning off the deposits in order to the exchanger to satisfactory operation (K.J. Bell, 2001).

4- Corrosion fouling

This type of fouling is also caused by some chemical reaction, but it has different from the last one. This fouling is a reactant and will be consumed. This surface will be reacts with the fluid to make corrosion on the surface (T.R. Bott, 1995).

5- Accumulation of biological fouling

This is occurring when the biological micro and macro organisms are stick to the heat transfer surface. In this case this attaching of the material will be growth so this is the common problem (T.R. Bott, 1995).

2.2.3 Fouling development stage

Fouling is developed with five stages (Epstein 1983a):

1- Induction

This is delay period for beginning of fouling. During the induction period, nuclei for crystallization of deposit are formed for biological growth. This period can take a long time, may be several weeks or a few minutes or even seconds. If the initial period decreases with increasing surface temperature, crystallization fouling will be change (Epstein, 1983b).

2- Transport

In this part, fluid transported toward a surface across the boundary layer.

This is depending on the physical properties of the system and concentration between the bulk fluid and fluid interface.

3- Attachment

In this stage, the deposits will be adhering to the surface and itself.

4- Removal

There is competition between removal and deposition, throughout the steady growth of the deposition on the surface.

5- Ageing

Ageing will be start as soon as deposition happens. During the ageing, there maybe transformation of crystal will be improve or decrease the deposition strength with time.

2.2.4 Change in deposition thickness with time

This graph has been shown the rate of growth of deposit on the surface (T.R. Bott 1995).

Region A: fouling is initiated in an induction period

Region B: a steady deposit growth on the surface. The rate of removal of deposit increases when the rates of deposition gradually fall.

Region C: in this region the rate of removal and deposition perhaps become equal and the thickness of deposition remains constant.

2.2.5 Composite fouling

Some of the common example for fouling are CaSO4 and CaCO3, CaCO3 and Mg(OH)2 , CaSO4 and SiO2. Solubility, crystal structure and strength will have influence composite scale formation on the fouling rat. Therefore, composite fouling needs to be more attention and research on further.

2.3 Common type of water formed deposits:

2.3.1 Calcium carbonate (CaCO3)

Calcium carbonate is a common laboratory and industrial chemical substance, and is one of the common dispersed minerals in the word, such as limestone, and chalk. Because of these thing calcium carbonate has harmless properties and low cost. It has been used for common purpose such as cement, fiber, and flux. Furthermore it also forms deposit on heat transfer surface. This is a common problem of the chemical industries, boilers, and water treatments (Sheikholeslami, 2003).

Calcium carbonate is identifying by CaCO3, with chemical properties as below:

Exact mass

100.0869 g/moll


White powder


2.271 g/cm3 (calcite)

2.83 g/cm3 (aragonite)

Melting point

825 (calcite)

1339 (aragonite)

Solubility in water

0.00015 mol/l (25)

Solubility product, Ksp


Acidity (PKsp)


Table 2.1 Chemical properties of calcium carbonate (Patanaik, pradyoy, 2003)

Calcium carbonate is a substance found in rock in all part of world. It is usually the principle cause of hard water.

Calcium carbonate shared the properties of other carbonates (Patanaik, pradyoy, 2003):

1- it react with strong acids:

CaCO3 (s) + 2 HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)

2- it release on heating above 840

CaCO3 (s) CaO + CO2

3- it react with water

CaCO3 + CO2 + H2O Ca(HCO3)2

Calcium carbonate is naturally in there crystal structures of Calcite, Aragonite, and Vaterite. The most formed of Calcium carbonate is Calcite. The aragonite is irreversibility change to Calcite while heated in 400 dry airs (Patanaik, pradyoy, 2003).

2.3.2 Calcium sulfate (CaSO4)

Calcium sulfate is a chemical compound with chemical formula CaSO4, another things about Calcium sulfate is:

Molar mass

136.14 g/moll (anhydrous)

145.15 g/moll (hemihydrate)

172.172 g/moll (dihydrate)


White powder


2.96 g/cm3 (anhydrous)

2.32 g/cm3 (dihydrate)

Melting point

1460 (anhydrous)

Solubility in water

0.0021 mol/100ml (20, anhydrous)

0.24 mol/100ml (20, dihydrate)

Solubility product, Ksp

2.4*10-5 (dihydrate)

Table 2.2 Chemical properties of calcium sulfate (Patanaik, pradyoy, 2003)

It is usually used as water absorber. The hemihydrate (CaSO4 . 0.5 H2O) is known as plaster of paris, when the dehydrate (CaSO4 . 2 H2O) occure naturally as can be exist in six different solid phase (a dehydrate, 2 hemihydrate, and 3 anhydrites). Calcium sulfate is naturally gypsum, and anhydrite which find at many part of the earth.

In addition, calcium sulfate is popular component of fouling in industrial heat exchanger because its solubility decrease with increasing water temperature ,and by adding equimolar solutions Na2SO4 and CaCl2 , NaHCO3 and CaCl2 , the supersaturation of CaCO3 ,and CaSO4 will be prepare (T.H. Chong 2001).

2.3.3 Nano Particles

The definition of nano particles is a particle defined as a small object which has a unit behaves in terms of its transport and properties. It is classified by diameter. Nanoparticles cover rang between 100 and 2500, and ultrafine particle size between 1 and 100 nano meters. At least nanoparticles have dimension between 1 and 10 nano meters.

Agglomerates of ultra fine particles, nanoparticles, or nano cluster are nano powders. Recently nanoparticles research is a new science field on fouling and effect of fouling on heat transfer surface and also effect of the crystallization of fouling.

Nano particles are effectively a bridge between bulk materials and molecular structures. Suspension of nanoparticles are possible while the interaction of the nanoparticles surface is strong enough to over come different density, in other hand usually result in a material is floating in a liquid. Nanoparticles have a very high volume ratio surface, especially at temperatures evaluative. Every days science have been found more nanoparticles to import some extra properties such as titanium dioxide, zinc oxide, clay, metal, dielectric, and semiconductor, semi-solid, and soft nanoparticles (Dubbs D.M. , 2000).

2.3.4 Gum Arabic

Gum Arabic is a kind of natural gum obtained from hardened sap taken from two species of the acacia tree. Gum Arabic is used in the food industry as a stabilizer which is complex mixture of polysaccharides and glycoprotein. Also gum Arabic is used in printing and glue in various industrial applications. Gum Arabic is using as viscosity control in textile industries ink, and using for water soluble binder in firework composition. One of the chemical properties of gum Arabic is effect on the surface tension of liquids, and further more it is used for control water treatment with high concentration, water quality improvement, and control of membranes fouling.

KIM H et al (KIM H, 2000) have been studied on Arabic gum powder and lignin, they are found that the raw water containing Arabic gum powder accelerated the membrane of fouling process by making a cake layer of PAC on the MF membrane. The initial cause of membrane fouling in the PAC-MF process was cake+gel layer formation (>98% of the total resistance), and the membrane pore blocking was very small (<2%), which was further reduced by PAC.

2.3.5 High-Density Polyethylene

High-Density Polyethylene is known as HDPE and made from petroleum. We can make 1 kilogram of HDPE by 1.75 kilograms of petroleum. It is normally recycled, and it recycle symbol's number is "2".

As properties of HDPE has little branching, giving it stronger intermolecular force then lower density polyethylene. HDPE is using in different and variety of application and including such as: telecom duct, strong sheds, plastic bags, and etc. also we can used HDPE ball in heat exchanger to control fouling rate.

2.3.6 Gum Tragacanth

Tragacanth is also known as Shiraz gum, gum Elector, and gum dragon. It is a natural gum, and made of the dried sap of several species of the genus Astragalus. It is including Adscendes, Gummifer, and Tragacanthus. Gum Tragacanth is a viscous, odorless, tasteless, and water soluble. Gum Tragacanth absorbs water to become gel, and used as stiffener in textiles industries (M. Grieve 1995). Gum Tragacanth is fewer products than other gums (e.g. gum Arabic or Guar gum). Gum Tragacanth is common grown in Middle Eastern countries (e.g. Iran). Commercial cultivation of Tragacanth plants has usually not proved worthwhile economically in the west and this gum like other gums can be employed for similar purposes.

If we use gum Arabic and Gum Tragacanth in water soluble, Gum Tragacanth is ideal for spread, and ease of working with. The maximum viscosity of Gum Tragacanth is reached only after 24 hours at room temperature or after 8 hours at high temperatures. The solution of Gum Tragacanth is stable to heat, and under a wide range of PH level (A.Yokoyama, 1987).

2.4 Basic principle of fouling

The accumulations of undesired deposit on the surface of heat transfer increase the overall heat resistance.

The temperature distribution effected by presence of the individual fouling layers. T1 and T4 are temperature of bulk hot and cold fluids. These temperatures under turbulent flow extend the boundary layer while there is a good mixing.

There is the region between the deposit and the fluid boundary layers because there is a resistance to heat flow. Heat transfer equipments are often limiting bye fouling (T.R. Bott, 1995). The effect of fouling on the heat transfer surface is accounted in design by overall fouling resistance. Rf, in to the basic heat transfer equation:

Where is clean, and U is fouled overall heat transfer coefficients.

The deposition surface roughness will be different from clean surface roughness, and the result will be change in the level of turbulent particularly near the surface. Therefore greater surface roughness will produce greater turbulent (Bott and Walker, 1971). Heat transfer coefficient has been change while the thermal resistance of foulant, roughness of foulant, and Reynolds (Re) cause by the presence of foulant change.

The purpose of any fouling model is to make, assist, and good operate the heat exchanger. An idealized of the development of a deposit with time is shown in 2.1 and other possibilities ideal.

In 2.3, curve A is a straight relation between deposit thickness and time, that means the rate of thickness of fouling will be increase by time. In curve B, fouling rate initiation has occurred, and is part of a similar cure C and if the fouling has been allowed to progress sufficiently would be produced. Curve C, same as 1.1., and it is a general model of fouling progress to fitting of equations the curves. The curve A, B, and C are shown in 1.3 have initiation period (initiation), that means this period is short and it is difficult the initiation periods, so the most mathematical models are ignore it.

2.5 Fouling mechanism of CaCO3 and CaSO4

2.5.1 Crystallization fouling

The formations of crystals are a common phenomenon. The fouling can be tenacious scale that cannot be removed easily that is referred to a softer fouling generally (T.R. Bott, 1995). Mineral deposit usually consists of calcium sulfate and calcium carbonate. The mechanism is crystallization fouling. The crystallization fouling has three continuous stages (T.R. Bott, 1995):

1- the occurrence of supersaturation

2- the formation of crystal nuclei and crystallites

3- the growth of crystal

Crystallization begins basically with the development of single discrete germs. These emerge either in the kernel of the less than cool and/or oversaturated fluid or at the firm limitation walls. The germs in the fluid kernel emerge, one speaks about homogeneous crystal germ development, are formed this at limitation walls or other surfaces, before a heterogeneous germ development. After stable germs formed and exists under the prerequisite, which further a driving descent, begin coming from to grow up immediately crystals the available germs. According to composition of the fluid, this entirely different crystal habit can show.

The development of stable germs does not use how the experience teaches, directly after exceeding of a phase boundary or balance boundary. It becomes rather a certain lower cooling and/or glut necessarily before it comes to the development more discretely none [Mersmann, 2001].

If first once stable germs formed, these catch immediately on to grow. The crystal growth continues as long as a sufficient lower cooling and/or glut exist and the growing crystals are not restricted spatially. The growth mechanism exact not yet completely is decoded until today. A multitude of models were developed in the last decades 2 state of the knowledge 14, of which each declares usually only an entirely certain experimental observation to be itself, [Gauze, 2001].

In fact the processes of formation in heat exchanger surfaces are different from industrial crystallization (Sheikholeslami, 2003b), and some basic theories in this filed are helpful to understanding the mechanism of crystallization fouling.

The supersaturation condition is not necessary to begin crystallization. The act of crystallization is prerequisite of the growth of crystals in the presence of minute solid boilers (Mullin, 2001). The main mechanism for the crystallization is sparingly soluble salt such as CaCO3 and CaSO4. The based on these various phenomena, nucleation is generally classified in two categories, primary, and secondary. Primary stage is classified in Homogenous (Spontaneous), and Heterogeneous (Induced by foreign particles). Secondary stage is induced by crystals (Mullin, 2001).

2.5.2 Solubility

It is equilibrium for soluble solid phase, and defined as saturated with this solid. Salt is classified in two categories of solubility characteristics. Some salts solubility is increased by temperature; these salts are called normal soluble salt (e.g. NaCl, and NaNO3). Some salts have lower solubility when the temperature is raised (e.g. CaCO3, and CaSO4). In addition solubility is also influence by the impurities in the systems. When a solution becomes supersaturation, it can be maintain detestable for a period of time. The time between achievement of supersaturation and appearance of crystal is referred to as an induction period. The induction period is greatly affected by level of supersaturation, state of agitation, presence of impurities, viscosity, and etc (Mullin, 2001).

The induction period is salt specification. The induction period for pure calcium sulfate is much longer than the calcium carbonate (Sheikholeslami, 2003b).

2.6 Scaling potential of calcium sulfate and calcium carbonate

2.6.1 Calcium sulfate

Calcium sulfate is in natural primary as Gypsum (CaSO4 . H2O), Anhydrite (CaSO4), and Hemihydrates (CaSO4 . 0.5H2O).

Calcium sulfate solubility decrease with the increase of temperature, the must solubility work in pure water. It is generally the calcium sulfate scaling indices can be assessed by the following equation:

SI (CaSO4) =

where SI is the scaling index, [Ca2+], and [SO42-] is the molar construction of Ca2+, and SO42- respectively (Yuan Wang, 2005).

SI value less than 1.0 denotes an undersaturated solution and CaSO4 scale will not form. If SI values exceed 1.0, the solution is oversaturated with CaSO4 and scale may from.

Calcium sulfate and its Hydrate belong to the probably most thoroughly investigated inorganic substances. Van published already 1912't Hoff a systematic investigation of the solution balances of the system CaSO4 - water [van't Hoff, 1912]. According to temperature area and solution concentration, calcium sulfate crystallizes as an Anhydrate, Hemihydrates or as a Dehydrate from. The Anhydrate occurs with in three modifications: a high temperature form. Natural Anhydrate (also - CaSO4, CASO4 II or dead burned plaster named) and a soluble form (the -CaSO4, CaSO4 III and/or Hemihydrates) are insolubilize. Just as different compositions were discovered in the Meta stable Hemihydrates. Especially the water salaries can values of up to 3/4 suppose per mol CaSO4. 2 H2O , and the Dehydrate occurs only in a crystal form. In natural form, CaSO4 · H2O is known as a plaster [Gmelin, 1961].

2.6.2 Calcium carbonate

Calcium carbonate is an inverse soluble salt and this salt deposition is depending on PH. It has three naturally structure, Calcite, aragonite, and vaterite (Sheikholeslami, 1999a). The formation of calcium carbonate is related to the following equations:

Carbonate acid is the species below PH6 and carbonate ions above PH9, and between PH6, and PH9, this water mostly contain bicarbonate ions.

Driving Force Index (DFI) is also widely used. It is related to concentration of calcium, carbonate, and Ksp value directly (Rossum, 1983):

DFI value 1.0 denotes solution saturation with respect to CaCO3. If DFI is exceeding 1.0, CaCO3 is undersaturated.

2.7 Cost of fouling

2.7.1 Scale of mitigation methods

Several practical are utilized to control scale of fouling. The most common existent for calcium sulfate and calcium carbonate is preventing from acid treatment. The strong effect of the PH is on crystallization fouling. Lowest fouling is at PH 7.0. The microscopic structure of crystallize layers have big different at different PH-values.

Volker Hofling et al. (Volker Hofling, 2004) have been study on the different PH-value. They found that the lowest fouling is occurred at PH 7.0, and below PH 6.0 only calcium sulfate was detecting by x-ray. At higher PH, calcium sulfate, and calcium carbonate was found in different modifications. They found the highest strength crystallization value in upper and middle layer were measured for growth crystalline scale at PH 7.0, and follow by layer at PH 6.5. In addition, PH value will influence on fouling of CaSO4/CaCO3. Scanning by election microscopy, and x-ray will be determine the strength of the layer crystallize fouling.

2.7.2 Influence of velocity

The prevailing velocity is important parameters in the scale of deposition rate, and fluid velocity might be increase overall fouling should go down. For example a biofilm might be more compact and dense under high velocities. In the presence of low velocity the structure might be more open and "fluffy" (T.R. Bott, 1995). Thono et al. (Thono, 1999) have been studied on particulate fouling in plate heat exchanger. They found that the fouling was inversely proportional to the velocity square.

Velocity is normally has effect on fouling in two different ways. Firstly, high velocity is helping the transportation of ions to the fouling on the wall and crystallization at the wall. Secondary, by increasing velocity, the removal rates are increase, Cause of higher shear rate at the at the liquid-solid interface (P. Walker, 2003). Velocity has also affects on chemical reaction mechanisms by influence on residence times but suggests that the impact is significant if the reaction rates are relatively large (T.R. Bott, 1995).

Williamson (Williamson, 1990) has been studied on data that show the dependence of haematite deposition (i.e. 0.2 particles) on velocity. 2.5 is shown the rapid fall in asymptotic deposit mass (mg/m2) as velocity is increased. This also illustrates the corresponding value of friction velocity u* and Reynolds number. This suggestion employing data about velocities > 2m/s would minimize the fouling due to particle deposition (T.R. Bott, 1995).

Cleaver and Yates (Cleaver, 1957; Yates, 1976) have been studied on "Turbulent bursts" near the heat transfer surface. The fluid velocity and the presence of suspended particle can be decrease the fouling rate. The most influence of velocity is different than these researches.

Malik Alahmad (Malik Alahmad, 2004) had study on a fluid velocity, and he found increasing fluid velocity reduce the fouling deposit. He has complete work on different velocity from 0.05 and 0.37 m/s, and with different Reynolds number 400 to 29600 and keep the other factors constant.

2.7.3 Effect of temperature

The fluid temperature is one of important thing on scale of fouling, and this effect on supersaturation of inverse solubility salt (e.g. calcium sulfate). The influence by change temperature brought about by presence of the deposit itself. Normally induction period decrease by increasing the temperature. While accumulation of undesired deposits increase, the outer surfaces of undesired deposit will be increase in temperature, therefore the chemical composite deposition will change as fouling develops. The flow temperature will increase in supersaturation, which produce early the nuclei calcium sulfate for starting the fouling process (Malik Alahmad, 2004). Some researches have been done in the mitigation fouling area of co-precipitation of CaCO3 and CaSO4; most of these researches are conducted by batch rig test system and have been studied at relatively high temperatures (e.g. 60-80°C).

2.6 shows how the boiling heat transfer coefficient change by different temperature by increasing the heat transfers surface and boiling liquid. It will be seen that the relative between increasing of temperature difference and heat transfer coefficient till a peak value is obtained. The heat transfer coefficient will be increase till the string action of the bubbles of vapor (steam). The movement of the bubbles of vapor by increasing of temperature difference becomes more violent as they escape into the bulk liquid. Cause of the low thermal conductivity of vapor, the vapor layer make an additional resistance to heat transfer surface, therefore the heat transfer coefficient fell down from the peak value ( 1.5). This phenomenon is known as "film boiling". In addition increasing temperature difference has another problem that is radiation heat transfer across the vapor layer (T.R. Bott, 1995).

2.7.4 Effect of PH

This parameter is also play an important in scale formation. Increasing PH value of fluids are reduced acid dosing to decrease the deposition rate, but in other hand in this case we have corrosion problems. The PH has investigated to distinguish between fouling, and corrosion. This parameter was varied form 4 to 9 (Malik Alahmad, 2004). Malik Alahmad and Sheikholesalmy (Malik Alahmad, 2004; Sheikholeslami, 1999a) have been study on PH. they found that increasing PH value are reduced the scale formation.

The motivation for crystallization processes is the supersaturation. The saturation grades are expressed by the saturation index, which is different, defined between the actual PH value and the saturation PH value, PHs (Langelier, 1946):

S = PH - PHs

As it is known the saturation index for CaCO3 strongly depends on the PH-value, at the same time for CaSO4 between PH 4 and PH 10 the saturation index is nearly constant and it is shown in 2.7:

The effect of PH-value on the fouling rate of pure CaCO3 was investigated by Augustin and Bohnet (1995). They found a big influence on the scaling tendency of this parameter.

Melo's reports (Melo and Pinheiro, 1988) was extended by Oliveira et al (Oliveira, 1988) to find influence of PH and chemical used to control PH on the deposition of kailin. 1.8 is shown some data about PH controlled by Na2CO3. Normally increasing the PH above 7 will decrease of thickness deposition. By increasing PH above 9 and using NaOH, the deposition thickness occurs dramatic fall.

Present study has been determined the affect of the PH-value on the fouling rate, and fouling behavior in the aqueous two-component system consisting of CaSO4/CaCO3 (Volker Hofling, 2004). Fouling experiments at different PH values were performed and the obtained scales were determined the performance of strength layers, and abrasion experiments investigated by using x-ray diffraction and electron- microscopy (Volker Hofling, 2004).

2.8 Pervious work on fouling

An important role in discovering dominate of mitigate fouling are need knowledge of fouling mechanism. For instance crystallization has been studied for several years, and many researches have been done in crystallization area, but it need more pay attention about fouling, and crystallization. The main reason, the scale forming process in heat exchangers in industrial crystallization, and fouling is more difficult to predict (Sudmails, 2000).

Hasson (Hasson, 1981; Hasson, 1997) reviewed literature on crystallization for recent fifty years. He has down an immense body of information available for pure salts, and has been developed to study of the co-precipitation phenomena. Also, co-precipitation study has an utmost important meaning, so this for two mechanism (Sheikholeslami, 1999a). One of the reason is the mechanisms of fouling may be different from deferent salts. The other reason that is the minimum presence of one salt might actually affect the thermodynamics and kinetics behavior of other salt. Therefore, the data for single salt may not be useful to the coexist salt. Some good research on co-precipitation on calcium sulfate, and calcium carbonate has been done by Bramson et al. (Bramson, 1996). They reported that the deposit purity was a factor affecting the scale tenacity. For calcium sulfate mixed with calcium sulfate, the higher impurities, is strongly tenacity. In addition, the impurities of calcium carbonate increase with decrease the adhesive strength. By contrast, they examined the crystals of co-precipitation calcium carbonate and calcium sulfate. They have suggestion, which is the co-precipitation calcium carbonate seems to function as bonding cement, to help the strength of calcium sulfate scale consider layer.

Sheikholeslami (Sheikholeslami, 1999a) has been investigated the effect of co-precipitation calcium carbonate and calcium sulfate at 60, 70, and 80while carbonate or sulfate was dominate. Sudmalis and Sheikholeslami (Sudmalis, 2000) has been studied on co-precipitation of calcium carbonate and calcium sulfate, when they exist in comparable ratios. This is shown that the scale morphology, solubility product, and precipitation rate of co-precipitation salts are different to the single salt. Furthermore, Sheikholeslami et al. made an effort to quantify co-precipitation process of calcium carbonate and calcium sulfate, when calcium carbonate was the predominate (Chong, 2001) or none dominated (Sheikholeslami, 2001).

Crystallization has been studied for many years. It has shown in two monographs by Mullin (Mullin, 1979; Mullin, 1993). An immense body of information is available on the thermodynamic and kinetic of crystallization of calcium carbonate ( Augustin, and Bohnet, 1995; Nancollas, and Reddy, 1971; Plummer, and Nancollas, 1992).

Middis et el (Middis, 1998) were working pulp fibers are efficient in mitigating fouling, even at small concentrations of 0.05 mass percent. Kazi et el (Kazi, 2001) conducted many experiments with fiber suspension of long softwood and short hard wood fiber at varying concentrations, where they found the effect of latter fiber in mitigation fouling, and at 0.25 mass percent of concentration, no fouling was abserve for 46.5 days and the experiment was terminate.

Epestein (Epstain, 1983) developed 5*5 matrixes, which adopted by Bohnet (Bohnet, 1987) to present on assessment of the exten knowledge in various elements. Ming Gao et al. (Ming Gao, 2006) have been study about fouling on different surface, and they found Ni-based implanted has anti fouling characteristic.

Abdul Quddus (Abdul Quddus, 2008) has laboratory study on the effect and influence of solution hydrodynamics on the deposition of calcium carbonate scale on stainless steel 316, and calcium sulfate scale on aluminum substrates using a rotating speed, atmospheric pressure, and temperature on deposition of calcium sulfate, and calcium carbonate.

Quan Zhenhua et al. (Quan Zhenhua, 2008) studied on force convection to investigate the fouling process on heat transfer surface. They did dynamic monitoring apparatus of fouling resistance. They found fouling rate and asymptotic fouling resistance increase and induction were shortened with the fluid velocity decreasing, hardness, alkalinity increase, solution temperature, cluded crystallization fouling, and particulate fouling.

Dydo et al. (Dydo, 2003) analyzed the scaling nano filtration system with CO32-/SO42- molar ratio ranging from 0.05 to 2.8*10-3. They found both balk and surface crystallization mechanism was identified; fouling is affected by water quality.

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