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Coagulation and flocculation

2 CHAPTER TWO

2.6 Arsenic treatment techniques in drinking water

2.6.3 Coagulation and flocculation

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coagulants (aluminium chloride and two types of polyaluminium chloride) during a coagulation process. The study showed with an initial As (V) concentration of 280 μg/L, all three coagulants reduced As (V) concentration below the MCL recommended by WHO. It further states that aluminium species regulate arsenic removal and thus arsenic removal efficiency can be improved by adjusting the pH.

The effect of operational variables such as coagulant dose, As (V) concentration and pH was conducted by Bilici Baskan and Pala (2010). Their findings showed more than 91 % As removal with an initial As (V) concentration of 10 μg/L and an Al2(SO4)3 coagulant concentration of 66 mg/L. Likewise almost 100 % removal of As (V) was achieved with an initial As (V) concentration of 500 – 1000 μg/L and a coagulant concentration of 42 – 56 mg/L. The high coagulant dose used in this study can increase the operational cost and generate a secondary waste. Iron (Fe) based coagulants have been used by several authors (Andrianisa et al., 2008; Lacasa et al., 2011; Lakshmanan et al., 2010; Song et al., 2006). Wickramasinghe et al.

(2004) use ferric chloride and ferric sulfate as a coagulant to remove arsenic and the study showed that the rate of removal depends on pH adjustment before coagulation and raw water quality.

According to (Ravenscroft et al., 2009), Fe based coagulants are more effective for arsenic treatment than aluminium based coagulants. Aluminium hydroxide (aluminium based coagulant) is stable over a very narrow pH range, whereas iron hydroxides are more stable over a wide pH range (Hering Janet G. et al., 1997).

Moreover, iron hydroxides have a high affinity for arsenic and thus, rapid precipitation/co-precipitation of arsenic takes place.

Several tools have been used for coagulation control and effectiveness such as determination of colloid charge, jar testing and pilot plant. Methods for measuring colloid charge include (1) charge titration, (2) zeta-potential and (3) streaming current potential.

There are different types of coagulants and polymers used in the water and wastewater treatment such as metal coagulants, polymers, activated silica and natural polyelectrolytes.

51 2.6.3.1 Metal coagulants

The commonly used metal coagulants fall into two major categories namely iron and aluminium based. The aluminium based coagulants include aluminium sulfate, aluminium chloride, sodium aluminate, aluminium chlorohydrate (ACH), polyaluminum chloride, polyaluminum sulfate chloride, polyaluminum silicate chloride and forms of polyaluminum chloride with organic polymers. The iron coagulants include ferric sulfate, ferrous sulfate, ferric chloride, ferric chloride sulfate, polyferric sulfate, and ferric salts with organic polymers. The popularity of these metal coagulants arises not only from their effectiveness as coagulants, but also from their ready availability and relatively low cost (Bratby, 2006).

2.6.3.1.1 Aluminium Sulfate: - this is probably the most used coagulant and has been in use for water treatment for several centuries. It is manufactured from the digestion of bauxite ores with sulfuric acid, so that in the final product no free acid is present. Evaporation of water in the process results in the dry product having the approximate formula Al2 (SO4)3.14H2O, with aluminium content ranging from 7.4 to 9.5 % by mass.

2.6.3.1.2 Aluminum Chloride: - this coagulant (AlCl3.6H2O) is normally supplied in solution form, containing 10.5 % as Al with a pH and density of approximately 2.5 and 1300 kg/m3, respectively (Bratby, 2006). It is widely used for sludge conditioning.

2.6.3.1.3 Sodium Aluminate: - (NaAlO2) is usually supplied as a viscous, strongly alkaline, and corrosive liquid. The solution strength is usually 13 % as Al. this coagulant differs from alum in that it is alkaline rather than acidic in its reaction. It is rarely used alone, but generally with alum to obtain some special result. NaAlO2

has also been used in the lime-soda softening process as an aid in flocculating the fine precipitates of calcium carbonate and magnesium hydroxide resulting from softening reactions. The reactions of NaAlO2 with Al2 (SO4)3.14H2O and with free CO2 produce insoluble aluminium compounds:

6NaAlO2 + Al2 (SO4)3.14H2O = 8Al(OH)3 + 3Na2SO4 + 2H2O 2NaAlO2 + CO2 + 3H2O = Na2CO3 + 2Al(OH)3

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2.6.3.1.4 Ferric Sulfate: - this coagulant (Fe2(SO4)3.8H2O) is available in both liquid and solid form. In the solid form, the material is granular and free flowing with the following typical specifications: 72 to 75 % Fe2 (SO4)3 and 20 to 21% Fe3+, by mass. In the liquid form, typical specifications are 40 to 42% Fe2(SO4)3 and 21%

Fe3+, by mass. Ferric sulfate is particularly used for color removal at low pH values and at high pH values, where it is used for iron and manganese removal and in the softening process.

2.6.3.1.5 Ferrous Sulfate: - this coagulant (FeSO4.7H2O) is available either as crystals or granules containing 20 % Fe, both of which are readily soluble in water.

Ferrous sulfate reacts either with natural alkalinity or added alkalinity to form ferrous hydroxide, Fe(OH)2, but since ferrous hydroxide is relatively soluble, it must be oxidized to ferric hydroxide in order to be useful. The important reactions for ferrous sulfate are:

FeSO4.7H2O + Ca(OH)2 = Fe(OH)2 + CaSO4 + 7H2O 4Fe(OH)2 + O2 + 2H2O = 4Fe(OH)3

2.6.3.1.6 Ferric Chloride: - this coagulant (FeCl3) is available commercially in the liquid, crystal, or anhydrous forms, although the liquid form is by far more common.

The liquid and crystal forms are extremely corrosive and must be handled in a similar fashion to hydrochloric acid. The reactions of ferric chloride with natural or added alkalinity may be written as follows:

2FeCl3 + 3Ca(HCO3)2 = 2Fe(OH)3 + 3CaCl2 + 6CO2

2FeCl3 + 3Ca(OH)2 = 2Fe(OH)3 + 3CaCl2 2.6.3.4 Polymers

Polymers refer to a large variety of natural or synthetic, water soluble, macromolecular compounds, which have the ability to destabilize or enhance flocculation of the constituents of a body of water (Bratby, 2006). A polymer may be described as a series of repeating chemical units held together by covalent bonds.

If the repeating units are of the same molecular structure, the compound is termed a homopolymer. However, if the molecule is formed from more than one type of repeating chemical unit, it is termed a copolymer.

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Polyelectrolytes are special classes of polymers containing certain functional groups along the polymer backbone which may be ionisable. All polyelectrolytes are typical hydrophilic colloids. They have molecular weights generally in the range 104 to 107 and are soluble in water due to hydration of functional groups. Some types of polyelectrolyte currently in use are discussed below.

2.6.3.5 Activated silica

This is probably the first polyelectrolyte to be used widely in water clarification. In preparing activated silica (which is an anionic polyelectrolyte) commercial sodium silicate solutions (pH approximately 12) at concentrations in excess of 2 x 10-3 M are neutralized with acid reagent (sulfuric acid, chlorine, aluminium sulfate etc.) to a pH less than 9.

2.6.3.6 Natural polyelectrolytes

Coagulation and flocculation could be achieved using either natural coagulants or chemical-based coagulants. Among the two, natural coagulants have long been acknowledged for their application in traditional water purification which is evident from various ancient records (Bratby, 2006). Natural coagulants include starch derivatives which can be natural starches, anionic oxidized starches, or amine treated cationic starches. Other classes include polysaccharides, such as guar gums, tannins, chitosan and the alginates.

2.6.3.6.1 Moringa oleifera seeds: - there are approximately 14 known varieties of moringa oleifera trees around the world, particularly in developing countries.

Different varieties appear to have differing coagulating properties that depend on the geographical location, climate, altitude, and soil characteristics. The seed contains up to 40 % by weight of oil. Narasiah et al. (2002) compared the efficiencies of two moringa seed extracts from Burundi, Central Africa, and from Mahajanga, Madagascar on the coagulation of a laboratory prepared kaolin turbid water. In both cases it was found that shelled seeds provide much higher turbidity removal than non-shelled ones and Burundi seeds were superior in quality than those of Madagascar. Ravikumar and Sheeja (2013) use moringa oleifera seed as a coagulant to remove heavy metal from water. The percentage removal by Moringa seeds were 95 % for copper, 93 % for lead, 76 % for cadmium and 70 % for chromium.

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2.6.3.6.2 Starches: - polymers may be processed from various sources of starches, including potato, corn, cassava, arrowroot, and yams. Starches are basically highly polymerized carbohydrates. These polymers may be non-ionic, cationic, or anionic depending on the form of processing and the substitutions. Choy et al. (2016) compared turbidity removal efficiency between rice, wheat, corn and potato starches to that of alum and polyaluminium chloride. Using kaolin suspensions, the effects of pH, dosage and need for starch gelatinization was studied.

2.6.3.6.3 Guar Gums: - these are neutral (non-ionic) polysaccharides relatively unaffected by pH and ionic strength. They are subjected to enzymatic degradation on storage, but this may be prevented by addition of citric or oxalic acid. Guar gum has been used in uranium ore processing.

2.6.3.6.4 Tannins: - these are complex polysaccharide tannin derivatives that have been used extensively in potable water, wastewater, and industrial effluent treatment applications. They are generally most effective under acidic conditions.

Care must be taken on storage as they are subject to degradation reactions, if left for lengthy periods (Bratby, 2006). Heredia and Martin (2009) tested the effectiveness of a new commercial tannin-based flocculant in order to remove Zn2+, Ni2+, and Cu2+ by coagulation-flocculation process.

2.6.3.6.5 Chitosan: - chitin is the skeletal substance of the shells of crustaceans, such as crabs, lobsters, and shrimps and it is described as a high nitrogen containing linear amino-polysaccharide polymer, with a molecular weight of several hundreds of thousands (Bratby, 2006). Chitosan is a cationic polyelectrolyte with a molecular weight of approximately 106. Sekine et al. (2006) applied 1.5 mg/L of a commercial chitosan solution directly to the river during a river construction project, to reduce the detrimental ecological effects arising from increased turbidity. Vogelsang et al.

(2005) demonstrated the effectiveness of chitosan on the removal of humic substances from Norwegian surface waters. They observed that the highest charged chitosan molecules tested were most effective, indicating that charged neutralization was an important mechanism for the coagulation of the humic matter.

Plant-based natural coagulants are safe, eco-friendly and generally toxin free (Bratby, 2006; Choy et al., 2014). Natural coagulants have been found to generate not only a much smaller sludge volume of up to five times lower but also with a

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higher nutritional sludge value (Fig. 2-12). As such, sludge treatment and handling costs are lowered making it a more sustainable option.

2.6.3.7 Synthetic polymers

Although natural polyelectrolyte products have the advantage of being virtually toxic-free, the use of synthetic polyelectrolytes is more widespread. They are, in general, more effective as flocculants principally due to the possibility of controlling properties such as the number and type of charged units and the molecular weight.

Figure 2-12: Advantages of natural coagulants (NC) over chemical coagulants (Choy et al., 2014).

2.6.3.7.1 polyDADMAC:- poly-diallyldimethylammonium chloride is the most commonly used primary coagulant (Hendricks, 2006). It is prepared by addition polymerization with molecular weights 50,000 – 200,000 being most common for water treatment. These polymers are completely quaternized and are linear in structure with repeating pyrrolidine rings, and they are chlorine resistant (Bratby, 2006). One of the polyDADMAC polymers is manufactured under the trade name

“Cat-Floc” and was the first to be approved by the Food and Drug Administration for use in potable water treatment (Hendricks, 2006).

2.6.3.7.2 Epi/DMA: - the epi/DMA group of polymers are used also as primary coagulants (the chemical name of the group is poly-2-hydroxypropyl-N,

N-Health Concerns

Sustainability Sludge

Generation

Cost Nature of

Coagulant Benefits

of NCs

Used as a traditional medicine in most cases

Generally not toxic

Plant based source

More environmentally friendly

Reduce chemicals dependency

Lower sludge handling and treatment costs

Available locally

No pH and alkalinity adjustments

Low procurement cost and generally abundant in source

Reduce sludge volume

Biodegradable

Higher nutritional sludge value

Non-corrosive

Safe for consumption

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dimethylammonium chloride). The group is referred to most often as polyamines.

Other names include quaternized polyamines, polyquatenary amines, and epi/DMA polymers, with the latter name referring to primary raw materials epichlorohydrin and dimethylamine.

2.6.3.7.3 Polyamines: - this is the third group of primary coagulant polymers, which includes several types. It has a cationic charge which is pH dependent and they are chlorine sensitive; because of these characteristics, they are used less frequently for water treatment than either polyDADMAC or epi/DMA.

2.6.3.7.4 Quaternized Polyamines: - polyamines may be quatenized, which means that all four hydrogens of ammonium, NH4+, are replaced by organic groups. Such result makes the monomers more resistant to chlorine; another important characteristic is that their charge does not change with pH (Hendricks, 2006).

2.6.3.8 Electrophoresis measurements and streaming current measurements Electrophoresis refers to the movement of a charged particle suspended in a fluid induced by an applied electrical force. When a direct-current electric field is applied across a suspension containing particles with a net double layer charge, the particles will migrate to the positive or negative pole depending on whether the particles carry a negative or positive respectively. The mathematical expression for measuring electrophoretic mobility (EM) is:

EM

=

δVv

δx

(2.37)

In which

EM = electrophoretic mobility (µm/s/volt/cm), v = velocity of particle in electric field (cm/s), δV = voltage drop across electrode plates (volts),

δx = distance of separation between electrode plates (m).

Electrophoretic mobility may be converted to zeta potential using an expression related to particle size and electrolyte concentration (Bratby, 2006). However, because of difficulties in assigning values to various terms in the appropriate equations, the calculated zeta potential may differ significantly from the true value.

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Therefore, for this reason, many workers express results solely in terms of electrophoretic mobility rather than convert to zeta potential.

When a colloid move in the electric field, some of the counter ions in the ion cloud around the particle move with it. A plane of shear is developed in the diffused layer as shown in Fig. 2-13. The electric potential in volts from the plane of shear to the bulk of the solution is the zeta-potential and is designated with the symbol, ζ, which is a measure of the particle charge causing the motion. The magnitude of the zeta-potential is calculated from measurements of electrophoretic mobility and is measured by a particle charge detector.

Figure 2-13: Schematic diagram showing the distribution of ions around a charged particle.

The Helmholtz-Smoluchowski equation is the usual equation used to convert EM to zeta potential. The relation is,

ζ = 4πμ

D . EM (2.38)

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ζ = zeta-potential (mv)

μ = viscosity of water medium (Ns/m2)

D = dielectric constant for medium (dimensionless)

At the electric point, the zeta-potential is zero (Hendricks, 2006). The iso-electric point can be demonstrated by plotting zeta-potential vs coagulant dose or zeta-potential vs pH.

The theory of the zeta-potential is that when the proper dosage of coagulant is added, the zeta-potential should be zero. Thus coagulant dosage can be determined using zeta-potential.

Sharp et al. (2005) investigated the applicability of zeta potential as a control tool on two waters high in natural organic matter (NOM). They found that with both waters, the window of zeta potential for minimum residual dissolved organic carbon (DOC) was approximately -5 to +5 mV. This optimum range was the same for both alum and ferric sulfate used as coagulants. Of the variables that affect zeta-potential, pH and coagulant dosage are very important.

A disadvantage cited of electrophoretic measurements is that they are relatively lengthy and subjective, requiring visual observation and timing of individual particles – although modern instruments do incorporate automatic particle tracking (Bratby, 2006).

Streaming current measurements, on the other hand, have the advantages of speed and are not as subjective as tests of electrophoresis. Furthermore, with the streaming current technique, results obtained are immediately in terms of average for the system. Streaming current devices measure the net residual charge surrounding particles in water. The particles have a net negative surface charge. Coagulant such as alum, ferric salts, or cationic polymers surround the particles with cations or positive charges and reduce or reverse the net surface charge. When in control mode, the streaming current monitor alters the coagulant dose until a preset end point is reached. This set point is determined by jar tests and confirmatory streaming current measurements.

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The streaming current is related to zeta potential as follows:

i = ZD/N (2.39)

Where

i = streaming current Z = zeta potential D = dielectric constant N = viscosity.