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2 CHAPTER TWO

2.6 Arsenic treatment techniques in drinking water

2.6.5 Flotation

Flotation technology has been used for a long time in ore processing in the mineral industry. Heavy metals are removed from a liquid phase by attaching them to bubbles (Fu and Wang, 2011). There are different types of flotation in which metal ions can be removed from solution such as dissolved air flotation (DAF), ion flotation, electrolytic flotation and precipitation flotation. The flotation method relies on factors such as wetting characteristics and surface properties of particles to separate particles from solution (Al-Zoubi et al., 2015a).

Bacteria

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Table 2-8: Different oxidizing agents and their removal efficiencies (Mondal et al., 2013).

Oxidants Operating

pH

Initial As Conc.

Type of water

Remarks Reference

Free available chlorine (HOCl + OCl-)

O3, pure air and oxygen

MnO4

-Cl2O

FeO4

2-H2O2

Hypochlorite

7

7.6 – 8.5

6.3 – 8.3

6.3 – 8.3

8.4 – 12.9

7.5 – 10.3

3 – 8

50 μg/L

46 – 62 μg/L

50 μg/L

50 μg/L

517 μg/L

0.65 μmol/L

3 mg/L

Real water

Groundwater

Synthetic water Synthetic water River water

Sea and river water Laboratory water

The oxidation of As (III) was very fast and the oxidation of As (III) can be achieved below 1 μg/L within 10 s by using 0.1 mg/l Cl2 when initial As concentration was 50 μg/L.

Oxidation with ozone is faster than by pure oxygen or air >96 % oxidation of As (III) was achieved within 10 min whereas to oxidize >50 % of As (III) by air and pure oxygen, 5 d oxidation is needed.

More than 95 % oxidation was observed in less than 20 s in the presence of three time of stoichiometric amount of MnO4

-Not very effective for oxidation but in presence of high ammonia, partial oxidation is possible

Total As concentration decreases below 50 μg/L from 517 μg/L due to the oxidation of As (III) by Fe (VI)

The rate of oxidation increases when the pH increases from 7.5 to 10.3

Highest oxidation (100%) achieved at pH 7 by in situ hypochlorite generation process (in presence of 125 mg/L chlorine, 0.04 mA/dm2 current density and 300 K temperature)

(Dodd et al., 2006)

(Kim and Nriagu, 2000) (Ghurye and Clifford, 2004) (Lee et al., 2003) (Pettine et al., 1999) (Vasudevan et al., 2006)

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MnO2 coated

nanostructured capsule Photocatalytic oxidation

UV-C/H2O2 and UV-A/TiO2

UV light in presence of oxygen and iron

Solar light and iron citrate complex

TiO2 photo-catalyzed oxidation

Photochemical oxidation in ferrioxalate

Photochemical oxidation

assisted by

peroxydisulfate ions

5.7 – 5.8

7

0.5 – 2.5

-

4.0 – 9.0

3 – 7

3

0.1 – 1.0 mg/L

500 μg/L

150 mg/L

2.09 – 5.58 μmol/L

40 – 200 μ M

1.3 – 13,500 μM

0.07 – 0.675 mM

Laboratory water

Ultrapure water (18.2 MΩ cm) Milli Q water

River and synthetic water Laboratory water Laboratory water

Milli Q water

100% oxidation for lower initial As concentration (0.1 and 0.3 mg/L) but for 0.7 and 1.0 mg/L initial As concentration, the maximum oxidation achieved was 90% and 73% even after 24 h

Removal percentage increases after peroxidation and there was no significant difference observed on As removal by Fe (III) or Al (III)

Iron based compounds used as photo-oxidant because the precipitate of iron hydroxides acts as an adsorbent for As (V) and > 0.97 mg/l/min rate of oxidation was achieved in presence of ~800 mg/l iron-complexing anions

Above 90 % total As was removed after 4 h irradiation. However As (III) removal was faster (80% removed in 1 h irradiation) than As (V) (4 h irradiation for 80 % removal)

This process is able to remove As below 10 μg/L

Maximum oxidation achieved at pH 5

UV light intensity and dissolved oxygen are important parameter for oxidation of As (III). Oxidation achieved via sulfate radical

(Criscuoli et al., 2012)

(Yoon and Lee, 2005)

(Emett and Khoe, 2001) (Lara et al., 2006)

(Liu et al., 2008) (Kocar and Inskeep, 2003) (Neppolian et al., 2008)

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Dissolved air flotation was first applied in the processing and dressing of ores at the end of the 19th century and was later introduced to the water industry in the 1920s (Kordmostafapour et al., 2006). Al-Zoubi et al. (2015a) studied the removal of Cd, Ni, Mn, and Pb from wastewater using economic polymeric collectors.

Kordmostafapour et al. (2006) also conducted research on arsenic removal from water using DAF where 99 % of arsenic was removed using polyaluminium chloride (PAC) with a flocculation time of 5 to 20 min, coagulant concentration of 40 mg/L and saturation pressure of 4.5 atm.

Ion flotation is another method which is showing great potential in removing heavy metals from drinking water. The process involves the attachment of hydrophilic ions on gas bubbles introduced into the solution and then removal of the ions by bubbles from solution (Hoseinian et al., 2015). Yuan et al. (2008) evaluated the potential of removing Pb, Cu, and Cd from a dilute aqueous solution under different operating parameters such as initial solution pH, the collector to heavy metal ratio and the ionic strength (NaCl). Ion flotation was also applied to removing Ni(II) and Zn(II) ions from low concentration synthetic wastewaters (Hoseinian et al., 2015).

Precipitation is another flotation method based on the formation of a precipitate and subsequent removal by attachment to air bubbles (Fu and Wang, 2011). Stalidis et al. (1989) investigate the selective precipitation and flotation of Cu, Zn, and As from dilute aqueous solution. A laboratory scale investigation was carried out by Medina et al. (2005) to remove Cr (III) by precipitate flotation from dilute aqueous solutions using sodium dodecylsulfate (SDS) as an anionic collector and ethanol as a frother. 96.2 % removal was achieved at pH 8.0.

2.6.5.1 Dissolved air flotation

DAF is a solid-liquid separation process in which nucleated microbubbles are introduced to a suspension comprising flocculated particles. Collision and attachment of bubbles and particles create low density bubble-particle agglomerates which rise to the surface to form a float layer and can be removed mechanically or hydraulically. In water and wastewater treatment plants (WTPs/WWTPs), DAF is used for the removal of low-density contaminants such as algae and natural organic matter (NOM) from reservoir water or waste stabilisation ponds (WSPs).

Coagulation-flocculation is conventionally applied to reduce particle and colloid

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charge, increase particle sizes, complex with NOM and ensure bubble-particle interactions and subsequent removal efficiencies are optimal (Edzwald, 2010).

Below is a typical conventional DAF plant.

Figure 2-14: A typical water treatment plant with DAF (Edzwald, 2010).

Performance evaluation and operating strategies of DAF systems for treating poultry slaughterhouse wastewater was carried out by (de Nardi et al., 2008).

Average removal efficiencies of 43 ± 15 % suspended solids (SS) and 49 ± 8 % oil and grease were achieved by using 24 mg Al3+/L, polyaluminium chloride (PAC) associated with 1.5 mg/L anionic polymer. Coagulation and DAF treatment of semi-aerobic landfill leachate was shown to have optimum operating conditions of 599.22 mg/L of FeCl3 at pH 4.76 and a saturator pressure of 600 kPa, flowrate of 6 L/min and injection time of 101 s (Adlan et al., 2011). Algae removal was conducted at Morton Jaffray water works, Harare, Zimbabwe (Hoko and Makado, 2011) where parameters considered included contact time, coagulant and algaecide doses. It was concluded that algae removal was better at pH 7.0 compared to 7.5 and also algae removal increases with increasing contact times, increasing algaecide dosage and increasing settling. DAF has also been used to remove zinc chloride, lead (II) nitrate, manganese (II) chloride, nickel chloride and cadmium chloride from wastewater with the aid of polymers (polyethylene alcohol, polyethylene glycol and chitosan). The studied heavy metals are (Al-Zoubi et al., 2015b). The results showed that chitosan performed better in affecting removal of Cd (29 %), Ni (27 %), Mn (31 %), and Pb (29 %).

Rapid Mixing

Flocculation DAF Filtration Disinfection

Saturator

Particle Separation &

NOM Removal by Conversion to particles Pre-treatment

Coagulant

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DAF can also be integrated into water treatment. Flotation can be placed above the filter in a vertical arrangement simply called flotation over filtration. It is also abbreviated as DAFF or DAF/F (Edzwald, 2010). This process has a smaller plant footprint thereby reducing the land area, which is important for large cities where land is expensive. It also has the advantage of construction cost savings having one structure for flotation and filtration compared to the conventional plants with a horizontal layout of separate units (Edzwald, 2010). DAF have been used extensively for removing algae, oil and greases and heavy metals from WTPs but not many studies have been carried out on removing the current emerging organic contaminants which can be toxic to humans and animals and also give the drinking water undesirable taste and odour. Table 2-9 shows the applications of dissolved air flotation.

In a DAF tank, the rise rate of an air bubble is a response of two opposing forces.

First, the differential densities of air and water generate a net upward buoyant force.

Second, the bubble encounters a drag force resisting the upward movement. For a constant rise rate, these forces need to be in balance (Edzwald and Haarhoff, 2012).

FB = FD (2.40)

w− ρb)gVb = CDAbρwvb

2

2 (2.41)

Where FB and FD are the forces due to buoyancy and drag, respectively; ρw and ρb

are the water and air bubble densities, respectively; Vb is the bubble volume; Ab is the projected area of the bubble in the direction of movement; g is the earth`s gravity acceleration (9.806 m/s2); vb is the uniform rise velocity of the bubble; and CD is the drag coefficient of the rising bubble.

Knowing that DAF bubbles are spheres, Vb is replaced with (πdb3)/6 and Ab with (πdb2)/4. The Reynolds number (Re) for the rising bubble is defined as:

Re = ρwvbdb

μw (2.42)

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Table 2-9: Dissolved Air Flotation Clarification Applications (Edzwald and Haarhoff, 2012).

Drinking Water Treatment

Clarification in a conventional water treatment plant

Clarification in low-pressure membrane treatment plants and nanotreatment-membrane plants Clarification in reverse osmosis desalination plants

Clarification in water reclamation/water reuse Treatment of spent filter backwash water

Municipal Wastewater Treatment

Primary clarification Secondary clarification

Tertiary treatment: Suspended solids removal, phosphorus removal following chemical precipitation Combined sewer water and storm water treatment

Wastewater reclamation

Thickening of waste suspensions

Industrial Water Supply and Industrial Wastewater Treatment

Chemical industry

Food waste: Vegetable waste, diaries, meat packing, poultry processing, vegetable oil production Oil production and refineries

Pharmaceutical plants Pulp and paper mills Steel mills

Soap manufacturing

Others

Separation of minerals from ores

Removal of PCBs at hazardous waste sites

In situ treatment of lakes for algae and seawaters for algae and oil spills.

The dynamic viscosity of water is μw. Laminar flow is indicated if Re ≤ 1. For laminar flow, the drag coefficient CD is estimated as a function of Re:

CD = K

Re (2.43)

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Separation of particles by flotation adheres to the same laws as sedimentation but in the reverse field of force. The governing equation in air flotation separation, as in all gravity-controlled processes, is Stoke’s Law which is used to compute the rise rate of bubble flocs, agglomerates, and bubble-oil aggregation.

For solid spheres, K = 24:

𝑣𝑏 = 𝑔(𝜌𝑤− 𝜌𝑏)𝑑𝑏2

18𝜇𝑤 (2.44)

Key design variables in the system controlling efficiency of removal are as follows:

• Gas input rate and volume of gas entrained per unit volume of liquid

• Bubble-size distribution and degree of dispersion

• Surface properties of the suspended matter

• Hydraulic design of the flotation chamber

• Concentration and type of dissolved materials

• Concentration and type of suspended matter or oils

• Chemicals added

• Temperature

• Residence time

• Recycle ratio

• pH

The key to DAF is the dissolution of air (or other suitable gas) under pressure and the reduction of this pressure to form bubbles. The amount of gas going into solution generally obeys Henry`s Law:

p = kC (2.45)

Where p = partial pressure of the gas k = Henry`s Law constant

C = concentration of the gas dissolved in the solution

The most important dependent variable in air flotation systems is bubble size. It affects the performance of collisions and attachment of particles to bubbles and bubble rise velocity. Bubbles are formed from cavitation from the pressure drop in the nozzle or injection device. Bubbles first form nuclei and then grow. For

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homogenous nucleation the critical bubble diameter (dcb) is predicted in Eq. (2.46).

Where σ is the surface tension of water and ΔP is the pressure difference across the injection device such as the nozzle.

dcb =

ΔP (2.46)

The actual bubble sizes in DAF are affected by heterogeneous nucleation, bubble growth, the injection flow rate, and the injection device, especially the type of nozzle – these are all important factors affecting bubble size.

Air-solids ratio is another important parameter governing the rise rate of bubble-particle agglomerates in solid-bubble-particle DAF systems. Eq. (2.47) indicates that as more air bubbles are incorporated into the aggregate, the aggregates net density decreases, and its rise velocity increases.

A

S = 1.3as

Ss (fPa− 1) (2.47)

Where A/S = air to solids ratio, mg/mg 1.3 = weight constant of air, mg/mL as = air solubility, mL/L

f = fraction of air dissolved at a given pressure usually 0.5 Pa = recycle system pressure, atm

Ss = suspended solids concentration, mg/L

For a system where pressurized recycle is used, Eq. (2.47) is modified as follows:

A

S = 1.3as

QSs (fPa− 1)R

(2.48)

Where R = recycle stream flowrate (m3/day) Q = water/wastewater flowrate (m3/day)

Table 2-10 below shows a typical design and operating parameters for conventional rate DAF plants.

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Table 2-10: Design and operating parameters for conventional rate DAF plants (Gregory and Edzwald, 2010).

Item Values Comments

Pre-treatment flocculation Mean detention time (min) Number of stages Mixing intensity (G) (s-1)

10 – 20 2 50 - 100

Some as low as 5 min Some with 3 stages

Some as low as 30 and some as high as 150 sec-1 Propeller or gate flocculators used

Some use of tapered flocculation Some use of hydraulic flocculation DAF Tank

Nominal hydraulic loading rate (m/h) Separation zone loading rate (m/h)

Contact zone detention time (min) Basin depth (m)

5 – 15 6 – 18

1 – 2.5 2.0 – 3.5

Based on the through-put flow and 10% recycle flow, and the separation zone area.

Recycle and saturator systems Air mass (g/m3)

Recycle rate (%)

Saturator gauge pressure (kPa) Saturator efficiency (%)

6 – 10 6 – 12 400 – 600

80 - 95

10% most typical

Higher pressures for unpacked saturators

For saturators with packing, unpacked saturators: 50-70%.

Higher efficiencies for higher temperatures.

Floated sludge Hydraulic removal

Chain and flight or reciprocating skimmer Beach drum

0.5 – 1 % solids 2 – 3 % solids 1 – 3 % solids

Some as high as 5%

Also called star wheel, sludge roller, and flipper

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