Parameters Governing the Efficiency and Economics of the Electrolytic Defluoridation

Process – Experience in India

Er. Subhash Andey

Senior Principal Scientist

CSIR- National Environmental Engineering Research Institute (CSIR-NEERI), Nehru Marg, Nagpur - 440020, (India)

Email: sp_andey@neeri.res.in

Health Impact of excessive fluoride in potable water

Dental Fluorosis

Skeletal Fluorosis

Global Scenario on Fluoride Pollution and Fluorosis

• Fluorosis relics as an endemic public health problem in at least 25 nations around globe

• The intensity of fluorosis problem is very badly felt in the two heavily populated countries of the world, India and China

Chemical Precipitation Adsorption

Alum Coagulation(Nalgonda Technique)

Alum, lime

ElectrolyticDefluoridat

-ion

Aluminium

Activated alumina

Activated alumina, Acid and

Alkali

Ion Exchange

Ion exchangerAcid, Alkali and Alum

Adsorbents

. Bauxite

. Gypsum

. Quartz

. Lime

. Zeolites

. Clays

. Sands

. Coal

. Red Mud

. Etc..

Ionic Separation

Reverse Osmosis

Electro-dialysis

Defluoridation technologies

Limitations of the Present Technologies for Defluoridation of Water

• High Cost, due to imported spare parts, continuous power supply, expensive chemicals, skilled operation or regeneration, etc. Reverse osmosis, ion exchange and activated alumina

• Unobserved break through : the fluoride concentration in the treated water may rise gradually or suddenly, typically when a medium in a treatment column is exhausted or even when the flow is out of control.

• Limited removal capacity : bone charcoal or activated alumina

• Deteriorated water quality: due to excessively high pH values, high doses of chemicals, poorly prepared medium (bone charcoal) or due to medium escaping from the treatment container, e.g. ion exchange, alumina, Nalgonda sludge, etc

• Taboo limitations in particular, the bone charcoal method is culturally not acceptable to Hindus. Bone charcoal originating from pigs may be questioned by Muslims.

Electrolytic Defluoridation Technique

• Electrolytic defluoridation is a process of water treatment in which active species of hydroxide of aluminium are produced by passing dc current through aluminiumelectrode

• The reactions involved during electrolysis for defluoridationAl Al3 + 3e

Al+3 + 6F- AlF63-

AlF63- + 3Na+ Na3AlF6

2H2O + 2e 2OH- + H2

Al3+ + 3OH- Al(OH)3

Electrolytic Defluoridation Schematic

Diagram

Reaction

Anode : Al 0 = Al 3+ + 3e-

Cathode : 2H2O + 2e- = 2 (OH) - + H2

Quantity of metal dissolved / deposited

Faraday’s Law : m = KIt

K = Electrochemical equivalent

I = Current

t = Time of electrolysis

Therefore m = 1/96500 x A / V x It

V = Valancy of Al = 3

A = Atomic mass of Al = 27

I = Current in Amp.

t = time in seconds

Factors Affecting the Fluoride Removal Efficiency

• Raw water quality

• Raw water fluoride concentration

• Current Density

• Initial pH of raw water

Effect of Variations of Initial Fluoride concentrations and Currents on Time Required for Fluoride reduction Below 1 mg/l

0

20

40

60

80

100

120

140

160

180

200

5 (mg/L) 8 (mg/L) 10 (mg/L) 15 (mg/L) 20 (mg /L)

1 AMP 1.5 AMP 2 AMP

Fluoride concentration

Tim

e (

Min

)

Effect of pHi on Aluminium-Fluoride Ratio at Various Initial Fluoride concentration

0

2

4

6

8

10

12

14

16

18

5 6 7 8 9

Al/

F R

ati

o

pHi

Initial F Conc. 8 mg/L

Initial F Conc. 10 mg/L

Initial F Conc. 15 mg/L

Electrical Power and Energy Consumption

Power = I ×VCELL (Watts)

I ×VCELL × t

Energy = ------------------------- (kW.hr)1000

The specific energy consumption (SEC)

Where,

U Applied voltage (V),

Iavg Average current (A),

t Electrolysis time (min),

C0 Initial concentration in milligrams per liter,

Ct Concentration value at time t in milligrams per liter,

V0 Initial volume of the treated water in liters,

Vt Volume of the treated water at time t in liters.

Effect of pHi on Energy Consumption at Various C0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

pHi 5 pHi 6 pHi 7 pHi 9

En

erg

y (

Kw

-Hr)

Per

gm

F r

em

ov

al

Initial F conc.8 mg/L

Initial F conc. 10 mg/L

Initial f conc.15 mg/L

Effect of pHi on Energy Consumption at Various C0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

pHi 5 pHi 6 pHi 7 pHi 9

En

erg

y (

Kw

-Hr)

Per

gm

F r

em

ov

al

Initial F conc.8 mg/L

Initial F conc. 10 mg/L

Initial f conc.15 mg/L

Residual Aluminium Concentration in Treated Water at Various C0 and Current Intensity

0

0.005

0.01

0.015

0.02

0.025

0.03

0.5 A 1.0 A 1.5 A 2.0 A

Resid

ual A

lum

iniu

m C

on

c.

(mg

/L)

Current Intensity

F = 8 mg/L F= 10 mg/L

F= 15 mg/L F = 20 mg/L

Residual Aluminium Concentration in Treated Water at Different C0 and pHi

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

5 6 7 8 9

Resid

ual A

lum

iniu

m C

on

c.

(mg

/L)

pHi

F= 8 mg/L

F =10 mg/L

F= 15 mg/LAluminium Limit (0.2 mg/L)

Comparative Data of Treated Water Quality Produced by Nalgonda and Electrolytic Defluoridation Processes

Parameters Raw Water

Treated water

Electrocoagulation Process Nalgonda Process

5 ppm 10 ppm 5 ppm 10 ppm

Final F (mg/l) - 1.03 ( 60 min) 1 (90 min) 1 0.9

Final pH 7.5 8.5 8.3 6.5 6.25

Alkalinity(mg/l) 120 136 144 40 28

SO4(mg/l) 136 90 94 172 256

NO3(mg/l) 2.6 2.1 2.2 2.69 2.81

Cl(mg/l) 134 134 134 145 165

Sludge (gm/ 5 L) - 1.15 2.42 1.89 (>64%) 4.24 (>75%)

Findings

Current (I) and electrolysis time (t) are the most important parameters forcontrolling the reaction rate in the reactor.

The highest current produced the quickest fluoride removal due to the readyavailability of Al3+ ions in the solution. However at higher currents, morecoagulant (aluminium) is available per unit time, which may beunnecessary, because not only excess residual aluminium is unsafe fordrinking water but also high current is also uneconomic in terms of energyconsumption

Minimum and maximum fluoride removal rates were observed at pHi 8 and 6respectively.

The residual Al concentration in the treated water is more than the permissiblelimits of 0.2 mg/L if the initial pH of the water is below 6

Variations in current intensities and initial fluoride concentrations (Co) has nosignificant impact on the residual aluminium concentration in the treated water.

Al/F ratio is in the range of 2.5 to 3.5 when initial raw water pH is in the range of5 to 6. When initial pH of the water is more than 7, there is significant increasein Al/F ratio (nearly 2 to 4 times) and it is maximum at pH 8.

No significant variation in energy required per unit weight of fluoride removal atdifferent initial fluoride concentrations was observed

Energy consumption per unit weight of fluoride removal is minimum in the pHirange of 5 to 6 and maximum at pHi 9

Electrolytic Defluoridation Technique

• Removal of fluoride by active species of hydroxide of aluminiumproduced by passing DC power through aluminium electrode

• Process is effective to remove excess fluoride and also bring down the bacterial load of the raw water

• Simple to fabricate, easy to operate with minimum maintenance

• Suitable for treatment of raw water with fluoride concentration upto 10 mg/L

• Produces potable water with palatable taste as against the other available chemical treatment methods

• Quantity of sludge produced is much less (60-70%) than conventional treatment methods

• Simultaneous reduction in bacterial contamination in treated water

• Treatment cost upto Rs. 20 per 1000 L

Flow diagram EDF process

Solar Energy Based Electrolytic Defluoridation Plants Installed by CSIR- NEERI

Usarvara village, Durg Dist. (C.G.)September 2010

Sargapur village, Seoni Dist. (M.P.)

June 2011.

Malgaon village, Balod Dist. (C.G.)January 2012.

Adivasi Kanya Shiksha Parisar, Dist. Chindwara (M. P.)August 2012

School based EDF plants

• 15 plants installed in residential

schools/hostels/ashrams in Dhar

Dist. M.P. in 2013

• Financial Support from

UNICEF, Bhopal

• Capacity 200 Litre per batch

• Raw water fluoride concentration

2.2 to 9.5 mg/L

• Cost of the plant: Rs. 15000

Awards

International Project Innovation Award (PIA)-

2011 instituted by International Water

Association (IWA) for which entries were

from various countries such as Sri

Lanka, Vietnam, Lebanon, Brazil, Republic of

Korea (South Korea), China, Tanzania, South

Africa, Kenya, Philippines, Jordan besides

India

The DST-Lockheed Martin India and FICCI

Innovation Growth Program Award 2012

New Delhi March 29, 2012