Iron

1. Rocks & soils Ferric oxides (Fe2O3)

Ferric hydroxides (Fe(OH)3)

2. Natural waters Ferrous bicarbonate Fe(HCO3)2)

Ferrous hydroxide

Ferrous sulphate(FeSO4)

Organic Iron

•GWs containing soluble iron (ferrous) are clear & colorless when it is first drawn

•Upon contact with air, a yellowish to reddish brown precipitate of ferric

hydroxide is formed.

•Stain the porcelain fixtures & laundry.

•Iron bacteria utilise ferrous iron as energy source & precipitate ferric hydroxide,

that may cause pipe clogging.

Manganese

1. Natural waters It appears with iron.

Manganous bicarbonate Mn(l+CO3)2

Manganous chloride(MnCl2)

Manganous sulphate(MnSO4)

•Causes Stain, bad taste & growth of microorganisms

•Rocks & soil

Iron & Mn Removal

•Oxidation & Ppn

•Aeration at high pH by lime addition coagulation & ppn

•Selective Ion exchange Resins

Iron & Mn Removal

1) Aeration, Sedimentation& Filtration

• Tray type aerators, frequently contain coke/Stone contain beds to speed up oxidation reactions.

2) Aeration, Chemical Oxidation, Sedimentation & Filtration:

•Aeration Strips out chemically oxidised gases & adds O2

•Iron & Mn are chemically oxidised by Cl2 /KMnO4

•1 mg/L of KMnO4 oxidises 1.06 mg/L of iron & 0.52 mg/L of Mn

(Fe2+ + M22+) (soluble Irons) + O2 Cl2/KMno4 (FeOx ↓ + MnO2↓) (Insoluble oxides)

•Filtration is needed to remove Flocculent metal oxides

3)Mn Zeolite Process:•It is a natural green sand coated with manganese dioxide that removes soluble

iron &Mn from soln

•Zeolite bed is regenerated with KMnO4

•A pressure filter with media i.e Anthracite & Manganese Zeolite bed.

Iron & Mn removal

•Impart a bitter characteristic, metallic taste

•Oxidised precipitates cause yellowish brown to black

•Staining of plumbing fixtures & laundered materials can also result•Carrying capacity of pipelines in distribution system is reduced due to

deposition of iron oxide & bacterial slimes due to growth of iron

bacteria in iron bearing water

•Con of iron in excess of 0.2 – 0.3 mg/L may cause nuisance

•Occur on certain underground water & springs alone or in association with

organic matter

•Discharge of industrial wastes or mine drainage

•Iron exits in water in two levels of oxidiation (Fe2+ & Fe3+)

•In λW, if it is present , it is in Fe2+ (Ferrous iron)

•Ferric iron is in precipitated form

•Manhanese is present in water in 2 oxidation states (biralent & quadrivalent,

which is sparingly soluble)

•Iron forms complexes with bicarbonate, sulphate, phosphate, cyaride or

halide

•Water of high alkalinity have low iron & Mn than water of low alkalinity

•If water contains H2O, little or iron or Mn is found in soln as it is precipitated

Sources & Nature

Removal Methods

•Oxidation by aeration

•Use of Cl2, ClO2, KMnO4 followed by filtration alone by settling &

filtration•Use of Zeolities as well as Catalytic Oxidation

1) Precipitation

4Fe2+ + O2 + 10H2O 4 Fe(oH)2 ↓ + 8H+

4X 56 mg Fe2+ =2X 16 mg O2

1 mg Fe2+ = 0.14mgO2

• Iron or Mn in water in reduced form os converted to insoluble ferric &

Mn compounds by oxidation

• Reaction time is 5 min, @ pH 7 – 7.5, 0.14 mg of O2 is needed to convert

1mg Ferrous iron to Ferric hydroxide• Water is allowed to trickle over coke or crushed stone

• Contact beds 2=3 m deep @ SLR of 40 – 70 m3/d/m2 with contact

medium of sizes 50-150mm• Accumulated iron & Mn are Flushed out by rapid drainage• Sedimentation before filtration is needed when iron content exceeds

10mg/L

• Settling period: 2-3 h• Water pass through filters (gravity or pr. Type) with 75 cm depth of sand or

sand & anthracite• Filters rates : 6-9 m3/h/m2

• All organic material to be oxidised before ppn. Of iron • Chlorination of many iron bearing water can bring about oxidation of organic

matter & other reducing agents facilitating oxidation of ferrous iron

• Addition of lime to raw or preaerated waters, co2 could be brought down to

zero, resulting high pH will promote flocculation of iron &Mn• Washing of filter medium 5-10 cm filter medium & washing it manually with

water to free it from sediment & replace the same in position• Coke medium needs washing once in 6-24 months• Mn removal requires pH adjustment upto 9.4 -9.6

• 0.29 mg o2 is needed to convert 1 mg Mn

• 6Mn2+ + O2 + 6H2O 2<n3O4 + 12 H+

• 2Mn3O4 + 2O2 6MnO2(Blau)

• 6Mn2+ + 3O2+ 6H2O 6MnO2 + 12H+

• 1mg Mn2+ = 0.29mgO2

• Prechlorination to free residuals upto 0.7 – 1mg/L will effect oxidation of ppn

of Mn

Contact beds

•To facilitate oxidation of iron or Mn through catalytic action of

previously precipitated oxides of these minerals on gravel or ore•Mn ore (pYro ludite), an oxide of Mn\•Upward flow rates @ 9.6 m/h•Bed depth 1.8m•Regeneration of beds by backwashing with potassium permanganate•Mn Zeolite, an effective contact material

•ClO2 & KMno4 are strong oxidants for Mn.o

•Percolation of water through bed of zeolite which takes up iron & Mn

by IX•Base exchanger : siliceous, carbonaceous, synthetic resin type•Air should be excluded

Zeolite

Catalytic Method

•Percolating water through contact materials that oxidise iron & Mn

•Dental fluorosis• skeletal or bone fluorosis•High con in AP, TN, Bihar, Gujarat, Kerala, etc.•Range 1.5 – 6 mg/L, 16 – 18 mg/L, 36 mg/L

1)Fluoride exchanges:•Degreased & alkali treated bones posses ability to remove fluorides•Bone charcoal•Tricalcium phosphate removes 0.7 kg of fluoride/n3

Defluoridation of Water

Removal Method

2) Anion Exchanges:• Basic formaldehyde resin quaternary aluminum type in hydroxide or chloride

from

3)Activated Carbon:• Carbonising paddy husk or saw dust, digesting under prenine with alkali &

quenching it in 2 % alum soln•The spent material could be regenerated by soaking it in 2 % alum soln for 14 h•A granular iron – exchange material (De fluoronz)a sulphunated coal.

4) Magnesium salts:

Excess lime treatment for softening effecrs removal of fluoride due to its

adsorption by Magnesium hydroxide floc.

5) Al. Salts:•Filter alum & activated aluminum & alum treated cation exchanges have shown

benefical effects•Filter alum during coagulation baring about some removal of fluorides from water•Coagulant aid like activated silice & clay (300 – 500 mg/L of alum) is required to

bring down fluoride from 4 to 1 mg/L•With coagulant aid, fluorides were reduced from 6 to 1 mg/L @ alum dose of 100

mg/L •Alum treated polystyrene cation exchanges & sulphonated coals were used•Calcinated or activated alumina in granular from can br=e osed for fluoride removal

Basic principles of iron & Mn removal are the same.

The basic approach for Iron & Mn removal involves

oxidation & removal of suspended material by either

sedimentation or filtration.

The most successful approach involves pH adjustment,

chlorination & direct filtration on a monomedia anthracite

filter.

Iron & Mn Removal

1) Oxidation & Removal:

4Fe2+ + 2H+ + O2 => 4Fe3+ + 2OH- ( 7mg Fe / mg O2)

2Fe2+ + Cl2 => 2Fe3+ + 2Cl- (1.6mg Fe / mg Cl2)

3Fe2+ + MnO4- + 4H+ => 3Fe3+ + MnO2 + 2H2O (1.6 mg Fe / mg MnO4)

Mn2+ + 2H+ + ½O2 => Mn4+ + H2O (3.5 mg Mn / mgO2)

Mn2+ + Cl2 => Mn4+ + 2Cl- (1.3 mg Mn / mg Cl2)

3Mn2+ + 2Mn7+ => 5Mn4+ (0.52 mg Mn / mg MnO4)

1) Oxidation & Removal (cont.)

Selection of media for filter unit is important.

Media should have large effective size (>1.5mm) to reduce head loss &

should have low UC.

Use of green sand, a natural zeolite allows for more rapid oxidation &

removal of iron & Mn

Coal works well for this application.

Recovery of backwash water & disposal of sludge.

2)Ion Exchange: Use of water softeners for iron & Mn removal is fairly

common. Due to divalent nature of iron & Mn, they are removed. The problem is that these materials are oxidised by DO in

water & thus media can be watered & fouled.

3)Polyphasphates: In special cases, use of polyphosphates is applied. Polyphosphates react with iron & Mn & hold it in solution,

so that consumer is not aware of its presence. Polyphosphates are dosed @ 2 times con of iron & Mn.

filter

Backwash

To System

Sludge to sewer

Backwash holding tankReclaimed backwash

Detention tank

Chemical injection

RawWater

Typical iron and manganese removal system

.

Lime – soda softening system

Sludge handling

Lime Soda ash

settling settling

CO2

Filter

Filter soft

effluent

Design Criteria for Iron and Manganese ControlDesign Criteria for Iron and Manganese Control

S.No Elemment Units

1. Detention time 5-30 min at average flow after chemical feed

2. Filter media

Uniformity coefficient Q/A

Backwash Q/A

1.5 mm effective size or greater anthracte

~1.2 – 1.4

5 gpm/ft2 to 10 gpm/ft2

15-25 gpm/ft2

3. Backwash tank size

20 min flow at 5 times normal Q/A

Defluoridation

1. 1.5 – 2 mg/L Dental Fluorosis

Discolored, blackened teeth permanent brown to grey discoloration of enamel

2. 3 – 6 mg/L Skeletal Fluorosis

Severe & permanent bone & joint deformations

3. > 10 mg/L Crippling Fluorosis

•High fluoride con. in GL in 23 countries•Fluoride bearing minerals viz., fluorite, apatite, rock phosphate, etc.•Fluorosis is an irreversible disease, there is no cure.•Non – skeletal fluorosis leads to gastro – intestinal problems & neurological

disorders, kidney & thyroid injury & death •Fluorosis can be detected in neck, spine, knee, pelvis, shoulder, small joints

of hands & feet•Gastro – intestinal problems include abdominal pain, diarrhea & constipation.

Methods of Defluoridation

1. Activated Alumina or Bone char2. RO3. Nalgonda Technique

Nalgonda Technique:•It uses Al. salt for removing fluoride

•Raw water is mixed with lime

•Alum soln. is then added & water is stirred slowly for 10 min & allowed to settle for 1 h

Activated Alumina or Bone char:•Water is percolated through insoluble granular media

•Regeneration of bone char consists of backwashing with 1 % soln. of caustic soda & then rinsing the bed.

•Regeneration of alumina involves backwashing with caustic soln.

Defluoridation

• Both lime softening & alum coagulation are effective.

• The only acceptable method of defluoridation is Adsorption onto

Activated Alumina or Bauxite.

• The water is filtered through a bed of Activated Alumina

• The regeneration of alumina bed involves backwashing,

regeneration by NaOH soln, Rinising with water neutralisation

• The major equipment include

1. An activated Alumina bed

2. Acid & base feed

3. pH adjustment & control system

4. 5raw water filtration & backwash system

5. Alumina – bed regeneration &

6. Neutralisation systems

Lime softening, Alum Coagulation & Ion exchange with Activated

Alumina.

Ion exchange with Activated Alumina is the only method that can

economically reduce fluorides levels to below drinking water stds.

Lime softening:An insoluble ppt is formed with Co-ppn. with Magnesium hydroxide

(Mg(OH)2)

Water high in Mg that would be softened can have significant fluoride

reduction by co-ppn. Fresidual = Finitial – (0.07 Finitial X √Mg)

To reduce Fluoride from 5 to 1.5 mg/L,100 mg/L of magnesium is

needed.

Fluoride Removal

2) Alum Coagulation:

It reduces fluoride levels to acceptable level, but requires very large

amounts of alum.Fluoride reduction 3.6 to 1.4 mg/L requires 250 mg/L of alum during

conventional treatment.Optimum pH 5.5 – 7.Large amount of sludge is produced.

3) Activated Alumina:

Since 1930s, this is practiced.

Activated Alumina is used in same way as IX resins .Activated Alumina is an amphoteric substance & its isoeletric point is pH

9.5.It will remove anions below this pH & cations above .When treated with an acid soln., alumina behaves like an anion

exchanger & will readily replace fluoride from alumina.

Optimum pH 5 – 8

Commercial available alumina is in 4 typical size ranges

8 – 10, 14 – 28, 28 – 48 & 48 – 100 mesh.

Registration methods include backwash (10 – 15 min)

followed by NaOH elution of fluoride & neutralisation of bed

by rinsing with the bed with water for removal of excess

NaOH followed by rinsing with an acid soln. (H2SO4/ HCl).

Disposal of regenerant waste is a problem.