Lin S.D. Water and Wastewater Calculations Manual

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14 Iron and Manganese Removal

Iron (Fe) and manganese (Mn) are abundant elements in the earth’s

crust. They are mostly in the oxidized state (ferric, Fe

3⫹

, and Mn

4⫹

) and

are insoluble in natural waters. However, under reducing conditions (i.e.

where dissolved oxygen is lacking and carbon dioxide content is high),

appreciable amounts of iron and manganese may occur in surface water,

groundwater, and in water from the anaerobic hypolimnion of stratiﬁed

lakes and reservoirs. The reduced forms are soluble divalent ferrous

(Fe

2⫹

) and manganous (Mn

2⫹

) ions that are chemically bound with

organic matter. Iron and manganese get into natural water from disso-

lution of rocks and soil, from acid mine drainage, and from corrosion of

metals. Typical iron concentrations in groundwater are 1.0 to 10 mg/L,

and typical concentrations in oxygenated surface waters are 0.05 to 0.2

mg/L. Manganese exists less frequently than iron and in smaller

amounts. Typical manganese values in natural water range from 0.1 to

1.0 mg/L (James M. Montgomery Consulting Engineers, 1985). Voelker

(1984) reported that iron and manganese levels in groundwaters in the

American Bottoms area of southwestern Illinois ranged from <0.01 to

82.0 mg/L and <0.01 to 4.70 mg/L, respectively, with mean concentra-

tions of 8.4 mg/L and 0.56 mg/L, respectively.

Generally, iron and manganese in water are not a health risk.

However, in public water supplies they may discolor water, and cause

taste and odors. Elevated iron and manganese levels also can cause

stains in plumbing, laundry, and cooking utencils. Iron and manganese

may also cause problems in water distribution systems because metal

depositions may result in pipe encrustation and may promote the growth

of iron bacteria which may in turn cause tastes and odors. Iron and

manganese may also cause difﬁculties in household ion exchange units

by clogging and coating the exchange medium.

To eliminate the problems caused by iron and manganese, the US

Environmental Protection Agency (1987) has established secondary drink-

ing water standards for iron at 0.3 mg/L and for manganese at 0.05 mg/L.

The Illinois Pollution Control Board (IPCB, 1990) has set efﬂuent stan-

dards of 2.0 mg/L for total iron and 1.0 mg/L for total manganese.

It is considered that iron and manganese are essential elements for

plants and animals. The United Nation Food and Agriculture Organization

recommended maximum levels for iron and manganese in irrigation waters

are 0.5 and 0.2 mg/L, respectively.

The techniques for removing iron and manganese from water are

based on the oxidation of relatively soluble Fe(II) and Mn(II) to the

insoluble Fe(III) and Mn(III,IV) and the oxidation of any organic-complex

compounds. This is followed by ﬁltration to remove the Fe(III) and

processes for iron and manganese removal are discussed elsewhere

(Rehfeldt et al., 1992).

Public Water Supply 435

Four major techniques for iron and manganese removal from water

are: (1) oxidation–precipitation–filtration, (2) manganese zeolite

process, (3) lime softening–settling–ﬁltration, and (4) ion exchange.

Aeration–filtration, chlorination–filtration, and manganese zeolite

process are commonly used for public water supply.

14.1 Oxidation

Oxidation of soluble (reduced) iron and manganese can be achieved by

aeration, chlorine, chlorine dioxide, ozone, potassium permanganate

(KMnO

) and hydrogen peroxide (H

). The chemical reactions can be

expressed as follows:

For aeration (Jobin and Ghosh, 1972; Dean, 1979)

(5.119)

(5.120)

In theory, 0.14 mg/L of oxygen is needed to oxidize 1 mg/L of iron and

0.29 mg/L of oxygen for each mg/L of manganese.

For chlorination (White, 1972):

(5.121)

(5.122)

The stoichiometric amounts of chlorine needed to oxidize each mg/L of

iron and manganese are 0.62 and 1.3 mg/L, respectively.

For potassium permanganate oxidation (Ficek, 1980):

(5.123)

(5.124)

In theory, 0.94 mg/L of KMnO

is required for each mg/L of soluble iron

and 1.92 mg/L for 1 mg/L of soluble manganese. In practice, a 1 to 4%

solution of KMnO

is fed in at the low-lift pump station or at the rapid-

mix point. It should be totally consumed prior to ﬁltration. However, the

required KMnO

dosage is generally less than the theoretical values

(Humphrey, and Eikleberry, 1962; Wong, 1984). Secondary oxidation

reactions occur as

(5.125)

(5.126)

and

(5.127a)

or (5.127b)MnO

XsH

2Mn

1 MnO

XsH

1 MnO

XsH

1 MnO

1 Mn

3Mn

1 2MnO

1 2H

5MnO

1 4H

3Fe

1 MnO

1 4H

MnO

1 2Fe

1 2H

MnSO

1 Cl

1 4NaOH

MnO

1 2NaCl 1 Na

1 2H

2FesHCO

1 Cl

1 CasHCO

2FesOHd

1 CaCl

1 6CO

1 O

MnO

1 2e

1 2OH

FesOHd

436 Chapter 5

For hydrogen peroxide oxidation (Kreuz, 1962):

1. Direct oxidation

(5.128)

2. Decomposition to oxygen

(5.129)

followed by oxidation

(5.130)

In either case, 2 moles of ferrous iron are oxidized per mole of hydrogen

peroxide, or 0.61 mg/L of 50% H

oxidizes l mg/L of ferrous iron.

For oxidation of manganese by hydrogen peroxide, the following reac-

tion applies (H.M. Castrantas, FMC Corporation, Princeton, NJ, per-

sonal communication).

(5.131)

To oxidize 1 mg/L of soluble manganese, 1.24 mg/L of 50% H

required.

Studies on groundwater by Rehfeldt et al. (1992) reported that oxidant

dosage required to meet the Illinois recommended total iron standard

follow the order of Na, O, Cl as Cl

> KMnO

> H

. The amount of

residues generated by oxidants at the critical dosages follow the order

of KMnO

> NaOCl > H

, with KMnO

producing the largest,

strongest, and densest ﬂocs.

For ozonation: It is one of the strongest oxidants used in the water

industry for disinfection purposes. The oxidation potential of common

oxidants relative to chlorine is as follows (Peroxidation Systems, 1990):

Fluoride 2.32

Hydroxyl radical 2.06

Ozone 1.52

Hydrogen peroxide 1.31

Potassium permanganate 1.24

Chlorine 1.00

Ozone can be very effective for iron and manganese removal. Because

of its relatively high capital costs and operation and maintenance costs,

the ozonation process is rarely employed for the primary purpose of oxi-

dizing iron and manganese. Since ozone is effective in oxidizing trace

toxic organic matter in water, preozonation instead of prechlorination

1 H

1 2H

1 2e

1 e 1 H

2Fe

1 H

2Fe

1 2OH

1 H

2Fe

1 H

1 2H

2Fe

1 2H

Public Water Supply 437

is becoming popular. In addition, many water utilities are using ozone

for disinfection purposes. Ozonation can be used for two purposes: dis-

infection and metal removal.

Manganese zeolite process. In the manganese zeolite process, iron and

manganese are oxidized to the insoluble form and ﬁltered out, all in one

unit, by a combination of sorption and oxidation. The ﬁlter medium can

be manganese greensand, which is a purple-black granular material

processed from glauconitic greens, and/or a synthetic formulated prod-

uct. Both of these compositions are sodium compounds treated with a

manganous solution to exchange manganese for sodium and then oxi-

dized by KMnO

to produce an active manganese dioxide. The greensand

grains in the ﬁlter become coated with the oxidation products. The oxi-

dized form of greensand then adsorbs soluble iron and manganese,

which are subsequently oxidized with KMnO

. One advantage is that

the greensand will adsorb the excess KMnO4 and any discoloration of

the water.

Regenerative-batch process. The regenerative-batch process uses man-

ganese-treated greensand as both the oxidant source and the ﬁlter

medium. The manganese zeolite is made from KMnO

-treated greensand

zeolite. The chemical reactions can be expressed as follows (Humphrey

and Eikleberry, 1962; Wilmarth, 1968):

Exchange:

NaZ ⫹ Mn

2⫹

→ MnZ ⫹ 2Na

⫹

(5.132)

Generation:

MnZ ⫹ KMnO

→

Z ⋅ MnO

⫹ K

⫹

(5.133)

Degeneration:

(5.134)

Regeneration:

(5.135)

where NaZ is greensand zeolite and Z ⭈ MnO

is manganese zeolite. As

the water passes through the mineral bed, the soluble iron and man-

ganese are oxidized (degeneration). Regeneration is required after the

manganese zeolite is exhausted.

MnO

1 KMnO

MnO

438 Chapter 5

One of the serious problems with the regenerative-batch process is the

possibility of soluble manganese leakage. In addition, excess amounts

of KMnO

are wasted, and the process is not economical for water high

in metal content. Manganese zeolite has an exchange capacity of 0.09 lb

of iron or manganese per cubic foot of material, and the ﬂow rate to the

exchanger is usually 3.0 gpm/ft

. Regeneration needs approximately

0.18 lb KMnO

/ft

of zeolite.

Continuous process. For the continuous process, 1% to 4% KMnO

solu-

tion is continuously fed ahead of a ﬁlter containing anthracite be (6- to 9-

in thick), manganese-treated greensand (24 to 30 in), and gravel. The

system takes full advantage of the higher oxidation potential of KMnO

as compared to manganese dioxide. In addition, the greensand can act as

a buffer. The KMnO

oxidizes iron, manganese, and hydrogen sulﬁde to

the insoluble state before the water reaches the manganese zeolite bed.

Greensand grain has a smaller effective size than silica sand used in

ﬁlters and can result in comparatively higher head loss. Therefore, a layer

of anthracite is placed above the greensand to prolong ﬁlter runs by ﬁl-

tering out the precipitate. The upper layer of anthracite operates basi-

cally as a ﬁlter medium. When iron and manganese deposits build up,

the system is backwashed like an ordinary sand ﬁlter. The manganese

zeolite not only serves as a ﬁlter medium but also as a buffer to oxidize

any residual soluble iron and manganese and to remove any excess unre-

acted KMnO

. Thus a KMnO

demand test should be performed. The con-

tinuous system is recommended for waters where iron predominates, and

the intermittent regeneration system is recommended for groundwater

where manganese predominates (Inversand Co., 1987).

Example 1: Theoretically, how many mg/L each of ferrous iron and soluble

manganese can be oxidized by 1 mg/L of potassium permanganate?

solution:

Step 1. For Fe

2⫹

, using Eq. (5.123)

3Fe

2⫹

⫹ 4H

⫹

⫹ KMnO

→ MnO

⫹ 2Fe

3⫹

⫹ K

⫹

⫹ 2H

MW 3 ⫻ 55.85 39.1 ⫹ 54.94 ⫹ 4 ⫻ 16

⫽ 167.55 ⫽ 158.14

X mg/L 1 mg/L

By proportion

167.55

158.14

X 5 1.06 mg/L

Public Water Supply 439

Step 2. For Mn

2⫹

, using Eq. (5.124)

3Mn

2⫹

⫹ 2H

O ⫹ 2KMnO

→ 5MnO

⫹ 4H

⫹

⫹ 2K

⫹

3 ⫻ 54.94 2 ⫻ 158.14

⫽ 164.82 ⫽ 316.28

y 1 mg/L

then

Example 2: A groundwater contains 3.6 mg/L of soluble iron and 0.78 mg/L

of manganese. Find the dosage of potassium permanganate required to oxi-

dize the soluble iron and manganese.

solution: From Example 1 the theoretical potassium permanganate dosage

are 1.0 mg/L per 1.06 mg/L of ferrous iron and 1.0 mg/L per 0.52 mg/L of man-

ganese. Thus

15 Activated Carbon Adsorption

15.1 Adsorption isotherm equations

The Freundlich isotherm equation is an empirical equation, which gives

an accurate description of adsorption of organic adsorption in water. The

equation under constant temperature equilibrium is (US EPA, 1976;

Brown and LeMay, 1981; Lide, 1996)

(5.136)

(5.136a)

where q

⫽ quantity of absorbate per unit of absorbent, mg/g

⫽ equilibrium concentration of adsorbate in solution, mg/L

K⫽ Freundlich absorption coefﬁcient, (mg/g) (L/mg)

1/n

n⫽ empirical coefﬁcient

The constant K is related to the capacity of the absorbent for the

absorbate. 1/n is a function of the strength of adsorption. The molecule

that accumulates, or adsorbs, at the surface is called an adsorbate; and

log q

5 log K 1 s1/ndlogC

5 KC

1/n

KMnO

dosage 5

1.0 3 3.6 mg/L

1.06

1.0 3 0.78 mg/L

0.52

5 4.9 mg/L

y 5

164.82

316.28

5 0.52 mg/L

440 Chapter 5

the solids on which adsorption occurs is called adsorbent. Snoeyink

(1990) compiled the values of K and 1/n for various organic compounds

from the literature which is listed in Appendix D.

From the adsorption isotherm, it can be seen that, for ﬁxed values of

K and C

, the smaller the value of 1/n, the stronger the adsorption capac-

ity. When 1/n becomes very small, the adsorption tends to be inde-

pendent to C

. For ﬁxed values of C

and 1/n, the larger the K value, the

greater the adsorption capacity q

Another adsorption isotherm developed by Langmuir assumed that

the adsorption surface is saturated when a monolayer has been

absorbed. The Langmuir adsorption model is

(5.137)

or (5.138)

where a ⫽ empirical coefﬁcient

b ⫽ saturation coefﬁcient, m

Other terms are the same as deﬁned in the Freundlich model. The

coefﬁcient a and b can be obtained by plotting C

versus C

on arith-

metic paper from the results of a batch adsorption test with Eq. (5.138).

Adsorption isotherm can be used to roughly estimate the granular

activated carbon (GAC) loading rate and its GAC bed life. The bed life

Z can be computed as

(5.139)

where Z ⫽ bed life, L H

O/L GAC

)

⫽ mass absorbed when C

⫽ C

, mg/g of GAC

␳ ⫽ apparent density of GAC, g/L

⫽ inﬂuent concentration, mg/L

⫽ average efﬂuent concentration for entire column

run, mg/L

would be zero for a strongly absorbed compound that has a sharp

breakthrough curve, and is the concentration of the nonadsorbable com-

pound presented.

The rate at which GAC is used or the carbon usage rate (CUR) is cal-

culated as follows:

(5.140)CURsg/Ld 5

2 C

Z 5

3 r

2 C

abC

1 1 bC

Public Water Supply 441

Contact time. The contact time of GAC and water may be the most

important parameter for the design of a GAC absorber. It is commonly

used as the empty-bed contact time (EBCT) which is related to the ﬂow

rate, depth of the bed and area of the GAC column. The EBCT is cal-

culated by taking the volume (V ) occupied by the GAC divided by the

ﬂow rate (Q):

(5.141)

(5.142)

where EBCT ⫽ empty-bed contact time, s

V ⫽ HA ⫽ GAC volume, ft

or m

Q ⫽ ﬂow rate, cfs or m

Q/A ⫽ surface loading rate, cfs/ft

or m

/(m

⋅ s)

The critical depth of a bed (H

) is the depth which creates the immedi-

ate appearance of an efﬂuent concentration equal to the breakthrough

concentration, C

. The C

for a GAC column is designated as the maxi-

mum acceptable efﬂuent concentration or the minimum datable con-

centration. The GAC should be replaced or regenerated when the

efﬂuent quality reaches C

. Under these conditions, the minimum EBCT

(EBCT

min

) will be:

(5.143)

The actual contact time is the product of the EBCT and the inter-

particle porosity (about 0.4 to 0.5).

Powdered activated carbon (PAC) has been used for taste and order

removal. It is reported that PAC is not as effective as GAC for organic

compounds removal. The PAC minimum dose requirement is

(5.144)

where D ⫽ dosage, g/L

⫽ inﬂuent concentration, mg/L

⫽ efﬂuent concentration, mg/L

⫽ absorbent capacity ⫽ KC

1/n

, mg/g

Example 1: In a dual media ﬁltration unit, the surface loading rate is 3.74

gpm/ft

. The regulatory requirement of EBCT

min

is 5.5 min. What is the crit-

ical depth of GAC?

D 5

2 C

EBCT

min

Q/A

EBCT 5

Q/A

The depth of GAC bed

Loading rate

EBCT 5

442 Chapter 5

solution:

From Eq. (5.143), the minimum EBCT is

and rearranging

⫽ EBCT

min

(Q/A)

⫽ 5.5 min ⫻ 0.5 ft/min

⫽ 2.75 ft

Example 2: A granular activated carbon absorber is designed to reduce

12 ␮g/L of chlorobenzene to 2 ␮g/L. The following conditions are given:

K ⫽ 100 (mg/g) (L/mg)

1/n

, 1/n ⫽ 0.35 (Appendix D), and .

Determine the GAC bed life and carbon usage rate.

solution:

Step 1. Compute equilibrium adsorption capacity, (q

)

⫽ KC

1/n

⫽ 100 (mg/g) (L/mg)

1/n

⫻ (0.002 mg/L)

0.35

⫽ 11.3 mg/g

Step 2. Compute bed life, Z

Assume:

Step 3. Compute carbon usage rate, CUR

5 0.00088 g GAC/L water

CUR 5

2 C

s0.012 2 0.002d mg/L

11.3 mg/g

11.3 mg/g 3 480 g/L of GAC

s0.012 2 0.002d mg/L of water

5 542,400 L of water/L of GAC

5 0 mg/L

Z 5

3 r

2 C

GAC

5 480 g>L

EBCT

min

Q/A

Q/A 5 0.5 ft

/min/1 ft

5 0.5 ft/min

Q 5 3.74 gpm 5 3.74

gal

min

1 ft

7.48 gal

5 0.5 ft

/min

Public Water Supply 443

Example 3: The maximum contaminant level (MCL) for trichloroethylene

is 0.005 mg/L. K ⫽ 28 (mg/g) (L/mg)

1/n

, and 1/n ⫽ 0.62. Compute the PAC min-

imum dosage needed to reduce trichloroethylene concentration from 33 ␮g/L

to MCL.

solution:

Step 1. Compute q

assuming PAC will equilibrate with 0.005 mg/L (C

)

trichloroethylene concentration

⫽ 28 (mg/g) (L/mg)

0.62

⫻ (0.005 mg/L)

0.62

⫽ 1.05 mg/g

Step 2. Compute required minimum dose, D

⫽ 0.027 g/L

or ⫽ 27 mg/L

16 Membrane Processes

In the water industry, the membrane systems have generally been uti-

lized in water and for higher quality water production. Membranes

also are being used for municipal and industrial wastewater appli-

cations, for efﬂuent ﬁltration to achieve high-quality efﬂuent for sen-

sitive receiving waters, for various water reuse applications, as a

space-saving technology, and for on-site treatment. In the last decade,

tremendous improvements of equipment developments, more strin-

gent effluent limit, and market economics have led to membranes

becoming a mainstream treatment technology. There has been sig-

niﬁcant growth and increase in application of membrane technologies,

and it is expected that membrane processes will be used even more

in the future as more stringent drinking water quality standards will

likely become enforced.

The use of semipermeable membranes having pore size as small as

3 Å (1 angstrom ⫽ 10

⫺8

cm) can remove dissolved impurities in water

or wastewater. Desalting of seawater is an example. The process that

allows water to pass through the membranes is called “osmosis” or

hyperﬁltration. On the other hand, the process in which ions and solute

that pass through a membrane is called “dialysis” which is also used in

the medical ﬁeld.

D 5

2 C

0.033 mg/L 2 0.005 mg/L

1.05 mg/g

5 KC

1/ n

444 Chapter 5