Water and Wastewater Engineering

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13-58 WATER AND WASTEWATER ENGINEERING

U.S. EPA (1999) Alternative Disinfectants and Oxidants Guidance Manual, U.S. Environmental

Protection Agency Publication EPA 815-R-99-014, Washington, D.C.

U.S. EPA (2006) Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface

Water Treatment Rule, U.S. Environmental Protection Agency Publication EPA 815-R-06-007,

Washington, D.C.

Watson, H. (1908) “A Note on the Variation of the Rate of Disinfection with Change in Concentration of

the Disinfectant,” Journal of Hygiene, vol. 8, p. 536.

Weast, R. C, (1983) CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, pp. 16–163.

Wilczak, A., L. L Hoover, and H. H. Lai, (2003) “Effects of Treatment Changes on Chlorine Demand and

Decay,” Journal of American Water Works Association, vol. 95, no. 7, pp. 94–106.

14-1

14-10 RADIONUCLIDES

14-11 SYNTHETIC ORGANIC CHEMICALS

(SOCs) AND VOLATILE ORGANIC

CHEMICALS (VOCs)

14-12 TASTE AND ODOR (T&O)

14-13 CHAPTER REVIEW

14-14 PROBLEMS

14-15 DISCUSSION QUESTIONS

14-16 REFERENCES

REMOVAL OF SPECIFIC CONSTITUENTS

14-1 INTRODUCTION

14-2 ARSENIC

14-3 CARBON DIOXIDE

14-4 FLUORIDE

14-5 IRON AND MANGANESE

14-6 NITRATE

14-7 NATURAL ORGANIC MATTER (NOM)

14-8 PERCHLORATE

14-9 PHARMACEUTICALS AND ENDOCRINE-

DISRUPTING COMPOUNDS (EDCs)

CHAPTER

14-2 WATER AND WASTEWATER ENGINEERING

14-1 INTRODUCTION

The focus of the previous chapters has been on removal of broad classes of constituents: turbid-

ity, color, hardness, and pathogens. The focus of this chapter is a selected list of specific constitu-

ents that are not addressed in the previous chapters. They were selected

because of their impact

on the potability and palatability of water. These include arsenic, iron and manganese, fluoride,

nitrate, NOM, pharmaceuticals, radionuclides, taste and odor (T&O), synthetic organic chemicals

(SOCs) and volatile organic compounds (VOCs). In addition, becaus

e of its economic impact on

the softening process, treatment to remove carbon dioxide is included.

14-2 ARSENIC

A r senic is almost exclusively a groundwater contaminant. Therefore, this discussion is limited to

a few selected processes deemed reasonable for treating groundwater at a centralized facility. A

more thorough discussion may be found in the Arsenic Treatment Technology Handbook (U.S.

EPA, 2003).

Arseni

c can occur in four oxidation states in water: 5, 3, 0, 3. The most common states

are trivalent arsenite [As(III)] and pentavalent arsenate [As(V)]. Most As(III)-containing water in

the pH range of 6 to 9 will have As in the form of H

A sO

. Arsenate will be in the form HAsO

2

in the pH range 7 to 11.5. At pH values less than 7.0, arsenate will be in the form H

A sO



Treatment Strategies

Preoxidation Processes. Reduced inorganic arsenite cannot be removed effectively. Preoxida-

tion to form As(V) at the head end of all of the unit processes described in the following para-

graphs is essential. Chlorine, permanganate, and ozone are highly effective. Chlorine dioxide,

monochloramine, and UV are ineffective as stand-alone oxidants for As(III).

Side reactions with iron,

manganese, and sulfide must be accounted for in determining the

dose for oxidation (Ghurye and Clifford, 2001).

The oxidation-reduction reaction with chlorine is

HAsOOCl

HAsO

HCl







→ (14-1)

This reaction is relatively independent of pH in the range 6.3 to 8.3. In a laboratory study, at an

excess of three times the stoichiometric amount of chlorine, 95 percent of the As(III) was con-

verted to As(V) in 42 seconds (Ghurye and Clifford, 2001).

The oxidation-reduction reaction with permanganate is

2 3

424

HAsO

MnO H AsO

HMnOHO





→

(14-2)

Like the chlorine reaction, this reaction is relatively independent of pH in the range 6.3 to 8.3.

At a similar threefold stoichiometric excess, 95 percent of the As(III) was converted to As(V) in

36 seconds. As with chlorine oxidation, side reactions with iron, manganese, and sulfide must be

accounted

for in determining the dose (Ghurye and Clifford, 2001).

Permanganate is difficult to handle. It is commercially available as a crystal that is corrosive

and stains nearly everything purple. Manganese particles are produced as a result of permanga-

nate oxidation reactions. Therefore, postoxidation filtration is essential to prevent a

ccumulation

of deposits in the distribution system.

REMOVAL OF SPECIFIC CONSTITUENTS 14-3

The oxidation-reduction reaction with ozone is

HAsOO

HAsO

333







→

(14-3)

Like the chlorine reaction, this reaction is relatively independent of pH in the range 6.3 to 8.3.

Using an excess of three times the stoichiometric dose, 95 percent of the As(III) was converted to

As(V) in 18 seconds (Ghurye and Clifford, 2001).

A comparison of preoxidation alternatives is shown in Table 14-1.

TABLE 14-1

Comparison of Preoxidation alternatives

Criteria

Liquid sodium

hypochlorite system

On-site hypochlorite

generation system

Permanganate solution

feed system Ozone generation

Safety and

regulatory issues

• HazMat regulations for

safety and handling apply

• Potential for corrosive

vapors in the presence of

moisture

• Em

ergency response plan

required with local fire

department

• Secondary containment

required

• Below 1% threshold for

hazardous classification

• Exempt from HazMat

regulations

• No secondary

containment requirements

• Solid permanganate poses

dust and inhalation hazard

• Liquid is very corrosive

• Poisonous and

reactive gas

Space

requirements

• Space requirements are

small, assuming the

Uniform Fire Code (UFC)

exempt criteria are met

• Space requirements are

large. There must be

room for salt storage,

brine tanks, hypochlorite

holding tanks, electrolytic

uipment, as well as

instrumentation & control

and power.

• Space requirements

are small. Additional

space may be required

for storage of solid

permanganate.

• Space requirements

are small

Chemical

characteristics

• 5¼ or 12½% sodium

hypochlorite solution.

Degrad

es over time.

• Decay of solution creates

chlorate byproduct

• Increases pH of water

slightly

• Stable sodium

hypochlorite solution

(0.8%)

• Constant application

concentration

• Chlorate formation low

to none

• Increases pH of water

slightly

• Stable permanganate

solution, generally 3–4%

• Reacts rapidly with

dissolved organics

• Gas

• Very strong oxidizer

Chemical

delivery

• Liquid hypochlorite

delivered by tanker truck,

55-gal drum, or 5-gal pail

• Salt delivered in 50-lb

bags or 2000-lb totes

• Solid permanganate

available in 25-kg pails,

50-kg kegs, and 150-kg

drums

• N/A

Labor • Periodic delivery

• Dilution procedures

• Salt d

elivery

• Weekly loading of salt

into brine tank

• Load dry feeder

• Dilution procedures

• N/A

(continued)

14-4 WATER AND WASTEWATER ENGINEERING

Example 14-1. Estimate the stoichiometric amount of hypochlorite in mg/L required to oxidize

arsenic in groundwater with the following constituents:

A s(III)  50  g/L

Fe(II)  1.2 mg/L

Solution:

1 . The redox reaction for oxidizing As(III) is given in Equation 14-1.

HAsOOCl

HAsO

HCl







→

Note that one mole of hypochlorite reacts with one mole of arsenic.

2. The redox reaction for oxidizing Fe(II) is

2 5 24

2 3

FeOClHOFeOH Cl H

 

 → ()

Note that one mole of hypochlorite reacts with two moles of iron.

3. Calculate the molar concentrations of arsenic and iron:

Moles/Lof As

g/ L

g/mole g/





74 92 10

()(. gg

moles/L

Moles/Lof Fe

mg/L

)







667 10

(()()55 85 10

215 10

g/mole mg/g

moles/L



4. The required stoichiometric addition is

667 10 05 215 10

7 5

... 



moles/L moles/L()( )11 141 10

. 



moles/L of OCl

5. In mg/L

()()1 141 10 5145

..



moles/L of OCl g/mole 55 871 10 0 59

..



g/L or mg/L

TABLE 14-1 (continued)

Comparison of Preoxidation alternatives

Criteria

Liquid sodium

hypochlorite system

On-site hypochlorite

generation system

Permanganate solution

feed system Ozone generation

Operation and

maintenance

• Low day-to-day O&M.

Long-term material

maintenance could be

a problem because of

corrosive effects of liqu

hypochlorite.

• Moderate O&M,

mainly associated with

salt handling. Change

electrode cells every five

years.

• Low day-to-day O&M for

automated systems

• Stains everything purple

• Low day-to-day

O&M

Off-normal

operation

• A temporary bleach

solution can be mixed in

the storage tank

• A temporary bleach

solution can be mixed in

the day tank

• N/A • N/A

Community

relations

• HazMat signage required • No HazMat regulations.

Hydrogen byproduct

vented to atmosphere.

• N/A • N/A

Adapted from U.S. EPA, 2003.

REMOVAL OF SPECIFIC CONSTITUENTS 14-5

Comment. According to Ghurye and Clifford (2001), three times this dose is required to oxidize

95 percent of the arsenic in less than one minute.

Ion Exchange. Limitations to the use of ion exchange are the concentration of total dissolved

solids (TDS) and sulfate. If the TDS is less than 500 mg/L (Wang et al., 2000) and the sulfate

concentration is less

than 50 mg/L (U.S. EPA, 2003), strong-base ion exchange is effective and

economical. Iron, manganese, or a combination of the two, should not exceed 0.3 mg/L because

the oxidized forms will plug the ion exchange resin (GLUMRB, 2003).

Designs generally employ empty bed contact times (EBCTs) of 1.5 to 3

minutes. Downflow

cocurrent regeneration has been shown to be more effective than countercurrent regeneration

(Clifford, 1999).

Activated Alumina. Activated alumina (AA) is a porous, granular material with ion exchange

properties. It is a mixture of amorphous and gamma aluminum oxide prepared by low- temperature

(300 to 600  C) dehydration of Al(OH)

. The media is placed in columns that are designed and

operated in the same fashion as ion exchange columns.

The pH must be tightly controlled in the range 5.5 to 6.0. The alumina capacity is signif-

icantly reduced in the presence of sulfate ions. Arsenic is diffi

cult to remove from alumina.

A 4 percent NaOH solution is recommended for regeneration (Clifford, 1999). Because of the

high pH of the regeneration process approximately 2 percent of the media dissolves during each

regeneration cycle.

Iron-Based Sorbents (IBS). Adsorption on IBS is considered an emerging technology by the

EPA. The processes are proprietary. They

are based on sorption on iron-impregnated media.

The process is described as irreversible chemis orption. The m edia is placed in columns that are

designed in the same fashion as ion exchange columns. The media is discarded when it becomes

aturated. Appropriate media disposal procedures are discussed in Chapter 15.

Reverse Osmosis (RO). Conventional RO design and operation results in highly effective

arsenic removal. While chlorine m ay be used to control biological growth, it is not necessary to

preoxidize the raw water to remove significant amounts of ar

senic. Some membranes are dam-

aged by chlorine and may be damaged by other oxidants as well. RO is capable of achieving

over 97 percent removal of As(V) and 92 percent removal of As(III) (NSF, 2001a and 2001b).

Because As(III) removal is incons istent, U.S. EPA (2002) recommends preoxidation. Careful

election of the membrane material is essential if chlorine preoxidation is considered a necessary

part of the process. Manufacturers should be consulted to obtain information on material war-

ranty requirements.

Enhanced Lime Softening. For those water systems employing lime softening, the adjustment

of the pH to values above 10.5 is effective in removing As(V) by coprecipitation.

Oxidation/Filtration. For those water systems employing oxidation and filtration for iron

removal, As(V) adsorbs onto the iron hy droxide precipitate. The removal efficiency is highly

dependent on the initial iron concentration and ratio of iron to arsenic. In general, if the Fe:As

mass ratio is greater than 20:1, the removal efficiency

will be on the order of 80 to 95 percent

(U.S. EPA, 2003).

14-6 WATER AND WASTEWATER ENGINEERING

Modified Iron Removal (MIR). Several alternatives are available when the iron-to-arsenic

ratio is less than 20:1. These systems work in a fashion similar to conventional iron removal

systems but use either a proprietary media or add iron (in the Fe

3

form) to maximize the iron

removal efficiency. These systems use either manganese greensand or other manganese dioxide

based systems.

Decision Trees and Technology Comparison. Figure 14-1 is an adaptation of EPA’s decision

trees for selecting a new treatment technology for arsenic removal. Figure 14-2 is an a

dditional

aid in process selection. Tables 14-2 and 14-3 on pages 14-9, 14-10, and 14-11 summarize a com-

parison of the technologies and some typical design and operating parameters.

14-3 CARBON DIOXIDE

R e moving CO

will improve neither the potability nor the palatability of the water. Its removal

is an economic issue in the lime-soda softening process and in posttreatment of NF/RO treated

water. As noted in Chapter 7, CO

and H

in water are essentially the same. Carbon dioxide

is the principal acid present in unpolluted, naturally occurring water. Raw water CO

concentra-

tions in surface water are generally negligible. In groundwater, concentrations ranging from a

few mg/L to nearly 100 mg/L have been reported (AWWA, 1990). Concentrations in the range of

20 to 40 mg/L are not uncommon. Carbon dioxide must be removed or neutralized before the pH

can be raised to precipitate hardness. When the concentration exceeds 10 mg/L as CO

(22.7 mg/L

as CaCO

or 0.45 meq/L), the economics of removal by aeration ( stripping ) are favored over

removal by lime neutralization (AWWA, 1990). In NF/RO treated water, air stripping in con-

junction with other treatment is used to increase the stability of the permeate.

Stripping

Air stripping of CO

may be accomplished by a variety of devices: spray aerators, multiple-tray

aerators, cascade aerators, cone aerators, and packed columns. Of these types, multiple-tray aera-

tors have found wide use in stripping CO

. The multiple-tray aerators consist of a series of trays

with slatted, perforated, or wire-mesh bottoms. The raw water is distributed at the top, flows

down over a series of trays, and is collected in a basin at the base of the unit. Although natural

ventilation may be used, artificial ventilation is more common because the u

nits are housed in

buildings. This is especially true in colder climates where freezing temperatures occur. Blowers

force air from the bottom of the tray system countercurrent to the flow of water. Scroll panels

provide good cross-ventilation, and the roof is open except directly over the distributing trays

Design Equation. The following empirical equation may be used to estimate the number of

trays (Scott, 1955):





10 (14-4)

where C

 concentration of carbon dioxide after passing through n trays, mg/L

 raw water concentration of carbon dioxide, mg/L

n  number of trays including distribution tray at top

k  coefficient dependent on ventilation, temperature, turbulence, installation

The value of k ranges from 0.12 to 0.16.

REMOVAL OF SPECIFIC CONSTITUENTS 14-7

Start

2

 50 mg/L



 5 mg/L as N

TDS  500 mg/L

pH  6.5 and  9

Fe  Mn  0.3 mg/L

Turbility  0.3 NTU

Fe  0.5 mg/L

Mn  0.05 mg/L

Cl  250 mg/L



 2 mg/L

SiO

 30 mg/L

2

 360 mg/L

TDS  1,000 mg/L

TOC  4 mg/L

Fe  1.0 mg/L

Mn  1.0 mg/L

TBLL  technology based local

limit for discharge of brine

IBS  iron-based sorbants

IX  ion exchange

MIR  modified iron removal

RSF  rapid sand filter

Key

Yes

Yes Yes

Yes

NoNo

Does

source water

meet water quality

criteria in

Box A

Adjust

Evaluate

IBS

Adjust

pH to

5.5–6.0

Use

Does

source water

meet water quality

criteria in

Box B

Does

source water

meet water quality

criteria in

Box D

Fe: As 

20:1

Is pH 5.5–8.5

Can

local TBLL

be met

Does

source water

meet water quality

criteria in

Box C

1.0

mg/L

6.0–8.5

Is pH 5.5–6.0?

Use

activated

alumina

Use

MIR

Use

RSF

greensand

Box A

Box B

Box C

Box D

3

FIGURE 14-1

Decision tree for treatment to remove arsenic.

( Note: The tree assumes oxidation to As(V) before all of the selections.)

(Adapted from U.S. EPA, 2003.)

14-8 WATER AND WASTEWATER ENGINEERING

0.0

Fe Secondary Maximum Concentration Limit

Arsenic, g/L

Iron, mg/L

or above

As MCL

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Media adsorption

Iron coag/filt

Ion exchange

Iron removal (M)

RO / NF

Modified iron removal process

Iron removal process

(optimized for maximium as removal)

20:1 F

e/A

atio

FIGURE 14-2

Arsenic treatment process selection guide.

( Source: Cook, 2005.)

Design Practice. Multiple-tray aerators are typically designed with three to nine trays. These are

spaced about 300 to 750 mm apart. The footprint of the structure ranges from 2 to 6 m

/1,000 m

of design capacity. The water application rates range from 60 to 75 m

/ d · m

(Dyksen, 2005).

14-4 FLUORIDE

When the raw water fluoride concentration exceeds the recommended limits (see Chapter 13

for limits), fluoride removal is required. Activated alumina is an excellent medium for fluoride

removal. It is far superior to synthetic organic ion exchange resins (Clifford, 1999). The design

fundam

entals are similar to those u sed for ion exchange columns with sy nthetic resins. Unlike

typical ion exchange column behavior, the breakthrough curve is not sharp and the design must

account for substantial leakage. Although countercurrent regeneration is the mos t efficient way

to minimize the leakage problem, most plant

s are designed with cocurrent regeneration because

of its simplicity.

A typical fluoride-removal plant utilizing activated alumina consists of two or more adsorp-

tion beds. The raw water pH is adjusted to 5.5 to 6.0, and it is passed downward through a 1 to

1.5 m deep bed of me

dium. The medium is specified by mesh size. For example 8  30 mesh

means that the activated alumina particles will pass a number 8 sieve and will be retained on a

number 30 sieve. Activated alumina medium is generally 28  48 mesh.

Following e

xhaustion, the medium is backwashed and then treated in a two-s tep regenera-

tion. Backwashing with raw water is to remove entrained particles, break up clumps of alumina,

14-9

TABLE 14-2

Arsenic treatment technologies summary comparison

Sorption processes Membrane processes

Factors Ion exchange Activated alumina

Iron-based sorbents Modified iron removal

Nanofiltration or reverse

osmosis

IX AA IBS MIR NF/RO

USEPA BAT

Yes Yes

No Yes

USEPA SSCT

Yes Yes

No Yes

System size

b,d

25–10,000 25–10,000 25–10,000 25–10,000 501–10,000

Removal efficiency

95%

Up to 98%

Up to 95%

 95%

Total water loss 1–2% 1–2% 1–2%  5%15–75%

Preoxidation required

Yes Yes

Yes

Likely

Optimal water quality

conditions

pH 6.5–9

 5 mg/L NO

1

 50 mg/L SO

2  1

 500 mg/L TDS

 0.3 NTU Turbidity

pH 5.5–6

pH 6–8.3

 250 mg/L Cl

1

 2 mg/L F

1

 360 mg/L SO

2  k

 30 mg/L Silica

 0.5 mg/L Fe

3 i

0.05 mg/L Mn

2 i

 1,000 mg/L TDS

 4 mg/L TOC

 0.3 NTU Turbidity

pH 6–8.5

 1 mg/L PO

3

 0.3 NTU Turbidity

pH 5.5–8.5

 0.3 NTU

turbidity

No particulates

Operator skill required Medium

Low

Low Low Medium

Waste generated Spent resin, spent brine,

backwash water

Spent media, backwash water Spent media, backwash

water

Spent media, backwash

water

Reject water, CIP water

Other considerations Possible pre & post pH

adjustment

Prefiltration required

Potentially hazardous brine waste

Nitrate peaking

Carbonate peaking affects pH

Possible pre & post pH adjust

ment

Prefiltration may be required

Modified AA available

Media may be very

expensive.

Prefiltration may be

required

Media may be expensive.

Prefiltration may be

required

High water loss

(15–75% of feed water)

Centrulized costMedium Medium Medium Medium High

Activated alumina is assumed to operate in a nonregenerated mode.

U.S. EPA, (2002). Implementation Guidance for the Arsenic Role, EPA Pub. No 816K02018.

IBS’s track record in the United States was not established enough to be considered as Best Available Technology (BAT) or Small System Compliance Technology (SSCT) at the

time the rule was promulgated.

Affordable for systems with the given number of people served.

U.S. EPA, 2000.

Preoxidation only required for As(III).

Some iron based sorbents may catalyze the As(III) to As(V) oxidation and therefore would not require a pre-oxidation step.

RO will remove As(III), but its efficiency is not consistent and pre-oxidation will increase removal efficiency.

AwwaRF, 2002.

Kempic, 2002.

Wang, et al., 2000.

AA can be used economically at higher pHs, but with a significant decrease in the capacity of the media.

Ghurye and Clifford, 2001.

Tumab, 2002.

With increased domestic use. IMS cost will significantly decrease.

Adapted from U.S. EPA, 2003.