Water and Wastewater Engineering

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14-10

TABLE 14-2 (continued)

Arsenic treatment technologies summary comparison

Precipitative Processes

Enhanced lime

softening

Enhanced

(conventional)

coagulation filtration

Coagulation-assisted

micro-filtration

Coagulation-assisted

direct filtration Oxidation filtration

Factors LS CF CMF CADF OxFilt

Best available technology

(BAT)

Yes Yes No Yes Yes

Small System Compliance

Technologies (SSCT)

No No Yes Yes Yes

System size

a,b

25–10,000 25–10,000 500–10,000 500–10,000 25–10,000

Removal efficiency

90% 95% (w/FeCl

)

 90% (w/Alum)

90% 90% 50–90%

Total water loss 0% 0% 5% 1–2% 1–2%

Preoxidation required

Yes Yes Yes Yes Yes

Optimal water

Quality conditions

pH 10.5 – 11

 5 mg/L Fe

3 e

pH 5.5 – 8.5

 0.3 mg/L Fe

Fe:As Ratio  20:1

Operator skill required High High High High Medium

Waste generated Backwash water,

Sludge (high volume)

Backwash water,

Sludge

Backwash water,

Sludge

Backwash water,

Sludge

Backwash water,

Sludge

Other considerations Treated water

requires pH

justment.

Possible pre & post

pH adjustment.

Possible pre & post

pH adjustment.

Possible pre & post

pH adjustment.

None.

Centralized cost

Low

High Medium Medium

U.S. EPA, 2002.

Affordable for systems with the given number of people served.

Depends on arsenic and iron concentrations.

Preoxidation only required for As(III).

AwwaRF, 2002.

Fields, et al., 2000.

Costs for enhanced LS and enhanced CF are based on modification of an existing technology. Most small systems will not have this technology in place.

REMOVAL OF SPECIFIC CONSTITUENTS 14-11

TABLE 14-3

Typical sorption treatment design and operating parameters

Parameter IX AA IBS MIR

Media bulk density, kg/m

640–700 640–750 1,100–1,200 N/A

Minimum column layers

Freeboard, %

a, b

50 N/A

Media, m

1–1.5

0.8–1.0 0.8–1.0

Operating conditions

Hydraulic loading rate m

/d · m

470–700

230–530

300–470 300–600

Empty bed contact time min 1.5–5

3–12

3.5 2–8

Downflow pressure drop,

kPa/m

50–100

N/A N/A

Maximum pressure differential, kPa 100 35 24 N/A

Backwash conditions

Backwashing flow rate, m

/d · m

180–230

400

N/A

Backwashing duration, min

5–20

N/A

Regeneration conditions

Brine strength, wt % 6–10 — — —

Downflow rate m

/d · m

120–350———

Regenerant volume m

resin 2.7 — — —

Rinsing conditions

Slow rinse rate, m

/d · m

25–270 — — —

Fast rinse rate, m

/d · m

120–1,200 — — —

Displacement requirements, BV 4–6 — — —

This will be very resin specific. Check with the resin manufacturer before design. Assumes AA is not regenerated.

Rubel, 2001.

Rubel, 2003.

This depends on temperature, type of media, and hydraulic loading rate.

For strong base anion exchange resin at 20 C and 25 m

/h · m

For AA at 5 m

/h · m

N/A—Not Available.

Source: Clifford, 1999; Cook, 2007; U.S. EPA, 2003.

and reclassify the medium. Following a 5 to 10 minute backwash, a 60 to 75 minute slow rinse

(0.020 m

/ min · m

) with a 1 to 4 perc ent NaOH solution is used. The NaOH is followed by a

slow rinse with water. A slow acid rinse follows the base (nominally 2 percent H

or HCl)

that, in turn, is followed by a s low water rinse before putting the unit back in service. Detailed

instructions on this complex regeneration process and schematic drawings are provided in the

AWWA manual (2004) as well as in Rubel and Woos ley (1979). Typical design criteria are

shown in Table 14-4 .

If lime-soda s

oftening is practiced, fluoride can be removed by suitable pH adjustment.

14-5 IRON AND MANGANESE

Iron and manganese often occur together, and the treatment technologies are similar, so they

are grouped together for this discussion. They are removed from water for aesthetic reasons. In

o xidized form [Fe(III) and Mn(IV)], they impart color to water and stain fixtures and clothing.

14-12 WATER AND WASTEWATER ENGINEERING

Iron stains are a red or rust color. Manganese stains are brown or black. For these reasons, the U.S.

EPA set secondary drinking water MCLs of 0.3 mg/L for iron and 0.05 mg/L for manganese.

While quite insoluble in their oxidized form , they are very soluble in their reduced forms

[Fe(II) and Mn(II)]. They are commonly ass

ociated with anoxic groundwater and hypolimnetic

water in eutrophic lakes. Although more commonly found in the low mg/L range, in groundwa-

ter with low alkalinity, total iron concentrations may reach 10 mg/L or more. Concentrations of

2

are on the order of 0.1 to 2.0 mg/L (Kawamura, 2000; MWH, 2005).

Treatment Strategies

Preoxidation. The most common method of removing iron and manganese is based on the

conversion of the soluble forms to insoluble forms by oxidation. Oxidation with air, chlorine,

chlorine dioxide, and permanganate are common in the United States. Ozone has been used suc-

cessfully in Europe.

The oxidation-re

duction reactions with air are (AWWA, 1990):

4248

3 22 2 3 2

FeHCO O HO FeOH CO() () →

(14-5)

TABLE 14-4

Typical design criteria for removal of fluoride by activated alumina

Parameter Typical range or value

Treatment

Media

Capacity 3,000 to 5,000 g/m

Size 28  48 mesh

Depth 1 to 1.5 m

Filtration rate 90 to 400 m

/d · m

Volume concentration 1,000 to 1,500 bed volumes depending on the F



Backwash flow rate 470 to 550 m

/d · m

Backwash time 5 to 10 minutes

Regeneration

Regenerant 1% NaOH

Flow rate 30 m

/d · m

Volume 5 bed volumes

Time 60 to 75 min

Rinse volume 2 bed volumes

Rinse flow rate 30 m

/d · m

Regenerant 2% H

Flow rate 30 m

/d · m

Volume

1.5 bed volumes typically

Rinse volume 2 bed volumes

Rinse flow rate 30 m

/d · m

To neutralize the bed to pH 5.5.

A dapted from Clifford, 1999; Kawamura, 2000.

REMOVAL OF SPECIFIC CONSTITUENTS 14-13

22 2224

4 3 22 2 4 2 2

MnSO Ca HCO O MnO CaSO H O CO () →

(14-6)

These reac tions are quite s low. At a pH of 7.5 to 8.0, the iron reaction m ay take between

15 minutes and one hour (Hand et al., 1999; MWH, 2005). GLUMRB (2003) recommends a min-

imum detention time of 30 minutes after aeration. At a pH of 9.5, manganese oxidation requires

an hour. At lower pH values, manganese oxidation with air is not practi

cal.

The oxidation-reduction reactions with chlorine are (AWWA, 1990):

226

3 2 3 22 3 2

Fe HCO Ca HCO Cl FeOH CaCl CO() () () →

(14-7)

Mn HCO Ca HCO Cl MnO CaCl H O CO()()

22 2 2 2

24 →

(14-8)

At a pH of 8.0 to 8.5, the oxidation time is about 15 to 30 minutes for iron. Oxidation of Mn

2

requires two to three hours at this pH. If ammonia is present it will consume chlorine and signifi-

cantly increase the time for oxidation (MWH, 2005).

The oxidation reactions with chlorine dioxide are (AWWA, 1990):

Fe HCO NaHCO ClO FeOH NaClO CO() ()

3 2 3 2 3 22

3 →

(14-9)

Mn HCO NaHCO ClO MnO NaClO H O()

3 2 3 22 22

22 224 → CCO

(14-10)

With a pH of 5.5, in the absence of natural organic matter (NOM), the oxidation time for iron

is about 5 seconds and about 20 seconds for manganes e. At higher pH values the reaction rate

is faster. NOM does not interfere with the oxidation of Mn

2

b ut inhibits the oxidation of Fe

2

almost completely (MWH, 2005).

The oxidation reactions with permanganate are (AWWA, 1990):

3 2 35

3 242 3 2 3

FeHCO KMnO H O FeOH MnO KHCO() () → CCO

(14-11)

3 2 5 224

3 2423 22

Mn HCO KMnO MnO KHCO H O CO()→

(14-12)

Both Fe

2

and Mn

2

are oxidized in less than 20 seconds at pH values greater than 5.5. Oxidation

of iron complexed with NOM requires over an hour. The applied dose of permanganate must be

controlled carefully because residual permanganate on the order of 0.05 mg/L results in an easily

detectable pink color to the water.

Filtration. Tra

ditional iron and manganese removal is by filtration of the preoxidized con-

stituents. All of the media types are similar with respect to particle size distribution. The media

differs from rapid sand filter media in that it is treated with KMnO

to provide a manganese

oxide (MnO

) coating.

The common media is called greensand because of the color of the media. Greensand is a

dull green iron potassium silicate called glauconite. For iron and manganese removal, it is syn-

thetically coated with a thin layer of MnO

. Glauconite exhibits an ion exchange capacity that

allows the surface to be saturated with manganous ions. F ollowing saturation, the glauconite is

soaked in a strong oxidizing solution (KMnO

) that converts the manganous ion to MnO

( s). As

with ion exchange, the greensand may be regenerated periodically by backwashing, or it may

be continuously regenerated with a low oxidant dose. Commercially available greensand has an

effective size of 0.30–0.35 mm, a uniformity coefficient of less than 1.6, and a specific gravity

of 2.4 (Somm

erfeld, 1999).

14-14 WATER AND WASTEWATER ENGINEERING

Silica sand and/or anthracite treated to provide a coating of manganese oxide have been used

as filter med ia. In addition, membrane (MF) filtration has been us ed to remove the precipitate

after preoxidation.

Ion Exchange. Ion exchange may be suitable for raw water with iron and manganese con-

centrations less than 0.5 mg/L, but this is not a common treat

ment technology. It is not recom-

mended for higher concentrations because the media becomes coated with oxides. The coated

media cannot be regenerated.

Nanofiltration. NF membranes are very efficient for the removal of soluble Fe

2

and Mn

2

However, a very small concentration of oxidized iron and manganese will foul the membrane. If

the raw water is anaerobic and great care is taken to prevent oxidation of the iron and manganese,

NF can be highly effective (MWH, 2005).

Lime-Soda Softening. As noted in Chapter 7 (Figures 7-11 and 7-12) at the high pH values

required for lime-soda softening, iron and manganese are effectively

removed.

Complexed Iron and Manganese. Preoxidation, followed by sedimentation and filtration is

ineffective for complexed iron and manganese. Typical circumstances that result in complexed

iron and manganese are (Sommerfeld, 1999):

• Shallow wells adjacent to rivers, streams, and lakes.

• Total organic carbon (TOC)

concentrations greater than 2.0 to 2.5 mg/L.

• Presence of ammonia in the raw water.

• Presence of hydrogen sulfide in the raw water.

Pretreatment to remove NOM, NH

, and H

S is one method to m ake the iron and manganese

oxidation removal processes effective. Alternative means of iron and manganese removal such as

ion exchange or NF may become competitive with oxidation/filtration when the complexes are

encountered.

Decision Trees. F igures 14-3 and 14-4 illustrate methods

to select alternative treatment trains

to deal with constituents that may interfere with the iron and manganese removal. Pilot testing of

alternatives is highly recommended.

14-6 NITRATE

High levels of nitrate are of concern because it is a precursor to nitrite, which c auses methemo-

globinemia, also known as “blue baby syndrome.” The nitrate is converted to nitrite in the stom-

ach. It complexes with hemoglobin which reduces its capability to carry oxygen. The U.S. EPA

has set an MCL of 10 mg/L as nitrate.

Of the

several processes that can remove nitrate (biological denitrification, RO, and ion

exchange), ion exchange is generally the most economical. Strong base anion exchange (SBA)

REMOVAL OF SPECIFIC CONSTITUENTS 14-15

Start

YesNo No

Yes

water

aerobic

Preoxidation

with Cl

Precipitation

Lime-soda

softening

Coagulation

flocculation

Preoxidation

Detention or

coagulation/

sedimentation

Greensand

filtration*

Settling

Filtration

Enhanced

coagulation

enhanced

softening

TOC

 20 mg/L

Disinfection

* When no Mn is present,

use plain sand filtration

FIGURE 14-3

Decision tree for treatment to remove iron/manganese at larger water treatment plants (  40,000 m

/ d).

resins are effective in removing nitrate. The preference for anion exchange for standard SBA

resins is sulfate  nitrate  bicarbonate  chloride. When the sulfate concentration is high,

nitrate-specific resins are selected.

14-7 NATURAL ORGANIC MATTER (NOM)

NOM is of concern because its reaction with chlorine forms carcinogenic disinfection byprod-

ucts and complexes with other constituents to inhibit their removal from water. One method

of quantifying the presence of NOM is by measuring the total organic carbon (TOC) in the

water. Unless they are under the influence of

surface water, most groundwaters have TOC

conc entrations less than 2 mg/L. Surface water TOC concentrations range from 1 to 20 mg/L

(MWH, 2005).

14-16 WATER AND WASTEWATER ENGINEERING

Start

S ?

TOC

> 2.0 mg/L ?

Greensand

filtration

Rapid sand

filtration

Oxidize

with

air or

ClO

KMnO

Oxidize

both

and

with

KMnO

Oxidize Fe

with Cl

GAC

remove

SAC

for NH

;

SBA

for

Fe/Mn

complex

Oxidize Mn

with KMnO

> 0.05 mg/L ?

Disinfection

Yes

FIGURE 14-4

Decision tree for treatment to remove iron/manganese at smaller water treatment plants (  30,000 m

/ d).

GAC  granular activated carbon

SAC  strong-acid cation exchange resin

SBA  strong-base anion exchange resin

REMOVAL OF SPECIFIC CONSTITUENTS 14-17

Treatment Strategies

The primary methods of NOM removal are enhanced coagulation, adsorption on activated

carbon, ion exchange, and NF/RO. As noted in Chapter 7, lime-soda softening is als o effective

in removing NOM.

Enhanced Coagulation. Alum or ferric chloride coagulation removes NOM by NOM-metal

ion interaction. The conventional coagulation/flocculation/sedimentation/filtration treatment train

is “enhanc

ed” by adjusting the dose based on TOC removal rather than on turbidity removal. In

general, this implies higher doses of coagulant. Adjustment of the pH to the range of 5.5 to 6.5

results in the optimum NOM removal for a given dose of alum. With the adjustment of pH and

alum doses on the order of 50 to 100 m

g/L, TOC removal on the order of 30 to 50 percent can be

achieved (MWH, 2005).

The U.S. EPA (2006) established requirements for TOC removal com pliance as shown in

Table 14-5 .

Adsorption. Although activated carbon is effective in NOM removal, a large amount of activated

carbon is required to remove a little NOM (MWH, 2005). Dvorak and Maher (1999) suggest that car-

bon usage rates c

an be minimized by blending effluent from parallel columns. For operational end-

points of 0.75 and 1.5 mg/L TOC and empty bed contact times (EBCTs) of 5.25 and 10.5 minutes, the

most dramatic improvement in carbon usage occurs by using two colum

ns. The usage rate decreases

slightly with the addition of a third and fourth column and then levels off. Increasing the EBCT from

5.25 minutes to 10.5 minutes reduces the usage dramatically for one column but only slightly for two

columns. They do not reco

mmend increasing the EBCT to 21 minutes based on their study.

Ion Exchange. Anion exchange resins are effective in removing NOM because it is highly

ionized in water. TOC reductions on the order of 50 percent can be achieved with conventional

packed bed ion exchange columns.

Magnetic ion exchange (MIEX) uses a magnetized bead that is

added to water as a slurry

( Figure 14-5 ). The exchange reaction is faster than in a column that allows a shorter contact time.

The magnetized beads coalesce during settling, which increases the settling rate.

NF and RO. While NF and RO systems have been used successfully for removing NOM from

groundwater while it was being softened, their use for surface water is limited because of fouling

problems.

TABLE 14-5

Enhanced coagulation TOC removal requirements

Source water alkalinity, mg/L as CaCO

Source water TOC,

mg/L 0 – 60

 60–120  120

0–2.0 No action No action No action

 2.0–4.0

35%25%15%

 4.0–8.0

45% 35%25%

 8.0

50% 40% 30%

14-18 WATER AND WASTEWATER ENGINEERING

14-8 PERCHLORATE

Perchlorate (

ClO

−

) contamination has become a major concern s ince the U.S. Environmental

Protection Agency (EPA) reported that its presence in drinking water poses a health hazard

because of its effect on thyroid hormone production. Although EPA has placed perchlorate

on the Contaminante Candidate List 2, a maximum contam inant level (MCL) has not been

set. However, several s

tates have set their own advisory levels. These range from 1 to 14  g/

L. In 2006, Massachusetts set an MCL of 2  g/L, and in 2007, California set an MCL of

6  g/L.

N umerous technologies for removing perchlorate from drinking water have been investi-

gated. These include ion exchange, biological reduction, reverse osmosis, granular activated

carbon adsorption, and chem

ical reduction. Of these, strong-base anion exchange (SBA) and

biological reduction have shown favorable results.

The presence of nitrate, sulfate, and uranium in the raw water require careful consideration in

the selection of an appropriate resin for ion exchange. The perchlorate-chloride separation factors

of commercially available resins vary over almost three orders of m

agnitude.

From limited research results with ion exchange resins, the following general conclus ions

may be made (Tripp and Clifford, 2006):

• Polyacrylic resins prefer sulfate over perchlorate and nitrate.

• Polystyrene SBA resins prefer sulfate over nitrate, but not over perchlorate.

• R e sins with high perchlorate selectivity (for e

xample tripropyl and bifunctional triethyl-

trihexyl) prefer perchlorate and nitrate over sulfate.

Raw

water

Waste brine

Brine

Make-up

water

Treated water

Resin

setting

tank

Spent resin

Regenerated resin

FIGURE 14-5

Magnetic ion exchange (MIEX

) process flow diagram.

REMOVAL OF SPECIFIC CONSTITUENTS 14-19

In each of these instanc es, the regenerant brine is a significant disposal issue. As an aid

to process selection, Tripp and Clifford (2006) presented the decision criteria outlined in

Table 14-6 .

Even at low concentrations ( 10  g/L), the presence of naturally occurring uranium [U(VI)]

complicates the decision process because SBA resins concentrate the uranium. The regeneration

of SBA with high concentrations of adsorbed uranium may result in the production of a hazard-

ous waste. Gu et al. (2005) c

oncluded that bifunctional resins can be used effectively to treat

water contaminated with both uranium and perchlorate. To m inimize the generation of hazard-

ous waste, the adsorbed U(VI) may be separated from the

ClO

−

b y using a dilu te acid wash to

remove the U(VI) prior to the regeneration of the spent resin loaded with

ClO

−

Large pilot-scale (270 m

/ d) fluidized bed bioreactor (FBR) results have demonstrated that

perchlorate concentrations on the order of 1,000  g/L can be reduced to less than 5  g/L (Web-

ster et al., 2009). Granular activated carbon (GAC) is suspended or fluidized by the upward flow

of water through a column. The GAC acts as the

medium for attached microbial growth. An

electron donor such as acetic acid is fed to the column. Under anoxic conditions, the microorgan-

isms perform an oxidation-reduction reaction in consuming all of the dissolved oxygen, nitrate,

and perchlorate. The byproducts of the proc ess

are nitrogen gas, chloride ions, carbon dioxide,

heat, and additional biomass. The FBR is self-inoculated by the natural flora of the incoming

groundwater. As with typical biological processes, an acclimation period is required after start-up

to achieve sufficient biomass to be effective. In this pilot study, the

start-up period was about one

month. Although the system required careful monitoring during start-up, the performance was

reliable even with simulated failures of power and pumps after the start-up period.

TABLE 14-6

Ion exchange process recommendations for perchlorate-contaminated groundwater

2−

mg/L

−

mg/L

Brine

disposal

available Suggested treatment process

 250

Any Yes Option 1: Use standard polyacrylic gel resin with

low perchlorate selectivity and operate until nitrate

breakthrough with partial exhaustion and partial

countercurrent regeneration without mixing. Use 6%

NaCl at 320 kg/m

, and no temperature adjustment.

Consider biological or chemical treatment of

perchlorate and reuse of brine.

 500  5.0

Yes Option 2: Use standard polystyrene gel resin with

medium-high perchlorate selectivity run to sulfate

breakthrough with partial countercurrent regeneration

at ambient temperature or elevated temperature of 50

to 60C.

 500  5.0

No Option 3: Use highly perchlorate selective resin, run

to perchlorate breakthrough, with resin regeneration

offsite.

Feed water nitrate as nitrogen concentrations as high as the 10 mg/L MCL are acceptable as long as several ion exchange columns are

operated in parallel at different stages of exhaustion. This is to dilute the the nitrate peak to less than the MCL when peaking occurs.