Water and Wastewater Engineering

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5-11

5 cm schedule 80 pvc fill line

7.5 cm over

flow line

Secondary

containment

enclosure

wall

Load cell

Liquid

chemical storage

Max level

10 cm

vent

Calibration

column

Pulsation

dampener

Sampling tap

Drain

To injection

point

Eccentric plug valve

Isolation valve

Strainer

Backflow preventer

Feedback

signal

transmitter

Utility

water

Sump

FIGURE 5-4

L i q uid chemical feed system.

Notes:

1. Tanks, horizontal or vertical.

2. Flow pacing—variable speed motor. Provide two in parallel.

3. Volume of enclosure around storage tank shall be 100% of liquid volume of tank  10%  freeboard.

4. Magnetic flowmeter for monitoring (optional).

5-12 WATER AND WASTEWATER ENGINEERING

Type of feeder Application

Capacity

Turn-down

ratio Remarks

Proportioning pump

Peristaltic Most solutions 10

6

to 10

3

10:1 Flow rate very sensitive to changes in head

Positive displacement

Piston at low feed Most solutions, light

slurries

3  10

4

to 5

10:1

Higher turn-down ratio leads to inaccuracy

Diaphragm at low feed Most solutions 1  10

4

4  10

3

10:1 Higher turn-down ratio leads to inaccuracy

Rotating dipper Most solutions or slurries3  10

3

to 0.8 100:1

Nonpositive displacement

Rotameter Clear solutions 1  10

4

5  10

3

10:1 Calibrated valve

Loss-in-weight Most solutions 6  10

5

6  10

3

30:1 Tank with control valve

Although manufacturers sometimes claim the capability of high turn-down ratios (i.e., 100:1) by using a combination of stroke length and speed, pumps

should be sized so that the turn-down ratio does not exceed 10:1 to ensure accuracy at low feed rates (Anderson, 2005).

TABLE 5-3

Liquid feeder characteristics

Peristaltic Pumps. These pu mps use a rotating cam to create successive waves of c ontrac-

tion on a flexible tube to move the fluid. They are particularly well suited to small flow rates of

chemical on the order of a few milliliters per minute up to about 1 L per minute.

Gas Feed System

A conventional gas feed system for chlorine, called a chlorinator, i s shown in Figure 5-5 . It

consists of an inlet pressure reducing valve, a rotameter, a metering control orifice, a vacuum

differential regulating valve, and a venturi injector. The vacuum created by the chlorine injector

moves the gas from the storage cylinder

s to the injection system. Evaporators may be used on

very large systems.

The chlorine passes through the rotameter that measures the gas flow rate, then through a

metering or control orifice. A vacuum differential regulator is mounted across the control orifice

to stabilize the flow for a particular setting of the orifice. Current design practic

e is to locate the

vacuum regulators as close as possible to the storage containers to minimize the amount of pres-

surized gas piping in the plant.

T ypically, the control orifice has a range of 20 to 1, and the vacuum differential regulator

has a range of about 10 to 1. The overall range is thus about 200 to 1. Because rotameter ranges

are generally limited to about 20 to 1, their s

election controls the actual operating range without

changing rotameters (Anderson, 2005).

To maintain an inventory of the chemical remaining in a cylinder, it is placed on a scale be-

fore being put into service. The weight is noted periodically.

CHEMICAL HANDLING AND STORAGE 5-13

Pressure relief valve

(relieves at 25 to 50 cm water

pressure)

Legend

Gas

Water

Solution

Vacuum gauge

(30 to 100 cm water vacuum)

Chlorine gas

feeder

Standard vacuum

regulator

check unit

Solution

discharge

Gas

supply

Remote from

control module

Automatic

switchoever

vacuum

regulator

check unit

(used in pairs)

Gas

supply

Injector

Drain

relief

Injector

treated water

supply

Differential regulating

valve

Rotameter

V-notch

variable

ifice

Injector vaccum

gauge

Manual

feed rate

adjuster

FIGURE 5-5

Flow diagram for conventional chlorinator.

5-14 WATER AND WASTEWATER ENGINEERING

5-5 CHEMICAL COMPATIBILITY

It is not intended here to present an exhaustive list of incompatible chemicals but rather to

highlight combinations that may lead to violent reactions . Many others used in water and waste-

water require care. In particular, the design should provide sufficient piping and storage vessels

that the potential for residues from

prior-use chemicals reacting with new-use chemicals is

minimized.

Because there is a tendency for water of crystallization from alum to slake lime, it is impera-

tive that mixture of these two chemicals be avoided. In a closed container, this combination ma

lead to a violent explosion. For the s ame reason, ferric sulfate and lime should not be mixed

(Anderson, 2005). Mixtures of potassium permanganate and ferric chloride will form toxic chlo-

rine gas (Kawamura, 2000).

An abbreviated list of incompatible chemicals is pre

sented in Table 5-4 . A rigorou s search

for chemical combinations not shown in Table 5-4 is recommended as part of the design

process.

5-6 MATERIALS COMPATIBILITY

For very corrosive chem icals s uch as ferric salts, sod ium hypochlorite, and sodium hydrox-

ide, nonmetallic materials are preferred. These include fiberglass-reinforced plastic (FRP)

and various forms of polyethylene (PE) such as high-

density, cross-linked polyethylene

(HDXLPE).

Polyvinyl chloride (PVC), polypropylene, rubber-lined steel and type 316 stainless steel are

used for alum. In warm climates where the temperature of liquid alum may exceed 50  C, chlori-

nated polyvinyl chloride (CPVC) is

recommended.

Lime and soda ash can be stored in concrete or mild steel silos.

S uitable materials for storage containers for other chemicals are listed in Appendix A. Rec-

ommended materials for piping are given in Table 5-5 .

ChemicalsKeep out of contact with: Remarks

Activated carbon Oxidizing agents such as chlorine,

hypochlorites, potassium permanganate,

sufuric acid

Potential for fire

Alum Lime, milk of lime—Ca(OH)

Violent exothermic reaction

Ammonia Concentrated chlorine and

chlorine compounds

Violent exothermic reaction

Ferric chloride Potassium permanganate Formation of chlorine gas

Fluorine compounds All chemicals Etches glass

Sulfuric acid Strong bases

, light metal compounds

containing potassium and sodium

Violent exothermic reaction

TABLE 5-4

A short list of incompatible chemicals used in water and wastewater treatment

5-15

Piping material

Chemical

Iron

steel

Type

316

stainless

Type

304

stainless Copper PVC—type 1

Fiberglass-reinforced

polyester

(FRP) Polypropylene

Rubber

tubing Glass

Activated carbon (slurry)X X X

Alum NR S NR S X X X

Ammonia, aquaSX

Calcium hydroxide (slurry)SXX X XX

Calci

um hypochlorite X X X

Carbon dioxide (dry)SXXXXXXX

Chlorinated copperas XX

Chlorine (dry gas)S XNR NR

Chlorine solution NR NR NR S X X

Chlorine dioxide (3% soln.) X X

Coagulant aids Consult manufacturer—generally not corrosive

Copper sulfate X S X X X

Dolomitic lime (slurry)XXX X XX

Ferric chlorideNRNRNRNRSXXXX

Fluos

ilicic acid NR NR NR X X NR

Hydrochloric acid NR NR NR NR X X X X

Potassium permanganate (2% soln.) X X X X X

Sodium carbonate (soln.) S X X X X

Sodium chlorideXXXXX

Sodium chlorite X X X X

Sodium fluoride (1% to 5% soln.) X X X X X

Sodium hexametaphosphate (soln.) X X X X

Sodium hydroxide (to 50% s

oln.) X X X X X X X

Sodium hypochlorite (to 16% soln.) S X X X

Sodium silicate S X X X X X X

Sodium silicofluorideXX X

Sulfur dioxide (dry gas)XXX X

Sulfur dioxide (soln.) X

Sulfuric acid (conc.) S

Sulfuric acid NR S X X X X

TABLE 5-5

Recommended materials for piping

Key: S  Industrial standard or excellent for handling

X  Suitable for handling

NR  Not recommended

Source: Anderson, 2005.

5-16 WATER AND WASTEWATER ENGINEERING

The following example illustrates the complete design of the storage and handling system for one

chemical.

Example 5-4. Design the coagulant chemical handling and storage system for Boiling Water,

Arizona using the following design data:

Average daily design flow rate  38,000 m

/ d

Coagulant  ferric chloride

M a ximum dosage  50 mg/L as FeCl

Shipping time  1 week

S ummer temperature frequently exceeds 40  C

Solution:

a . Off-loading piping

From Table 5-5, select a 100 mm diameter schedule 80 PVC pipe with a notation to

check manufacturer’s data for temperature limitations.

b. Storage tank

(1) From Appendix A select FRP for tank material. The tank

should be located indoors

in a cool location.

(2) From Appendix A, note that ferric chloride is shipped as a 40% solution with

100% active ingredient. At the maximum dosage, the d aily mass of ferric chloride

us ed is

()( )()(50 38 000 10 10

3336

mg/L m /d L/m kg/mg,



))1 900, kg/d

and the mass of solution required is

1 900

040

4750

kg/d

kg/d

(3) Noting, from Table 5-1 , that coagulants are noninterru ptible, the volum e to be held

in two tanks for redundancy is 30-days supply plus two times the shipping time.

( )( ( )( ))4750 30 2 7 209 0000,,kg/dd d kg

(4) U sing the density of ferric chlorid e from Appendix A, the volume of solution to be

stored is

209 000

1 440

145 14 150

kg/m

or m

c. Feeder

Two feeder pumps are required to meet redundancy requirements.

From Table 5-3 , a piston metering pump with PVC or PE coated piston is s elected.

Checking the capacity

CHEMICAL HANDLING AND STORAGE 5-17

()4750

1 440

kg/d

h/d

kg/m

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

014

. m /h

where 1,440 kg/m

i s the density of ferric chloride from Appendix A. This is in the op-

erating range of 3  10

 4

to 5 m

/h.

d. Transfer piping

From Table 5-5 , select a 50 to 100 mm diameter schedule 80 PVC pipe.

e. The arrangement of the system is shown in Figure 5-4 .

5-7 DESIGNING FOR SAFETY AND HAZARDOUS CONDITIONS

Table 5-6 provides a general overview of safety requ irements and protective measures for han-

dling chemicals. Many of thes e measures are to be implemented by the operators, but s everal

require design provisions. Material Safety Data Sheets (MSDS) provided by the manufacturer of

the chemical provide

more detailed information on its s afe handling. Another general reference

for chemical safety, expos ure limits, and incompatibilities is NIOSH Pocket Guide to Chemical

Hazards (NIOSH, 2003).

The Emergency Planning and Community Right-to-Know Act (EPCRA), also known as

Title III of the Superfund Amendments and Reauthorization Act (SARA), requires facilities

with chemicals above the thresholds given in Table 5-7 to report this to the State Emergency

Response Commission (SERC) and coordinate with the appropriate Local Emergency Planning

Commission (LEPC). Con

struction of a new facility exceeding these amounts requires that the

owner notify the SERC and LEPC. Operating and maintenance manuals should address these

issues.

I n many communities, chlorine gas is the most hazardous substance in substantial q

uan-

tity in the community. Not only is it a hazard because of potential accidental release from

delivery through application to the water supply, but it also is a security hazard. Although it

is more expensive, sodium hypochlorite (NaOCl) is being used to replace gaseous chlorine to

reduce the hazar

d that gaseous chlorine poses. Many water treatment plants are using alterna-

tive disinfectants, su ch as ultraviolet (UV) radiation and ozone, to reduce the need for large

amounts of chlorine. Wastewater treatment plants have implemented the use of UV for the

same reason.

5-8 OPERATION AND MAINTENANCE

The major issues in operation and maintenance are safety programs and training, preventive

maintenance, good housekeeping, and good record keeping.

Because the concentrated chemicals used in water and wastewater treatment are for the most

part harmful to human health, formal safety programs are essential. This includes period

ic hands-

on training, provision of appropriate safety equipment in accessible locations, and provision of

personal protective equipment (PPE).

5-18

Protective equipment required

Chemical (D = dry;

L = liquid; G = gas)

Positive

ventilation

Protective

clothing

Neck cloths

Gloves

Rubber boots

Rubber gloves

Goggles

Face shields

Rubber aprons

Respirator

Gas mask

Avoid skin

contact

Safety shower

and eye baths

Remarks

Activated alumina (D)

■

Store away from gasoline, mineral or

vegetable oils, calcium hypochlorite (HTH),

lime, sodium chlorite, or potassium

permanganate

Activated carbon

Powder (D)

■■ ■■ ■■ ■

Granulate (D)

■■ ■

Alum sulfate (D)

■■ ■■ ■ ■ ■

Similar to other acids

Alum sulfate (L)

■■ ■

Ammonium hyd

roxide (L)

■ ■■

Moist NH

reacts with many metals and

alloys—liquid contact produces burns

Ammonium sulfate (D)

■■ ■■ ■ ■ ■ ■

See alum sulfate above

Anhydrous ammonia (G)

■■ ■ ■ ■■■ ■

Fire sprinklers and water hoses effective in

removing gas

Bauxite (D)

■■■ ■

Bentonite (D)

■

Calcium carbonate (D)

■

Calcium h

ypochlorite (D)

■■■ ■■

Carbon dioxide (G)

■■■

Chlorine (G)

■■■■■■

Avoid contact with hydrogen or organic

compounds or other ﬂammable materials

Chlorine dioxide (G)

■ ■ ■■ ■■ ■■■ ■

Solution is corrosive

Copper sulfate (D)

■■ ■■ ■ ■ ■ ■

Very corrosive

Ferric chloride (D)

■ ■ ■ ■■■■■ ■ ■

Very corrosive

Ferric sulfate (D)

■ ■ ■ ■■■■■ ■ ■

Ferrous sulfate (D)

■■ ■■ ■ ■ ■

Ferrous sulfate (L)

■■■■■■

Fluorosilicic acid (L)

■ ■■■■■■■ ■ ■ ■

Have lime slurry on hand

Fluorspar (D)

■■ ■ ■ ■■■ ■ ■

Etches glass when moist

TABLE 5-6

Protective measures for water & wastewater treatment chemicals

5-19

Protective equipment required

Chemical (D = dry;

L = liquid; G = gas)

Positive

ventilation

Protective

clothing

Neck cloths

Gloves

Rubber boots

Rubber gloves

Goggles

Face shields

Rubber aprons

Respirator

Gas mask

Avoid skin

contact

Safety shower

and eye baths

Remarks

Hydrated lime (D)

■■ ■■ ■■ ■ ■ ■

Can burn eyes or skin

Hydrochloric acid (L)

■■ ■■ ■■ ■ ■

Iron-exchange resins (D)

■■■ ■ ■

Hydrogen cation resins are acidic

Ozone (G)

■ ■■

Potassium permanganate (D)

■■ ■■■ ■ ■ ■

Large quantities present ﬁre hazard

Quicklime (D)

■■ ■■ ■■ ■ ■

Can burn eyes or skin

Sodium aluminate (D)

■■■ ■

Sodium aluminate (L)

■■■ ■

Sodium bisulfate (D)

■■■

Sodium carbonate (D)

■■ ■ ■ ■

Sodium chloride (D)

■■ ■ ■ ■ ■

Can dehydrate skin

Sodium chlorite (D)

■■ ■ ■ ■ ■

Rinse any spills immediately with water

Sodium ﬂuoride (D)

■■ ■■ ■ ■ ■ ■

Sodium polyphosphate, glassy (D)

■■ ■■ ■ ■

Sodium hydroxide (D)

■ ■ ■ ■■■■■ ■

Sodium hydroxide (L)

■■■■■■■

Sodium hypochlorite (L)

■ ■ ■■■■ ■ ■ ■

Sodium silicate (D)

■

Sodium ﬂuorosilicate (D)

■■ ■■ ■■ ■ ■

Sodium sulﬁte (D)

■■■ ■

Sodium dioxide (G)

■■■■■

Sulfuric acid (L)

■■ ■■■■■ ■ ■

Adapted from Anderson, 2005.

5-20 WATER AND WASTEWATER ENGINEERING

Preventive maintenance includes regularly scheduled times for equipment to be taken out

of service for replacement of worn parts, calibration, and so on. Frequently, this type of work

is scheduled in the winter to take advantage of low flows. In addition, feeders, feed lines, and

instrumentation are to be checked routinely during ea

ch shift.

Good housekeeping includes prompt cleaning of spills and removal of chemical dust.

Although regulatory agencies will dictate that certain records be kept, operation of the plant

often requires more information than is reported. For example, the status of the chemical inven-

tory and the operating performance of each piec

e of the chemical handling and feeding equip-

ment should be logged and conveyed to the next shift operator (Kawamura, 2000).

Hints from the Field. Operation and maintenance personnel who have to live with the results

of the engineer’s design have offered the following suggestions:

• S chedule 80 PVC and CPCV are the most commonly used materials for s

odium hypochlo-

rite piping. Early installation of these systems failed because of leaks at the solvent welded

joints. Special glues designed for use with NaOCl must be used to guard against this type of

failure.

• Q uicklime storage silos should always be c

ylindrical. Because of its hygroscopic nature,

lime will invariably cake in the silo. In one case, the working volume of a 200 Mg square

silo was effectively reduced to 35 Mg. Vibration and other attempts to loosen the caked

material were ineffective. The additional expense of a cylindrical silo will be repaid many

times by the redu

ced O&M costs of trying to loosen the caked lime.

• As shown in Figure 5-6 , place the slaker directly beneath the lime storage silo to minimize

dust in transporting the lime to the slaker.

• Grit in the lime can be removed after slaking by the simple expedient of placing a milk

crate lined with hardware cloth in the exit stream ( Figure 5-6 ).

• Transport the slaked lime to the mixing device with an eductor and flexible hose ( Figure 5-6 ).

Pumps will cake with lime, and rigid pipes will clog. The eductor elim

inates moving parts,

and the flexible pipe makes it easy to spot blockages and either break them in place or quickly

replace a section for out-of-service cleaning.

TABLE 5-7

EPCRA threshold planning quantities

Chemical

Threshold planning

quantity, kg

Chlorine 45

Chlorine dioxide Not listed

Anhydrous ammonia 225

Aqua ammonia Not listed

Hydrogen peroxide (52%) 450

Sulfuric acid 450

Ozone 45