United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-00-018

September 2000

Wastewater

Technology Fact Sheet

Chemical Precipitation

DESCRIPTION

Chemical precipitation is a widely used, proven

technology for the removal of metals and other

inorganics, suspended solids, fats, oils, greases, and

some other organic substances (including

organophosphates) from wastewater. Generally

speaking, precipitation is a method of causing

contaminants that are either dissolved or suspended

in solution to settle out of solution as a solid

precipitate, which can then be filtered, centrifuged,

or otherwise separated from the liquid portion. A

voluminous precipitate can capture ions and

particles during formation and settling, in effect

“sweeping” ions and particles from the wastewater.

(Tchobanoglous and Burton, 1991). Precipitation

is assisted through the use of a coagulant, an agent

which causes smaller particles suspended in

solution to gather into larger aggregates.

Frequently, polymers are used as coagulants. The

long-chain polymer molecules can be either

positively or negatively charged (cationic or

anionic) or neutral (nonionic). Since wastewater

chemistry typically involves the interaction of ions

and other charged particles in solution, these

electrical qualities allow the polymers to act as

bridges between particles suspended in solution, or

to neutralize particles in solution (Amirtharajah and

O’Mella, 1990; Jacangelo, 1987). The specific

approach used for precipitation will depend on the

contaminants to be removed, as described below.

Metals Removal

Water hardness is caused primarily by the

dissolution of calcium and magnesium carbonate

and bicarbonate compounds in water, and to a lesser

extent, by the sulfates, chlorides, and silicates of

these metals. The removal of these dissolved

compounds, called water softening, often proceeds

by chemical precipitation. Lime (calcium oxide),

when added to hard water, reacts to form calcium

carbonate, which itself can act as a coagulant,

sweeping ions out of solution in formation and

settling. To do this with lime alone, a great deal of

lime is typically needed to work effectively; for this

reason, the lime is often added in conjunction with

ferrous sulfate, producing insoluble ferric

hydroxide. The combination of lime and ferrous

sulfate is only effective in the presence of dissolved

oxygen, however. Alum, when added to water

containing calcium and magnesium bicarbonate

alkalinity, reacts with the alkalinity to form an

insoluble aluminum hydroxide precipitate.

Soluble heavy metal ions can be converted into

insoluble metal hydroxides or carbonates through

the addition of hydroxide compounds.

Additionally, insoluble metal sulfides can be

formed with the addition of ferrous sulfate and

lime. Once rendered insoluble, these compounds

will tend to precipitate and settle. The solubility of

the metal compounds thus formed is pH dependent;

most tend to be least soluble in alkaline solutions.

Since the optimal pH for precipitation depends

both on the metal to be removed and on the counter

ion used (hydroxide, carbonate, or sulfide), the best

treatment procedure must be determined on a caseby-case basis. Metal solubility data are available in

Benefield and Morgan, 1990, as well as in many

other sources.

Once the optimal pH for precipitation is

established, the settling process is often accelerated

by addition of a polymer coagulant, which gathers

the insoluble metal compound particles into a
coarse floc that can settle rapidly by gravity.

Removal of Fats, Oils and Greases

Fats, oils, and greases are typically organic

substances which tend to bead together or form

“slicks” on the surface of aqueous solutions. They

behave in this way because these organic, nonpolar substances are typically insoluble in water,

which is inorganic and polar. Because they tend to

be less dense than water, they float to the surface

rather than settling to the bottom. In situations

where the oily substance is free floating in slicks,

skimming the surface of the solution is often the

best way to remove most of the material. However,

oils, fats, and greases can become emulsified in

aqueous solution, meaning that small globules of

the oily product can become suspended throughout

the water. These globules are localized, particlelike aggregations of compatibly charged molecules

existing in an incompatible aqueous

medium—which is to say that these molecules are

hydrophobic (“water-fearing”). Often times, other

substances (especially products like soaps and

detergents) in solution can act as aids to making

hydrophobic substances soluble in water.

To remove emulsified oils and greases, the

emulsion must be brokenup by destabilizing the

electrical charge attractions that keep the localized

clusters of oily molecules stable in solution. This

can be done with the addition of a polymer

designed for charge neutralization. In this way, the

charge attraction of the oily particles is disrupted,

allowing them to separate from the aqueous

solution.

Phosphorus Removal

Metal salts (most commonly ferric chloride or

aluminum sulfate, also called alum) or lime have

been used for the removal of phosphate compounds

from water. When lime is used, a sufficient amount

of lime must be added to increase the pH of the

solution to at least 10, creating an environment in

which excess calcium ions can react with the

phosphate to produce an insoluble precipitate

(hydroxylapatite). Lime is an effective phosphate

removal agent, but results in a large sludge volume.

When ferric chloride or alum is used, the iron or

aluminum ions in solution will react with phosphate

to produce insoluble metal phosphates. The degree

of insolubility for these compounds is pHdependent. Moreover, many competing chemical

reactions can take place alongside these, meaning

that the amount of metal salt to add to the solution

cannot simply be calculated on the basis of the

phosphate concentration, but must be determined in

the laboratory for each case (Tchobanoglous and

Burton, 1991).

Suspended Solids

Finely divided particles suspended in solution can

elude filtration and other similar removal processes.

Their small size allows them to remain suspended

over extended periods of time. More often than

not, the particles populating wastewater are

negatively charged. For this reason, cationic

polymers are commonly added to the solution, both

to reduce the surface charge of the particles, and

also to form bridges between the particles, thus

causing particle coagulation and settling

(Tchobanoglous and Burton, 1991).

Alternatively, lime can be used as a clarifying agent

for removal of particulate matter. The calcium

hydroxide reacts in the wastewater solution to form

calcium carbonate, which itself acts as a coagulant,

sweeping particles out of solution.

Additional Considerations

The chemical agents most frequently used for

chemical precipitation are shown in Table 1. The

amount of chemicals required for treatment depends

on the pH and alkalinity of the wastewater, the

phosphate level, and the point of injection and

mixing modes, among other factors. Competing

reactions often make it difficult to calculate the

quantities of additives necessary for chemical

precipitation. Accurate doses should be determined

by jar tests and confirmed by field evaluations.

Chemicals are usually added via a chemical feed

system that can be completely enclosed and may

also include storage space for unused chemicals.

Choosing the most effective coagulant depends on

jar test results, ease of storage, ease of

transportation, and consideration of the operation

and maintenance costs for associated equipment.
Although chemical precipitation is a well

established treatment method, research continues to

enhance its effectiveness. Much recent research

concentrates on combining chemical precipitation

with other treatment methods such as

photochemical oxidation, reverse osmosis, and

biological methods to optimize performance.

APPLICABILITY

Chemical precipitation can be used to remove

contaminants from both municipal and industrial

wastewaters. It can be used for water softening,

heavy metal removal from metal plating wastes, oil

and grease removal from emulsified solutions, and

phosphate removal from wash-waters and other

wastewater. It is an effective tool for wastewater

polishing and removal of particulate matter.

ADVANTAGES AND DISADVANTAGES

Before deciding whether chemical precipitation

meets the needs of a municipality, it is important to

understand the advantages and disadvantages of this

methodology. 

Advantages

C Chemical precipitation is a well-established

technology with ready availability of

equipment and many chemicals.

C Some treatment chemicals, especially lime,

are very inexpensive.

C Completely enclosed systems are often

conveniently self-operating and low

maintenance, requiring only replenishment of

the chemicals used. Often times, a

sophisticated operator is not needed.

Disadvantages

C Competing reactions, varying levels of

alkalinity and other factors typically make

calculation of proper chemical dosages

impossible. Therefore, frequent jar tests are

necessary for confirmation of optimal

treatment conditions. Overdosing can

diminish the effectiveness of the treatment.

C Chemical precipitation may require working

with corrosive chemicals, increasing operator

safety concerns.

C The addition of treatment chemicals,

especially lime, may increase the volume of

waste sludge up to 50 percent.

C Large amounts of chemicals may need to be

transported to the treatment location.

C Polymers can be expensive.

Table 2 provides a summary of properties and

considerations appropriate to chemicals commonly

used for precipitation.

TABLE 1 CHEMICALS USED IN

WASTEWATER TREATMENT

Lime – Calcium Oxide, CaO

Produces calcium carbonate in wastewater which acts as

a coagulant for hardness and particulate matter. Often

used in conjunction with other coagulants, since: (1) by

itself, large quantities of lime are required for

effectiveness, and (2) lime typically generates more

sludge than other coagulants.

Ferrous Sulfate – Fe(SO4)3

Typically used with lime to soften water. The chemical 

combination forms calcium sulfate and ferric hydroxide. 

Wastewater must contain dissolved oxygen for reaction

to proceed successfully.

Alum or Filter Alum – Al2(SO4)3.14H2O

Used for water softening and phosphate removal. 

Reacts with available alkalinity (carbonate, bicarbonate

and hydroxide) or phosphate to form insoluble aluminum

salts.

Ferric Chloride – FeCl3

Reacts with alkalinity or phosphates to form insoluble

iron salts.

Polymer

High molecular weight compounds (usually synthetic)

which can be anionic, cationic, or nonionic. When added

to wastewater, can be used for charge neutralization for

emulsion-breaking, or as bridge-making coagulants, or

both. Can also be used as filter aids and sludge

conditioners.

Source: U.S. EPA, 1980.
DESIGN CRITERIA

Chemical precipitation is normally carried out

through a chemical feed system, most often a totally

automated system providing for automatic chemical

feeding, monitoring, and control. Full automation

reduces manpower requirements, allows for less

sophisticated operator oversight, and increases

efficiency through continuous operation.

An automatic feed system may consist of storage

tanks, feed tanks, metering pumps (although

pumpless systems do exist), overflow containment

basins, mixers, aging tanks, injection quills, shot

feeders, piping, fittings, and valves.

Chemical feed system storage tanks should have

sufficient capacity to run for some time without

running out and causing downtime. At least a one

month supply of chemical storage capacity is

recommended, though lesser quantities may be

justified when a reliable supplier is located nearby,

thus alleviating the need for maintaining substantial

storage space. Additive chemicals come in liquid

and dry form (see Table 2). Figure 1 shows a

simplified flow chart of a chemical feed system.

Source: Adapted from U.S. EPA, 1980.

FIGURE 1 FLOW DIAGRAM OF A

CHEMICAL FEED SYSTEM

When working with dry chemicals, a volumetric

feeder or a gravity feeder can be used to measure

the amount of chemical to be dissolved in water.

Gravimetric feeders measure the chemical as a

weight per unit time; volumetric feeders, by

contrast, measure the chemical volume per unit

time. While gravimetric feeders are more

expensive than volumetric ones, they are also more

accurate. Even so, volumetric feeding systems are

more commonly used. In either case, the type of

feeding mechanism required depends on the feed

rate anticipated. Table 3 summarizes the types of

feeding mechanisms available with associated feed

rates.

In choosing a feed system, one must be certain that

the materials used to build the system are

chemically compatible with the chemicals to be

used. Equipment manufacturers’ chemical

resistance charts should be used in selecting

appropriate construction materials.

TABLE 2 COMMERCIAL FORMS OF

CHEMICAL PRECIPITATION CHEMICALS

Chemical Commercial Characteristic

Alum Alum is an off-white crystal which, when

dissolved in water, produces acidic

conditions. As a solid, alum may be

supplied in lumps, but is available in

ground, rice, or powdered form. Shipments

range from small 100 lb bags, to bulk

quantities of 4000 lbs. In liquid form, alum

is commonly supplied as a 50% solution

delivered in minimum loads of 4000 gal.

The choice between liquid and dry alum

depends on the availability of storage

space, the method of feeding, and

economics.

FeCl3 Ferric chloride, or FeCl3, is available in

either dry (hydrate or anhydrous) or liquid

form. The liquid form is usually 35-45%

FeCl3. Because higher concentrations of

FeCl3 have higher freezing points, lower

concentrations are supplied during the

winter. It is highly corrosive.

Lime Lime can be purchased in many forms, with

quicklime (CaO) and hydrated lime

(Ca(OH)2) being the most prevalent forms.

In either case, lime is usually purchased in

the dry state, in bags, or in bulk.

Polymer Polymers may be supplied as a prepared

stock solution ready for addition to the

treatment process or as a dry powder.

Many competing polymer formulations with

differing characteristics are available,

requiring somewhat differing handling

procedures. Manufacturers should be

consulted for recommended practices and

use.

Source: U.S. EPA, 1980. 

Water

Feeder Dissolver Pump

Holding

Tank

Wet

Chemical

Dry

Chemical

Storage

Mixer
PERFORMANCE

Jar Testing

For any given wastewater, the optimal treatment

strategy should be determined by jar testing.

Commercial chemical vendors provide testing

guidelines to determine the most appropriate

chemical(s) and the most effective dosage.

Laboratory bench-scale jar testing apparatuses

(available through scientific product supply

companies) typically allow for six samples, each

one liter in size, to be tested simultaneously. One

central control operates the mixing of all jars, hence

one variable (for example, polymer dosage) can be

manipulated in a test group while all other factors,

including mixing rates and times, can be kept

constant. Inconclusive and incorrect interpretation

of the results may be the result of using too small a

sample for stock solution (1-2 ml of stock solution

equals a 1-20 percent error in final concentration),

adding chemicals inconsistently, erroneous data

recording, using old chemicals, choosing improper

flocculation and settling conditions (time, duration),

using different people to perform tests, and

choosing too narrow a dosage range. (Molina, et al,

1998).

DoD Facility Reduces Plating Waste and

Reduces Costs Using Chemical Precipitation. 

A recent study of a Department of Defense (DoD)

facility demonstrated the ability to use chemical

precipitation to reduce plating waste and costs

(Hewing, et al., 1995). The decision to treat

concentrated plating waste as well as to dilute

wastes already being treated at the facility was cost

driven. Chemical precipitation was selected over

reverse osmosis, demineralization, electrodialysis

reversal, evaporation, and electrolytic precipitation,

primarily because of space limitations. The

decision also reflected sludge disposal costs,

equipment requirements and cost, and safety

considerations.

The facility compared the performance of ferrous

sulfate, dithiocarbamate, borohydride, and

aluminum as precipitating additives. Treatment

was needed for waste streams containing high

concentrations of potassium persulfate, copper

sulfate, ammonium chloride/hydroxide, and

pyrophospate, with copper concentrations ranging

from 20,000 to 150,000 ppm. Persulfates

accounted for more than half of the generated waste

and presented numerous safety challenges since

they are strong oxidizers.

Table 4 shows the final treatment costs for the four

chemicals studied. Overall treatment costs included

initial chemical costs, stoichiometric volumes, and

dry sludge weight.

Ferrous sulfate generated large sludge volumes,

making it too expensive for consideration.

Dithiocarbamate produced less sludge but presented

an occupational health hazard due to generation of

noxious carbon disulfide. Sodium borohydride

TABLE 3 TYPES OF FEED MECHANISMS

WITHIN VOLUMETRIC AND

GRAVIMETRIC FEEDERS

Dry Feed Mechanism Feed Rate (lbs/hr)

Rotating disk 10

Oscillating 10 - 100

Rotary gate 200 - 500

Belt 500 - 20,000

Screw* 10 - 24,000

* Typically for volumetric feeders

Source: Benefield et al, 1990.

TABLE 4 COMPARATIVE TREATMENT

COSTS

Chemical Chemical

Cost/lb

Treatment

Cost/gal

Ferrous sulfate $0.17 $1.03

Dithiocarbamate $0.95 $0.82

Borohydride $2.86 $0.76

Aluminum $0.50 $0.04

Source: Hewing, et al., 1995
produced less sludge but presented a safety problem

due to its chemical properties.

Aluminum presented no occupational health

problems, generated the least amount of sludge, and

resulted in a final wastewater concentration of 5-10

ppm copper. The overall cost was $0.04 per gallon.

Further testing helped identify optimum conditions

for the use of chemical precipitation by the facility

using aluminum.

OPERATION AND MAINTENANCE

A routine O&M schedule should be developed and

implemented for any type of bulk chemical

feed/handling system. Many systems are now

completely enclosed, factory mounted/piped/wired

systems. All manufacturer O&M recommendations

should be followed, including testing and

calibration. Regular O&M includes the following:

C Occasional flushing of the system, if this is

not provided automatically.

C Inspecting and replacing pump seals, bags,

dust filters, pH and ion specific electrodes,

and other components..

C Periodically lubricating bearings, motor, and

other moving parts.

C Developing an emergency response plan for

onsite storage of chemicals. 

When using chemical precipitation to treat

wastewater, it is important to properly and safely

store all chemicals. For further details on the safe

use and storage of chemicals, refer to Material

Safety Data Sheets (MSDSs) provided by the

chemical manufacturers.

COSTS

The overall cost of chemical precipitation depends

on many variables, including the characteristics of

the wastewater, the chemicals and dosages to be

used, the volume of water to be treated, and the

level of water purity desired. Moreover, chemical

costs can vary widely depending on the form and

quantity of material to be procured. Material prices

fluctuate according to the region of the country. 

Chemicals provided in bags or measured batches

are more expensive than when purchased in bulk

quantities. 

Table 5 summarizes some chemical prices as of

January, 2000, as reported by the Chemical Market

Reporter (ChemExpo, 2000).

It should be noted, however, that estimation of

treatment cost cannot be determined solely on the

price of the chemicals. For example, one study

found that while it was less expensive to purchase

alum than ferric sulfate, overall treatment costs

were less using ferric sulfate (Hook et al, 1992). 

Generally speaking, lime is readily available and

the least expensive of common treatment options.

A completely enclosed lime chemical handling

system costs between $110,000 and $130,000, with

the lower prices reflecting gravity versus pump

systems. The more expensive lime systems include

a slaker to convert calcium oxide to calcium

hydroxide. These systems can consume up to 2,000

lbs of lime per hour and include storage for up to

60,000 lbs of lime. The trade-off in using lime,

however, is in the large sludge volumes that result

from its use. For this reason, lime is often used in

tandem with more expensive additives. These

TABLE 5 COSTS OF SELECTED

CHEMICALS

Product Description Cost (per Ton)

Aluminum sulfate, liquid, in tanks,

iron-free

269

Aluminum sulfate, liquid, in tanks,

NOT iron-free

152

Aluminum sulfate, dry, 100 lb

bags, iron-free

250

Aluminum sulfate, dry, 100 lb

bags, NOT iron-free

245 - 280

Ferric chloride, technical grade, in

tanks

255 - 300

Ferrous sulfate, monohydrate,

granulated, bulk

222.50 - 240

Lime, chemical, hydrated, bulk 70

Source: ChemExpo, 2000.
trade-offs must be kept in mind when performing

jar tests and developing the optimal treatment

strategy for each situation.

REFERENCES

Other Related Fact Sheets

Other EPA Fact Sheets can be found at the

following web address:

http://www.epa.gov/owmitnet/mtbfact.htm

1. U.S. EPA, 1980. Innovative and

Alternative Technology Assessment Manual.

U.S. EPA 430/9-78-009 Washington, D.C.

2. Amirtharajah, Appiah and O’Mella, Charles

R., 1990. Coagulation Processes:

Destabilization, Mixing, and Flocculation.

Water Quality and Treatment, A Handbook

of Community Water Supplies. ed, Pontius,

Frederick W., AWWA 4

th Ed. McGrawHill, Inc. NY. 

3. Benefield, Larry D. and Morgan, Joe M.,

1990. Chemical Precipitation. Water

Quality and Treatment, A Handbook of

Community Water Supplies. ed, Pontius,

Frederick W., AWWA 4

th Ed. McGrawHill, Inc. NY. 

4. ChemExpo, 2000. Online Chemical

Exposition, Archives.

http://www.chemexpo.com/news/

newsframe.cfm?framebody=/

news/archives.cfm. May 2000.

5. Dowbiggin, William B.; Richardson,

Michael; Langley, Ricky, Management

Perspectives of 50 Water Treatment Plant

Superintendents. In AWWA Proceedings,

1998 Annual Conference, NY.

6. Hammer, Mark J., 1995. Water and Wastewater Technology. John Wiley & Sons,

Inc., New York.

7. Hewing, Alvin N.; Nethercutt, Richard, and

Andrews, Ted, 1995. “Reducing Plating

Line Metal Waste.” Pollution Engineering.

November 15, 1995.

8. Hook, Mark A.; Habraken, Joseph;

Gianatasio, James; Hjersted, Lawrence J.;

Benefits of Enhanced Coagulation for

Improved Water Quality and Beneficial

Reuse of Residual Materials. In AWWA

Proceedings, 1992 Annual Conference, NY.

9. Jancangelo, J.G.; Demarco, J.; Owen, D.M.;

Randtke, S.J. 1987. Selected Processes for

Removing NOM: An Overview. Journal of

the AWWA. 87(1):64 - 77.

10. U.S. EPA Guidance Manual for Enhanced

Coagulation and Enhanced Precipitive

Softening. Pirnie (Macolm), Inc.

September 1993, NY.

11. 1994. Drew Principles of Industrial

Wastewater Treatment. 11th ed. Ashland

Chemical Company, NJ.

12. Molina, Servando; West, Tom; Daniel,

Phillippe; Wong, Alan; Sebastiani, Enio;

Labonte, Julie; Manouchehr, Boozarpour

1998. Refining Jar Test Procedures to

Improve Coagulant Optimization. In

AWWA Proceedings, 1998 Annual

Conference, NY.

13. Tchobanoglous, George and Burton,

Franklin L., 1991. Wastewater Engineering

Treatment, Disposal, Reuse Metcalf and

Eddy Inc. 3rd ed. 

ADDITIONAL INFORMATION

Greg Falk 

US Filter, Inc.

301 West Military Road

Rothschild, WI 54474 
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

1200 Pennsylvania Avenue, NW

Washington, D.C., 20460

Chris Litz 

Ashland Chemical Company

Drew Industrial Division

One Drew Plaza 

Boonton, NJ 07005 

Richard Taylor 

Environmental Solutions

1339-165 Bennett Dr.

Longwood, FL 32750

The mention of trade names or commercial

products does not constitute endorsement or

recommendation for use by the U.S. Environmental

Protection Agency.