vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA832-F-00-018
September 2000
Waste water
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 case-
by-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
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coarse floe 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, non-
polar 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, particle-
like aggregations of compatibly charged molecules
existing in an incompatible aqueous
medium—which is to say that these molecules are
hydrophobia ("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 pH-
dependent. 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.
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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 - AI2(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 - FeCI3
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.
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
Chemical precipitation is a well-established
technology with ready availability
equipment and many chemicals.
of
Some treatment chemicals, especially lime,
are very inexpensive.
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
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.
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.
Chemical precipitation may require working
with corrosive chemicals, increasing operator
safety concerns.
The addition of treatment chemicals,
especially lime, may increase the volume of
waste sludge up to 50 percent.
Large amounts of chemicals may need to be
transported to the treatment location.
• Polymers can be expensive.
Table 2 provides a summary of properties and
considerations appropriate to chemicals commonly
used for precipitation.
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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 Ib bags, to bulk
quantities of 4000 Ibs. 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.
FeCI3 Ferric chloride, or FeCI3, is available in
either dry (hydrate or anhydrous) or liquid
form. The liquid form is usually 35-45%
FeCI3. Because higher concentrations of
FeCI3 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.
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.
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TABLE 3 TYPES OF FEED MECHANISMS
WITHIN VOLUMETRIC AND
GRAVIMETRIC FEEDERS
Dry Feed Mechanism
Feed Rate (Ibs/hr)
Rotating disk
Oscillating
Rotary gate
Belt
Screw*
10
10-100
200 - 500
500-20,000
10-24,000
* Typically for volumetric feeders
Source: Benefield et al, 1990.
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, etcil,
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.
TABLE 4 COMPARATIVE TREATMENT
COSTS
Chemical
Ferrous sulfate
Dithiocarbamate
Borohydride
Aluminum
Chemical
Cost/I b
$0.17
$0.95
$2.86
$0.50
Treatment
Cost/gal
$1.03
$0.82
$0.76
$0.04
Source: Hewing, et al., 1995
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
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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:
• Occasional flushing of the system, if this is
not provided automatically.
Inspecting and replacing pump seals, bags,
dust filters, pH and ion specific electrodes,
and other components..
Periodically lubricating bearings, motor, and
other moving parts.
• 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).
TABLE 5 COSTS OF SELECTED
CHEMICALS
Product Description
Cost (per Ton)
Aluminum sulfate, liquid, in tanks, 269
iron-free
Aluminum sulfate, liquid, in tanks, 152
NOT iron-free
Aluminum sulfate, dry, 100 Ib 250
bags, iron-free
Aluminum sulfate, dry, 100 Ib 245 - 280
bags, NOT iron-free
Ferric chloride, technical grade, in 255 - 300
tanks
Ferrous sulfate, monohydrate, 222.50 - 240
granulated, bulk
Lime, chemical, hydrated, bulk 70
Source: 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
Ibs of lime per hour and include storage for up to
60,000 Ibs 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
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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 4th Ed. McGraw-
Hill, 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 4th Ed. McGraw-
Hill, 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 Waste-
water 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
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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.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, D.C., 20460
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