United States
                        Environmental Protection
                        Office of Water
                        Washington, D.C.
September 2000
Waste water
Technology  Fact Sheet
Chemical  Precipitation

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

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

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.


 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

 Ferric Chloride - FeCI3

 Reacts with alkalinity or phosphates to form insoluble
 iron salts.


 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
Source: U.S. EPA, 1980.

Before  deciding whether chemical  precipitation
meets the needs of a municipality, it is important to
understand the advantages and disadvantages of this
      Chemical precipitation is a well-established
      technology  with  ready   availability
      equipment and many chemicals.
      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.
      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.


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.


 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

 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
Source: U.S. EPA, 1980.

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.


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

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.

 Dry Feed Mechanism
Feed Rate (Ibs/hr)
 Rotating disk


 Rotary gate




    200 - 500


* Typically for volumetric feeders
Source: Benefield et al, 1990.


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,

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

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
Ferrous sulfate
Cost/I b
                         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

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.


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.


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

Table 5 summarizes some chemical prices as of
January, 2000, as reported by the Chemical Market
Reporter (ChemExpo, 2000).

 Product Description
Cost (per Ton)
 Aluminum sulfate, liquid, in tanks,    269
 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
 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

trade-offs must be kept in mind when performing
jar  tests  and developing the optimal  treatment
strategy for each situation.


Other Related Fact Sheets

Other EPA Fact Sheets can be found at the
following web address:

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.
      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.


Greg Falk
US Filter, Inc.
301 West Military Road
Rothschild, WI 54474

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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