Pollution Prevention and Control Technologies for Plating
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
6.2.4 Metals Removal
126.96.36.199 Metals Removal Process Limitations
6.2.4 Metals Removal
Hydroxide precipitation is the standard method of removing heavy
metals from wastewater. This is achieved by adjusting the pH of
the wastewater with an alkaline reagent to reduce the solubility
of the dissolved metals and settling and removing the resultant
metal hydroxide precipitates. Seventy-five percent of the respondents
to the Users Survey employ this process. The metals removal process
is often divided into four steps: (1) pretreatment; (2) precipitation;
(3) flocculation; and (4) settling. Each of these steps will be
discussed in this subsection, with reference to the methods and
chemicals employed by survey respondents.
Prior to metals precipitation, many shops use "pretreatment"
processes to affect substances that require special treatment
or interfere with subsequent treatments. Common metal complexing
compounds which will cause interference are listed in Exhibit
6-8. Some shops combine this step with the precipitation and/or
flocculation steps. Exhibit 6-9 (PS 064, PS 116, PS 287) shows
several configurations for pretreatment or co-treatment using
various chemicals. Approximately 45% of the survey respondents
use some form of precipitation aid (not including polymer, which
is employed by nearly every shop to aid in flocculation). Chemicals
used by survey respondents prior to or during metals precipitation
include ferrous sulfate, sodium hydrosulfite, aluminum sulfate,
soda ash and sodium dithiocarbamate (DTC). Ferrous sulfate is
sometimes added to function as a reducing agent, but more often
to provide co-precipitation for the removal of metals from chelated
wastewaters. With the latter application, complexed metals are
co-precipitated with ferric hydroxides. While being an effective
metals removal method, this process generates approximately four
times as much sludge as sodium hydroxide alone (ref. 144). Calcium
chloride and aluminum sulfate are typically used during a pretreatment
step (they are often used together1 because the combined results
are better than for either one used alone) for the removal of
fluoride, phosphate, silicates and/or emulsified oil (ref. 38).
Precipitation is the process by which dissolved metals are made
insoluble, usually as metal hydroxides. With metal finishing wastewaters
this is brought about by the addition of alkali treatment reagents.
Two alkalis are most commonly used for hydroxide precipitation:
sodium hydroxide (i.e., NaOH, or, as it is commonly referred to,
caustic soda or simply caustic) and lime (i.e., calcium hydroxide
or hydrated lime, Ca(OH)2). Of the survey respondents that have
metals precipitation processes, 84% use caustic only, 5% use lime
only and 9% use a combination of caustic and lime. Several other
treatment reagents are used as either the sole alkali ingredient
or as an adjunct to caustic or lime. The other primary alkalis
are magnesium hydroxide (used by 4.3% of all respondents that
perform metals precipitation, but not as the sole source of alkali)
and calcium chloride (used by 11.9% of respondents that perform
metals precipitation, but only 2% use it as their sole source
of alkali). Other alkalis used by metal finishers are sodium carbonate
and sodium bicarbonate (ref. 38). When magnesium hydroxide and
calcium chloride are used in conjunction with either caustic or
lime, this is sometimes done as a pretreatment step prior to metals
precipitation, as discussed previously.
Each alkali has advantages and disadvantages. For example, of
the two most common alkalis, hydrated lime has the advantage over
caustic soda of lower cost per unit of neutralizing capacity.
Also, the metal hydroxide precipitants produced with the use of
lime have much faster settling rates because of co-precipitation
of calcium solids. Further, the settled sludge from lime treatment
is higher in solids content and much more amenable to dewatering.
On the other hand, lime takes longer to react in the neutralizer
than caustic soda, has a more complicated feed system and, most
significantly, it generates a considerably higher mass of sludge
solids (ref. 39). Sulfide precipitation, which precipitates metals
as sulfides instead of hydroxides, has been found capable of achieving
low levels of metal solubility in highly chelated wastestreams.
However, sulfide precipitation is not used by any of the respondents
to the Users Survey. This process has been proven as an alternative
to hydroxide precipitation or as a method for further reducing
the dissolved metal concentration in the effluent from a hydroxide
precipitation system. Two processes are used for sulfide precipitation:
the soluble sulfide process uses sodium sulfide as the treatment
reagent, and the insoluble sulfide process uses ferrous sulfide.
Both processes generate metal sulfide sludge and the sludge may
be less amenable to off-site recovery than hydroxide sludge. An
additional drawback of the insoluble sulfide precipitation process
is that it generates a significantly larger volume of sludge compared
with conventional hydroxide treatment. The large sludge volume
is caused by the liberation of ferrous ions from the treatment
reagent. These ions are converted to ferrous hydroxide and add
to the sludge volume (ref. 39). Additional information and data
on treatment reagents can be found in the literature (e.g., ref.
38, 39, 348, and 409).
The metals precipitation process begins with pH adjustment, as
previously shown in Exhibit 6-1. The pH is usually adjusted to
between 8.5 and 10.0, with 9.2 being the most frequent target.
The optimal pH for precipitating metals from a wastestream will
depend mostly on the species of regulated metals present. Each
metal hydroxide has a characteristic solubility that is dependent
on pH. Exhibit 6-10 shows the solubility curves for the hydroxides
of commonly regulated metals (metal sulfides have different curves
with minimum solubilities at much lower concentrations than hydroxides,
ref. 348). The lowest point of each curve corresponds to the pH
at which that metal species will be removed to its minimum solubility
point. For example, copper will be removed to its minimum concentration
if the pH of the precipitation step is held at 9.0. Since the
low points of the curves occur at different pHs, it is necessary
to make a compromise when selecting a target pH (pH control set-point).
Often, the pH control set-point selection process is based on
the need to reach a low concentration for a particular metal species
(e.g., cadmium) that is regulated to a lower concentration than
Exhibit 6-11 shows the process schematic of a single-stage neutralizer
with provisions for adding both acid and base. Since most combined
waste streams are acidic when entering the metals removal step,
many facilities do not provide for the addition of acid. However,
it is useful in instances when caustic has been inadvertently
added in excess and therefore is present in most system designs.
With a single-stage system, proportional control for reagent addition
is required to maintain a reasonably constant effluent pH. The
elevation of the reagent storage and gravity feeding through a
proportioning valve is one mode of control. Use of variable speed
pumps will achieve the same level of control. If incoming wastes
are subjected to wide swings in reagent demand, a two-stage neutralization
system should be used. The first stage would control at a pH of
approximately 6 to 7 (ref. 39).
Required residence volume in the neutralizer depends on the reagent
used. Assuming good mixing, a minimum of 15 minutes residence
volume is required for sodium hydroxide; with lime as the alkali,
a minimum of 30 minutes is required (ref. 39).
The level of residual dissolved metals after pH adjustment depends
on the pH control set-point, the mixture of metals in the wastewater,
and whether any compounds are present that interfere with metal
Nearly every metals removal system uses chemical aids to foster
particle growth before the wastewater enters the clarifier. By
adding coagulating/flocculating agents in slow-mix reactors, the
solids in a wastewater can be agglomerated into sturdy, fast-settling
particles easily separated in the clarifier. Coagulants/flocculants
in broad commercial use include inorganic chemicals such as alum
and ferrous sulfate (previously discussed as pretreatments and
co-precipitants) and a highly diverse range of organic polyelectrolytes
with varying characteristics suitable for different wastewaters.
Organic polyelectrolytes are superior to inorganic compounds because
both the charge density and valence have been synthetically introduced
to the large polymer molecule. The length of the polymer also
allows the particles to "knit" together. Normally the
appropriate polyelectrolyte is selected by testing a range of
different polymers and observing settling behavior after mixing.
Removal of solids by gravity settling (clarification) is the most
common method of separating insoluble particles from a waste stream
before discharge. Clarification is a relatively simple process
that relies on a density difference between the particles and
water and the presence of gravity. However, it is often the unit
operation of the waste treatment process most subject to upsets.
With effluent limits placing strict control on the level of suspended
solids in the wastewater, many modifications to the original circular
clarifier have resulted from research and development activities.
The two most successful approaches include the sludge blanket
clarifier and the plate settler. In the sludge blanket unit, the
clarifer inlet first passes through a sludge blanket of agglomerated
particles. The mixing tends to promote particle growth and reduce
the concentration of slow-settling particle fines. The plate settler
(Exhibit 6-12) relies on a series of inclined plates between which
the wastewater flows in an upward direction. In essence, the particles
must only settle a few inches before impinging on the plate surface.
The particles then slide down the plate surface to the base of
the separator. In a chamber of equal size, plate settlers can
provide considerably greater effective settling volume than a
Performance of clarifiers varies significantly depending on the
type of waste, the design of upstream components, and the design
of the clarifier itself. According to EPA, a "properly operating
system" has a total suspended solids (TSS) concentration
of 50 mg/l or less and the mean concentration of such systems
is 16.8 mg/l TSS (ref. 386). A well-operated clarifier will have,
at best, 5 to 10 mg/l of suspended solids in the overflow (ref.
477). Frequently, turbidity in the overflow contributes to the
metals content of the discharge and makes strict pollutant guidelines
difficult to meet. Consequently, many advanced waste treatment
systems employ a polishing filter that uses a sand bed or mixed
media filter to remove suspended solids not effectively removed
by clarification (ref. 39). Another option is the use of membrane
filtration, which either supplements or replaces the clarification
step (see Section 6.4.4).
Granular media polishing filters are used for the removal of suspended
solids in the 5 to 50 mg/l range where an effluent of less than
1 Jackson Turbidity Unit2 (JTU) is desired (ref. 362). These units
typically contain graded sand or multimedia such as garnet, ground
anthracite and silica sand. The principal solids removal mechanism
of these units is straining. When a single medium such as sand
is employed, it will classify in the filtration tank according
to size, with the smallest grains at the top. When water flows
downward through the sand, which was common with older designs,
the solids would form a mat on the surface and filtration only
occurred in the top few inches. The sand bed was then cleaned
by an upward washing of the bed with water or with water and air
(ref. 362). A patented alternative design permits upward feed
flow and a continuous backwash of the sand bed as described in
Multimedia filters employ two or more filter media with different
grain size and densities. The media are selected such that the
smaller particles are the most dense (e.g., garnet with a specific
gravity of 4.5 and particle size of 0.2 to 0.4 mm), the medium-sized
particles have an average density (e.g., sand grains with a specific
gravity of 2.65 and a grain size of 0.5 mm) and the largest particles
are the least dense (e.g., anthracite grains with a specific gravity
of 1.6 and a grain size of 1.0 mm). When mixed and permitted to
settle, the multimedia bed will grade itself according to the
density of the material. Therefore, the smallest particles will
locate at the bottom and the largest particles at the top. When
the feed stream flows from top to bottom, the courser suspended
solids will be removed in the upper layers of the filter and the
smaller suspended solids near the bottom. Backwashing is performed
in the opposite direction.
The results of the Users Survey indicate that 58 respondents (or
19.3%) employ polishing filters.
Some respondents to the Users Survey use alternative methods of
removing metal hydroxides rather than using standard clarification.
PS 036 and PS 168 pump the entire volume of treated wastewater
containing precipitated metals through a filter press (use of
the filter press is normally reserved for settled sludge). Both
of these shops have wastewater flow rates below 3,000 gpd, which
is the reason they are able to use this method of separation.
As indicated by Roy (ref. 38), when direct filtration is used,
the gelatinous character and the shear bulk of solids results
in premature blinding or blockage of the filter, causing minimal
hydraulic throughput. One respondent to the survey uses dissolved
air flotation (DAF) rather than settling (PS 240). A DAF unit
generates fine air bubbles that attach to the hydroxide particles
and cause them to rise to the surface of a tank, where they are
removed by skimming. It is a treatment technology commonly used
by industries with oily wastestreams (e.g., petroleum refining
and industrial laundries). Several shops use membrane filtration
to separate precipitated metals. This method of treatment is a
viable alternative to conventional clarification for plating shops,
but currently is more common to the printed circuit board industry.
Microfiltration is discussed in Section 6.5.3.
188.8.131.52 Metals Removal Process Limitations
As a summary for the conventional metals removal subsection, the
following limitations of the hydroxide precipitation/clarification
process are presented:
The process cannot precipitate metals to low levels of solubility
in the presence of chelating compounds unless additional treatment
reagents are employed. The use of additional treatment reagents
may significantly add to the volume of sludge generated by the
The precipitation/clarification process cannot remove suspended
solids below approximately 5 to 10 mg/l TSS and TSS can range
up to 50 mg/l for a well operated system unless polishing filtration
is employed. Because the suspended solids remaining in the effluent
contain metals, this factor limits the attainable effluent concentration
Certain metal hydroxides are amphoteric; that is, solubility is
at a minimum at a specified pH and increases if pH is higher or
lower. The solubility minimum for different metals occurs at different
pH levels (ref. 39). Therefore, a compromise is necessary with
mixed metal wastes that limits attainable effluent concentrations.
The metal hydroxide sludge resulting from treatment of electroplating
wastewater has been classified as a hazardous waste (F006) and
must be managed, handled, transported, and recovered or disposed
of under the provisions of RCRA.
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