vvEPA
United States
Environmental Protection
Agency
Technology Transfer
Industrial Environmental Research
j Laboratory
i Cincinnati OH 45268
- 003
Summary Report
Control and Treatment
Technology for the
Metal Finishing Industry
Sulfide Precipitation
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Technology Transfer
EPA 625/8-80-003
Summary Report
Control and Treatment
Technology; for the
Metal Finishing Industry
Sulfide Precipitation
April 1980
This report was developed by; the
Industrial Environmental Research Laboratory
Cincinnati OH 45268 i
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Sludge dewatering filter press
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Overview
Today more than 13,000 firms in
the United States are engaged either
wholly or partially in electroplating
or other metal finishing opera-
tions. These firms discharge their
spent process water to either water-
ways or publicly owned treatment
works (POTW's)i and they comprise
more individual wastewater dis-
charges than any other industrial
category. The pollutants contained
in this process water are potentially
toxic; therefore, to comply with
the Clean WaterjAct of 1977 (Public
Law 95-217), th'e water must
be treated beforp being discharged
to a waterway or a POTW. The
regulations require oxidation of
cyanide, reduction of hexavalent
chromium, removal of heavy metals,
and control of piH.
Sulfide precipitation is one among
many methods available for
removing metals from metal finish-
ing process wastewaters. This
summary report series presents
information on various technologies
that have been demonstrated. Other
publications in the series discuss
different control; alternatives. By
providing process descriptions,
advantages and 'disadvantages, and
economic characteristics of each
system, these reports can facilitate
the evaluation of effective means
of pollution control by those
involved in metal finishing waste-
water pollution control.
Metals are usually removed by
adding an alkali,; such as hydrated
lime [Ca(OH)2] or caustic soda
(NaOH), to adjust the pH of the
wastewater to the point where the
metals exhibit minimum solubilities.
The metals precipitate as metal
hydroxides and can be removed from
the wastewater by flocculation
and clarification; In many cases,
the addition of a postfiltration
step can reduce ;further the total
metal concentration in the effluent
by removing any metal hydroxide
carryover. I
Some common limitations of the
hydroxide process follow:
• The theoretical minimum solubil-
ities for different metals occur
at different pH values (Figure 1).
For mixtures of metal ions, it
must be determined whether a
single pH can produce sufficiently
low, though not minimum,
solubilities for the metal ions
present in the wastewaters.
• Because hydroxide precipitates
tend to resolubilize if the solu-
tion pH is increased or decreased
from their minimum solubility
points, maximum removal
efficiency will not be achieved
unless the pH is controlled within
a narrow range.
• The presence of complexing
ions—such as phosphates,
tartrates, EDTA,1 and ammonia—
that are commonly found in
cleaner and plating formulations
may have an adverse effect on
metal removal efficiencies when
hydroxide precipitation is used.
Figure 2 shows the solubility
of nickel ions as a function of pH
when precipitated with other
metal ions in the presence of
certain complexing ions used in
a proprietary electroless nickel
plating bath.
Despite these limitations, hydroxide
precipitation (particularly when
followed by flocculation and
filtration) produces a high-quality
effluent when applied to many
waste streams. Often coprecipita-
tion of a mixture of metal ions will
result in residual metal solubilities
lower than those that could be
achieved by precipitating each
metal at its optimum pH. In other
cases, modification of the hydroxide
process has improved its perform-
ance in treating waste streams
containing complexed heavy metals.
This improved performance is
usually realized by dissolving
another positively charged ion—
such as Fe+2 or Ca+2—into the
wastewater and then precipitating
the metals. High-pH lime treatment
1 Ethylenediaminetetraacetic acid.
-------
100 I—
10
1
o
o
z
1
1.0
0.1
0.05
I I I I
89 10 11 12
pH
SOURCE: U.S. Environmental Protection Agency, Waste Treatment: Upgrading Metal-
Finishing Facilities To Reduce Pollution, EPA 625/3-73-002, July 1 973.
Figure 1.
Metal Solubility as a Function of pH
10 r
=• 8
z
a
UJ
3 4
O
tn
ta
o
2
8
pH
10
Figure 2.
Solubility of Complexed Nickel When Precipitated With Caustic Soda
and ferrous sulfate (FeS04) pre-
cipitation techniques use this
principle.
Sulfide precipitation has been
demonstrated to be an effective
alternative to hydroxide precipita-
tion for removing various heavy
metals from industrial wastewaters.
The high reactivity of sulfides
(S~2, HS~) with heavy metal ions
and the insolubility of heavy metal
sulfides over a broad pH range
are attractive features compared with
the hydroxide precipitation process
(Figure 3). Sulfide precipitation
can also achieve low metal solu-
bilities in the presence of certain
complexing and chelating agents.
The main difference between the
two processes that currently
use sulfide precipitation is the
means of introducing the sulfide
ion into the wastewater. In the
soluble sulfide precipitation (SSP)
process, the sulfide is added in the
form of a water-soluble sulfide
reagent such as sodium sulfide
(Na2S) or sodium hydrosulfide
(IMaHS). A more recently developed
process adds a slightly soluble
ferrous sulfide (FeS) slurry to the
wastewater to supply the sulfide
ions needed to precipitate the
heavy metals.
In the past, operational difficulties
prevented more than minimal
application of the SSP process.
Recent investigations, however,
have eliminated or reduced these
problems. Technological advances
in the area of selective-ion elec-
trodes have provided a probe that
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102
101
10°
10"
10"
=• 10"
ra
O
CO
CO
a
O
o
I
10"
10"
10"
10"
10"
10"
10"
10"
10"
10'
,-13
Zn(OH)
Pb(OH),
PbS
Legend:
I metal hydroxide
I metal sulfide
Ag2S
789
pH
10 11 12 13
Note.—Plotted data for metal sulfides based on experimental data listed in Seidell's
solubilities. j
SOURCE: Solubilities for metal hydroxides are taken from curves' by Freedman and
Shannon, "Modern Alkaline Cooling Water Treatment," in Industrial Water Engineering
(p. 31), Jan.-Feb. 1973. i
I
Figure 3. j
Solubilities of Metal Hydroxides and Sulfides as a Function of pH
has proven successful in pilot-
scale evaluations for controlling
the addition of soluble sulfide
reagent to match reagent demand.
Eliminating sulfide reagent overdose
can prevent the odor problem
commonly associated with these
systems. In currently operated
soluble sulfide systems that
do not automatically adjust reagent
dosage to match demand, the
process tanks must be enclosed and
vacuum evacuated to minimize
sulfide odor problems in the work
area. The formulation of poly-
electrolyte conditioners that
effectively flocculate the fine rnetal
sulfide particles has eliminated
the difficulty in separating the
precipitants from the discharge and
has resulted in sludges that are
easily dewatered.
Recently, a patented sulfide pre-
cipitation process called Sulfex™
has proven.effective in separating
heavy metals from plating waste
streams. The process uses a
freshly prepared ferrous sulfide
slurry (prepared by reacting
FeS04 and NaHS) as the source of
the sulfide ions needed to precipitate
the metals from the wastewater.
The process operates on the
principle that FeS will dissociate
into ferrous ions and sulfide ions
to the degree predicted by its
solubility product. As sulfide
ions are consumed, additional
FeS will dissociate to maintain the
equilibrium concentration of
sulfide ions. In alkaline solutions,
the ferrous ions will precipitate as
ferrous hydroxides. Because
most heavy metals have sulfides
less soluble than ferrous sulfide,
they will precipitate as metal
sulfides.
An advantage of the insoluble
sulfide precipitation (ISP) process
is the absence of any detectable
hydrogen sulfide (H2S) odor—a
problem historically associated with
SSP treatment systems. Another
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(a)
Ca(OH)
Polymer
Wastowoter
Neutralization
Clarification
Wastewater
discharge
Heavy metal
sludge
(b)
Ca(OH)
NaHS
Polymer
Wastowator
First
stage
Neutralization
Clarification
Wastewater
discharge
Heavy metal
sludge
(C)
Ca(OH).
Wastewater
First
stage
Neutralization
Clarification
Wastewater
discharge
Heavy metal
sludge
Figure 4.
Wastewater Treatment Processes for Removing Heavy Metals: (a) Hydroxide Precipitation, (b) SSP, and (c) ISP
advantage is that the ISP process
will reduce hexavalent chromium
to the trlvalent state under the
same process conditions required
for metal precipitation, thus
eliminating the need to segregate
and pretreat chromium waste
streams. Disadvantages of the
ISP process include considerably
higher than stoichiometric reagent
consumption and significantly
higher sludge generation factors
than either the hydroxide or
soluble sulfide treatment processes.
Figure 4 compares typical process
flow diagrams of a hydroxide
treatment system and both types
of sulfide systems. Most of the
elements of the sulfide systems are
common to the hydroxide precipita-
tion treatment sequence. The
sulfide treatment processes also
can be used as a polishing system
after a conventional hydroxide
precipitation/clarification process to
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significantly reduce the consump-
tion of sulfide reagent.
The final selection of a hydroxide
or sulfide process should also
consider any different constraints for
disposal of the resulting sludge.
Preliminary studies have indicated
that metal ion teachability is
lower for metal sulfide sludges
than for hydroxide sludges. How-
ever, the long-term impacts of
weathering and of bacterial and
air oxidation of sulfide sludges
have not been evaluated.
At this time, the. necessary precau-
tions for environmentally safe
disposal of sulfide sludge have
not been established. This lack of
information prevents an evaluation
of the impact of generating a
sulfide sludge instead of a hydroxide
sludge.
This report will evaluate the
sulfide precipitation process,
assuming that disposal of the resi-
due incurs the sjame constraints
as the hydroxide process.
The importance jof design safe-
guards to avoid |the potential
hazards associated with sulfide
precipitation processes cannot be
overemphasized. For example,
a sulfide reagent coming into con-
tact with an acidic waste stream
can result in the evolution of
toxic H2S fumes in the work area.
The potential danger can be
minimized by fajirly conventional
design safeguards, but the safe-
guards must be|well maintained
to be effective. Another potential
problem for plants discharging
to enclosed sewers is the danger
associated with!residual levels
of sulfide in the; wastewater. This
problem occurs primarily with
the SSP processes because the
low solubility of FeS in the ISP
process controls the residual
sulfide concentration at a very low
level. Elimination of the H2S hazard
to sewer workers would require
either oxidation.of the wastewater
before discharge or process con-
trols to ensure aj low sulfide residual
in the discharge.
This summary report is intended
to promote an understanding
of the use of sulfide precipitation
for the removal of heavy metals
from industrial waste streams.
The report includes a general
discussion of sulfide precipitation
process theory and an evaluation
of both soluble and insoluble
sulfide treatment systems in terms
of state of development, perform-
ance, cost, and operating reliability.
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Dual-bed effluent polishing filters
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Process Theory
The precipitatiop °f a dissolved
metal ion as a metal sulfide (MS)
occurs when the metal ion (M+2)
contacts a sulfide ion (S~2):
M+2 + S~2 -*• MS i
Most heavy metals encountered in
electroplating w;astewater will
form stable metal sulfides; common
exceptions include the trivalent
chromic and ferric ions.
The two processes currently
employed to precipitate metals
as sulfides differ mainly in the
method used to introduce the sulfide
ions into the wastewater. The SSP
process uses a water-soluble sulfide
compound; consequently, the
concentration of dissolved sulfide
depends on the;quantity of reagent
added. The ISP process mixes the
wastewater with a slurry of slightly
soluble FeS, which will dissociate
to satisfy its solubility product,
yielding a dissolved sulfide con-
centration of approximately 0.02
ppb in the wastewater. Use of
FeS as the source of sulfide ions
controls the level of dissolved
sulfide at a concentration low
enough to eliminate any detectable
emission of H2S but still provide
an inventory of undissolved sulfide
i
Table 1.
Solubilities of Sulfides
that automatically replaces the
sulfide consumed in precipitation
reactions.
In the ISP process, the dissolved
sulfide ions will precipitate as a
metal sulfide any metal with a sulfide
solubility less than that of FeS.
As shown in Table 1, the only heavy
metal with a sulfide more soluble
than FeS is manganese. In an
alkaline solution, the ferrous ions
generated in the dissociation of the
FeS will precipitate as hydroxides.
Maintaining low levels of ferrous
ions in the effluent requires that
the pH be controlled between
8.5 and 9.5.
One advantage of the ISP process
is the ability of the sulfide and
ferrous ions to reduce hexavalent
chromium to its trivalent state,
which eliminates the need to
segregate and treat chromium
wastes separately. Under alkaline
conditions, the chromium will then
precipitate as chromium hydroxide
[Cr(OH)3]. The overall reduction
reaction is:
H2Cr04
FeS + 4H2O
:--, - ----- --M Cr(OH)3
+ Fe(OH)3 4 + S I + 2H2O
Metal sulfide
KSP
(64° to 77° F)a
Sulfide
concentration
(mol/l)
Manganous sulfide.. .j.
Ferrous sulfide j.
Zinc sulfide J.
Nickel sulfide !.
Stannous sulfide ....'.
Cobalt sulfide j.
Lead sulfide I.
Cadmium sulfide ... J.
Silver sulfide....
Bismuth sulfide .
Copper sulfide ..
Mercuric sulfide.
I
• r
1.4X
3.7 X
1.2X
1.4X
1.0X
3.0 X
3.4 X
3.6 X
1.6 X
1.0 X
8.5 X
2.0 X
TO"
io-
io-
10~
10~
io-
10~
10~
3.7 X 10~a
6.1 X '10~10
3.5 X 10~12
1.2 X '10~12
3.2 X IO"13
1.7X
1.8X
6.0 X
3.4 X
4.8 X
9.2 X
4.5 X
,—13
,-14
,-15
,-17
,-20
,-23
,-25
"Solubility product of ;a metal sulfide, Ks , equals the product of the molar concentrations of the
metal and sulfide. ,
SOURCES: Robert C. Weast (ed.). Handbook of Chemistry and Physics, 50th ed., West Palm
Beach FL, The Chemical Rubber Company (p. B252), 1969. Louis Meites (ed.). Handbook of
Analytical Chemistry, jNew York NY, McGraw-Hill (pp. 1-15, 1-19), 1963.
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Ferrous sulfide makeup and feed system
In the SSP process, the sulfide
ion is capable of reducing hexa-
valent chromium as follows:
2H2Cr04 + SNaHS -f 8H2O -*
2Cr{OH)3 I 4- 3S I
7H2O
3NaOH
The question of whether a soluble
sulfide reagent can reduce and
precipitate hexavalent chromium
in one step was addressed in a
pilot study conducted for the U.S.
Navy. The study concluded that
the reduction could be accom-
plished in the presence of ferrous
ions (or conceivably some other
suitable secondary metal). The
ferrous ion acts principally as a
catalyst for chromium reduction.
Less than stoichiometric dosages2
of iron are required to effect
^Stoichiometric dosage = dosage of reagent
required per unit of contaminant based on the
chemical formula and relative atomic
weights as predicted by the treatment reaction.
reduction of most of the chromium.
Nearly stoichiometric dosages,
however, are required to achieve
levels typical of other reduction
processes. No operating systems
currently employ this one-step
process.
SSP Process Chemistry
The addition of a sulfide reagent
that has a high solubility in waste-
water will yield a relatively high
concentration of dissolved sulfide,
compared with the ;ISP process.
This high concentration of dissolved
sulfide causes a rapid precipita-
tion of the metals dissolved in
the water as metal sulfides, which
often results in the'generation of
small particle fines :and hydrated
colloidal particles. The rapid
precipitation reaction tends more
toward discrete particle precipita-
tion than toward nucleation precipi-
tation (the precipitation of a
particle from solution onto an
already existing particle). The
resulting poor-settling or -filtering
floe is difficult to separate from
the wastewater discharges. This
problem has been solved by
the effective use, separately or
combined, of coagulants and floc-
culants to aid in the formation of
large, fast-settling particle floes.
Another disadvantage of an SSP
system is the H2S odor often
associated with it. The odor detec-
tion levelof hydrogen sulfide—0.1
to 1.0 ppm—is very low compared
with the workplace H2S con-
centration limit of 10 ppm specified
by the Occupational Safety and
Health Administration (OSHA)
for worker safety.
The rate of H2S formation in a
water solution is a function of pH
8
-------
(concentration of hydrogen ions)
and sulfide ion concentration. The
formation of H2S from dissolved
sulfide ions proceeds as follows:
S+2 + H+ -> HS-
HS- + H+ -> H2S
Actually, the strong base S~2 is not
present in any significant amount
except at high pH. For example,
at a pH of 11, less than 0.05 percent
of the dissolved sulfide is in the S~2
form; the remainder is in either the
HS~ or H2S form. Figure 5 is a
graph for determining the percent-
age of the dissolved sulfide in
the form of H2S as a function of
the pH of the solution. The relation-
ship shows that at a pH of 9, H2S
accounts for only 1 percent of the
free sulfide in solution. The rate
of evolution of H2S from a sulfide
solution per unit of water/air
interface will depend on the tem-
perature of the solution (which
determines the H2S solubility), the
dissolved sulfide concentration, and
the pH. In practice, considering
typical response lags of instruments
and incremental reagent addition,
control of the level of dissolved
sulfide and pH would require fine
tuning and rigorous maintenance
to prevent an.H2S odor problem
in the work area. In currently operat-
ing treatment systems, the H2S
odor problem is eliminated by
enclosing and vacuum evacuating
the process vessels.
Adding sulfide reagent to waste-
water containing precipitated
metal hydroxides will result in re-
solubilization of the metal hydrox-
ides. The dissolving of the metal
hydroxides occurs because the
dissolved metal ion concentration is
now lower than the equilibrium
level predicted by the hydroxide
solubility. These newly liberated
metal ions will be precipitated by
any excess sulfide present. The
following reactions occur:
M+2 + 5-2 -»• MS 1
M(OH)2 -»
M+2 + s-2
M+2 + 2(OH)-
-» MS 1
pH (if pK = 7)
8 9
10
11
100
10
0.1
0.01
T
-1
(pH - pK)
Note.—pK (logarithmic practical ionization constant) is used to measure the degree of
dissociation of ^veak acids, in this case H2S.
Specific electrical conductance
ofj solution at 77° F (/iohm/cm)
Value of pK
50° F 68° F 104° F
Oa
100
1,000 ..
50,000b
7.24 7.10 6.82
7.22 7.08 6.80
7.18 7.04 6.76
7.09 6.95 6.67
bSeawater.
Figure 5. j
i
I
Percent of Dissolved Sulfide in the
I
i
j
Normally, the precipitated solids
are in contact with the wastewater
long enough to result in an almost
complete conversion of metal
hydroxides to metal sulfides.
Therefore, the sulfide reagent
demand depends on the total metal
concentration contained in a
wastewater. Consequently, a
significant reduction in sulfide
reagent consumption could be
jmpt
I
H2S Form
achieved by separating the precipi-
tated metal hydroxides from the
wastewater before adding the
sulfide reagent.
ISP Process Chemistry
The Sulfex™ process precipitates
dissolved metals as sulfides
by mixing the wastewater with
an FeS slurry in a solid/liquid contact
chamber. The FeS dissolves to
9
-------
20
10
|
CD
D.
CL.
o
u
0.1
0.01
No sludge
blanket
Legend:
10-min mixing time
20-min mixing time
45-min mixing time
60-min mixing time
5,000-mg/l
sludge blanket
0.001
I
I
I
1x
2x
3x
FeS DOSAGE3
4x
5x
6x
"x — stoichiometric equivalent concentration of FeS required to precipitate 20 mg Cu+2/l = 27.7 mg FeS/l.
Noto.—Results of jar tests with complexed Cu influent; pH values ^maintained between 7 and 8 during tests.
SOURCE: U.S. Environmental Protection Agency, Treatment of Metal Finishing Wastes by Sulfide Precipitation, EPA 600/2-77-049,
Feb. 1977.
Figure 6.
Influence of FeS Dosage, Sludge Blanket Concentration, and Mixing Time on Copper Solubility
maintain the sulfide ion concentra-
tion at a level of 0.02 ppb.
The following reactions occur when
FeS is introduced into a solution
containing dissolved metals
and metal hydroxide:
FeS
Fe+2 + S~2
+ s-z -» MS
M(OH)2 -» M+2 + 2(OH)~
Fe+2 + 2(OH) -- > Fe(OH}2
The addition of ferrous ions to
the wastewater and their precipita-
tion as ferrous hydroxide [Fe(OH)2]
results in a considerably larger
quantity of solid wa>te from this
process than from a conventional
hydroxide precipitation process.
As with SSP, the ISP process
achieves an almost complete
conversion of previously precipi-
tated metal hydroxides to metal
sulfides. The reaction goes to
completion because of the long
residence time of the solids in
the treatment system before
discharge.
Figure 6 shows three different
factors that affect the ability of FeS
10
-------
20
18
16
s: 14
0!
£ 12
53
q 10
8
6
4
2
CO
8
Filtered effluent Cu
vs. Ca(OH)2
dosage
Effluent pH vs.
Ca(OH)2 dosage
No sludge
blanket
3,000-mg/l
sludge
blanket
Legend:
I 45-min mixing time
I 60-min mixing time
12
10
100
200
300
|400
500
600
700
800
93% Ca(OH)2 DOSAGE (mg/l)
]
Note.—Results of hydroxide process jar tests with complexed Cu influent.
SOURCE: U.S. Environmental Protection Agency, Treatment of Metal Finishing Wastes by Sulfide Precipitation,
EPA 600/2-77-049, Feb. 1977. i
I
I
Figure 7. |
I
Influence of Ca(OH)2 Dosage, Sludge Blanket Concentration, and pH on Copper Solubility
to precipitate copper from a solu-
tion containing metal complexing
compounds. The design criteria
that must be addressed are:
• A dense sludge blanket must be
maintained in the solid/liquid
contact zone.
• Adequate mixing time is required
for the precipitation reaction
to reach equilibrium.
From 2 to 4 times the required To illustrate the relative effective-
stoichiometric amount of reagent
is needed to realize the low levels
of dissolved copper achievable
by sulfide precipitation.
ness of sulfide precipitation,
Figure 7 represents the solubility
of copper in the same complexing
compound solution as a function of
pH. Even at a pH of 12, the level
of dissolved copper cannot be
reduced below 2 mg/l.
11
-------
Hydroxide and sulfide mixer/clarifiers
12
-------
Soluble Sulfide
Precipitation
Use of a water-jsoluble sulfide
compound Jo reduce the solubility of
heavy metals in a waste water
discharge is an! effective method
of improving the performance of a
hydroxide precipitation treatment
system. This section describes
the results of a;recent investigation
of the use of SSP and presents
information on isystems currently
using the technology.
Pilot Plant Evaluation
Test Description. Currently, there
are only a few applications of SSP
to treat metal finishing waste-
water. To provide a source of the
data needed by firms interested in
using the treatrjnent process, the
U.S. Environmental Protection
Agency's (EPA's) Industrial Envi-
ronmental Research Laboratory
funded a pilot study by the Boeing
Commercial Aircraft Company
to compare and evaluate five
treatment systems using variations
of SSP and hydroxide precipita-
tion processes jto treat metal
finishing wasteWater. The pilot
tests were designed to simulate
the three basic, process systems
shown in Figure 8. The five process
variations testejd were:
• Lime only, clarified (LO-C)—the
conventional!process using
lime as a neutralizing agent to
precipitate the dissolved
metals and clarification to
separate the Suspended solids
from the discharge (System A)
• Lime only, clarified, filtered
(LO-CF)—the; LO-C process with
a filtration step downstream of
clarification to improve the
suspended solids removal
(System A) I
• Lime with sujfide, clarified
(LWS-C)—the LO-C process with
controlled addition of a soluble
sulfide reagent in the neutralizing
chamber (System B)
• Lime with sulfide, clarified,
filtered (LWS-CF)—the LWS-C
process with;a filtration step
downstream of clarification to
improve the Suspended solids
removal (System B)
• Lime, sulfide polished, filtered
(LSPF)—a polishing sulfide
precipitation process featuring
lime neutralization and clarifica-
tion to remove the metal hydrox-
ides followed by addition of a
soluble sulfide reagent to reduce
the metal solubility and a fil-
tration step to remove the
precipitated solids (System C)
These process variations were
evaluated with 14 actual raw waste-
water feed samples obtained
from various industrial firms en-
gaged in electroplating and rnetal
finishing. The pilot plant could
operate in any of the five modes
and could process 0.034 gal/min
(1 30 ml/min) of wastewater in a
continuous treatment sequence.
Samples were pretreated as
required for chromium reduction
and cyanide oxidation. Attempts
were not made to reduce hexavalent
chromium with sulfide reagent.
In the sulfide process variations,
the soluble sulfide reagent addi-
tion was controlled automatically by
a specific-ion sulfide reference
electrode pair to maintain a
preselected potential of —550 mV
with respect to the reference
electrode. The value of —550 mV
corresponds to about 0.5 mg/l
of free sulfide, which was selected
as the control point because at
that concentration:
• The curve of electrical potential
versus sulfide concentration
has its maximum gradient.
• The wastewater solution has
no detectable sulfide odor.
The study reported that the de-
pendability of the sulfide specific-
ion electrode was excellent
during the 6-month test period.
13
-------
SYSTEM A: LIME
SYSTEM B:
LIME WITH SULFIDE
SYSTEM C:
LIME, SULFI
Feed ' >
DE POLISHED
Input
filter
Input
filter
Sulfide
controller
i i —
Input
filter
—
Lime
slurry
Lime
slurry
pH
controller
1 j |
1
Flash A
mix
i
Polyelectrolyte
Soluble
sulfide
-1 |
1 1
Settling
clarifier
Effluent
- Final -0- -l»
filter *~
1 r ,i
Sample point Sample point
for LO-C for LO-CF
Lime
slurry
PH
controller
4 j f
4
1
r
• Flash •
mix
i
Polyelectrolyte
pH
controller
1 1
f r
Flash •
mix
t ,
Polyelectrolyte
No
LO-
wit
LSF
Sett
colu
clar
e. — Inpu
C=lime
i sulfide
Settling
column
clarifier
Effluent
filter
1 ' 1 '
Sample point Sample point
for LWS-C for LWS-CF
1
1
1
j
ling
fier
Soluble
sulfide
Sulfide
controller
1 J !
T J
1 i — J
Sulfide •
polishing
Effluent
— a. Final _.» *.
filter T
Sample point
for LSPF
t filter not required for industrial applications. Abbreviations:
only, clarified; LO-CF= lime only, clarified, filtered; LWS-C= lime
c arified; LWS-CF = lime with sulfide, clarified, filtered;
, sulfide polished, filtered.
Figure 8.
Metal Precipitation Processes Evaluated in Pilot Study
14
-------
Table 2.
Wastewater Treatment Process Details of Pilot Tests
Characteristic
Raw feed before treatment:
pH :.
Conductivity (/imho/cm)
Color
Precipitation pH for LO and LWS processes . . .
Sludge volume (%):°
LO process
LWS process
Process consumables (mg/l):
Sulfuric acid for Cr"1"6 reduction
Sodium sulfite for Cr"1"6 reduction
Calcium oxide for neutralization . .
Sulfide for LWS process
Sulfide for LSPF process
1
1 7
10 600 at 7
Yellow
85
18
16
0
226
1 530
8
1
Pilot test3
2b
1.2
2°F 149,000 at 68° F
Colorless
6.2/9.0
78/23
78/13
0
31
14,380
381
5
3
6.4
1 2,1 00 at 77° F
Colorless
9.0
0
(d)
0
0
911
0
(d)
4
2.4
5,600 at 66° F
Colorless
10.0
43
37
0
41
2,680
400
141
5
7.1
1 ,500 at 70° F
Pale green
8.5
5
6
339
25
145
91
67
=Wastewater by pilot test: 1 —high-chromium rinse from aluminum cleaning, anodizing, and electroplating; 2—chromium, copper, and zinc rinse from
electroplating; 3—high-zinc rinse from electroplating; 4 and 5—mixed heavy metal rinse from electroplating.
bBecause of the exceptionally large volume of sludge generated by this wastewater, precipitation was accomplished in two stages. First- and
second-stage values are separated by a diagonal line; single values apply to the total process.
°Sludge volume per solution volume, percent after 1 hour settling. I
dData not available.
Note.—LO = lime only; LWS= lime with sulfide; LSPF= lime, sulfide polished, filtered.
SOURCE: Sulfide Precipitation of Heavy Metals, EPA Grant 580541 sj in preparation.
Test Results. Results of five of
the pilot tests are presented in
Tables 2 and 3. Table 2 lists the
characteristics of the wastewater
before treatment, the volume of
sludge generated, and the amount
of reagents consumed in the
treatment. Table 3 compares the
amount of metal per liter of raw
feed before treatment and waste-
water after treatment using the
five process variations.
Pilot Test 1 simulated treatment of
a wastewater containing a high
concentration of chromium and
moderate levels of copper and
zinc. As can be seen from the
effluent quality of the LO-CF process,
the hydroxide solubilities of the
metals in this wastewater were
quite low and use of a sulfide
reagent to achieve lower metal
solubilities was not required. The
significant reduction in the chro-
mium concentration across the
filter can be seen by comparing the
effluent quality of the LO-C and
LO-CF processes. This situation
points out how| poor solids removal
can have significant adverse
effects on an otherwise effective
metal precipitation treatment
system.
Pilot Tests 2 arjd 3 were performed
with wastewaters that were not
effectively treated by hydroxide
precipitation. In these tests,
significantly improved effluent
quality was achieved by sulfide pre-
cipitation treatment. In Pilot Test 2,
the effluent produced by the LO-CF
process contained relatively
high levels of zinc and copper,
2.3 and 0.8 mgi/l, respectively.
Treatment withja soluble sulfide
compound considerably reduced
the effluent concentration of
these metals. In Pilot Test 3, soluble
sulfide treatment of wastewater
with a high zinc concentration
was significantly more effective
than hydroxide precipitation.
Tests also were conducted on
wastewaters containing an assort-
ment of heavy metals at relatively
high concentrations. The results
of Pilot Tests 4 and 5 (shown in
Table 3) indicate that low levels of
all metal pollutants could not be
achieved by treatment of these
particular wastewaters with either
hydroxide or sulfide precipitation.
In Pilot Test 4, sulfide precipitation
removed the cadmium, copper,
and zinc to considerably lower
levels than the hydroxide precipita-
tion process, but both processes had
a high residual nickel concentra-
tion in the effluent. A similar
situation occurred with nickel
in Pilot Test 5.
15
-------
Table 3.
Chemical Analysis of Raw and Treated Wastewater Used in Pilot Tests
I
Contaminant (ju.g/1)
Mirfefil *
Load .—
Nickel
Silver
Nickel
|_eac|
fjn
Ammonium.
Nickel .
Tin
Nickel .
2jnc ft . • • • •
Silver
Raw feed
before
treatment
• 45
1 63,000
,4,700
185
2,800
119
' 58
6,300
<5
1,100
: 1 60
650,000
<1
, 16
': 34
: 3
20
64
440,000
<10
45
61
200
'' (b)
58,000
! 5,000
2,000
3,000
290,000
740,000
<0.3
14
5,000
! <40
1 ,700
21 ,000
11! 9,000
13,000
NA
13
6
Wastewater after treatment3
LO-C
15
3,660
135
30
44
119
7
4
<1
860
30
2,800
NA
NA
21
NA
7
29
37,000
NA
13
4
<10
NA
1,130
138
909
2,200
1,200
2,000
<0.3
14
129
<1
109
1,300
1 2,000
625
2
7
NA
LO-CF LWS-C
Pilot Test 1
8 11
250 1 ,660
33 82
38 33
10 26
88 104
Pilot Test 2
12 <5
2 5
<1 <1
848 13
34 33
2,300 104
NA NA
NA NA
Pilot Test 3
21 1
NA NA
8 2
29 72
29,000 730
NA NA
14 9
4 1
<10 <10
NA NA
Pilot Test 4
923 26
103 49
943 60
2,300 1,800
510 216
334 563
<0.3 <0.3
10 7
81 71
PilofeTest 5
<1 <1
39 187
367 2,250
9,400 1 1 ,000
10 192
<2 5
5 4
NA NA
LWS-CF
7
68
18
31
2
59
<5
7
<1
13
23
19
NA
NA
1
NA
1
34
600
NA
11
3
<10
NA
<10
50
160
1,900
38
229
<0.3
7
71
<1
17
169
3,500
8
<2
3
NA
LSPF
20
159
3
18
11
120
<5
3
<1
132
34
242
NA
NA
1
NA
4
31
2,000
NA
13
4
<10
NA
<10
37
929
2,600
12
305
<0.3
8
71
<1
20
11
5,300
5
<2
3
NA
"LO-C = lime only, clarified; LO-CF = lime only, clarified, filtered; LWS-C = Ijme with sulfide, clarified; LWS-CF = lime with sulfide, clarified, filtered;
LSPF ** lime, sulfide polished, filtered; NA = not applicable.
bQualitative tests indicated the presence of significant amounts of ammonium.
Nous.—Wastewater by pilot test: 1—high-chromium rinse from aluminum cleaning, anodizing, and electroplating; 2—chromium, copper, and zinc rinse
from electroplating; 3—high-zinc rinse from electroplating; 4 and 5—mixed heavy metal rinse from electroplating.
SOURCE: Sulfide Precipitation of Heavy Metals. EPA Grant 5805413, in preparation.
16
-------
The data on effluent quality from
this study suggest the following
general conclusions about the
treatment of wastewater with either
hydroxide or sulfide precipitation
for removal of heavy metals:
• In most cases, metal removal
can be improved by precipitating
metals as sulfides rather than
as hydroxides.
• Some wastewaters can be
effectively treated to low residual
concentrations of all metals
present by either hydroxide or
sulfide precipitation processes;
some wastewaters cannot be
effectively treated by either hy-
droxide or sulfide precipitation.
• Consistent removal of metals
to effluent concentrations of less
than 1 mg/l requires filtration
to remove residual suspended
solids. Because fine particles
(which include precipitated
metals) are only minimally differ-
ent in density from water, they
cannot be effectively separated
by clarification and therefore
contribute to the effluent metal
concentration.
Another significant finding of
the study is the quantity of sulfide
reagent consumed in precipitating
the metals as sulfides. In the
LWS processes, the bulk of the
test runs consumed between 1.0
and 2.5 times the stoichiometric
sulfide reagent demand based
on the total mass of metals that
form sulfides in the wastewater.
This reagent demand factor supports
the belief that all metals are pre-
cipitated as sulfides and that
any metals initially precipitated as
hydroxides are converted to metal
sulfides. ,
In the LSPF process, the metals
precipitated as hydroxides are
separated by clarification before
addition of the sulfide reagent. The
sulfide reagen^ demand for most
of the LSPF process tests ranged
from 2 to 6 times the stoichiometric
sulfide reagenjt demand. The
stoichiometrici demand in this case
can be calculated from the con-
centration of metals in the LO-C
effluent. The study contained no
conclusions as to the cause of
the significantly higher sulfide
reagent demand relative to the
stoichiometric! requirements.
j
SSP System Description and
Performance
Although onlyja few plants currently
treat their wastewater with SSP,
this process has proved effective for
precipitation of many of the metals
typically encountered in electro-
plating wastewater. At present,
however, no cbmmercial units are
demonstrating! the treatment of
heavy metals at the high concentra-
tions typical of metal finishing
industry wastewater. The primary
application of |SSP has been for
waste streams] containing low con-
centrations of imetals and complex-
ing agents, wrjich interfere with
effective metal removal by hydrox-
ide precipitation.
i
Figure 9a is a |schematic of a
continuous SS;P system used to
treat a heavy metal waste stream
discharged frorn a large mechanical
equipment mahufacturer. Part
of the wastewater results from
electroplating land surface finishing
operations. The wastewater pH
is adjusted to J7.5 in the first-stage
neutralizer and is maintained at
approximately J8.5 in the second-
stage neutralizer. If the pH falls
below 7 in the: first stage, a low-pH
alarm sounds and the pump feeding
the second-stage neutralizer is
shut off. Consequently, a surge
volume is required in the system
to store the wastewater until the
pH returns to the control set-point.
I
Sodium hydrosulfide is added in
the second-stage neutralizer at
a rate set to maintain a dosage of
5 to 10 mg of free sulfide per liter
of wastewater. Automatic controls
are not used to adjust sulfide
reagent feed rate to account for
changes in demand. The required
sulfide reagent addition rate is deter-
mined by periodic testing.
The system shown in Figure 9a
uses a separate hexavalent chro-
mium reduction system, although
the free sulfide can potentially
accomplish the reduction. This
approach was not evaluated because
performing chromium reduction in
the second-stage neutralizer
would increase sulfide reagent
demand to approximately 35 to 50
mg/l of feed (based on consumption
equal to twice the stoichiometric
reagent demand) and would make
sulfide reagent demand considerably
more variable. Without an automatic
sulfide reagent addition system
to match supply with demand,
the increased variability in reagent
demand would reduce the reliability
of the treatment system. The
existing chromium reduction unit,
which uses sodium bisulfite
(NaHSO3) as the reducing agent,
reduced the hexavalent chromium to
the required level. Therefore,
sulfide precipitation was used only
to achieve the superior metal
removal required by the discharge
permit.
The reduction in the metal solubility
achieved by adding NaHS to this
plant's wastewater is shown in
Table 4. The data indicate that
the metal solubility decreases as the
sulfide reagent dosage increases.
17
-------
(a)
Concentrated dumps
Chroma Acid/alkali waste
waste Na"S°3
Chromium reduction
unit
First-stage neutralizer
(pH = 7.5, r= 10 min)
Second-stage neutralizer
(pH = 8.5, r= 10 min)
Wastewater
discharge
Sludge to
dewatering filter
Legend:
pHC= pH controller
pHA= low-pH alarm
r= retention time
V, = surge volume
Polyelectrolyte
feed tank
Figure 9.
SSP Treatment Systems: (a) Continuous and (b) Batch
18
-------
Table 4.
Sulfide Precipitation of Cadmium, Zinc, and Mercur}
i Supernatant3
Metal (mg/l)
Cadmium
Zinc
Raw
waste
2 1
3 0
0006
Hydroxide
solubility
at pH of 8.5
2.0
2.25
0.0027
Sulfide
1
1.6
1.8
0.001 3
addition
5
0.39
1.5
0.001
(mg/l)
10
0.06
1.1
0.0008
"Polyelectrolyte dose = 1 mg/l; settling time of 2 hours. j
Note.—Stoichiometric sulfide requirement to precipitate mixture given is 2.1 mg/l of sulfide based
on raw waste composition. !
Table 4 also shows the solubilities
of the metal hydroxides after
pH adjustment to 8.5. Effective
metal removal is achieved by this
treatment system with a sulfide
reagent in the sulfide dosage range
of 5 to 10 mg/l.
Figure 9b shows a commercially
operated batch wastewater treat-
ment system using a soluble sulfide
reagent. The system includes two
batch treatment tanks, each
sized to hold 1 day's wastewater
flow. The sequence of treatment
follows:
1. The pH of the full, off-stream
tank is raised automatically to a
value of 11 by the addition of
hydrated lime.
Table 5.
Removal of Complexed Copper and
Other Metals From Electroplating
Wastewater
Metal (mg/l)
Nickel
Lead
Tin
Untreated
wastewater
17
03
1 85
086
429
Filtrate
04
<02
<02
04
<1 0
2. Dependingj on the volume of
wastewater in the tank, a quantity
of IMaHS is |metered into the tank.
3. The tank is' agitated for approxi-
mately 30 [ninutes and a sample
is taken, filtered, and analyzed
for the metal that is character-
istically most difficult to remove.
4. If the meta) concentration is low,
enough, this contents of the
tank are piimped through a
diatomaceous earth precoat
pressure filjter and, after final
pH polishing {to 8), are dis-
charged. lf|the reference metal
level is nol; low enough, addi-
tional NaHS is added and Steps
3 and 4 are repeated.
The performance of the batch
system in reducing the level of total
metals in the Wastewater discharge
is presented ifi Table 5. As shown,
the pH of the wastewater is raised
to 11 before iSlaHS is added.
Experimentally, it was found that
the sulfide addition would reduce
the dissolved jmetal concentration
to equally low levels at a pH
of 8.5. Removjal of fluorides present
in the plant's wastewater, however,
required elevating the pH to 11.
i
The continuous and batch SSP
systems described in this section
Note.—Batch treatment sequence: lime added
to pH of 11; NaHS added to equivalent sulfide
ion concentration of 20 mg/l (stoichiometric
requirement =10 mg/l); filtered through
diatomaceous earth filter; final pH adjustment
to 8 before discharge.
are located in segregated waste
treatment areas. Despite careful
control of the wastewater pH and
sulfide addition rate, the H2S odor
in the area was a nuisance. To
reduce the ambient level of H2S,
the open-top treatment tanks
where the sulfide reagent is added
to the wastewater were modified
into closed-top, vacuum-evacuated
tanks. In the batch system shown
in Figure 9b, the final pH adjust-
ment tank contributed to the odor
problem and was modified similarly.
The exhaust from these tanks,
which contains a low level of H2S,
is vented outdoors. These changes,
plus rigid control of pH and sulfide
dosage levels, have resulted
in an almost undetectable H2S
odor in the waste treatment area.
SSP Polishing Treatment System
Sulfide reagent demand for the
SSP treatment system shown in
Figure 9a is a function of the total
metal concentration of the raw
wastewater. Sufficient reagent must
be supplied to convert all entering
metals to metal sulfides. In
treating wastewater containing
high metal loadings, significant
sulfide reagent cost savings can be
realized by using SSP to polish
the effluent after a conventional
pH adjustment/clarification treat-
ment sequence (Figure 10). The
LSPF process evaluated in the
pilot studies discussed earlier
simulated the use of SSP as a
polishing system. There are no
commercially operated SSP polish-
ing systems currently in operation.
In addition to reducing sulfide
reagent consumption, using sulfide
precipitation as a polishing system
will reduce the variability of re-
agent demand. The reagent demand
for the polishing system will be
a function of wastewater flow
and the concentration of metals
in the overflow from the first-stage
clarifier. The metal concentra-
tion in the wastewater at this point
19
-------
Ca(OH).
Wastewater
First-stage Second-stage
neutralizer neutralizer
Wastewater
discharge
Hydroxide sludge
Clarifier
NaHS
Filter
|-
Figure 10.
SSP Polishing System
should not be subject to the wide
variability that often characterizes
the raw Wastewater feed metal con-
centration. Without an automatic
reagent addition control loop,
dosing the Wastewater with a
predetermined amount of sulfide
reagent would be considerably
more reliable in a polishing treat-
ment application.
The plant operating the treatment
system shown in Figure 9a evaluated
the use of SSP as a polishing
treatment to reduce the variability
of sulfide reagent demand. It
was found that clarifying the waste-
water before adding the sulfide
reagent resulted in the formation
of poor-settling particles that
were difficult to remove from the
Wastewater. The current treatment
sequence, in which the sulfide
reagent is added in the second-
stage neutralizer, removes pre-
cipitated metal more effectively. It
was concluded that the presence of
the precipitated metal hydroxides
and lime solids in the wastewater
entering the seconds-stage neu-
tralizer provided nucleation sites,
which promoted the coagulation
of the precipitated metal sulfides.
An SSP pilot study reports success
in forming metal sulfide particles
that were easily removed from the
wastewater despite precipitation in
a solution lacking njjcleation sites.
The researchers found that con-
ditioning the colloidal metal sulfide
precipitants with a cationic coagu-
lant to increase the particle size
and then adding an.anionic floccu-
lant to link the particles produced
large, fast-settling particles
when flocculated. In the pilot
study discussed previously, the
sulfide polishing process precipi-
tated metals as sulfides after the
wastewater had been clarified to
remove suspended solids. The
study indicated that the metal
sulfide solids were removed effec-
tively by filtration.
The additional equipment require-
ments of a polishing treatment
system include a second mixing
tank to add the sulfide reagent
and a second solids separation unit
(using either a clarifier or a filter)
installed downstream of the metal
hydroxide clarification step. A
second polyelectrolyte addition
system also may be required to
enhance the efficiency of the metal
sulfide solids separation step.
Hydroxide System Modifications
for SSP
Augmenting a hydroxide precipita-
tion wastewater treatment system
20
-------
pn Flocculation
adjustment
First-stage Second- Flocculation ]
pH stage pH
adjustment adjustment
First-stage Second- Flocculation
pH stage pH
adjustment adjustment
Figure 11.
I
Conversion of Hydroxide Treatment System To Use S;SP: (a) Hydroxide Precipitation System, (b) SSP System, and
(c) SSP System With Automatic Control of Sulfide Rbsidual
with SSP to achieve a lower
level of metals in the effluent can
be a cost-effective means of
achieving compliance. The cost of
using soluble sulfide treatment
will be significantly affected
by the reliability and dependability
of using the specific-ion sulfide
reference electrode to control
the sulfide reagent addition. If the
I
residual sulfide concentration
can be maintained consistently at
a level of 0.3 jto 0.5 mg/l in the
wastewater, itjshould not be neces-
sary to modify existing treatment
tanks to eliminate sulfide odor
in the work area. Because the
reliability of thje control system has
not been established, two alter-
native approaches emerge for
converting a hydroxide system
to use SSP.
With no automatic control of
the level of residual sulfide in the
wastewater, converting the
conventional hydroxide precipita-
tion system (Figure 11 a) to an SSP
system (Figure 11 b) requires
21
-------
several process modifications. The
modifications, which are discussed
in the following paragraphs, include:
• NaHS reagent feed tank and
feed pump
• Second-stage neutralizer/soluble
sulfide treatment tank
• Clarifier enclosure and vacuum
evacuation
• Control system
• Sand filter or other polishing
filtration unit
• Aeration system
The NaHS feed tank should have
a closed top with a vent connect-
ing to an exhaust system. In
installations where venting any
odor is considered a public nui-
sance, the vent can be connected
to a scrubber system. Using a
scrubber eliminates the discharge
of any odor, whereas simply venting
outdoors eliminates any hazard
to the worker during reagent
preparation. The feed pump should
be a positive displacement pump
with a variable stroke to facilitate
the metering of reagent into
the system.
The second-stage neutralizer/
soluble sulfide treatment tank is
used for adding the sulfide reagent
to the wastewater. The tank also
provides improved pH control
to ensure that the sulfide reagent
does not come into contact with
acidic wastewater. The tank
contents should be agitated. The
tank should be sized to provide
a minimum retention time of
20 min, and it should be equipped
with a pH control loop and alkali
neutralizing reagent feed system. To
minimize any H2S odor associated
with the treatment, the tank
should be totally enclosed and
vacuum evacuated.
To convert the conventional
hydroxide precipitation system to
an SSP system, it is also necessary
to totally enclose and vacuum
evacuate the clarifier.
A control system is needed to
avoid mixing of the sulfide reagent
with low-pH wastewater. An
instrumentation !oop;that interrupts
the wastewater feed ,to the sulfide
treatment tank if the pH of this
stream falls below set-point is
one way of minimizing the potential
hazard. Low-pH conditions also
should sound an alarm and interrupt
the sulfide feed to the system.
This type of control will result in
the need for surge volume upstream
of the sulfide treatment tank
to store the volume buildup until
the pH is brought back above
the set-point.
A sand filter or other polishing
filtration unit that removes sus-
pended solids in the clarifier
overflow to very low levels is
recommended for any treatment
system that must achieve very low
levels of metals in the effluent.
The significance of reducing
the solubility of a metal pollutant
by means of sulfide precipitation
will be lost unless the level of
suspended solids, which include
insoluble metals, is also controlled
at a low level.
An aeration system may be needed
to oxidize residual splfide before
wastewater discharge. If wastewater
is discharged into a :sewer system,
precautions must be: taken to
ensure that the discharge does
not contain high levels of sulfide.
Discharge of wasteWater containing
significant quantities of sulfide
could be hazardous to individuals
working in a poorly vented sewer
system. No specific limit exists
for direct discharge of sulfide, but
its presence contributes to the
biochemical oxygen demand (BOD)
of the wastewater. The easily
oxidized sulfide compounds can
be treated in an air-sparged
tank with a retention time of
approximately 30 mm. If indoors,
this tank also should be totally
enclosed and vacuum evacuated.
For a process using automatic
control of the sulfide reagent
addition (Figure 11c), the required
modifications to convert the
hydroxide system to an SSP system
would include the following:
• NaHS reagent feed tank and
feed pump—identical to the tank
and pump required for the
previous case, except the feed
pump is actuated by a signal from
the sulfide reagent control
system to maintain a constant
residual sulfide concentration
in the wastewater
• Second-stage .neutralizer/
soluble sulfide treatment tank—
for addition of the sulfide reagent
to the wastewater, but in this
case the residual free sulfide
ion concentration is maintained
at a level belowO.5 ppm by means
of a sulfide ion control loop
• Control system to avoid mixing
of the sulfide reagent with low-pH
wastewater
• Sand filter
The second-stage neutralizer/
sulfide treatment tank and the
downstream process tanks will not
need to be1 enclosed and vacuum
evacuated if careful control of
pH (between 8 and 9.5) and sulfide
ion concentration is maintained.
Control of sulfide ion concentra-
tion also should eliminate the need
to aerate the wastewater before
discharge. The other elements
of the sulfide system shown in
Figure 11—first-stage pH adjust-
ment, polyelectrolyte conditioning,
and clarification—are common
to hydroxide precipitation systems.
For batch treatment SSP systems,
a two-tank system for alternately
collecting and treating the waste-
water would be required. The
treatment sequence for a batch
system was presented earlier. If
the residual level of sulfide cannot
be controlled, aeration of the
wastewater after chemical treatment
may be required in addition to
enclosing and vacuum evacuating
22
-------
the tanks during treatment. The
wastewater could be aerated in
the treatment tank before floccula-
tion (if required) and solid/liquid
separation.
Retrofitting a hydroxide system
to use soluble sulfide polishing
would require a mixing tank to add
the sulfide reagent to the waste-
water downstream of the existing
clarifier and a second solids
separation unit. Because the solids
generation rate in the soluble sulfide
polishing step should be low, a
sand or mixed-media filter should
be suitable for removing the
suspended solids from the waste-
water before discharge.
Polyelectrolyte conditioning and
flocculation may be required
between the sulfide reagent addition
tank and the solids removal filter.
Without instrumentation for
reliable control of the residual
sulfide concentration, the sulfide
reagent mixing tank and down-
stream equipment would need
to be enclosed and ventilated, and
aeration of the effluent might
be required.
SSP Cost Estimating
Improving the performance of a
hydroxide precipitation system
through the use of SSP will require
investment capital to modify
the treatment system and will
increase the cost to operate the
system.
There is some uncertainty in
predicting the extent of the modi-
fications needed to convert a
hydroxide system to use SSP.
Demonstration of the reliability of
automatic control of the sulfide
reagent feed is needed to eliminate
this uncertainty. Table 6 presents
the costs (including hardware
and installation) of the different
Table 6. j
Equipment Cost Factors for SSP Treatment Systems
!
Equipment component
1
i
Sodium hydrosulfjde feed tank and metering pump
Automatic sulfide! reagent addition control
Low-pH prevention control loop
Second-stage pH|adjustment and sulfide reagent mixing tank:
Open top . . |
Totally enclo'sed and vented
Suspended solids polishing filter. ...
Aerator I
Installed cost ($1,000),a
by wastewater flow rate
(gal/min)
30
3.3
3.5
1 .5-2.0
18
23
24
4
60
3.3
3.5
1.5-2.0
22
28
33
7
90
3.3
3.5
1 .5-2.0
24
30
41
9
aMarch 1979 cosi basis. Installed costs of different components are presented. Engineering and
design costs, sitfe preparation, and equipment freight charges are not included.
equipment components that may
be required: ]
• NaHS feed
tank and metering
pump
• Automatic control of sulfide
reagent addition
• Low-pH prevention control loop
• Mixing tank
• Suspended solids polishing filter
• Aerator
The cost for a sodium hydrosulfide
feed tank is based on a 400-gal
(1,514-1), closed-top, carbon-steel
tank that hasj a removable lid,
exhaust vent,; and appropriate
nozzles. The |diaphragm metering
pump is rated,to deliver 0 to 20 gal/h
(0 to 76 l/h).j
A specific-ion sulfide reference
electrode pair automatically
controls the sulfide reagent feed
pump. A control loop prevents
low-pH conditions in the sulfide
treatment tanik by automatically
shutting down the wastewater feed
pump and sulfide reagent feed pump
if the wastewater pH falls below
the control set-point. The cost
presented assumes the prior
existence of a pH probe and a
surge volume to hold the diverted
flow. ;
i
I
Second-stage pH adjustment
and sulfide reagent addition occur
in an agitated tank sized for 20-min
retention of wastewater. Costs
are given for both an open-top
and a totally enclosed and vented
tank.
The suspended solids polishing
filter costs presented are for
dual mixed-media filters, skid
mounted and sized so that one filter
can process the maximum flow
during backwash. The unit is
equipped with a blower for low
pressure air scouring, a backwash
storage tank, and a pump to bleed
the wash back into the treatment
system.
The aerator cost is based on an
enclosed, vacuum-evacuated
tank sized for 30-min retention
of wastewater and equipped with
an air sparger.
Higher operating costs—for operat-
ing labor and treatment reagents—
will result from incorporating
SSP into an existing treatment
system. Additional operating labor
will be required to prepare sulfide
reagent and to maintain and operate
the additional equipment com-
ponents. Additional expense
will result from consumption of
sulfide reagent. The consumption
rate will depend on the volume
of wastewater treated and the
23
-------
LOO r~
0.75 -
8
I- 2
= 5
9l
So
31
0.50 -
0.25 -
—I 5.40
— 4.05
a
z
d t!
55 S
— 2.70 9
9 M
oc o
8
™ '
Cl
1.35
100
150
200
250
WASTEWATER METAL CONCENTRATION (ppm)d
"Based on NaHS (7296 flake) at $370/ton.
"Total treatment at 2 times the stoichiometric reagent demand.
'Polishing treatment at 4 times the stoichiometric reagent demand and a total metal hydroxide solubility of 10 ppm.
dlnoludes all metals that form sulfides, based on metal with molecular weight of 62.5 (average of Ni, Cu, and Zn).
Figure 12.
Soluble Sulfide Reagent Cost
required dosage. The dosage per
volume of wastewater treated will
be a function of the wastewater
metal concentration. Figure 12
presents the sulfide reagent cost per
1.000 gal (3,800 I) of wastewater
treated as a function of metal
concentration for an SSP system
used to treat the total metal load
as well as for polishing treatment.
Sludge generation rates will
increase with the use of SSP com-
pared with a conventional hydroxide
treatment system because of
improved metal removal, but the
increase should be insignificant. For
example, precipitating an additional
5 ppm of dissolved metals from a
waste stream will increase the
clarifier underflow rate by less than
1 gal of sludge per 1,000 gal of
wastewater treated, based on an
underflow concentration of 1 per-
cent solids by weight. Also, the
dewatering properties of sulfide
sludges are believed to be superior
to those of hydroxide sludges.
although limited information is
available to support this view.
If the pH of the neutralized waste-
water is increased to minimize odor,
more alkali will be consumed,
causing an increase in cost. The
increased cost of alkali should
not be significant except for high-
volume treatment systems. Use of a
pH above 10 would necessitate
a final adjustment to lower the pH
to the acceptable discharge range.
24
-------
Insoluble Sulfide
Precipitation
A commercially available ISP
wastewater treatment system was
developed to] provide a treatment
process that offers the superior
metal removal of sulfide precipita-
tion systems Without the unpleasant
H2S odor often associated with
soluble sulfide systems. Since the
first commercial demonstration of
the process in 1978, additional
installations have become opera-
tional. The process is patented,
and its use requires payment of a
licensing fee to the patent holder.
This section describes the process,
presents performance data on
three currently operating systems,
and evaluates use of the process
for treatment' of electroplating
wastewaters.:
Process Description
i
Process Equipment Components. A
hydroxide neutralization/ISP treat-
ment system :for control of pH and
precipitation of heavy metals
is depicted in Figure 13. In this
system, the hexavalent chromium is
reduced to its trivalent state by
the sulfide arid ferrous ions present
in the mixer/clarifier, thus eliminat-
ing the needier a separate chro-
mium reduction unit. With the
exception of chromium and iron,
all other heavy metals in the
wastewater precipitate as sulfides.
The key elements of the system are:
pH control!
Mixer/clarifier
Reagent addition to mixer/
clarifier
FeS feed rate control
Sand filter j
!
Effective metal removal by sulfide
or hydroxide precipitation requires
that the pH of the wastewater
be controlled within the neutral
to slightly alkaline range. Although
the dependence of metal solubility
on pH is not jcritical for sulfide
precipitation systems, it still
affects metal Removal (see Figure 3).
It is more important to eliminate
the danger of the FeS slurry coming
into contact with acidic waste-
water; FeS is| soluble in acidic
solutions, and mixing it with low-pH
wastewater would result in the
emission of toxic H2S fumes in the
work area. The risk is minimized by
installing a recycle control on
the feed to the mixer/clarifier. If
the pH of the feed stream drops
below 7, valves automatically
reroute the feed back to the second-
stage neutralizer. For this reason,
a surge volume, shown as Vs in
Figure 13, is required to store the
accumulated wastewater until
the control set-point is reestablished.
The mixer/clarifier shown in
Figure 13 serves two purposes.
First, it provides the solid/liquid
contact volume between the
wastewater and the FeS slurry
necessary to maintain the waste-
water sulfide-ion concentration at its
saturation point. As illustrated
in Figure 6, both mixing time and
sludge blanket density in the
solid/liquid contact zone affect
metal removal. Second, it clarifies
the effluent of suspended solids.
To achieve low concentrations
of dissolved metals, which are
characteristic of metal sulfides,
the liquid residence time in the solid/
liquid contact zone of the mixer/
clarifier must be sufficient for
the metal precipitation reaction to
reach completion. Proper agitation
in the contact zone will enhance
the degree of reaction completion
achieved as well as promote
particle growth of the precipitated
metal sulfides. The formation of
large, rapid-settling particles -
facilitates removal of the solids
by clarification.
Reagent addition to the mixer/
clarifier is controlled by a flow-
measuring device that monitors
the feed to the mixer/clarifier
and sends a signal to a counter,
which computes the cumulative
flow. The additions of fresh FeS
and polymer are controlled to
provide a set quantity of each when
I
25
-------
Signal
to start
polymer
and FeS
pumps
Dump
sump
Rinse
sump
Logond:
pHC = pH controller
pHA= low-pH alarm
RC — recycle control
MFM — magnetic flow meter
FC — flow counter
Vj= surge volume in
second-stage neutralizer
N/C~ normally closed
Filtrate to
socond-stage
noutralizor
Sludge filter
press
Figure 13.
Sulfex™ ISP Treatment System
26
-------
Ptant feed
(PF) (gal/h)
3 —
LU
Q
Blended feed
(BF) (gal/h)
Averaging
tank volume
(V) (gal)
Averaging tank retention
time in hours (r= V/PF)
PF instantaneous reagent demand
BF reagent demand with r= 1 h
BF reagent demand with r = 4 h
0
Figure 14. I
Impact of Averaging Tank Volume on Reagent Demand Variability
the counter records a set volumetric
throughput. The dosage rate
is determined for both reagents
by performing a series of jar tests.
A sample is taken from the second-
stage neutralizer and tested to
determine the required addition
of FeS.
Jar tests are conducted on approxi-
mately four samples to determine
the lowest FeS dosage that provides
optimum metal removal. Because
polyelectrolyte demand should
be proportional to the demand
for FeS, it is fed at a constant ratio
of the demand for FeS. Jar tests
normally are conducted once
or twice per shift to determine the
required addition rate.
The FeS feed rate control loop
automatically adds a preset amount
of reagent each jtime an increment
of wastewater enters the mixer/
clarifier. The amount of reagent
added is set manually based on
the results of the jar tests. The
inability to adjust the FeS reagent
dosage automatically in response
to changes in re-agent demand
complicates operation of ISP treat-
ment systems. To compensate for
the lack of autorpatic control,
two features must be considered in
design of the system:
• FeS reagent djemand averaging
• Maintaining arji inventory of
unreacted FeS in the mixer/
clarifier !
Reagent demand averaging requires
the elimination of sharp deviations
in wastewater flow rate and
pollutant concentration entering
the treatment system. Flow variabil-
ity normally is eliminated by
providing a surge volume upstream
of the treatment process and
treating the wastewater at a con-
stant average rate. The variability
of pollutant concentration can
be reduced by use of an averaging
tank—an agitated tank that stores
and blends the treatment system
feed before processing. The
impact of averaging tank volume
and retention time on reagent
demand variability is presented
graphically in Figure 14. As shown,
with 1 hour of retention time in
upstream process tanks, the
27
-------
variability of the mixer/clarifier
(blended feed) reagent demand is
equal to 54 percent of the plant
feed reagent demand variability;
with 4-hour retention time in
upstream process tanks, the mixer/
clarifier reagent demand variability
is reduced to 15 percent of the
plant feed variability. The graph
presents an idealized situation of
reagent demand fluctuating
around a constant average demand.
In actual practice, however, the
deviations may be long term and
may not average out to a constant
demand rate. The relationship
between retention time in upstream
blending tanks and demand
fluctuations is a key to operating
any treatment process that does
not adjust reagent supply to
changes in demand automatically.
Maintaining an inventory of
unreacted FeS in the mixer/clarifier
is needed to provide the sulfide
reagent when reagent demand
exceeds supply. Because demand
fluctuations are inevitable, an
inventory of reagent is essential
to consistently achieve maximum
removal of metals. The quantity of
FeS stored in the mixer/clarifier
is proportional to the quantity of
solids maintained in the unit
and to the concentration of FeS
in those soltds.
A sand filter is included in the
system to ensure that the waste-
water discharge contains a minimum
concentration of suspended solids.
To meet strict metal discharge
requirements, the level of dissolved
and insoluble metals in the effluent
discharge must be reduced to a
minimum. For both sulfide and
hydroxide precipitation systems,
a sand filter ensures that upsets
in the treatment system causing
turbidity in the clarifier overflow
will not jeopardize effluent quality.
FeS Reagent Consumption. As
shown in Figure 6, precipitation of
dissolved metals to the low solu-
bility level characteristic of metal
sulfides normally requires 2 to 4
times the stoichiometric amount of
FeS. The ratio of the amount
of reagent added to the stoichio-
metric demand establishes the
equilibrium concentration of
FeS in the sludge blanket solids.
The FeS added in excess of the
stoichiometric demand provides
the inventory of unreacted reagent
that is consumed when reagent
demand exceeds supply.
The concentration of FeS in the
sludge blanket as a function of the
ratio of reagent addition to stoichio-
metric reagent demand is shown
in Figure 15. The quantity of
reagent consumed as a function
of this ratio also is shown. Because
the underflow rate is set to balance
the solids loading rate, the con-
centration of FeS in the sludge
blanket also determines the
amount lost in the sludge underflow.
By defining the volume of the
solid/liquid contact zone and the
density of the sludge blanket in
this zone, the amount of FeS
stored can be approximated. The
larger the quantity iof unreacted
FeS maintained in the blanket, the
greater the ability of the system
to compensate automatically
for increases in reagent demand.
The FeS supply can be increased by:
• Increasing the FeS reagent
feed rate
• Designing larger solid/liquid
contact volume 'into the system
• Maintaining the maximum
sludge blanket solids concentra-
tion in the solid/liquid contact
volume that is compatible with
good clarification in the settling
zone of the mixer/clarifier
The first two methods of increasing
the FeS inventory have economic
penalties: reagent cost and sludge
volume rise as dosage is increased,
and the initial cost and space '
requirements increase as larger
mixing volume is designed into
the system. Therefore, maintaining
a dense sludge blanket in the
mixing zone is the most efficient
way to achieve good reagent use
and to provide the inventory
of FeS needed for reagent demand
increases. In practice, this requires
monitoring the blanket level
and adjusting the sludge drawoff
rate to match the solids accumula-
tion rate in the system.
Operating Procedure. The ISP
system shown in Figure 13 required
a full-time operator during one
shift and approximately 2 to 4 hours
of operator attention during other
shifts. Operator duties are as follows:
• Once each shift, a sample of
mixer/clarifier feed is removed
from the second-stage neutralizer
for jar testing to determine
the required FeS addition rate.
• Based on the jar test results, the
FeS and polyelectrolyte addition
control system is set to feed
the needed quantity of reagents
each time a set feed increment
has entered the mixer/clarifier.
• The timer that controls the sludge
blowdown is adjusted to reflect
any change in the solids loading
rate. (This relates to the jar
test performed in the first step.)
• The level of solids in the mixer/
clarifier is monitored periodically
(normally every 1 or 2 hours)
by performing a settling test on
samples removed from the
mixing zone of the mixer/clarifier.
The sludge blowdown rate is
adjusted to maintain the maximum
solids concentration in the
28
-------
-Q
to
C/3
Q
LU
O
<
L1J
CC
to
RATIO OF FeS SUPPLIED |TO STOICHIOMETRIC DEMAND3
8x = the stoichiometric
b
S! reagent requirement.
Based on treatment of wasjtewater containing 100 ppm Cu+
Figure 15. I
Sludge Blanket FeS Concentration and Associated Reagent Demand
mixing zone that is compatible
with low levels of turbidity in the
clarified effluent.
Other operator duties generally
required for operation of this
system and most treatment systems
include:
• Preparation of treatment
reagents—in this case, reagents
include lime slurry, Sulfex™
reagent (Figure 16), and polyelec-
trolyte j
Operation of sjudge dewatering
filter i
• Periodic back-flush cleaning
of the sand filter
• Periodic calibration of pH probes
• Collection of samples required
for discharge permit
© Regularly scheduled lubrication
of system elements
29
-------
210 Ib NaHS"
Sulfex" reagent:
0.21715 FaS/gal
0.02 Ib Fo(OH)2/gal
0.23 Ib CaSO4/gal
0.06 Ib Na+/gal
0.09 Ib SOJ2/gal
0.02 Ib OH"7gal
800 Ib FeSO4 • 7H2O
Batch volume = 1,080 gal. FeS = 0.217 Ib/gal.
Reagent cost of $0.43/lb FeS based on:
Ca(OH)2 = $163/tonb-c
NaHS = $370/tona'd
FeS04-7H20 = $144/tond
a70% to 72% flake.
"93% pure.
"Includes shipping and palletizing.
dlncludes shipping.
"One in use, one for batch preparation.
Figure 16.
FeS Feed System
ISP Polishing Treatment System
The FeS reagent demand for the
system shown in Figure 13 is a
function of the total metal load
entering the mixer/clarifier. Sufficient
FeS must be added not only to
precipitate the dissolved metals
but also to convert the precipitated
metal hydroxides to metal sulfides.
For treatment systems with a high
mass flow of metals, FeS consump-
tion will be high and considerable
waste solids (a combination
of metal sulfides, metal hydroxides,
and unreacted FeS) will be gen-
erated. For these applications, the
reduction in reagent consumption
and solid waste disposal charges
may justify using ISP to polish
the clarified overflow after a
conventional hydroxide precipita-
tion/clarification treatment sequence
(Figure 17).
In this polishing system, the
FeS demand is based on the metals
contained in first clarifier overflow.
If hexavalent chromium is present
in the wastewater, it will be
reduced in the second-stage
mixer/clarifier and precipitated
along with the dissolved metals.
Two advantages of this approach,
compared with the system shown
in Figure 13, are reduced FeS
reagent demand and reduced
sludge generation, which is a func-
tion of metal load.ing and reagent
consumption. Another advantage
is that the concentration of metals in
the first-stage clarifier overflow
will not be subject to the wide
variation that often characterizes
the wastewater feed metal con-
centration. The metal hydroxide
equilibrium solubility will determine
the concentration of dissolved
metals in the overflow; this concen-
tration will establish reagent
demand. Again, because reagent
supply is not adjusted automatically
for changes in demand, this
feature increases reliability. The
concentration of hexavalent
chromium, which is unaffected
by the hydroxide treatment, will
still be subject to variation, but
the variability should be reduced
because of the larger volume of
upstream process tanks in a polish-
ing treatment system.
30
-------
Ca(OH)
Wastewater
discharge
Metal hydroxide
sludge
Metal sulfide
sludge
Figure 17.
ISP Polishing System
Identification of the optimum
system—polishing sulfide precipi-
tation or treatment of the total
metal load—requires determining
whether the operating cost savings
of the polishing system offset
the additional cost of a second
mixer/clarifier and polyele'ctrolyte
feeder.
ISP System Performance
Three plants currently use the
Sulfex™ system to remove heavy
metals from wastewater discharge.
All three systems were placed
in plating shops where no waste-
water treatment systems existed.
Two of the plants (Plants A and B)
treat the total metal load with
FeS, whereas the third (Plant C)
employs ISP as a polishing step
after hydroxide precipitation/
clarification.
Plant A performs copper, nickel,
and chromium plating (both
electroplating and electroless
plating) of plastic components. The
Table 7.
Plant A Discharge Permit Requirements
Discharge limits8
Item
Mass (Ib/d)
Concentration (ppm)
Averageb Maximum0 Average13 Maximum0
Total copper J
Total nickel \
Total chromium . . . J
Hexavalent chromium
35 3
089
089
0 89
0.089
53 0
1 77
1 77
1 77
0 177
NAd
1 0
1 0
1 0
005
NAd
1 5
1 5
1 5
0 10
aRequired pH level is between 6.0 and 9.5.
bMonthly average of daily 24-hour composite samples.
°Highest daily 24-hojur composite in the month.
dNot applicable. !
heavy metals imthe wastewater are
complexed with a variety of chelating
agents. During the pilot evaluation,
it was apparent [that hydroxide
precipitation would not remove
the metals to the levels required
in the discharge permit (Table 7).
After a pilot evaluation showed
that ISP could achieve the required
discharge limitations, the firm
hired a vendor to design a treatment
system using this technology.
The vendor guaranteed that the
system would meet all discharge
regulations.
31
-------
200 I—
100
10
o
I
UJ *
U I
8
0.1
Mixer/clarifier feed
Plant effluent
0.01
12 18 24 30 36 42 48 54 60
Legend:
ITotaJ chromium
I Hexavalent chromium
Figure 18.
Plant A's Performance in Removing Chromium
The system was designed to
treat 40 gal/min (151 l/min) of
wastewater and is essentially
identical to the system shown in
Figure 13. The performance
of the system in removing copper,
nickel, total chromium, and hexa-
valent chromium (Cr+6) during
a 60-hour test period is shown
in Figures 18 and 19. Figure 20
shows the corresponding sample
point locations.
The performance in chromium
removal shows a deviation from
normal removal efficiency between
hours 16 and 28 that corresponds
to an increase in the level of
hexavalent chromium in the mixer/
clarifierfeed during hours 8 through
28. By comparing the stoichiometric
FeS demand with the quantity
supplied and the associated
mixer/clarifier removal efficiency
(Figure 21), it is obvious that
the FeS feed was not increased
sufficiently to compensate for the
increased demand. Consequently,
the level of unreacted FeS in the
sludge blanket was gradually
depleted, and at hour 1 6 insufficient
FeS was present in the blanket
to achieve the normal high level
of removal. This condition persisted
until hour 28. The FeS stored
in the sludge blanket maintained the
high removal efficiency between
hours 8 and 16, despite a low
FeS reagent supply/demand ratio.
32
-------
100
10
0.1
0.01
Mixer/clarifierfeed
Plant effluent
6 12 18 24 30 36 42 j 48 54 60
TIME(h)
Legend:
Nickel
Copper
Figure 19. '
I
Plant A's Performance in Removing Nickel and Copper
Figure 21 shows that optimum
removal efficiency for the chromium
is achieved with an FeS dosage
of approximately 3 times the
stoichiometric demand. Stoichio-
metric demand was determined
by laboratory analysis of mixer/
clarifier feed samples. The removal
efficiencies for nickel and copper
were relatively constant and
showed no discernible trends over
the dosage ratios encountered
during the test period.
Based on an FeS dosage rate of
3 times the stoichiometric demand
and the observed consumption
of other treatment reagents, the cost
of treatment chemicals and sludge
generation factors for the ISP
system at this facility are shown
in Table 8.
Plant B manufactures carburetors
for the automotive industry.
Wastewaterfrom the metal finishing
portion of the process contains
varying quantities of chromium
(hexavalent and trivalent), zinc,
and iron in solution with phosphates,
organic chelating agents, and
assorted chemicals used in the
process baths. The wastewater is
treated in a neutralization/ISP/
clarification treatment sequence
similar to that shown in Figure 13.
Then it is mixed with the remainder
of the wastewater from the plant
and is discharged to the city waste-
water treatment system.
33
-------
Mixer/clarifier
feed
Mixer/clarifier
overflow
Plant
effluent
Socond-stage
noutralizer
Mixer/clarifier
Filter feed tank
Figure 20.
Sample Points
Anionic and oationic polymer feed systems
34
-------
The wastewater flow rate to the
system averaged 20 gal/min
(76 l/min). The performance of the
system during a 2-day test in
removing chromium (total and
hexavalent), zinc, and iron from the
wastewater is shown in Figures 22
and 23. The same sample location
designation used in Figure 20
applies. Figure 24 defines the ratio
of FeS supply to stoichiometric
demand for the same test period.
The ratio varied from 3 to 5 times
the stoichiometric demand during
the test period. The quality of
the effluent, which contained lower
pollutant levels than those specified
in both local and State guidelines,
showed no discernible trends within
this range of reagent supply/
demand ratios.
The cost of treatment chemicals
and the sludge generation factors for
the ISP system at this facility
are shown in Table 8. Chemical
costs were approximately $1.77'/
1,000 gal of wastewater treated.
Plant C uses the ISP process to
polish the clarified overflow from a
conventional hydroxide precipita-
tion/clarification treatment
sequence. The system treats
approximately 15 to 18 gal/min
(57 to 68 l/min) of wastewater from
a programed, barrel-dip, zinc-
phosphatizing plating line. The
system is similar to the one shown
in Figure 17; it has a second
mixer/clarifier and polymer feed
system, installed after the second-
stage neutralizer, to remove
the precipitated metal hydroxides
and phosphates. Dual polyelec-
trolyte feed systems are needed
because an anionic polymer
is used in the hydroxide removal
clarifier and a cationic polymer is
used to enhance the settling
of the precipitated metal sulfides.
The sludge production and FeS
consumption are reduced consider-
ably compared with a system
(a)
Q
m 0
J E,
9 I cc
"- O =,
(b) 0.7 ,-
I
2 0.1
o
o
0.01
I
12
18
24 30 36
TIME (h)
42
48
54
60
TIME (h)
Figure 21. |
i
Impact of FeS Supply/Demand Ratio on Reduction of Hexavalent Chromium
at Plant A: (a) Fe'S Supply vs. Stoichiometric Requirement and (b) Mixer/
Clarifier Overflow Chromium Concentration
35
-------
Table 8.
Wastewater Treatment Process Characteristics for Plants A, B, and Ca
Waslewaten
Average flow rate (gal/min)
pHi
Feed
Effluent
Average feed concentration (ppm):
Nickel
Copper
Hexavalent chromium
Total chromium
Zinc
Iron
Phosphorus
Treatment chemicals:
Lime:"
Ib/h
S/h
Calcium chloride (for phosphate removal):
Ib/h
S/h
Calionic polymer:5
Ib/h
S/h
Anionic polymor.b
Ib/h
S/h
Ferrous sulfida:
Ib/h
S/h
Total chemicals (S/h)
Chemical cost (S/1 .000 gal)
Sludge generation factors:
Dry solids generation:
Ib/h
First stage
Second stage
lb/1,000 gal wastewater
Underflow volume (gal/h at 0.75% solids)
Filler cake volume (gal/h at 30% solids)
Plant A
39
2.0-4.0
9.0-10.0
31
28
76
88
NA
NA
NA
8.8
0.28
NA
NA
0.1
0.14
NA
NA
12.5°
5.37°
5.78
2.47
23.7
NA
NA
10.1
380
7.9
Value
Plant B
21
4:5-6.0
8.5-9.5
1
NA
;NA
27
39
: 48
1.4
NA
2.0
0.06
:NA
.NA
0.17
0.23
NA
NA
4.5d
' 1 .94d
2.23
! 1.77
! 7.2
, NA
NA
5.7
114
2.4
Plant C
16
2.5-3.0
7.5-8.5
NA
NA
0.07
8
24
127
289
8.1
0.60
17.0
1.70
0.02
0.04
0.01
0.03
0.30b
0.1 1b
2.48
2.58e
16.4
16
0.4
17e
262
5.3
'All three plants use an ISP process to remove metals from wastewater, but Plant C uses ISP as
o polishing system.
bObsorved rates.
'Based on 3 times the stoichiometric requirement.
dBased on 4 times the stoichiometric requirement.
"Without the presence of phosphates, treatment cost equals $0.81/1,000 gal, solids generation
equals 6.4 lb/1,000 gal.
Note.—1979 cost basis, NA = not applicable.
treating the total metal load
with sulfide precipitation. Less
than 5 percent of the waste solids
removed from the system are
attributed to the sulfide precipita-
tion step.
Table 8 presents the chemical
consumption and sludge generation
rates for Plant C. Treatment of
the phosphates in the wastewater
accounts for a large percentage
of the treatment cost, and the
phosphate solids constitute the
bulk of the sludge generated.
The chemical cost associated
with removal of the heavy metals
contained in the wastewater
was estimated at $0.81/1,000 gal.
Without the presence of phosphates,
the solids generation rate would
equal 6.4 lb/1,000 gal (0.76
kg/m3) of wastewater.
Table 9 presents the pollutant
concentrations in Plant C's raw
waste and effluent discharge and
shows the effluent quality required
by the discharge permit.
In this polishing application,
FeS is fed into the second-stage
mixer/clarifier to yield a concentra-
tion of approximately 40 ppm
in the wastewater. The dosage rates
for the insoluble solids systems
treating the total metal load
for Plants A and B are approximately
640 ppm and 430 ppm, respectively.
36
-------
: _ .
100
10
=•
.1
z
o
< 1
£ '
z
UJ
0
o
o
0.1
-
~ Mixer/clarifierfeed
* ^**^fc, «te
^^*^^ ^ ^
¥
—
~
-
~
-
Bk.
^^^k
- ^V ^^^^
^ ""^
" - ^^^^
- Plant effluent
J ^Sm ^. *
/, N /
^<^»^^
r^
^r ^^^^
L
•••I Iron
•• ••Zinc
aummmmmmm
^^
t \
f \
0 4 8 12 16 20 24 28 32 36
TIME (h)
Note. — The plant operates two shifts per day; there was no wastewater flow between
hours 1 6 and 24.
Figure 22.
Plant B's Performance in Removing Iron and Zinc
Hydroxide System Modifications
for ISP
The metal removal efficiency of a
hydroxide precipitation system
can be improved by incorporating
ISP into the system. Sulfide
precipitation can be used either to
convert the metals to metal sulfides
before the clarifier or as a polishing
system to precipitate dissolved
metals from wastewater after
the insoluble metal hydroxides have
been removed by clarification.
Equipment Requirements. The key
component of an ISP system
is the solid/liquid contact chamber
where the wastewater is mixed
thoroughly with the insoluble
sulfide contained in the sludge
blanket. Three design criteria must
be addressed in specifying this
piece of equipment:
• Liquid residence time in the
mixing zone
• Sludge blanket volume and
density
• Mixing efficiency
Figure 25 is a schematic of the
mixer/clarifier designed specifically
for this application. In the systems
currently using ISP, the unit is
sized to provide approximately
1 hour of liquid residence time in
the mixing zone. Because the
mixing zone volume is equal to the
solids retention volume, a large
inventory of unreacted FeS can be
maintained in the unit. The agitator
is designed to maintain a dense.
fluidized sludge in the mixing
zone. Sample ports are located
in the different zones of the unit
to check the sludge density.
The unit also has a timed sludge
drawoff valve that can be set to
balance the blowdown to the solids
accumulation rate automatically.
37
-------
100
10
z
o
I
0.1
0.01
Mixer/clarifier feed
Legend:
I Total chromium
I Hexavalent chromium
Plant effluent
I I I I I
0 4 8 12 16 20 24 28 32 36
TIME (h)
Note.—There was no wastewater flow to the system between hours 16 and 24.
Figure 23.
Plant B's Performance in Removing Chromium
Other elements needed to augment
a treatment system with ISP include:
• FeS reagent preparation tanks,
reagent storage, and feed pumps
• A reagent feed control system
that matches reagent dosage to
wastewater flow rate
• A control loop to interrupt
the wastewater feed during
low-pH conditions
In converting a hydroxide system
to use sulfide precipitation, the
addition of a polishing filtration
system to remove residual sus-
pended solids from the clarifier over-
flow could significantly reduce
effluent metal concentrations.
Meeting strict effluent metal
discharge limits will require an
effluent with low levels of both
suspended and dissolved metals.
Treatment System Evaluation. The
cost advantages of using ISP
as a polishing system must be
weighed against the higher equip-
ment costs and space requirements
of a second clarifier. It might
be more cost effective for plants
with small metal loadings to
incorporate ISP upstream of the
existing clarifier and thus avoid
the expense of a second clarifier.
Retrofitting a hydroxide treatment
system that already has a flocculation
zone to enhance the settling
properties of the precipitated
metals before clarification can
be accomplished simply and with
minimum investment. Many existing
systems include a flocculation
chamber either in a separate vessel
or as part of the clarifier itself.
As shown in Figure 26, sulfide
precipitation can be incorporated
38
-------
Q L1J
LLJ QC
•t
CO CC
I
y 2
!e
CO
o
12
ije
20
24
28
32
J
36
TIME (h)
Note.—There was no wastewater flow to the system between hours 16 and 24.
Figure 24. I
FeS Supplied vs. Stoichiometric Requirement at Plant; B
Table 9. i
Influent and Effluent Wastewater Characteristics for ISP Polishing System
Item
pH
Phosphorus (mg/l)
Total suspended solids (mg/l)
Total chromium (mg/l)
Hexavalent chromium (mg/l)
Nickel (mg/l)
Zinc (mg/l)
Iron (mg/l)
'Monthly average of daily composite samples.
Wastewater analysis
Influent
2 9
289
320
8
007
0 77
24
1 27
Effluent
8.5
j 0.3
i 6
]<0.10
: <0.02
i<0.1
, 0.12
j 0.60
i
I
Permit
requirements8
6.0-9.5
<1.2
<23
<0.6
<0.06
<0.6
<0.6
<1.2
into this type of treatment system
by installing:
• An FeS reagent addition system
and feed control system to
feed FeS into the flocculation
chamber in proportion to the
volume of wastewater processed
A sludge recirculation loop
(if not already! existing) to
recycle solidsjfrom the clarifier
underflow back to the flocculator
A low-pH feed interrupt control
loop to stop the feed to the
flocculator if the pH of this stream
falls below the set-point
Pilot tests must be performed to
determine if the residence time,
agitation, and blanket density
in the flocculation chamber are
conducive to effective metal
removal. Figure 6 defined the differ-
ent variables for evaluation by
pilot testing or jar testing. Deficien-
cies in the flocculator residence
time, mixing efficiency, and
the like can be tolerated, although
they generally result in increased
reagent consumption.
An approach for treatment systems
that do not have flocculation
zones is either to add a flocculator
or to replace the existing clarifier
with the mixer/clarifier designed for
this application (Figure 25).
The most reliable approach to
using ISP as a polishing system
would be to install a mixer/clarifier
downstream of the existing clarifier.
39
-------
Influent » ~^*-^
Legend:
Jje =|sampling port
Figure 25.
Cross Section of Mixer/Clarifier
ISP Batch Treatment Systems. Batch
treatment systems currently
are not demonstrated for ISP.
As with continuous treatment
systems, batch treatment using
ISP would require contact between
the wastewater and a dense sludge
blanket to achieve maximum
metal removal. Consequently, a
large volume of solids would be
needed for each batch, necessitating
storage of the settled sludge
after batch treatment. Figure 27
shows a possible configuration
of an ISP batch treatment system
and the associated treatment
sequence. The major process com-
ponents of the system are:
• Two tanks equipped with
mechanical agitation
• A precipitation tank
• Reagent storage and feed
systems to add the lime (or
caustic), FeS, and polymer
The two agitated tanks alternate
as the wastewater collection tank
and pretreatmenttank. Pretreatment
is required to neutralize the
acidic wastewater before mixing
it with the metal sulfide sludge.
A precipitation tank is needed
to bring the wastewater into contact
with the FeS slurry and to provide
storage volume for maintaining an
inventory of sludge solids in
the system. Gentle agitation is
required to suspend the sludge
solids during mixing and to promote
particle growth of the precipitated
solids.
ISP Treatment Costs
Operating Costs.The following costs
associated with using ISP are in
addition to the operating costs of
40
-------
Ca(OH),
Wastewater
discharge
Clarification
Legend:
pHC = pH controller
Solids
bleed-off
to disposal
Figure 26. I
Retrofit of a Hydroxide System With Insoluble SulfidJ Treatment
a conventional hydroxide pre-
cipitation system:
• Reagent costs for FeS and
polyelectrolyte
• Labor cost of additional opera-
tional duties described earlier
• Disposal cost of any additional
solid waste generated
• Licensing fee charged by the
patent holder to use the process
Reagent costs' for FeS depend on
the quantity of metals to be
precipitated (or, in the case of
hexavalent chromium, the quantity
to be reduced chemically) and
the ratio of reagent needed for
effective removal to the stoichio-
metric reagent requirement. Figure
28a shows the FeS consumption
rates and reagent cost for various
metal concentrations in the
wastewater and jtypical ratios
of reagent demand to stoichiometric
requirement. The wastewater metal
concentration is1 defined as
the metals other than iron that
will form sulfides. To compute
reagent consumption rates, it was
assumed that the metals have a
"plus 2" valence and a molecular
weight equal to the average
molecular weight of copper, nickel,
and zinc. Although determination
of the optimum dosage ratio
requires testing, waste waters
with no heavy metal complexing
agents generally require 1.5 to 2
times the stoichiometric reagent
requirements, whereas wastewaters
containing complexed heavy
metals will require 3 to 4 times
the stoichiometric reagent dosage.
Figure 28b presents the FeS
reagent demand and cost for
wastewater treatment over a range
of hexavalent chromium con-
centrations.
At the three plants currently
operating, labor requirements
for the ISP systems varied only
slightly. Each plant employed a full-
time operator for one shift and
required 2 to 6 hours of operator
attention on other shifts.
41
-------
FeS and polymer •
Wastowatcr containing
metals (Cr+6)
Ca(OH)2
Tank1°
(wastewater
collection)
*Tanks 1 and 2 alternate in process function.
Noto —Treatment sequence: When Tank 2 is filled to capacity, incoming wastewater is diverted to Tank 1. The pH of the wastewater in Tank 2
is adjusted to 8.5. A sample is removed from Tank 2 and analyzed by jar test procedure to determine required FeS dosage. The wastewater
in Tank 2, along with the required amount of FeS and polymer, is charged into Tank 3. The wastewater/sludge mixture in Tank 3 is agitated for
1 hour Agitation is stopped and the solids are allowed to settle. A sample of the clarified wastewater is analyzed to check water quality.
The wastewater in Tank 3 is decanted and discharged. A portion of the settled sludge is discharged to sludge disposal to maintain a constant
sludge inventory.
Figure 27.
Batch Wastewater Treatment Using ISP
The ISP systems generate consider-
ably more sludge in treating a
volume of wastewater than the
conventional hydroxide precipitation
scheme. The additional sludge
results from precipitation as
hydroxides of the ferrous and ferric
ions liberated as the sulfide
reagent is consumed and from
the excess FeS that is used in
treatment. Figure 29 compares the
solids generation rates for ISP
systems with those for treatment
systems using hydroxide precipita-
tion for metal removal and sulfur
dioxide (SO2) for chromium re-
duction. The graph also shows
solid waste disposal charges,
assuming the sludge is disposed
of at 25 percent solids by weight
and at a cost of $0.10/gal. For
plants with different sludge disposal
cost formulas, the disposal cost
can be calculated by multiplying
the cost indicated in Figure 29
by the ratio of the;actual disposal
cost to the assumed rate of
$0.10/gal.
Owing to the high cost of sludge
disposal—normally from $0.05/gal
to $0.20/gal—it is cost effective
to invest in mechanical dewatering
equipment to reduce the sludge
volume. At the three plants
currently operating, recessed
plate filter presses were installed
42
-------
(a)
Q
<
uj CTJ
(T, 0)
< O
UJ O
oc q
co 17
9 r-
6 -
3 -
100
150 |
WASTEWATER METAL CONCENTRATION (ppm')b
200
(b)
< s
5 o
QJ +2
Q a
r
50 100 150
WASTEWATER Cr+6 CONCENTRATION (ppm) i
200
i- s
co S
• o
8
Legend:
I 4 times the stoichiometric
reagent requirement
3 times the stoichiometric
reagent requirement
2 times the stoichiometric
reagent requirement
'Based on FeS at $0.43/lb.
bOnly includes those metals, other than
iron, that form sulfides; based on metal with
molecular weight of 62.5 (average of
Ni, Cu, and Zn).
Figure 28.
I
FeS Consumption and Cost Factors for: (a) Precipitatior^ of Metals and (b) Hexavalent Chromium Reduction
to dewater the sludge before
transport to the disposal site.
The presses dewatered the under-
flow from less than 1 percent solids
by weight to 25 to 30 percent
solids by weight.
Total sludge generation for both
hydroxide and su'lfide systems
will be somewhajt higher than
the rates shown in Figure 29. The
additional solids are caused by the
presence of lime Isolids, suspended
solids in the wastewater feed,
and insoluble byproducts resulting
]
from neutralization. For treating
waste streams to remove heavy
metals, the additional solids should
be approximately the same for
43
-------
(a) 9 .-
_ 0.36
§
cc 5
m o
Z n
UJ 91
O S
-------
Equipment Costs. The actual total
installation costs for the three ISP
treatment systems described earlier
are presented in Table 10. All
three systems were installed in
plants that had no existing treatment
systems. The systems in Plants A
and B are similar to the one illustrated
in Figure 13. The costs presented
also include duplexing of many of
the pumps and reagent storage
tanks, a control panel, and additional
instrumentation not shown on the
flow diagram. Plant C is a sulfide
polishing system similar to the
one shown in Figure 17. The installed
cost of this system includes the
additional equipment required by
a polishing system—a second
clarifier (to separate the insoluble
compounds resulting from hydroxide
neutralization) and a second
polyelectrolyte feed system.
Much of the equipment in an ISP
system is common to hydroxide
systems. Cost data on wastewater
treatment equipment for the
metal finishing industry are
presented in the EPA report,
Economics of Wastewater Treat-
ment Alternatives for the Electro-
plating Industry. Converting
a hydroxide system to use ISP
in many cases will require only the
installation of a mixer/clarifier
downstream of the existing clarifier
and a feed system to meter the
FeS and polyelectrolyte into
the wastewater.
Table 11 presents the cost (including
installation and hardware) of
installing the following ISP process
Table 10.
Installation Costs for Three Sulfex™ ISP Treatment Systems
Cost component
ISP system cost ($1,000)
Plant A Plant B Plant C
Installation costs: 1
Process equipment . '. . . . .
Underground tanks
Shipping and installation
Additional building space
Startup expenses
Engineering . . j
Other ,. J...
j
Total j
j
Current installation cbstsd
i
a|OD 1
1 75
36
29
20
3
NA
NA
303
QJ
1 7
bISP polishing system design flow = 35 gal/min; installed in April 1978.
°ISP polishing system design flow = 15 gal/min; installed in March 1978.
Costs escalated to fj/larch 1979 based on Chemical Engineering Plant Cost Index.
Note.—NA= not available.
Table 11.
i
Equipment CostjFactors for ISP Treatment System Components
Equipment component
Installed cost
($1,000)
Mixer/clarifier:
I
30-gal/min wastewater flow rate.
60-gal/min wastewater flow rate.
90-gal/min wastewater flow rate
Ferrous sulfide reagent preparation and feed system-
5-lb/h FeS feed r[atea
10-lb/h FeS feed, rate
15-lb/h FeS feed! rate
Polymer feed system. |
Control loops: j
Reagent additionjsystem
Low-pH feed interruption control
Suspended solids poli'shing filters:
30-gal/min wastewater flow rate
60-gal/min wastewater flow rate
90-gal/min wasteyvater flow rate
18
22
24
16
20
24
4-6
4-5
1.5-2.5
24
33
41
aFor lower feed rates, less automated systems are available for approximately $12,000.
Note.—March 1979 cost basis. Costs are basic installed costs of different components. Engineering
and design costs, site 'preparation, and equipment freight charges are not included.
SOURCE: Equipment vendors.
45
-------
(a)
Chromium wastewater
(pH^4)
(b)
Chromium wastewater
(pH^4)
^
Note.— Table
H2SO4 SO2 or NaHS03 Ca(OH)2 Polymer
1 i I 1
Acidify
(pH = 2)
Ca(OH)2
I
Neutralize
(pH = 8.5)
*^
Reduce
Cr+6 to Cr«
FeS
|
Reduce
and
precipitate
• i*r.
-
Neutralize
precipitate
Polymer
|
Clarify
^^
Dewater
sludge
disposal
^ Dewater ^^
"^ sludge W^
Sludge
disposal
fc~
12 presents cost basis for comparison of chemical and insoluble sulfide chromium reduction systems.
Figure 30.
Comparison of Chromium Reduction Treatment Sequences: (a) Chemical and (b) Insoluble Sulfide
equipment components in an
existing treatment system:
Mixer/clarifier
FeS reagent preparation and
feed system
Polymer feed system
Control loops
Suspended solids polishing filters
The installed costs presented
for a mixer/clarifier are for a
preassembled, skid-mounted com-
ponent requiring only piping
and electrical connections for
installation. The FeS reagent
preparation and feed system in-
cludes two FeS feed tanks with
low-level alarms, two reagent
pumps, a mixing tank, and a
transfer pump; the costs are for
skid-mounted, preassembled units,
constructed of carbon steel (see
Figure 16).
The costs presented for the polymer
feed system are based on a system
with two plastic polymer feed
tanks and two positive displacement
pumps with adjustable stroke.
The skid-mounted,.preassembled
components are equipped with
a low-level alarm dnd dilution
water-mixing apparatus. Costs are
given for two control loops: a
reagent addition control system
with a magnetic flow meter and
flow counter (to match the addition
of FeS and polymer; with wastewater
volumetric throughput) and
a low-pH feed interruption control.
The costs for suspended solids
polishing filters are for dual
mixed-media filters, skid mounted
and sized so that One filter can
process the maximum flow during
backwash. The filters are equipped
with a blower for low pressure
air scouring, a backwash storage
tank, and a pump to bleed the
wash back into the system.
Cost Comparison of Conventional
Chemical Reduction and ISP
Chromium Reduction. Replacing
a conventional chromium reduction
system with reduction by FeS can
be advantageous. In some cases,
an operating cost benefit will
result. Another advantage of
reducing chromium with FeS is that
the hexavalent chromium wastewater
does not need to be segregated
for individual treatment; it can be
treated in the common neutraliza-
tion/precipitation treatment
sequence. Figure 30 defines typical
treatment sequences for reduction
of chromium by chemical means
and using FeS. The FeS treatment
46
-------
process eliminates the need
to lower and raise the pH of the
wastewater and results in a
significant saving in acid and
alkali reagent. Table 12 presents
treatment and sludge disposal costs
for the two chromium reduction
systems shown in Figure 30. The
chemical consumption factors
assume that lime consumption is
twice the stoichiometric amount
required to neutralize the wastewater
and precipitate the dissolved
metals. The excess lime is needed
to overcome buffering normally
encountered when neutralizing
waste streams. It is further assumed
that lime solids equal to 50 percent
of the mass of lime used in neu-
tralization are present in the
sludge. These lime solids result
from precipitation of insoluble
byproducts in the neutralization
reaction as well as from the
tendency for some portion of the
lime used not to dissolve and
add to the sludge volume. Conse-
quently, the lime required in the
chemical reduction treatment
sequence to raise the pH from 2 to
8 results in considerable sludge
generation.
Figure 31 compares the cost of
treatment chemicals and sludge
disposal for the two chromium
reduction systems shown in
Figure 30 over a range of hexavalent
chromium concentrations in the
wastewater. A cost savings can be
realized for FeS reduction compared
with conventional chemical
reduction. Forwastewaters requiring
twice the stoichiometric FeS
dosage, a treatment cost advantage
exists over treatment of wastewater
containing less than 50 ppm Cr+e
by SO2 reduction and that containing
less than 100 ppm Cr+6 by NaHSO3
Table 12.
I
Cost Basis for Comparison of Chemical and Insoluble Sulfide Chromium
Reduction Treatment Systems Shown in Figure 30
Cost"
Para'mete
Treatment6
Sludge disposal0
$/lbCr+6 «/1 .°00 gal +B $/1,OOOgal
wastewater wastewater
Chemical reduction:!
Sulfur dioxide, j 0.43
Sodium bisulfite ' 0.82
Insoluble sulfide reduction:
Ferrous sulfidej at dosage equal to
2 times stoichiometric require-
ment , 1.58
Ferrous sulfidejat dosage equal to
4 times stdichiometric require-
ment \ 3.12
0.57
0.68
0.03
0.03
0.16 0.12
0.16 0.12
0.21 0.01
0.33 0.01
"Total treatment cosf is based on both mass of chromium reduced and volume of wastewater
treated. j
bBased on lime at $0.035/lb, sulfur dioxide at $0.15/lb, sodium bisulfite at $0.20/lb, sulfuric acid
at $0.05/lb, and ferj-ous sulfide at $0.43/lb.
C8ased on disposal at 25 percent solids by weight at a cost of $0.10/gal sludge.
Note.—1979 cost basis. Sulfur dioxide and sodium bisulfite consumption is equal to 2 times the
stoichiometric requirement at a hexavalent chromium (Cr+6) concentration of 50 ppm. Lime
consumption is equal to 2 times the stoichiometric requirement for unbuffered waste streams. Lime
solids are 50 percent of lime dosage and contribute to sludge volume.
reduction. For FJeS reduction
systems requiring twice the
stoichiometric dosage rate, a
savings in solid iwaste disposal
costs also would be realized
for treatment of .wastewater con-
taining less than 150 ppm Cr+6. At
higher FeS dosage requirements,
such as 4 timesjthe stoichiometric
demand, chromijjm reduction
using FeS is mofe economical
for treatment of jdilute chromium
waste streams, j
|
It is important to point out that
the preceding comparisons are
based on typical] operating condi-
tions and reagent costs; a compara-
tive analysis for £ specific plant
should use actual operating data
(e.g., reagent consumption and
sludge generation).
I' ' '
Cost Comparison of ISP Polishing
and Total Metal Treatment.
Converting all metals in a waste
stream to metal sulfides via sulfide
precipitation uses considerable
FeS and results in a large volume
of waste solids. Separation of
the precipitated metal hydroxides
from the wastewater before
polishing with sulfide precipitation
can reduce both reagent consump-
tion and solid waste generation.
In a polishing application, the
FeS reagent demand is a function
of the dissolved metal concentration
in the wastewater after hydroxide
precipitation/clarification. Conver-
sion of a sulfide precipitation system
47
-------
2.0 I-
1.5
uj 3
S 1
M
ll 1.0
Q •
UJ
cc ,
0.5
EC
X
u
V
Reduction with SO-
_L
Legend:
50 100 150
Cr+8 CONCENTRATION (ppm)
50 100 150
Cr+6 CONCENTRATION (ppm)
200
-a 0.40
- 0.30
- 0.20
- 0.10
200
FeS reduction at 4 times
the stoichiometric requirement
0 FeS reduction at 2 times
the stoichiometric requirement
8
03
If
"Based on disposal at 25% solids by weight
at $0.10/gal sludge.
Note.—Based on treatment parameters
defined in Table 12.
Figure 31.
Costs of Treatment Chemicals and Sludge Disposal for Chemical and Insoluble Sulfide Chromium Reduction
to a polishing system requires
installation of a second clarifier
and polyelectrolyte feed system
to separate the precipitated
metal hydroxides from the neutralized
wastewater before adding the
sulfide reagent.
The reagent consumption and solid
waste generation'factors associated
with treatment of the total metal
load were presented in Figure 28.
To estimate reagent requirements
for a sulfide polishing system,
it is necessary to determine the
concentration of ;the metals in the
wastewater after hydroxide
neutralization/precipitation/clarifi-
cation. Reagent consumption
ranges between 1.5 and 4 times
the stoichiometric demand for
polishing systems. Compared with
the reagent consumption factors
presented in Figure 28, the sulfide
precipitation polishing system at
Plant C required an FeS dosage
rate of 40 ppm in the wastewater.
Note, however, that this system
did not have a significant level
48
-------
Table 13. '
I
Potential Benefits for Use of ISP Polishing System at (plant A
Item
Value
Wastewater characteristics: j ;
Average flow rate (gal/min) I
pH: ] 39
Feed j
Effiuem :;;:;;;::;;:;;:;; 3:::::::::::::::::::;::;; 9 :Q
Average feed concentration (ppm): '
Nickel !
copper ;;;;;;.;;;:;;;;;;;;;;;;;;;;;;;;;;;;;:;;;;;;;;;;;;"" 28
Hexavalent chromium j
Total chromium ;
I ' oo
j _ Current Polishing
! system system
Treatment chemical costs ($/h): | "
:::::::::::::::::::::::::::::::::;;;::;;;t;;;;;;;;;;;;;;;;;;; ^
| ::::::::::::::::: °5fs 37
• | 6.08 4.20
Cost savings ! — :—
Sludge generation factors: j
Dry solids generation (Ib/h): ;
^irStS^>e j. NA 62
Second stage , NA ^
T°tal | ." 23.6 19.3
Sludge cake volume (gal/h at 30% solids) i 7q
Disposal cost at $0.19/gal sludge ($/h) i ..'5O :*,,
Disposal cost savings ($/h) j '.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'. NA' 028
Net savings: treatment chemical cost savings plus disposal cost savings1 ($/h) ... MA 2°«fi
Annual savings based on 6,000-h/yr operation ($/yr) I '.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'. NA 13000
"Observed rates. ]
bDesign rate. j
°Based on 3 times the stoichiometric requirement. j
Note.—1979 cost basis, NA= not applicable.
of hexavalent chromium in the
wastewater; hexavalent chromium
is not removed by hydroxide
precipitation, and reagent demand
for chromium reduction will
be the same for sulfide polishing
or sulfide precipitation systems.
Plant A uses ISP for total treatment
of the metals in the wastewater.
Table 13 presents the costs
of wastewater treatment using
ISP as a polishing step compared
with its use to prpcipitate the
total metal load at Plant A. The
major cost saving results from
reduced FeS consumption; the re-
quired FeS dosage is reduced
by separation of precipitated metal
hydroxides before the addition
of the sulfide reagent.
Based on the savings indicated in
Table 13, a profitability analysis
of the investment required to
convert to a polishing system is
presented in Table 14. The $26,000
investment required for the
conversion would have an average
49
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Table 14.
Economics of Converting Plant A ISP Treatment System to ISP Polishing System Operating 6,000 h/yr
Item
Value
Installation costs ($):
Equipment: 18,000
40-gal/min mixer/olarifier • 5 Q00
Polyaleotrolyte feeder. • '
23 000
Total equipment installation ;
o rvrjn
Additional installation: estimated freight, site preparation, and miscellaneous '
26 000
Total installation costs : ===
Additional annual operating costs ($/yr): ; 8QO
Labor (100 h/yr at S8/h) : Q
Supervision •• • •' 1 600
Maintenance (6% of investment) '80Q
General plant overhead
Utilities: 200
Electricity 200
Water (polymer feeder)
Total operating costs '
Annual fixed costs (S/yr): . 2 600
Depreciation (10% of investment) • '26Q
Taxes and insurance (1 % of investment) -
2,860
Total fixed costs ; _____
Total operating and fixed costs ($/yr) • _J
Annual savings (S/yr): - 11 280
Chemicals t 1 ggg
Sludge disposal • '
12,960
Total annual savings t
Not savings: annual savings minus operating and fixed costs ($/yr) 3330
Not savings after taxes, 48% tax rate ($/yr) >• ' ' 13 Q
After-tax average return on investment (%) ggo'
Cash How from investment: net savings after taxes plus depreciation ($/yr) • ^ g
Payback period: total investment/cash flow (yr) • •
Noto.—1979 cost basis.
after-tax return on investment
of 13 percent.
The costs of FeS reagent and
solid waste disposal for ISP
systems and sulfide polishing
systems are compared further in
Figure 32 for each 1,000 gal (3,785 I)
of wastewater treated at various
metal concentrations. The solid
waste disposal cost estimate
assumed disposal of the sludge
at 25 percent solids by weight at
a cost of $0.10/gal of waste and
that both sludges would dewater
to the same level. The FeS reagent
cost for the polishing system
was derived from a required FeS
dosage rate of 40 mg/l of waste-
water. Figure 32 presents the
difference in cost, rather than total
treatment costs, of sulfide reagent
and solid waste disposal for
sulfide precipitation and sulfide
polishing systems. Other costs
associated with treatment should
be similar for both systems.
50
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o
CO o
CO g
o -
<
LLJ
en
CO
3.00
2.40
1.80
° 1.20
0.60
50 100 150!
METAL CONCENTRATION (ppm)b :
200
Legend:
I 4 times the stoichiometric
reagent requirement
2 times the stoichiometric
reagent requirement
"Solid waste disposal at 25% solids by
weight and $0.10/gal.
Based on total metal concentration in
wastewater; includes only metals, other
than iron, that form sulfides; based on a
metal with a molecular weight of 62.5
(average of Ni, Cu, and Zn).
I
Figure 32. I
j
Treatment Cost of ISP vs. Insoluble Sulfide Polishing j
A polishing system can achieve
significant savings at higher
wastewater metal concentrations.
As an example. Figure 32 reveals
that a system treating 3,000 gal/h
(11,340 l/h) with a metal con-
centration of 100 ppm and requiring
twice the stoichiometric amount
of FeS would save $2.80/h—
(B minus A) X 3,000 gal/h. At the
same flow rate and metal concen-
tration, the savings would be
$5.70/h if the wastewater required
4 times the stoichiometric amount
of FeS. :
Using the savings shown in
Figure 32, Figure 33 presents
the return on investment for
installing the additional treatment
hardware needed for a polishing
system over a range of metal
concentrations and wastewater
flow rates.
51
-------
i
Ul
cc
I
oe
UJ
t
g
h-
z
z
o
oc
I
cc
60 i-
40
20
60
40
20
(a)
SO 100 150
METAL CONCENTRATION (ppm)
200
Legend:
: 100 gal/min
I 50 gal/min
I 25 gal/min
(b)
50 100 150
METAL CONCENTRATION (ppm)
200
Note.—Based on operating 4,000 h/yr.
Return on investment calculated using same
basis as Table 14; chemical and sludge
disposal savings from Figure 32, and
equipment cost from Table 11.
Figure 33.
Return on Investment of Additional Capital Required for Insoluble Sulfide Polishing System: (a) Treatment Requiring
Two Times the Stoichiometric FeS Requirement and (b) treatment Requiring Four Times the Stoichiometric FeS
Requirement
52
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Bibliography
Lantz, J. B. "Evaluation of a
Developmental Heavy Metal Waste
Treatment Systepn." Technical
report prepared for Civil Engineering
Laboratory, Naval Construction
Battalion Center|and U.S. Army
Medical Research and Development
Command, May )979.
Lisanti, A. F., and S. 0. Megantz.
"Selecting the Proper Unit Processes
for the Treatment of Electroplating
Wastewaters." Irj Proceedings
of EPA/AES Second Conference
on Advanced Pollution Control for
the Metal Finishing Industry.
EPA 600/8-79-014. June 1979.
Metal Finishers Foundation,
Treatment of Metal Finishing
Wastes by Sulfide Precipitation.
NTIS No. Pb 267-284. EPA 600/
2-77-049. Feb. 1|977.
Robinson, A. K., jand J. C. Sum.
"Sulfide Precipitation of Heavy
Metals." Draft report prepared for
Industrial Pollution Control Division,
Industrial Envirorimental Research
Laboratory, U.S. Environmental
Protection Agency. Seattle WA,
Boeing Commercial Aircraft
Company, undatejd.
U.S. Environmental Protection
Agency. Environmental Pollution
Control Alternatives: Economics
of Wastewater Treatment
Alternatives for the Electroplating
Industry. EPA 625/5-79-01 6.
June 1979.
Wing, R. E. "Case History Reports
on Heavy Metal Removal Processes."
In Proceedings of 66th Annual
AES Conference and Exhibit of
Industrial Finishing. June 24-28,
1979.
Yeligar, M. B., G. Bagenski, and
R. M. Schlauch. "Treatment of
Metal Finishing Wastes by Sulfide
Precipitation." Draft report prepared
for Industrial Environmental
Research Laboratory, Office of
Research and Development, U.S.
Environmental Protection Agency.
Paris TN, The Permutit Company
for Holly Carburetor Division
of Colt Industries, undated.
53
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Environmental res.earch and development in the metal finishing industry
is the responsibility of the Metals and Inorganic Chemicals Branch,
Industrial Environmental Research Laboratory, Cincinnati OH. The U.S.
Environmental Protection Agency hired the Centec Corporation, Fort
Lauderdale FL and Reston VA, to prepare this report. Mrs. Mary K. Stinson
is the EPA Project Officer.
EPA thanks the following companies and organizations for providing
information and technical review: American Electroplaters' Society; Amron
Corporation, Waukesha Wl; The Chester Engineers, Coraopolis PA; Holly
Carburetor, Division of Colt Industries, Paris TN; The Permutit Company,
Incorporated, Paramus NJ; and Phillips Plating Company, Phillips Wl.
Photographs were supplied by The Permutit Company, Incorporated,
Paramus NJ.
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, Cincinnati OH, and
approved for publication. This summary report presents only one of many
control alternatives for wastewater control. Approval does not signify
that the contents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency. Mention of trade, names or commercial
products does not constitute endorsement or recommendation for use
because other existing and future systems may be as acceptable as those
mentioned in this document.
COVER PHOTOGRAPH: Sodium sulfide and sodium bisulfite mixing tanks
54
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