-------
Short Stop—
Following the development step, the film is immersed in an
acid stop bath which neutralizes and, to some extent,
removes the developer absorbed in the emulsion. A dilute
solution of a weak acid, e.g., 0.5 percent acetic acid, is
usually used.
Fix—
The fix step removes unexposed silver halide by converting
it into a soluble complex. The fixer solution contains
either sodium or ammonium thiosulfate ("hypo") as the
principal ingredient. Usually sodium bisulfite or potassium
metabisulfite is added to cause an acid condition in the
solution to neutralize any developer contamination. The
sodium bisulfite or potassium metabisulfite also acts to
prevent oxidation of the thiosulfate.
Harden—
This step serves to check emulsion swelling and raise its
melting point, thus allowing a higher temperature to be used
for drying. Potash alum (potassium aluminum sulfate) is
commonly used as the hardening agent though chrome alum
(potassium chromium sulfate) is sometimes used. It is
common to add the alum compound to the fix solution so that
the fixing and hardening are accomplished in a single step.
Wash—
A water wash is the final solution step. Its purpose is to
remove the processing chemicals absorbed in the emulsion or
substrate.
Dry—
The final processing step is drying the photographic film or
paper in a dust-free atmosphere.
III-6
-------
Reversal Process
The reversal process forms a direct positive image on the
same material used for the original exposure and is most
often used for the production of motion picture films and
projection slides. It is common to pre-harden the emulsion
in the first processing step because more steps are used in
the reversal process which tend to soften the emulsion. The
pre-hardening solution contains hardening agents such as
formaldehyde or succinaldehyde, an antifoggant
(6-nitrobenzimidazole and sodium bromide) and sodium sulfate
to retard emulsion swelling. After pre-hardening, a
neutralizing bath is used to prevent developer
contamination. A typical neutralizing solution contains
hydroxylamine sulfate and sodium sulfate. The exposed
material is fully developed to a negative using a developer
solution similar to that used in the negative process except
that a small quantity of thiocyanate is added to aid in
dissolving small non-light sensitive silver grains. The
film is then washed and the silver negative image removed by
bleaching in an acidic permanganate or dichromate solution.
A clearing bath (for example, bisulfite) is used to remove
the bleaching agent and reaction products, leaving the
undeveloped silver halide crystals in the emulsion. The
emulsion is then given a uniform light exposure, and the
remaining silver halide is developed and fixed to provide
the positive image. As an alternative to this procedure, a
highly fogging developer or non-selective reducer may be
used for the second development.
Certain black and white materials, commonly referred to as
reversal products, yield a direct "reversal" (positive)
image but are processed by the conventional negative
process, i.e., no bleach step is used. This is accomplished
by incorporating appropriate chemicals into the emulsion
during the manufacturing process. These subsequently
produce a reversal image during developing.
III-7
-------
Color Photographic Processing
General
Unmodified silver halide emulsions are sensitive only to a
limited range of wave lengths, including the blue-^violet
region of the visible spectrum, ultraviolet, and shorter
wave lengths. However, certain organic dyes can be included
in the emulsion to extend sensitivity to longer wave lengths
through the visible spectrum into the infrared or to make
the emulsion sensitive to a particular region of the visible
spectrum. Color films have three separate light sensitive
emulsion layers, which after inclusion of the appropriate
dyes, record an image of the blue light components on one
layer, the green light components on another, and the red
light components on the third layer.
The commonly used color materials are color negative film,
color reversal film, color print film, and color print
papers. The three basic processes for the processing of
color materials are negative, reversal with couplers in the
emulsion (1C), and reversal with coupler in the developers
(DC). The commonly used color materials, uses, and process
types are given in Table II1-3.
III-8
-------
TABLE II1-3
Common Color Materials
Color Material Use Process image
negative film original exposure, negative negative
intermediate for
copying positive trans-
parencies
positive print positive transparency negative positive
film from negative film
reversal film original exposure, reversal (1C) positive
intermediate or or
reversal (DC) negative
positive print print from negative negative positive
paper film
reversal paper print from positive reversal (1C) positive
transparency
Descriptions of these color processes follow. They apply to
both film and paper base materials as the steps and chemical
solutions are basically the same for each. Where major
differences occur, they are noted.
Color Negative Process
The first step in the negative process is color development.
Color development produces in each layer a dye image and a
silver image, the amount of dye generated being proportional
to the silver developed. The images are negative with
respect to the exposure sources. The dye image is formed by
a reaction between the developer oxidation products and a
group of organic molecules called couplers to form dyes of
the appropriate color in each layer. The developer solution
commonly contains salts of diethylpara-phenylene diamine or
its derivatives as the developing agent. The salts may be
the hydrochloride or the sulfate. Certain newer developing
agents, such as 4-amino-N-ethyl-N-[beta-methane-
sulfonamidoethyl]-m-toluidine sesquisulfate monohydrate,
III-9
955 aiS 2 at-PP
-------
produce better color rendition and have reduced toxicity.
In addition to the developing agent, the developer solution
usually contains color-fog restrainers such as hydroxylamine
hydrochloride, a solvent such as benzyl alcohol and a
contrast and color balance control agent such as
ethylenediamine tetraacetic acid or citrazinic acid.
The next process step stops development and removes excess
developer. This can be done by washing, but usually a weak
acid stop bath is used, e.g., 0.5 percent acetic acid
buffered with sodium acetate to control pH, followed by a
brief wash. In some cases a stop bath is used with a
hardening agent.
The film or paper is then bleached to convert the developed
silver image back to a silver halide in preparation for the
subsequent removal of all silver from the final product by
the fix solution. The color dye image remains unaffected in
each layer. The bleach most commonly used in the negative
process is the ferric salt of ethylenediamine tetraacetic
acid (ferric EDTA). It is possible to substitute
ferricyanide bleach for the ferric EDTA in some processes.
This is done by some large plants incorporating centralized
ferricyanide bleach systems which supply bleach to multiple
processes.
After bleaching, the film or paper is fixed to remove the
silver compounds and washed to remove all excess chemicals.
Finally the emulsion is hardened and stabilized using a 2
percent formalin and 3 percent sodium carbonate solution,
with the formalin acting as the dye stabilizer. Most color
paper processes use a combination bleach-fix solution which
converts silver to the halide and dissolves the halides in
one operation. The typical solution contains ferric EDTA
and sodium thiosulfate as major ingredients.
Color Reversal Processes
There are two different types of color reversal materials.
In one, the color couplers which form the color dye image
are incorporated into the emulsion layers at the time of
manufacture (1C). Most color reversal materials are of this
type. The second type has three black and white layers,
111-10
-------
each sensitive to a different color. For this type of
material, the color couplers are added during development
(DC). The (1C) process applies to film and paper materials,
and the (DC) process applies to film. The procedures and
chemistry for reversal color processing are described below.
Color Reversal Process (1C)—
The first step in the color reversal process for (1C)
materials is to pre-harden and develop the film or paper in
a highly alkaline negative developing solution to produce a
negative silver image in each layer. This developer is
similar to that used for black and white reversal
development. Color couplers are not affected during this
step. A small amount of thiocyanate added to the developing
solution aids in dissolving small, non-light sensitive
silver halide grains, thus eliminating a source of image
fog. Following the negative development step, the material
is washed or treated in stop and hardening solutions. The
emulsion is then re-exposed to a strong light source,
exposing the undeveloped silver halide. The material is
further developed in a color developer solution which
reduces the remaining silver halide to silver and reacts
with the couplers to produce a positive dye image of the
appropriate color in each of the three layers. The color
developer formulation is similar to the developer used for
color negative materials. In some processes, a highly
fogging color developer is used in place of the light
re-exposure. The dye development is usually followed by
hardening, stop and wash steps, which in turn are followed
oy bleaching. A commonly used film bleach is a potassium
ferricyanide and potassium bromide solution which converts
the developed silver to silver bromide. EDTA bleach is also
used for certain film processes, and an EDTA based
bleach-fix is commonly used for reversal papers. The film
or paper is then fixed to remove the silver bromide, washed,
stabilized, and dried.
Color Reversal Process (DC)—
Color reversal (DC) film processing is complicated and
requires rigorous chemical control of solutions. The first
development step, as with the reversal (1C) materials, forms
III-ll
-------
a negative silver image in all three layers. After this,
all three layers in the emulsion are treated separately.
First, the red-sensitive layer is prepared for development
by exposure to red light through the base of the film. This
exposes the remaining undeveloped silver halide in that
layer. The other two layers, which are not sensitive to red
light, are unaffected. The film is treated with a color
developer that contains, among many other ingredients, a
coupler which forms a red-absorbing (cyan) dye in the
red-sensitive layer. As the color developing agent reduces
the silver halide and forms an image, the oxidized color
developer in the vicinity of the developed silver grains
forms the positive cyan dye image.
After washing, the film is exposed from the top with blue
light exposing the undeveloped silver halide in the top
blue-sensitive layer. A yellow filter layer protects the
middle green-sensitive layer. A second color developer,
containing a soluble yellow coupler, produces both a silver
and blue-absorbing (yellow) positive dye image in the top
layer.
After a second wash, the middle layer is developed and
chemically fogged in a third color developer containing a
fogging agent and a magenta coupler which forms the final
positive silver and green-absorbing (magenta) dye image.
The film at this point has a negative silver image, a
positive silver image, and a positive color dye image in
each layer. Following a third wash, the silver images are
removed as in the other color processes by bleaching and
fixing followed by washing and drying.
PROCESSING CHEMISTRY
To make up the required solutions for processing a
particular film or paper, the processor has a choice of (a)
using a kit where all the necessary solution formulations
are included, (b) using bulk formulations for each solution,
or (c) using bulk raw chemicals. In general, small
processors use kits and larger processors use bulk
formulations and raw chemicals. The processor has a wide
latitude in selecting formulations for the processing of
black and white materials. For example, many different
111-12
-------
developer formulations made by any one of several
manufacturers can be used to develop a given black and white
film. In color processing, the choice is more limited,
because process solutions are formulated specifically for a
particular emulsion type.
Because of the broad range of black and white processing
formulations, individual processes are not identified by
name; rather, formulations are named by the various
manufacturers (for example, developers such as Dektol, D-76,
53-D, etc.). In color processing, where each solution
formulation is more limited, the processes generally have
names which define the process steps and the solution
formulations. For a given color emulsion, several
manufacturers may provide kits or formulations with
different names, but the chemical content is quite similar.
In references to black and white processing in this report,
no process names are used, only the descriptors, negative or
reversal. Where appropriate, color processes are referenced
by name. The process name is used to identify a specific
composition and sequence of processing solutions for which
certain photosensitive materials are designed. Any
manufacturer of photosensitive materials may produce films
or papers designated for processing in any specific process.
Any manufacturer of photographic chemicals may produce
chemicals intended to be used for specific processing
solutions in any designated process. The following is a
list of the color processes most often encountered during
this study.
111-13
-------
Material Type Process Name
Color negative process film C-22, C-41, MC-42, ECP-1, ECP-2
ECN-1, ECN-2
Color reversal process film (1C) E-3, E-4, ME-4, EM-25,
CRI, ECO-3, E-6, E-7M, EA-5
Color reversal process film (DC) K-12, K-14
Color negative process paper EP-2, EP-3, MCI 11, 85/86
Color reversal process paper EPR-5, EPR-100, P-10
P-18
Over the period of this study the manufacture of
photographic materials designed for some of these processes
has been discontinued. As the stocks of these materials are
depleted there will no longer be a need for the
corresponding processes. The processes included in this
category are C-22, ECN-1, E-3, E-4, K-12, and EPR-5.
The schematic process diagrams for the listed processes are
included in Figures III-l through 111-19. Dashed lines
represent optional steps or operations. The optional
recycle of fix is applicable only when electrolytic silver
recovery methods are used. The additions of make-up
chemicals, regeneration chemicals, or air required in the
various bleach regeneration processes are not shown in the
following schematics. This information is given in the
discussion of the regeneration methods in the following
subsections.
The process solution chemicals lose activity, are
transformed by chemical reaction, and are contaminated by
chemicals from the emulsion and by drag-in from previous
solutions. There is also volume reduction or concentration
changes due to absorption of solution into the emulsion and
evaporation. Drag-in and drag-out of solution are usually
not factors because they are approximately equivalent and
cause no net volume change. To alleviate the effects of
chemical transformation and volume reduction, it is common
practice to replenish the solutions with fresh or
111-14
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-------
reconstituted chemicals. For example, with use, a developer
gradually loses its effectiveness. The concentration of the
developing ' agent and the pH decrease, and the halide ion
accumulates. The developer replenishing solution usually
contains a higher concentration of developing agents than
the original solution and little or no bromide. It also
contains alkali to help maintain a constant pH.
The replenishment is at a continuous or programmed rate on
continuous process machines. The rate of replenishment is
greater than the normal volume reduction resulting in an
overflow of solution. This reduces accumulation of unwanted
chemicals in the process tank. Batch replenishment may be
used on "dip and dunk machines" or with manual processing.
Some processors practice a type of replenishment called
"replenishment on demand" whereby the critical constituents
(for example, chromate) of the solution are monitored and
replenishment is done by batch in the required amount when
these constituents are reduced to predetermined levels.
Although overflow is still allowed to occur to reduce
unwanted chemical accumulation, proponents of this method
claim that the waste load is reduced from that found in
continuous replenishment because of higher efficiency. A
further explanation of this method is given in a subsequent
section of this report, Reconstitution of Dichromate Bleach.
PROCESSING EQUIPMENT
Photographic materials may be processed either manually or
in automatic processing machines. Manual processing and
certain automatic "drum" machines are suited to custom
processing or processing small quantities of material.
Manual processing is predominately suited to very small labs
but may be found in the larger labs as well. Automatic
processing machines (with the exception of drum processors)
have the capability for high production rates and are suited
to the larger labs.
A manual processing method, sometimes called "sink line"
processing, consists of manually placing, for the correct
time and in proper sequence, exposed photo material in each
of a series of trays or tanks containing the required
111-34
-------
chemical solutions. A second manual technique utilizes a
single tank or drum. The photo material is placed in the
tank or drum and the first processing chemical solution is
poured into the container. After remaining for the
proscribed time, the solution is dumped and replaced with
the second solution. This process is continued until all
solutions have been used. An automated version of this
process, called automatic drum processing, consists of a
motorized rotating drum and preprogrammed, electrically
operated solution fill and dump valves.
Most photoprocessors, as included in this study, handle
large quantities of film and paper with automatic processing
machines. The three types most widely used are "dip and
dunk," roller transport, and continuous length processors.
The "dip and dunk" processor, an automated version of "sink
line" processing, is generally used for roll films. The
machine consists of a series of deep tanks, elevator or
lifter mechanisms, and a movable track or chain drive. The
film is attached to hangers, automatically transferred from
tank to tank by the lifter mechanism and deposited in the
tank by the movable track. A limited adjustment of the
processing times in individual tanks is possible. The film
is dried in a drying tunnel that is part of the machine. A
second type of dip and dunk processor is an automatic
version of manual basket processing. The film, plates or
paper are loaded onto reels or baskets. The entire assembly
is moved on an overhead gantry that raises and lowers the
basket into each process tank at preprogrammed times.
Roller transport machines consist of a series of solution
tanks and use a combination of pinch rollers and belts to
feed the photo materials through each solution tank in
succession. Roller transport machines are commonly used for
sheet film, narrow widths of professional and aerial film
and large format (up to 1.3 meters wide) prints. They
require no leader and are suited to the processing of single
pieces of film or paper at high production rates.
The continuous length processor is generally used for movie
films and long rolls of film and paper. Short rolls can
also be accommodated by splicing the short rolls together
111-35
-------
end on end to make a long length. The continuous length
processor consists of a series of deep tanks, roller racks
or transports in each tank, and feed and take-up mechanisms.
The feed and take-up mechanisms usually include a slack box
to facilitate splicing without stopping the main drive. The
film travels back and forth in each processing tank over a
series of rollers on the rack. The path length and linear
film speed are adjusted to give the desired residence time
in each tank. Upon exiting a tank, the film travels to the
next tank over roller connections between racks. Continuous
processing units are time-consuming to thread and are
commonly threaded with leader stock. The starting end of a
material to be processed is attached to the leader end which
then guides the material through the machine. New leader is
attached to the end of the processed material and remains in
the machine ready for the next run. It is common to run
leader before and after production runs for quality control
purposes to inspect for mechanical abrasion problems and
chemical balance. A short photosensitive test strip is
attached to the leader for the chemical balance check.
During leader run there is generally either no replenishment
or reduced rate replenishment of some solutions. Wash water
usually runs at the normal production rates. The wastewater
hydraulic load is comparable to production loads. Pollutant
loading is reduced to essentially that caused by solution
carryover into the wash waters. No additional silver
loading occurs since the leader has no silver emulsion.
The ratio of leader to production varies widely and depends
on the machine type and the materials processed. "Dip and
dunk" machines require no leader. Roller transport machines
have pinch roller feed guides which require no leader.
Plants that predominately use these machines have a very low
leader to production material ratio. On the other hand,
plants using continuous machines and processing amateur
materials consisting of short lengths of narrow film and
paper may have a leader to production material ratio of one
or more, i.e., they may run more leader than production
material. If possible, the processor will splice the short
film end on end to create a long continuous length and
reduce the need for leader. However, this is often not
possible because of the promise to the customer to return
the finished product quickly. A quick turnaround time
111-36
-------
requires machines to be continually "at ready," resulting in
numerous short runs, relatively large amounts of leader run
and frequent quality control checks. The movie labs, which
use similar machines and process similar film widths, have
leader to film ratios on the order of 0.1 to 0.5 because
they process comparatively long lengths and can plan their
production more efficiently.
Most automatic processing machines have automatic agitation
and solution replenishment. The method most commonly used
for agitation is called gas-burst agitation which consists
of releasing gas through tiny holes in a distributor plate
in the bottom of the solution tank at controlled intervals.
The gas bubbles formed during release provide the random
agitation pattern necessary for uniform results. Oil-free
compressed air is usually used except in the developers
where nitrogen is used to prevent oxidation of developing
agents.
IN-PROCESS CONTROL TECHNOLOGIES
In-process controls are used in the photographic processing
industry primarily for the conservation or recovery of raw
materials for economic purposes and secondarily for
pollution abatement purposes. Controls include: (1) the
recovery of raw materials such as silver and organic
couplers, (2) the regeneration of processing solutions for
recycle such as fix and bleach, and (3) various housekeeping
practices such as water saver controls and squeegees.
Effect of In-Process Controls on Product Quality
In the consideration of in-process controls, which involve
the reuse of solutions, recycle of wash water, or other
modif icat ions of standard!zed procedures, two major
precautions should be taken to ensure that (1) the control
is properly applied and (2) once applied, the control is
properly maintained. Any of these controls, if not properly
applied and maintained, can cause immediate or long-term,
adverse effects on product quality. For"" example, dirt
build-up can immediately cause pinholes, spots, and
scratches; trace chemical build-up, improper chemical
balance in the solution, and insufficient chemical removal
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from the emulsion can cause (1) stains and improper color
balance in the short term and (2) poor dye stability and
stains in the long term.
The first precaution concerns proper application of the
in-process control on a process specific basis, i.e.,
successful application on one particular process does not
guarantee success on another. When a control is applied to
a particular process, it should be established, by testing
product quality, that there are no short- or long-term
adverse effects. The short-term tests include inspection
for stains, spots, and scratches; color balance checks
against standards; and hypo retention tests. Long-term
effects are somewhat more difficult to evaluate. Current
techniques involve subjecting processed test material to
accelerated aging conditions prior to performing quality
tests.
The use of many of the various in-process controls, which
are discussed in detail in the following subsections, has
been demonstrated and accepted. Ferricyanide and EDTA
bleach regeneration have been established on most color
processes, and fix recycling has been successfully used on
many processes. On a more limited basis, countercurrent
washing and wash water recycle have been established for
some processes. Developer recycle has been successfully
used on a few color paper processes. Low flow prewash is
still being evaluated.
The second precaution concerns proper use and maintenance of
the in-process control to ensure continued high standards of
product quality. In some cases, strict adherence to
recommended maintenance and use procedures may be sufficient
to ensure a quality product. In others, careful chemical
monitoring of process solutions or wash water and periodic
product tests may be required in addition to in-process
control equipment maintenance and use requirements. (Note
that in-process control use procedures may require chemical
monitoring of some solutions.) Finally, the origin or
end-use of the material being processed may have some
bearing on the use of a particular in-process control.
Solution regeneration and wash water recycle do increase the
risk of reduced long-term stability or product damage. In
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cases where a process is used for unique film originals or
where the product must have archival storage capability, the
risks imposed by use of an in-process control may be
unacceptable.
Silver Recovery
Two photoprocessing waste solutions contain essentially all
silver removed during processing: (1) the fix or bleach-fix
overflow and (2) the post-fix wash water.
The state-of-the-art of silver recovery from the fix and
bleach-fix processing solutions includes metallic
replacement, electrolytic recovery, and chemical
precipitation. Ion exchange and reverse osmosis are other
methods that can be used alone or in combination with
conventional silver recovery systems. .However, these are
generally considered suitable only for dilute solutions of
silver, such as the polish desilvering of effluent from a
silver recovery unit or wash water desilvering.
These silver recovery systems can be used in a variety of
ways. It is typical to have a primary silver recovery unit,
which removes the bulk of silver, in combination with a
"tailings" unit. Tailings consist of the relatively low
silver concentration effluent from a primary silver recovery
system. A tailing unit is used as a secondary or polishing
unit for additional silver recovery. The typical system
consists of an electrolytic primary unit and a metallic
replacement tailing unit. A silver recovery system can be
devoted to a single process line or be used to remove silver
from the fix from several or all process lines in a plant.
The multiple use systems are found in the larger plants.
Sometimes a separate fix system is used for processing
unique original film to reduce the possibility of
inter-process contamination (when desilvered fix is recycled
to the process) and the resultant damage to irreplaceable
originals.
Metallic Replacement
Metallic replacement occurs when a metal, such as iron,
comes in contact with a solution containing dissolved ions
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of a less active metal, such as silver. The dissolved
silver, which is present in the form of a thiosulfate
complex, reacts with solid metal (iron); the more active
metal goes into solution as an ion, and an ion of the less
active metal becomes solid metal (silver).
Silver ions will displace ions of many of the common metals
from their solid state. Because of its economy and
convenience, iron in the form of steel wool is used most
often. Zinc, as a replacement metal, can also be effective,
but it is not used because of its relatively higher toxicity
and greater cost. Aluminum has also been used as a
replacement metal but is not commonly used because the
simultaneous generation of hydrogen gas could be hazardous.
For most efficient operation, the pH of the solution passing
through the metallic replacement unit should be between 4
and 6.5, with an optimum between 5 and 5.5. Below a pH of
4, the dissolution of the steel wool is too rapid. Above a
pH of 6.5, the replacement reaction may be so slow that an
excessive amount of silver would be lost because of the long
reaction time required.
Silver recovery by metallic replacement is most often
carried out using commercially available units consisting of
a steel wool filled plastic bucket with appropriate
plumbing. Typical practice is to feed waste fix to two or
more canisters in series or series-parallel combinations.
For two canisters in series, the first canister removes the
bulk of the silver and the second unit polishes the effluent
of the first and acts as a safety factor if the first unit
is overused. When the first unit is exhausted, it is common
to replace it with the second and put a new unit in the
place of the second. Silver concentrations in the effluent
from a single unit average 40 to 100 mg/1 over the life of
the unit versus a range of 0.1 to 50 mg/1 when two canisters
are used in series.
Desilvered fix is not recycled because of the iron
contamination. The average iron concentration in the
cartridge effluent, over the life of the cartridge, is 4,000
mg/1. However, this is not a problem with bleach-fix which
contains iron complexed with EDTA as an active ingredient.
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Because of this, metallic replacement silver recovery is .a
commonly used first step in the regeneration of bleach-fix.
Electrolytic Recovery
This silver recovery method requires the application of
direct current across two electrodes in a silver-bearing
solution causing metallic silver to deposit on the cathode.
Sulfite and thiosulfate are oxidized at the anode as
follows:
HZ0 + 803-2 = S04~2 + 2e- + 2H+ (Anode)
S03~2 + S203-2 = S306~2 + 2e- (Anode)
Ag+ + e~ = Ag° (Cathode)
Approximately 1 gram of sodium sulfite is oxidized for each
gram of silver plated. Considerable agitation and large
plating surface areas are necessary to achieve good plating
efficiency and high quality silver up to 96-98 percent pure.
Lower silver purity levels are usually achieved in tailing
cells. The cathodes are removed periodically, and the
silver is stripped off. Care must be taken to prevent the
current density in the cell from becoming too high to
prevent "sulfiding." Sulfiding is the result of
decomposition of thiosulfate at the cathode. The sulfide
contaminates the deposited silver and reduces recovery
efficiency. The higher the silver concentration the higher
the current density can be without danger of sulfiding. As
the silver is plated out of solution, the current density
must be reduced.
Electrolytic units can be used for primary or tailing silver
recovery. Primary electrolytic systems are typically
installed in two basic ways. One is a batch recovery system
where overflow fix from a process line or lines is collected
in a tank. When sufficient volume is reached, the waste fix
is pumped to an electrolytic cell for the silver removal
process. The desilvered fix is either discharged or reused.
For reuse, it is pumped to a mix tank where chemicals are
added to bring it to replenishment strength. Primary batch
system cells are usually designed to desilver the fix at
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fairly high starting silver concentrations of about 5,000
mg/1. The silver concentration in the effluent is typically
about 200-500 mg/1 but can be reduced to 20-50 mg/1 with
additional treatment time and careful control of current
density. An electrolytic tailing cell typically achieves
the lower range because the process can be optimized for low
starting silver concentrations.
The second primary electrolytic recovery method is to remove
silver from the fix solution from one or more process
machines in a continuously recirculating system at
approximately the rate at which silver is being added by
processing. The recovery cell is included "in-line" as part
of the recirculation system. This continuous removal
technique has the particular advantage of maintaining a
relatively low silver concentration in the fix processing
solution so that the amount of silver carried out with the
processed material into the wash tank is minimized. The
silver concentration in the fix can be maintained in the
range of 500 to 1,000 mg/1, the lower limit being primarily
a function of residence time in the cell, i.e., system flow
rate.
The recycling of desilvered fix solution, whether by an
"in-line" continuous system or by batch, requires adequate
monitoring and process quality control to protect product
quality. Parameters which should be monitored to maintain
the physical and chemical properties of the fix solution
include pH, silver, and sulfite concentrations.
Chemical Precipitation
Chemical precipitation is a relatively uncommon method for
recovery of silver from photographic processing waste
solutions but is practiced widely in the photographic
supplies manufacturing point source category. Silver may be
precipitated from fixers with sodium sulfide:
2Ag+ + S-2 = Ag2S
The precipitation is quantitative in an alkaline solution,
and the resultant silver sulfide has a solubility product of
10-50 making it one of the most insoluble substances known.
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The physical characteristics are not as favorable as the
chemical characteristics. Precipitation must be carried out
in alkaline media to avoid the generation of hydrogen
sulfide. Silver sulfide tends to form colloidal
suspensions. Its very small particle size makes filtration
difficult and the filter cake produced is extremely dense.
Diatomaceous earth filter aid can be used to improve
filtration. About three grams of filter aid are required
for each gram of silver, if a conventional filter press is
used.
Sodium borohydride is also an effective precipitant for
silver:
BH4~ + 2HZ0 + 8Ag+ = 8Ag + 8H+ + B02-
This precipitating agent can also be used to treat
photographic processing wash waters. Sodium sulfide
requires very little excess, while borohydride requires
significantly more than the stoichiometric reaction quantity
to complete the reaction. A major difference between the
two precipitating agents is the resulting silver quality.
Sodium borohydride produces elemental silver of 90 to 95
percent purity. The sulfide addition generates silver
sulfide containing 87 percent silver. Silver concentrations
as low as 0.1 mg/1 can be achieved by either method.
In a typical system, the precipitating agent is mixed with
the silver-bearing waste solution in a batch reaction tank
equipped with automatic pH control. The pH is maintained
above 7 to avoid releasing toxic hydrogen sulfide gas when
sodium sulfide is used.
The optimum pH range for sodium borohydride precipitation is
6.5 to 6.8. The solid particles formed (1-2 microns) are
allowed to settle before filtering.
Solutions treated by sodium sulfide or sodium borohydride
cannot be reused in the photographic process.
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Silver Recovery from Wash Waters
Even with an efficient fix.solution silver recovery system
and squeegee use at the exit of the fix tank, up to 10
percent of the available silver is lost to the after-fix
wash water by carryover. The silver concentration in the
wash water is typically in the range of 1 to 50 mg/1 and it
has not been found practical to use the previously described
conventional silver recovery methods for recovery of this
silver. Thus, the concentration of silver is generally too
low for effective use of electrolytic or metallic
replacement recovery methods. In addition, the iron
by-product from metallic replacement precludes the possible
wash water reuse without extensive treatment. Although
precipitation is technically satisfactory, it is too slow
and too expensive because of raw material and filtration
costs to be economical.
Two methods have been found to be effective and are
currently in use for recovery of silver from wash water,
namely resin ion exchange and reverse osmosis (RO). A third
method called "low flow prewash" has been used on an
experimental basis at two plants. This consists of
apportioning the after-fix wash water into two segments, a
low volume, high silver concentration prewash and a final
wash of low silver content. Silver can be removed from the
prewash by conventional methods.
Resin ion exchange is the reversible exchange of ions
between a solid resin and a liquid. A variety of weak and
strong base anion resins are effective in silver recovery.
Using chloride as the mobile ion, the following is
representative of the reaction:
[Resin] Cl + AgS203- = [Resin] AgS203 + Cl~
The silver complex has a high affinity for the resin and
consequently it is difficult to reclaim the silver and
regenerate the resin. Recently, a concentrate has been
developed that accomplishes this task economically. Other
problems such as suspended matter, particularly gelatin,
plugging the resin have also been solved by equipment design
and operational procedures.
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Reverse osmosis is a general separation technique. It
involves a wastewater stream flowing under pressure through
an appropriate porous membrane. Water passes through the
membrane as product and the pollutants remain upstream from
the membrane as concentrate. Reverse osmosis is governed by
membrane surface phenomena and pore size, as well as
wastewater characteristics. The membrane surface must be of
such a chemical nature that it has a preferential sorption
or repulsion for one of the constituents of the fluid
mixture. Consequently, the membrane-liquid interface is
enriched in one of the solution constituents. A continuous
flow under pressure through the membrane capillaries results
in a production solution (permeate) whose composition is
different from that of the bulk solution. For recovery of
silver from wash water by reverse osmosis, after-fix wash
water is equalized, filtered, and then pumped through a
reverse osmosis unit. Silver can be removed from the
concentrate by conventional silver recovery methods
depending on silver concentration and presence of other
pollutants. Potential problems encountered with the use of
reverse osmosis equipment for recovering silver from wash
water are fouling of the membrane and biological growth.
Proper maintenance and control are required to alleviate
these problems. One plant reported difficulties with
membrane fouling which required frequent replacement of the
membrane with a resulting high maintenance cost. The
problem was alleviated by the use of sand filtration in the
waste stream prior to the reverse osmosis unit.
The low flow prewash system is a relatively new concept and
is still being evaluated. This system concentrates most of
the fix carryout in a low volume after-fix prewash tank.
The system consists of segmenting the after-fix wash tank to
provide a small prewash section with separate wash water
make-up and overflow. The wash water flow can be optimized
depending on the carryover silver concentration and the
level of treatment. By design, the concentrations of fix,
silver, and other chemicals reach high levels in the prewash
tank under steady-state conditions. There is concern
expressed by some investigators that this may cause problems
with the quality of the processed material. In effect, the
processed material receives additional fix time and exposure
to potential contaminants while immersed in the prewash.
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Dye stability tests, performed by one investigator on color
paper processed using the prewash system, have shown an
increase in yellow stain six months after processing. There
is also a requirement for increased maintenance of the wash
tank because of biological growth.
Coupler Recovery
The three color developers used in the K-14 color reversal
process (DC) contain organic dye couplers. Since the
couplers are in solution, it is common practice to recover
these organic couplers from the waste developer overflow for
economic reasons. The couplers are not recoverable from the
1C process because they are not in solution. Instead, the
color couplers are put into the three emulsion layers during
manufacture of the film or paper and remain to form the
color image after processing.
The (DC) couplers are soluble in the normally alkaline
developers but will precipitate at neutral or acid pH. To
recover the couplers, the pH of the waste developer is
adjusted to 7 or less with sodium bisulfate and then the
precipitated couplers are extracted by centrifugation.
Carbon dioxide has also been used for pH adjustment. The
photoprocessor may reuse the couplers in the appropriate
developer solution. The recovery and reuse of couplers
requires proper testing and quality control procedures to
avoid problems with color balance and saturation in the
processed films.
Bleach Regeneration
Bleaches are used in the black and white reversal process to
dissolve the negative silver image and in the color process
to oxidize the developed silver image to a silver halide
which is subsequently dissolved in the fix solution. It is
common practice in color processing to regenerate the
reduced bleach for reuse by oxidizing the bleach back to its
original state or by discharging a portion of the bleach and
adding fresh chemicals to restore it to original
specifications. The main active ingredient contained in
color processing bleaches is either sodium or potassium
ferricyanide, ferric EDTA, ferric chloride, or sodium
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dichromate. A dichromate bleach is commonly used in black
and white reversal processing and is not regenerated.
Ferric chloride bleach is also not regenerated. The reuse
of regenerated bleaches require various quality control
measures, depending on the bleach type and regeneration
method, to ensure against adverse effects in the product
from improper chemical balance, chemical impurities, and
dirt. These problems are addressed in more detail in a
preceding subsection on product quality.
Ferricyanide Regeneration
The basis for all the ferricyanide regeneration methods is a
sufficiently strong oxidizing agent that has reaction
products compatible with or used in the process. Current
regeneration processes which fulfill these requirements are:
A. Persulfate regeneration
B. Ozone regeneration
C. Electrolytic bleach regeneration
D. Miscellaneous chemical regeneration methods
Persulfate Regeneration—
Persulfate regeneration is a batch process consisting of
collecting the bleach overflow in a tank and adding
potassium persulfate to oxidize the ferrocyanide ion back to
ferricyanide by the following reaction:
2[Fe(CN)6]-* + SaOe-2 = 2[Fe(CN>6]-* + 2SQ4~*
Bromide is also added to replenish that taken up by the
silver during bleaching. One problem with the persulfate
method is a build-up of the sulfate ion which slows the
bleaching action. To alleviate the problem, about 10
percent of the bleach is directly discharged before
regeneration or the bleach drag-out rate in the process tank
is regulated by squeegee adjustment so that a comparable
sulfate reduction occurs.
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Ozone Regeneration—
The use of ozone for ferricyanide bleach regeneration
minimizes the salt build-up problems encountered with
persulfate regeneration and reduces the need for bleach
blowdovm during regeneration cycling. The ozone process is
characterized by the following reaction:
2Na4Fe(CN)« + H20 + 03 = 2Na3Fe(CN)6 + 2NaOH + 02
Stoichiometrically, 12.7 kg of sodium ferrocyanide are con-
verted to 11.7 kg of sodium ferricyanide using 1.0 kg of
ozone. Under varying conditions of pH, the ozone oxidation
efficiency is near 100 percent for ferrocyanide
concentrations above 1.0 gram per liter.
The pH of the bleach increases as the reaction proceeds, and
bromide additions are required to replace the bromide taken
up by the silver. Therefore, hydrobromic acid is added to
accomplish both pH adjustment and bromide ion addition.
Theoretically, one bromide ion is required for each
ferrocyanide ion that is oxidized to ferricyanide. The
hydrobromic acid avoids all build-up of sulfate and other
unwanted by-products. If, in practice, there is a slight
build-up of bromide ion, small amounts of sulfuric acid can
be added for pH adjustment with little danger of high
sulfate build-up.
Some photoprocessing plants have installed continuous
in-line ozonation for ferricyanide bleach regeneration.
This technique permits a significant reduction in the
necessary ferricyanide concentration in the bleach tank,
since the ferrocyanide level in the solution is kept low and
the ferri to ferro ratio is the controlling factor in
effective bleaching. With a lower ferricyanide bleach
concentration, there is less drag-out and less pollution of
the waste wash water.
Ozone can be somewhat hazardous because of its toxicity, but
methods have been developed to use it safely. Safe
practices include the use of ozone detectors to monitor the
air in the vicinity of the ozonation tanks.
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Electrolytic Regeneration—
Ferrocyanide can be oxidized to ferricyanide at the anode of
an electrochemical cell. ' Because the reverse reaction
occurs at the cathode, the cell must be divided by a
semi-permeable membrane, or some other method of minimizing
the cathode reaction must be employed. The electrolytic
cell reactions are given below:
Anode: [Fe(CN)6]~4 = [Fe(CN)6]~3 + e~ (primary reaction)
4{OH)~ = 02 + 2H20 + 4e- (secondary reaction)
Cathode: 2HZ0 + 2e~ = H2 + 2(OH)- (primary reaction)
[Fe(CN)6]~3 + e- = [Fe(CN)tj-* (secondary reaction)
With improved cell technology minimizing the undesirable
secondary reactions having become available recently, this
method of ferricyanide bleach regeneration has found wider
acceptance in the photoprocessing industry. Like ozonation,
there is no requirement for bleach blowdown during the
regeneration cycle.
Bromine and Peroxide Chemical Regeneration Methods—
Probably the simplest ferricyanide bleach regeneration tech-
nique, from a chemical point of view, is the use of bromine.
The bromine performs the ideal role of oxidizing the
ferrocyanide and replacing the bromide ions removed by the
film as it passes through the bleach. It is an economical
method, there are no unwanted reaction products, and there
is minimal dilution. Because of the hazard and
corrosiveness associated with the handling of 1iquid
bromine, however, the method has not been widely accepted.
Bromate and bromite compounds have been used with
satisfactory results and are somewhat easier to handle. The
main reason for their not being used more widely has been
hesitation by the industry to handle chemicals of this
nature.
Hydrogen peroxide has also been used successfully.
Hydrobromic acid is added to control pH and supply the
required bromide ions. However, the problem of handling
peroxide has restricted the use of this method.
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Summary of Ferricyanide Bleach Regeneration Methods—
The most popular methods are persulfate and ozone
regeneration. Both methods are efficient with the
persulfate yielding about 90 percent of the used bleach for
recycle and the ozone method allowing 100 percent recycle.
Both methods require analytical monitoring of the
regeneration process. The persulfate method consists of a
single chemical addition requiring no special equipment
beyond tanks and plumbing. It does have the problem of
sulfate build-up which must be corrected by up to a 10
percent discharge. Ozonation allows essentially 100 percent
recycle and is adaptable to continuous in-line regeneration
but requires a larger investment in equipment and
maintenance. Electrolytic regeneration is a relatively new
method but appears to have advantages and disadvantages
similar to ozonation.
Ferric EDTA Regeneration
Modifications in the processing of certain photographic
materials have resulted in the substitution of ferric ions
chelated with EDTA for ferricyanide as the bleaching agent.
The ferric-EDTA bleach relies upon the oxidative power of
the ferric ion which is reduced to ferrous ion in the
process. Ferric EDTA is used alone as a bleach for certain
color films and is used in combination with sodium or
ammonium thiosulfate fixer for color paper processing. The
combined material is known as bleach-fix.
Regeneration of the bleach involves the oxidation of the
ferrous ion back to the ferric ion and the replenishment of
various chemicals, principally bromide. Typically the
oxidation process is performed as a batch process in a tank
with aeration. Some plants aerate the bleach in the process
bleach tank using compressed air which serves the dual
purpose of solution agitation and oxidation.
Regeneration of the waste bleach-fix solution involves three
steps. First, the silver must be recovered. Second, the
reduced iron EDTA complex must be oxidized back to ferric
EDTA. Finally, certain chemicals lost through carryover
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with the film or paper must be added to bring the solution
up to replenisher strength.
As with the desilvering of waste fixer solutions, there are
several techniques available to remove the silver from
bleach-fix waste. The most widely used technique is the
silver recovery cartridge containing steel wool where the
silver is replaced by iron, and the silver is retained in
the cartridge. The metallic replacement method is most
widely used both for economic and practical operating
considerations. After passing the bleach-fix overflow
through the silver recovery cartridge, the resulting
solution is aerated to oxidize the ferrous EDTA complex to
the ferric form. After aeration, the bleach is returned to
a holding tank where make-up chemicals are added as required
to restore the solution to replenishment strength.
It is also possible to use electrolysis to recover silver
from bleach-fix baths. Equipment has recently appeared on
the market which is designed for desilvering bleach-fixers
electrolytically. This method has some advantages, such as
better control of iron concentration and higher quality
recovered silver, over metallic replacement, particularly in
larger processing laboratories.
Irrespective of technique, it is possible to reuse between
70 and 80 percent of the bleach-fixer by regeneration. The
20 to 30 percent loss is due to a combination of carryout
with the paper or film and excess bleach discharged to
prevent contaminant build-up.
Reconstitution of Dichromate Bleach
Bichromate bleach is found in processes for motion picture
color negative print film and in processes for black and
white reversal film. The dichromate bleach used in color
negative processing has the same function as the
ferricyanide and EDTA bleaches, that is, it oxidizes the
silver image to silver bromide. However, the function of
the dichromate bleach used in black and white processing is
to dissolve the silver image. To do this, the bleach
formula contains no bromide and is very acidic. As a result
the waste bleach contains silver. Before disposal of the
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dichromate bleach, silver is usually recovered. This is
generally done by adding sodium chloride which precipitates
the silver as silver chloride.
A portion of the dichromate color bleach can be reused by
discharging approximately 50 percent of the used solution
and then adding chemicals to restore its original strength
and volume. As discussed in the section on product quality,
controls must be used to prevent dirt and unwanted chemical
build-up.
Another practice that is used to reduce the waste bleach
load is to modify the normal replenishment system to one of
"replenishment-by-demand." During the color dichromate
bleaching process, the bleach is chemically reduced. The
amount of dichromate ion reduction per square foot of film
depends on the relative amounts of light and dark areas in
the film image. By adding replenishment chemicals "on
demand," only the amount of dichromate reduced is replaced,
minimizing waste. Demand is determined by monitoring the
bromide level. Dichromate ion depletion is proportional to
the bromide ion depletion. Normal replenishment rates are
based on the maximum potential rate of dichromate depletion
per square foot. This practice commonly results in the
generation of some excess bleach which must be discharged.
Developer Regeneration
Developers become exhausted both by loss of active
developing agents and by increase of reaction products. The
limiting factor is usually the increased bromide concen-
tration. Two approaches may be taken to reuse developers:
(1) the reaction products can be removed by a technique such
as ion exchange so that the bulk of the solution may be
reused; or (2) specific chemicals can be separated from the
bulk of the solution by precipitation or extraction
techniques and the treated solution reused with or without
further purification. As an example, bromide and developer
decomposition products can be removed by ion exchange from
color developers; other constituents are not affected.
After passing through an ion exchange column, the developer
is reconstituted and reused.
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Currently, only the ion exchange method is used on a limited
basis on some color paper developers. Its applicability to
other developers is yet to be determined. Because of the
complexity of the developing solution and process, proper
control of chemical balance i-s difficult and may not be
possible in some cases.
Water Conservation
In the photoprocessing industry, the three major areas of
plant water use occur in solution preparation, water washing
of film and paper, and equipment cleanup. Of these three!
l£*h *arg?st sinale use occurs in film and paper washing!
Methods for reduction and conservation of wash water
include:
A. Countercurrent washing
B. Squeegee use
C. Automatic wash water controls
D. Chemical prewash
E. Water recycle
Countercurrent Washing
The Countercurrent wash system referred to here is the use
of a segmented wash tank after one of the process steps,
usually the final wash after fix. It does not refer to the
practice of pumping the same wash water from one wash tank
atter a process step to another wash tank after a different
chemical processing solution. This practice may interfere
with the process chemistry. In the segmented tank system,
wash water is cascaded progressively from one tank segment
to the next against the movement of the film. Fresh wash
water enters the last wash tank segment in the system, and
the overflow flows to each preceding segment in succession.
AS the film or paper moves forward, it progressively comes
into contact with cleaner and cleaner wash water. It is
reported to be up to a hundred times more efficient than
deep-tank washing. The net result is a lower total water
input to remove chemical by-products from the film or paper.
Countercurrent washing is not universally applicable to all
process machine configurations. When used, proper controls
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must be applied to prevent quality problems from biological
growth, dirt accumulation, and inadequate washing.
Squeegees
Squeegees are devices designed to remove the liquid from the
surfaces of film and paper as they move from one process
tank to another to reduce solution carryover. They are
placed at the exit of process solution and wash tanks where
such placement is compatible with the process chemicals and
emulsion. With proper placement and adjustment, squeegees
conserve raw materials and reduce water use and
replenishment volume {waste effluent). The amount of
carryover reduction varies depending on a number of factors
including squeegee type, process material area and machine
speed, but typically ranges up to 95 percent. This
reduction in carryover can result in a significant reduction
of wash water. For example, the recommended wash water rate
for the C-41 process without squeegee use is 183 1/sq m and
with squeegee use is 91 1/sq m, a reduction of 50 percent.
Squeegee use is not universally applicable to all process
machines, chemistry, or products. Proper placement,
adjustment, and maintenance are extremely important to
prevent physical quality problems from scratches and
abrasions.
Among a wide variety of squeegee types, the most common are
rubber or polyurethane wiper blades (usually used in pairs
with the blades opposed to each other on opposite sides of
the film or paper), air knives, Venturi, rotary buffer, and
soft rollers. Wiper blades can have rigid mountings or, as
more recently used, they are attached to the machine with
plastic leaf springs which provide a constant self-adjusting
blade pressure on the film. The air knife squeegee consists
of slits cut into two opposed tubes from which air impinges
on each side of the material at an angle of 20° to 45° off
normal. The Venturi squeegee also utilizes air impingement
where the material effectively becomes one side of a Venturi
orifice. The rotary buffer consists of two opposed soft
felt rollers rotating in opposition to the linear motion of
the material. Soft rollers are used in opposing pairs. The
roller has an inner shaft covered with a soft polyurethane
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foam, which in turn is covered with a thin layer of silicone
rubber having a very smooth surface. Soft roller squeegees
are typically used for wide format materials. In addition
to their squeegee action, the rollers aid in transporting
the material through the machine and allow self threading
which, in turn, means no leader is required.
Automatic Wash Water Controls
Excessive wash water flows can be avoided simply and
effectively by the installation of automatic solenoid
operated shutoff devices which completely stop the flow of
water into the processor when it is not being used. A
shutoff delay of a few minutes is needed so that time is
allowed for removal of excessive chemical by-products
carried into the wash water by the film. Further
efficiencies can be accomplished by the installation of
maximum flow regulation valves which prevent greater flow
than needed.
Chemical Pre-Wash
After-fix wash water volume can be reduced by 60 to 70
percent with the addition of a salt bath between the fix and
final wash. The role of the salt bath is to remove the fix
from the emulsion chemically at a faster rate than can be
done by washing. The salt bath provides a resultant
reduction in washing time and water volume. A satisfactory
bath of this type is a 20 grams per liter mixture of sodium
sulfate and sodium bisulfite at a pH of 8 to 9.
Water Recycle
Wash water recycle has the potential for a significant
reduction in a plant's water use. With effective
application, along with other in-process controls, total
plant process water use has been observed in actual
applications to be reduced by about 70 percent. As
discussed in the section on product quality, proper quality
control is extremely important when recycling wash water
Wash water recycle is not necessarily applicable to all
processes or practical in all plants because of product mix.
When recycled water is used, the current practice is to
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recycle the water to print processes and use fresh water for
processing of originals so that in case of control problems
irreplaceable originals are not damaged. Methods used for
preparation of the water for recycle include treatment with
resin ion exchange or reverse osmosis, which were described
previously in the section on silver recovery from wash
waters.
Housekeeping
The overall effectiveness of most of the previously
discussed in-process controls is greatly reduced if a plant
does not have a commitment to good housekeeping practices.
The attendant results are a decreased economic benefit and
increased pollutant loads.
For example, silver recovery units must be regularly
maintained to prevent silver loss. The effluent from
metallic replacement units must be carefully monitored and
the units replaced when exhausted. The unit is usually
considered to be exhausted when the effluent silver
concentration is 1,000 mg/1 as determined by the use of
silver test paper or chemical analysis. Care must be
exercised in the operation of electrolytic units to control
cell current density to prevent sulfiding, if too high, and
the loss of silver, if too low.
Plumbing leaks and the use of excess chemicals and water
must be prevented to reduce hydraulic and chemical loads.
The replenishment rate used should be the minimum required
for proper process operation. The replenishment and wash
water control valves should be calibrated and periodically
checked for proper flow rate. Automatic control equipment
should be used for cut-off of replenishment and wash water
at the appropriate time at the start and end of a product
run. Squeegees should be used at every solution exit where
possible and should be regularly checked for proper squeegee
action.
END-OF-PIPE TREATMENT TECHNOLOGIES
End-of-pipe treatment is the treatment of wastewater from a
process just prior to discharge for the purpose of reducing
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pollutant loads to the receiving stream. Although benefit
may be gained from material recovery, the primary purpose is
pollutant reduction. A number of establishments in the
photographic processing industry use various types of
end-of-pipe treatments.
In many cases the wastewaters are segregated into con-
centrated chemical wastes and wash water wastes and then
treatments are applied to one or both waste streams. The
size, complexity, and corresponding costs of the treatment
equipment may be reduced by separation of wastewaters.
Descriptions of the end-of-pipe treatment technologies
encountered in this industry follow.
Precipitation involves the reaction of two or more soluble
chemicals to produce an insoluble product. This technique
is used to reduce the amount of iron-cyanide complex being
discharged by treating waste fix containing the complex with
a flocculant and ferrous sulfate as a reducing agent. This
results in the formation of insoluble ferrous ferrocyanide
which settles with the aid of the flocculant. Sulfide
precipitation is also used in a proprietary process for the
reduction of metals such as silver, cadmium, lead, iron, and
zinc from photoprocessing wastewater. Because these metals
exist as complexes, it is necessary in some cases to break
down the complex before effective precipitation can take
place. This problem has been overcome by use of proprietary
chemical additions.
Precipitation can be used for the reduction of chromium. As
used in the electroplating industry, the amount of chromium
in the wastewater is reduced by chemical reduction of
hexavalent chromium to trivalent chromium and then
precipitation of the chromium followed by filtration. The
precipitation of chromium involves the addition of caustic
soda or lime to the wastewater to increase the pH to 8-10.
This decreases the solubility of the chromium, which
precipitates as the hydroxide.
Cadmium can be precipitated as the hydroxide by adjustment
of pH. Alkalinity has a significant effect on the
solubility of cadmium. The theoretical solubility values of
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cadmium hydroxide, according to Pourbaix, are approximately
the following:
Solubility,
pH mq/1
8 3,000
9 30
10 0.03
11 0.003 (minimum)
The insolubility of cadmium carbonate suggests that
precipitations with soda ash may reduce soluble cadmium to
very low levels in effluent. Since many combined
wastewaters contain some carbonate, it is very possible that
cadmium carbonate rather than cadmium hydroxide is
precipitated when wastewaters are neutralized with caustic
or lime. Some reported values that seem unrealistically low
for hydroxide precipitation actually may be achieved by this
mechanism.
Cadmium sulfide is very insoluble (solubility product, K =
10-z»), so that a precipitation system based upon sulfides,
combined with efficient removal of dissolved solids, may
provide acceptable effluent.
Settling involves the concentration of particulate matter in
wastewater by collecting the wastewater in tanks or ponds
under quiescent conditions and allowing the suspended matter
to settle with time. Waste streams in this industry do not
normally contain large amounts of suspended solids; however,
settling is commonly used in conjunction with precipitation
to remove the resultant solids. Settling was observed to be
used for (1) collection of precipitated ferrous
ferrocyanide, (2) collection of precipitated metal sulfides,
(3) reduction of suspended solids in aerated wastewater, and
(4) preliminary settling of wastewater prior to discharge to
a POTW.
Ozonation is a treatment process where ozone is bubbled
through the wastewater. The wastewater is usually collected
in tanks and the ozone added through sparging tubes in the
bottom of the tank. Sometimes the wastewater is cascaded
through two or more tanks connected in series. The ozone
111-58
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provides a source of oxygen for oxidizable compounds and is
used for the general reduction of BOD and COD in the
wastewater. It is also .used as a pretreatment for
wastewater that is to be further treated by aeration. Plant
7781 has demonstrated that the addition of ozone prior to
aeration enhances the effectiveness of the aeration process.
Filtration is used for reduction of waterborne suspended
solids. It is accomplished by passing the wastewater stream
through solid media such as retaining screens, cloths or
papers or through particulate media such as sand, gravel,
carbon, or diatomaceous earth using gravity, pressure or
vacuum as the driving force. The filter equipment includes
plate and frame presses, cartridges, and sand or mixed media
beds. Filtration is employed to dewater precipitated
ferrous ferrocyanide and metal sulfides and as a wastewater
preconditioner prior to treatment by ion exchange or reverse
osmosis. Diatomaceous earth filtration is used in the
electroplating industry for reducing the amount of the
previously precipitated chromium hydroxide in the treated
wastewater.
Clarification is a unit operation for reduction of suspended
solids. A clarifier is a tank with internal baffles,
compartments, sweeps, and other directing and segregating
mechanisms to separate the solids from the liquids. The
solids are contained in the underflow, and the overflow
consists of wastewater with reduced solids. Often the
underflow, having a high solid content, is sent to a second
clarifier or sent directly to a centrifuge or filter device
for further concentration to sludge or cake solids One
facility was observed to use the clarifier as both a
reaction vessel to precipitate ferrocyanide and as a
clarifier to settle the precipitate from the liquid.
Aeration involves the treatment of wastewater with air to
cause the reduction of oxygen demand. This is commonly done
in ponds or large tanks. The aeration action is enhanced by
pumping the wastewater into the air as a fountain or
bubbling air through the water by means of sparging tubes in
the bottom of the pond. Aeration is used to a limited
extent in the Photographic Processing Industry for reduction
of BOD and COD.
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Neutralization involves the adjustment of the pH of a waste
stream with acid or alkali to produce a near neutral
condition. The most common method is to treat acidic
streams with alkaline materials such as limestone, lime,
soda ash, or sodium hydroxide. Alkaline streams are treated
with acids such as sulfuric. There is no particular pH
control problem in the industry. For example, developer
wastes are alkaline and stop and fix wastes are acid. The
combined wastes are often nearly neutral. Neutralization is
employed at one plant to adjust the pH of wash water treated
by reverse osmosis prior to discharge to a POTW.
Equalization involves the collection of wastewater in tanks
or ponds for the purpose of equalizing or controlling the
flow quantities prior to discharge or other treatment steps.
Equalization is necessary and practiced prior to reverse
osmosis.
Chlorination involves the addition of chlorine gas or a
hypochlorite salt to the wastewater to cause breakdown of
certain compounds by oxidation. It is used in the industry
to reduce chlorine demand loads in cooperation with the
local POTW and for control of slime organisms. It can also
be used for odor abatement or as a specific reactant.
Flocculation is used to cause or accelerate the settling and
concentration of suspended solids. Solids often settle
slowly, or not at all, because of small size and electrical
charge. Addition of flocculants such as alum, ferric
chloride, and polymeric electrolytes promotes coagulation of
particles and gives faster settling rates and improved
separation. This process is used as an aid in the settling
of precipitated iron-cyanide complexes.
Reverse Osmosis is a physical separation technique that
involves a wastewater stream passing under pressure through
a membrane. Water passes through the membrane as product
(permeate) and the pollutants remain upstream from the
membrane as concentrate. Reverse osmosis is used in the
industry to reduce pollutants in the relatively dilute wash
water wastes. This reduction of pollutants allows the
recycle of the permeate and is practiced at several plants.
111-60
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Evaporation is a technique that is used to reduce the volume
of a wastewater stream. The one plant using this system
recycled most of the processing chemicals and wash water to
reduce the hydraulic load input to the evaporation system.
The evaporation system consists of two stages of evaporation
in a spray film evaporator with a third and final
evaporation stage in a hot oil wiper film evaporator.
Thermal energy is conserved by pre-heating the incoming
water by directing it through the condensation tubes in the
first evaporator and by storage of the wastewater between
stages in thermally insulated tanks. The solids content of
the wastewater is concentrated from 2 to 65 percent in the
first two evaporation stages. The sludge remaining after
the third stage of evaporation is about 85 percent solids.
The evaporated water is condensed, purified, and reused in
the process as wash water. The purification process,
consisting of ion exchange, is necessary to remove ammonia
compounds.
Chemical Reduction of Hexavalent Chromium - Chemical
reduction is used for the treatment of wastewater by the
electroplating industry for the reduction of hexavalent
chromium to trivalent chromium. The reduction enables the
trivalent chromium to be separated from solution by alkaline
precipitation followed by diatomaceous earth filtration.
Reduction is a chemical reaction in which one or more
electrons are transferred to the chemical being reduced from
the reducing agent. Hexavalent chromium (CrVI) is usually
reduced to trivalent chromium at a pH of 2 to 3 with sulfur
dioxide (S02)f sodium bisulfite, other sulfite-containing
compounds, or ferrous sulfate. The reduction makes possible
the removal of chromium as the trivalent hydroxide which
precipitates under alkaline conditions. Typical reactions
for 302 reduction are:
S02 + H20 = H2S03
2H2Cr04 + 3H2S03 = Cr2 (S04)3 + 5H20
Representative reactions for reduction of hexavalent
chromium under acid conditions using sulfite chemicals
instead of S02 are:
111-61
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(a) sodium metabisulfite with sulfuric acid:
4H2Cr04 + 3Na2S205 + 3H2S04 = 3Na2S04 + 2Cr2(S04}3
+ 7H20
(b) sodium bisulfite with sulfuric acid:
4H2Cr04 + 6NaHS03 + 3H2S04 = 3Na2S04 + 2Cr2(S04)3
+ 10HZ0
(c) sodium sulfite with sulfuric acid:
2H2Cr04 + 3NazS03 + 3H2SO4 = 3NazS04 + Cr2(S04}3
+ 5H20
Reduction using sulfur dioxide is the most widely used
method in the metal finishing segment of the electroplating
industry, especially with larger installations. The overall
reduction is readily controlled by automatic pH and
oxidation-reduction potential instruments. Treatment can be
carried out on either a continuous or batch basis.
Hexavalent chromium is also reduced to trivalent chromium in
an alkaline environment using sodium hydrosulfite as
follows:
2H2Cr04 + 3Na2S204 + 6NaOH = 6Na2S03 + 2Cr(OH)3 + 2H20
Data from the specific plants employing these technologies
are presented in Section V and their effectiveness is
discussed in Section VII.
INDUSTRY CHARACTERISTICS
Information on the general characteristics of plants in the
photographic processing industry has been collected in two
industry surveys. The first, performed by the Eastman Kodak
Company during the period 1969 through 1974, contains
information on 237 plants. The results of this survey are
summarized in Table III-4. Within a given category, e.g.,
method of waste disposal, the parts may add up to more than
100 percent. This is because in many cases more than one
method of disposal may be used in a given plant. The totals
111-62
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for each category do not necessarily equal 237 because some
plants were unresponsive for some categories. The
information included in . this survey was collected from
plants which had requested information from Kodak on waste
disposal or effluent calculations. It is recognized that
the information included in Table III-4 is six to eleven
years old. Many of the processes in use at that time are
obsolete, and it is likely that many of the individual plant
characteristics have changed over this time period.
Nevertheless, the data were valid at the time of the survey
and can serve as a reference point for comparison with the
more current information which follows.
A more recent survey conducted by EPA in the spring of 1977
is summarized in the following tables. Table II1-5
correlates production and water use with the plant type,
arranged by SIC code. SIC codes were not directly addressed
in the telephone survey, rather the plant representative was
asked what kind of customer the plant served. This
information was used to place the plant in a SIC code using
best available judgment. Tables III-6 to 111-15 correlate
various plant characteristics with production cate9ories.
The 139 plants in this survey were contacted by telephone
and asked questions according to a fixed format. The
answers were recorded on a Telephone Survey Form, a blank
sample copy of which is included in Appendix A. Prior to
contacting the plant, a letter was sent to the plant
explaining the purposes of the EPA program, and giving
notification that they would be called and the type of
information that would be requested. The list of plants
contacted was compiled from trade magazine advertisements,
membership directory of the Photographic Marketing
Association (PMA), information supplied by the National
Association of Photographic Manufacturers (NAPM), previous
surveys, and personal contacts. The methods used to obtain
plant lists tended to bias the selected plant sample towards
relatively large plants [production greater than 93 sq m
(1,000 sq ft) per day]. The information available at the
time on the identity of the almost 10,000 smaller plants was
insufficient for the selection of a random sample of the
entire industry.
111-63
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