Upgrading Metal -Finishinc
                           Facilities to Reduce Pollutior
:FA Technology Transfer Seminar Publication


EPA 625/3-73-002
                                             WASTE TREATMENT

                                   Upgrading Metal-Finishing Facilities

                                                   to Reduce Pollution
                           U.S. Environmental Protection Agmey
                           Region 5, Ubrary (PL-12J)
                           77 West Jackson Boulevard, 12th float
                           Chicago. tl 60604-3590

                                  July 1973

     This seminar publication contains materials prepared for the
U.S. Environmental Protection Agency Technology Transfer Program
and presented at industrial pollution-control seminars for the metal-
finishing industry.

     Materials for the publication were prepared by Dr. L. E. Lancy,
President, and Mr. R. L. Rice, P.E., Executive Vice President, Lancy
Laboratories, Division of Dart Industries, Inc., Zelienople, Pa.
     The mention of trade names or commercial products in this publication is
for illustration purposes, and does not constitute endorsement or recommenda-
tion for use by the U.S. Environmental Protection Agency.
 Revised January 1974


Introduction	   1
     The Need for Waste Treatment	   1
     Engineering Considerations	   2
     Sources of Waste	   2
     Processes Requiring Pollution Control	   5

Chapter I. Commonly Used Waste-Treatment Systems	   9
     Batch Treatment	   9
     Continuous Treatment	 12
     Integrated Treatment	 14
     Ion-Exchange Treatment	 16

Chapter II. Process-Solution Regeneration and Recovery and Metal Recovery	 21
     Ion Exchange	 21
     Evaporative Recovery	 21
     Reverse Osmosis	 23
     Metal Recovery With the Integrated Waste-Treatment System	 24
     Acid-Pickle and Etch-Solution Regeneration by Crystallization or Electrolytic
          Continuous Maintenance Programs	 24

Chapter III. Economic Considerations	 25

Chapter IV. Solid-Liquid Separation, Solids Concentration, and Sludge Disposal	 27
     Introduction	 27
     Precipitation of Metal Salts in Wastewater	 28
     Gravity Separation of Precipitated Metal Salts	 29
     Design for Good Settling	 30
     Improved Clarifier Designs	 33
     Sludge Thickening	 39
     Filtration of Sludges	 41
     Ultimate Disposal	 44

References	 47

                          THE NEED FOR WASTE TREATMENT

     Metal processing in manufacturing includes a number of finishing steps that improve and con-
dition the surface for further processing for the intended final purpose of an article. Most of these
finishing steps employ wet processes and require rinsing steps. Water pollution is caused by the
deliberate or accidental discharge of the processing solutions and the contaminated rinse water. The
process may be directed toward:

     • Cleaning, which is the removal of surface oils, greases, buffing compounds, and the like

     • Removal of oxides, rust, scale, and other materials

     • Electrochemical or chemical processing to provide the basis metal with a surface coating con-
       sisting of a plated metal or a chemically deposited, so-called conversion coating such as phos-
       phate and oxide film as in blackening

     The general aims are to change the surface of a product to:

     • Improve corrosion  resistance

     • Make the appearance more pleasing

     • Improve hardness

     • Increase wear resistance

     • Change surface conductivity

     • Finish to suit specific engineering applications.

     It is evident that the first steps in the process—cleaning and oxide removal—are mainly prepar-
atory steps for good adhesion and receptivity for the subsequent finishes to be employed.  These
preparatory  steps may be similar to the procedures used in the primary metal-manufacturing
industries or by the manufacturers of certain finished products (such as automobiles and appliances).
They are, however, a major part of the activity in an electroplating plant, while they are steps of
relatively minor importance for the manufacturer.

     An electroplating plant may also be engaged in mechanical finishing activities, such as polish-
ing and buffing, sandblasting,  or wire brushing. It may have cleaning or painting processes employ-
ing solvents, mainly various chemical solutions using water as a solvent for the chemicals and as a
rinsing medium between the various process solutions through which the work progresses.

     In an activity centered around various processes employing water as a solvent, it is evident that
water-pollution problems will be encountered whenever an effluent is discharged.  The severity of
the pollution naturally will depend on the source of waste, the type of process employed, the size
of the installation, the relative concentration of the effluent, and so forth.

                             ENGINEERING CONSIDERATIONS
     Waste-treatment processes aim to eliminate from the effluent stream the polluting ingredients.
An engineering function selects and coordinates the various treatment steps, decides the required
size of equipment, selects the best site to meet plant requirements, and provides the necessary plans,
specifications, and operating information, aiming for a treatment plant that, when properly operated,
will insure that the effluent will meet the design criteria. The engineer who designs such a plant
must have information from many sources to be able to meet his objectives.

     Each plant uses a finishing program unique to itself. The processes used and the makeup of
processing solutions are highly variable. The processing equipment employed will differ in individ-
ual features, drainage rates, foundation, floor contours, age—all factors to be considered. The pro-
duction volume, type, and shape of the articles will determine the surface area to be processed and
the anticipated dragout rates per unit of surface area.  The various finishing processes contain a
variety of contaminants; these must be known to judge the type of material to be removed from the
waste stream.  The applicable chemical processes must be selected to insure the desired reactions.

     Because the economy of the manufacturing process is affected, the treatment plant and its
anticipated operating costs are of paramount importance.  Process solution regeneration, chemical
and metals recovery, water reuse and water savings, and operating and supervisory labor costs are
all factors to be considered.

     The flow rate in the receiving stream, the capacity of the municipal sewage-treatment plant,
and the applicable Federal,  State, and local requirements will have great bearing on  the formulation
of design criteria for the effluent to be discharged.  Plant safety, Occupational Safety and Health
Act (OSHA) regulations, and specific local requirements—for example, the Pennsylvania Pollution
Incident Prevention Plan—must be considered.
                                   SOURCES OF WASTE
Dumping of Waste Process Solutions

     The cleaning- and descaling-process solutions are so formulated that they will remove soil or
scale and surface metal film, and will hold the removed material without depositing it back on the
work being processed. Naturally, the capacity for additional soil or metal removal will slowly be
reduced as the soil or metal content of the cleaning solution rises, until finally the particular clean-
ing solution is considered spent and is dumped. The dumping can occur as a batch waste discharge,
or perhaps as a continuous, slow wastage to maintain a certain uniform concentration of active clean-
ing compounds or acids with a uniform contaminant loading or metal concentration to avoid the
necessity of batch dumping.

     Batch discharges occur periodically.  The relative volume of waste is usually not large; but as
the chemical concentration is relatively high, the pollution effect may be considered relatively serious.

     The cleaners employed in metal finishing usually are compounded with various alkali phosphates
and relatively high concentrations of wetting agents to provide fast and complete oil, grease, and soil
removal. The various  acid solvents for metal removal may contain higher concentrations than normal
in the primary manufacturing industry because  of the greater demands for utmost cleanliness of the
metal film for subsequent processing and because the usual demands for bright finishes require high
concentrations of acid solutions and frequent dumping to maintain a low metal contaminant level in
the acid cleaners in use.

     Most of the electrochemical process solutions in use—such as electroplating and anodic treat-
ment solutions—can be maintained in working order by periodic or continuous filtration, purification,
or additions of various chemicals. But many process solutions employed in finishing either cannot
be completely purified or are uneconomical to purify, in which case the process solution itself will
reach a point where dumping is necessary and a new process solution must be made up. This
category would include, for example, chromium plating solutions contaminated by iron, copper,
nickel, etc., anodizing solutions for aluminum processing, some of the cyanide-type plating solu-
tions, and chromating- or phosphating-type conversion-coating processes.

     From a pollution-hazard standpoint, these wasted process solutions may be considered primary
subjects for waste treatment. In view of the periodic or infrequent discharges, the relatively small
volume, and ample time available for proper treatment, the  waste-treatment effort is relatively small.
Because of the high concentrations of chemicals to be discharged, the pollution effect can be most
severe. It will be evident, then, that the chemical consumption for treatment may also be significant.
The considerable time available between batch discharges and the usually small total volume to be
treated, on the other hand, may allow small-sized equipment to be used.

Accidental Discharges of Process Solutions

     The second most severe pollution hazard in connection with metal-finishing operations is from
accidental discharges of key process solutions.  The concern shown for the treatment of the period-
ically dumped process waste should be multiplied with regard to the accidental loss of process
solutions, because not only is this hazard present for the few process solutions that are assumed to
have finite  life, but it may affect the contents of processing vats that under normal conditions could
be maintained by the usual purification maintenance practiced in the particular plant.

     Nearly every process solution in the plant is prone to accidental discharge, mainly because in
the past no engineering effort was directed to the avoidance of such discharge.  The usual plant lay-
out is such that the entire processing area drains onto the floor, and the floor is only an extension
of the sewer system leaving the area.

     While it is not common for a plating tank to spring a leak of such magnitude that the entire
plating solution could leak away undetected, many plants are operated haphazardly so  that a slow
leak amounting to a solution loss of 1 to 2 inches per day could go  undetected for months. And it
is common practice to make up evaporation losses by adding water with a hose to a process solution
or by  turning on a spigot to the process tank, which  may be neglected until the solution overflows
the rim.

     Filter hoses, heat-exchanger connections, and pumped  process lines are all prone to leak.  Hoses
deteriorate, and so forth. Waste-treatment engineering would anticipate a certain frequency of
accidental spillage, depending on the general maintenance in the particular plant.

     Steam coils or heat exchangers undergo slow corrosion reactions, and it may be anticipated  that
pinpoint corrosion or a corrosion cracking will perforate the barrier between the process solution
and heat-exchange medium—that is, between solution and steam condensate or water.  As the steam
condenses, a vacuum forms in the heating coil or in the jacket of the heat exchanger, drawing in the
process solution through  the voids created by the corrosion  action. Proper waste-treatment
engineering would therefore concern itself with the accidental contamination of either  the steam
condensate or the cooling water used in the particular process.

     Some  State codes require containment of the most toxic process solutions, such as cyanide and
chromic acid plating  solutions, by a surrounding outer container capable of holding the entire volume
of the process solution in case of accident. No doubt these  State codes reflect experience accumu-
lated with various plating operations. The fallacy of these regulations is that, as discussed earlier,

 there are many other ways for serious accidents to occur and cause severe pollution conditions
 against which the outer container for the process vat would provide no insurance.  The container
 only protects against the least common occurrence—a serious leak in the process tank itself—while
 adding to the cost of the plant installation.

      On the other hand, the awareness of the regulatory agencies of some of the potential hazards
 with the so-called accidents in a metal-finishing installation should help in a waste-treatment
 engineering effort aiming for the utmost safety considering the particular plant and proc^sses under
 scrutiny. The Pennsylvania Pollution Incident Prevention Program regulations require that the
 engineer submit detailed plans for each installation that stores, uses, or processes toxic or potentially
 harmful materials. Compliance naturally requires that facilities be incorporated and value judgments
 be made not only for the processing area, but for lagoons, containment sumps, chemical storage
 tanks, storage areas, and so forth.

      There are also special conditions created by plant locale that may cause accidental pollution;
 for example, earthquake hazards should be considered in areas such as California, Japan, and Mexico.

      Table 1 lists some of the more common accidental discharges, with methods for detection and
 prevention or correction.
                       Table 1 .—Common waste discharges due to accidents in metal-finishing plants
                                             Method of detection
                                                                                Correction or prevention
1. Process tank overflow
    a. Unattended water additions.
    b. Leak of cooling water into solu-
      tion from heat exchanger or cool-
      ing coil.
2. Process solution leakage
    a. Tank rupture or leakage.
    b. Pump, hose, pipe rupture or leak-
      age, filtration, heat exchanger,
    c. Accidental opening of wrong
3. Normal drippage from workpieces
  during transfer between process tanks.
4. Process solution entering cooling
  water (heat-exchanger leak).
5. Process solution entering steam
  condensate (heat exchanger or
  heating coil leak).
6. Spillage of chemicals when making
  additions to process tanks or spillage
  in the chemical storage area.
1. High-level alarms in floor collection
  system to signal unusual discharges.
2. Integrated floor-spill treatment.
Same for process tank overflow.
1. Conductivity cell and bridge to actu-
  ate an alarm.
2. Use of the cooling water as rinse water
  in a process line where the contamina-
  tion will be immediately evident.
Conductivity cell and bridge to actuate
  an alarm.

Solution maintenance man responsible
  for chemical additions.
1. Proper floor construction for floor-
  spill segregation and containment
  (curbs, trenches, pits).
2. Treatment facilities for collected floor
3. Integrated floor-spill treatment system.
4. Use of spring-loaded valves for water
5. Automatic level controls for water
Same as 1-^ for process tank overflow.
1. Drainage pans between process tanks
  so that drippage returns to the tanks.
2. Floor-spillage collection.
3. Integrated floor-spill treatment.
Conductivity controller to switch con-
  taminated condensate to a waste-
  collection and -treatment system.
1. Careful handling and segregation of
  chemical stores.
2. Segregation and collection of all floor
3. Integrated floor-spill treatment.

Contaminated Rinse-Water Effluent

     When generally discussing waste treatment in connection with metal-finishing processes, it is
normally assumed that the topic will be the elimination of the toxic constituents from the rinse-
water effluent. As discussed earlier, the most severe hazards are not from the discharge of an un-
treated rinse-water effluent.

     Metal finishing requires copious quantities of water to wash away the remaining chemical film
on the work surface dragged out from one process solution before the workpiece enters the next
process.  First, water is the common solvent for this purpose.  Second, the removal of the tenaciously
adhering chemical film can be more easily accomplished with fast-flowing water, which provides
agitation around the work surface. A chemical film dragged out from one process and remaining on
the work surface may react with the next process solution to precipitate insoluble salts on the metal
surface as barriers for good adhesion, causing subsequent roughness and other defects.  The chemicals
that may be dragged from one process into the next could cause contamination of the  second
process solution by the slow accumulation of dragged-in impurities, chemical constituents of the
first process. Good rinsing is also needed to eliminate any chemicals remaining in the pores of
finished work that may lead to later discoloration, tarnishing, or corrosion, and that this  may
destroy the desired finish.

     The rinse-water effluent from a metal-finishing plant will contain the various dissolved solids
which were originally carried on the work surfaces.  The dragout from alkali cleaners becomes mixed
with the dragout from acids, pickling solutions, and the various plating processes. While the total
dissolved salt concentration in the water may not increase significantly, the effluent carries—either
as dissolved or as  precipitated suspended solids—the various metal salts, cleaning compounds, and
anions of the acids used and possibly a small quantity of the oils and greases originally removed by
the cleaners from the  work surface.

     The relatively large volume of effluent discharged makes the treatment of the rinse-water
effluent the major problem. It is an additional problem that after all the various process  rinses are
mixed, the proper chemical treatment becomes much more complicated, or even impossible.  In
such a case, it may be necessary to segregate the rinse waters into various chemical groupings to pro-
vide proper treatment. Mixing of the total effluent occurs only after the segregated effluent waste
streams have received specific chemical treatment.
     There follow some typical metal-finishing processes that require in-plant or waste-treatment

     • Effluents containing only solid, mechanically produced impurities, such as rolling or
       annealing scale, sand, or sludge

     • Effluents containing liquid impurities that are immiscible with water, such as greases, oils
       and their solvents including kerosene, benzene, trichlorethylene, and similar solvents

     • Effluents from acid pickling solutions for ferrous and nonferrous metals

     • Effluents from chemical or electropolishing solutions formulated with mineral acids such as
       phosphoric, sulfuric, chromic, nitric, and organic acids—such as acetic, citric, gluconic—with
       high concentration of dissolved metals

     • Effluents from phosphating solutions that contain phosphoric acid and metals such as iron,
       zinc, or manganese

     • Effluents from alkaline pickling solutions for aluminum arid zinc, including the alkaline

     • Effluents from various types of electroplating solutions, including acid, alkaline, and cyanide
       solutions from which various metals may be deposited

     • Effluents from chromic acid and chromate solutions in the form of electroplating, etching,
       arid anodizing solutions—including chromating solutions for magnesium, zinc, and aluminum—
       and electropolishirig solutions

     • Effluents from metal heat treatment, such as cyanide hardening operations, quench waters
       after brazing, metal and paint stripping, water-wash paint-spray booths, metal etching,
       typographical and rotogravure operations, and nonelectrolytic plating systems used for metal
       deposition on nonconductors

     Almost all of the chemical-processing solution must be discarded, owing to the buildup of
foreign metal impurities. As a general rule, electroplating solutions are seldom, if ever, discarded.
When it is necessary to discard spent processing solutions, they are generally batch treated before
disposal. The rinse waters following these processing operations usually contain impurities in dilute
form. Except in the case of purely mechanical contamination, most of the contamination constit-
uents are highly toxic and these toxic effects usually persist even at low concentrations. Even with
large volumes of water it is impossible to dilute the contaminate sufficiently below the toxic levels.
For example, the normal commercial chromic acid plating solutions may, on the average, contain
300 g/1 chromic acid.  These solutions would require dilution by a factor of about 1 million to meet
U.S. Public Health Service Drinking Water Standards, which specify less than 0.05 ppm for chromium.

Effluents Containing Solid Impurities  From Mechanical Operation

     To obtain a good-quality final finish in a metal-processing operation, whether it  be an organic,
metallic, or a chemical coating, it is imperative that the surface of the workpiece be completely free
from oils, greases, and rust or other oxide films.  Thus, it is obligatory that certain cleaning opera-
tions be performed before subsequent chemical processing. It is necessary often to subject the work-
pieces initially to a mechanical operation, such as tumbling, blast cleaning, polishing,  or buffing.  A
mechanical operation can produce an effluent containing solid impurities.  Water discharged from
the dust collectors, tumbling, or vibratory finishing after the solids separation may contain impuri-
ties requiring chemical or biological treatment.

Effluents Containing Liquid Organic Impurities

     Workpieces received in a metal-finishing plant are often covered with oils and greases left by
machining operations or applied to protect their surface during storage and shipping.  Oils and
greases usually are removed by subjecting the workpieces to the action of organic solvents and/or
inorganic alkaline cleaning solutions. Effluents generally are contaminated with these water-
immiscible materials, due to dragover or batch dumping of the processing media.

     The solvents used in vapor or the immersion types of degreasing—for example, the nonflamma-
ble chlorinated hydrocarbons or the flammable solvents (such as kerosene) used in emulsion
cleaners—can form an emulsion in water or a floating film that not only detracts from the appear-
ance of the water but also presents danger to all living organisms. In addition, these organic

contaminants in effluents may be flammable or may liberate toxic gases, which would also prohibit
their discharge to a storm or sanitary sewer system.  The BOD (biochemical oxygen demand) con-
tent may be sufficiently high to require biological treatment.

Effluents From Acid Pickling, Polishing, or Phosphating Solutions

     The effluents from pickling operations yield large quantities whenever any type of metal is
processed, owing to rinsing following processing or to dumping of spent processing solutions.  Pick-
ling solutions are usually strong acids. The acids are consumed by the dissolution of oxides and
metals processed. The acid must be replenished and metal ion content controlled for an effective
operation.  In most pickling operations the solutions cannot be controlled and must be discarded.
The effluent from batch dumping or from rinsing following processing is toxic because of the acidity
or the metal ion content. Exposure to acid water will damage masonry and iron structures. The
metal ion content of the effluent from pickling areas may be high in copper, zinc, nickel, cadmium,
iron, and other heavy metals, which are dangerous poisons to all living  organisms and may be fatal
at low concentrations.
Effluents From Alkaline Pickling Solutions

     Alkaline pickling solutions are used primarily for etching aluminum and zinc. These solutions
are generally highly caustic and must be neutralized with acid or spent-acid pickling solutions to a
slightly alkaline pH for the precipitation of the metal ions.

Effluents From Electroplating Solutions

     Effluents from many of the alkaline plating solutions contain complex metal cyanides that must
be treated to destroy or remove the cyanide radical; and the heavy metals must be removed before
the effluents can be safely discharged. The primary toxic constituent of the non-cyanide-containing
processing solutions, whether acid or alkaline, is the heavy metal ion content.  The heavy metal ions
must be removed by alkaline precipitation, electrolysis, or ion exchange.  The processing solution
formulation may contain complexing agents that will not  allow the complete precipitation of the
metal salts upon neutralization.

Effluents Containing Chromium Ions

     Chromium-containing chemicals are used in many processing solutions for plating, etching,
anodizing, electropolishing, and chromating.  The chromium ion content of many of these process-
ing solutions is quite high; consequently, the rinse effluents following processing are high in
chromium ion, which is toxic even in the most dilute of concentrations and must be reduced to a
safe level.  Some of the more acceptable methods are reduction and alkaline precipitation,  ion
exchange, and evaporation.

Quench Water Following  Cyanide Metal Hardening

     These quench waters usually contain ferrocyanides in addition to the simple cyanides, and
require treatment of both compounds.  Paint- and metal-stripping solutions and nonelectrolytic
plating systems may contain a wide variety of organic acids and salts, complexing agents, and
cyanides. Both chemical  and biological treatment may be required; the complexity of the  problem
may  require consultation with the supplier of the process  or with the waste-treatment specialist.

                                      Chapter I

                                   BATCH TREATMENT
     For dumped processing solutions containing high concentrations of chemicals, batch treatment
may be the best system. Dumps usually are infrequent and sufficient time is available for the slow
addition of the needed chemicals.  Rapid treatment of concentrates may generate considerable heat,
and this again may cause the release of toxic fumes.  With collected floor spill, the time needed to
analyze for the contents and to provide the necessary chemicals for treatment indicates the suitabil-
ity of batch handling.

     Concentrated process solutions containing cyanides will be handled most economically through
a batch electrolytic destruction^>2>3 treatment and subsequent chemical processing.  High cyanide-
content waste solutions should be otherwise diluted to provide a waste containing less than 1 oz/gal
CN, to avoid overheating and the release of the highly toxic CNC1 gas.  Also, some of the metal-
cyanide complexes cannot be treated rapidly with the usual chlorination. Nickel and silver cyanides,
for example, require a long time for treatment and as the soluble free cyanide is destroyed in the
chlorination, there is danger that the metal cyanides will precipitate as the insoluble salts and be-
come available for further chemical destruction. Sludges containing slowly soluble metal cyanides
would then result, making the solid waste discharged by the plant unsuitable for land disposal.

     Batch treatment for rinse water should be considered only for small-sized plants, owing to
economical considerations, and because the insoluble metal precipitates cannot be easily separated
from the treated wastewater. Filtration is complex in view of the gelatinous nature of the metal
hydroxides, and decantation easily disturbs the precipitated flocculent particles separated from the
water in quiet condition.

     Batch treatment is suitable  for the neutralization of acidic and alkaline effluents and also for
the treatment of cyanide- or chromium-containing wastes.

Cyanide-Containing Effluents

     The dilute cyanide wastes from the rinse waters following plating operations, cyanide dips, or
other cyanide-containing processing solutions are drained to a treatment tank of large enough
capacity to hold the waste accumulated in one shift plus the volume of treatment chemicals.  With
two tanks, waste can be collected in one while treatment is taking place in the other. Each tank is
baffled to provide adequate mixing and to prevent short circuiting during treatment, and each can
be equipped with high- and low-level alarms.

     During all stages of the treatment, the contents are continuously circulated and/or vigorously
stirred to provide rapid  and intimate mixing of the reaction mass.  When the level alarm signals that
a treatment tank is full, the wasteflow is diverted to the other tank. If plating or concentrated
processing solutions are dumped very frequently, a separate holding tank should be provided. The
concentrated waste from the holding tanks would  be bled into the dilute waste-treatment tanks by
means of a metering pump. The size of the holding tank is determined by the frequency  of dumping.

     Treatment consists of first elevating the pH by the addition of caustic, and second, simultane-
ously adding chlorine and caustic or sodium hypochlorite. 4>5  After treatment, the contents of the
tank are discharged.

     Manual Batch Cyanide Waste.  A typical manual batch system for cyanide waste is shown in
figure 1-1. In operation, the circulating pump is started and the pH of the waste is adjusted by adding
caustic to the system through the caustic feed pump. The pH is checked at approximately 10-minute
intervals using either universal or narrow-range pH test paper.  When the pH reaches 11.5, the caustic
feed pump is stopped. A sample of the waste must then be collected for a determination of the
cyanide content in order to establish the approximate additions of chlorine or sodium hypochlorite
required. The treatment chemicals are fed continuously for a predetermined time. At the end of
this time, the chemical feeds are stopped, but circulation of the batch is continued.  At the end of
45 minutes, a sample of the batch is tested for residual chlorine or cyanide. If cyanide is still  present,
the chemical treatment is continued by starting the chemical feed pump. The batch is then tested
at intervals approximating 15 minutes until it has been determined that all cyanide is destroyed.
When the absence of cyanide has been confirmed, the pumps are stopped. The treated batch may
then be dumped to a clarifier for settling the precipitated metal salts prior to the discharge of the
treated effluent.

     When the second tank  has become full, the wasteflow is diverted to the now empty first tank.
The batch in the second tank is then treated in the same manner.

     Instrumented Batch Cyanide Waste. In the  instrumented batch system, semiautomatic control
is provided by the addition of pH and oxidation-reduction potential (ORP) controllers.6 The opera-
tion  is as follows:

     With the pH controller switched to manual control, caustic soda is injected by the caustic feed
pump into the untreated waste as it is pumped into the circulation system. The pH is raised to 11.5
initially.  When the pH has been adjusted, the booster pump is started and sodium hypochlorite or
chlorine is added at the desired flow rate.

     At the same time,  the pH  instrument is switched to automatic control and, under the propor-
tional throttling control of the pH instrument, the caustic feed pump  continuously adds enough
reagent to maintain a pH of 11.5 during the reaction. The pH controller is equipped with a low-pH-
limit switch for alarm and is interlocked with the booster pump to stop chlorine or sodium hypo-
chlorite feed in the event of a low-pH condition.
                              TREATMENT TANK NO I
                                                                            TO CLARIFIER
                                                                TREATMENT TANK  NO. 2
                              Figure 1-1. Manual batch cyanide system.

     The progress of the chlorination is continuously indicated by the ORP recorder-controller.
When this instrument senses that the endpoint of the reaction has been reached, after about an hour,
it shuts down the booster pump through a limit switch and signals the operator.

     The batch continues to circulate for 30 minutes, and if the ORP recorder still shows the reac-
tion to be complete, the treated waste can be dumped. Should the reaction be  incomplete after the
30-minute mixing period, the  chlorine booster pump is started and continues to feed chlorine or
sodium hypochlorite until the limit switch on the ORP controller is again tripped, indicating that
the reaction is complete.  The pH recorder-controller will maintain the proper pH automatically
during chlorination.

     Kastone Process.7'8  The Kastone process was developed by the Du Pont Company and is
recommended for the treatment of sodium, potassium, zinc, and cadmium cyanide only. The solu-
tion pH is adjusted to 10.0-11.5, the solution is heated to 120-130° F, and hydrogen peroxide and
formalin are added according  to recommendations of the supplier, depending on a previous analysis.
In approximately 60 minutes, the treatment is completed and the waste may be checked for un-
reacted cyanide. If it is established that all the cyanide content is treated, the waste may be decanted
or filtered and discharged. The filterability of the metal solids is improved by the process, but there
is greater danger for entrapment of the insoluble metal cyanides in  the sludge.  The chemical costs
are somewhat higher with this process; on the other hand, there is no danger of generating cyanogen
chloride gas during treatment. The effluent requires biological treatment, so  it  must be discharged
to a sanitary treatment facility.

Chromium-Containing Effluents

     The batch treatment of chromium is handled in a similar manner to cyanide batch treatment.
The chromic acid wastes from the rinses following plating solutions, bright dips, and conversion
coatings are collected in two duplicate receiving tanks alternately used for collection and treatment.
As in the case of cyanide, it is desirable  to provide a third tank for holding dumped spent processing
solutions and a metering pump to bleed a predetermined portion of the solution from this tank into
the treatment tanks containing dilute wastes.

     Treatment consists of first lowering the pH by addition of acid, second, adding sulfur dioxide or
sodium metabisulfite to reduce the hexavalent chromium, and third, elevating pH adding caustic to
precipitate the trivalent chromium.

     Manual Batch Chromium Waste. In operation, the circulation pump is started and concentrated
sulfuric or hydrochloric acid is added to the batch manually or by means of the acid feed pump.
The pH is checked at frequent intervals  and, when it reaches 3.0, acid addition  is stopped. A sample
of the waste must then be collected for  determination of the hexavalent chromium content of the
waste in order to establish the required feed rate for sulfur dioxide or sodium metabisulfite. A
procedure can be set up for this  purpose, using a color comparator.  For each installation, curves can
be plotted for the quantity of chemical  required versus hexavalent chromium.

     When the required chemical feed rate has been determined, the pump is started and the pre-
viously dissolved sodium metabisulfite is added or the sulfur dioxide feed rate set on the sulfonator.
The batch is then circulated continuously for about 3 hours. At the end of this time the sulfonator
booster pump is stopped, but  the batch is circulated for another  15 minutes.

     At the  end of the 30-minute period,  a sample of the treated batch is tested for hexavalent
chromium.  If the test indicates that hexavalent chromium is still present, the treatment is con-
tinued; more chemicals are added from the sodium metabisulfite stock solution tank, or the sulfo-
nator booster pump is started again. The batch is then tested at 15-minute intervals until all the

hexavalent chromium has been reduced, at which time the sulf onator booster pump is stopped.
Caustic is then added manually or by means of a caustic feed pump until the batch attains a pH of
8.0, at which point the trivalent chromium will precipitate out of solution. The treated batch can
be discharged to a clarifier, or decanted to the sewer.

     When the second tank has become full, the wasteflow is diverted to the now empty first tank.
The batch in the second tank is then treated in the same manner.

     Instrumented Batch Chromium Waste.  A semiautomatic batch treatment system can be pro-
vided by the addition of pH and ORP controllers.9 In operation, the pH controller is switched to
acid control and acid is injected by the acid feed pump into the untreated waste as it is pumped into
the recirculation system.  The pH is lowered to 3.0, at which point the acid feed pump is cut off.
The chromium booster pump is then started and sulfur dioxide is fed at a preset flow rate, main-
tained constant by the manually set sulfonator. Using sodium bisulfite or ferrous sulfate as a
reducer requires controlled additions of acid to maintain the preset pH.

     The progress of the reduction reaction is continuously indicated by the ORP recorder-controller.
When this instrument senses that the endpoint of the reaction has been reached, it automatically  shuts
down the booster pump.  The pH recorder-controller is then switched to alkali control and the
caustic feed pump is started. When a pH of 8.0 is reached, the pH controller cuts off the caustic feed
pump. The trivalent chromium will precipitate and the batch can be discharged to a clarifier to settle
the precipitated chromium salts.
                                CONTINUOUS TREATMENT
     In contrast to batch treatment, continuous treatment for rinse-water effluents allows closer
instrumental control, better mixing of the reacting chemicals, and a uniform rate of flow pre-
requisite for the successful performance of a clarifier, usually following the chemical treatment as
the first step in the liquid-solids separation to remove the precipitated metal salts.

     The rinse-water streams are separated according to the various chemical treatments needed.
Each segregated stream passes through a reaction tank of suitable size with mixing and reaction
chambers and instrumentation to allow the required chemical feed and retention time to provide the
optimum conditions for the completion of the intended reaction. Subsequently the treated individual
rinse-water streams are mixed for self-neutralization, final pH control, possible polyelectrolyte dosing
for best flocculation, and discharge to a clarifier or lagoon to settle the precipitates and skim
potential floating solids.

Continuous Cyanide Treatment

     The dilute wastes are drained directly to the first reaction tank. A separate holding tank is pro-
vided for concentrated wastes, which are slowly bled into the first reaction tank by means of a
metering pump. The reaction tank is baffled to insure positive  mixing and prevent short circuiting
of the waste through the tank. In the first reaction tank, the cyanides are converted by  the chlorine
addition into cyanogen chloride, which will hydrolyze to cyanate in 10-15 minutes, at a pH of 11.0-
11.5. The pH is maintained constant by the injection of caustic through the metering pump, which
is proportionally controlled by the pH recorder-controller. Chlorine is fed to the system by a
chlorinator or added from a sodium hypochlorite stock solution tank through a control  valve. As
mentioned earlier, the first reaction—cyanide to cyanate—takes place at a pH of 11.5.  If required,
the cyanate may be completely broken down to nitrogen and carbon dioxide gases in a second

reaction that occurs more rapidly at pH 7.5-8.0.  The pH reduction is accomplished by the addition
of concentrated mineral acid. The acid is fed to the reaction tank by a control valve.

     From the pH adjustment tank the waste flows to a larger, baffled tank where the complete
destruction of the cyanate is accomplished. Either chlorine and caustic are added to this tank, con-
trolled by pH and ORP instruments, or sodium hypochlorite solution is fed by the ORP controller.
The effluent from this final reaction tank is discharged from the plant directly to a settling tank or
Continuous Chromium Treatment

     As in the case of cyanides, continuous treatment of chromate wastes is more practical than
batch treatment for plants handling large amounts of dilute wastes.

     The continuous system for the treatment of chromates is basically similar to that for cyanide
treatment.  In operation, the dilute wastes are drained directly to the reaction tank. Concentrated
wastes from the holding tank are slowly bled into the tank by means of a metering pump. The
hexavalent chromium is converted to the trivalent state in the reaction tank by the addition of a
sodium metabisulfite solution or sulfur dioxide. The reaction is virtually instantaneous at a pH of
3.0 or less. The pH is maintained constant by the addition of mineral acid through a control valve
regulated by the pH recorder-controller. Sulfur dioxide may be fed to the system by an SO2 feeder.

     From the first reaction tank the flow of the waste is directed to the small pH adjustment tank.
Here, the pH is increased to 8.0 by alkali addition.  The pH recorder-controller automatically main-
tains the pH constant by the addition of the caustic soda solution. From the pH adjustment tank,
the waste can be discharged to a settling tank for separation of the precipitated solids. Figure 1-2
shows the schematic arrangement for continuous treatment of a cyanide- and chromium-containing
                                   RINSE WATER
                                   WITH CYANIDE
                                                                     >-TO CLARIFIER
                      Figure 1-2. Continuous treatment of cyanide and chromium.

     The advantages of the continuous system are:

     • Design is simplified. Assuming the presence of only conventional metal-finishing processes
       with well-known chemical treatment requirements, engineering problems are minimized.

     • With proper instrumentation and incorporation of safety features, the supervisory attention
       is minimal; therefore, no separate waste-treatment operator is needed.

     The disadvantages of the system are:

     • Opportunities are minimal for water saving or reuse.

     • The precipitated metal salts are mixed, eliminating the opportunity for economical metal
       recovery from the sludges.

     • Chemical costs are high because, with the metal precipitation and water hardness, solids have
       to be precipitated and the chemicals for pH adjustment for the various treatments can become

     • Unless care is exercised and complexing agents are rigorously kept out of the rinse water,
       meeting of effluent limits may be marginal or not possible.
                              INTEGRATED TREATMENT10-11
     The integrated treatment system was devised primarily to meet the need of the plating industry
for improved rinsing, and to create savings to offset the costs of waste treatment. The basic concept
of the integrated system is the segregation and treatment of the waste at the source. To accomplish
this purpose, the liquid film of plating solution that adheres to the part as it is removed from the
bath is simultaneously treated and removed from the plated or processed part.  The waste treatment
is integrated into the processing sequence and no separate treatment plant is required.  The system
can be employed following any processing step that would result in toxic waste carryover, regard-
less of its position in the processing line. The simplicity and reduced space requirements of this
system make it easily adaptable to existing processing lines.

     In operation, a treatment wash tank is substituted for the first rinse tank following the plating
operation.  A treatment wash solution is recirculated continuously through this tank, where it phys-
ically removes the dragout and at the same time reacts chemically with it. The part is then rinsed
with fresh water in the subsequent rinse tank. The effluent from this rinse tank is now uncontam-
inated with toxic dragout or precipitated metal hydroxides and requires no additional treatment.

     The treatment solution is continuously recirculated between the treatment wash tank and a
larger reservoir. The reservoir tank serves three major functions.

     • It is the all-important  buffering component in the system that neutralizes the shock load
       caused by sudden and irregular changes in the quantity of plating or processing solution
       dragout treated.

     • It serves as a clarifier,  settling out the insoluble metal oxides and hydroxides formed in the
       first stage of the reaction.

     • It serves as a retention tank, providing adequate time for the desired chemical reaction, such
       as oxidation of cyanates to carbon dioxide and nitrogen, and treatment of nickel cyanide,
       silver cyanide complexes, all of which take hours for completion.

     Only one reservoir tank is required; several treatment wash tanks can be served by a common
reservoir tank.  Various metal wastes should not be mixed if metal recovery from the sludges is the

     The integrated system can be completely automated, but in smaller plants it operates without
the need for close control since high excesses of treatment chemicals are used in the closed loop.
Relatively simple paper and spot tests are sufficient for control of the treatment process.  Treatment
chemicals are added either continuously or by batch as they are consumed by the toxic materials.
A typical integrated system is shown in figure 1-3.

     The system has many advantages, including the following:

     • Equipment costs are low.  The equipment is integrated into the finishing line,  occupying a
       minimum of floor space and requiring no separate waste-treatment plant.

     • Supervision and control are simplified, because control is restricted to simple checks to
       ascertain the availability of excess treatment chemical in the system.

     • Rinsing is improved, staining is eliminated, and rinsing rejects are reduced. Reduced quantities
       of water are used in view of the prerinse with chemicals, allowing 80-90 percent reuse of the
       wastewaters, since the contaminating chemicals are kept out of the rinse-water flow and no
       treatment chemicals are added to increase the salt content of the wastewater.

     • Waste-treatment chemical cost is minimal, because one of the major chemical consumption
       factors—the addition of caustic or acid to bring the waste rinse water into the  correct pH
       range for treatment—is eliminated.

     o Sludge handling is simplified. The  chemical system is  so formulated that the precipitates are
       densely settled.  The metal sludges are segregated, allowing simple and economical recovery
       of the metal values.
                                                                  REUSE WATER
                      CYANIDE WASTE
                      TREATMENT RESERVOIR


. n
                                                                        - TO SLUDGE BED
                             Figure 1-3.  Integrated treatment system.

     • The individual chemical rinse stations, operated with high content of excess reacting chemicals,
       provide a fast and more complete treatment. Only the dragout from the treatment rinse
       reaches the rinse water; therefore it is easier to meet effluent quality requirements.

     The disadvantages of the system are:

     • Additional rinse tanks may be required where such are presently not used, unless one of the
       rinse tanks in a double-rinse system can be converted to chemical rinsing.

     • While the integrated rinse, if properly maintained, leads to a quality improvement in the
       finish, the improperly operated plant may affect the desired finish quality and may be the
       cause for reworks.

     • The metal finisher is made responsible for waste treatment and has to attend to the mainte-
       nance of additional process solutions. It would be more convenient to leave these problems
       to a remote waste-treatment-plant operator.
                              ION-EXCHANGE TREATMENT
     Ion exchange is one method of concentrating the chemical contaminants in rinse waters so that
they can be treated more easily. It also makes possible the recovery of valuable materials. As a by-
product, ion exchange produces deionized water that is useful in rinse tanks and in preparing new
plating solutions.

     Basically, ion exchange is a system for removing one ion from solution and substituting it with
another ion to produce a solution that has a more desirable composition than the one being treated.
The basic material involved is a granular solid known as an ion-exchange resin, which has the prop-
erty of exchanging one of its ions for one of those in the solution being treated.  The process itself
is cyclic. The solution being treated passes through the exchanger until it exhausts the resin.  In
essence, the resin itself can be used indefinitely. Ion exchange can  be used to concentrate rinse-water
wastes that must be neutralized before discharge or, when economics dictates, it can be used to
recover metals and regenerate process solutions.

     Theoretically, it is possible to remove continuously the chemicals contained in the process-
solution dragout from the rinse-water effluent, and to recover these chemicals in a useful form when
backwashing the ion-exchange column. The backwash waters, because they are usually more dilute
than the original process solution, would only have to be reconcentrated by evaporation before
returning them to the process.  At the same time, the rinse waters that have been passing through
the ion-exchange column would be returned for repeated rinsing. Waste treatment operated in this
manner becomes a closed cycle, greatly reducing the quantity of water that has to be purchased, but
at the cost of chemicals that are required to backwash the ion-exchange column—that is, to free the
ion-exchange resin of the process chemicals that were picked up when the waste rinse water was

     When an ion-exchanger installation is used for the purification of rinse-water effluent from
various processes, the reclamation of the process chemicals contained in the backwash water can no
longer be used for return to the original process from which they originated. In an installation of
this type, the main function of the ion-exchange installation is to avoid waste treatment of large
volumes of rinse-water effluent. By backwashing the exchanger, all the chemicals that would have
required treatment become available in a far more concentrated form for waste treatment. The
functions of the ion exchanger in an application of this type would aim to return nearly all the rinse

water to the process for repeated use and allow a simplified waste treatment with regard to the
volume of the total waste to be treated. The chemical and maintenance cost of the ion-exchange
installation would have to be balanced against the water savings.

     In a system of this nature, consideration has to be given to the fact that the ion-exchange resin
bed is capable of repeated regeneration without deterioration if it is loaded with chemicals that the
resin can release on simple back washing with the chemicals used for this purpose. With a mixed
rinse-water waste stream, careful engineering is required to insure the success of the installation.
Oil that may be carried by the rinse water after cleaning should be removed. Some wetting agents
and organic brighteners may also foul the resin bed. It is best,  therefore, to insert a carbon filter
into the recirculation system to remove all deleterious organic  materials before passing the waste
rinse water through the ion exchanger. Another problem is the precipitated water hardness and
metal hydroxides clogging the resin bed.  In a pool of mixed rinse-water effluent, the pH of the
stored water will determine the extent to which the metal salts in the waste may  have been precip-
itated. The ion-exchange resin is capable only of removing ionized substances; therefore, solids
such as precipitated metals and soaps will mechanically clog the resin bed. Some compounds may
not be released as easily by some of the ion-exchange resin materials as some other resin formula-
tions.  It is necessary, therefore,  to consider all the possible organic and inorganic materials that
will be contained in the rinse water and select the resin that is least affected and easiest to regenerate
for a successful installation.  Figure 1-4 provides a schematic  presentation of an ion-exchange
installation of this type.

     Assuming that the precipitated metal salts are removed by previous pressure filtrations,  and
that the organic content of the waste stream was purified, through the use of a carbon filter, of all
the materials suspected of fouling of the ion-exchange resin bed, the rinse water as it is passed
through the ion exchanger is freed of all salt content and the water can be reused in a closed  loop.
The cation bed will remove all cations until exhausted and, similarly, the anion exchanger will
remove the anions.  For example, sodium, calcium, the metal ions, and trivalent chromium would
                                                             TO CLEAN WATER
                                                             RESERVOIR AND
                                                             PROCESS RINSE
                                                                 TO WASTE
                                  Figure 1-4.  Ion-exchange system.

be retained on the cation resin, while the anion exchanger would retain the sulfates, chlorides, and
so forth, hexavalent chromium, and cyanide complexes.

     Each ion-exchange resin column has a certain capacity to combine with equivalent weights of
the various chemicals contained either in the process solution or in the rinse water. Calculations
for each installation must be made to establish the volume of resin that should be used to provide
a reasonable length of time for use between backwashings.  Normally, parallel ion-exchange systems
are used for wastewater treatment to allow the backwashing of one while the other is on stream.
The size of the installation will depend on these calculations and a certain percentage of deteriora-
tion in time should be calculated to  allow for an efficiency drop of 20-25 percent in the use of the
ion-exchange column.

     Because the capacity of the exchanger is based only on the chemical equivalent weights of the
various chemicals to be removed from the recirculated rinse water, the volume of water recirculated
is immaterial. This nondependence  on recirculation rate allows accelerated water recirculation for
better rinsing without economical detriment.  While it may not be significant that the recirculated
water entering the rinse tank is desalted, because desalted water does not have  buffering capacity,
the fact that the flow rate can be accelerated is an advantage.

     The performance of the ion-exchange system usually is monitored with a conductivity con-
troller.  When the exchange capacity of the system nears exhaustion, the salt content increases the
conductivity of the water. This breakthrough of dissolved salt in the recirculated effluent is an
indication that the system requires regeneration.  Usually dilute hydrochloric acid is used for the
regeneration of the cation exchange column, while caustic soda solution is used for the regeneration
of the anion exchanger. Cyanide and wetting agents may not be retained completely and may be
present in the recirculated rinse water.  The complete release of chromic acid from the anion bed
can be  another difficulty to guard against.

     The regenerant and backwash waters carry all the salts earlier retained on the resin. Usually
a batch waste-treatment system is used to treat this mixed waste.  Since the acid and alkali values
are near balance, the regenerant and backwash waste is mixed. After cyanide treatment the
chromium is reduced and precipitated in the alkaline range, the metal salts are  precipitated and
settled, and the clear effluent is decanted to allow the separation of the solids from the water.

     The chemical treatment of the  batches must be tailored to the particular installation and to
the anticipated constituents of the mixed waste. Process changes  and variable processing in the
finishing plant may require variations in the treatment process. Complex salt formation is always
a danger if the potential of iron cyanide and nickel cyanide formation is present; the cation and
anion regenerant waters may have to be segregated. Complexing agents should be kept out of the
effluent in a manner similar to that indicated for continuous or batch treatment.  Sometimes it is
necessary to pass the treated effluent through  a secondary cation exchanger to remove the metal
salts that could be only incompletely precipitated in the treatment.

     Dumped processing solutions and floor spill are batch treated, sometimes mixed with the
regenerate and backwash from the ion-exchanger system. Inconsiderate mixing of these wastes may
make the treated effluent or sludges, or both, unacceptable for discharge.  There is great potential
hazard of creating a sludge high in slightly soluble metal cyanides, insoluble iron cyanides, or soluble
metals  that cannot be precipitated or separated from the water phase.

     Moving-bed ion-exchange systems are theoretically similar to the fixed-bed installations.12
Their advantage may be obvious when large capacities are needed.  As shown in figure II-1, the resin
is recirculated in pulses and only a short column length is used for absorption, while simultaneously
regeneration and backwashing take place at other areas. Besides potentially large capacity, the
greatest advantage of the moving-bed systems may lie in the fact that resin is in the loading cycle

only for a few minutes, which permits it subsequently to be washed, then regenerated and washed
again—the changes of clogging the bed with insoluble precipitates and fouling with organic com-
pounds are greatly reduced.

     The advantages of the ion-exchange treatment system are:

     • Water savings are significant, up to 90 percent, owing to water recirculation. At the same
       time, sewer rental charges are equally reduced.

     • The volume of the effluent discharged is reduced greatly, thereby allowing potential reduc-
       tion of the polluting substances.

     The disadvantages of the ion-exchange-type treatment are:

     • Design considerations have to be very carefully weighed.

     • Batch-treatment chemical costs, labor and supervisory expenses, and equipment maintenance
       costs are high. Sludge handling can be expensive since the metal separation can be complex.

     • The investment in equipment and installation is relatively high.

                                      Chapter II

                             AND METAL  RECOVERY

                                     ION EXCHANGE
     The best opportunities for ion-exchange systems in metal-finishing waste treatment may lie in
the field of valuable metal recovery or regeneration of process solutions.13  As an example, when
rinse waters after chromium plating are passed through a cation exchange column, the system may
serve to recover the valuable chromium chemicals by removing the impurities—such as trivalent
chromium, copper, zinc, nickel, and iron—in the cation exchange column, the backwash waters
from which would go to waste treatment. An evaporation system allows the reconcentration of
valuable chemicals and the reuse of rinse waters.

     An example of the maintenance  of a process solution would be the removal of aluminum from
a chromic acid anodizing bath, avoiding the necessity of periodic disposal of the bath.14  Chromic
acid, a strong oxidizer, will deteriorate the resin to some extent; therefore, concentrated chromic
acid solutions should first be diluted with water before regeneration through an ion exchanger is

     An ion exchanger receiving rinse waters from only one process can retain the desired cation or
anion in a sufficiently pure condition so that the regenerant could serve as replenishment source
back to the process, especially after reconcentration by evaporation.  Nickel sulfate, for example,
can be reclaimed in this manner.

     Process-solution regeneration is perhaps the field that is economically best suited for the use
of ion exchangers. The limited capacity of the ion-exchange systems and the necessity for large
installations to provide the necessary salt absorbancy between backwashings to some extent has
limited more common use of this type of equipment. Moving-bed ion-exchange columns overcome
the common limitations assumed regarding the capacity of ion-exchange installations (fig. II-l). Suc-
cessful, economical installations have  been accomplished using this type of system for such process-
solution recovery as bright dip solutions used for aluminum, which require resins to be able to
accept high-strength oxidizing acids and which have  large removal rates for aluminum, maintaining
the process solution at the optimum aluminum concentration.
                               EVAPORATIVE RECOVERY
     There are basically two systems used in the recovery of chemicals by the evaporation technique.
The uacuum evaporator concentrates the process-solution waste at reduced temperature by depress-
ing the boiling point, maintaining a vacuum in the evaporative vessel.15  By reducing the pressure
in the evaporator the boiling will take place at a lower temperature and the oxidative breakdown of
cyanide compounds may be reduced.  The atmospheric evaporator forces air through a chamber
into which the processing solution is sprayed to accelerate evaporation rates and remove water
vapor with the air that is discharged through a stack.  The water removed by the vacuum evaporator
can be recondensed and reused for rinsing.


             CONDUCTIVITY  PROBE ^-^

                                                        RESIN RESERVOIR
                RE6ERATION COLUMN
                                                       PULSING CHAMBER
                                                          XI	*" PROCESS WASTE 'OUT1
                                                           REGENERATION OF
                                                           PROCESS SOLUTION
                                                          1X1—"-PROCESS WASTE  IN
                               Figure 11-1. Moving-bed ion exchanger.
     Two modes and their variants are usually employed, depending on the number of counter-
current rinse stations available and the waterflow rate required for good rinsing.16

Closed-Loop System

     The closed-loop system is an effective way to recover cyanide, metal cyanides, chromium- and
other metal-containing chemicals from plating operations so that chemical treatment of rinse water
is eliminated or minimized.  This technique is applied only to processing lines using countercurrent
rinsing. In a typical system  (fig. II-2), a single-effect evaporator concentrates flow  from the rinse-
water holding tank.  The concentrated rinse solution is returned to the plating bath and the distilled
water is returned to the final rinse tank.

     In the closed-loop system, no external rinse water is added for makeup except that required
by atmospheric evaporation. The only chemicals added to the plating bath are those required for
replacing what is actually deposited on parts and what is lost by spillage or other accident.  The
system is designed to recover 100 percent of the plating chemicals normally lost in  dragout for
reuse in the plating cycle.

Open-Loop System

     The open-loop system is adaptable  to those plating installations where there are insufficient
countercurrent rinse tanks, and where there is a system for partial recovery of plating chemicals.
A small portion of the chemical dragout that accumulates in the final rinse tank is not circulated to
the evaporator for concentration. The circulation loop through the evaporator is opened by
creating another flow path for the chemical dragout. This small fraction of dragout solution not
returned to the evaporator can be treated by an appropriate  chemical method before disposal.  A
typical open-loop system with only two rinse tanks can be operated economically.  In this system
a small percentage of the dragout is not returned to the evaparator and must be treated.

              PLATING  TANK
                                                      CONCENTRATE     V_xl
                                                      SETURN PUMP      4
                                                                                *-STEAM CONDENSATE
                             Figure 11-2.  Evaporative recovery—closed loop.
                                     REVERSE OSMOSIS
     Functionally, reverse-osmosis applications in metal finishing are very similar to the opportuni-
ties available by evaporation.  Theoretically, reverse osmosis aims to apply high pressure to a suit-
able thin membrane, overcoming the osmotic pressure and passing water through the membrane,
which at the same time rejects the salt molecules and thereby separates a relatively salt-free water
stream and a salt solution at a higher concentration than that in the original input.

     Rinse waters  from a specific process can thereby be treated, the water product returned for
rinsing, and the concentrates,  possibly after further concentration by evaporation, returned to the
process.  If the recovered water stream is sufficiently free of the dragout salts, and if a sufficient
number of countercurrent rinse stations are available, the level of the original contaminant con-
centration can be reduced sufficiently so that a last rinse does not require treatment. Otherwise
the configuration is similar to  the open-loop system described for evaporative recovery.17'18

     Suitable membrane materials for cyanide- and chromium-type rinse-water reconcentration are
not yet commercially available.

     Care must be exercised with reverse-osmosis systems so that the rinse water does not contain
precipitated salts; otherwise, these may in time reduce the permeability of the membrane.

     As discussed earlier with the various waste-treatment systems available, the integrated waste
treatment aims to follow closely the processing step and is segregated with respect to the general
metal-treatment process. The design theory assumes that only the metal is worth recovering to
achieve economy, to reduce the sludge volume, and to avoid returning contaminants to the process
and unduly increasing the metal content of the solution.
Electrolytic Recovery

     After certain processes—for example, gold,19 silver, and tin—a dragout solution is maintained
as a suitable electrolyte to allow the continuous recovery of the metal in the process and to main-
tain the electrolytic system at such metal level that the dragout loss of the metal into the follow-
ing treatment wash system is negligible economically.

Metal Recovery Values From the Sludges

     The relative integrated closed loop segregates the metal in a dense sludge collected in the
respective reservoir tank.  Since the sludge is uncontaminated, it has a good market and can be sold
to the suppliers of the plating chemicals or anodes for nearly the full value of the metal content.
Especially for smaller plants this marketable sludge provides a simple and unsophisticated means to
recover the dragout losses from, for example, nickel- and copper plating installations.20'21  A sig-
nificant metal loss also occurs at times of purification  of these plating solutions and from filter
drains. Up to 100 percent of all these wastes can  be recovered  easily with the integrated technique.

     Larger installations may avail themselves of further savings by recovering the full metal values
with a highly economical  automatic electrolytic recovery plant, operating at near 100 percent effi-
ciency, to reclaim all the nickel and copper lost.  This  step is obvious, for instance, for the non-
ferrous metals processing  plants, even though some waste sludges are mixed; for example, copper
sludge containing zinc, tin, nickel, and so forth. 22
     Processes depending on cooling or heating of the acid solutions to avoid dumping and recover-
ing the crystallized metal salts have been known for many years.  Their wider use may come in the
future with added emphasis on the need to recycle and with increasing costs of sludge handling.
Consider, for example, the crystallization recovery for pickling solutions for steel, the removal of
ferrous sulfate, or the regeneration of ammonium persulfate copper-etch solutions as used for
printed circuit boards. The removal of the copper-ammonium sulfate extends considerably the
usefulness of the processing solution.

     Continuous electrolytic maintenance of a desired metal concentration has been practiced for
many years for pickling acids in nonferrous metal manufacturing plants.  With greater emphasis on
waste treatment the processes expanded, recycling the metal that was dumped earlier with signifi-
cant reduction in acid-handling costs and improved processing.

                                      Chapter III

                         ECONOMIC CONSIDERATIONS
     From the foregoing, it will be obvious that waste-treatment costs will be greatly dependent on
many factors.23  Under plant-management control are, for example: dragout control, economical
water use, good housekeeping, as well as the selection of processes from the standpoint of waste-
treatment needs.  Investment for solution, regeneration, purification, reduction of dumping rates,
closer chemical control of processing systems—all these issues require a new look from the waste-
treatment cost angle.

     The engineer in charge of waste-treatment design should also consider the question from many

     • The design should meet not only today's requirements, but should provide the best treat-
       ment in view of the mounting restrictions anticipated in the future and should allow the
       safest possible margin for the client.

     • Water consumption and sewer rental charges should be considered as much part of the overall
       cost as the chemicals used in treatment.

     • Savings achieved in water-reuse opportunities and from chemical and metal recovery steps
       built into the waste-treatment scheme may allow economies to offset treatment costs,
       reducing the overall operating costs.24

     • Increasing the  supervisory and operating labor costs can  be avoided only if the waste-
       treatment system is integral, or at least close, to the processing area.

     • An extensive and expensive waste-treatment plant does not necessarily insure better treat-
       ment, more flexibility, or freedom from upsets.

     • Design criteria should not be established solely on the basis of the needs of the receiving
       stream or the leniency or restrictive attitudes of the local regulatory agency.

     A case history is reported elsewhere in detail.~° The report covers cost comparison between
three types of general  waste-treatment systems  as related to the  cost of rinse-water treatment  for
the same size plants with identical dragout rates, production  volumes, and other basic costs. The
report is specific regarding a plating process consisting of cyanide copper strike, followed by acid
copper, nickel, and chromium plating.

     The following general conclusions may be  drawn from the  example:

     Water costs and sewer rental charges are the most significant cost factor. Sludge handling is the
second important cost item, followed by amortization charges.

     Chemical costs for the treatment of the dragout are not a major item. When large volumes of
water are processed, the precipitation of water hardness may exceed the chemical cost of treating

the chemicals from the dragout. The study does not reflect what the chemical costs of treatment
would have been if solution dumping and excessive floor spill would have to be treated. Chemical
costs could be a major item in a sloppy operation.

     Treated rinse water due to water-reuse options may cost less than the untreated rinse water and
sewer rental.

     The cost comparison study did not take into consideration that all the copper and nickel are
recovered in the plant under study. The actual savings far exceed the reported values.  We may say,
therefore, that suitable waste-treatment design in many installations may significantly reduce oper-
ating costs.

                                       Chapter  IV


                             AND SLUDGE  DISPOSAL


     The removal of solids from the rinse water effluent is a complex problem because there are
no general rules regarding the means of separation; the ability to make this separation depends on
the various solids and the form in which they are dispersed. Other influences are the size and
shape of the receiving vessels, the flow rate, and the content of the effluent—the mixture that
causes the various particles to have a relative effect on each other and makes their separation from
the water easier or more difficult.

     The solids in the effluent may be large particles, such as scale, sand, or tumbling media. At
the other end of the scale, they may be colloidal particles that could come from fine suspensions
as used in nickel plating, cleaning, and so forth.

     Regulatory interest is mainly directed toward

     • The removal of precipitated and suspended metal salts, in view of their potentially toxic

     • Esthetic appearance of the effluent, if it is discharged to the surface waters—a dye solution
       or dye suspension and the cloudy appearances of emulsified oil are conditions that may
       cause rejection of a permit

     • Floating  oil and debris,  which are esthetically unacceptable and may be considered a pollu-
       tion hazard

     The following discussion will be  concerned mainly with the removal of the precipitated metal
salts because, in metal-finishing waste treatment, the precipitation of these metals to reduce the
toxic conditions is only the first step; equally important is the removal of the precipitates from
the effluent before it is discharged. In general, the removal of these suspended metal precipitates
is the greater problem and requires more experience and skills than the chemical precipitation  step.

     Removal of the precipitated metal solids is needed because, significant metal sludge accumu-
lation in the bottom sediment of rivers and lakes could cause toxic conditions to develop in  the
organisms living there even though insoluble metal precipitates have no toxic effect on the biota
in the water.  Also, when the effluent is discharged to the sanitary sewer, the precipitated metal
salts accumulate in the sanitary sewage sludge creating possible toxic effects to the bacteria in  the
sewage treatment plant, soil contamination and toxic vegetation if the sludge is discharged as
fertilizer on the land, and, finally, atmospheric and ash pollution if the sanitary solids are

     Waste treatment requirements usually stipulate removal of the precipitated metals, and
usually set limits for both soluble metal content and the metal content of the suspended solids.
In some areas, the allowable metal content is expressed as total metal content of the effluent,
considering equally the dissolved and  suspended metal content.


     The nomenclature of water and waste treatment distinguishes among the following:

     • "Total solids" (TS) refers to the total solid content of the water after evaporation and con-
       siders the total weight of the residue after drying at 100° C.

     • "Settleable solids" (SS) describes the settleable solid material content of the water.  This
       content usually is measured by settling the sample in an Imhoff cone for 1 hour and usually
       is expressed in millilitres per litre.

     • "Dissolved solids" (DS) include those solids which will pass through the filter and either are
       truly dissolved  in the water or are present as colloidal solids that, therefore, cannot be
       separated easily from truly dissolved solids.

     • "Total dissolved solids" (TDS) indicates both  colloidal and dissolved matter.

     • "Total suspended solids" (TSS) expresses the suspended solids content of the water under
       investigation, measured when weighing the residue retained on the filter after drying at
       100°-105° C.

     The first step in the removal of metal from metal-finishing wastes is commonly called "neu-
tralization."  The term means that the pH of acid solutions is raised toward neutrality or the pH
of alkaline solutions is lowered to the same condition.

     Metal salts from simple inorganic compounds will tend to become insoluble in the neutral pH
range, but not all metals will precipitate on neutralization and not all metals will precipitate at the
same pH point and to the same extent. In view of the low discharge limits with regard to soluble
metal content of a metal-finishing effluent, the initial problem in separation may be to decide what
pH to aim for to reach the most complete precipitation of the metals present in the waste. Some
of the metals that may be present are amphoteric, and therefore are soluble at alkaline pH; exam-
ples of such metals are aluminum and zinc. Some other metal content requires a relatively high pH
to reach minimum solubility; this category would include, for example, nickel and copper.

     With mixed waste streams, the best pH for the most complete separation will be that which
limits the content of the most toxic of the metal salts and for which the regulatory limits are most
stringent.  To meet regulatory requirements, it may be  necessary to segregate streams to allow the
best  pH conditions for the particular metals to be precipitated.  As a general rule, pH 8.0-8.5 is
better (although not truly neutral) than is a pH closer to 7.00. Figure IV-1 shows some typical
examples of the pH conditions at which the particular metals will be found at minimum concen-

     The various salts dissolved in the water also affect the solubility of the typical metal salts
encountered.  Table IV-1 gives a sampling of the usual metSl content at various pH's if the precipi-
tation occurs in distilled water or hard water.  It will be evident from this variance that with many
salts  that may be encountered in a metal-finishing effluent the solubility limit at the various pH's
may  not be clear cut and predictable. Some of the metals may form more insoluble compounds
when precipitated as carbonates rather than as the hydroxides.  Chelating agents, organic acid salts,
and the various wetting agents sometimes encountered  in a mixed rinse-water stream may make
the quantitative precipitation of the various metals nearly impossible.

               1 0 —
               0 1  —
                    Figure IV-1. Precipitation of metal salts versus pH. See R. Weiner.28

     The precipitated metals usually form a so-called floe, which is a voluminous solid particle with
a specific density very close to that of water, and which therefore tends to float in the water. The
flocculent particles do not have sufficient weight to settle by themselves, but must agglomerate into
larger particles before they gain sufficient weight to allow settling.

     Gravity settling is the most economical and simplest way to effect separation of flocculated
salts.  Thus, efforts must be made to cause sufficient increase in flocculent-particle size and weight
to make gravity separation possible. The higher the concentration of the metal content of the
effluent and the more particles per  unit volume, the better the chance to effect the increase in parti-
cle size necessary for settling.  It is relatively easy to effect gravity separation and  precipitation when
neutralizing a high-concentration waste solution, such as a dumped pickling solution relatively high
in the metal salts.


                   Table IV-1.— Metal residue after precipitation, distilled and hard water


6.5 . .
8.5 .
90 . .


12 5
19 2

17 8
13 7
7 1

1 3
1 6


1 3
1 7
 Source R Werner, Die Abwasser der Galvanotechnik und Metatlmdustne, 4th edition, Eugen G Leuze Verlag, 1973

                            DESIGN FOR GOOD SETTLING29

     The engineering of a waste treatment system has to aim both for precipitates that will tend to
agglomerate, floe out, and settle, and for equipment design that lends itself to good separation.


     The higher the metal concentration in the effluent, the larger the floe formation and the weight
gain by the flocculent particles.

     High concentrations of wetting agents and cleaner compounds, such as silicates and phosphates,
that tend to suspend the flocculent materials and create adverse conditions should be avoided.

     Precipitation with calcium hydroxide, while sometimes creating a larger volume of sludge, will
tend to provide a faster settling precipitate.

Equipment Considerations

     Flocculation.  During neutralization it is necessary to stir the effluent to provide good mixing
and to effect fast chemical reaction between the neutralizing chemicals and the contents of the waste
stream. After 5 minutes, or sometimes less, the flocculent particles will start to appear; excessive
stirring at this point tends to break down the small floe into colloidal  material that will be nearly
impossible to settle.

     After neutralization is complete, 5-10 minutes should be allowed for flocculation to occur. A
typical flocculation chamber would have slow agitation to permit  the flocculent particles to contact
each other and thereby be preconditioned  for the next step.

     Clarifier Design. The clarifier is basically a vessel in which the flocculent particles may settle
to the bottom of the container and remain in the clarifier without being stirred  and carried over the
effluent weir. The  basic clarifier aims for quiet conditions and laminar flow of the effluent over the
effluent weir, with  a minimum current affecting and stirring the contents of the clarifier.

     The depth of the clarifier is important because good depth allows a floe blanket (cloud of small
flocculent particles) to persist and grow,  slowly settling toward the bottom of the clarifier tank,
without being affected by the flow conditions near the surface of the clarifier.  As the sludge content
of the clarifier increases, the floe blanket, which can be 4-5 feet deep, comes increasingly closer to
the surface of the clarifier until the top of the floe blanket may be disturbed by the flowing stream.
As soon as this condition occurs suspended solids are carried out of the clarifier, indicating that
sludge should be  removed from the bottom of the clarifier to allow sufficient room for the sludge
and the floe blanket to remain safely below the effluent stream.  Some clarifier design parameters
are shown in figure IV-2.

     Polyelectrolyte Feed.  The slow agglomeration of the precipitated metal floe is caused to a
great extent by electrostatic charges carried by the floe particles.  The same electrical charge polarity
causes these particles to repel each other. The electrostatic charge most often is described as "zeta
potential," an expression of the potential required to move the particles as an electrophoretic stream
toward one of the electrodes. Because some particles have a positive charge and others a negative
charge, there also can be a positive or negative sign indicating the zeta potential of a particular pre-
cipitate.  Zeta potential close to neutrality is the desired condition for floe aggregation.

     Polyelectrolytes are  long-chain,  organic molecules that have the capability, owing to their fila-
mentous-polymer form, to bridge and neutralize the potential differences between the various floe
particles and, as a filament,  tie these particles together30 (fig.  IV-3). The charges  of available poly-

r M u t

1 t *

                                        SURFACE AREA  A
                            DIRECTION OF FLOW Q
             Figure IV-2. Parameters of concern in the design of a clarifier. (a) Functional zones
             in an idealized sedimentation basin. See R. Weiner.28  (b) Idealized settling paths of
                     discrete particle in horizontal flow tank. See Burns and Roe.29

               Figure IV-3.  Electron micrograph of polystyrene latex and a cationic polymeric
                                flocculent.  From Ries and Meyers.30

electrolytes appear to be different.  Negatively charged, positively charged, and neutral polyelectro-
lytes can be distinguished,  and the difference can be significant. An empirical procedure normally is
used to establish which particular polyelectrolyte to use, in what concentration, and under what con-
dition.  The empirical test is the so-called jar test, which relies on slowly stirring a container of floc-
culent particles, timing the settling rate of the floe and the volume of settled material, and com-
paring the performance of  one polyelectrolyte addition with another or the concentration of one
addition with more or less  of the same material. As a rule, an addition of Vz-l mg/1 polyelectrolyte
is found best for metal-finishing effluents.  Excessive amounts of polyelectrolyte can cause harm as
great as the improvements  that may result from  proper use.

     Since polyelectrolyte  feed is creating additional concern and additional cost, it is used only
when the settling problem  cannot be solved without this aid.

     Coagulants.  Some of the metal precipitates form very good hydroxide floe, capable of removing
other solids by adhesion. When the waste stream is very dilute, or when some content of the waste
has to  be removed owing to its colloidal nature or to us organic content of large molecules that can-
not be separated because they are soluble, coagulants are added to the waste stream to provide addi-
tional flocculent precipitate. Most often such salts  as ferric sulfate, ferric chloride, or aluminum sul-
fate are used in the range of 100-300 mg/1 as coagulant additions to the waste stream (fig. IV-4
and IV-5).

     It will be evident that coagulant additions greatly increase the amount of sludge to be handled
as a final solids removal step.  The usual metal content of a metal-finishing effluent stream is far

                                        SETTLING TIME-MIN

              Figure IV-4. Coagulations and settling time versus concentration of coagulant.
                                     See Burns and Roe ^9

less than the quoted quantities of metal that are fed  as coagulant to be subsequently precipitated
and settled with the metal that is present. As the metal salts precipitate, the anion becomes free;
therefore, the pH tends to be lower when coagulants are added.  Either the initial pH should be high
enough to allow for this depression of the pH, or additional alkali has to be provided. With these
coagulant additions, large volumes of total flocculent precipitate are created. If the mixing of the
coagulant into the waste stream for subsequent flocculation is not designed carefully, all the bene-
fits that were to be gained thereby can be easily lost.

     Other Considerations. As the flocculent particles coalesce and settle to the bottom of the
clarifier, additional sludge settling occurs and a sludge slurry is slowly formed. To avoid handling
a very thin,  dry-solids-content sludge, water has to be squeezed out of the slurry so that the dry
solids content will increase.  Normally, the sludge accumulated on the bottom of the clarifier con-
tains 1-2 percent dry solids.  "Dry solids content" is an expression of the metal salt content of the
wet sludge, and is established after the water is filtered out and the wet filter cake is dried in an
oven for 2-4 hours at 100° C.

     Aging is a very important factor in increasing the dry solids content of the accumulating sludge.
The greater the depth of the clarifier, the greater  the capacity in relation to the accumulating total
solids, and the more days the sludge may accumulate and age on the bottom of the clarifier.

     Typical design criteria for the sizing of the clarifier would provide for 4 hours' retention time
for the wastewater and 1 weeks' retention time for wet sludge accumulation.

                              IMPROVED CLARIFIER DESIGNS

Solids Contact Clarifiers
     Solids  contact clarifiers (fig. IV-6) are clarifiers with mechanical aids to help the flocculent
particles make contact with each other and with provisions  for possible addition of polyelectrolytes
and for their contact with precipitating floe. Some of these clarifiers slowly agitate the floe blanket

                 + 10

            z 2
            U-' "^
            H J -10
            ^ ~
            < 5


                                100          200          300

                                          ALUM DOSAGE, (mg/1)
               Figure IV-5.  Coagulation and particle removal versus coagulant and zeta potential.  See
                                          Burns and Roe.29

and have provisions for polyelectrolyte addition. Others lead the incoming effluent carrying the
freshly precipitated floe through the floe blanket before the effluent is allowed to leave the floe-
blanket area, using the flocculent cloud somewhat as a filter to trap the small incoming particles and
filter them out of the effluent stream.  If the floe blanket is dense and cannot be lifted  out of the

                                                         CHEMICAL FEED INLETS
                             •' V ;fV4 r
      LINE    1
                                                              PRECIPATOR DRAIN
        Figure IV-6. Solids contact clarifier with sludge blanket filtration.  (Courtesy of the Permutit Co.)

clarifier, since it is isolated from the flowing stream on the top of the clarifier, the floe blanket can
be used most efficiently as a filter.

Tube Settler and Lamella Thickener Systems

     Tube Settler. The tube settler, a system first developed by the Neptune MicroFloc Co., is based
on the theoretical concept that if the path of the settling particle is shortened, and the bottom of
the* container is brought close to it, the settling and separation of the particle will be accelerated.
These designs allow a significant reduction in the size of the equipment owing to the inclined tubes
that reduce the distance particles must fall to be removed. The clarifier design can be  changed to
shorten retention time to approximately 1 hour, while the true design parameter is the 1.5 gal/min/ft2
rising rate of the floe particles.  The tube settler allows sufficient contact to get good filtration of
the rising particles through the floe blanket. Second, there is a sludge collection area sufficiently
large to provide time for aging of the sludge.  A conveyor system is provided to move the sludge to
the discharge hopper, from which it can be discharged automatically by either pumping or gravity.
Finally, tube  modules impede the floating out of the suspended solids from the clarifier  because
the even flow through the clarifier is closely regulated with an extensive effluent weir design, and
because the suspended particles can settle faster if floating in the tube modules since the bottom of
the individual container (the tube) is brought very close.  Because the tubes are inverted at a 60°
angle, the accumulating and settled sludge is discharged automatically as it slides down the incline
into the sludge collection area (figs. IV-7 to IV-11). This design is very efficient for metal-finishing
waste-treatment effluents.

                             TO SLUDGE
    Figure IV-7. Tube settlers: (a) flow pattern; (b) inclined tube. (Courtesy of Neptune MicroFloc, Inc.
           Figure IV-8. Module of steeply inclined tubes.  (Courtesy of Neptune MicroFloc, Inc.

                      Figure IV-9.  Tube settlers in existing clanfier. (Courtesy of Neptune
                                              MicroFloc, Inc.)
      Figure IV-10.  Plan view of modified clanfier.
         (Courtesy of Neptune MicroFloc, Inc.)
                                                     SUPPORT MODULE
                                                     TUBE SETTLER
                                                                 CLARIFIED EFFLUENT
                                                                              "• '
     Lamella Thickener.  The Lamella thickener (fig. IV-12) was introduced to this country by the
Swedish Axel Johnson Company and their U.S. subsidiary, the Parkson Company.  The Lamella
thickener operates by separating the feed, sludge, and effluent and allowing the sludge particles to
slide down the 55° incline and fall through a quiescent zone into a sludge hopper in the bottom.
The sludge is further thickened in the hopper by use of a plate vibrator before being withdrawn by
pumping or gravity.  The effluent flows out the top through flow-control orifices, which evenly dis-
tribute the flow between plates.  The Lamella thickener is best applied for new settling basin appli-
cations because it comes in a self-contained unit with attached flocculator.

     Lamella thickener designs allow a further reduction in the size of the necessary equipment
over tube settlers, because the  clarified water (effluent), the feed (influent), and the sludge  (under-
flow) are completely separated as they enter or exit the equipment. The sludge does not have to
fall through the feed to be removed as with a tube settler. Thus, the entire  horizontal projected sur-
face area of all the plates becomes the effective settling area.  The result is a significant reduction
in the size of the equipment compared with a conventional clarifier, because the Lamella thickener
can be designed for a flow rate of 4-5 gal/min/ft2 of projected settling area.
                      Figure IV-12. Lamella thickener. (Courtesy of Parkson Corp.

                                      SLUDGE THICKENING

     Subsequent handling of the sludge accumulated and removed from the clarifier requires the
removal of additional quantities of water because the sludge will have only 1-2 percent average dry-
solids-weight content. The magnitude of the problem and the importance of sludge thickening can
be perceived more clearly in  terms of the volume of sludge generated in metal finishing.

     A dry weight content of 1-2 percent means 50-100 times the weight in the water to be moved
when disposing of sludges  from metal-finishing operations. For example, in aluminum finishing
approximately 1.5 percent of the aluminum processed will be dissolved and will remain in the metal-
finishing waste-treatment system of a plant anodizing aluminum extrusions. A plant processing
100,000  pounds of aluminum per week therefore will dissolve 1,500 pounds of aluminum, which
will form 4,500 pounds of aluminum hydroxide upon neutralization.  If the plant can only dewater
the sludges from the waste-treatment system to 2 percent dry weight content, there will be 4,500
pounds of dry solids multiplied by  50—that is, 22-5,000 pounds of sludge—for disposal for each
100,000  pounds of aluminum processed.  The ratio is no different for the other processes, except
where metal removal is not as great. On the other hand, the iron sludges are less easily dewatered,
and chromium-containing processes, owing to drag-out, create far more metal waste to be disposed
of than does the aluminum-processing example cited.

     Minimizing sludge-handling  problems can begin early in the processing step—hence this discus-
sion of the value of process solution regeneration and solution and metals recovery.  Because sludge
handling is the most expensive part of metal-finishing waste treatment, sludge elimination in the
engineering stages is the best design aim that can be followed.

     Very little can  be done  to minimize sludge-handling problems if the waste-treatment plant is
designed just to follow the traditional chemical processes without attention to segregation for the
best and  most economical  solutions to sludge-conditioning problems, as well as for the chemistry of
the various treatment processes.  There are various ways in which the sludge-conditioning costs can
be greatly minimized, of which the following are only a few:

     • Segregated treatment of concentrated dumped processing solutions—such as waste pickles,
       chromate dips, and aluminum-anodizing solutions—will allow simple and inexpensive thicken-
       ing of these sludges without further effort. For example, waste from an aluminum  etch or
       dumped anodized solution can be easily conditioned so that it will have 20 percent dry weight
       content, instead of the 2  percent discussed earlier, by neutralizing the  batch dump separately.
       Actually, an aluminum-processing solution containing more than 4 oz/gal of aluminum will
       jell because there is not sufficient water content to complete the neutralization reactions and
       provide inert sludge.  As sludge from such waste does not have to be dewatered, it is similar
       to a concrete paste; if not discharged it can harden in the neutralization tank to a rocklike
       consistency.  The same process solution discharged with the rinse water, on the other hand,
       will create additional costs owing to handling through the clarifier, sifting out of the floccu-
       lent material, settling, sludge pumping,  and, finally, thickening.

     • Segregation of the  individual rinse waters and segregated precipitation of the metals, as prac-
       ticed in integrated  waste treatment, allows simple and inexpensive metal recovery.  In addi-
       tion, sludge in the integrated system—since the metal hydroxides are allowed to settle and
       are kept in the closed system for weeks or months—will not only grow into large clumps,
       but will age to the  extent that the accumulated sludges can be shoveled from the container.
       The integrated treatment is concerned to a great extent with reducing  sludge-handling costs.
       For this reason, the precipitation reaction is so designed that the precipitates are not floccu-
       lent material holding on to an extensive sheath of water molecules; if possible, precipitation
       occurs in the form of the oxide of the metal—for example, cupric or cuprous oxides in copper
       precipitation. In these instances, sludges with 50 percent dry weight content are not uncommon.

                                   SLUDGE FILTER
                                   DECANT PANEL
                                                                          TANK TRUCK
                                                                           SLUDGE PUMP

       Figure IV-13. Sludge filter tank. (Courtesy of Lancy Laboratories, Division of Dart Industries, Inc.)
     • Countercurrent rinsing also will greatly reduce the dilution of the wastewater; thus, it tends
       to increase the flocculent metal content in the clarifier, yielding a significantly higher dry-
       solids-weight content in the sludge subsequently removed.

     The first step in sludge thickening may be a simple and inexpensive sludge-holding tank into
which the sludges removed from the clarifier are pumped and allowed to settle and age further.  If
sufficient time is available, because the sludge is removed from the clarifier only two or three times
a day, the clear supernantant water in the gravity sludge-holding tank may be decanted.  Care must
be exercised that the settled sludges are not stirred up; decanting must be slow, and only sufficient
water must be removed to allow room for the subsequent wet sludge discharges.  As one sludge layer

is deposited on top of another, water is squeezed out from the sludges by the accumulating weight
and considerable aging occurs, allowing thickening of the sludges so that the 1-2-percent dry-weight-
content sludge originally removed from the clarifier may be dried by water decantation to 5-15 per-
cent dry solids content. Sludge with a 15-percent dry solids content is reasonably thick and can be
compared to a thick mud.

     A proprietary system developed by Lancy Laboratories uses a horizontal dewatering technique,
which allows somewhat simpler dewatering than does manual decantation (fig. IV-13) and yields an
even thicker sludge. The design is based on the fact that metal hydroxides are slimey precipitates
and clog all filter areas if pressed into them.  Horizontal dewatering—as it occurs in a porous concrete-
block sludge filter  (fig. IV-14) in a porous plastic-walled container, or through the use of a porous
plastic decant  panel inside the sludge holding tank—allows gravity water removal and evaporation
by capillary seepage into the porous walls of the filter.31 These vertical walls or internal decant
panels do not  clog, even though the metal hydroxides are slimey  and prone to clog porous membranes,
because no pressure is allowed to develop. The water separation  surfaces are designed for a maximum
water head of  3 feet. If any solids accumulate on the filter surface, they can be removed easily by
mechanical means.

     Dewatering to more than 15-20 percent dry solids content should be avoided in the sludge
thickener because the sludges will not be pumpable. A 15-20 percent dry-solids-content sludge can
be hauled in a  tank truck or in drums to a disposal site and dumped. The sludge will dry completely
within days.

     Centrifuges also will provide a dry-solids-content sludge—usually 15-20 percent. Sludges should
be of approximately 2-5 percent dry solids content before centrifuging for best performance. The
liquid waste discharged by the centrifuge during the solids-liquid  separation processes will be high in
suspended solids; therefore, it is best to direct this discharge back into the clarifier for resettling and
separation of the liquid and solid phases.
                                 FILTRATION OF SLUDGES
Vacuum Filter
     The most commonly used filter for metal-finishing sludges is the vacuum filter (fig. IV-15).  As
explained earlier, metal hydroxides are not easily filtered, and it is general practice to conduct some
filtration tests on typical sludge samples before deciding whether a certain filtration technique will
be successful.  The sludges should be preconditioned, and a dry  solids content of 2.5-5 percent is
necessary for reasonable chance of success. The usual dry solids content of the cake discharge from
the vacuum filter will be in the range of 20-25 percent.
                                                                        FRENCH DRAIN TILE
                                                                        TO EFFLUENT

   Figure IV-14. Concrete-block sludge filter.  Courtesy of Lancy Laboratories, Division of Dart Industries, Inc.)

Figure IV-15. Vacuum filter.  (Courtesy of Filtration Engineers Division, Ametek/Process
 Figure IV-16.  Plate and frame pressure filter.  (Courtesy of Johns-Manville Corporation.)

Pressure Filters

     The following two types of pressure filters are used in industry:

     • Plate and frame filter (fig. IV-16)

     • Chamber filter (fig. IV-17)

     The operation of both filter types is similar. The chamber filter differs mainly in that it allows
far greater cake thickness to be developed and, therefore, for large operations where the sludge
volume warrants, the chamber filter will be the preferred equipment. Chamber filters were devel-
oped in Germany for these types of applications, and automatic cake discharge filters are available
for large capacity applications.  The pressure filter can further reduce the water content; 40-60 per-
cent dry solids content can be achieved, depending on the type of metal hydroxide sludges proc-
essed.  (See also fig. IV-18.)

     The dry cakes from these various mechanical filters can be discharged into dump containers.
The sludge from the chamber filter will be sufficiently dry to be conveyorized into dump trucks.
             ROLLER CHAIN
             WITH SIDE TRACK
    A	V




                                            ELECTRO HYDRAULIC
                                            CLOSING DEVICE
                                                                        tl       OUTLET
                               FILTER CLOTH    FILTER
                               ATTACHMENT     PLATE
                      HEAD PIECE
              TURBIDITY INLET.
                                              /    __/
1 *•
\ f ,
•• ^

; ,
' t

' f
> f
•: •




•» r


                                          SiVfci] j^
                                       ' I -  \  • \ •  I    1 •
                                       CLEAR FILTRATE OUTLET
                                                                 FILTER CAKE
                                                                  END SECTION
                 Figure IV-17. Chamber filter. (Courtesy of Eberhard Hoesch & Sohne.)

           Figure IV-18. Vertical leaf pressure filter, horizontal tank.  (Courtesy of Johns-Manville
                                   ULTIMATE DISPOSAL

     For many years conventional disposal for metal-finishing sludges by the various industries
operating waste-treatment systems was by dumping the thickened slurries or dry cakes on land areas
prepared for receiving such wastes, with the aim of landfill soil buildup.

     Today, with accelerated concern for the environment and for the safety of metal-waste solids
disposal, the very best practices of some operations are just as much questioned as the injurious prac-
tices of others. It takes experience, discernment, and time to establish safe practices, and many local
solid waste managers, under whose jurisdiction these concerns come, do not yet have either the
experience or the accumulated knowledge to provide technical advice on the question of the suita-
bility of disposal sites or disposal practices.  The practice—of spreading the properly treated and inert
metal-waste sludges on suitable ground area and  periodically covering them to prepare a new deposit
layer base—has worked without any known accident, ground contamination incident, or known harm.
It must be acknowledged, of course, that the industries carrying on waste-treatment operations were
few, and the consulting engineers advising them may have been well experienced. The aim usually
was to find suitable ground area—possibly an abandoned  quarry or excavation not suitable for any
other purpose, or an area requiring fill to make it useful for future use.  Spreading on land where
growing vegetation may pick up toxic amounts of metal content should be carefully avoided.

     Properly treated metal-finishing waste solids are not soluble by rainwater washing; these metals,
therefore, cannot infiltrate the ground and contaminate the ground water. This statement is true
only if the sludges come from a well-operated treatment system. The common test to establish the
suitability of a metal-finishing sludge for land disposal is the so-called leach test, which consists of
stirring a preweighed amount of solid  waste in distilled water and subsequently testing the water for
significant constituents to establish the leachability of the waste. Distilled water has a far greater
solubilizing effect than does the rainwater infiltrating the ground. Metals precipitated as hydroxides,
carbonates, and trivalent chromium hydroxide are all sufficiently insoluble to indicate around  1 mg/1
or less metal content in these leachate waters. An equally suitable and probably more economical
test is to analyze the decant water over the sludge before filtration  or discharge. If this filtrate is
free of the metal constituents, chromic acid, cyanide, and so forth, the sludge may be considered safe
for land disposal.

     In general, sulfides should not be used for metal precipitation  because the metal sulfides, in a
manner similar to the typical sulfide commonly found in mine-waste residue, undergo  bacterial
decomposition, creating an acid condition if in contact with air and water. The acid so generated
can redissolve the metal salts earlier precipitated.32

     The greatest danger of ground contamination is from hexavalent chromium, which will infil-
trate and can contaminate the aquifer if disposed on land. When ground water contamination occurs,
it is not known how many years it will take until the chemicals are  slowly leached out.  It must be
remembered that drinking water is considered unsafe for humans or animals  if it contains more than
0.05 mg/1 hexavalent chromium.  It is evident from this example that the quantities of chromic acid
that may cause significant ground contamination are very small. Another waste that must be care-
fully handled is precipitated calcium fluoride in solid form. Calcium fluoride has a residual solubility
sufficiently high that it will continue to  dissolve  if washed by rainwater, and can infiltrate the ground,
which may cause the fluoride content of the aquifer to exceed the established maximum limit of
1-1.5 mg/1 allowed for drinking water. The best  way to keep calcium fluoride solid discharges from
infiltrating the ground is to prepare an impervious clay-layer disposal area and cover the waste with
clay soil to provide the necessary foundation for subsequent calcium fluoride layers. Large quanti-
ties of calcium fluoride solids are, for  instance, discharged by the video-tube-manufacturing and
glass-working companies.  Stainless steel processing also creates this type of solid  wa=te problem.

     Disposal of metal-finishing solid waste with garbage and other organic waste materials may cause
limited resolubilization of the metal content, because the organic waste, as it undergoes anaerobic
decomposition, releases organic acids  which at pH 4.0-5.0 will  have a limited solubilizing effect on
the metal salts.  While ground  water contamination from these causes has not been observed, it is
recommended that metal-waste solids be deposited in an area separate from that used for trash and
other organic waste. To avoid the chance  of an acid coming in contact with  the metal salts and the
subsequent leaching of metals, it is best  to provide a first layer of limestone on which the metal
solids are deposited. As the dump increases in size, some additional layers of limestone will aid the
stabilization of the soil. The soil itself has good  ion exchange capacity for metal salts, and this may
be the reason that no investigations of the performance of the  old metal-sludge lagoons and metal-
disposal areas showed any evidence of potential injury to the water table.

     The greatest danger lies in disposal  of metal solids into the sanitary sewer systems, where they
may accumulate with organic waste.  As the digestion of the organic solids proceeds, the metal con-
tent in the organic residue constantly  increases.  The digested and stable organic wastes from the
sanitary sewage treatment plant may be  deposited on the land, as they are a  most desirable fertili-
zer; but, at the same time, many of the metal salts, such as nickel, zinc, lead, and cadmium, are
present in toxic quantities and may be taken up  through the root system by  growing vegetation.33"39

     Incineration of metal waste, or of the sanitary organic solids mixed with metal  waste, is also a
potential and significant pollution danger.  Zinc, lead, and cadmium will escape as gases into the


atmosphere, while trivalent chromium sludge will be reoxidized and will remain in the ashes as
hexavalent chromium, with the usual solubility and contamination potential.40

     Today many areas are faced with the problem of suitable disposal sites for the  metal sludges
from waste-processing operations. The best approach is to recognize that sludge disposal is a local
problem that has to be solved by securing the cooperation of the regulatory agencies as close as
possible to the locality where the treatment plant is operating. Some areas require the filtration of
solids for maximum dryness. As this discussion shows, the required filtration adds to waste-treatment
cost and is economical only for large plants where the savings in hauling costs will amortize the
investment for the filter installation.  The solid waste discharged on the land disposal site will not be
distinguishable from the high-solids-content slurry after the first rain. Also, the slurry will dry to
powdery consistency if the weather is dry.   Solid waste disposal cannot be regulated as easily at the
Federal level as at the local, even though general guidelines would be desirable.  Under any circum-
stance, the decision regarding the land disposal site and its suitability will be a local  question. It is
hoped that the State regulatory agencies will offer guidance and advice for local decisionmaking
bodies.  In view of the recently significant portions of the metal-finishing wastes being treated, and
the staggering volumes of sludges being created, it is only natural that there are problems of safe land
disposal and of suitable disposal sites, and that they cannot be solved overnight.

      *R. W. Oyler, Plating, 36, p. 341, 1949.
      2R. W. Oyler, Proceedings of the 4th Industrial Waste Conference, Purdue University, p. 250,
      3 J. M. Conrad and G. P. Beardsley, Metal Finish., 59(5), 54, 1961.
      4J. G. Dobson, Sewage Works J., 19, 100, 1947; Metal Finish., 45(2), 78, and 45(3), 68, 1947.
      5B. F. Dodge and W. Zabban, Plating, 38,  561, 1951, and 39, 385, 1952.
      6L. E. Lancy and W. Zabban, ASTM-STP No. 337, p.  32, 1962.
      7B. C. Lawes and E. I. DuPont, U.S. Patent No. 3,617,582.
      8B. C. Lawes, L.  B. Fournier, and O. B. Mathre, "A New Peroxygen Method for Destroying
  Cyanide in Zinc and Cadmium Electroplating Rinse Waters," 58th Annual Convention, American
  Electroplaters' Society, Inc., Buffalo, N.Y.,  1971.
      9W. N. Greer, Sewage Indus. Wastes, 25, 930, 1953.
     l°L. E. Lancy, U.S. Patent No. 2,725,314.
     11L. E. Lancy, Sewage Indus. Wastes, 26, 1117, 1954.
     12I. R. Higgins, U.S. Patent No. 3,492,092.
     L3R. Kunin,Prod. Finish., Apr. 1969.
     L4J. Thompson and V. J. Miller, Plating, 58, 809, 1971.
     15J. M. Culotta, Plating, 52,  545, 1965.
     16J. M. Culotta and W. F. Swanton, Plating, 58, 783, 1971.
     17A. Golomb, Plating, 57, 1001, 1970.
     18 A. Golomb, Plating, 59, 316, 1972.
     19L. E. Lancy, U.S. Patent No. 3,431,187.
     20R. Pinner, IMF,  Symposium Report 14, p. 272, 1968.
     21W. Nohse and D. Wystrach, "Nature of the Effluent Sludge from Lancy Treatment of Rinse
  Waters from Acid Copper Plating," Electroplat. Metal Finish., 19, 146, 1966.
     22 Water Pollution Control Research Report No. 12010DPF, Nov. 1971.
     23L. E. Lancy, Plating, vol. 54, p.  157, 1967.
     24R. Pinner and V. Crowle, Electroplat. Metal Finish., Mar. 1971.
     25 L. E. Lancy, W. Nohse, and D. Wystrach, Plating, 59, 126, 1972.
     26L. Hartinger, Bander Bleche Rohre, Dusseldorf, pp. 535 and 638, 1963, and p. 14 (1964).
     27H. Schlegel, Metalloberflache, 17(5), 129, 1963.
     28 R. Weiner, Die Abwasser der Galvanotechnik und Metallindustrie, 4th edition, Eugen G.
  Leuze Verlag, 1973. (English translation in press.)
     29Burns and Roe,  Inc., Process Design Manual for Suspended Solids Removal, EPA Technology
  Transfer Program  No. 17030 GNO, Oct. 1971.
     30H. E. Ries, Jr., and B. L. Meyers, Science, 160, 1449, June 28, 1968.
     31L. E. Lancy, U.S. Patent No. 3,325,008.
     32H. P. Larsen, J. K. P. Shou, and L. W. Ross, Water Pollut. Cont. Fed., 45(8), 1682, 1973.
     33D. Tarvin, Sewage Indust.  Wastes, 28(11), 1371, 1956.
     34 J. M. Craddock, Water Sewage Works, 74, June 1973.
     35L. E. Lancy, "The Effects of Metal Finishing Effluent on the Sanitary Sewage Treatment Proc-
  esses and Sewer Structures," 49th Annual Technical Proceedings, American Electroplaters' Society,
  Inc., 1962.
     36M. K. John, H. H.  Chuah, and C. J. Van Laerhoven, Environ. Sci. Technol. 6(6), 555, 1972.
     37M. J. Buchauer, Environ. Sci. Technol., 7(2), 131, 1973.
     38C. D. Gish and R. E. Christensen, Environ. Sci. Technol.,  7(11), 1060, 1973.
     39F. E. Hemphill et al., Science, 172, 1031, 1971.
     40S. R. Nietz, Galvanotechnik, 64(11), 998, 1973.

S  GOVERNMENT PRINTING OFFICE: 1976-757-056/S'(32 Region NO. 5-11  47


Recommended Units







Moment or




square metre

square kilometre

square millimetre

cubic metre


tonne or




newton metre
















River flow

Flow in pipes,
conduits, chan-
nels, over weirs,

Discharges or

Usage of water



cubic metre
per second

cubic metre per

litre per second

cubic metre
per day

cubic metre
per year

litre per person
per day

kilogram per
cubic metre









Basic SI unit

The hectare (10000
m2) is a recognized
multiple unit and
will remain in inter-
national use.

The litre is now
recognized as the
special name for
the cubic decimetre.

Basic SI unit

1 tonne = 1 000 kg
1 Mg = 1 000 kg

Basic SI unit
Neither the day nor
the year is an SI unit
but both are impor-

The newton is that
force that produces
an acceleration of
1 m/s2 in a mass
of 1 kg.

The metre is
measured perpendicu-
lar to the line of
action of the force
N. Not a joule.

of Units

For meteorological
purposes it may be
convenient to meas-
ure precipitation in
terms of mass/unit
area (kg/m3).
1 mm of rain =
1 kg/m2

Commonly called
the cumec

1 l/s = 86.4 m3/d

The density of
water under stand-
ard conditions is
1 800 kg/m3 or
1 000 g/l or
1 g/ml.
39.37 m.=3.28ft=
0.62 mi
0.03937 in.
3.937 X 10 3=103A

10.764 sq ft
= 1.196sqyd
6.384 sq mi =
247 acres
0.00155 sq in.
2.471 acres

35.314 cu ft =

1. 057 qt = 0.264 gal
= 0.81 X104acre-

2.205 Ib
0.035 oz = 1 5.43 gr
0.01 543 gr
0.984 ton (long) =
1.1023 ton (short)

0.22481 Ib (weight)
= 7.233 poundals

0.7375 ft-lbf

0.02089 Ibf/sq ft
0.14465 Ibf/sq m



Flow (volumetric)




Work, energy,
quantity of heat


Recommended Units


metre per
per second
per second

radians per

cubic metre
per second

litre per second

pascal second

newton per
square metre
or pascal

kilometre per
square metre
or kilopascal

deqree Celsius



joule per second


















Commonly called
the cumec

Basic SI unit
The Kelvin and
Celsius degrees
are identical.
The use of the
Celsius scale is
recommended as
it is the former
centigrade scale.

1 joule = 1 N-m
where metres are
measured along
the line of
action of
force N.

1 watt = 1 J/s


3 28 fps

0.00328 fps

2.230 mph

1 5,850 gpm
= 2.120 cfm

15.85 gpm

poundals/sq ft

0.000145 Ib/sq in

0.145 Ib/sq m.

14.5 b/sq in.

7 ~17-77

2.778 X 10 7
kwhr =
3.725 X 10 7
hp-hr = 0.73756
ft-lb = 9.48 X
2.778 kw-hr

Application of Units

35.314 cfs


1.83 X 10-3gpm

0.264 gcpd

0.0624 Ib/cu ft


BOD loading

Hydraulic load
per unit area;
e.g. filtration

Hydraulic load
per unit volume;
e.g., biological
filters, lagoons

Air supply


Optical units

milligram per

kilogram per
cubic metre
per day

cubic metre
per square metre
per day

cubic metre
per cubic metre
per day

cubic metre or
litre of free air
per second


lumen per
square metre










If this is con-
verted to a
velocity, it
should be ex-
pressed in mm/s
(1 mm/s = 86.4
m3/m2 day).

1 ppm

0.0624 Ib/cu-ft

3.28 cu ft/sq ft

0.03937 in.
39.37 in. =

0.092 ft
candle/sq ft

                MAY 3  1977
           RESEARCH & Ik
                EPA. REGION V