Conserving Water
And Segregating Waste Streams
By Patrick M. Dowd
The benefits and methods of conservation and segregation in
metal finishing shops are reviewed. Case histones show
substantial savings.
Plant Z is ai
, 7rlsphating an((
In metal finishing and electroplating plants, the growing shortage of good-quality water and its increased cost, combined with
higher sewer-use rates, makes it highly desirable to conserve
water Likewise, the segregation of wastewater streams can
result in large cost savings
Several studies of water conservation in finishing plants
document savings ranging from $1 0,000 to $99,000 annually
Studies of wastewater segregation made at two plants resulted
in estimated capital savings of $1 4,000 and $1 30,000 Tables
1-4, compiled from these studies, indicate the overall water
usage of plating lines prior to, and after, conservation measures
were implemented The lines are grouped according to coating
type-zinc phosphate, zinc, brass and nickel plate No distinction is made between the different types of zinc plating solutions
(eg., cyanide or chloride)
Conservation measures adopted by Plant X included drain
boards for dragout in one plating line and countercurrent rinsing
for several others. In one case, a dragout recovery tank was
supplemented by three counterflow rinses Flow control valves
were installed on all fresh water inlets
In Plant Y, flow control valves were also installed on all fresh
water inlets, but on counterflow systems that were already in
place At Plant Z, flow control valves were implemented on spray
rinses and other fresh water inlets Five rinse tanks were
connected to the outlet of a fume scrubber
The tables show that effective conservation techniques
substantially reduce water consumption in plating operations,
resulting in a number of benefits
1 Lower sewer-use charges due to a smaller volume of
water being discharged
2 Lower water billings due to the smaller volume of water
required
3 Lower energy costs on hot rinses due to the lower volume
requiring heating
4 Lower capital and operating costs of a wastewater treat
ment facility
5 Less space required for a treatment facility
Table 5 shows the estimated annual cost savings realizeG
from conservation programs implemented by the four plants
The estimated annual cost savings are based on water and
sewer billing only, but include the positive impact of Water
conservation measures from additional processes not PIp
sented in Tables 1-4
Two case studies document the capital cost savings that may
be obtained from a wastewater segregation program The firs1
involves Plant Y, ajobshop engaged mostly in zinc plating using
alkaline cyanide baths The wastewater was segregated intc
three main streams-(1 )those not requiring treatment, (2)those
requiring cyanide oxidation, and (3) those requiring heavy
metal removal The estimated cost of the segregation syster
was $1 7,000 The estimated capital cost for the entire treatmen'
system, including segregation piping, was $367,000 The est'
mated capital cost for this system without segregation was Ir
excess of $500,000 Thus, by segregating the waste streams
x
an estimated capital savings of at least $130,000 could t
realized
'71splant was fc
stimated capit!
ijstems were $)
:.aters were nott
1 removal sys,
530000 Segrec:
Jpital costs
As these cases
an result in lai
mservation cai
\',ingent effluentt
J npliance proc:
Recommende?
Rinsing workpiecc
Plating process c
is removed from)
Rinsing must rerr
solution in the ne
Poor rinsing
staining, spotting,
?sing also incre:
:rGess solutiom
rt'Pleted Therefc
:"de
adequatE
A first step in
Wamination in ii
-rude (1) con)
I'ating the work
' h e surface te:
' crease soiutior'
the drat
ath
After the drago)
''OUld be investic
use during
:'kPieCe and ttt
'OtaCt between
"ume of water d'
'Its rinsed from ti
Agitation is nee
that the rtr
W i o n may caL
from being ut
Volume of w
'@
104
PLATING AND SURFACE FINISHIN(
A
"'1985
L
ms
ion in
low
ler volume of
lume of water
lower volume
dewater treat-
vings realized
\e four plants
on water and
pact of water
xes not prevings that may
jram. The firs!
c plating using
?gregated into
nent, (2) those
luiring heavygation system
ntire treatment
,000. The est!.gation was in
laste strearns.
,000 could be
plant Z is a metal finishing facility that performs zinc
osphating and zinc plating The only segregation required at
plant was for emulsified oil and heavy-metal removal The
timated capital costs of the segregation and oil-removal
Slems were $5000 and $1 1,000, respectively If the wastellers were not segregated, the estimated capital cost of the
removal system alone would have been approximately
0000 Segregation at this facility saved about $14,000 in
pita1 costs
Asthese case studies illustrate, segregation of waste streams
n result in large capital savings Segregation and water
nservation can greatly reduce the cost of complying with
Ingent effluent limitations and should be an integral part of any
mpliance program
?commendedConservation Measures
,sing .torkpieces to prepare them for the next step in the
,ling process creates a major demand for water As the work
removed from a process bath, a film of solution clings to it
ising must remove this film sufficiently to ensure that the
Mon in the next process tank will remain uncontaminated
poor rinsing greatly reduces product quality by causing
ning, spotting, blistering and peeling of the coating Inefficient
sing also increases production costs because contaminated
)cess solutions must be dumped before they are fully
2leted Therefore, the goal of the cost-conscious finisher is to
,vide adequate rinsing with a minimum volume of water
A first step in reducing rinsewater usage and eliminating
intamination in process baths is to minimize dragout Methods
:lude (1) controlling the workpiece withdrawal rate, (2)
ating the work above the tank, (3) adding wetting agents to
Yuce surface tension, (4) increasing the bath temperature to
crease solution viscosity, (51 adding a drain board or drip tank
‘ecover the dragout so that it may be recycled to the process
ilh
411er the dragout has been minimized, the rinsing operation
ould be investigated The following techniques can minimize
aler use during rinsing using turbulent motion between the
3rkpiece and the water, providing an adequate period of
Jdactbetween the work and the water and using a sufficient
,ume of water during contact to reduce the concentrations of
311s rinsed from the surface
%itation is needed to provide the turbulence necessary to
:sure that the rinse solution is completely mixed Insufficient
ltatlon may cause short-circuiting, which prevents the rinse
:’I( from being utilized efficiently Vigorous agitation increases
volume of water contacting the workpiece, thereby in-
-
1985
creasing the transfer of contaminants from the work surface to
the main body of the rinse tank Adequate mixing can be
induced by mechanical or air mixing or by pumping water
through the tank at a high rate Of these methods, air mixing is
the most efficient because it provides the highest degree of
agitation
Because some fresh water makeup to the rinse tank is
necessary, it is important to hold this flow rate to a minimum in
the interest of water conservation Flow control valves will
maintain a constant flow regardless of fluctuations in line water
pressure These valves also prevent the operator of the plating
line from increasing the rinse rate
Controlling the water makeup to the rinse tank also can be
achieved using conductivity controllers These devices are set
to maintain the rinsewater conductivity at the highest possible
level without adversely affecting product quality When solution
conductivity rises above a pre-set limit, the controllers will signal
for water additions until the conductivity falls below the setpoint
This control method results in a very low rate of water usage
because water is added only as needed However, conductivity
controllers have one major deficiency When organic additives
such as brighteners are used in the plating line, they will be
carried into the rinse tanks with the dragout and may contaminate the rinse without increasing conductivity In such
situations, conductivity controllers are ineffective and therefore
their use should be evaluated on a case-by-case basis
Limit switches provide an excellent means of controlling
rinsewater additions on barrel plating lines When a barrel is
lowered into a rinse tank, it activates a limit switch that turns on
the rinsewater When the barrel is removed, the flow turns off so
that water is added only when the barrel is actually in the rinse
tank
After the flow rate has been controlled, the configuration of
the rinse tank should be examined to determine if additional
water savings can be achieved through changes in the rinsing
operation If space is available, all rinse operations should be
done in countercurrent tanks Countercurrent rinsing uses the
water from previous rinses as feed for the more contaminated
ones Fresh water is introduced at the final tank, then overflows
into the preceding tank Thus the dilute rinses are usedas the
feed for the more contaminated rinses, thereby reducing the
input of fresh water required
In some situations, where space is restricted, it may be
impossible to install separate rinse tanks to implement countercurrent rinsing In such cases it may be possible to convert an
existing rinse tank to a countercurrent one by dividing the tank
with a steel plate
105
-im
difficult-to-tii
,- utions are allc
d:ents For these
:'din
Similar results may be obtained by installing spray rinses
above the rinse tank Using this technique, the workpiece is first
rinsed in the tank and, as it is being withdrawn, it is rinsed a
second time with the spray The last water to contact the work is
uncontaminated Since the onlyfresh water input into the rinse
tank is from the spray rinse, the result is the equivalent of a
two-station countercurrent system However, this method is not
effective on barrel plating lines
Hot rinses also reduce water usage A combination of
countercurrent and hot rinses can produce a zero-discharge
system This efficiency results because the heated rinse has a
lower kinematic viscosity, which reduces the volume of dragout
to the next rinse This reduction prevents the second rinse from
being contaminated as rapidly, thus requiring less fresh water
input In this way, the overflow from the second rinse replaces
the evaporative loss from the hot rinse
Recycling is another technique that minimizes rinsewater
use In many instances, the rinsewater following an acid pickle
can be reused directly after an alkaline cleaner This flow
arrangement is advantageous because alkaline cleaners
usually have a high viscosity, thus facilitating rinsing In some
instances, it may be possible to use the final rinse overflow as
the feed to the acid rinse, which is then used as the feed to the
alkaline cleaner rinse Prior to implementation, these types of
recycling systems should be evaluated on a case-by-case
basis for their effect on product quality
One simple, yet highly effective, modification to a rinsing
system is the installation of a master contrql valve In many
instances, water is left running unnecessarily on the plating line
This practice often occurs because of the numerous valves that
must be closed to turn off all water sources Moreover, when all
of the valves are reopened, it is difficult to reestablish the
previous flow rate This problem can be eliminated by the sing,
valve approach
Another important conservation method involves soun
housekeeping practices, which require little or no capital outla,
Examples include installing spring-loaded hose nozzles t h i
shut off when not hand held, repairing leaky valves, installin;
drip-catching boards between tanks, and cleaning up spil!.
using dry instead of wet methods Furthermore, periodic inspec
tions should be performed on all tanks, pumps, racks, barre.
and auxiliary equipment to detect and repair leaks
In addition to the substantial savings in capital and annui
costs, a conservation program may facilitate the use of recow.
processes by reducing the volume of water used for rinslnu
This reduction will increase the contaminant concentration
the rinse tank-a desirable approach because most recove'.
processes require a fairly high feed concentration to beffective
should inve
The first catec
"arged without 11
3ie been impltt
,inbination ther
' Jr example, if t h
wper and ano)
mentration, thi
"argcd without 11
'wastewater IC(
, * d d be applie
'+ uent limits Tyf
' '*vo-thirdsof thl
: ilnarge withoul
After identifyinl!
'"out treatmentt
a-cording tothe t!!
'm frequency au
'
Segregation of Wastewater
After the wastewater flow has been reduced by implementing a
conservation program, further cost reductions may be achieve:
by segregating streams requiring different types of treatmen'
A good approach is to segregate wastewater into a low-volume
highly concentrated stream A low-volume stream will requfi- '
small treatment facility, and its highly concentrated nature Us
ally improves the effectiveness of the treatment process a'
reduces chemical requirements
Segregation is also desirable from a safety standpo'
because combinations of different waste streams can result
the generation of toxic gases For example, the combinatlono'
cyanide rinse with an acid pickle rinse can liberate hydrost
cyanide gas In addition, the combination of waste streams ma,
--------.
***TEWATER REOURII
METAL REMOVI
--2___
106
PLATING AND SURFACE F I W "
-
A
.
ivolves SOU' :
)capital outi,i,
,e nozzles t h + t
ilves, instalilfj ,
ming un SF:
ertodic insper
, racks, barrc s
iks
ita1 and annua,
use of recovery
sed for rinslng
oncentration irmost recover.
ntration t9 ti,
.
implementing
lay b e achievr
3s of treatmer-'
o a lOW-VOlUlTlc
i m will require c j
ited nature usb
nt process aP :
fety standpoif.'
ms c a n resul' * '
:ombination cf A
Ierate hydrog?'
jte streams ma,
dlfficult-to-treatcompounds, especially if metal-containing
are allowed to contact baths containing complexing
For these reasons, a comprehensive segregation proshould investigate various categories of wastewater
first Category consists of wastewater that may be disged without treatment Even after flow reduction measures
2 been implemented, there are usually some rinses, or
bination thereof, that may be discharged without treatment
?xample,if there is a rinse relatively high in zinc and low in
jer and another of similar volume with just the reverse
:entration, the combination of the two may likely be disged without treatment
wastewater is discharged without treatment, a safety factor
j d be applied to guarantee that the discharge is within
ent limits Typically, a stream should not exceed one-half
{o-thirdsof the effluent limits for all parameters to qualify for
harge without treatment
Iter identifying the wastewaters that may be discharged
out treatment, the remaining streams should be segregated
xdingto the type of treatment necessary Depending on the
frequency and volume of these discharges, provisions for
WASTEWATER NOT
KOUIRNQ TREATMENT
CILNIDE OXIDATION
CYANPE DESTRUCTWN
SYSTEM
CHROMUM REDUCTION
CHROMNJM REDUCTWN
SYSTEM
WASTEWATER REOURNQ
COMPLEXED METALS
TREATMENT
TREATMENTSYSTEM
Ill
I
I
EMULSIFIED OILS
* I V Y METAL REMOVAL
l
l
EMULSFED O L
EOUALTUTWN
TANK WlOL
SKMMER
1
c
solution storage may be required
The first type of treatment is cyanide oxidation Segregation
for cyanide destruction is necessary for several reasons If
allowed to contact an acidic solution, cyanide baths will release
hydrogen cyanide gas Also, a very stable rron-cyanide complex, which is difficult to treat, will form if the cyanide solution is
mixed with one containing iron
It is also advantageous to segregate cyanide solutions to
conserve chemicals Complete oxidation of cyanide is usually
performed in a two-stage alkaline chlorination system The
reactions occurring in this process are pH, dependent, and
chemical addition is required to adjust the pH The quantity of
chemicals required for pH adlustment is directly proportional to
the flow rate Therefore, minimizing the flow rate through segregation will reduce the quantity of chemicals required to destroy
the cyanide
Another pH-dependent treatment is the conversion of hexavalent chromium to its trivalent form This process, which results
in the formation of a hydroxide precipitate, is accomplished at a
very low pH by adding sulfuric acid Therefore, if the volume of
wastewater requiring chromium reduction is reduced through
segregation, the amount of sulfuric acid required will be reduced
proportionately
An additional consideration in chromium treatment is batch
dumping A large portion of the wastewater containing hexavalent chromium is due to the discharge of spent chromate solutions These baths are usually highly concentrated and will
overload most continuous chromium reduction systems Batch
dumps containing hexavalent chromium should be segregated
from the rinsewater
Complexing agents should also be isolated If these discharges are allowed to mix with solutions containing metals, the
metals will react with the agents and become bound up in the
solution Such bound compounds are difficult to remove in
conventional treatment systems and normally require specialized treatment If this waste stream is segregated, a higher
concentration of the complexing agents will be present, making
any specialized treatment more efficient than if it were applied to
a dilute waste stream
Many cleaners contain very high concentrations of complexing agents These solutions normally require replacement on a
regular basis, and if discharged directly to a waste-treatment
facility will most likely cause a plant upset To prevent this, these
solutions should be collected in holding tanks and either
metered slowly into the treatment system or treated separately
One type of discharge that can be segregated or treated in a
combined waste stream is free oil The most common method of
oil removal is skimming, which usually can be applied at any
point in the treatment process prior to settling Oil is generally
removed prior to clarification to avoid fouling the plates or tubes
of the clarifiers However, emulsified oils cannot be removed by
simple skimming and therefore waste streams containing such
oils should be segregated for separate treatment
After the waste streams have been segregated into the various categories, the remaining ones should be combined for
heavy-metal removal In some instances, the remaining
streams will contain metals that precipitate at significantly different pH levels If this situation exists, the streams may be either
segregated for metals removal at different pH levels or combined and treated in a two-stage precipitation system
All of the previously segregated waste streams, except for the
discharges not requiring treatment, should be combined after
specialized treatment, then routed to a heavy-metal removal
system Figure 1 represents a typical segregation scheme
Another candidate for segregation is any discharge where
recovery may be feasible Segregation is required in this
107
situaun so that recovery processes wilJ be free of impurities
found in other waste streams. Recovery processes such as ion
exchange and reverse osmosis (RO) are relatively sensitive to
contamination. Ion-exchange columns will foul from the presence of organics or suspended solids. The membranes used in
RO systems may be attacked by various contaminants. On the
other hand, evaporative recovery processes are relatively
insensitive to impurities and make complete segregation unnecessary. Recovery systems should be evaluated on a case-bycase basis to determine their feasibility.
The costs associated with implementing a segregation
scheme are highly variable. Factors such as age,of the facility,
existing piping, and the location of the treatment facilities can
greatly affect the cost of segregation. In some instances, it may
be impossible for the wastewater to flow by gravity to the
treatment facility. In such cases, pumping systems would be
required for each segregated waste stream. This pumping
requirement will increase the capital cost and add to the
operating expense of the segregation system. Generally, any
additional costs due to the segregation piping and pumping
systems are more than offset by reductions in the size and costs
of the equipment needed to treat each waste stream.
Reference Sources
Conclusions
About the Author
The cost of having a conservation and segregation study by a
competent consulting engineering firm may well be offset by
savings in water usage, reduced treatment costs, and strict
compliance with effluent regulations. A good study and implementation of its recommendations may prove to be one of the
answers in remaining competitive in the price-sensitive marketplace. 0
Patrick Dowd is an environmental engineer for Baxter &
Woodman, Inc ,8678 Ridgefield Rd , Crystal Lake, IL60014 The
firm specializes in civil and environmental engineering Mr
Dowd holds a BS in civil engineering from the Illinois Institute of
Technology and is pursuing an MS degree in environmental
engineering Previously, he was employed in the Water Control
Department of Republic Steel
-
B O T H C- O
- ST YOU:
- I-*+
LUX solution
0 Waste treatme
A
I
1 . Joseph B. Kushner, Water and Waste Control for the
Plating Shop, Gardner Publications, Cincinnati, OH (1 976).
2. H.L. Pinkertonand K.A. Graham, Electroplatingfngineering
Handbook, 3rd ed., Van Nostrand Reinhold, New York, NY
(1 962).
3. C.H. Roy, In-Plant Conservation, AES Illustrated Lecture
(1 980).
4. US. EPA, Controland Treatment Technology for the Metal
finishing lndustry (In-Plant Changes), EPA 625/8-82-008
(1 982).
5. US. EPA, Development Document for Effluent Limitations
Guidelines and Standards for the Metal finishing Point
Source Category, EPA 440/1-83/091 (1 983).
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108
PLATING AND SURFACE FINISH IN^
L
I