Pollution Prevention and Control Technologies
for Plating Operations
Section 2 - General Waste Reduction Practices
2.5 RINSEWATER REDUCTION
2.5.3 Alternative Rinsing Configurations
184.108.40.206 Generating Rinsing Data
220.127.116.11 Counterflow Rinsing
18.104.22.168 Cascade, Reactive, and Dual Purpose Rinsing
22.214.171.124 Chemical Rinsing
126.96.36.199 Spray Rinsing
188.8.131.52 Combined Drag-Out Loss/Rinse Water Reduction Rinsing Arrangements
184.108.40.206 Generating Rinsing Data
To evaluate rinsing alternatives effectively, the plater needs
drag-out and water use data, rinse quality criteria and equations
that utilize these data and criteria. Of these items, the drag-out
data are the most difficult and expensive to obtain. The high
cost is due to a need for expensive analytical work. A well proven
method of generating drag-out data, that is easy and inexpensive
to apply, is discussed in this subsection. This method, which
utilizes conductivity measurements, was originally described by
Mohler (ref. 2).
In order to utilize this methodology, a conductivity meter is
required. Using a conductivity meter greatly reduces the analytical
costs of generating rinsing data. In a matter of hours a plater
can generate a quantity of data equal to that generated by days
of laboratory work. Also, unlike using laboratory analytical methods,
with the conductivity meter approach there is almost no lag time
associated between the taking of samples and generating of results.
Most plating shops have combination pH/conductivity meters in
their shops that can be used for this purpose. Alternatively,
a portable unit of sufficient quality can be purchased for $200
to $300. Exhibits 2-17 and 2-18 present the conductivity values
and rinsing criteria used in the following method.
1. Use conductivity measurements to determine the present concentration
of a plating solution in a rinse tank (example: sulfuric acid
- Measure the conductivity of your tap water (or DI water if
used): a = 100 µmho (micromhos)
- Measure the conductivity of the rinse water. If a multiple
rinse tank arrangement is present (e.g., counterflow), make the
conductivity measurement in the last rinse (i.e., least contaminated): b = 5,500 µmho
- In a container, add one ml of the pickle solution to one liter
of rinse water and measure the conductivity: c = 6,250 µmho
- The increase in conductivity for one ml of pickle solution
per one liter of rinse water is: c - b = 6,250 - 5,500 = 750
- The concentration of pickle solution in the rinse water is:
e = (b - a)/(c - b) = 5,400/750 = 7.2 ml/l
- The equivalent sulfuric acid in the rinse is (see Exhibit
2-17, 6,300 µmho = 1,000 mg/l): 1,000 (c - a)/6,300 =
6,150,000/6,300 f = 976 mg/l equivalent H2SO4
- Compare this concentration to the criteria in Exhibit 2-18
to determine if the proper rinse flow rate is being used. If necessary,
adjust the rinse flow and repeat the measurements until the rinse
criteria is met.
2. Use conductivity measurements to determine the drag-out
- First determine the rinse water flow rate. Record the number
of racks processed per hour. Note the valve setting, turn off
the flow, bail out six or more inches of water, set the flow and
record the rate of rise of the water in inches per minute. From
the number of racks processed and the rate of rise of the water
calculate the flow as liters per rack (alternatively flow can
be expressed in l/ft2 of wetted area, l/barrel, or another relevant
unit of measure): g = 8.0 l/rack
- Second, measure the drag-in per rack. Turn off the flow, agitate
the rinse, measure the conductivity, process a number of racks
of work through the rinse, agitate the rinse, and measure the
conductivity again. From the µmohs increase per rack calculate
the drag-in ml/rack: h = 50 ml/rack
3. Use the equation presented in Section 220.127.116.11 to evaluate
the effect of adding additional rinse tanks.
18.104.22.168 Counterflow Rinsing
Electroplaters have long reduced water use by employing several
rinse tanks connected in series. Fresh water flows into the rinse
tank located farthest from the process tank and overflows, in
turn, to the rinse tanks closer to the process tank (see Exhibit
2-13). This technique is termed counterflow (or countercurrent)
rinsing because the work piece and the rinse water move in opposite
directions. Over time, the first rinse becomes contaminated with
drag-out and reaches a stable concentration which is lower than
the process solution. The second rinse stabilizes at an even lower
concentration which enables less rinse water to be used then if
only one rinse tank were in place. The more counterflow rinse
tanks (three-stage, four-stage, etc.), the lower the rinse rate
needed for adequate removal of the process solution.
Counterflow rinsing systems are not without drawbacks. The negative
aspects of counterflow rinsing include: (1) cost of additional
rinse tanks; (2) loss of valuable production space; and (3) an
increase in production time/labor.
The rinse rate needed for adequate cleaning is governed by an
exponential equation that depends on the concentration of plating
chemicals in the drag-out, the concentration of plating chemicals
that can be tolerated in the final rinse tank before poor plating
results, and the number of counterflow rinse tanks. The mathematical
rinsing models are based on complete rinsing (i.e., removal of
all drag-out from the part/fixture) and complete mixing (i.e.,
homogeneous rinse water). These conditions are not achieved or
even approached unless there is sufficient residence time and
agitation in the rinse tank. More typically, each added rinse
stage reduces rinse water use by 50 percent.
The most commonly applied counterflow rinsing model follows:
Rc = (Ct/Cr)1/n
where, Rc = rinsing ratio; Ct = concentration in the preceding
tank; Cr = required concentration in the last rinse tank; n =
number of rinse tanks
This model does not predict required rinse rates accurately when
the value of Rc falls below 10. Also, complete rinsing will not
be achieved unless there is sufficient residence time and agitation
in the rinse tank.
The following example demonstrates the use of the counterflow
rinsing equation. A typical Watts nickel plating solution contains
270,000 mg/l of total dissolved solids, and the selected final
rinse concentration is 37 mg/l of dissolved solids. The ratio
of Ct/Cr is 7,300 and approximately 7,300 gallons of rinse water
are required for each gallon of process solution drag-in with
a single-tank rinse system. By installing a two-stage rinse system,
water requirements are reduced to 86 gallons of water per gallon
of process solution drag-in (assumes 100% rinsing efficiency).
The same degree of dilution is obtained in the final rinse, and
the rinse water consumption is reduced by 99 percent. The mass
flow of pollutants exiting the rinse system remains constant,
however, the pollutants are much more concentrated with a two-stage
rinse system than with a single-tank rinse system.
From the Users Survey results, 217 respondents (or 68.2%) employ
counterflow rinsing. These respondents gave this method of water
use reduction a success rating of 4.21, the highest for any water
use reduction method.
22.214.171.124 Cascade, Reactive, and Dual Purpose
Cascade rinsing refers to the practice of reusing rinse water
multiple times in different rinse tanks for succeeding less critical
rinsing. Reactive rinsing is similar, but it refers to cases where
a chemical reaction takes place as a result of using the rinse
water for multiple purposes. An example is reusing the rinse water
following acid cleaning as rinse water following alkaline cleaning.
In this case, the acid rinse water helps to remove the viscous
alkaline film remaining on a part after alkaline cleaning.
Dual purpose rinsing refers to the practice of using the same
rinse tank for rinsing following more than one process tank. It
provides essentially the same results as cascade and reactive
rinsing but uses a fewer number of rinse tanks. Often, the employment
of dual purpose rinsing means transporting a dripping rack/part
over a considerable distance. This can result in dripping onto
floors and/or the accidental contamination of other tanks. An
exhaustive evaluation of dual purpose rinsing is presented by
Mohler (ref. 3). Mohler presents methods and guidelines for ascertaining
the accumulated concentration of chemicals in counterflow, dual-purpose
rinses in order to determine the feasibility and economics of
Typically, cascade and reactive rinsing are used for automatic
plating machines whereas dual purpose rinsing is used for manual
Use of any of these methods must closely consider the combined
chemistry in the rinse tank to prevent undesirable reactions that
may impact worker safety (e.g., hydrogen cyanide formation) or
work quality (e.g., precipitation of solids).
Reactive or cascade rinsing is used by 76 (or 23.9%) of the survey
respondents to reduce water usage. These respondents gave this
method an average success rating of 3.79.
126.96.36.199 Chemical Rinsing
The technique of chemical rinsing has been used by the metal finishing
industry for many years. One of its earliest applications was
to eliminate staining from chromium solution, which is notoriously
difficult to rinse. By simply making the first rinse after chromium
plate a stagnant rinse containing sodium bisulfite, the drag-in
of hexavalent chromium was converted to trivalent chromium. The
rinsability of the workpiece in the second rinse was improved
considerably by changing the chemical nature of the film on the
workpiece in the stagnant rinse and by reducing film concentrations
before attempting to rinse by diffusion. The same principles are
frequently employed in "neutralizing" dips.
The application of chemical rinsing to plant effluent treatment,
known in the industry as integrated waste treatment, has been
described by Lancy and Pinner (ref. 305). Aside from the environmental
benefits, this type of rinsing also prevents the majority of heavy
metal solids formed in the chemical rinse from reaching the succeeding
water rinses by removing these materials in an external settling
vessel. Removal of these solids is accomplished by flowing the
chemical rinse solution to a treatment reservoir. The overflow
from the reservoir is pumped back to the rinse tanks, forming
a complete closed-loop system. Integrated treatment gained some
popularity in the 1970's, but is believed to be in little use
today, mostly due to high maintenance requirements.
There were no questions in the Users Survey regarding chemical
rinsing, nor did any respondents indicate that they currently
use it or have used it in the past. Due to this lack of data,
the presumed decline of integrated waste treatment use has not
188.8.131.52 Spray Rinsing
Spray rinsing is employed in various manners to reduce drag-out
losses and rinse water use. Spray rinsing over process tanks (Section
184.108.40.206) provides direct recovery of drag-out. Spray rinse tanks
can be used as drag-out tanks, single rinses, or multiple rinses.
A common use of spray rinsing is to substitute a spray rinse tank
for an overflow rinse tank. Depending on the part configuration,
spray rinsing generally uses from one-eighth to one-fourth the
amount of water that would be used for equivalent dip rinsing
(ref. 305). Spray rinsing is most effective for flat-surfaced
parts and is less effective with recessed and hidden surfaces.
Although less commonly used, a spray rinse can substitute for
a drag-out tank. Several shops indicated that they consider this
a very successful pollution prevention measure (e.g., PS 177,
PS 242, PS 275). Using this method, one shop has reduced their
discharge from 15,000 gpd to an average flow of 800 gpd (PS 242).
PS 275 employs a fog spray rather than a common spray rinse (success
rating of "4").
Combined spray and dip rinse tank designs are employed where the
bottom portion of a rinse tank acts as a dip tank and the upper
portion a spray rinse. A weir is located at approximately the
middle of the tank which maintains the solution level in the tank.
In operation, the rack is lowered into the dip rinse, raised above
the solution level, and sprayed with fresh water. This combination
rinse can be nearly as effective as a counterflow rinse, but takes
up the floor space of one tank.
The design of spray rinses must consider the size and shape of
the part. Spray nozzles are available in many sizes and spray
patterns, and should be selected appropriately. Usually, the pressure
in the waterline is sufficient to operate an effective spray rinse,
however, higher spray velocities can be obtained by pumping.
A special application of the spray rinse is a patented unit (ref.
6) that contains five to seven progressively cleaner rinse solutions
in separate compartments. The solutions are successively pumped
(up to 20 gpm) to a spray rinse tank and drain back to the unit.
During each cycle, only the water use in the first spray is discarded
or processed for recovery. The subsequent sprays are collected
for reuse in the following cycles. The advantage of this unit
is that it provides the effect of multiple counterflow rinsing
with use of a single rinse tank. The floor space requirement of
the unit is 7.5 square feet (five stage rinse unit) or 11.0 square
feet (seven stage rinse unit).
Although it is not widely used, spray rinsing is applicable to
barrel plating operations as previously shown in Exhibit 2-11.
One shop reported that their platers are mistrustful of the efficiency
of spray rinses. Whenever plating quality problems arise, the
platers target the spray nozzels as the cause (PS 176). Another
shop complained that spray rinses often clog and must be cleaned
or replaced (PS 230).
The Users Survey showed a moderate to high usage rate of spray
rinsing. Most frequently it was used as a water use reduction
method and to a lesser degree for drag-out loss prevention (fog
or spray rinse over process tank). As a water use reduction method
124 (or 39.0%) of the shops employed spray rinsing. These respondents
gave spray rinsing an average success rating of 3.82.
220.127.116.11 Combined Drag-Out Loss/Rinse Water
Reduction Rinsing Arrangements
Many plating shops combine drag-out tanks and overflow rinsing
in the same rinse systems (see Exhibit 2-13). For example, a four
rinse system could consist of two drag-out tanks connected in
series and two free-flowing rinses connected in series (counterflow).
Alternatively, the system could consist of three drag-out tanks
in series and a single overflow rinse or a drag-in/drag-out arrangement
and two counterflow rinses. Various rinsing configurations can
also be combined with chemical recovery technologies, as discussed
in Section 3. The optimal rinse configuration will depend on numerous
factors including: (1) the evaporation rate in the plating tank,
(2) the drag-out rate, (3) the rinse water quality requirement
(final rinse), (4) process chemical costs, (5) alternative technology
recovery costs, (6) water costs, and (7) wastewater treatment/sludge
In general, when more of the available rinse tanks are used as
drag-out tanks, the process chemical and wastewater treatment
operating costs are lowered and water use costs are increased.
The reverse is true when more tanks are used for counterflow rinsing
than for drag-out recovery tanks.
The optimal configuration can be determined through mathematical
means which must be supported by data collection (i.e., drag-out
and evaporation measurements, production rates, etc.) for producing
accurate results. Examples of such analyses are presented in ref.
305 for chromium and nickel plating. As an alternative to using
the rinsing equations to perform the calculations, a modeling
program can be employed. A commercially available software program
(ref. 317) permits an analysis considering up to five rinse stations
and the use of supplemental evaporative recovery. This program
permits the user to add recovery rinses, change tank volumes,
experiment with process chemistries, add evaporators, and change
workload to find the combination that makes the most environmental
and economic sense (ref. 398).
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