Chrome Plating and Anodizing Operations 

Cal/EPA – January 2006 1 

306 EMISSIONS CONTROL 

TECHNIQUES

 

TABLE OF CONTENTS

306 EMISSIONS CONTROL TECHNIQUES

 306.1 FUME SUPPRESSANTS 

 306.1.1 Chemical Fume Suppressants

 306.1.2 Mechanical Fume Suppressants

 306.2 ADD-ON CONTROL DEVICES

 306.2.1 Capture Techniques

 306.2.2 Pack-Bed Scrubbers

 306.2.3 Mist Eliminators 

 306.2.4 HEPA Filters 

 306.2.5 Combination Systems

 306.2.6 Encapsulating Tank Covers

 

 

 

306 EMISSIONS CONTROL TECHNIQUES

To comply with the State ATCM regulation, a plating or anodizing facility must either

1. Meet a surface tension limit using chemical fume suppressants containing a wetting agent or; 

2. Meet an emission limit (mg/amp-hr or mg/dscm) demonstrated by a performance test. 

Techniques to control hexavalent chromium emissions from chrome electroplating and anodizing 

tanks include fume suppressants and add-on control devices. In the first, an anti-mist additive is 

added to the plating bath to reduce the amount of misting that takes place. In the second, the mist is

captured, and the entrained chromium is extracted from the exhaust air stream prior to discharge to 

the stack. 

In California, decorative platers typically use an anti-mist additive to achieve compliance with the 

State's hexavalent chromium emissions limit. About 25% of decorative platers in the State uses addon control equipment in addition to an anti-mist additive. About 5% use only add-on control equipment 

to reduce hexavalent chromium emissions.

Hard chrome platers in California use add-on controls to achieve compliance with the State's 

hexavalent chromium emissions limit. However, about 50% of hard chrome platers in the State use an 

anti-mist additive in conjunction with add-on controls to reduce hexavalent chromium emissions from 

their tanks. 
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About 40% of chromic acid anodizing operations in the State use an anti-mist additive alone to 

achieve compliance. Another 40% use an anti-mist additive in conjunction with add-on controls. The 

remaining 20% use only add-on controls to reduce hexavalent chromium emissions from their tanks.6

306.1 FUME SUPPRESSANTS

Fume suppressants are mist suppressant or physical barriers at the surface of the plating bath used 

to control chromic acid mists. There are two categories of fume suppressants: chemical fume 

suppressant and mechanical fume suppressant. 

306.1.1 Chemical Fume Suppressant

A chemical fume suppressant is any chemical agent that reduces or suppresses fumes or mists at the 

surface of an electroplating or anodizing bath. Another term for chemical fume suppressant is mist 

suppressant. Fume suppressants can be temporary or permanent depending on the mechanism of 

depletion from the baths. Temporary fume suppressants are depleted mainly by the decomposition of 

the active chemical components. Permanent fume suppressants are depleted by drag-out of the 

solution. 

Chemical fume suppressants are added directly to the bath to reduce or inhibit misting. Chemical 

fume suppressants include: wetting agents, foam blankets (Figure 306-1), and combinations of both 

(Figure 306-2). Most chemical fume suppressants in use today contain wetting agents. The difference 

between wetting agents and foam blankets is the way in which they reduce emissions. Wetting agents 

control misting by lowering the surface tension of the bath, while foam blankets physically suppress 

the mists produced on the surface of the bath.

Fume suppressants are manufactured in liquid, powder, or tablet form. Some of the main advantages 

of using chemical fume suppressants include:

 1. Minimization of plating solution evaporation losses; 

 2. Low cost of chromium emission control; 

 3. Very little energy consumption; and 

 4. No solid waste generation.

Limitations in using chemical fume suppressants include:

 1. Some fume suppressants have a tendency to aggravate gas pitting and defects in base metals; 

 and 

 2. Foam blankets that are too thick can entrap hydrogen gas, which poses a risk of explosion.

Chemical fume suppressants typically reduce chromium emissions between 93 and 99 percent.1

306.1.1A Foam Blankets

A foam blanket is a type of chemical fume suppressant that generates a layer of foam across the 

surface of a solution when current is applied to that solution. Foam blankets do not prevent the 

formation of chromic acid mist, but instead trap the mist under a blanket of foam. The foam blanket is
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Cal/EPA – January 2006 3 

formed by the agitation produced by hydrogen and oxygen gas bubbles generated during 

electroplating. Once formed, the foam blanket is usually maintained at a thickness of 1.3 to 2.5 

centimeters (0.5 to 1.0 inches) and covers the entire surface of the plating bath (see Figure 306-1). 

The ability of the foam layer to contain mist is greatly reduced if proper thickness is not maintained. If 

the foam blanket is too thin, less than 1.3 centimeters (0.5 inches), it will no longer trap the chromium 

containing mist. Conversely, a thickness greater than 2.5 centimeters (1.0 inches) will cause 

hydrogen gas to build up in the foam layer, creating a hydrogen explosion hazard should a spark be 

generated from the contacting equipment. These problems tend to occur more frequently with higher 

current densities and longer plating times.1

Figure 306-1. Foam Blanket

The characteristics of foam blanket solutions are determined by the surface area of the solution, 

amount of current applied, and temperature and chromic acid concentration of the plating bath, (e.g. 

less suppressant is required at lower temperatures). Visual monitoring of the thickness of the foam 

blanket is the most common method for determining when to add foam blanket solution to the bath. 

The frequency of adding foam blanket solution, also, depends upon the drag-out rate of the solution 

and the amount of work being performed in the plating tank. Chemical decomposition, to a greater 

extent than drag-out, is the primary mechanism for depletion of foam blankets.

306.1.1B Wetting Agents

A wetting agent is a type of chemical fume suppressant that reduces the surface tension of a liquid. 

When wetting agents lower the surface tension of a plating bath, gases escape at the surface of the 

solution with less of a "bursting" effect, causing less mist formation. In essence, a wetting agent 

reduces bubble size, and smaller bubbles burst with less impact on the surface.

Plating bath typically have a surface tension of 70 dynes/cm. With the addition of sufficient wetting 

agent, the surface tension of the plating bath can be reduced to 45 dynes/cm or less where it has 

been shown to create a rapid reduction in chromic acid emissions.1

 The two methods used to 

measure the surface tension of the plating solution are the stalagmometer (the drop weight method) 
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and the tensiometer (the ring method). Most plating facilities use a stalagmometer to measure the 

surface tension. Go to the Photo Gallery to view pictures of the stalagmometer and the tensiometer.

Figure 306-2. Fume Suppressant - Combination of Wetting Agent and Foam

306.1.2 Mechanical Fume Suppressant

A mechanical fume suppressant is any device that reduces fumes or mist at the surfaces of an 

electroplating or anodizing bath by direct contact with the surface of the bath. Polyballs (Figure 306-3) 

are the most commonly used mechanical fume suppressant. Polyballs are hollow, plastic floating 

spheres with typical sizes range from 3/4 to 2 inches in diameter. They are fabricated from plastics 

that are resistant to chemical attack and are used to reduce heat loss, bath evaporation, and misting.

In order to be most effective, balls must cover the entire surface area of the bath. 
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Figure 306-3. Polyballs 

The U.S. EPA has examined the use of these floating spheres for air pollution control purposes, and 

found them to be of limited use. In one such test, a two to three inch layer of 1.5 inches in diameter

spheres achieves a removal efficiency of only about 75 percent.1

 The effectiveness of polyballs is also 

reduced by the fact that the evolution of hydrogen gas at the workpiece tends to push the spheres 

away, exposing a portion of the plating bath surface to the air above.

 

306.2 CONTROL DEVICES

This section discusses the capture techniques to capture the chromium misting at the surface of the 

tank and the add-on control devices available to extract the entrained chromium from the exhaust air 

stream prior to discharge to the stack. An add-on air pollution control device is any equipment 

installed in the ventilation system of chrome electroplating and anodizing tanks for the purposes of 

collecting and holding chromium emissions from the tank(s). Add-on control devices include: packbed scrubbers, mesh pad mist eliminators, fiber bed mist eliminators, and combination systems. Also 

discussed is the encapsulating tank cover which does not require an exhaust stack to vent process air 

outside of the facility.

306.2.1 Capture Techniques - Lateral Exhaust System

Local exhaust ventilation is the most common method used to capture chromic acid mist from chrome 

plating and chromic acid anodizing baths. Typical local exhaust ventilation equipment includes lateral

(slot) and push-pull exhaust hoods. Canopy hoods or overhead exhaust hoods and enclosures are 

normally not used because they restrict access to the plating or anodizing tank. In addition, overhead
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hoods would draw mists from the tank surface into the breathing zone of anyone who was working at 

the tank. However, there is a control system called the encapsulating tank lid or cover used by hard 

chrome platers that contains the mist while allowing other gasses to vent. This control system is 

further discussed in Subsection 306.2.6, Encapsulating Tank Covers.

A tank is said to be "laterally exhausted" if the hood does not project over the entire tank and the 

direction of the air movement into the hood is substantially horizontal. Lateral exhaust systems 

typically used to control chromic acid mist from open-surface tanks include both single-sided and 

double-sided slot hoods. The slot velocity affects the uniformity of airflow distribution along the length 

of the hood. 

Single-sided hoods have a maximum reach across the width of the tank of about 30 inches (2 and 1/2 

feet) while double-sided hoods have a maximum reach of about 60 inches (5 feet). In this system, the 

chromic acid mists are collected by slot vents which usually run along the long side (single-sided) or

along both long sides of the tank (double-sided) (see Figure 306-4). 

Figure 306-4. Slot Vents on a Lateral Exhaust System

Push-pull ventilation is recommended for tanks greater than 42 inches (3 and 1/2 feet) in width. In a 
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push-pull system (Figure 306-5), one slot vent is an intake ("pull") while the other supplies air under 

pressure ("push"). Push air supplied through slots, orifices, or nozzles is blown across the width of the 

tank to direct contaminated air toward the exhaust hood, the "pull" vent, located on the opposite side

of the tank. 

Figure 306-5. Push-Pull System 

306.2.2 Packed Bed Scrubbers

A packed-bed scrubber (PBS) consists of a single or double packed bed that contains packing media 

on which the chromic acid droplets attach. First, the gas stream is wetted by spraying water against 

the flow of the gas to make the droplets larger. Then the gas stream passes through the packed 

bed(s) where the droplets attach onto the packing media. A mist eliminator section follows the packed 

bed section of the scrubber. The mist eliminator removes any water entrained from the packed bed 

section. Treated gases then pass through an induced draft fan and out a stack of exhaust vent. The 

scrubber water is recirculated and periodically tapped and discharged to the electroplating tanks as 

makeup solution. Figure 306-6 shows a packed-bed scrubber. 
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A PBS has a typical efficiency of 97 percent for decorative chromium electroplating and anodizing 

tanks and 99% for hard chromium electroplating tanks. Factors influencing efficiency include: liquidto-gas ratio (the flooding or starving of the media with scrubbing liquor), gas velocity, surface area

and type of packing media, distribution of the media in the bed, and effectiveness of washdown 

procedures.3

 

Figure 306-6. Packed-Bed Scrubber 

(Courtesy of Midwest Air Products Co., Inc. (MAPCO), adapted)

306.2.3 Mist eliminators

Mist eliminators generally remove droplets from gas streams by inertial impaction of droplets on these

devices and centrifugal force applied to the gas stream. Many types of mist eliminating devices have 

been developed, but most are categorized within the following types: chevrons, mesh pads, and fiber 

beds. 

Mist eliminators may be used in a horizontal or vertical gas flow configuration. When mist eliminators

are used on a vertical gas stream, liquids removed from the gas will drip from the eliminator from the

force of gravity on the liquid against the flow of the gas stream. Liquids collected by mist eliminators 

used on horizontal gas flows will not drip against the flow of the gas, so horizontal mist eliminators

can handle higher gas velocities and liquid loadings.
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Mist eliminators are usually equipped with liquid sprays to prevent the accumulation of solids. An 

accumulation of solids can plug part of the eliminator, reduce the area of flow, create excessive 

pressure drops and increase liquid entrainment. Properly operating mist eliminators usually have a 

pressure drop between 0.5 and 4.0 inches of water. Type, size, and composition of the pads are 

dependent on the system they are incorporated in (i.e. gas density, flow velocity, etc.).

306.2.3A Chevron-Blade Mist Eliminators

Chevrons are devices composed of zigzag baffles which force a gas stream to turn sharply. Liquids 

collect on the blades of the chevron and drain downward by gravity. Basically, a series of parallel 

zigzag blades (see Figure 306-7) are positioned in the body or housing of the device, in order to direct 

the air flow through narrow passageways that repeatedly change its direction by approximately 30 

degrees. At regular intervals, the extruded blades have narrow vertical channels which serve to 

capture some of the chromium droplets by inertial forces. Since the channels are oriented in a vertical 

plane, condensed chrome is transported to a drain at the bottom of the housing. Common blade 

designs are the overlapping and sinusoidal wave designs (see Figure 306-7). 

Figure 306-7 Blade Configurations Used in Chevron Mist Eliminators 

(EPA Capsule Report) 

Blade-type horizontal mist eliminators have demonstrated a wide efficiency range. Removal 

efficiencies ranged from about 83 to 99 percent. Some of the factors influencing efficiency were: gas 

velocity, blade spacing, seal integrity, and wash down frequency.1, 3 

306.2.3B Composite Mesh Pad Mist Eliminators

A composite mesh pad mist eliminator (CMP) consists of several layers of interlocked fibers densely 

packed between two supporting grids. The composite mesh pad was developed to remove small 

particles (< 5 micrometers or 0.2 mils) that were not effectively controlled by other control devices.

There are usually at least three mesh pad stages (see Figure 306-8). The first stage removes large 

particles. The second stage removes smaller particles using a composite mesh pad. The final stage 

may remove any reentrained particles not collected by the composite mesh pad in the second stage. 
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A reentrained particle is a particle which was caught by the mesh pad, but some force removed it 

from the mesh pad and returned it to the air stream.

The layers of material in composite pads are arranged with the smallest diameter fiber layer located in 

the center of the pad and progressively larger diameter layers located on both sides of the center. 

Particles collide with the fibers in the pad and attach to the fiber surfaces. These captured particles 

combine into larger particles as they travel through the small diameter fiber layers in the center of the 

pad. These larger particles either drain to the bottom of the unit or are reentrained in the gas stream. 

The reentrained particles are then captured by the large diameter fiber layers in the back of the pad.

CMP systems incorporate a larger particle removal system located upstream from the composite 

mesh pad, to reduce the plugging potential of the pad. The large particle removal system can either 

be a series of larger diameter mesh pads or a packed bed scrubber section. Typical removal 

efficiencies associated with this control device are greater than 99 percent.1, 3 

 

 

 

Figure 306-8. Mesh Pad Mist Eliminator 

(Courtesy of Midwest Air Products Co., Inc. (MAPCO), adapted) 

 
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306.2.3C Fiber Bed Mist Eliminators

A fiber bed mist eliminator (FBME) removes contaminants from a gas stream through the 

mechanisms of inertial impaction and Brownian diffusion. FBMEs have most often been used to 

reduce acid mists from sulfuric, phosphoric, and nitric acid plants. An FBME consists of one or more 

fiber beds. Each bed consists of a hollow cylinder formed from two concentric screens (see Figure 

306-9). The fiber between the screens may be made of glass, ceramic, plastic, or metal. Fiber bed 

units are designed for horizontal, concurrent gas liquid flow through the fiber bed. The contaminated 

gas stream flows toward the downstream face of the bed. The acid mist in the gas stream impacts on 

the surface of the fibers and drains down the outer face of the bed to the sump while the clean gas 

flows up and out the top of the unit.

Figure 306-9 Fiber Bed Mist Eliminator 

(EPA Capsule Report) 

 

A FBME device is typically installed downstream from another control device that can prevent 

plugging of the FBME device. The upstream device removes the majority of the emissions and thus 

prevents plugging of the fiber bed.

To date, there is no adequate test data available to accurately quantify the control efficiency of FBME. 

The U.S. EPA believes that these systems can achieve the same emission limits as CMP systems 

and chemical fume suppressants.1, 3
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306.2.4 HEPA Filters

A High Efficiency Particle Arresting or Particulate Air (HEPA) filter is defined by the Department of 

Energy (DOE) as a throwaway, extended-media, dry type filter with a rigid casing enclosing the full 

length depth of the pleats. The filter shall exhibit a minimum efficiency of 99.97% when tested at an 

aerosol of 0.3 micrometers diameter.5 These filters were originally designed to trap microscopic 

particles such as radioactive dust in atomic plants and are commonly used for critical environments 

like hospitals, clean rooms, and lead abatement projects. Figure 306-9 illustrates a HEPA filter 

system.

Figure 306-9. HEPA Filter System
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306.2.5 Combination Systems

Combination systems are various arrangements of multiple control devices, as discussed above, in a 

common exhaust system. Some of the variations include:

1. Mesh pads installed in exhaust hoods at the tank, followed by an end-of-line mesh pad or 

packed bed scrubber;

2. In-line mesh pad mist eliminators, located after the exhaust hoods, followed by an end-ofline mesh pad or packed bed scrubber;

3. Blade-type horizontal mist eliminators followed by an end-of-line mesh pad or packed bed 

scrubber;

4. Mesh pad mist eliminator system followed by a high efficiency particulate air (HEPA) filter 

as a final polishing device; and

5. Mesh pad mist eliminator system followed by a filter bed as a final polishing device.

Some of the advantages in using multiple devices include: segregating chemicals, keeping duct work 

clean, and reducing the inlet loading on the end-of-line unit.1

 

306.2.6 Encapsulating Tank Covers

An encapsulating tank cover involves the use of a special tank hood or cover that contains the mist 

while allowing other gasses to vent (see Figure 306-10). The system does not require an exhaust 

stack to vent process air outside of the facility. The hydrogen and oxygen gas, generated during the 

chrome plating process, pass through a selective membrane system to the plating room environment. 

Water vapor and chromic acid mist do not penetrate the membrane, but will condense within the 

confines of the tank's cover. At the end of the plating batch, an evacuation cycle removes latent 

fumes or mists prior to opening the cover for partial retrieval. This approach can reduce the building's 

energy requirements significantly, since the unit does not exhaust conditioned air outdoors, and there

are no large fan motors. An additional benefit is that no chromium-bearing fluids are generated, as 

with the washdown cycles of mesh pad mist eliminator systems or the recirculated liquor in packedbed scrubbers.
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Figure 306-10. Encapsulating Tank Cover 

(EPA Capsule Report, PF Online) 

 

Because water vapor is not removed from the plating bath through surface evaporation, reclaim 

rinsing strategies and cooling systems may be adversely affected. Also, parts should either have 

identical start/stop times (i.e., batch plated) or be capable of multiple plating cycles, since all plating in 

the tank must be stopped before any parts can be added or removed.1, 4

 

 
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References

1.U.S. EPA, 1998, Capsule Report-Hard Chrome Fume Suppressants and Control Technologies, 

Office of Research and Development, Washington DC, Document No. EPA/625/R-98/002, 

December 1998, Available online at: 

http://www.epa.gov/ORD/NRMRL/pubs/625r98002/625r98002.pdf

2. U.S. EPA, 1993, Chromium Emissions from Chromium Electroplating and Chromic Acid Anodizing 

Operations-Background Information for Proposed Standards, Document No. 

EPA 453/R-93-030b, p. 4-1 - 4-70

3. Colorado Department of Public Health and Environment (CDPHE), 1999, CDPHE Chromium 

MACT - Control Measures, Last updated April 21, 1999, Accessed January 20, 2005, 

http://www.cdphe.state.co.us/ap/chromium/html/control/index.html 

4. PF Online an online component of Products Finishing magazine, 1997, Reducing Hexavalent 

Chromium Emissions, December 1997, Accessed on May 25, 2004, 

http://www.pfonline.com/articles/129705.html

5. U.S. Department of Energy, 1997, DOE Standard Specification for HEPA Filters Used By DOE 

Contractors, Document No.: DOE-STD-3020-97, Washington, D.C., January 1997, p. 4, Accessed on 

September 21, 2005, http://www.eh.doe.gov/hepa/docs/std3020.pdf

6. California Air Resources Board, telephone discussion with Shobna Sahni, Lead staff on ATCM for 

Chrome Plating and Chromic Acid Anodizing Operations, September 2005

7. California Air Resources Board, 2000, Toxics Enforcement Manual, Part B: NESHAP/MACT 

Guideline Documents, Volume 2, Chapter I, Chrome Electroplating and Anodizing Operations, 

Compliance Assistance Program, Sacramento, CA, November 2000, p. 200-12 - 200-18