Wet Electrostatic Precipitator (WESP) Control for Meeting Metals Emission 

Standards 

Paper # 585 

Andrew C. Bartocci 

Envitech, Inc. 

2924 Emerson Street- Suite 320 

San Diego, CA 92106 

Ph: 619-223-9925, ext. 203 

ABSTRACT

Several industrial processes face the challenge of meeting increasingly aggressive metals 

emission standards. In many cases these standards exceed the capability of existing air pollution 

control equipment including bag-houses, packed bed absorbers, and Venturi Scrubbers. 

The metals of concern can include mercury, arsenic, lead, cadmium, nickel and others, 

depending on the process. A control strategy using a wet electrostatic precipitator (WESP) in 

conjunction with sub-cooling was used on a secondary lead smelter to meet more stringent 

emission standards. This approach achieved > 98% removal of arsenic and > 92% removal of 

lead and other condensed metals downstream of a bag-house. This substantially reduced the 

plant’s cancer risk index and helped to meet reduced fence line lead emission limits. This 

control strategy will be discussed and how it can be applied to other industrial processes that face 

similar challenges, including lead refiners, refinery sludge incinerators, hazardous waste 

incinerators, and geothermal energy plants. 

INTRODUCTION

A secondary lead smelter recovers lead from used automobile batteries by using a combination 

of combustion processes including kilns and furnaces. Emissions from the combustion sources 

are highly controlled for particulate and acid gases. Despite these controls, stack tests show low 

amounts of various heavy metals including mercury, arsenic, lead, cadmium, nickel, and several 

other metals. Concentrations for some of the compounds are non-detectable, while others may 

be in the range of 0.002 to 0.2 mg/dscm. These low concentrations result in a cancer risk index 

that is higher than new state standards allow. The pollution control challenge is therefore a 

“polishing step” to significantly reduce toxic air emissions from very low inlet concentrations. 

This challenge was met with a wet electrostatic precipitator (WESP) with an integrated packed 

bed condenser/absorber (C/A). Stack tests indicate greater than 98% removal of arsenic and 

greater than 92% removal of lead was achieved. This substantially reduced the plant’s cancer 

risk index and helped to meet more stringent fence line lead emission standards. There are many 

other industrial process that face similar challenges including lead refiners, refinery sludge and 

hazardous waste incinerators, and geothermal energy plants. The results achieved on the 

secondary lead smelter suggest the same approach can be applied to these other industrial 

processes.
ADDITIONAL CONTROL REQUIREMENTS FOR HEAVY METLS 

There are several industrial processes that emit hazardous air pollutants (HAPs) which can 

include lead, arsenic, cadmium, and/or mercury. These processes are highly controlled for 

particulate and/or acid gases. Some facilities, however, are being subject to emission limits that 

exceed the capability of existing control equipment. 

Figure 1 provides a summary of some of these processes including the additional removal 

efficiencies that can be required downstream of existing air pollution controls. Most of the 

processes are from combustion sources and use a range of air pollution controls including baghouses, packed bed absorbers, and Venturi scrubbers. In some cases a combination of controls 

are used. Despite existing controls, very low concentrations of heavy metals can be emitted. In 

the case of bag-houses for instance, the operating temperature may be in a range that some of the 

metals are in a gas phase. In such case they will not be collected by particulate control devices. 

In other cases, the concentrations of submicron, condensed phase heavy metals may exceed the 

removal capability of controls like packed bed absorbers or Venturi scrubbers. 

The performances achieved on a secondary lead smelter using a WESP, suggest the approach can 

be used on these other processes to remove residual concentrations of condensed metals. In the 

case of mercury, the ability to remove it with a WESP depends on whether it is in a condensed 

form. This requires reliable speciation data to make that determination. More specifically, the 

mercury must be in a particulate or oxidized form for it to be removed by a WESP. 

Figure 1: Additional control requirements for heavy metals by process type. 

PROCESS Upstream Controls

Compounds

Requiring Polishing

Add‐on

Removal

Efficiency1

Secondary Lead Smelters Bag‐houses,

packed bed absorbers

Lead (Pd)

Arsenic (As)

92% ‐ 98%

Primary Lead Smelters Bag‐houses Arsenic (As) > 85%

Hazardous Waste Incinerators Venturiscrubbers,

packed bed absorbers

Lead (Pd)

Cadmium (Cd)

Mercury (Hg)

80% ‐ 90%

Refinery Sludge Incinerators Venturiscrubbers,

packed bed absorbers

Cadmium (Cd) 98% ‐ 99%

Geothermal Plants Packed bed absorbers Arsenic (As)

Mercury (Hg)

> 90%

1

Refers to additional removal efficiency after the upstream controls. 
WESP SYSTEM DESIGN

A WESP is primarily a particulate removal device. Its main advantage is the ability to collect 

submicron particulate with high removal efficiency. Figure 2 illustrates this capability which 

compares the particle removal efficiency of a WESP with a 40 inch W.C. pressure drop Venturi. 

The curves show that the Venturi removal efficiency drops off dramatically for particles below 1 

micron in size. The WESP, however, has removal efficiencies greater than 90% for most particle 

sizes below 1 micron. The dip in collection efficiency that occurs around 0.4 micron is due to a 

transition in particle collection from inertial to diffusion forces. Because the particle sizes of 

condensed metals are predominately submicron, the WESP is ideally suited for collecting these 

compounds as a polishing step. 

Figure 2: WESP Particle removal efficiency 

Figure 3 shows a schematic for an upflow WESP arrangement. The inlet gas enters the 

conditioning section at the bottom of the WESP. The conditioning section ensures the gas is 

saturated and evenly distributes the flow to the collection section above it. A packed bed 

condenser/absorber (C/A) can be integrated into the conditioning section. This enables the 

WESP to be used as a multi-pollutant device for particulate and acid gas removal. Acid gases 

are removed by a process of absorption and neutralization by metering a caustic solution into the 

packed bed recirculation line. A liquid cooling circuit consisting of a cooling tower and a plateand-frame heat exchanger sub-cools the gas to condense the water vapor in the gas stream. 
Figure 3: Upflow WESP arrangement. 

Sub-cooling provides several advantages. The gas volume is reduced which allows the 

collection section to be smaller and saves on capital and operating cost, and space. Additionally, 

sub-cooling maximizes condensation of gas phase heavy metal compounds prior to entering the 

collection section. This is important because condensed heavy metals can be collected in the 

WESP. This is illustrated by the vapor pressure curve for arsenic trioxide in Figure 4. At a subcooled temperature of 100°F, the vapor phase As2O3 concentration is negligible at 0.0001 ppmv. 

This means any As2O3 present in the gas will be in a condensed form that can be collected by the 

WESP. At a higher temperature of 300°F, there can be as much as 29.8 ppmv in the gas phase 

which will pass through a bag-house. Lastly, sub-cooling eliminates the water vapor plume 

under favorable climatic conditions. This helps to lower the profile of the plant within the 

surrounding community. 
Figure 4: Arsenic trioxide vapor pressure curve. 

0.0001

0.001

0.01

0.1

1

10

100

0 50 100 150 200 250 300 350

Vapor Concentration, ppmv

Temperature,°F

As2O3Vapor PressureCurve

As2O3

After passing through the conditioning section, the gas enters the collection section for 

particulate collection. The collection section is an array of hexagonal shaped tubes with an 

electrode protruding in the center of each tube. Figure 5 shows a picture of a hexagonal 

collection tube array and a WESP collection tube illustration. 

Particulate collection entails three steps which are depicted in the collection tube illustration. 

First, particles are given a strong negative charge by an ionizing corona produced by a high 

voltage electrode. Second, the electric field between the electrode and the tube wall causes the 

charged particles to migrate to the grounded tube wall where they accumulate. Finally, a falling 

water film inside the tube removes the collected material to a discharge drain. The falling water 

film is created by a periodic flush that occurs above the collection section. 
Figure 5: WESP Collection Tube Section 

Picture of a hexagonal, cross-secitonal array 

looking from bottom to top. 

Collection Tube Illustration 

The final section of the WESP is the outlet section. The outlet section supports the high voltage 

grid and houses the wash header to provide the wet film for particulate removal. It includes an 

air purge system to keep the high voltage insulators dry. As the gas exits the outlet section, it 

passes through an entrainment separator for water droplet removal. 

WESP PERFORMANCE 

The size of the WESP is a function of the desired removal efficiency and the inlet volumetric 

flow rate. The specific collection area or SCA is the active collection tube surface area per 1,000 

cubic feet per minute of gas. It is a metric used to size a WESP to meet a given performance 

requirement. Figure 6 shows the WESP removal efficiency for various specific collection areas 

(SCAs) represented by the letters A through E. The curve illustrates that more collection area 

(higher SCA) is required to achieve greater removal efficiency. 
Figure 6: Predicted WESP Efficiency 

STACK TEST RESULTS 

Table 2 compares the stack test results to the performance guarantee for a secondary lead smelter 

WESP system. The removal efficiencies are calculated from WESP inlet and outlet testing. The 

results confirm the system is above the performance guarantee for both compounds and achieved 

greater than 92% removal. 

Table 2: System Performance 

Compound 

Approximate 

Inlet, mg/dscm1

Guarantee 

Efficiency, % 

Performance Test 

Efficiency, % 

Arsenic a x 10-02 92% 98.7% 

Lead b x 10-02 92% 92.4% 

1

Indicates order of magnitude for actual value. 

CONCLUSION 

Several industrial processes including lead refiners, secondary lead smelters, refinery sludge 

incinerators, hazardous waste incinerators, and geothermal energy plants face challenges to meet 

more stringent emission standards for heavy metals. In many cases, new standards exceed the 

capability of existing air pollution control equipment including bag-houses, packed bed 

absorbers, and Venturi scrubbers. 

A control strategy using a wet electrostatic precipitator (WESP) in conjunction with sub-cooling 

was used on a secondary lead smelter to reduce the plant’s cancer risk index. This approach 

successfully removed greater than 92% of various condensed metals from very low inlet 

concentrations in the range of 0.002 to 0.2 mg/dscm. Stack test results indicate greater than 98% 

removal of arsenic and greater than 92% removal of lead was achieved. 
The results confirm that a WESP can be an effective control device to remove condensed heavy 

metals with low inlet concentrations. This approach can be used on similar industrial processes 

facing the same challenge. 

REFERENCES 

1. Crawford, M. Air Pollution Control Theory, 1976. 

2. Calvert, S.; Englund, M. Handbook of Air Pollution Technology, 1984 

KEY WORDS

Wet electrostatic precipitator, electrostatic precipitator, WESP, hazardous air pollutants, HAPs, 

heavy metals, geothermal plants, secondary lead smelters, refinery sludge incinerators, hazardous 

waste incinerators, primary lead smelters.