UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

Office of Air Quality Planning and Standards

Research Triangle Park, NC 27711

July 10, 2002 

Mr. Edward R. Herbert III

Director of Environmental Affairs

National Ready Mixed Concrete Association

900 Spring Street

Silver Spring, MD 20910

Dear Mr. Herbert:

Your April 30, 2002, letter requests a review from the Environmental Protection 

Agency (EPA) regarding the inclusion of control devices on pneumatically loaded 

cement silos in the “potential to emit” calculations for ready mixed concrete plants. We 

agree with your assessment that, for potential to emit calculations, the control devices on 

the silos generally should be considered as an inherent part of the process for loading 

ready mixed cement silos. 

Criteria for Determining Whether Equipment is Air Pollution Control Equipment or 

Process Equipment 

For purposes of calculating a source’s potential to emit, it is necessary to consider 

the effect of air pollution control equipment. Current EPA regulations and policy allow 

air pollution control equipment to be taken into account if enforceable requirements are 

in place requiring the use of such air pollution control equipment. There are, however, 

situations for which case-by-case assessments are needed regarding whether a given 

device or strategy should be considered as air pollution control equipment, or as an 

inherent part of the process. The EPA believes that the following list of questions should 

be considered in assessing whether certain devices or practices should be treated as 

pollution controls or as inherent to the process: 

1. Is the primary purpose of the equipment to control air pollution? 

2. Where the equipment is recovering product, how do the cost savings from the 

product recovery compare to the cost of the equipment? 

3. Would the equipment be installed if no air quality regulations are in place? 

If the answers to these questions suggest that equipment should be considered as 

an inherent part of the process, then the effect of the equipment or practices can be taken 
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into account in calculating potential emissions regardless of whether enforceable 

imitations are in effect. 

Analysis of the criteria for control devices on pneumatically loaded cement silos 

The equipment used for pneumatic loading is commonly referred to as bag 

houses or dust collectors. Based on the information supplied to date by you, the EPA 

believes that, overall, the above criteria are satisfied as follows: 

Criteria 1. The primary purpose of the control devices on pneumatically loaded cement 

silos is not to control air pollution but to provide a restricted air flow from the silo so that 

the silo will fill properly without excessive loss of product. 

Criteria 2. The cement collected by the filters falls into the silo and is recovered for use 

as product. The cost savings from this product recovery varies depending on such factors 

as silo capacity, amount of product in the silo, and the efficiency and cost of the control 

device. 

Criteria 3. The information you have provided suggests strongly that air quality 

regulations are not the driving factor for installation of the control equipment. The 

control devices would be installed regardless of air quality requirements. 

Cautions 

The views expressed above regarding the use of the control devices for loading 

cement silos are specific for ready mixed concrete facilities using pneumatic loading. 

While we believe the views in this letter are applicable for the majority of ready mixed 

concrete facilities with pneumatic loading, there may be circumstances that would need 

to be considered on a case-by-case basis. For example, there may be situations where air 

pollution control regulations or a company’s desire to limit its potential to emit for 

regulatory purposes result in the company’s installation or use of bag houses with a 

greater collection efficiency than would be the case if product recovery or other process 

considerations were the only factors at work. Should such circumstances arise, source 

owners and operators are encouraged to work with their permitting authorities if they 

have questions. 

This letter is not intended to set a precedent for control equipment for other 

source types, which must be reviewed separately. This letter also does not assess the 

control efficiency or emissions from the baghouses. Also, this determination does not 

exempt these sources from otherwise applicable permitting or other regulatory 

requirements. These requirements are determined by the appropriate permitting 

authority. 
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If you have any further questions regarding this matter, please call me at (919) 

541-4718, or Mike Sewell at (919) 541- 0873. 

Sincerely,

original signed by Robert Kellam for

William T. Harnett

Director, Information Transfer and Program

Integration Division 

cc: Regional Air Division Directors 

Mario Jorquera, OECA 

Greg Foote, OGC 

Karen Blanchard, IIG 

Steve Hitte, OPG 

Kirt Cox, OPG 

Mike Sewell, IIG