Clearing the air, consistently

Operators of auto shredding plants can benefit from a nationwide framework on how to measure and suppress regulated emissions.

material handler feeding auto shredder

Photo by Vicki Blayney

Photos by Vicki Blayney

Air emissions from the auto shredding industry have come under increasing scrutiny from the U.S. Environmental Protection Agency (EPA), as well as from state and local environmental regulatory authorities. Currently, no nationally applicable standards are designed with the auto shredding industry in mind. Consequently, efforts to quantify air pollution emissions associated with shredding operations and associated control requirements have been inconsistent.

No one wants to be the target of additional regulation, but the EPA is continuing to explore the auto shredding industry and is targeting individual shredders, one-by-one, for investigation, enforcement and control requirements. More regulations are surely on the way, and the auto shredding industry is best served if it exercises its rights as a stakeholder in the rule-making process.

Regulation is pending, in other words; but, without stakeholder involvement, it is likely that many of the complexities of industry processes will not be considered. This article provides considerations and a framework for what beneficial regulation and controls could look like and how they could be fairly applied to shredders of all sizes, as well as recognize the immense environmental and economic contributions this industry provides.

Why shred?

Shredding of end-of-life vehicles and other metallic scrap is an environmentally efficient and economically important form of recycling.

The recycling industry ranks 16th in the nation by earnings, contributing about $25 billion per year to America’s gross domestic product and employing more than 100,000 people. Almost half of recycled steel processed in the U.S. originates at auto shredders. As the industry has evolved, separation techniques have been developed that enable shredders to recover increasing amounts of nonferrous metals that meet high purity standards.

The shredding process generates some air pollutions emissions. The majority of these emissions are associated with hammer mill activity. The air pollutants of most importance released from the hammer mill are volatile organic compounds (VOCs) and, to a lesser extent, particulate matter (PM). While it is true that shredders emit these pollutants, any discussion of regulation should begin with recognition of two important facts about the shredding sector. First, it is, by some measures, the largest form of recycling in the U.S. About 14 million tons of steel are recovered by shredders every year. Second, shredding’s environmental impact is small relative to the initial process of manufacturing metals from mined materials.

The U.S. is home to more than 300 shredding operations. Many of these shredding operations are small, consisting of shredders that process less than 50 tons per hour (tph). At the other end of the spectrum are “super-sized” shredders capable of processing more than 300 tph.

Super-sized shredders typically are near large metropolitan areas where feedstock supplies are plentiful. Intermediate-sized shredders, processing feedstock in the range of about 100 to 200 tph, also typically are found near metropolitan areas. Economies of scale also come into play in the shredding industry. A 400-tph super-sized shredder can process scrap more economically and with greater energy efficiency than eight 50-tph shredders.

Finally, the metals recycling industry has a substantial effect on America’s energy usage. It is estimated the industry saves the equivalent of 85 million barrels of oil in energy consumption each year.

Something in the air

It is only relatively recently that auto shredding operations have been considered sources of VOC emissions. Auto shredding began to attract the attention of the EPA and state agencies several years ago. Two reasons for this are local siting issues and shifting EPA priorities.

Ideally, each shredder yard would be in a remote or heavily industrial part of its municipality. However, this is not always the case. As neighborhoods shift and demographics change, it was inevitable that new residential districts would be developed near some shredders that had previously been isolated.

When a scrap yard attracts the attention of neighbors, events generally follow a pattern. Concerned neighbors band together to form a citizens group. Formation of such a group comes to the attention of local media outlets. The publicity and direct outreach from voters brings elected officials into the picture, who turn to EPA or their state agency (or both) as the regulatory body with the authority and expertise to attempt to help with perceived issues.

The EPA sent out (and likely will continue to send out) official Requests for Information to shredder operators. The EPA (and state and local environmental authorities) are trying to address key questions:

  1. how to identify and quantify air pollutant emissions from shredders;
  2. how to regulate shredder operations; and
  3. how to determine what add-on controls should be required of shredders if deemed necessary.

The EPA’s approach during its ongoing investigation of shredder emissions has been to treat each shredder as a unique emissions source. Testing has been performed on several shredders, and the data subsequently were used to develop emissions factors that apply to each shredder.

For example, imagine a 200-tph shredder is tested, and an emissions factor of “n” pounds of VOC per ton of feed is developed based on the data gathered. Calculations using this emissions factor show that the shredder does not emit enough VOC to require a construction permit or installation of add-on controls.

Five years later, a second test series is conducted. Emissions from a shredder are a function of the type and condition of feedstock at any given point in time. If the feedstock used during the second test series contains double the VOCs of the feedstock used during the first test series, a new emissions factor likely will be higher than in the previous test.

The increase in emission rates could lead to the existing plant needing to obtain a permit or to add pollution controls to keep VOCs below the level originally predicted. Based on similar cases in other industries, EPA (or the state authority) could issue a violation notice to the owner of the shredder, claiming the shredder was in violation of applicable regulations.

The more you know

EPA has focused on a number of air pollutant categories:

  • VOCs, with emission rates that can vary based on the test methods used and the feedstock being processed on the test day;
  • PM, with some shredding activities producing insignificant amounts;
  • organic hazardous air pollutants (organic HAPs), which are also regulated as HAPs, and include individual VOCs that can have additional regulatory applicability under the Clean Air Act in light of their unique level of toxicity; and
  • inorganic hazardous air pollutants (inorganic HAPs) can contain certain pollutants that are classified as HAPs because of their unique level of toxicity.

Efforts made to work with EPA on regulatory development should consider these pollutants and how they vary by plant, process and material shredded.

When controls are required, control regimens typically will consist of up to four distinct operations:

  • exhaust fume collection – The collection of fumes from the mill involves some type of enclosure that encompasses the mill. The most cost-effective option will enclose the mill, portions of the infeed system and the under-mill oscillator on the outflow side of the mill. More expensive options would enclose the mill and most or all downstream conveyance and separation equipment under one roof.
  • PM removal – Regulated PM is considered to be less than 75 microns in diameter and is small enough to be suspendable in the air for long distances. In the case of shredders, processing produces little or no regulated PM. However, larger particles of solids can be in the exhaust stream that can damage downstream control equipment. These larger particles must be removed. Downstream fabric or paper filters are sometimes used as a polishing step.
  • VOC/organic HAP removal – If VOC and organic HAPs reach levels that require controls, thermal oxidation (combustion) is a control strategy. The most efficient form of thermal oxidizer is a regenerative thermal oxidizer (RTO), which has become the preferred method for VOC/organic HAP control at some facilities. An additional consideration for RTOs is the construction material, as the combination of the materials in the exhaust stream, moisture and metal ductwork/stacks can, in some cases, be a significant corrosion concern. The cost of the RTO can be high because of the additional cost of advanced, corrosion-resistant materials. RTOs also can require safety interlocks that automatically shut down the RTO if the incoming air stream exceeds certain setpoints for incoming combustible gases that could damage the process.
  • acid gas control – If acid gases are formed in the process and emitted in quantities that exceed applicable state or federal thresholds, additional prevention or control might need to be considered. Potential solutions include dry scrubbing and wet scrubbing. Wet scrubbers have proven to be effective, but they require a high volume of water, which can be a challenge in some areas of the country.

Despite the important role shredding plays in the recycling process, of concern is the level to which regulators and the general community understand the industry. As shredding is likely to be pushed toward industry-specific regulation, this article seeks to provide a summary of the key information collected and can help focus stakeholders in developing a regulatory framework that can be sufficient from an environmental perspective and workable from an industry perspective.

The author is a Nebraska-based environmental, health and safety consultant with Trinity Consultants who can be contacted via email at or through

April 2021
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