Like the term “solid waste”, “sludge” when used by the EPA can be confusing.  It can be found in 40 CFR 261.2 as part of EPA’s description of what wastes may and may not be a solid waste (more on that below) .  The reason for this confusion is that the EPA definition of a sludge differs from the common English definition:

EPA definition of sludge from 40 CFR 260.10:  Sludge means any solid, semi-solid, or liquid waste generated from a municipal, commercial, or industrial wastewater treatment plant, water supply treatment plant, or air pollution control facility exclusive of the treated effluent from a wastewater treatment plant.

Whereas the standard English definition:  Thick, soft, wet mud or a similar viscous mixture of liquid and solid components, esp. the product of an industrial or refining process.

As used by the EPA, sludge identifies a waste generated from one of several specific processes (in this case the removal of contaminants from air or water) and not the characteristics of the waste.  In addition, note the following about the EPA definition of sludge:

• May be a solid, semi-solid, or liquid.
• Generator of waste must be municipal, commercial or industrial entity.
• May be generated as a result of wastewater treatment, treatment of a water supply, or an air pollution control device.
• It does not include the treated effluent (or discharge) from a wastewater treatment plant.

A sludge as defined by EPA may include any of the following:

The material captured by this dust collector is a sludge and therefore a solid waste.

• Zinc oxide dust (RO11275)
• Ion exchange resin (Faxback 14017)
• Filters & baghouse dust (RO11933)
• Electroplating sludges (RO11338)
• Air scrubber waste (RO13742)
• Chemical recovery cartridges (RO11210)

In short, no matter its physical state (ie. liquid, solid, semi-solid, dust, manufactured article) if a waste is generated as a result of a process to remove contaminants from air or water, it is likely a sludge as defined by EPA.

Why do sludges matter?  The knowledge of what is and isn’t a sludge is necessary to understand the exemptions from the definition of solid waste (another misleading term) allowed by EPA at 40 CFR 261.2.  A full explanation of these exemptions will have to wait for a future article, but suffice to say that some sludges (characteristic hazardous wastes) are not solid wastes if they are reclaimed whereas others (listed hazardous waste) are solid wastes when reclaimed.