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.

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