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the land. For most FOTWs, however, 40 CFR Part 503 is the
applicable regulation.
For additional information on the Part 503 regulations,
refer to EPA/G25/R-92-013, Environmental Regulations and
Technology: Control of Pathogens and Vector Attraction in Sludge
(Including Domestic Septage) Under 40 CFR Part 503.
2.4.3.1
Solids Definitions. The Part 503 regulations
promulgated the word "sludge" to describe a variety of solids
residuals from wastewater treatment processes. The wastewater
treatment industry and regulatory agencies have recently tried to
minimize the use of the word "sludge" because the term is too
general and its negative connotations do not accurately reflect
the industry's goal: to promote the beneficial reuse of properly
treated wastewater solids as useful soil amendments for
agricultural users and the general population. In keeping with
current industry practices, this document avoids the word
"sludge" except when directly referred to in Part 503 regulations
or a widely accepted process name such as the "activated sludge
process." Figure 1 shows a secondary wastewater treatment plant
and identifies the terminology used by industry and this document
to replace the word "sludge."
The primary solids referred to in Figure 1 are those
derived from primary treatment processes. Solids drawn from the
secondary treatment system are referred to as "waste activated
sludge" or "biosolids." The word "biosolids" refers to the
residual treatment bacteria and inert solids contained in the
biological treatment process. Solids that have undergone
treatment for beneficial reuse are generally referred to as
"residual solids" or can be classified according to their level
of treatment, such as "Class A Solids." In some cases, treatment
facilities do not further treat primary or secondary solids and
dispose of these in a permitted landfill; in this case, the
residuals are referred to as "sludge," meaning the product has
not received treatment to reduce pathogens or vector attraction.
The phrase "other residual solids" refers to the dense, grit-like
solids that accumulate in process tanks and are removed when the
tanks are periodically emptied and cleaned.
2.4.3.2
Protection of Public Health and the Environment.
In the judgment of the Administrator of EPA, Part 503 protects
public health and the environment through requirements designed
to reduce the potential for contact with the disease-bearing
microorganisms (pathogens) and heavy metals in biosolids applied
to the land or placed on a surface disposal site. These
requirements are divided into the following categories:
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