settle are called colloidal particles. Colloidal
particles are often removed in the biological treat-
ment units. They may also be removed by
chemical treatment followed by sedimentation.
All the solids discussed above may be either
organic or inorganic. Organic solids always con-
tain carbon and hydrogen and when ignited to
high temperatures (500°C to 600°C) burn to form
carbon dioxide, water, and sometimes various
other compounds. The burning or volatilization
of organic solids has led to the term volatile solids.
All solids that burn or evaporate at 500°C to
600°C are called volatile solids. These solids serve
as a food source for bacteria and other living
forms in a wastewater treatment plant. Most
organic solids in municipal waste originate from
living plants or animals.
Those solids that do not burn or evaporate at
500°C to 600°C, but remain as a residue, are
called fixed solids. Fixed solids are usually
inorganic in nature and may be composed of grit,
clay, salts, and metals. Most inorganic solids are
from nonliving sources. Table 10-4 summarizes
the types and amounts of the solids discussed in
the preceding paragraphs.
The chemical characteristics of wastewater of
special concern to the Utilitiesman are pH, DO
(dissolved oxygen), oxygen demand, nutrients,
and toxic substances.
The term pH is used to describe the acid or
base properties of water solutions. A scale from
Table 10-4.-Solids of a Typical Domestic Wastewater