Environmental Engineering Reference
In-Depth Information
7
ChaPTer
seCondary
TreaTMenT
7.1 inTroduCTion
The main purpose of
secondary treatment
(sometimes referred to
as
biological treatment
) is to provide biochemical oxygen demand (BOD)
removal beyond what is achievable by primary treatment. Three com-
monly used approaches all take advantage of the ability of microorgan-
isms to convert organic wastes (via biological treatment) into stabilized,
low-energy compounds. Two of these approaches, the
trickling ilter
or
its variation, the
rotating biological contactor
(
rBC
), and the
activated
sludge process
, sequentially follow normal primary treatment. The
third approach,
ponds
(oxidation ponds or lagoons), however, can pro-
vide equivalent results without preliminary treatment. In this chapter,
we present a brief overview of the secondary treatment process followed
by a detailed discussion of wastewater treatment ponds (used primar-
ily in smaller treatment plants), trickling filters, and RBCs. In the next
chapter, we shift focus to the activated sludge process, the secondary
treatment process that is used primarily in large installations and is the
main focus of this handbook.
Secondary treatment
refers to those treatment processes that
use biological processes to convert dissolved, suspended, and colloidal
organic wastes to more stable solids that can be either removed by set-
tling or discharged to the environment without causing harm. Exactly
what is secondary treatment? As defined by the Clean Water Act (CWA),
secondary treatment produces an effluent with not more than 30 mg/L
BOD
5
and 30 mg/L total suspended solids.
