TM 5-814-8
from 2 to 5 times the conventional rate while still
and remove the nonsettleable colloidal solids and
maintaining about the same suspended solids
to stabilize the organic matter. There are many
effluent level. Manufacturer's bulletins and U.S.
alternative systems in use and each uses biologi-
EPA Technology Transfer series documents pro-
cal activity in different manners to accomplish
vide data on design criteria.
treatment. Biological processes are classified by
(2) Sedimentation with chemical coagulation.
the oxygen dependence of the primary microor-
Sedimentation using chemical coagulation has
ganism responsible for waste treatment (125). In
been implied mainly to pretreatment of industrial
aerobic processes, waste is stabilized by aerobic
and facultative microorganisms; in anaerobic pro-
or process wastewaters and removal of phospho-
rus from domestic wastewaters. Chemical usage
cesses, anaerobic and facultative microorganisms
as a pretreatment step for industrial wastes and
are present. The discussion of biological treat-
phosphorus removal is discussed later. The use of
ment processes has been further divided into the
following two categories:
chemical coagulating agents to enhance the re-
moval of BOD and suspended solids has not been
--Suspended growth processes.
used extensively on domestic wastewaters, since
--Fixed growth processes.
(1) Suspended growth processes refer to
it is not usually economical or operationally
desirable. However, special applications may exist
treatment systems where microorganisms and
at some installations. Advantages of increased
wastewaters are contained in a reactor. Oxygen is
introduced to the reactor allowing the bilogical
solids separation in primary sedimentation facili-
ties are:
activity to take place. Examples of suspended
A decrease in organic loading to second-
growth processes include ponds, lagoons and
ary treatment process units.
activated sludge systems.
A decrease in quantity of secondary
(2) Fixed growth processes refer to systems
sludge produced.
where a biological mass is allowed to grow on a
medium. Wastewater is sprayed on the medium
An increase in quantity of primary sludge
produced which can be thickened and
or put into contact in other manners. The biologi-
dewatered more readily than secondary
cal mass stabilizes the wastewater as it passes
sludge.
over it. Examples of fixed growth processes
Chemicals commonly used, either singularly or in
include trickling filters and rotating biological
combination, are the salts of iron and aluminum,
contractors.
lime, and synthetic organic polyelectrolytes. It is
b. Suspended growth processes.
desirable to run jar studies to determine the
(1) Ponds. Ponds have found wide-spread us-
optimal chemicals and dosage levels. The use of a
age in the U.S. In 1968, 34.7 percent of the
nearly 10,000 secondary treatment systems oper-
given chemical(s) and effluent quality must be
ating in the U.S. were in the category of stabiliza-
carefully balanced against the amount of addi-
tional sludge produced in the sedimentation facil-
tion ponds (49). Waste treatment ponds can be
ity. Design information and guidance is contained
divided into three general classifications: aerobic
in the U.S. EPA Technology Transfer series
ponds, aerobic-anaerobic (facultative) ponds, and
documents.
anaerobic ponds. Ponds are sized on an average
(3) Other methods. F o r some industrial
BOD loading or detention time basis and are
wastes which contain large amounts of floatable
quite sensitive to climate and seasonal variations.
and finely suspended matter, flotation may be
(a) Aerobic ponds. Photosynthetic ponds
used in lieu of sedimentation as a cost-effective
are 6 to 18 inches deep with BOD loadings
means of primary treatment. Some wastewater
ranging from 100 to 200 lb per acre per day and
treatment alternatives, including ponds and ex-
detention times of 2 to 6 days. These are usually
tended aeration, do not require primary treatment
mixed intermittently, generally by mechanical
as a distinct process step. Other secondary treat-
means, to maximize light penetration and algae
ment processes could operate without primary
production. A very high percent of the original
treatment but it is cost-effective to remove the
influent BOD is removed, but due to algae
suspended organics physically rather than biologi-
growth and release to the effluent, overall remov-
cally.
als are in the 80 to 95 percent range. Suspended
solids in the effluent are also mainly due to algae.
6-2. Biological
Wastewater
Treatment
Lower efficiencies occur during warmer periods of
Processes
the year due to algal growths, and during ex-
tremely cold periods due to decreased biological
a. Introduction. Biological treatment processes
are those that use microorganisms to coagulate
activity and freezing. Aerated aerobic ponds uti-
6-6
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