TM 5-814-8
treatment or as a polishing step after biological
ranged from about 50 percent to as high as 90
treatment. It is particularly applicable for re-
percent. Maintenance of the units can be costly,
moval of the weaker chemical floe particles while
since they require periodic cleaning. For further
surface straining devices such as rapid-sand fil-
information, the U.S. EPA "Process Design Man-
ters and microstrainers work well with the stron-
ual for Suspended Solids Removal", and "Process
ger biological floes. Use of the filters for the dual
Design Manual for Upgrading Wastewater Treat-
purpose of solids removal and as a fixed media
ment Plants".
for denitrification should also be considered where
(4) Filtration. Secondary effluents normally
both processes are necessary. A summary of
contain minerals which range from the easily
information on effluent suspended solids to be
visible insoluble solids to colloids. Filtration is
expected from a multi-media filtration system is
one means of removing the suspended solids (and
indicated in table 6-5.
the BOD associated with the suspended solids)
remaining after secondary sedimentation to a
Table 6-5. Expected effluent suspended solids
level which will meet effluent or water quality
from multi-media filtration of secondary effluent*
criteria. Filtration methods most applicable to
Effluent Suspended
military facilities are the multimedia filter and
Solids, mg/L
Effluent Type
the diatomaceous earth system. For information
10-20
High-Rate Trickling Filter
on design criteria and operating considerations,
6-15
Two-Stage Trickling Filter
6-15
Contact Stabilization
the U.S. EPA Process Design Manual for Sus-
3-10
Conventional Activated Sludge
pended Solids Removal should be consulted.
1--5
Extended Aeration
(a) Multi-media. Recently, dual-media,
mixed-media and multi-media filtration units
*Adapted from the U.S. EPA "Process Design Manual for
have basically replaced the conventional single
Suspended Solids Removal".
medium filter otherwise known as the "rapid-sand
(b) Diatomaceous earth. Filtration by
filter" for wastewater applications. These filters,
diatomaceous earth consists of mechanically sepa-
widely utilized in advanced wastewater treatment,
rating suspended solids from the wastewater
are sometimes referred to as "deep-bed" filters.
influent by means of a layer of powdered filter aid
Single medium filters have a fine-to-coarse grada-
or diatomaceous earth, applied to a support
tion in the direction of flow which results from
medium. The use of the system for clarification of
hydraulic gradation during backwash. This grada-
domestic secondary treatment effluent has been
tion is not efficient, since virtually all solids
demonstrated only at pilot scale facilities. Multi-
removal must take place in the upper few inches
media filters are more costeffective for domestic
of the filter with a consequent rapid increase in
wastewaters from military installations. However,
headloss. A coarse-to-fine gradation, as used by
the diatomaceous earth system is applicable and
multi-media units, is more efficient since it pro-
currently being used as part of a treatment step
vides for greater utilization of filter depth, and
in munitions wastewater treatment.
uses the fine media only to remove the finer
e. Membrane processes. Other feasible methods
fraction of suspended solids. The multi-media
of advanced wastewater treatment consist of
filter is capable of producing effluents with sus-
what are generally known as the membrane
pended solids of less than 10 mg/L from typical
processes, and include electrodialysis, ultrafil-
feed concentrations of 20 to 50 mg/L. This also
tration and reverse osmosis. These processes can
reduces the BOD since about one-half of the
remove over 90 percent of the dissolved inorganic
BOD of a secondary effluent is normally associ-
material to produce a high quality product suit-
ated with the suspended solids. The feed concen-
able for discharge or reuse. Considerable pretreat-
tration must be kept below 100 mg/L of sus-
ment is required. Use of these membrane pro-
pended solids for practical backwash cycles. A
cesses in the field of wastewater treatment is at
typical multi-media system consists of three or
the present time limited because the costs are
more materials, normally anthracite (coal), sand
very high and applications will be to small flows
and garnet, with carefully selected specific gravi-
at best. For example, a possible application is the
ties. Dual-media filters usually utilize anthracite
treatment for reuse of small process discharges at
and sand. The filtering system is supported by a
military field installations. Three different reverse
few feet of gravel or other support means. Addi-
osmosis units were evaluated at a field location
tion of small amounts of coagulant chemicals
b y the U.S. Army Environmental Hygiene
Agency (1). This study was initiated to determine
such as alum or polymer enhances filtration.
the feasibility of treating and reusing wastewater
Multi-media filtration is a process normally asso-
from field laundries, showers and kitchens. Where
ciated either with physical-chemical wastewater
Previous Page
