TM 5-814-8
efficiency of the plant. Bypassing of the entire
mings are combined with sludge for disposal.
wastewater treatment plant through an emer-
Removals from domestic wastewaters undergoing
gency overflow structure during periods of ex-
plain sedimentation will range from about 30 to
traordinarily high flow must be provided. In all
40 percent for BOD and in the range of 40 to 70
cases, this diverted flow shall be disinfected and
percent for suspended solids. With optimum de-
screened, and the quantity of flow measured and
sign conditions for sedimentation, BOD and sus-
recorded. The appropriate regulatory agency shall
pended solids removal efficiency is dependent
be notified of every bypass occurrence. When the
upon wastewater characteristics and the propor-
wastewater is discharged to a waterway which
tion of organics present in the solids. One of the
could be permanently or unacceptably damaged
most important design parameters if the overflow
by the quantity of bypassed wastewater, such as
rate, usually expressed in gal/day/sq ft, which is
shellfish waters, drinking water reservoirs, or
equal to the flow in gal/day divided by the
areas used for water contact sports, provision
settling surface area of the basin in square feet.
shall be made to intercept the bypassed flow in a
Usually average daily flow rates are used for
holding basin. The intercepted flow shall then be
sizing facilities. The flow rates, detention time,
routed back through the treatment facility as
and other factors which shall be employed for
soon as possible. Bypasses for diversion of flow
design purposes are documented in TM 5-814-3.
around treatment plants will be locked in a closed
(a) Secondary treatment sedimentation fa-
position. The bypass must be controlled by super-
cilities. It should be recognized that design princi-
visory personnel.
ples of secondary sedimentation tanks are signifi-
c. Primary treatment. Primary treatment for
cantly different than those for primary tanks, the
fundamental difference being in the amount and
the purposes of this manual will be limited to
nature of solids to be removed. Primary sedimen-
sedimentation with and without chemical addi-
tion. Other unit processes are usually combined
tation facilities are basically designed on overflow
rate alone; secondary units must be designed for
with sedimentation as a part of "primary treat-
ment", including some degree of preliminary
solids loading as well as overflow rate. Reference
treatment, sludge treatment and disposal, and
should be made to TM 58 143 for design
chlorination as a disinfection step. For many
criteria.
(b) High-rate settlers. In recent years, the
years, water quality criteria specified only the use
of primary treatment for domestic wastewaters.
development of high-rate settlers has proven
Primary treatment is no longer acceptable as the
quite promising for both primary and secondary
sedimentation applications. These have been used
total wastewater treatment step prior to dis-
primarily to improve performance and to increase
charge to a receiving body of water and second-
ary treatment must now be employed to meet
treatment capacity of existing plants and should
regulatory criteria. Therefore, the discussion pre-
receive attention for upgrading military facilities.
The theory is that sedimentation basin perfor-
sented herein on primary treatment shall be
mance can be improved by introducing a number
utilized by military personnel concerned with:
--Alternatives that must be considered for
of trays or tubes in existing facilities, since
existing treatment facilities which are to be
efficiency is independent of depth and detention
upgraded to meet effluent limitations and
time. Until recent years, use of trays or tubes
water quality criteria.
was unsuitable on a practical basis because of
--Design factors and alternatives that must
difficult sludge collection and removal. These
be considered when planning a new
problems have been largely overcome although
wastewater treatment facility.
slime growths may cause flow restrictions and
(1) Plain sedimentation. Wastewater, after
require periodic cleaning. The principal advantage
preliminary treatment, undergoes sedimentation
of the settlers is their compactness which reduces
material costs and land requirements. For most
by gravity in a basin or tank sized to produce
near quiescent conditions. In this facility, settle-
military installations, the land savings is not
able solids and most suspended solids settle to
critical but cost reductions will be important.
the bottom of the basin. Mechanical collectors
Settlers do not improve the efficiency of primary
should be provided to continuously sweep the
sedimentation facilities that are already achieving
sludge to a sump where it is removed for further
reasonably high removals of suspended solids.
treatment and disposal. Skimming equipment
Available data indicate that where the settlers
have been installed in existing units, it has been
should be provided to remove those floatable
substances such as scum, oils, and greases which
possible to increase the surface overflow rate of
accumulate at the liquid surface. These skim-
both primary and final sedimentation systems
6-5
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