TM 5-814-8
weeds. A study (154) indicated that of 21 differ-
lize oxygen mixed with the wastewater either
ent pond installations studied, none would consis-
from diffused air or mechanical means, with
tently meet the secondary treatment effluent
photosynthetic oxygen generation not playing a
requirement of 30 mg/L BOD. Similarly, of 15
major role in the process. These ponds are 6 to 20
installations reporting effluent suspended solids
feet deep with BOD loadings ranging from 100 to
values, none would consistently meet the 30 mg/L
300 lb per acre per day and detention times of 2
effluent limit. New wastewater treatment pond
to 7 days. BOD and suspended solids removals in
designs and existing installations being upgraded
the range of 80 to 95 percent are obtained if a
must recognize and provide methods which will
quiescent cell is provided to effect solids removal
achieve required effluent levels. Definitive design
after aeration. Aerated aerobic ponds may be
criteria for all situations are beyond the scope of
considered for military applications where flow is
this manual. EPA Technology Transfer series
variable or land is precious. Without the aerators
documents and similar publications should be
operating, the system might function as an
consulted when planning a new wastewater treat-
aerobic-anaerobic (facultative) pond during low
ment pond facility or when assessing alternatives
loads.
for upgrading an existing pond system. Locally
(b) Aerobic-anaerobic (facultative) ponds.
applicable design criteria considering the effect of
These ponds consist of three zones: a surface
climate should be used when planning new or
zone of algae and aerobic bacteria in a symbiotic
upgrading existing facilities. Wide variations in
association; an intermediate zone populated with
criteria are followed in the U.S. in terms of
facultative bacteria (aerobic or anaerobic); and an
loading rates, detention times, depths and num-
anaerobic bottom zone where settled organic sol-
ber of cells required. While most States in the
ids are decomposed by anaerobic bacteria. The
midwest relate to a BOD design loading criteria
ponds, operated in natural aeration mode, are 3 to
in pounds BOD per acre per day, the principal
8 feet deep with BOD loadings ranging from 10
design factor in northern states is retention time,
to 100 lb per acre per day and detention time of
10 days to 1 year. BOD removals of 80 to 95
primarily because of the extreme winter tempera-
tures. In terms of organic loading, pounds of
percent are obtained with proper operation and
BOD per acre per day, State design criteria range
loadings, but suspended solids removals vary
from less than 20 in the northern states to as
because of algal carryover. These ponds may also
high as 75 in the southern, southwestern or
be partially mixed using mechanical or diffused
western states, reflecting temperature effects on
aerators to supply some oxygen. Mechanically
performance.
mixed ponds normally have BOD loadings rang-
(2) Activated sludge. Activated sludge is an
ing from 30 to 100 lb per acre per day; detention
efficient process capable of meeting secondary
times of 7 to 20 days; operational depths of 3 to
treatment effluent limits. In recent years, this
8 feet; and, BOD removals of 90 to 95 percent.
process has undergone significant changes and
(c) Anaerobic ponds. These ponds have
improvements from the conventional activated
BOD loadings in the range of 10 to 700 lb per
sludge process. For further information on the
acre per day and can provide removals of 50 to 80
process itself or its modifications, reference
percent. Detention times range from 30 days to 6
should be made to TM 5-814-3. The principal
months and operational depths range from 8 to
factors which control the design and operation of
15 feet. Anaerobic ponds have been used princi-
activated sludge processes are:
pally in industrial waste applications and particu-
--Detention time.
larly in meat packing wastes. Due to the nature
--BOD volumetric loading.
of the pond environment, these treatment units
--Food to microorganism (F/M) ratio.
generally produce severely offensive odors. They
--Sludge age or solids retention time (SRT).
are normally not used by themselves and in order
While all of these parameters have been used to
to produce a higher quality effluent, must be
size facilities, the most commonly used are the
followed by an aerobic pond. Anaerobic ponds
F/M ratio and the SRT. Reference should be made
should not be utilized for military wastewaters
to textbooks or TM 5-814-3 for further explana-
except under special circumstances.
tion and limitations to be considered when deal-
(d) Other considerations. In treatment of
ing with these parameters. Secondary sedimenta-
principally domestic wastes, there are additional
tion is particularly important for activated sludge
factors to consider (44)(154). Aside from not
systems. The design of these units is based on
meeting effluent criteria, operating problems in-
overflow rate and solids loading. Design criteria
clude odors, colored effluent, high effluent sus-
for various size plants and process modifications
pended solids, mosquito and insect problems and
6-7
Previous Page
