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512 Chapter 14 Design of Sewer Systems
14.4.3 Interceptors

Intercepting sewers (Fig.10.1) are generally designed to carry away some multiple of the
dry-weather flow in order to bleed off as much stormwater and included wastewater as can
be justified by hygienic, aesthetic, and economic considerations. Where rainfalls are in-
tense and sharp, as in most of North America, it is not possible to lead away much
stormwater through reasonably proportioned interceptors. Consequently, they are designed
to transport not much more than the maximum dry-weather flow, or 250 to 600 gpcd
(946 to 2,271 Lpcd). A more informative measure of interceptor capacity in excess of
average dry-weather flow is the rate of rainfall or runoff they can accept without over-
flowing. Studies of rainfalls in the hydrologic surroundings of communities in the
United States usually lead to the conclusion that most precipitation in excess of 0.1 in.
(02.54 mm) is spilled; that spills can occur as frequently as half a dozen times a month;
and that interception is not improved greatly by going even to 10 times the dry-weather
flow. However, where rains are gentle and long, as in the United Kingdom, six times the
dry-weather flow comprises much of the runoff from rainfall and becomes a useful de-
sign factor.
The total yearly pollution reaching an interceptor-protected body of water is a signif-
icant fraction (3%) of the total annual volume of sanitary sewage. During the periods
when spilling occurs, a very high percentage of the sanitary wastewater can be carried
through combined sewer overflows. If solids have accumulated in the sewer during the in-
terval between rains, these may be washed out also. Thus overflows from combined sew-
ers can be heavily charged with solids. They present a serious pollution problem that will
be difficult to correct. Detention, settling, and chlorination are useful, and under some cir-
cumstances it may be desirable to work toward full separation of sanitary wastewater
from surface runoff.

14.4.4 Retarding Basins

Interception can be improved by introducing into combined systems retarding devices—
for example, up-system detention basins or equalizing tanks. Constructed in advance of
junctions between submains and interceptors, they store flows in excess of interceptor ca-
pacity until they are filled. After that, they continue to retard and equalize flows in a lesser
degree, but they do function as settling basins for the removal of gross and unsightly set-
tleable matter. Depending on local conditions, detention periods as short as 15 min can be
quite effective for settling basins and more so for chlorine-contact basins. Operating ranges
extend from the dry-weather flowline of the interceptor to the crown of the conjoined com-
bined sewer. After storms subside, the tank contents are flushed or lifted into the intercep-
tor, and the accumulated solids eventually reach the treatment works. Where much
stormwater is carried as far as the treatment plant—in the British Isles, for example—
stormwater standby tanks, which serve as primary sedimentation tanks, become useful ad-
juncts to the works.

14.4.5 Overflows
The amounts of water entering interceptors at junctions with submains must be con-
trolled. Only as much should be admitted as individual interceptor reaches can carry
without being surcharged. Higher flows must be diverted into stormwater overflows.
As shown in Fig. 14.12, admission and diversion can be regulated hydraulically or
mechanically.