Encyclopedia of Environmental Science and Engineering, Volume I and II

URBAN RUNOFF 1183

up to 30% of the total annual dry-weather sewage loadings.

Sewer flushing during dry weather is designed to periodi-

cally remove the material, as it accumulates, and hydrauli-

cally convey it to the treatment facilities, thus preventing

resuspension and overflow of a portion of the solids during

storm events and lessening the need for CSO treatment.

Flushing is particularly beneficial for sewers with grades

too flat to be self-cleansing and also helps ensure that

sewers can carry their design flow capacities. Sewer flush-

ing requires cooperation between the authorities with juris-

diction over collection system maintenance and wastewater

treatment.

For developing sewer-flushing programs, it is necessary

to be able to estimate deposition buildup. Predictive equa-

tions have been developed, based on field studies in Boston,

to relate the total daily mass of pollutant deposition in a

collection system to the system characteristics, such as per-

capita waste-production rate, service area, total pipe length,

average pipe slope, and average pipe diameter. A simple

model is given by the equation

TS= L′ S q R =

0 0076.()()() ( .)1 063.. .−−0 4375 0 51^2 0 845

where

TS 
 deposited solids loading, lb/d

S

• 
  mean pipe slope, ft/ft
  L 
  total length of sewer system, ft
  q
  per-capita waste rate (plus allowance for infiltra-
  tion), gpcd

(EPA-600/2-79-133)

The total pipe length ( L ) of the system is generally assumed

to be known. In cases where this information is not known,

and where crude estimates will suffice, the total pipe length

can be estimated from the total basin area, A (acres), using

the expressions that follow.

For low population density (10–20 people/acre):

LAR′==168 95 0 821

.()0 928. (^2. ).

T A B L E 3

Catch basin cleaning costs (ENR 
 5,000)

Manual Eductor Vacuum

$/catch $/catch $/catch

basin $m^3 $/yd^3 basin $/m^3 $/yd^3 basin $/m^3 $/yd^3

19.20 47.07 36.21 14.77 13.40 10.14 20.00 28.10 21.44

(EPA-600/2-77-051).

0.0

0.0

0.5

1.0

1.5

2.0

1.0 2.0 3.0

STORAGE CAPACITY (SUMP), YD

COST FACTOR

(MULTIPLE OF STANDARD BASIN* COST)

FIGURE 4 Catch basin cost factors versus storage capacity (EPA-600/2-77-051).

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