Environmental Engineering FOURTH EDITION

154 ENVIRONMENTALENGINEERING

Flow

12 12 12

midnight noon midnight

Figure 8-1. Typical dry-weather wastewater flow for a residential area.

proportion of the flow depth. Industrial flows often vary considerably throughout the
day and continuous recording is necessary.
Injiltration is the flow of groundwater into sanitary sewers. Sewers are often
placed below the groundwater table, and any cracks in the pipes will allow water
to seep in. Infiltration is least for new, well-constructed sewers, but can be as high as
500 m3/km-day (200,000 gallmileday). For older systems, 700 m3/km-day (300,000
gaVrnile4ay) is the commonly estimated infiltration. Infiltration flow is detrimental
since the extra volume of water must go through the sewers and the wastewater treat-
ment plant. It should be reduced as much as possible by maintaining and repairing
sewers and keeping sewerage easements clear of large trees whose roots could severely
damage the sewers.
Injow is storm water that is collected unintentionally by the sanitary sewers.
A common source of inflow is a perforated manhole cover placed in a depression, so
that stormwater flows into the manhole. Sewers laid next to creeks and drainageways
that rise up higher than the manhole elevation, or where the manhole is broken, are
also a major source. Illegal connections to sanitary sewers, such as roof drains, can
substantially increase the wet weather flow over the dry weather flow. The ratio of dry
weather flow to wet weather flow is usually between 1 : 1.2 and 1 :4.
The three flows of concern when designing sewers are the average flow, the peak
or maximum flow, and the extreme minimum. The ratio of the average flow to both the
maximum and minimum flows is a function of the total flow, since a higher average
daily discharge implies a larger community in which the extremes are evened out.
Figure 8-2 shows commonly experienced ratios of average to extremes as a function
of the average daily discharge.

SYSTEM LAYOUT

Sewers that collect wastewater from residences and industrial establishments almost

always operate as open channels or gravity flow conduits. Pressure sewers are used in