Domestic wastewater collection systems are generally composed of pipes buried underground at elevations that permit
the collection of wastes by gravity. It is common practice to design on the basis that the pipes will be completely full
when accommodating peak flows. For this reason, open - channel flow conditions prevail most of the time the pipes are
in service. Although several empirical equations have been used in the selection of pipes for collection systems, the
Manning equation has come into more general use for such purposes.
Types of Collection Systems...
Sanitary sewers : Often identified as separate sewers, sanitary sewers were developed to remove domestic wastes
from residential areas. Originally, the flow in sanitary sewers was by gravity. More recently, both pressure and vacuum
sewers have been used to serve areas where gravity sewers would be difficult and costly to install and maintain. Combined
sewers : Domestic wastewater and stormwater are collected together in combined sewers. Although the use of combined
sewers persist in many of the older cities in the world, they are seldom constructed today.
Types of Sewers...
Seperate systems are composed of several elements ; Building sewers : Building sewers, sometimes called "building
connections", connect to the building plumbing and are used to convoy wastewater from the buildings to lateral or branch
sewers, or any other sewer except another building sewer. These pipes generally have a diameter of 15 cm or more and are
placed at grades no less than 1 or 2 %. Building sewers discharge into collector sewers (lateral or branch). Lateral or
branch : Lateral sewers form the first element of a wastewater collection system and are usually in streets or special
easements. They are used to collect wastewater from one or more building sewers and convey it to a main sewer. Main
sewers : Main sewers are used to convey wastewater from one or more lateral sewers to trunk sewers or to intercepting
sewers. Trunk sewers : Trunk sewers are large sewers that are used to convey wastewater from main sewers to
treatment or other disposal facilities or to large intercepting sewers. Intercepting sewers : Intercepting sewers
are larger sewers that are used to intercept a number of main or trunk sewers and convey the wastewater to treatment or
other disposal facilities.
Manholes are used to interconnect two or more sewers and to provide entry for sewer cleaning. For sewers that are 1,200
mm and smaller, manholes should be located at changes in size, slope, or direction. Sometimes the slope of collector
pipes coming into a manhole from streets other than the one under consideration will govern the elevation of the bottom of
a manhole. Unless the manhole is specifically designed as a drop manhole, the inverts of all pipes coming into a manhole
should be located at the elevation of the bottom of the manhole.
The following should be covered in the design guidelines for manholes ; (1) All manholes should be constructed with a drop
to self - cleanse. The minimum drop for an outlet at right angles to an inlet of the same diameter should be 5 cm. (2) All
manholes in drainage, gutter, or similar low - lying points outside the crown of the street should be fitted with water -
tight gasketed covers, (3) All manholes on easement outside travelled rights - of - way such as sidewalks, side - lot, and
back - lot areas should be fitted with tamper - proof locking lids. Locking manholes on easement in parking lots or along
public roadways could be waived at the city's discretion, (4) Connections to existing manholes where no stubout is available
should be made with a core drill or concrete saw, never with a jack - hammer. The new invert elevation should conform with
25 cm depth, and a minimum of 5 cm above the existing manhole outlet. It will help avoid delays and disputes to add a note
by each manhole tap in the plans with words to the effect of "tap existing manhole by core - drilling or sawing". Install
approved commercial boot adapter with latex concrete bonding agent and non - shrink grout per specs. Rechannel manhole base
per standard drawing, (5) Wherever manholes are to be constructed over existing sewers, the existing pipe material and
diameter should be determined in the field and shown on the plans, (6) Building drains (service laterals) should be
connected to wyes or tees, instead of manholes. At the city's discretion, house connections to new pre - cast manhole bases
could be allowed using the specified manhole adapters and casting self - cleansing channels in the pre - cast base. If
allowed, house connections per manhole should not exceed 2, (7) Preliminary rim elevations for manholes should be shown.
Manholes under 15 cm deep from invert to rim should be flattop to facilitate access for inspection and maintenance, (8)
Installing toning wire into manholes is discouraged because the holes through each manhole are too often a source of
groundwater leakage. Sewer pipe installations between manholes normally do not require toning wire to locate the sewer
or services. Where necessary, commercial metallic tracer tape could be used, (9) The manholes on the project should be
numbered on both plan and profile, using a consistent system of designation and (10) Whether to use or exclude alternative
manhole materials such as plastic is the prerogative of the city or district responsible for maintenance of the system,
and of the engineer stamping the plans.
Many cities allow manhole testing after the first lift of asphalt - cement, instead of requiring total completion of all
the paving. Sometimes postponing manhole tests until paving may be unreasonable, as when bad weather prevents paving for
an extended period. Some cities will now test for acceptance prior to paving if the contractor will first anchor the
manhole neck and casting with a substantial concrete collar. Collars 12" thick by 6' to 8' square or 6' to 8' diameter
have been used.
"Definition Sketch for Types of Sewers"...
