Environmental Engineering Reference
In-Depth Information
a wide variety of treatment systems, each costing about the same to construct
but each posing separate needs for parts replacement (which may take many
months in many cases) and for O&M training. The economic answer is for each
DC to standardize the design of treatment units so that one of several parts
depots can readily service all plants in the country and so that O&M training
is greatly simplified. The recommended procedure is to utilize a system of four
types of plants, proceeding from the first to the fourth progressively as land prices
increase, as shown in Figure 4.7. Note that three options (A, B, C) are indicated
for System (i) and 2 options (A, B) for System (iii). The selection depends on
the particular situation. For example, for System (iii), Option B will often be
adequate for very small plants, as for schools in rural areas and for large plants
discharging to open (unconfined) ocean water. See
105
for details on the selection
process.
Experience indicates that the IAAs will generally be quite willing to give
grants to international DCs who wish to follow these guidelines to prepare a
national sewerage system plan, including provisions for collection as well as
treatment, to be progressively implemented as the country's urbanization and
industrialization continue to grow and land prices correspondingly increase. The
same study can also examine potentials for achieving regional pollution control
systems for protecting affected waterways.
Sewage Collection (Including Interceptors, Pumping, Transmission)
The most important design consideration here is to recognize that many DC
municipalities will not be able to afford immediate construction of sanitary sew-
ers for servicing the entire community area, which will have to be achieved
progressively with a series of stages, so that use is made of the existing storm
drainage conduits for the collection role on an interim basis. Again, the national
agency responsible for sewerage facilities should standardize use of materials
and equipment for simplifying parts replacement and O&M training, including
sewer-cleaning equipment.
Pumping Stations
An especially difficult problem is design of pumping plants
(which have always been a headache in sewerage history everywhere), but for-
tunately a design manual produced by Robert Sanks et al.
130
is available that
is invaluable for helping the designer with this problem. Another important
design aspect for collecting sewers is to utilize not the old-fashioned bell and
spigot pipe lengths, which generally result in entry into the sewers of large
amounts of unwanted groundwater, but instead the modern type of joint (now
generally available) using rubber rings, which virtually eliminate groundwater
infiltration.
Curved Sewers
Another valuable change in design of sewers applicable to
both ICs and DCs was development in the United States following World War II
of curvilinear sewer alignments, thus greatly reducing sewer construction costs,
