|
|
|
|
|
|
|
the head of each sewer line. Ventilation was provided through manholes with open grates spaced at a minimum of 300 m apart. Waring's system worked well, the only problems he reported being obstructions caused by objects such as 'a splinter of wood, a carpenter's rule, a bone, a bottle, or some such thing a little longer than the diameter of the sewer'. They occurred primarily in areas near schools and shops. It is interesting to note that most of the original design criteria survived intact (or have become more conservative) in Brazil, with very few exceptions such as the flush tanks and the open grate manholes which have long disappeared. The idea of self-cleaning sewers had become the central design criterion. Unfortunately, the costs of sewer systems based on these century-old criteria have become excessive, prompting engineers in Brazil to question their applicability in the context of their cities. |
|
|
|
|
|
|
|
|
Consequently, a thorough and critical review of the justification for conventional sewerage design standards was made. This review led to sweeping changes in conventional sewer design standards. The changes were based on a variety of factors such as findings of recent research in hydraulics, satisfactory experience, and redundancy. The outcome of these new standards is a lower-cost sewer system with smaller, flatter, and shallower sewers with fewer and simpler manholes. |
|
|
|
|
|
|
|
|
11.3
Hydraulic Design Method |
|
|
|
|
|
|
|
|
As the major concern in the operation of sanitary sewers is the immediate removal of suspended particles, their design is based on ensuring that a sufficient velocity is constantly maintained to keep the suspended particles moving. In 1940, the Sanitary Section of the Boston Society of Civil Engineers conducted a theoretical analysis of the relationship between bed-load movement and the critical tractive force required to initiate motion of the bed-loads (Boston Society of Civil Engineers, 1942). The committee was able to develop an empirical model for removal of single grain particles of varying specific gravities: |
|
|
|
|
|
|
|
|
where V = wastewater velocity, m/s |
|
|
|
|
|
