|
|
|
|
|
|
|
laying sewers at shallow depth; |
|
|
|
|
|
|
|
|
laying sewers within plots at the rear of premises, and |
|
|
|
|
|
|
|
|
providing interceptor tanks for settlement of solids |
|
|
|
|
|
|
|
|
Other chapters in this book illustrate to what extent conventional criteria may be simplified and reduced, and with what effect. However, the following approaches have distinct differences from conventional systems, and the hydraulic criteria that apply are summarized. |
|
|
|
|
|
|
|
|
In this system, also known as small bore sewerage, domestic sewage flows through an interceptor tank prior to discharge to the sewer. This means that the sewerage system receives only the liquid portion of the sewage for conveyance. Solids are intended to accumulate in the interceptor tank for periodical removal and disposal. Otis and Mara (1985) describe many practical considerations for effective operation of the interceptor tank. Provided the tank operates as intended, the sewer may be laid at a nominal fall, and may include sections of pressure flow where the sewer drops below the hydraulic grade line. In that respect, the hydraulic design becomes like that for gravity water supply pipelines, which may follow the ground contours, provided the point of discharge is at a lower elevation than the point at which flow enters the system, and consideration is given to the possibility of air locks. |
|
|
|
|
|
|
|
|
Unless specialized cleaning equipment is available, a minimum pipe size of 100 mm was recommended, with periodic flushing from manholes to create a flow velocity of at least 0.5 m/s. With solids removed from the sewage, there is no requirement for a daily self-cleansing velocity. With considerable attenuation of individual peak discharges by the interceptor tanks, a reduced design peaking factor of 2 was recommended pending the availability of more field data. Flat gradients may be accommodated by allowing the pipeline to follow within reason the natural ground contours, in what is referred to as the inflective gradient design approach. |
|
|
|
|
|