Sewerage Systems

Question 1

Development of Sewerage systems

Answer 1

1870-1920 - Local collecting systems
1890-1995 - Provision of interceptor sewers and combined sewer overflows and waste water treatment plants
1960-now - rehabilitation of existing systems, reduction of polluting discharges and optimising performance

Question 2

Surface Water

Answer 2

Rainwater or storm runoff from roads, roof and gardens - not contaminated with foul sewage

Question 3

Combined Sewage

Answer 3

Foul sewage mixed with surface water - the old system

Question 4

Foul water

Answer 4

Contaminated water either domestic or industrial

Question 5

Sewerage

Answer 5

The infrastructure system which conveys sewage to a treatment works or for disposal providing good sanitation and protecting human health.

Question 6

Combined Sewerage Systmes

Answer 6

Convey foul flow and surface runoff in the same pipe

Question 7

Combined sewerage overflows

Answer 7

Are within the system and at the sewage treatment works to relieve the system of flows in excess of a selected rate. The excess flow is diverted to a local water course.

Question 8

Separate sewage system

Answer 8

Drainage system where sewage and surface water are conveyed in separate pipe systems. Surface runoff conveyed by surface water sewer to a suitable receiving water.

Question 9

Partially separated systems

Answer 9

A proportion of the surface water is discharged to the combined system.

Question 10

Inspection manholes

Answer 10

Are required at each junction, change of direction, change of gradient,at 150 m intervals.

Question 11

Impermeable area

Answer 11

Paved area, roofs, roads where water washes to the drains and does not infiltrate.

Question 12

Modified Rational Method

Answer 12

For surface water of combined sewerage system. Takes into account analysis of flow through sewers, ascertain peak flow and determine pipe sizes and gradients.

Question 13

Stages in the Modified Rational Method

Answer 13

1. Assume an initial diameter.
2. Obtain pipe full velocity from tables and calculate the full bore flow from tables
3. Calculate time of flow and time of entry.
4. Calculate the time of concentration
5. Look up rainfall intensity from tables
6. Calculate cumulative catchment area
7. Calculate peak discharge
8. Calculate proportion of pipe full
9. Calculate part full velocity using graphs