Sewer Systems / Urban Drainage

Shows a direct relation with the amount of water consumed

If consumption data is available 80 to 90% of the water consumption is taken as wastewater flow

Wastewater flow varies from hour to hour and also seasonally. Since sewers must accommodate the maximum rate of flow, the variation of flow must be studied -> peak factor

Peak factors depend on population density, topography of the site and hour of water supply

If available use monitoring data e.g. specific data on water consumption - if no specific data are available, capita specific wastewater flow allows a first assessment

For design purposes usually not less than 150 l/c*d

As a rule of thumb: 5 l/(s*1000c)

By-product of industrial or commerical activities

Quality and quantity depends on industrial sectors

Daily, weekls or seasonal variation result from production pattern (8, 12, 24h)

As a rule of thumb for industrial wastewater peak flow

• Industries with low water consumption: 0,2-0,5 l/(s*ha)

• Industries with average and high water consumption: 0,5-1,0 l/(s*ha)

Water that enters into the sewers through poor joints, cracked pipers, walls and covers of manholes

For an first estimate: 0,05-0,15l/(s*ha)

Surface runoff generated from rainfall

Intensity- Duration-Frequency Curves

• Design strom (block rain)

• intensity as a function of D (duration) and T (return period) -> rD,T

Selection of Design Storm

• Duration depends on time of flow (concentration time)

• Minimum time of runoff generation and concentration 10 to 15 minutes

• Return Period T depends on risk assessment based on land use

Peak runoff coefficients for sizing of sewers

• Values depend on degree of pavement, slope and rein intensity

Combined Sewer Systems

Seperate Sewer System

Pros and Cons for the Sewer System Types

Horizontal alignment of sewer pipes

• Shortest possible way

• Preferably following surface slope on public ground (along streets)

• Avoid unnecessary changes of direction (manhole necessary)

• Best use of surface slope (-> vertical allignment) -> avoid pumping stations or drop manholes

Competing goals!

• Note: Urban design and traffic infrastructure define the layout of the drainage system to a large extend

• -> Need for integrated planning

Vertical alignment:

Depth

• Static reasons: > 0,5m

• Frost protection: > 1,3 m (in Germany)

• Draining of basements: > 2,5 m (necessary)

• Avoid extreme depths (> 5-7 m -> pumping station)

• Diameter

Slope

• Maximum slope to prevent abrasion (-> drop manholes)

• Minimum slope to prevent sedimentation

• Recommended self-cleansing velocity is 0,5 m/s

Choice of Pipe shape

• Circular

  • + standard

  • + cheap

  • + all materials

• Egg-shape

  • + sedimentation limited

  • - large depth

  • - expensive

• Horse-shoe

  • + low depth

  • - prone to sedimentation

  • - expensive

pumps used instead of gravity forces to transport wastewater

The primary effluent is delivered to the collection tank by gravity where is grinded before being transported into the pressurized system by pumps

The system can be built with only shallow trenches and relatively small-diameter pipes

System components need regular service to avoid damage

Electricity needs to be available all the time

Unwanted anaerobic degradation in the pressurized pipes

Use a central vacuum source to convey sewage from individual households to a central collection station

Use differential air pressure (negative pressure) to move the sewage

The collection system is hold on permanent level of vacuum

Redces the amount of flushing water considerably

It allows for the use of alternative wastewater handling (blackwater and greywater)

Needs energy to create permanent vacuum

Requires skilled engineers operators

It is flushing system, requires additional treatment

Storage as primary function, partial treatment through sedimentation process + treatment in WWTP (comined system)

Regulation

• Not precise

• Depends on water level

• Smooth system behavior

Upstream Head Control

• Stable and constant flow except in low ranges

• Controlled upstream water level

• Risk of failure with small diameters

Measurement and Control

• Flow meter

• Regulating slide

• Very stable and constant flow possible