Wastewater Management

Obj 6.3: Sewage Generation

Water Supply Usage

for most municipalities the amount of sewage will be about 60-75% of the water supply used.

demand for fresh water peaks around 6:00 pm

this is typically when PEAK SEWAGE FLOW RATE occurs.

Peak Sewage Flow Rate

Is the total flow from all generators of sewage and includes:

domesticcommercialinstitutionalinfiltration

Domestic Sewage

Qdomestic = A x PD x SGR x PF

A = residential area (ha)PD = population density

(persons/ha)SGR = sewage generation rate

(m3/person/day)PF = peaking factor

Peaking Factor

Peaking Factor

If sewage flow monitored from a subdivision was measured at 8,640,000 L for a 24 hour period and the measured flow rate at the time of peak usage was 0.22 m3/s:Average flow rate =

Peaking Factor =

/sm 0.1 L/s 100606024

8640000 3

2.2/sm 0.1

/sm 0.223

3

Peaking Factors (PF)

small towns have higher peaking of water use (high peaking factor)

200% to 500% of average flow rate large towns have lower peaking of

water use (low peaking factor) 150% to 250% of average flow rate

Commercial and Institutional Sewage wide variety of land

use types and densities

great variation in sewage generation

Domestic Equivalent Estimation Method

Sewage Generation Rate Method

Commercial & Institutional Sewage

Qcom/inst= AC x SGR x PF

AC = area characteristic SGR = sewage generation rate

(m3/characteristic/day)PF = peaking factor

Infiltration

groundwater seeps into sanitary sewers through cracks and joints

surface runoff finds its way into manholes

infiltration amounts depend on quality of construction and age of system

Infiltration

defective joints cracked pipes water table level manhole integrity construction and

inspection maintenance and

repair

Factors that affect infiltration rates

Infiltration Flow Rates

Qinfiltration = A x IF

A = surface area (ha) IF = infiltration rate (m3/ha/day)