Encyclopedia of Environmental Science and Engineering, Volume I and II

727

MUNICIPAL WASTEWATER

Sewage is the spent water supply of a community. Because of

infiltration of groundwater into loose sewer pipe joints, the

quantity of groundwater is frequently greater than the quan-

tity of water initially consumed. Sewage is about 99.95%

water and 0.05% waste material.

A weaker (more dilute) sewage results from greater per

capital water consumption. Industrial wastes will contribute

to sewage strength. Sewage flow varies with time of day

and will be higher during daylight hours. Larger cities will

exhibit less variation with time than smaller cities. Many

small communities will have a flow in the late night hours

that is due almost entirely to infiltration.

Per capita production will vary from less than 100 gal-

lons per day for a strictly residential community to about

300 gallons per day for a highly industrialized area. The

concept of “population equivalent” is frequently applied

in evaluating industrial waste contributions to sewage flow

and planning for hydraulic, solids, and biochemical oxygen

demand loadings.

Wastewater treatment facilities have high initial capital

costs due to the need for large tanks, equipment and land

area. The designed life of a treatment plant is normally

equal to the life of the bonded indebtedness of the project.

It is expected that capacity will be just reached at the end

of this period. In a community with combined sanitary and

storm sewers it is often found necessary to bypass waste-

water flows during periods of heavy rain or, in low lying

areas, during high tides. Excess water may be held in deten-

tion basins until normal flow resumes and then treated in

the plant. Bypassed flow can be screened and chlorinated

before discharge. This subject is receiving increasingly

close attention.

Strength of sewage is usually expressed in terms of the

following parameters: total solids (sometimes called volatile

suspended solids, VSS), filterable solids, and biochemical

oxygen demand (BOD). Many methods are available for

determination of the foregoing. However, in order that oper-

ating and research data reported by workers in widely sepa-

rated areas be comparable, it is necessary to have analytical

methods which are simple, give consistent results and are

widely accepted. This need is filled by Standard Methods

for the Examination of Water and Wastewater. Contained in

each edition are methods for analysis of water and waste-

water which have been accepted by committees of experts

in various fields. Standard Methods is revised about every

five years.

Analysis of a typical American sewage is given below:

Total solids 600 mg/l

a) suspended solids 200 mg/l

1) settleable solids 120 mg/l

2) colloidal solids 80 mg/l

organic 60 mg/l

mineral 20 mg/l

b) Filterable solids 400 mg/l

1) colloidal solids 40 mg/l

organic 30 mg/l

mineral 10 mg/l

2) dissolved solids 360 mg/l

organic 100 mg/l

mineral 260 mg/l

c) BOD (5 days 20°C)

1) suspended 42 gms/cap. × day

settleable 19 gms/cap. × day

non-settleable 23 gms/cap. × day

2) dissolved 12 gms/cap. × day

Total 54 gms/cap. × day

Assuming an average daily flow of 100 gallons per capita,

54 gms/cap. × day = 0.12# BOD/cap. × day. This is a strictly

domestic sewage. Per capita BOD values can vary from about

0.10#/day to above 0.25#/day. A commonly accepted value

used for estimation is 0.17#/cap. × day.

In the above analysis, the determination of solids, min-

eral and organic, gives an indication of the loadings to be

placed on the plant. Volatile solids give the organic loading

and mineral solids are those which must be accommodated

by sedimentation equipment.

BOD is a measure of the loadings placed on the oxygen

resources of the receiving waters. Several methods have been

used over the years for determination of the amount of oxygen

necessary for stabilization of the waste. Permanganate and

other oxidizing agents were formerly used. One method still

finding application, but only as a general indication, is the

putrescibility, or methylene blue, test. Methylene blue, a dye,

decolorizes in the absence of dissolved oxygen. This test is

only quasiquantitative, but finds use in day to day operation

control of wastewater treatment plants. The method used

today is an incubation and dilution method. Dissolved oxygen

is determined at the beginning and end of a 5 day period by

the Alsteberg azide modification of the Winkler method.

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