Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.1 Wastewater characteristics (Topic a)
4.1.1 Cultural and technical practices
In general, human waste in North America is collected using water as the
transport medium. High water use flush toilets, showers, appliances (e.g.
clothes washing machines), dishwashers, and garburators result in relatively
dilute wastewater in comparison to that generated in other parts of the world.
To save water and wastewater treatment costs, as well as to save water in arid
areas, there is now a growing trend towards the legislated use of water saving
devices such as water meters and low flow toilets, shower heads and other
fixtures, especially in new developments, which can reduce wastewater flows up
to 30 percent.
Dry washing after urination and defecation using toilet paper (which is
disposed of to the wastewater stream) is standard practice and is the cause of
the relatively high amounts of cellulose fiber found in North American
Most large cities, except those where the bulk of development occurred in the
latter half of the twentieth century, have combined stormwater and wastewater
sewers which affect wastewater collection and treatment. Since the use of the
automobile is widespread, runoff from large paving areas often contributes to
wastewater flow in urban areas. Trace contaminants from automobile exhaust, oil
and fuel, and tires then become part of the wastewater stream.
Wastewater from large industries is generally treated on-site and then
discharged into nearby water courses. Smaller industries, especially when
located within or very near to a city boundary, may discharge into municipal
sewers and contribute to municipal wastewater flows. In such cases,
pre-treatment is generally required to adjust pH, reduce corrosiveness, reduce
high solids content (to prevent obstruction of flow), reduce temperatures to
below 40 degrees C, and prevent discharge of flammable materials that could
create a fire and explosion hazard. Volatile organic compounds are also now
largely regulated due to concern for air pollution, and the health of collection
system and treatment plant workers. The release of volatile and toxic
contaminants, and high strength conventional contaminants is often controlled
through the use of Sewer Discharge Bylaws which restricts such discharges or
applies heavy charges to discourage them.
4.1.2 Wastewater flows
Domestic wastewater is generated by: dwellings, commercial facilities (e.g.
stores, office buildings, airports, hotels, laundries, restaurants and shopping
centres), institutions such as hospitals, prisons, rest homes and schools, and
recreational facilities (e.g. pools, camps, resorts, golf clubs and parks). It
is that part of the waste stream resulting from washing, bathing, culinary use,
and human waste disposal.
There are generally two peaks in wastewater flow each day corresponding to
sewage generated in the early morning as people get ready for the day, and early
evening when they return home. A mid-day peak in wastewater flow may also occur.
The number of peaks often depends on the size of the sewage collection system
and the time it takes for sewage to reach the treatment facility. Lowest
wastewater flows typically occur in the very late night to very early morning
hours. Seasonal flow variations also occur due to inflow and infiltration of
stormwater and groundwater into the sewer or due to changes in population (i.e.
tourist and resort communities such as Banff, Alberta, and Aspen, Colorado, and
in small communities serving colleges or universities).
The amount of domestic wastewater generated from residential dwellings ranges
from 100 to 600 litres/capita/day with a typical average value of 260 litres/capita/day.
Lower per capita volumes typically occur in areas with higher population
densities, and areas with water use restrictions and controls. Areas with
unreliable or limited water resources such as resorts with individual holding
tank systems, also have characteristically low per capita sewage volumes. Higher
per capita volumes are generated by lower density high cost housing with
reliable water supplies, which tend to make heavy use of water using luxury
appliances. Geography, such as water conservation in arid regions, does affect
overall wastewater generation rates, but property value is a better indicator of
expected per capita wastewater flow in individual cases.
Large industries such as pulp mills, refineries, and power plants tend to
treat (and often dispose of) their wastewater separately. This may be due to
being situated in a remote location, or the prohibition of discharge to the
municipal sewers due to the adverse impacts upon wastewater treatment systems of
the associated high flows, high organic loads, high temperatures, acid or
caustic solutions, or the presence of metals and other toxic contaminants.
Smaller industries such as metal plating operations, or food processing
industries that are located in urban areas often may discharge to municipal
sewers. Flows from such sources may be highly variable, including batch
operations and seasonal operations. Peak flows can be strongly affected in small
communities with large industrial contributors. Many municipalities levy charges
based on flow and contaminant loading from industrial users, which gives these
industries economic incentives to investigate options for internal reuse and
recycling of process water.
Sewers built in North America prior to 1970, using mortar or mastic joining
materials, are particularly susceptible to infiltration from leaking joints,
cracks and breaks in the collection system. Newer plastic and cast iron piping
with elastomer gaskets have greatly reduced the amount of infiltration.
Infiltration rates can thus range from 93 to 92,590 L/d/cm/km, with the higher
rates usually in areas with high groundwater levels (Metcalf and Eddy Inc.
1991). An infiltration rate of 185 L/d/cm/km is considered to be reasonably
acceptable for new systems.
Inflow resulting from direct connection to storm drains, roof leaders,
foundation and basement drains, and leaking manhole covers, is highly seasonal
and geographical. In areas with combined sewer systems, all such flow is
directed to the treatment plant, resulting in higher peak flows than expected
from sanitary sewage flow alone. Wet to dry ratios in combined sewers (sewage to
stormwater) range from 1:1 to 8:1, with median 4:1 (Metcalf and Eddy Inc. 1991).
Although newer systems are separate, illegal connection of roof leaders,
foundation and basement drains is common in small communities where building
inspection may not be rigorous. A typical per capita allowance for infiltration
and inflow in a separate sewer is about 150 L/capita/d .
A common design tool in use in North America is the Storm Water Management
Model (SWMM), developed in 1971 and updated since by the U.S. Environmental
Protection Agency (Athens, Georgia) [http://www.ccee.orst.edu/swmm].
This and other computer models are in general use, and require calibration with
the system being modelled.