WASTEWATER COLLECTION AND TREATMENT
of wastewater is a relatively modern practice.
While sewers to remove foul-smelling water were common in ancient Rome,
it was not until the 19th century that large cities began to understand that
they had to reduce the amount of pollutants in the used water they were
discharging to the environment. Despite
large supplies of fresh water and the natural ability of water to cleanse itself
over time, populations had become so concentrated by 1850 that outbreaks of
life-threatening diseases were traced to bacteria in the polluted water.
that time, the practice of wastewater collection and treatment has been
developed and perfected, using some of the most technically sound biological,
physical, chemical, and mechanical techniques available.
As a result, public health and water quality are protected better today
than ever before.
modern sewer system is an engineering marvel.
Homes, businesses, industries, and institutions throughout the modern
world are connected to a network of below-ground pipes which transport
wastewater to treatment plants before it is released to the environment.
Wastewater is the flow of used water from a community.
As the name implies, it is mostly water; a very small portion is waste
a typical wastewater plant, several million gallons of wastewater flow through
each day -- 50 to 100 gallons for every person using the system.
The amount of wastewater handled by the treatment plant varies with the
time of day and with the season of the year.
In some areas, particularly communities without separate sewer systems
for wastewater and runoff from rainfall, flow during particularly heavy rains or
snowmelts can be much higher than normal.
happens in a wastewater treatment plant is essentially the same as what occurs
naturally in a lake or stream. The
function of a wastewater treatment plant is to speed up the process by which
water cleanses (purifies) itself.
treatment plant uses a series of treatment stages to clean up the water so that
it may be safely released into a lake, river, or stream.
Treatment usually consists of two major steps, primary and secondary,
along with a process to dispose of solids (sludge) removed during the two steps.
primary treatment, sand, grit, and the larger solids in the wastewater are
separated from the liquid. Screens, settling tanks, and skimmering devices are most
commonly used for the separation. Primary
treatment removes 45 to 50 percent of the pollutants.
primary treatment, wastewater still contains solid materials either floating on
the surface, dissolved in the water, or both.
Under natural conditions, these substances would provide food for such
organisms as fungi, algae, and bacteria that live in a stream or lake.
public wastewater treatment plants now provide a second stage of treatment known
as secondary treatment to remove more of the pollutants--up to 85 or 90 percent
treatment is largely a biological process.
Air is supplied to stimulate the growth of bacteria and other organisms
to consume most of the waste materials. The
wastewater is then separated from the organisms and solids, disinfected to kill
any remaining harmful bacteria, and released to a nearby lake, river, or stream.
Stuff That’s Left Behind
may have figured out by now that while treatment of wastewater solves one
problem-- cleaning the water that is released from the treatment plant to the
stream--it can generate others. For
example, the material that is removed from wastewater doesn’t just disappear.
It is called sludge. Sludge
requires proper treatment and disposal, and can often be reused.
Sludge handling methods are designed to destroy harmful organisms and
remove water. The end product of
the sludge handling process is a relatively dry material known as “cake.”
It can be applied to agricultural land as a soil conditioner, placed in
landfills, or cleanly burned. At
some plants, sludge serves as a fuel to produce energy.
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