Since 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.
The 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 material.
At 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.
What 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.
A 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
In primary treatment, sand, grit, and the larger solids in the wastewater are separated
from the liquid. Screens, settling tanks, and skimming devices are most commonly used
for the separation. Primary treatment removes 45 to 50 percent of the pollutants.
Secondary Treatment
After 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.
Most 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
altogether.
Secondary 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.
The Stuff That’s Left Behind
You 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.
