NUTRIENTS - Marine Pollution: What Everyone Needs to Know - page 22

and industries to the treatment plant. Screens let water pass,
but not trash (such as rags, diapers, etc.), which is collected
and disposed of. The sewage is then held temporarily in a
settling basin where heavy solids settle to the bottom while
oil, grease, and lighter solids float to the surface. The settled
and floating materials are removed and the remaining liq-
uid may be discharged to the environment (if primary treat-
ment is all there is), or it flows to the next (secondary) stage
of treatment.
Secondary treatment involves biological processes by
which microorganisms break down organic matter (just like
microbes do in the natural environment) in the separated liq-
uid and solid phases. Air is pumped in to the mixture of pri-
mary wastewater and microorganisms, whose growth is sped
up by the aeration. Final settling basins allow the clumps of
microorganisms to settle from the water by gravity. Most of
this mixture, called activated sludge, is returned to the aera-
tion basins to maintain the needed amount of microorgan-
isms. Secondary treatment may include a separation process
to remove the microorganisms from the treated water prior to
discharge or tertiary treatment. The final effluent (liquid por-
tion) may be disinfected (e.g., with chlorine, ozone, UV) before
it is discharged, to reduce the microorganisms in the water
before it is released into the environment.
Some sewage treatment plants include tertiary processes
to remove more pollutants before the effluent is released. In
some cases it is pumped to constructed wetlands (with cat-
tails or other plants) for further treatment. Plants in treatment
wetlands take up some of the remaining nutrients from the
effluent. Other forms of tertiary treatment use biological nutri-
ent removal technology to remove most of the organic N by
converting it to N 2 gas, which is harmless and released into
the atmosphere (which is mostly N 2 anyway). Effluents must
meet EPA's National Pollutant Discharge Elimination System
(NPDES) criteria. Improving wastewater treatment systems is
a major way to improve water quality.