NUTRIENTS - Marine Pollution: What Everyone Needs to Know - page 23

The sludge from primary treatment is pumped to a sepa-
rator, where inorganic solids (grit) are separated from the
lighter weight organic solids, which are then concentrated and
pumped to the anaerobic digesters with bacteria that work
in the absence of oxygen. Stabilized sludge has little odor.
Methane gas is produced by this anaerobic digestion and
can be used as fuel. The final sludge (biosolids) can be used
in an environmentally acceptable manner as a fertilizer and
soil conditioner except in urban areas, where the sludge may
contain high concentrations of metals and other toxic pollut-
ants from industry and must be disposed of. In the past, it was
barged out to certain sites in the ocean and dumped, but due
to its negative impacts, dumping became illegal. (Virtually all
material dumped in the ocean in the United States today is
sediments dredged from the bottom of water bodies in order
to maintain navigation channels.)
What is Combined Sewer Overflow (CSO)?
Older cities like New York and those in Europe have com-
bined sewer systems built in the nineteenth century that
combine sewer pipes from buildings with pipes and sew-
ers for stormwater from streets. This was a major advance
from the cesspools and gutters that formerly carried waste to
nearby waterways. In the first half of the twentieth century,
many sewage treatment plants were designed and built. At
that time, combined sewers seemed like a good way to carry
away stormwater along with the garbage, animal waste, and
other refuse that collected on city streets. Sewage plants were
designed to handle twice the average flow of wastewater,
but because of population increases, what was considered
excess capacity is no longer adequate after a storm. While
the volume of water entering sewage treatment plants can be
managed during dry weather, during storms the increased
volume of water from the storm sewers combined with the
wastewater is more than the plants can process. Many sewer