Destroys organic materials
PHOSPHATE: Lessens pipe corrosion
FLUORIDE: Lessens tooth decay
CARBON: Controls taste and odor
ALUM: Forms "floc" (snowflakes) to trap impurities
LIME: Helps "floc" formation; lessens pipe corrosion
]

The basic and usual damage comes from oxygen depletion. A stream has a
natural capacity for hastening the decay of organic wastes, which is
determined by such things as the volume of its flow, the pollution
already in it, its velocity and depth, and its temperature. When that
capacity is exceeded, as we have noted, too much of the stream's oxygen
is used up by the process of decay and the stream, which is an
intricately complex work of living things, begins to die. Under really
bad conditions, the waste solids themselves cannot all be assimilated,
and hence may build up in layers of stinking sludge at the bottom of
the stream and continue to seize available oxygen for a long time
thereafter.

Conventional waste treatment, in plants built by towns or by industries
whose raw materials are animal or vegetable in origin, is aimed at
removing the solids in the wastes and reducing the bio-chemical oxygen
demand--called B.O.D. It is a speeded-up version of the same process of
purification that goes on normally in any stream when loads are not too
heavy. "Primary" treatment removes such solids as will readily settle
out and passes the rest on back to the stream as part of the effluent.
"Secondary" treatment plants, after settling out the gross solids, speed
up decay by furnishing air to the bacteria that eat up dissolved and
finely suspended materials; a good secondary plant, under much more
skillful supervision than is usual, can get rid of 85 or 90 percent of
the organic materials and the associated B.O.D. by the time it turns its
effluent into a stream. How damaging that effluent will be depends on a
number of things, chief among them being the size and condition of the
receiving stream and the volume of organic materials that went into the
treatment plant in the first place. A riverside town of 1000 with a
secondary treatment plant operating at 75 percent efficiency is going to
inflict on its river a daily load roughly equivalent to the raw sewage
from 250 people.

Over the years a lot of hard effort, notably on the part of the
Interstate Commission on the Potomac River Basin, has resulted in some
degree of treatment for about 85 percent of all munici