Although most photographic
processing facilities use silver-
recovery techniques that are efficient
(>90% recovery), there will usually be
a small amount of silver discharged,
as a tightly bound chemical complex
along with wastewater from the
processing activities.
These discharges are treated by
wastewater treatment plants, along
with other domestic and industrial
waste. Most wastewater treatment
plants operate by biological treatment
of the waste, a process where the
waste is treated with naturally
occurring microorganisms that break
down the waste products. During this
waste treatment, the silver complex is
converted into silver sulfide and
silver metalboth nearly insoluble
forms of silverand separated from
the water as part of the normal
treatment process. Typically, over 90%
of the silver reaching wastewater
treatment plants is removed. The
silver that is separated from the water
is then contained in the biological
(biosolids) solids (called sludge),
which are disposed of through land
application, landfilling, or
incineration. Because of the very low
water solubility of the silver
contained in the sludge, it does not
leave (leach from) the sludge to any
significant extent in landfills or soil
when used as a fertilizer.
The very small amount of silver that
is released from treatment plants to
bodies of water is in the form of either
tightly bound, soluble silver
complexes or nearly insoluble silver
forms such as sulfide. The soluble
silver complex may react with a
variety of naturally occurring
substances such as chemical
constituents of the water (e.g.,
chloride), organic constituents of the
water (e.g., humic acids) reactive
sulfides, and solid particles
suspended in the water. These
processes quickly render any soluble
silver forms nearly insoluble, and
remove them from the water. Silver
sulfide and silver particulates settle to
the bottom (sedimentation).