AND SULFIDE PRECIPITATION
treatment systems for metals are pretty well defined for precipitation
systems. The incoming solution is pH adjusted to the optimum range
for precipitating the metal as a hydroxide. In difficult situations
a sulfide is added to increase recovery. The treated water is run
through a clarifer to settle the solids. The effluent water is pH
adjusted, if needed, to meet city limits and the hydroxide sludge
is filter pressed to a cake for recycling.
precipitation systems operate on the principal that give enough
time and a low enough flow, a solid will settle out of a liquid.
A pretty simple concept but much harder in practice than in theory.
The way this is accomplished in practice is with a lamellae plate
clarifier. By using a series of plates about 1" apart set at
a 45-60 degree angle and forcing the water to run through the channels,
a short settling distance is created. Using a flow rate of about
1/4 gpm/surface sq. ft. of plate, the solids drop to the plates
before the water can carry them out. As they accumulate on the plates,
they slide to the bottom and off into a sump where they are eventually
standard heavy metal wastewater treatment methodology for metals
removal has been hydroxide precipitation with or without the addition
of a sulfide. The sulfide results in a lower solubility than hydroxide
precipitation alone. In some cases, neither seems to be able to
consistently obtain satisfactory results. We have found in most
cases, this was cause by either and undersized system (flow too
high for the settling area) or incompatible constituents resulting
in a carryover of the flock. The two solutions are obvious, size
the system correctly in the first place or add a polishing filter
to the end of the system. Our polishing filters
can take a 20 NTU (NTU is a measure of optical clarity) effluent
stream down to 0.2 NTU and operate completely automatic.
major problem with this technology is with chelates. Chelates are
organic compounds that hold metals in solutions at high pH. Hydroxide
precipitation depends on the insoluble metal hydroxide forming and
the chelating agent prevents this. Some sulfides and strong reducing
agents can break weak chelates but EDTA is more difficult.
method of breaking chelates is to do a substitution. A non hazardous
metal is added that the chelate prefers over the target metal. For
EDTA at high pH, Calcium, Magnesium, and Iron are all preferred
over the hazardous divalent metals such as Copper, Nickel, and Zinc.
Adding any one of these would increase the precipitation of the