Section 11. PLATING WASTES
TREATMENT OF PLATING WASTEWATERS
BY FERROUS REDUCTION, SULFIDE PRECIPITATION,
COAGULATION AND UPFLOW FILTRATION
Thomas E. Higgins. Assistant Professor
Department of Civil Engineering
Arizona State University
Tempe, Arizona 85281
Stephen G. TerMaath, Chief
Environmental Engineering Branch
Air Force Engineering and Services Center
HQ AFESC/REVW
Tyndall AFB, Florida 32403
The U.S. Air Force has electroplating facilities ranging from the small, 3-4 plating bath
operation, to the very large operation with over 40,000 square feet of floor space. These
shops plate a variety of metals in support of both local maintenance, and periodic, major
overhaul of engines and aircraft at Air Logistics Centers. Although recovery and reuse are
used to the extent economically feasible, low concentrations of mixed metals are present
in the wastewater from electroplating facilities. The variety of metals and the normally
low flow rates make total segregation and treatment of the waste streams impractical.
This research was undertaken to develop simple treatment operations applicable to small
Air Force plating facilities and improve treatment already being accomplished at large
plating shops. Variable workloads generate mixed metal bearing wastewaters which are
difficult to treat with conventional hydroxide precipitation. This is further complicated
at small plating shops where full time treatment plant operators are not practical; thus,
an efficient and simple treatment methodology was desired.
Past Air Force success with ferrous reduction of hexavalent chromium at near neutral pH
coupled with the lower solubilities of sulfide precipitation were judged to be candidate processes. Upflow filtration offered additional potential to eliminate rapid mixing and flocculation steps. Therefore, a single process was conceived where chemicals to reduce the hexavalent chromium and to precipitate metals are added just ahead of an upflow filter. Before
such a process could be instituted, ferrous reduction of hexavalent chromium at neutral
and alkaline pH had to be proven, since literature reports reduction is essentially nonexistent
above pH 3. Other factors examined were: effectiveness of sodium sulfide as a reducing
agent and as a precipitant for mixed metals; and effects of competing ions (hardness, alkalinity, EDTA, cyanide) often present in plating wastewaters on chromium reduction and metal
precipitation.
LITERATURE
Conventional treatment of mixed metal wastewaters containing hexavalent chromium is a
multi-stage process. Reduction of hexavalent chromium is performed under acidic conditions
(pH 3) utilizing either a ferrous salt or a reduced sulfur compound (S02, H2S03, etc.).
The pH is then raised to 8-10 for hydroxide precipitation. The need to use processes which
require these major adjustments in pH complicates the treatment system and results in
relatively high chemical consumption.
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