PERFORMANCE OF BUOYANT MEDIA FILTER IN METAL PLATING
SHOP WASTEWATER TREATMENT
John S. Hunler, Coordinator
Environmental Technology Development
3M Company
St. Paul, Minnesota 55133
INTRODUCTION
A heretofore little known technology called buoyant media filtration has been found to provide
some unique benefits to the electroplating/metal finishing industry when used in conjunction with
conventional metal hydroxide precipitation wastewater treatment. Because of their ability to accommodate very high influent suspended solids concentrations, buoyant media filters can be used to
replace conventional clarifiers and filters operated in series. When the buoyant media filter is used
in the more conservative application of polishing clarifier effluent, it will continue to function even
when a severe upset is occurring in the upstream clarifier that would preclude operation of a conventional sand filter. Additional advantages include a low operating headloss (2 to 4 inches of water)
that permits gravity flow operation and a built-in backwash water holding tank that further reduces
capital costs and space requirements.
The purpose of this paper is to report on a field study of a full-scale buoyant media filter used
as a part of an actual metal hydroxide precipitation wastewater treatment system at a plating shop.
The filter was studied both while operating as a substitute for a clarifier and while operating as a
polishing filter for clarifier effluent. Its performance is compared to that of a slant tube clarifier and
to United States Environmental Protection Agency effluent limitations.
Buoyant Media Filtration
Buoyant media filtration refers to liquid filtration in which the liquid to be filtered is passed up
through a bed of granular media that has a specific gravity less than that of the liquid being filtered.
A line drawing of the buoyant media filter used for this study appears in Figure 1. The flow of
wastewater through the filter during the filtration cycle is shown in this drawing. Backwashing is
accomplished by rapidly draining water out the bottom of the filter. Any sludge accumulated on the
bottom of the tank is removed and filtered water flows downward through the filter bed providing
&\A
Figure 1. Schematic of buoyant media filter.
BACKWASH OUTLET
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