Section 18. MISCELLANEOUS
AN INVESTIGATION OF FLUORIDE REMOVAL
FROM SEMICONDUCTOR WASTEWATER
USING WATER SOFTENING SLUDGE
John L. McDonough, Corporate Facilities Engineer
Millipore Corporation
Bedford, Massachusetts 01730
James C. O'Shaughnessy, Associate Professor
Department of Civil Engineering
Northeastern University
Boston, Massachusetts 02115
INTRODUCTION
Historically, the removal of fluoride from acidic industrial wastewater has been performed with
the use of commercial lime. The importance of lime is to supply the divalent calcium ion, Ca2 + ,
necessary to precipitate calcium fluoride, CaF2. In the semiconductor industry, acidic fluoride-bearing wastewater is formed by a chemical etching process based on hydrofluoric acid.
Similarly, the removal of hardness from municipal and industrial water supplies is also accomplished using lime. The resulting water softening process generates precipitated calcium carbonate
(CaC03) sludge which is usually thickened and disposed of offsite. One of the least practiced methods
of water softening sludge disposal is codisposal [1].
The codisposal process is based on the following: (1) the softening sludge is of some value in the
treatment/disposal of another waste; or (2) the lime sludge is of no value and simply takes advantage
of the economies of scale of joint disposal. The first condition is attractive because it uses a waste
to treat a waste thereby eliminating the need to purchase quantities of chemical reagents. Using high
pH liquid or semisolid alkaline waste, such as water softening sludge for neutralizing an acidic waste,
has major opportunities for use in the industrial sector.
— The objective of this investigation was to determine the feasibility of using water softening sludge
to treat a fluoride bearing wastewater generated by a semiconductor manufacturer. The underlying
objective of this project therefore was to investigate the potential for using a waste to treat a waste.
This treatment method would apply to industries that process their own water supplies for hardness
removal and who may also generate fluoride wastes or be proximate to other industries that do.
Another potential application would be in a municipal/industrial waste exchange agreement. The net
effect of using one waste to treat another would be a reduction in treatment costs.
A review of the literature indicated few documented codisposal methods.
REGULATORY REQUIREMENTS
The 1962 United States Public Health Service (USPHS) Drinking Water Standards established
recommended optimum, as well as maximum fluoride concentrations. The 1975 EPA Interim Primary
Drinking Water Regulations, established under the provisions of the Safe Drinking Water Act (PL
93-523), promulgated maximum contaminant levels (MCL) for 10 inorganic chemicals, including
fluoride. The MCL for fluoride was the same as the maximum concentration previously established
by the USPHS. Since water consumption, and thus fluoride intake, increases with increasing air
temperature, the standards were established as a function of annual average maximum daily air temperature.
Recently, EPA proposed regulations under the Clean Water Act to limit effluent discharges to
waters of the United States and the introduction of pollutants into publicly owned treatment (POTW's)
843