75    PRETREATMENT OF INDUSTRIAL WASTEWATER
CONTAINING PHTHALATE ESTERS BY
CENTRIFUGATION
Curtis J. Petrosky, Graduate Research Assistant
Radisav D. Vidic, Assistant Professor
University of Pittsburgh
Department of Civil and Environmental Engineering
Pittsburgh, Pennsylvania 15261
INTRODUCTION
The use of centrifugation in wastewater treatment to separate liquids has not typically been an
economical process due primarily to the high capital cost1 compared to other liquid/liquid separation processes such as gravity settling. However, its use may be justified in wastewater treatment applications involving stable mixtures of immiscible liquids. Centrifugation has been
proven by the petroleum2 and dairy3 industries as a successful physical separation process for
immiscible liquid mixtures.
In this study, a full-scale commercial centrifuge was used to treat, on a continuous basis, the
entire wastewater stream generated by a chemical manufacturing facility which produces a variety of phthalate, adipate, maleate, and trimellitate esters. The wastewater from this facility is
comprised of process water, equipment wash water, and rain water runoff containing varying
concentrations of bis(2-ethylhexyl) phthalate (BEHP), di-n-octyl phthalate (DNOP), and di-n-
butyl phthalate (DNBP) esters in addition to mono-ester salts and alcohols. The wastewater is
discharged to the local Publicly Owned Treatment Works (POTW) under pretreatment regulations which specify an effluent limitation of 5.0 mg/L on the total toxic organic (TTO) concentration4 which can be placed on the combined BEHP, DNOP, and DNBP ester concentration.
Various esters and long chain alcohols present in the wastewater have very low water solubilities and are considered immiscible. They form a dispersed phase in the wastewater that has a
specific gravity in the range of 0.88 to 0.93. Separation of the dispersed phase containing the
regulated esters from the heavier water phase to consistently below 5.0 mg/L poses a challenge
due to the stability of this colloidal suspension. The objective of this study was to evaluate the
effectiveness of centrifugation in meeting the 5.0 mg/L effluent limit on the total BEHP, DNOP,
and DNBP ester concentration.
In addition to the POTW limits, the Environmental Protection Agency (EPA) has established
future effluent pretreatment guidelines which mandate much stricter limits on BEHP and DNBP
esters.5 As a result of these EPA effluent guidelines, an activated sludge treatability study was
conducted on this wastewater. The results from the study showed biological treatment alone
could not produce an effluent with BEHP and DNBP levels consistently below the EPA effluent
limits. The poor performance of the activated sludge process in the removal of phthalate esters
was due to the extremely high BEHP and DNBP concentrations in the wastewater which were
over one thousand times the median concentrations used by EPA to develop their guidelines.6
Previous studies have evaluated a number of pretreatment processes in the removal of BEHP,
DNOP, and DNBP from the wastewater, but none have proved effective or economical. Gravity
separation with and without polymer addition and ester hydrolysis were not effective methods
due to poor phthalate removal and/or significant cost to install and operate these processes.
Granular activated carbon (GAC) successfully removed the phthalate esters, but also removed
other organic compounds present in the wastewater. The adsorption capacity of the GAC was
50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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