Use of centrifuge for pretreatment of combined wastewaters from a plasticizer manufacturing facility

Wastewater generated by a chemical manufacturing facility required a significant reduction in the bis(2-ethylhexyl) phthalate, di-n-octyl phthalate, and di-n-butyl phthalate ester concentration before discharge to a publicly owned treatment works (POTW). These three phthalate esters along with other esters and alcohols make up a dispersed phase in the wastewater that has lower density than water. Centrifugation was evaluated as a pretreatment process for the removal of the phthalate esters to within the effluent limits set by the POTW. The wastewater was treated by a full-scale commercial centrifuge at different operating conditions (effluent back pressure and flow rate) to determine the optimum conditions for phthalate ester removal. The results of this Study suggest that the degree of phthalate esters removal is independent of wastewater flow rate and centrifuge effluent back pressure but depends solely on the stability of the phthalate ester emulsion that appears to be directly related to the concentration of emulsifying agent (sodium monoester salts) in the wastewater. Centrifugation provided better phthalate ester separation from the wastewater than gravity separation alone; however, the periodic occurrence of heavily emulsified phthalate esters in the wastewater necessitates the use of an equalization tank downstream of the centrifuge to consistently meet the effluent limit of 5.0 mg/L of the three phthalate esters. In addition to the wastewater effluent, the centrifuge produces a light-phase effluent and a sludge effluent. The light-phase effluent contains esters, alcohol, and water and can be recycled back to the ester facility for reprocessing into salable product. The sludge effluent contains a dilute concentration of solids with a significant amount of adsorbed phthalate esters. The phthalate esters can be separated from the solids into an oily layer under acidic conditions. The oily layer can then be removed by reprocessing the acidified solids stream in the centrifuge, thereby enabling the disposal of the solids with the treated wastewater. This integrated waste management process would eliminate the need for additional solids processing equipment.