The process performance of an activated sludge system during acclimation to phthalate esters was examined. Concentration of the phthalate esters was increased gradually, and over about a two-week period, the acclimation to a mixture of 410 mg/L diethyl phthalate and 540 mg/L dimethyl phthalate was complete. It was observed that the effluent Chemical Oxygen Demand (COD) was not affected adversely in spite of increasing influent COD. Other indicators of process performance, for example, effluent suspended solids, were at acceptable levels at the completion of acclimation. The system responded well when 2,4-dinitrophenol, up to 4 mg/L, was included in the influent in addition to phthalate esters. Actually an improvement in system effluent COD was observed possibly due to increasing Mixed Liquor Suspended Solids (MLSS). At a 2,4-dinitrophenol concentration of 8 mg/L, system performance deteriorated considerably but the recovery was fast, and within a two-week period system performance returned to normal.

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