Degradation of phthalate esters in an activated sludge wastewater treatment plant

Abstract

Efficient removal of phthalate esters (PE) in wastewater treatment plants (WWTP) is becoming an increasing priority in many countries. In this study, we examined the fate of dimethyl phthalate (DMP), dibutyl phthalate (DBP), butylbenzyl phthalate (BBP), and di-(2-ethylhexyl) phthalate (DEHP) in a full scale activated sludge WWTP with biological removal of nitrogen and phosphorus. The mean concentrations of DMP, DBP, BBP, and DEHP at the WWTP inlet were 1.9, 20.5, 37.9, and 71.9 microg/L, respectively. Less than 0.1%, 42%, 35%, and 96% of DMP, DBP, BBP, and DEHP was associated with suspended solids, respectively. The overall microbial degradation of DMP, DBP, BBP, and DEHP in the WWTP was estimated to be 93%, 91%, 90%, and 81%, respectively. Seven to nine percent of the incoming PE were recovered in the WWTP effluent. Factors affecting microbial degradation of DEHP in activated sludge were studied using [U-(14)C-ring] DEHP as tracer. First order rate coefficients for aerobic DEHP degradation were 1.0 x 10(-2), 1.4 x 10(-2), and 1.3 x 10(-3) at 20, 32, and 43 degrees C, respectively. Aerobic degradation rates decreased dramatically under aerobic thermophilic conditions (<0.1 x 10(-2)h(-1) at 60 degrees C). The degradation rate under anoxic denitrifying conditions was 0.3 x 10(-2)h(-1), whereas the rate under alternating conditions (aerobic-anoxic) was 0.8 x 10(-2)h(-1). Aerobic DEHP degradation in activated sludge samples was stimulated 5-9 times by addition of a phthalate degrading bacterium. The phthalate degrading bacterium was isolated from activated sludge, and maintained a capacity for DEHP degradation while growing on vegetable oil. Collectively, the results of the study identified several controls of microbial PE degradation in activated sludge. These controls may be considered to enhance PE degradation in activated sludge WWTP with biological removal of nitrogen and phosphorus.