Treatment of a 2,4-dichlorophenoxyacetic acid (2,4-D) contamined wastewater in a membrane bioreactor

Abstract

2,4-Dichlorophenoxyacetic acid (2,4-D) is used for commercial herbicides preparation. In Ecatepec, Mexico, there is an industrial plant that manufactures 2,4-D and other herbicides that discharges a low pH, high salinity and 2,4-D rich effluent: a very aggressive and toxic wastewater. This research aimed at acclimating a 2,4-D degrading culture from wild inocula, and implementing and determining the performance of an extractive membrane bioreactor (EMB) for 2,4-D removal from an effluent similar to that industrial wastewater. The experimental work consisted of two phases. In the first phase a mixed, aerobic microbial culture was acclimated to 2,4-D in submerged culture. In the second phase, a lab scale EMB was implemented. The acclimated culture was able to use 2,4-D as the sole source of carbon and energy with an optimum at pH = 8. Gram negative microorganisms predominated in the consortium. The culture growth in 2,4-D followed an Andrews-like substrate inhibition kinetic model with μmax = 0.06 1/h, Ks = 17 mg/L, Ki = 128 mg/L. The hydrophobic membrane had a partition coefficient membrane/water of 131 Laq/Lmembr, which suggested an important adsorption effect/solution in the membrane. The permeability coefficient determined at pH 0, 1 and 4 was practically independent of pH with an average value of 3.2=10-8 m/s. After inoculation, continuous, steady-state experiments were also run at influent 2,4-D concentration of 300 mg/L. The reactor showed 2,4-D unit reductions in the range of 110 to 120 mg 2,4-D/L lumen.h (32 to 34 mg/h.m2) at lumen retention times of 4.1 to 15 min. The unit surface removal rate values in this work compared favourably with literature data for EMB degrading other xenobiotics (for instance, phenol, retention time 353 min, removal rate 94.1 mg/h.m2). Studies are underway to determine 2,4-D removals at longer retention times. Overall, the EMB seems to be an effective configuration for removing 2,4-D while at the same time avoiding the active biomass exposure to the low pH and high salinity concentration of the influent.