Chlorinated hydrocarbons such as trichloroethylene(TCE), tetrachloro ethylene (PCE) are critical pollutants because of their toxic property and widespread occurrence in soil, air, and water. These compounds tend to resist biological degradation in conventional biological wastewater treatment system as well as in natural ecosystem. Recently, cometabolic conversions of several chlorinated aliphatics have been found successful under aerobic conditions. Also, advanced oxidation processes(AOPs) have been found effective in the destruction of refractory organic pollutants due to effective oxidation potential of hydroxyl radicals. In this study, the removal efficiencies of TCE by advanced oxidation processes(AOPs) as well as the cometabolic degradation of TCE by a biofilm process were investigated for the development of combined treatment process of physico-chemical and biological technologies in laboratory- and pilot-scale experiments.
In ozonation, TCE was removed by a pseudo first-order reaction. The rate of TCE removal increased with the increase of pH up to critical value. In the PEROXONE($O_3/H_2O_2$) AOP, TCE was removed most effectively at peroxide-to- ozone dosage ratio of 0.8. The mineralization of TCE was made faster by continuous injection of peroxide than by batch injection. Unlike ozonation, the rate of TCE removal was the highest at the neutral pH condition. In $O_3$/UV AOP, the utilization efficiency of ozone and the removal efficiency of TCE increased with increasing UV intensity. About 40\% of TCE was photolyzed directly by the UV irradiation. In $H_2O_2$/UV AOP, the removal rate of TCE increased in proportion to the $H_2O_2$ concentration up to 45 ppm at UV intensity of 5 W/L. Destruction of TCE was better at an acidic pH condition than neutral or basic conditions. The PEROXONE process appeared most effective in degrading concentrated TCE among various AOPs studied in lab-scale.
To evaluate the feasibility of cometabolic degradation of TCE by mixed microbial culture, three packed-bed bioreactors filled with GAC, Celite, and Ceramic beads respectively were operated for 285 days. Phenol-oxidizing microorganisms were immobilised effectively in the bioreactors which degraded TCE cometabolically utilizing phenol as a carbon and energy source. However phenol degradation was inhibited significantly when TCE concentration increased. In same manner, high phenol concentration over 50 mg/L inhibited TCE degradation in both the Celite and Ceramic bead bioreactors. When influent phenol was reduced to 50 ppm, 4 ppm of TCE was degraded effectively in both reactors. However, due probably to a large adsorption capacity of GAC, both TCE and phenol were removed most effectively in the GAC reactor, degrading over 85% of 9 ppm TCE and 87% of 50 ppm phenol. The activities of toluene dioxygenase, catechol-1,2-dioxygenase(C12O), and catechol-2,3-dioxygenase (C23O) were observed in the cells isolated from the bioreactors. From the results of the enzyme assay, it was demonstrated that immobilised microorganisms adapted to phenol possessed enzymes that were known to be responsible for the degradation of TCE.
In continuous operation of the packed-bed bioreactors for 285 days, the pretreated influent pretreated by PEROXONE AOP at pH 9 and 0.8 of $H_2O_2/O_3$ ratio was supplied to three bioreactors. As microorganisms were not inhibited by the oxidation intermediates of TCE and residual hydrogen peroxide, residual TCE of 4 ppm and 65 ppm phenol were degraded effectively in the Celite and the Ceramic bead bioreactors.
In piolt-scale, the PEROXONE AOP and two packed-bed bioreactors filled respectively with GAC and Celite were operated continuously with the influent 60 ppm of TCE. The influent was pretreated by PEROXONE AOP to 4 and 8 ppm of TCE for the Celite and the GAC bioreactors, respectively, and 50 ppm of phenol was degraded simultaneously with no competitive inhibition, to discharge in acceptable concentration. Also, a biodegradable compounds including glucose could be removed effectively with out inhibiting degradation of TCE and phenol. In conclusions, the combined treatment process of PEROXONE AOP and a biofilm process could treat the chlorinated solvent wastewater effectively.
