Fundamental Research And Comparison Of Four TCE Removal Processes

Along with the rapid industrial and agricultural development of the modern society, trichloroethylene (TCE) and other chlorinated hydrocarbons have entered environment due to improper disposal practices and accidental leaks resulting in serious contamination of groundwater in many parts of the world. Granular activated carbon (GAC) adsorption has often been employed for removing persistent organic pollutants (POPs) to produce a clean effluent meeting a very low discharge limit (<10μg/L); taking advantage of the concurrent biodegradation of the adsorbed POPs in many GAC adsorbers, biological activated carbon (BAC) process can be used to extend the GAC service life; expanded granular sludge bed (EGSB) process has been employed for treating influents containing high concentrations of toxic pllutants; biotrickling filter (BTF) process is capable of removing POPs from the exhaust stream of the stripping operation. This research has been conducted to determine the feasibility of the four treatment processes for removing TCE, to identify the potential applications, and to produce results useful for remediation of TCE contaminated groundwater.Ten GAC samples of different raw materials and preparation were employed in the batch adsorption isotherm runs for measuring their TCE capacities and the continuous flow breakthrough runs for simulating the TCE removal in actual treatment applications. The GACs'adsorptive capacities for TCE were in the same order as their phenol numbers; they were not affected by the starting TCE concentration and/or the presence of small amount of methanol. The higher utilization rate found for the meso pore rich coal based GAC makes it competitive despite its relatively low TCE capacity. The GAC made from toxicant free, low cost and renewable bamboo is attractive because of its relatively high TCE capacity and availability. GACs'capacities for TCE in pure water were reduced by competitive adsorption of other organic constituents of the water samples; small organic compounds in tap water were more competitive than the NOM in the higher TOC well water sample. The MCRB data confirmed the GACs'available TCE capacities and that the serial bed treatment is desirable because most of the first absorber's capacity can be utilized for removing the pollutants.Under the aerobic condition, TCE can be biodegraded only in the presence of a co-substrate such as phenol. Phenol can induce the production of oxidase and the energy necessary for TCE cometablism; high degree of TCE degradation would take place after most of the phenol has been biodegraded. Acclimated mixed phenol degraders originated from activated sludge were employed for the batch experiments to assess their TCE degrading capability. Inoculation of highly acclimated TCE degraders quickly established the BAC system which removed 20-40% TCE of the feed during the 170d treatment run. Using a GAC with a high TCE capacity ensured the treatment performance during the start-up stage and/or under the shock loading conditions. The TCE removal rate was dependent on the phenol concentration of the feed; a higher than the optimum concentration of phenol would cause competitive inhibition while too low a concentration would result in reduced rate of TCE degradation; up to 40% TCE removal was obtained in treating a feed of 2-4mg/L of phenol and a phenol/TCE loading ratio of 15. Based on the mechanism of cometabolism in the TCE-phenol system, sequential feeding of phenol (2h) and TCE (22h), which avoided competitive inhibition in the BAC and thus would enhanced the growth rate and activity of the TCE degraders, was employed to acheive a better treatment performance of 65% TCE removal (influent concentration=200ug/L) with a substantially higher max removal of 72% (0.39g/m3/h).TCE can be anaerobically dechlorinated to dichloroethylene, vinyl chloride and ethylene. In treating an influent with 2-10mg/L of TCE under a hydraulic retention time of 6h, the addition of the commercial SDC-9 bacteria to the anaerobic granular sludge in an EGSB bioreactor enhanced its performance with a faster start-up and a stable higher TCE removal of about 85%(max removal capability=1.5g/m3/h). More ethylene was produced in a longer treatment time when the EGSB treatment was conducted in the sequential mode. The results also have demonstrated that anerobic granular sludge was a good medium for retaining and growing the supplemented active bacteria which enabled the outstanding long term stable TCE removal of the EGSB process.Inoculation of SDC-9 accomplished rapid start-up of the BTF process; operating at an empty bed residence time of 12min, more than 99% of TCE was removed from the nitrogen influent (feed concentration=700mg/m3) with an outstanding TCE removal capability of up to 9.0g/m3/h; the bed plugging and other operating problems which would adversely affect the excellent treatment performance were not observed during the 150d+ test period. TCE removal capability of BTF sections was dependent on the type, density and activity of the residing TCE degraders; more TCE was degraded in the front part as the treatment progressed. The TCE removal capability of an ineffective BTF (2% of the former activity associated with the 50% remaining microorganisms 60d after the treatment ended) was mostly restored 8 days after the system was restarted. For the BTF which was suspended for 28d, its TCE removal capability was basically recovered 1d after the treatment was resumed; however, a longer time was required to achieve the same degree of ethylene yield.Overall, the results have confirmed the feasibility of the four researched processes for cost effective TCE removal from contaminated water bodies. The highly effective GAC process is the one to choose for achieving total TCE removal; the long lasting aerobic BAC process may be beneficially employed for treating a low concentration (<500μg/L) influent; the stable anaerobic EGSB process is preferred for treating a higher concentration (up to 10mg/L) influent; the high loading anaerobic BTF is the best for removing gaseous phase TCE after nitrogen stripping of TCE contaminated water body.The features and applications of the effective GAC process and the three innovative biological TCE removal processes are presented. The adsorption study results have demonstrated that the GACs'adsorptive capacities for TCE is in the same order as their phenol number, that the capacities for TCE in pure water are reduced more by the small molecule organic consistutents of tap water relative to the larger constituents of the well water, that the efficient MCRB method can be use to conclude the essential breakthrough in about 1% of the time required by the conventional method, and that the two-adsorber-in-series mode of treatment is the key to the cost effective application of the GAC process. The BAC study results have shown the aerobic TCE degradation was achieved by cometabolism and that the sequential addition of the co-substrate (phenol) and TCE produced better TCE removal due to avoid competitive inhibition. The TCE removal capabilities of the SDC-9 enhanced anaerobic EGSB and BTF processes have been defined and that advanced molecular biology techniques can be employed to identify the type, the dechlorination genes, the population dynamics and the distribution of the TCE degraders in the treatment systems.