This paper reviews ten years of research on on-site and in situ bioremediation of chlorophenol contaminated groundwater. Laboratory experiments on the development of a high-rate, fluidized-bed process resulted in a full-scale, pump-and-treat application which has operated for several years. The system operates at ambient groundwater temperature of 7 to 9°C at 2.7 d hydraulic retention time and chlorophenol removal efficiencies of 98.5 to 99.9%. The microbial ecology studies of the contaminated aquifer revealed a diverse chlorophenol-degrading community. In situ biodegradation of chlorophenols is controlled by oxygen availability, only. Laboratory and pilot-scale experiments showed the potential for in situ aquifer bioremediation with iron oxidation and precipitation as a potential problem.

Groundwater contaminants including 2,4,6-trichlorophenol (TCP), 2,3,4,6-tetrachlorophenol (TeCP), and pentachlorophenol (PCP) were mineralized in three aerobic fluidized-bed reactors (FBRs) employing sand, volcanite, and diatomaceous earth as biomass carriers. The effect of temperature on chlorophenol degradation kinetics was studied in FBR batch tests at temperatures ranging from 4 to 16.5°C. TCP and TeCP degradation was modeled using the Michaelis-Menten kinetics. Specific maximum degradation rates for TCP and TeCP varied with temperature from 0.46 × 10−3 to 31 × 10−3 mg mgVS−1 h−1 and Ks varied from zero to 7.1 mg l−1. Degradation of PCP was affected by the presence of TCP and TeCP and followed competitive inhibition kinetics. Specific degradation rates for PCP degradation varied with temperature from 0.24 × 10−3 to 1.7 × 10−3 mg mgVS−1 h−1 and were always lower than for other chlorophenols. The Arrhenius equation described the temperature effects on biodegradation of chlorophenols. The activation energies (kJ mol−1) for TCP and TeCP varied from 126 to 194, and for PCP from 59 to 130. In the studied temperature range, a 10°C decrease in temperature generally resulted in over seven times slower degradation rates. The volcanite reactor had the highest and the sand reactor the lowest biomass accumulation. © 1998 Elsevier Science Ltd. All rights reserved

This paper summarizes a five-year research effort on the development of a high-rate fluidized-bed biofilm system for remediation of chlorophenol contaminated groundwater. Laboratory-scale experiments with model compounds and actual contaminated groundwater included studies on aerobic and anoxic treatment, strategies for selective enrichment of chlorophenol degrading microorganisms, kinetics and stoichiometries of chlorophenol conversions, toxicity removal, growth and culture characterization, recovery from process upsets, and operation at suboptimal temperatures. The aerobic process using chlorophenols as the sole source of carbon and energy showed the greatest versatility and efficiency. The contaminated groundwater consisted of 2,3,4,6-tetrachlorophenol, 2,4,6-trichlorophenol and pentachlorophenol with total chlorophenol concentrations ranging from 45 to 55 mg/l. At room temperature, groundwater remediation resulted in over 99.9% mineralization at chlorophenol loading rates of 1000 mg/l/d and hydraulic retention times of less than 1h. At the groundwater temperature (7°C), similar removal efficiencies were obtained at chlorophenol loading rates of up to 740 mg/l/d. The laboratory results were successfully repeated in an on-site, pilot-scale demonstration.

Data on anaerobic degradation of chloroaromatic compounds in Upflow Anaerobic Sludge Blanket Reactors (UASB-reactor) are presented and compared. Special attention is given to the metabolic pathways for degradation of chlorinated phenols by granular sludge. Results indicate that PCP can be degraded in UASB-reactors via stepwise dechlorination to phenol. Phenol will subsequently be converted to benzoate before ring cleavage. Dechlorination proceeds via different pathways dependent upon the inocula used. Results are further presented on the design of special metabolic pathways in granules which do not possess this activity using the dechlorinating organism, Desulfomonile tiedjei. Additionally, it is shown that it is possible to immobilize Dechlorosporium hafniense, a newly isolated dechlorinating anaerobe, into granular sludge, thereby introducing an ability not previously present in the granules.

Aerobic fluidized-bed treatment was employed for psychotrophic bioremediation of chlorophenol-contaminated groundwater. Laboratory-scale, continuous-flow reactors were inoculated with nonacclimated activated sludge, the groundwater was amended with inorganic nutrients and a phosphate buffer, and continuous groundwater feed was started at 14-17-degrees-C. Chlorophenol concentrations (in mg/L) in groundwater were as follows: 7-11 for 2,4,6-trichlorophenol, 32-36 for 2,3,4,6-tetrachlorophenol, and 1.8-2.3 for pentachlorophenol. After the startup period, the treatment temperature was gradually decreased to the ambient groundwater temperature (7-degrees-C) and further to 4-degrees-C. Steady-state fluidized-bed remediation at 5-h hydraulic retention time resulted in effluent concentrations of less than 0.003 mg/L of each chlorophenol at all temperatures tested. At 5-7-degrees-C, over 99.9% chlorophenol biodegradation was achieved at a chlorophenol loading rate of 740 mg L-1 d-1. Inorganic chloride releases were in conformity with the chlorophenol removals indicating mineralization. In conclusion, this system used higher loading rates than previously reported for bioremediation, and the effluent quality was close to drinking water standards. Further, this is the first paper on high-rate bioremediation at ambient groundwater temperatures or lower (4-10-degrees-C).