Biodegradation of chlorinated aliphatics and aromatic compounds in total-recycle expanded-bed biofilm reactors

The widespread use of chlorinated ethenes and aromatic hydrocarbons and their improper disposal has led to the occurrence of compounds like trichloroethylene. tetrachloroethylene, dichloroethylene isomers. vinyl chloride, benzene, toluene, xylene, etc., in ground water at spill sites and at disposal sites. These compounds are recalcitrant in nature, and certain toxic intermediates formed due to their incomplete mineralization, may accumulate in the environment. The presence of these compounds in drinking water aquifers is a cause for serious concern because of their toxicity and suspected carcinogenicity. The currently used techniques for the cleanup of these contaminated sites include incineration, land filling, and air stripping. The disadvantages of existing technology include; (i) incineration although a widely used technique is a potentially hazardous waste management technique; (ii) land filling is not really a treatment process, this only moves the problem from one location to another; and (iii) air stripping leads to deterioration of air quality. Degradation of hazardous chemicals by microorganisms is very promising. The degradation of trichloroethylene by bacterial systems under aerobic and anaerobic conditions has been reported. Aerobic transformation of trichloroethylene results in harmless products. Methanotrophic microorganisms degrade TCE by a process of co-metabolism. This study examined the biodegradation of chlorinated ethylenes like trichloroethylene, tetrachloroethylene, vinyl chloride, vinylidene chloride, and aromatic compounds like benzene, toluene, and xylene in a continuously-recycled expanded-bed biofilm reactor. One of the reactors (reactor 1) was inoculated with a microbial consortium isolated from a TCE contaminated site (the SRP consortium) capable of utilizing methane and propane as a carbon and energy source. Another reactor (reactor 2) was inoculated with a known mixed culture (P. fluorescence, P. putida (strains RB1401 and pWWO) and M. trichosporium OB3b). A third reactor served as an uninoculated control. More than 97% of initial levels (20 mg/L) of TCE was degraded in the reactors within 12 days. Benzene, toluene and xylene were degraded (>99%) within the first 8 days. The reactor inoculated with the SRP consortium degraded PCE, VC, and VDC. No significant degradation of PCE, VC, and VDC was observed in reactor 2. The indigenous SRP consortium performed better than pure cultures, in terms of degrading a wider variety of mixture of toxicants. Measurement of dissolved oxygen levels in the effluent from the expanded-bed column indicated that the bulk liquid phase was aerobic. Additional experiments were performed to study the effect of protozoan grazing on TCE degrading microbial consortia. Preliminary results indicate that degradation potential of indigenous consortia that contain protozoa can be improved by treatment with cycloheximide.