The most feasible bioremediation technologies of unsaturated soils contaminated by Petroleum are (1) the on-site controlled remediation of the excavated contaminated plume in the contained system; and (2) the in situ sub-surface, highly controlled process in which nutrients and microbes or air are injected into the plume volume via a grid of wells and reversing the direction of the pumping. The detailed results of ex situ (controlled aerobic bioremediation) of ten sites with differing petroleum contamination and four in situ cases are reported and compared. It is concluded, that bioremediation of petroleum contaminated soil is an effective, safe and economically feasible remediation technique for the remediation of such contaminated sites, contingent on the providing of the site-specific appropriate conditions. However, the biodegradation rate of the NAPL is more variable in the in situ than in the ex situ aerobic process, correlating best in the latter with the molecular weight of the petroleum contaminants.

Biodegradation studies of 1 mM aqueous pentachlorophenol (PCP) solutions have been conducted in completely mixed batch reactors using a selected microbial consortium (SMC) and a stock culture of activated sludge (AS). Comparable biodegradation has also been carried out following chemical oxidation pretreatment by hydrogen peroxide alone and by Fenton's reagent (a solution of hydrogen peroxide with ferrous ion) in order to observe any enhancement of biodegradation.
The microbial species isolated from the soil contaminated by PCP were gram‐negative, rod‐shaped bacteria, identified as Pseudomonas putida and Pseudomonas aeruginosa. The stock culture of activated sludge was developed from a wastewater treatment plant treating domestic wastewater only and maintained on tryptic soy broth substrate.
Chemical oxidation pretreatment experiments were conducted in the absence of SMC and AS, and the chemical oxidation reaction was almost complete after four hours of reaction. The highest PCP removal by chemical oxidation tested in this study was 2.97% with Fenton's reagent at a molar ratio of 1:1 (H202 :PCP) and 50 mg/L iron.
The biodegradation process was carried out on both pretreated and untreated PCP for 12 days, but the biodegradation was almost complete by the sixth day. Inoculation of SMC or AS into the aqueous PCP solutions following pretreatment by Fenton's reagent resulted in a higher PCP uptake rate than did the system treated by hydrogen peroxide alone. The enhanced biodegradation rate was highest when the system was pretreated by Fenton's reagent with the highest chemical oxidant dose, a 1:1 molar ratio of hydrogen peroxide to PCP and 50 mg/L iron. The biodegradation rates of untreated PCP, degraded by either SMC or AS, were each similar and much slower than for any of the pretreated systems.
Comparison of the biodegradation rate of the un‐pretreated PCP with the rates of pretreated systems indicated that chemical oxidation by Fenton's reagent enhanced the biodegradability of PCP by both SMC and AS.

Land disposal is required for industrial chemicals which are not readily biodegraded. Such compounds lead to adverse effects on the environment if they escape containment. Recalcitrant and persistent hydrocarbons and chlorinated chemicals are inherently resistant to any degree of biodegradation and cause a growing threat to underground aquifer quality. Hydrogen peroxide is a potentially economical method of pre-oxidation utilized to enhance the biodegradation of persistent and recalcitrant organics in contaminated soil systems. This pre-oxidation technology was examined in a laboratory respirometer using three model toxic organic chemicals: toluene, trichloroethylene and pentachlorophenol. Microbial cultures were selected from contaminated sites for the degradation of each model organic chemical. The rate at which the microbes degraded the organic chemicals in unoxidized aqueous systems was compared to the rate of degradation in peroxide pre-oxidized aqueous systems. Results indicated that pre-oxidation enhanced the biodegradation of trichloroethylene and pentachlorophenol. Toluene, in contrast, was not significantly oxidized by pretreatment with hydrogen peroxide, and its biodegradation rate was not enhanced by the oxidation pre-treatment process.

Nuclear magnetic resonance relaxation times (T, and T2) were measured for flocculated and unflocculated samples of activated sludge. The weight of water and solids in the sludge samples was found and related to T, to find the relative percentage of bound water. The results suggest that the amount of bound water increases as the samples become more unflocculated. The values of T, and T2 also indicate that unflocculated individual particles are characterized by loose packing of shorter molecules and that the addition of larger molecules may induce flocculation.

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