Ozone Treatment of Contaminated Groundwater

Biological pretreatment degrades contaminants into forms that can be treated more effectively by photochemical oxidation, thus optimizing the cost and performance of the remediation.

From 1963 to 1972 indeterminate chemical wastes were dumped into an abandoned limestone quarry. In 1980, contaminated groundwater and resulting bad odors were noticed from water percolating out of a natural seep at the base of a 20 m limestone hill down-gradient from the quarry, near the Autumn River valley. Under the limestone is a layer of fine sand underlain by a thick impermeable clay. The sand layer is a drinking water aquifer. The contamination plume has penetrated the limestone and sand to a depth of 40 m beneath the quarry. Clean up was initiated in 1991 when the French Energy and Environment Agency assumed responsibility for the project. The annual flow of the 20m-thick contaminated plume is estimated to be 50-80,000 m(3), or 25 m(3)/hr. Over 20 toxic chemicals were identified in the groundwater, including chlorinated and non-chlorinated solvents, alcohols, petroleum hydrocarbons, aliphatics and aromatics, and volatiles. Ethanol, phenols, chlorinated solvents, BTEX, acetone, and several aliphatic hydrocarbons were noted in concentrations exceeding milligrams per liter. Chemical oxygen demand was 250 mg/l; total organic carbon was 60 mg/l; and AOX was 5.5 mg/l.

A 600 m long, 6 m deep drainage trench was constructed at the base of the cliff to intercept the groundwater flow. Contaminated water was pumped to an carbon reactor, where organics were microbially degraded. Activated sludge was separated from the treated water using a membrane filter, resulting in 5 times greater biomass retention and 5 times lower sludge production. Concentrated chlorinated aliphatic and aromatic contaminants are then oxidized to the point where they can be naturally biodegraded.

During the pilot testing, experiments were conducted with and without biological pretreatment, and with oxidation with ozone, peroxide, and UV in various combinations. After pretreatment and ozonation, significant reduction in chlorinated hydrocarbons-to below detection limits-was reported. Concentrations of COD were 239 ppm in the raw water, reducing to 28 ppm with ozonation alone, and undetectable after pretreatment and ozonation. Dissolved constituents were reduced by 80-85%, and 100% of VOCs were eliminated. Estimated costs for reducing VOCs to acceptable drinking water limits were FF15 million (capital cost) plus FF23.7 million annually. Meeting acceptable surface water discharge levels would cost about FF10 million initially, with an annual cost of FF1.7 million. Ozone consumption was higher in an O(3)/H(2)O(2) system than in 0(3)/UV, but the results were the same.

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