APPLICATION SCALE BIOREMEDIATION OF CONTAMINATED SOIL AND GROUND WATER

Risto Valo, Soil and Water Ltd. Itälahdenkatu 2, FIN-02100 Helsinki, Finland.

Volatile aromatic hydrocarbons (benzene, toluene, xylenes and ethylbenzene) often contaminate soil and ground water at old industrial sites and at gasoline stations, where they are major components of gasoline.

An old chemical manufacturing site at Keyport, NJ, USA was contaminated by these aromatic compounds after a storage tank had been injured . This accident was the main reason for contamination of both soil and ground water. Ground water was analyzed to contain 20-130 mg/l of total BTEX and soil 100-8000 mg/kg. Soil was rich in silt and clay with low hydraulic conductivity. The principle of soil remediation was to clean the soil on-site without carrying anything out and the target level of BTEX had to fullfill acceptable level set for residential area. Several treatment alternatives were evaluated; vacuum extraction, in-situ degradation, ex-situ treatment and landfilling. Relatively low permeability limited in-situ methods and vacuum extraction.

The work was started with laboratory tests of the site soil. The soil was tested for BTEX degradation and microbes responsible for this were isolated. Microbes were counted to know the amount of BTEX-degrading microbes, several of the microbes were isolated and their BTEX-degrading activity was tested. Two of the isolates were selected for full scale remediation of contaminated water. The microbes were fermented in large scale and the cells used to inoculate fluidized bed reactor for cleaning of BTEX contaminated water.

Second phase of the project was pilot scale experiments on-site, soil treatment in a 5 m³ windrow and water in a 50 l bioreactor. The five month experiments proved that BTEX compounds can be degraded also in the field. A permit for full scale remediation was obtained from local Environmental Agency in 1993.

Full scale soil and water treatment started in 1993. A bioreactor for treatment of vapor phase compounds of compost exhaust air was planned and started operation at the same time as soil and water bioreactors. The permit level of benzene in soil was 1 ppm and in water 0.01 ppm, total xylenes in soil 10 ppm, ethylbenzene in soil 100 ppm and toluene in soil 500 ppm. Activated carbon filters were installed to remove traces of BTEX present after bioreactors.

BTEX contaminated water game from ground water wells, soil excavation, rain and leaching water from compost windrow. Water had first to be oxidated followed by sedimentation of iron, which would otherwise precipitate on biofilter and tube surfaces and inhibit the process. Four fluidized bed water biofilters ( 800 liters each) were run in series. Biofilter water was aerated with pure oxygen to minimise exhaust gas volumes and loss of the volatiles. After treatment water was returned to subsoil in infiltration trenches. Treatment capacity was gradually increased from 1 000 l/d to 10 000 l/d. Benzene feed concentration of 20-100 mg/l was decreased to < 1 ppm in the biofilters and to < 0,01 ppm in the GAC unit.

Soil BTEX compounds were degraded in compost and partly in air biofilters. Benzene level of 1 ppm was reached in 10-30 days, depending on initial concentration. 60 to 80 % of the volatiles in gas biofilter feed was degraded in the reactors.