Vapour extraction systems

Description

A vapour extraction system is capable of removing volatile and semi-volatile contamination, such as petroleum hydrocarbons, from nonsaturated soil. The system consists of a network of air withdrawal wells installed throughout the contaminated soil. The wells contain vacuum pump systems which create a region of low pressure within the soil. This encourages the volatilisation of the contaminants, which are then drawn to the surface as a gas stream. The withdrawn gas is directed through a treatment system, such as a catalytic incinerator or an activated carbon bed, to remove the organic contaminants prior to release to the atmosphere.

Dual vapour extraction systems incorporate groundwater pumps that locally depress the groundwater table. This has three effects: (i) contamination that has entered the groundwater can be pumped and treated; (ii) oil-based contamination lying on the surface of the groundwater collects in the depressions and can be removed by the pumps; and (iii) the hydrocarbons in the so-called smear zone at the original groundwater level can be volatilised and removed by the vacuum pumps.

Gas flow through the soil can be increased by injecting air or nitrogen into the soil or groundwater to increase the rate of volatilisation of target contaminants. Pneumatic soil fracturing can be used to improve the air penetration of heavy soils. The increased air flow through the soil caused by any type of vacuum extraction has the additional benefit of improving the rates of biodegradation.

Application

Vapour extraction systems are used to clean up soil contaminated with volatile and some less volatile organic compounds. Typical compounds that can be removed through vapour extraction are petrol, diesel and chlorinated hydrocarbons and industrial solvents such as benzene, toluene, xylene and styrene. Vapour extraction is a successful in situ remediation technology and has applications at most oil storage site and refineries and in metal processing plants. Up to 999 % removal of hydrocarbons and solvents can be achieved. However, the full treatment of the gas stream must be considered.

Sizing

Vacuum extraction is seen as a relatively rapid technique for the in situ treatment of oil-based land contamination. It is compatible with bioremediation systems. A typical vapour extraction system sized to treat an area of contaminated land of about 500 m3 to a depth of about 4 m would require an electrical supply of about 20 kW.