Physical Treatment Methods

Dig and haul is not a treatment method per se, but rather the practice of excavating contaminated soil and hauling it to an off-site location for incorporation with other contaminated soil or treatment. The practice can be performed year-round (excavator with frost bucket in winter), is routinely used in the Arctic and Antarctica where roads and infrastructure exist, and is expensive. Permitting may limit the practice in sensitive environments (e.g., tundra, tundra lakes or marshes, and Arctic river and stream drainages) or when seed material for site reclamation is in short supply.

Incineration is a high-temperature treatment process that affords complete destruction of petroleum hydrocarbons in soil. Rotary kiln, multiple-hearth, or flu-idized bed, fixed-base or mobile incinerators can treat up to 200 tons per day of petroleum-contaminated sand or gravel. Incineration is not amenable to cohesive soils, and can produce incomplete combustion products and residual ash that may have to be treated as hazardous waste. Thermal incineration is expensive as operating costs are high, and since few fixed-base facilities operate in the Arctic; mobile units incur high mobilization/demobilization and permitting costs. The Arctic operating season is from April to mid-October. Incineration is very expensive, and the environmental risks are considered too high for use in Antarctica.

Thermal desorption, or hot-air vapor extraction, is a thermal process that removes oxidizable hydrocarbons with low boiling points. The typical thermal treatment plant comprises mechanical pretreatment of the excavated soil, followed by thermal treatment in a rotary kiln, and with auxiliary treatment of exhaust gas. Operating temperatures for petroleum hydrocarbons range from 600°C to 900°C, with increased desorption rates realized at higher temperatures. The process is amenable to granular soils and non-cohesive silts. Removal efficiency is a function of temperature, residence-time volatility, and purge-gas velocity (Riser-Roberts 1998). Thermal desorption is cost-prohibitive for Antarctic use; the Arctic operating season is from May to early October. Mobile units are available that can be disassembled and reassembled at remote sites. However, expect mobilization/demobilization, permitting, energy, and labor costs to be high.

Incorporation and encapsulation refers to use of petroleum-contaminated soil in roads and airport runways and tarmacs. The contaminated soil is first screened to remove unsuitable fractions (i.e., aggregate greater than 5 cm across). Soil gradations are then performed on useable material to assess suitability for asphalt or tarmac incorporation, or highway or runway encapsulation. Highway/runway design specifications, potential long-term liability, wetlands issues, and required work plans limit use of this method in North America. Plan submittals include a runway or pavement structure design study and a leachate assessment or migration model. Modeling must demonstrate that contamination will not migrate off-site. The method is usually expensive because of high hauling costs between contaminated and use sites, and the additional expenses associated with plan submittals and long-term monitoring. Incorporation and encapsulation is more appealing when contaminated and use sites are located near each other.

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