In a rotating-drum bioreactor (or revolving tubular reactor), the reactor itself acts as a rotating conveyer in which the contaminated solids are mixed (Fig. 11.3). In a (se-mi)continuous operation the solids are transported (screwed) through the system. Loading at the head of the process is combined with unloading at the back. The solids residence time in the system is determined by the shape of the screw blades, the rotation speed, the scale, and the amount of material present. Experiments have been carried out in a bioreactor with a length of 25 m and a diameter of 3.5 m. A warm air blower provided an appropriate temperature and oxygen, a sprinkler system was installed and nutrients added; the residence time was 14 d. For starting concentrations of mineral oil contaminant ranging from 1000 to 6000 mg kg-1, at a temperature of 22 °C, the contaminant concentration decreased to 50-350 mg kg-1. Most breakdown activity took place in the first few days (Munckhof and Veul, 1990).
In a revolving tubular reactor developed by BioteCon in Germany, having a length of 18 m and a diameter of 2.5 m, between 0.5 and 1 t h-1 of soil can be treated (Kieh-ne et al., 1995). Model drums with diameters of 0.5 m and 1 m were tested. The typical moisture content was 15%, the revolution speed was 1 h-1, and the aeration rate 0.125 VVM. In the trials the breakdown of motor fuel, kerosene, and phenol was investigated. Under batch conditions kerosene was degraded from 8300 mg kg-1 to 500 mg kg-1 in 300 h. When the same soil was treated in a repeated batch (25% of the treated soil was returned and mixed with new soil), within 180 h end concentrations of 180 mg kg-1 were reached. The increase in microbial activity due to the reuse of biomass was considered responsible for the enhanced breakdown.
In an experiment a combination between heap leaching and reactor processing was tested. After treatment in the reactor for 120 h, kerosene-contaminated soil was deposited in a heap for four weeks. The treatment was finalized by a second reactor treatment for 100 h. With a total residence time in the reactor of 220 h, an overall conversion of 98.5% was achieved. It was concluded that for "contaminated soils in the order of 500-3000 t the revolving tubular reactor especially demonstrates its advantage" (Kiehne et al., 1995). The technology has been developed to operate on-site.
Another solid-state process was developed by Umweltschutz-Nord (Bremen, Germany). Using a steel tube having a length of 45 m and an diameter of 3 m, the contaminated soil is screwed through the Terranox system. For mineral oil-contaminated soils a treatment period of 6-8 weeks is used; the system can also be used for a combination of anaerobic and aerobic treatments.
On a smaller scale (3, 6, and 90 L) a blade mixing bioreactor was operated as a solid-state process (Hupe et al., 1995). The authors concluded that, in addition to the benefit of adding compost, moisture enhances the process; however, the water content should be <65% of the maximum moisture content to obtain solid-state processing conditions. The aeration rate is also a key parameter.
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