• polycyclic aromatic hydrocarbons (PAH) having up to four aromatic rings
Each of the different compounds requires some modifications to the basic technique. Especially if volatile substances have to be treated, emissions must be controlled and waste air has to be treated. An example of how to manage this problem is shown in Figure 10.6 for the remediation of a soil contaminated with phenols and aromatics.
For efficient extraction of volatiles from the soil, extraction pipes are installed in the heap for soil air extraction. During setup of the heap and the turning procedure in the dynamic system, the air of the tent is exhausted and treated, instead of extracting the soil air. The treatment system consists of four biofilter units and two activated-carbon filters. In addition to the usual tent material, another plastic cover having a thin layer of aluminum is installed inside the tent as an effective barrier to the volatile compounds. This example demonstrates that the heap technique is very flexible and can be modified from a very simple installation to a high-performance technology.
Another modification has been used for the bioconversion of explosives, especially TNT and its derivates. For biological detoxification, TNT (trinitrotoluene) is converted to TAT (triaminotoluene) under anaerobic conditions (Lenke et al., 1997). Under aerobic conditions TAT is fixed to the soil matrix. To establish strictly anaerobic conditions in the heap, organic material is added to the soil to a high extent. During degradation of the organic material, all oxygen is consumed and the temper-
ature reaches levels of about 60 °C. By turning the soil, aerobic conditions appear for a short time so that TAT is bound to the soil matrix and cannot be detected after treatment. With this technology, the treatment time can be reduced to less than four weeks to achieve target values (Fig. 10.7). The concentration of PAH is also reduced in the heaps. This example shows that the heap technology can be used for thermophilic degradation processes and that, under thermophilic conditions, degradation times can be reduced significantly.
Efficiency has to be discussed in relation to the economics of the process. Because of the low investment costs for installation and the simple operation procedures, the heap technique is of course a low-cost technology compared with other biological technologies like bioreactors or soil washing and incineration. Therefore, about 80% of the soil treatment plants installed in Germany are biological treatment plants (Schmitz and Andel, 1997), most of them working with the heap technique. However, because of the high number of treatment plants in Germany, with an annual capacity of about 2 500000 t the competition is extremely strong. During the last decade prices for soil remediation have decreased from nearly 100 US$ per ton to 30-50 US$ per ton. Even for the simple heap technique, this is the absolutely lowest limit for any reliable operation.
On the other hand, legislative regulations regarding soil handling and plant operation have become stricter, so that nearly half the costs for soil bioremediation by the heap technique are due to measures set by the authorities. In contrast to this high level of environmental-safety and health-care concerns regarding treatment plants that destroy contaminants and lead to recycling of soil for different uses, dumping contaminated soil and leaving the problem for the next generation is still allowed.
Nevertheless, soil treatment by the heap technique is suitable for many different situations involving contaminants, soil quality, or climatic conditions. It is adjustable by technical modifications to meet any requirement of the degradation process. Therefore, although it was the first large-scale remediation technique, it still needs further development to become the biological treatment technology of the future.
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