Besides the different technical solutions for below-surface preparation and heap cover, various treatment systems have been developed to establish and maintain suitable conditions for microbiological degradation of toxic compounds. Figure 10.3 shows several approaches concerning humidity, agitation, aeration, and temperature and their impact on the area needed for treatment.
The first technologies used for large-scale biological soil remediation were open-air heap installations with facilities for water recycling (Altmann et al., 1988). The moisture in the heaps is above the maximum water holding capacity (whc), and the seepage water is collected via a drainage system in a pond at one end of the heap (Fig. 10.4).
From the collecting pond, the water is pumped to the surface of the heap and spread over the heap. Nutrients and other soluble additives are mixed with the water, and the microorganisms are supplied with the added substances and oxygen via the water phase.
In contrast to these 'wet' systems, comparable 'dry' technologies have been developed and are mainly used today. The moisture in the dry heaps is below the maximum whc, so that water seepage is prevented and the soil pores are filled with water and air.
Systems with low water content can be operated with (dynamic) and without (static) agitation, but the wet systems are all static. By agitation, i.e., turning and mixing the soil at time intervals of days to weeks depending on the level of biological activity, the heap is aerated and the water or nutrient content can be readjusted. With the static and dry systems, no additional supply of additives is possible, so all ingredients must be added during pretreatment. Therefore, special additives such as slow-release fertilizers must be used with these technologies. It is also necessary to install some kind of aeration system to supply the microorganisms with oxygen.
One advantage of the static and dry system is the height of heaps, which can be up to 3-5 m, whereas in dynamic systems the heaps are no higher than about 2 m, because the special turning machines (Fig. 10.5) have a limited turning depth.
The 'wet' systems require the greatest area, because the limited capacity of water to transport oxygen leads to anaerobic layers if the height of the heaps is >0.5 m.
Supply with oxygen is the most critical factor in biological soil remediation by the heap technique. Because of the high oxygen consumption during aerobic degradation of hydrocarbons, an aeration system must be installed in all static heaps. The 'wet' technologies supply oxygen through the water phase. Therefore, the efficiency of oxygen transport to the different parts of the heap is limited by the concentration of dissolved oxygen and cannot exceed saturation. Investigations of changes in oxygen concentrations in different parts of the heaps show a rapid decrease during the first phase of degradation, with creation of anaerobic zones and methane production inside the heaps (Koning et al., 1999). Recent results of large-scale experiments show that high-pressure injection of air can solve this problem and may be an efficient alternative to dynamic treatment technologies.
If oxygen supply is sufficient, the temperature in the heaps increases by biological activity to a level of 30-35 °C. Especially in closed systems this temperature level can be maintained independent of outside conditions, so that optimum mesophilic conditions can be established by the heap technique. Research on extremophile microorganisms and their practical biotechnological application indicates a high potential of thermophilic bacteria for hydrocarbon degradation (Sorkoh et al., 1993). The establishment of thermophilic conditions in heaps is one approach to increasing degradation efficiency. Besides using the existing climatic conditions, in some regions of the world, such as Arabia, Africa, and South America, it is possible to increase the temperature by adding easily degradable organic matter. This leads to enhanced oxygen consumption, which must also be ensured by the technical design.
Today the dry and dynamic solution is the common technology for treating petroleum hydrocarbons by the heap technique. 'Wet' solutions have failed because of their long degradation times of (1-2 years) and the large demand for space. If the dry and static approach could overcome problems with limited oxygen supply, it may be the technology of the future because of the small area needed for the installation and the possibility of thermophilic process design, which together can enhance the efficiency of the heap technique.
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