The heap technique for biological soil treatment is an ex situ technology, that is, the contaminated soil is excavated and separated from the uncontaminated material. In contrast to so-called 'landfarming', where the contaminated material is spread over a large area in a comparably thin layer and mixed with the existing soil cover, in the heap technique the contaminated soil is prepared by homogenizing and mixing with additives and then piled up in heaps. Figure 10.1 shows the different steps of treatment, as implemented in the Terraferm system (Henke, 1989).
Before starting the treatment, samples of the excavated contaminated soil are tested for biological degradation of the pollutants by standardized laboratory procedures (Dechema, 1992). After a positive result is obtained in these tests, the first step in soil preparation is the separation of non-soil material like plastics, metals, etc., as well as stones having a particle size of more than 40-60 mm. Although non-soil material has to be disposed of or treated by other methods, the stones can be crushed and added back to the soil. Often the stones are not contaminated to the same extent as the fine particles, because the pollutants did not penetrate into them. If so, the separated stones can be reused directly. Therefore, separation is not only important for the degradation process but is also a way to reduce the volume of material that has to be treated.
The most important steps in pretreatment are homogenization of soil material and mixing with additives. Homogenization means that the normally inhomogene-
10.2 Principles of the Heap Technique | 277
ous distribution of the pollutants is changed into an average concentration of contaminants in the total volume of soil. Statistical evaluation of 35 large-scale remediation projects using the heap technique (Krass et al., 1998) shows that the standard deviation of the hydrocarbon concentration is reduced significantly after homogen-ization and that the average level of concentration decreases to about 50% of that calculated for the original material. This second effect is due to overestimation by the analysis of the original material because samples are normally taken from more-contaminated parts of the soil.
Determination of requirements for and addition of additives of suitable quality and quantity is another important step for successful soil treatment with the heap technique. As shown in Figure 10.1, additives can be divided into different categories. Here, 'substrate' indicates all additives that improve the physical and chemical soil structure. Depending on soil quality parameters like particle size, pH, organic matter, etc., and the results of laboratory testing, materials such as compost, bark, lime, tensides, etc. are added to create optimal environmental conditions for the degrading microflora. 'Substrate' can also be used to enhance the soil temperature by including a high amount of easily degradable organics, although with the risk of a high level of carbon sources that are competitive to the contaminant. Otherwise, the added carbon source can be used as cosubstrate for energy supply or an inducer for degrading enzymes; this use has been investigated for the degradation of chlorinated hydrocarbons like tetrachloroethylene and trichloroethylene (Ewers et al., 1990; Koziollek et al., 1999; Meyer et al., 1993).
The second class of additives are the nutrients for the degrading microflora. Because contaminated soil often comes from industrial sites and is often excavated from layers several meters deep, it usually has no significant content of nutrients like nitrogen, phosphorous, or potassium. Of course, degrading microorganisms need these substances for growth and metabolism. Most often, mineral fertilizers are used as liquids or in granular form to supplement the soil with these compounds. Because nutrients normally remain in the soil after treatment, it is important not to overdose with these additives. The level of fertilizer should not exceed that used in conventional agricultural practice.
As a third element, specialized microorganisms can be added to the soil during the mixing and homogenization procedure. Inoculation with bacteria, fungi, or enzymes to enhance the degradation process is controversial in the scientific community. In contrast to many studies in the United States, investigations in Germany have shown no significant effects of added cultures of specialized microorganisms on petroleum hydrocarbon degradation (Dott and Becker, 1995), which is the prior application of the heap technique. The potential of the autochthonous microflora is normally sufficient for effective degradation.
Good results have been obtained by adding complex mixtures of 'substrates', nutrients, and microorganisms, e.g., compost (Hupe et al., 1998) or activated sludge from wastewater treatment plants. The overall goal of the mixing and homogenization process is to obtain optimum conditions for aerobic metabolism of the contaminating substances.
For sufficient homogenization and mixing of contaminated soil, special machines and aggregates have been developed, which have to combine powerful homogenization and mixing units with controlled and sophisticated dosing of the various additives. Today, specialized equipment is available (Fig. 10.2) with capacities of 50 t h-1 or more and which have been adapted to different soil qualities from sand to clay. Crushing units can be integrated directly in this machinery.
After pretreatment, the soil is transferred to the degradation area and piled in heaps. According to the environmental regulations for treating hazardous wastes in Germany, the heaps must be located in a closed space. Depending on local climatic conditions, locating the process in a closed system is not only necessary for complying with environmental regulations but is also an important tool for controlling the degradation process, especially the water content and temperature.
The area must be prepared below the ground surface to prevent contaminated seepage water from penetrating into the subsoil. This can be done by compacting the soil or by installation of an area sealed with concrete or asphalt. Often, layers of 1.0-1.5-mm-thick high-density polyethylene (HDPE) are used to ensure safe and sustainable enclosure of the contaminated material.
To minimize emissions of volatile compounds to the air, the heaps are set up in structures such as tents or sheds or covered with plastic sheeting or membranes. With this measure it is also possible to protect the heaps from unsuitable weather conditions like rainfall or extreme temperature. The design, construction, and material of the cover depend on the kind of heap technique that is used. In the past five years in Germany, soil treatment has changed from on-site to off-site installations. Therefore, the heap technique is used mainly in stationary treatment centers, which are permanent installations and normally equipped with a treatment shed or a similar building in which the heaps are set up.
During the process of degradation, the soil is monitored continually by analyzing samples from different parts of the heap. The main control parameters are:
• concentration of the contaminants
• water content
• concentration of available nutrients
• biological activity (soil respiration)
Depending on the monitoring results, the degradation conditions are optimized by aeration, addition of water or nutrients, and further homogenization. The treatment
ends after reaching the target values, which are sufficient for reuse of the cleaned soil. The time of treatment differs greatly, depending on the kind and concentration of the contaminants, the target values that have to be reached, and the soil quality. The normal residence time is in the range of several months. For example, Krass et al. (1998) detected average halftimes of 85 d with a 95% confidence interval in a range from 75.4 to 94.6 d.
After treatment by the heap technique, the soil quality is adequate for it to be used as topsoil for landscaping or as dumpsite cover. Because of the treatment procedure the soil is free of larger stones, very homogeneous, and enriched in nutrients and humic substances.
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