Heap technique

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.

1. HEAP TECHNIQUE PRASANNA R KOVATH,DEPARTMENT OF BIOTECHNOLOGY

2. HEAP TECHNIQUE 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 the heap technique the contaminated soil is prepared by homogenizing and mixing with additives and then piled up in heaps. Before starting the treatment, samples of the excavated contaminated soil are tested for biological degradation of the pollutants by standardized laboratory procedures .

3. 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.

4. The most important steps in pretreatment are  homogenization of soil material and mixing with additives . 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 . 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

5.   'substrate' indicates all additives that improve the physical and chemical soil structure. Depending on soil quality parameters like particle size, pH, organic matter, etc . 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

6. The added carbon source can be used as co substrate for energy supply or an inducer for degrading enzymes; this use has been investigated for the degradation of chlorinated hydrocarbons like tetrachloroethylene and trichloroethylene. 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.

7. 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 Good results have been obtained by adding complex mixtures of 'substrates', nutrients, and microorganisms, e.g., compost 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 .

8. 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 . After pretreatment, the soil is transferred to the degradation area and piled in heaps. According to the environmental regulations for treating hazardous wastes , 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.

9. 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.

10. 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 .

11. 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

12. 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 . 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 .