Treating Contaminated Soil by Biological Means (Biomrediation).
Contaminated soil contains soil particles and the contaminant that flowed though them, contaminants absorbedin the soil and volatile contaminants are part of the ground gas. Contaminated soil constitutes a health and environmental hazard that is liable to contaminate water sources, impact on flora and fauna, the inhalation of contaminated dust, the contamination of underground caverns, etc. Soil contamination is generally only detected when large quantities of contaminants accumulate. The major sources of soil contamination are Gasoline filling stations, industry, transport, motor vehicle workshops, building rubble and defense industries. When suspicion of a contaminated site is raised, a number of preliminary steps are taken prior to the actual restoration. Such steps include conducting a historical review of the site and a ground survey, the objective of which is to define the concentration and types of the contaminants. After the contaminant has been identified, a plan to rehabilitate the soil is drawn up pursuant to the findings.
The restoration can be conducted at the site itself (in situ) or off site (ex situ). Prevalent in situ treatments include biological treatment, oxidization, and bounding solidification of metals, Ex situ treatments are generally carried out by removing the contaminated soil to an approved site or by incineration.
The Easytop Company carried out ex situ treatment, from the level of excavation and transport of the contaminated soil to approved sites all over the country up to and including the treatment and internment of the soil. Contaminated soil is generally transported to the largest site of its type – the Ef-eh Site, located on the Mishor Rotem Plain, where Easytop also developed and set up the largest pit of its type for biological treatment of contaminated soil. The pit contains, at any given time, some 40,000 tons of contaminated soil, at contamination levels of over 5,000 TPH, with no upper limit.
Using advanced technologies, Easytop reduces the level of contamination to under one half of a percent, and, from this stage on, the soil is used to coat mixed organic waste land fills. The size of the pit and the innovative treatment methods have drastically reduced the cost of treatment of contaminated soil over recent years.
Restoration of soil by biological means can be divided into the following main methods:
1. Biological treatment based upon the encouragement of the activity by microorganisms, which feed on the contaminant and break it down. Generally, biodegradation of a specific material is the result of the activities of a number of different microorganisms. Such treatment is generally carried out on soil contaminated with light refined fuels (for example, gasoline), or contaminants of similar attributes. Such treatment is not suitable for contaminants such as heavy metals. The biological restoration process makes use of local or non local bacteria. In order to survive, the bacteria requires basic conditions of nutrients, water, oxygen, temperature and redox (oxygen reduction) potential.
The effectiveness of the process depends upon the enzyme activity of the microorganisms, which break down the contaminants into harmless by products. Adding oxygen to contaminated soil increases the activities of the bacteria and hastens the biological restoration. The level of saturated oxygen must be higher than the critical concentration required to promote aerobic activity (the span ranges from 0.2 mg. per liter to 2 mg. per liter, with the most common value being 0.5 mg. per liter). The rate of supply of the oxygen is determined pursuant to the breakdown rate, with an insufficient rate leading to an extended restoration period, while an excessive rate will lead to high restoration costs. The majority of biological restoration systems operate under aerobic conditions (that is, in the presence of oxygen) but there are also anaerobic systems that facilitate the breakdown of specific contaminants.
There are several types of in situ biological soil treatments:
Bioventing – Streaming air and nutrients through boreholes in the contaminated ground, which encourages bacterial activity.
Biodegradation – the supply of oxygen and nutrients by using solutions circulated through the contaminated ground.
Biosparging – injecting low pressure air under the surface of the ground water. This causes an increase in the oxygen content of the water, thereby increasing the rate of bacterial breakdown activity.
Bioaugmentation – adding microorganism cultures from an external source to the soil.
Composting – The use of compost (fertilizer) increases the level of organic material in the soil and provides nutrients for the organisms. Combining contaminated soil with fertilizer leads to an optimal environment for the break down process. The fertilizer contains materials such as carbon, nitrogen, oxygen and water. The carbon is consumed by the bacteria as cellulose which is converted into sugars and heat. The nitrogen is used to build protein. The active fertilization method encourages the bacteria to break down and remove pathogenic entities, thus creating passive fertilizing that enables nature to do its thing at a slower rate.
Landfarming – A treatment whereby the contaminated soil is transferred and spread over a prepared bedding and periodically processed. Thus the bacteria is excited and the rate of breakdown increases.
Biopiles – A combination of the above two methods (by creating a heap of soil).
Bioreactors – The use of a special pit that accelerates the breakdown process. Preliminary treatment of the soil is required.
2. Phytoremediation – Restoration by means of vegetation – based upon the ability of vegetation to absorb toxins. The vegetation functions as a sort of filter and absorbs contaminants through the roots and leaves. This type of restoration is suitable for very large sites where other biological methods are not suitable. The treatment requires a long period of time but is reasonably inexpensive and does not harm the environment.
The Easytop Company has accumulated experience in treating ground and marshes using the wetlands method, the objective of which is to purify effluent and water using natural absorption and breakdown processes in an artificially established water saturated and highly vegetated environment. The bedding on which the vegetation grows serves as a biological filter.
3. Mycoremediation – restoration using mushrooms Based on the ability of mushrooms to exude enzymes which cause the breakdown of the contaminants.
The control and monitoring of the biological restoration processes are carried out by gauging the redox potential, the temperature, the level of oxygen and the pH level in the soils and in the ground water. These parameters affect the rate and scope of the breakdown of the contaminant materials. In addition, the structure of the soil plays a role – the nutrients, the oxygen and the water will pass through more easily in highly permeable soil. Therefore, low permeability soil is not suitable for in situ treatment.
The advantages of biological restoration are lower costs, facilitating treatment at the site itself without any ramifications vis-à-vis the environment, enable the removal of a broad range of contaminants and also the fact that the process is a natural one.
The disadvantages of biological restoration are that not all types of contaminants can be biologically treated. Occasionally the processes are very specific for a particular material, the duration of the treatment is liable to be long and, frequently, it is difficult to assess the effect of the treatment on the contaminated soil.