Soil Treatment Technologies through Bioremediation

Abstract

Bioremediation includes processes such as bioventing, bioaugmentation, phytoremediation, biopiles, and composting. In this chapter, we details the characteristics, utilization and operating conditions of each process. Bioremediation is understood, according to the general definition, as the use of living organisms (microorganisms, plants, etc.) to improve and restore the ecological condition of a polluted or degraded substrate (area, land, aquifer, etc.) to better, favorable quality parameters life, harmless, non-polluting or to return it to its previous state. Soil treatment technologies through bioremediation include two types of treatments: in situ biological treatments (bioventilation, bioaugmentation, phytoremediation in soil) and ex situ biological treatments of polluted soils (biopiles and soil cultivation).

1. Introduction

Bioremediation is a modern pollutant treatment technology that uses biological factors (microorganisms) to transform certain chemicals into less harmful/hazardous final forms, ideally CO₂ and H₂O, which are non-toxic and are released into the environment without altering substantially the balance of ecosystems. Bioremediation is based on the ability of some chemical compounds to be biodegraded.

The concept of biodegradation is accepted as a summation of the decomposition processes of some natural or synthetic constituents, through the activation of some strains of specialized microorganisms resulting in useful or acceptable final products from the point of view of environmental impact [1, 2, 3].

In the last decades, the term bioremediation is used in a more specific way, which is reflected by the two specific definitions:

• The use of living organisms to degrade environmental pollutants, to prevent pollution or in the waste treatment process;

• Application of biological treatments for cleaning, decontamination and degradation of dangerous substances [3, 4, 5].

Bioremediation can be applied “in situ” (on the area, the polluted substrate, on the place where a contamination occurred) or “ex situ” (in specially arranged systems/installations, where it is brought the polluted substrate to be treated by biological methods).

2. Bioventing

Through bioventilation, oxygen is introduced into unsaturated contaminated soils through a forced air circulation (extraction or air injection) to increase the oxygen concentration and to stimulate biodegradation (Figure 1).

Figure 1.

Installation of bioventing [1] which includes: 1- analysis trailer, 2- blower, 3- emissions control, 4- vertical ventilation network and 5- lateral ventilation network.

Bioventing is a technology that stimulates the natural in situ biodegradation of any aerobically degradable compound by providing oxygen to microorganisms existing in the soil. Compared to suction vapor extraction, bioventilation uses weak air flows to provide just enough oxygen to support microbial activity. Oxygen is most often applied by direct injection into residual soil pollutants. In addition to the degradation of adsorbed fuel residues, the volatile components are biodegraded as the vapor slowly circulates through the biologically active soil. Applicability of bioventilation is a medium and long term technology.

Cleaning can take from a few months to a few years. Bioremediation techniques through bioventing have been used successfully to remediate soils contaminated with petroleum hydrocarbons, non-chlorinated solvents, certain pesticides, wood preservatives and other organic chemicals.

Bioremediation can be used to change the valence state of inorganic substances and to cause adsorption, assimilation, accumulation and concentration of inorganic substances in micro- or macro-organisms. These bioremediation techniques are promising from the perspective of stabilizing or removing inorganic substances from the soil, while biodegradation cannot degrade inorganic pollutants. Limitations. Among the factors that can limit the field of application and the efficiency of the bioventilation process are: the mass of water at a few decimeters of the surface, saturated soil lenses or soils with low permeability reduce the efficiency of bioventilation. Vapors can collect in basins within the radius of influence of the air injection probes. This problem can be eliminated by vacuuming the air near the structure.

A low degree of moisture in the soil can limit biodegradation and the efficiency of bioventilation. It is necessary to monitor residual gases at the soil surface. Aerobic biodegradation of certain chlorinated compounds cannot be efficient unless there is a co-metabolite or an aerobic cycle.

Cold temperatures can slow down the cure, although successful cures have also been achieved in extremely cold environments. The condition is that a successful bioventilation is based on the fulfillment of two basic criteria. First, air must enter the soil in sufficient quantity to maintain aerobic conditions; and secondly, the microorganisms that naturally degrade hydrocarbons must be present in sufficiently high concentrations to achieve adequate biodegradation percentages.

The initial tests aim to determine both soil permeability and in situ respiration percentages. The grain size of the soil and its humidity have an important influence on the permeability of soil gases. The greatest restriction in terms of air permeability is represented by excessive soil moisture. The combination of high water tables, high humidity, and fine-grained soils prevented successful bioventing in certain locations. Among the soil properties that have an impact on microbial activity are pH, moisture and basic nutrients (nitrogen and phosphorus) and temperature. It has been calculated that the optimum soil pH for microbial activity falls between six and eight. The optimum humidity is established for each soil separately.

Too much humidity can reduce air permeability and decrease oxygen transfer capacity. Too low humidity will inhibit microbial activity. Several biovent tests have indicated biodegradation rates with moisture levels between 2% and 5% by weight. And yet, in extremely arid climates, it is possible to increase the rate of biodegradation through irrigation or humidification through injected air.

Pollutants degrade faster through bioventilation during the summer, but remediation can also occur at temperatures of 0°C. Hydrocarbon biodegradation rates are always estimated based on percentages of oxygen utilization, assuming that oxygen loss is due to microbial mineralization of hydrocarbons.

The main cost elements are the following: surface area; the number of installed injection/extraction wells matters. The number of wells (and related costs) increase proportionally with the area. Soil type; soil types with sand and gravel content reduce costs due to the smaller number of injection/extraction wells that need to be installed. Indicative prices for bioventilation fall between 25 and 200 Euros per cubic meter of soil. Costs can be influenced by the type of soil and its chemical properties, the type and amount of amendments used, and the type and extent of contamination.

3. Bioaugmentation

Bioaugmentation is a process in which local or inoculated microorganisms (such as fungi, bacteria and other microbes) degrade (metabolize) organic pollutants in soil and/or groundwater and neutralize their harmful effect (Figure 2). The activity of microbes that occur naturally is stimulated by the aqueous solutions that circulate through contaminated soils and that increase the degree of in situ biological degradation of organic pollutants or the immobilization of inorganic ones. Nutrients, oxygen and other amendments can be used to increase the bioremediation and desorption of pollutants from underground materials [1].

Figure 2.

Installation of bioaugmentation [1] which includes: 1- monitoring probe, 2- groundwater pumping probe, 3- spray irrigation, 4- groundwater re-jection wells.

Bioaugumentation process is usually performed aerobically. In the presence of a sufficient amount of oxygen (aerobic conditions) and other nutrients, microorganisms will transform many organic pollutants into carbon dioxide, water and masses of microbial cells. Bioaugmentation of a soil normally involves the percolation or injection of groundwater or uncontaminated water mixed with nutrients and saturated with dissolved oxygen. Sometimes acclimatized microorganisms (bio-augmentation) and/or other sources of oxygen such as oxygenated water can be added. Sprinkler irrigation is regularly used for shallow contaminated soils, and injection wells for deep contaminated soils. Although in situ bioremediation has also proven successful in cold climates, low temperatures slow down the remediation process. Warm layers that cover the soil surface can be used for contaminated soils with low temperature to increase its temperature and the rate of degradation.

Bioaugmentation in the anaerobic process in the absence of oxygen (anaerobic conditions), organic pollutants will transform into methane, small amounts of carbon dioxide and tiny amounts of hydrogen. Under conditions of reduction with sulfates, sulfate is transformed into sulfide or elemental sulfur, and under conditions of reduction with nitrates it is finally produced into hydrogen sulfide.

Pollutants can sometimes degrade into intermediate or final products more or less dangerous than the original pollutant. For example, TCE (Trichlorethylene) can anaerobically biodegrade into vinyl chloride which is more persistent and toxic. To avoid such problems, most bioremediation projects are done in situ. Vinyl chloride can degrade further under aerobic conditions.

Bioaugmentation is a long-term technology that can take years to clean up a pollutant plume. Applicability in bioremediation techniques have been successfully used to remediate soils and sludge; remediation of groundwater polluted with petroleum hydrocarbons, solvents, wood preservatives and other organic chemical products. Bioremediation is especially effective in remediating low-level residual contamination related to removal of the pollution source. The pollutant groups that were treated most often are PAHs, non-halogenated SVOCs (without PAHs), and BTEX (Benzene, Toluene, Ethylbenzene, Xylene (volatile organic compounds).

The types of contaminated sites treated most often were polluted by processes or through waste from wood preservation, oil refining, and recycling. Wood preservation involves the use of creosote, which contains a high concentration of PAHs and other non-halogenated SVOCs.

Similarly, oil refining and recycling processes frequently rely on BTEX. Given that the pollutant groups most often treated by bioremediation are SVOCs (PAHs and other non-halogenated SVOCs), treating them with volatility-based technologies such as SVE (soil vapor aspiration) could prove difficult. Bioremediation treatment does not frequently require heat treatment, involves few cost-effective elements such as nutrients, and does not normally generate residues requiring further treatment e or eliminations. Also, when done in situ, it does not require excavation of the contaminated environment. Compared to other technologies, bioremediation is advantageous in terms of price in treating non-halogenated SVOCs. Although bioremediation cannot degrade inorganic pollutants, it can instead be used to change the valence of inorganic substances and cause adsorption, immobilization in soil particles, precipitation, assimilation, accumulation and concentration of inorganic substances in micro- and macro-organisms.

These techniques show promise for stabilizing or removing inorganic substances from soil. Among the factors that prevent the applicability and efficiency of the process are: Cleanup objectives cannot be achieved if the soil base mass prevents pollutant-microorganism contact.

The circulation of aqueous solutions through the soil can increase the mobility of pollutants and may require treatment of the underground water table. Preferential colonization with microbes can cause clogging of nutrient and water injection wells. Many of the above factors can be controlled by paying attention to good technical practices.

The duration of the treatment can be from 6 months to 5 years and depends on factors specific to the respective site.

If bioaugmentation can achieve the cleanup goal in a compatible time frame, it can significantly reduce costs without excavation and transportation. Also, both contaminated groundwater and soil can be treated simultaneously, which is another cost advantage. In situ processes generally require longer periods of time, there is no certainty regarding the uniformity of treatments due to the inherent variability in the soil, the characteristics of the aquifer and the difficulties related to the monitoring process.

The remedial procedures can sometimes last for years depending especially on the degradation rates of the specific pollutants, the characteristics of the site and the climate. It could take less than a year to clean up certain pollutants, while higher molecular weight compounds take longer to degrade. Indicative prices for bioaugmentation fall between 25 and 220 Euros per cubic meter of soil. Costs can be influenced by the type of soil and its chemical properties, the type and amount of amendments used, the type and size of contamination [1].

4. Phyitoremediation

Phytoremediation is a process of using plants to remove, transfer, stabilize and destroy pollutants from soil and sediments. Pollutants can be organic or inorganic (Figure 3).

Figure 3.

Phytoremediation in soil organic and inorganic [1].

Phytoremediation is a process of using plants to remove, transfer, stabilize and destroy pollutants from soil and sediments. Phytoremediation mechanisms include advanced rhizosphere biodegradation, phytoaccumulation, phytodegradation and phytostabilization [1, 2, 3].

In addition, poplars can draw large amounts of water (compared to other plant species) when it passes through the soil or directly from the aquifer. This means absorbing a large amount of dissolved pollutants from contaminated environments and reducing the amount of pollutants dispersed through or out of the soil or aquifer. Phytoaccumulation represents the assimilation of pollutants by plant roots and their movement/accumulation (phytoextraction) in the trunk and leaves. Phytodegradation is the metabolism of pollutants in plant tissues.

Plants produce enzymes such as dehalogenase and oxygenase that help catalyze degradation. Investigations will determine whether aromatic and chlorinated compounds respond to phytodegradation. Phytostabilization is a phenomenon of the plant’s production of chemical compounds that serve to immobilize pollutants when the roots come into contact with the soil.

Phytoremediation can be applied to remediate metals, pesticides, solvents, crude oil, PAHs and landfill leachate. Some plant species have the ability to store metals in their roots. These plants can be transplanted to contaminated sites to filter metals from wastewater. When the roots become loaded with metal pollutants, these plants can be removed.

Plants that accumulate large amounts can remove or store significant amounts of metal pollutants. Currently, trees are being tested to determine their ability to remove organic pollutants from groundwater, translocation and transpiration and their possible metabolism into CO₂ or plant tissues. Soil phytoremediation can be limited by: the depth of the treatment zone is determined by the plants used in the phytoremediation. In most cases, this method can be used on shallow soils. High concentrations of hazardous substances can be toxic to plants. It has the same mass transfer limits as other biotreatments.

Sometimes it can only be done in certain seasons, depending on the locations. It can transfer pollutants between environments, such as from soil to air.

It is not effective for strongly absorbed pollutants (such as polychlorinated biphenyl PCBs) and poorly absorbed ones. The toxicity and bioavailability of degradation products are not always known. The products can be mobilized in groundwater or bioaccumulated in animals. Detailed information is needed to determine the soil types used in phytoremediation projects. Flow, oxygen-reducing concentrations, root growth and their structure affect plant growth and must be taken into account when implementing phytoremediation. Performance data. Several phytoremediation demonstrations are currently being done (e.g., oak species were planted in the middle of a TCE pollutant plume to assess TCE transpiration and TCE transformation rates). Evaporation-transpiration rates were measured equally.

This is a long-term test of the ability of trees to control the circulation of underground water. Important cost elements Degree of effort; the contaminated area influences the costs the most. Sampling density; essential cost element for determining sample costs; can be directed by regulatory requirements. Phytoremediation is a long-term remedial process. Costs are mainly caused by procurement of plants, investments related to trees with subsequent testing and sampling costs. Costs can vary from 10 Euros for a lightly contaminated site to 150 Euros for a difficult site, per cubic meter treated [1].

5. Biopiles

The excavated soils are mixed with amendments and placed in enclosures on the surface. It is a composting process with aerated static piles in which the compost is raised in piles and aerated with blowers or vacuum pumps (Figure 4).

Figure 4.

Typical biopile system for solid phase bioremediation [1].

Suppliers have developed proprietary nutrient and additive formulas, as well as methods of incorporating the formulas into the soil to promote biodegradation. Formulas are usually modified from soil to soil. Soil mounds and piles usually have an air distribution system buried underground to allow air to pass through the soil either through vacuum or positive pressure. In this case, the mounds of soil can have a maximum height of 2–3 meters.

These mounds can be covered with plastic to control scattering, evaporation and volatilization and to stimulate solar heating. If there are VOCs in the soil that will evaporate into the air, the air that is emitted from the soil can be treated to remove or destroy the VOCs before entering the atmosphere. Biopile is a short-term technology that can last from a few weeks to a few months. Treatment options include static processes such as treatment bed preparation, biotreatment cells, soil mounds, and composting.

Biopile treatment has been applied to non-halogenated VOCs, hydrocarbons from fuels, halogenated VOCs, SVOCs, pesticides can also be treated, but the efficiency of the process will vary and may only be applicable to a few compounds within these pollutant groups.

Among the factors that can limit the applicability and efficiency of the process are:

• Excavation of contaminated soils is necessary.

• Treatment grade testing should be performed to determine pollutant biodegradability, adequate oxygenation, and nutrient loading rates.

• Solid phase processes are likely effective on halogenated components and may prove ineffective in degrading explosives transformation products.

• Piles of similar size take longer to clean than sludge phase processes.

• Static treatment processes may result in less uniform treatment than processes involving periodic mixing. The first steps in preparing a well-argued project for the biotreatment of contaminated soils include: site characteristics and soil samples and characteristics.

Biopile treatment has been demonstrated on several fuel-polluted sites. Costs depend on the pollutants, the procedure to be applied, the need for additional or post-treatment and the need for air control equipment. Biopiles are quite simple and require little personnel for maintenance and handling. Typical indicative costs with one coat and a prepared liner are between 35 and 100 Euro per cubic meter [1].

6. Composting

Contaminated soil is excavated and mixed with fill materials and organic amendments such as wood scraps, hay, natural fertilizers and plant waste (e.g., potatoes). Selecting the right amendments ensures sufficient porosity and provides a balance between carbon and nitrogen to promote thermophilic microbial activity (Figure 5).

Figure 5.

Coposting scheme [1] which includes: Excavation and soil screening, composition of furrows/soil amendaments, periodic overturning of abrasions, furrow monitoring, compost analysis and opening furrows arrangement.

Composting is a controlled biological process through which organic pollutants (for example PAH) are transformed by microorganisms (in aerobic and anaerobic conditions) into harmless, stabilized by-products. Normally, thermoelectric conditions (54 to 65°C) must be maintained to properly fertilize soil contaminated with hazardous organic pollutants. The high temperatures result from the heat produced by microorganisms during the degradation of organic matter in the waste. In most cases, this is achieved by using local microorganisms. Soils are excavated and mixed with filling materials and organic amendments such as sawdust, animal and vegetable waste, in order to increase the porosity of the mixture that will be decomposed. Maximum degradation efficiency is achieved by maintaining oxygenation (such as daily furrow turning), irrigation if necessary, and careful monitoring of soil moisture and temperature [1].

There are three types of processes used in composting: fertilization of static aerated mounds (the compost is raised in piles and aerated with blowers or suction pumps), by composting in mechanically stirred vessels (the compost is placed in the reactor vessel where it is mixed and aerated), and furrow composting (compost is placed in long mounds known as furrows that are periodically mixed with mobile equipment). Furrow fertilization is usually considered the most cost-effective composting option. At the same time, it can also present thousands of transient eruptions.

If VOCs or SVOCs are present in the soil, off-gas control may be required. The composting process can be applied to soils polluted with biodegradable organic compounds. Aerobic, thermophilic composting can be used for PAH-contaminated soils. Any material or equipment used in composting is available on the market.

Factors that can limit the applicability and efficiency of the process include: A large space is required. Excavation of contaminated soils that may cause uncontrolled VOC emissions is required. Composting leads to an increase in volume of the material due to the addition of amendments. Although metal levels can be reduced by dilution, heavy metals cannot be treated by this method. High concentrations of heavy metals can be toxic to microorganisms.

Among the specific data needed to evaluate the composting process are pollutant concentration, excavation requirements, availability and cost of amendments required for the compost mixture, space required for treatment, soil type and pollutant response to composting. Furrow composting has been demonstrated as an effective technology for treating soils contaminated with explosive substances.

The cost of providing a treatment base with the collection of polluted seepage water is included. The main cost elements are: The type of pollutant is the key element in determining composting costs. Soil type/total organic content (TOC); soils with higher density (generally fine sands and gravel) cost less to compost, while soils with higher TOC would require more expense. The density influences the mass of the soil to be treated, and the percentage of TOC indicates the level of contamination. Costs depend on the pollutants, the procedure to be applied, the need for additional or post-treatment and the need for air control equipment. Biopiles are quite simple and require little personnel for maintenance and handling. Typical indicative costs with one layer and a prepared liner are between 35 and 100 Euro per cubic meter [1, 2, 3].

7. Conclusions

Bioventilation has become a common technology, with most of the technical components already available. Bioventing is receiving a lot of remedial attention from the consulting community, especially regarding the use of this technology in conjunction with soil vapor extraction (SVE). As in the case of all biological technologies, the time required to remediate a site through bioventilation depends to a large extent on the characteristics of the soil and the chemical properties of the contaminated environment. With bioaugmentation there is a risk of increasing the mobility of pollutants and their infiltration into the water table. Bioaugmentation has been selected for corrective and emergency actions on a large number of contaminated sites. In general, petroleum hydrocarbons can be immediately bioremedied at relatively low cost by stimulating local microorganisms with nutrients.