Csaba Juhász (2013)
University of Debrecen
The used mainly EPA based text, focuses on engineering-related considerations for evaluating some important technology.
Remediation ex-situ methods involve excavation of effected soils and subsequent treatment at the surface, In-situ remediation methods seek to treat the contamination without removing the soils.
Soil vapor extraction (SVE), also known as soil venting or vacuum extraction, is an in situ remedial technology that reduces concentrations of volatile constituents in petroleum products adsorbed to soils in the unsaturated (vadose) zone. In this technology, a vacuum is applied to the soil matrix to create a negative pressure gradient that causes movement of vapors toward extraction wells. Volatile constituents are readily removed from the subsurface through the extraction wells. The extracted vapors are then treated, as necessary, and discharged to the atmosphere or reinjected to the subsurface (where permissible) (Fig. 7-2, 7-3).
This technology has been proven effective in reducing concentrations of volatile organic compounds (VOCs) and certain semi-volatile organic compounds (SVOCs) found in petroleum products at underground storage tank (UST) sites. SVE is generally more successful when applied to the lighter (more volatile) petroleum products such as gasoline. Diesel fuel, heating oils, and kerosene, which are less volatile than gasoline, are not readily treated by SVE but may be suitable for removal by bioventing. Important indicator of the volatility of a constituent is by noting its Henry’s law constant. Henry’s law constant is the partitioning coefficient that relates the concentration of a constituent dissolved in water to its partial pressure in the vapor phase under equilibrium conditions. This describes the relative tendency for a dissolved constituent to partition between the vapor phase and the dissolved phase. Therefore, the Henry's law constant is a measure of the degree to which constituents that are dissolved in soil moisture (or groundwater) will volatilize for removal by the SVE system. Constituents with Henry’s law constants of greater than 100 atmospheres are generally considered amenable to removal by SVE. SVE is generally not successful when applied to lubricating oils, which are non-volatile, but these oils may be suitable for removal by bioventing. Soil vapor extraction is a well-known and effective technology when applied in permeable soils but has not been effective in tight impermeable soils, particularly those soils containing significant amountsof silt and clay. The most technologies operate as a closedloop hot air recirculation process in the vadose(unsaturated) zone. Since there are no externalemissions, no air permit is required. Warm airfrom a blower is injected directly into theimpermeable soil, which desiccates the soil. Thedry soil then readily desorbs its volatile organiccompounds, which are collected through aconventional soil vapor extraction system. Thevapors from the soil extraction system then passthrough an activated carbon system and recirculateback to the blower, where the processbegins again. The heat of compressing the air flowing through the blower creates the hot airthat is introduced back into the contaminatedvadose zone treatment area. The closed loopenhanced soil vapor extraction system takesadvantage of the fact that desiccated soils aremuch more permeable.Surface seals might be included in an SVE system design to prevent surface water infiltration that can reduce air flow rates, reduce emissions of fugitive vapors, prevent vertical short-circuiting of air flow.
MECX succesfully applied the catalyzed hydrogen peroxide to effectively desorb contaminants such as gasoline, fuel oils and coal tars. The exothermic reaction reduces the viscosity of highly viscous petroleum products. The catalyzed hydrogen peroxide breaks down larger molecules into smaller molecules. In addition to these chemical effects, the physical action of hydrogen peroxidecreated bubbles facilitates the separation of free product from the soil matrix. The first step is to pre-condition low permeabilitysoils using either chemical and/or mechanicalmeans. Mechanical augers and direct pushprobes are now available with specializedchemical injection devices to facilitate breakingup tight soils. The second step is to apply an enhancement of the traditional catalysed hydrogen peroxide chemical oxidation process. The use of exothermic free radical reactions to destroy contaminants in the dissolved (saturated zone) phase is well documented. High temperature applications (greater than 75 ˚C) in the saturated zone have resulted in inefficient use of hydrogen peroxide and runaway exothermic reactions. Likewise, low temperature applications (less than 40 ˚C) have resulted in problems with dissolved phase contamination rebound. Further oxidation and, ultimately, significant mass contaminant destruction occurs, which facilitates the third and fourth steps, which involve bio-treatment polishing. The third optional step is an advanced aerobic treatment processes, which optimally produces stable concentrations of dissolved oxygen to biodegrade petroleum hydrocarbons in the contaminated groundwater using indigenous bacteria. MECX is also teamed with FMC Technologies to provide calcium peroxide-based PermeOx® Plus, an oxygen release compound able to deliver up to 18% oxygen, versus the usual 10% oxygen with magnesium peroxide. In addition, MECX is teamed with Solvay Chemical to provide sodium percarbonate, which upon dissolving in groundwater will provide hydrogen peroxide as an additional oxygen releasing compound(Fig. 7-4).
7.4. ábra - Fig. 7.4: The application of catalyzed hydrogen peroxide to application wells, Pensacola USA. Source: MECX
MECX used this method in case former dry cleaning activities contaminated (tetrachlroethylene-PCE) a groundwater zone beneath the site. Within 30 hours after beginning the application,parameters which indicate an oxidative state (i.e.increased dissolved oxygen, increased oxidationreductionpotential, etc.) were observed.Monitoring wells further down-gradient alsodisplayed an increase in dissolved oxygen.
If chlorinated contaminants are also present, the treatment train process is followed by a fourth advanced anaerobic treatment step that employs emulsified food-grade soybean oil that is injected into the aquifer to stimulate reductive dechlorination. The edible oil, with a very low viscosity, slowly dissolves over several years providing a carbon and hydrogen source to accelerate the anaerobic biodegradation of contaminants.
MECX, LP performed an in-situ chemical oxidation (ISCO) application at a fuel bulk storage terminal located northwest of Dallas, Texas. An underground transfer pipe associated with a fuel storage tank was suspected to have leaked and was subsequently cleaned and abandoned in-place. Nonaqueous phase liquid (NAPL) and high concentrations of dissolved petroleum hydrocarbon constituents including methyl tert-butyl ether (MTBE) benzene, toluene, ethyl-benzene, xylene (BTEX) and were identified. Rather than excavate affected soils, near utilities, and compromise the tank berm and floor, a rapid and effective in-situ chemical oxidation remediation solution was selected to remove the source area (i.e. NAPL) and reduce the BTEX and MTBE levels(Fig. 7-5).
7.5. ábra - Fig. 7-5: The applied in-situ chemical oxidation to eliminate NAPL, Dallas USA. Source: MECX
MECX successfully eliminated NAPL and significantly reduced total BTEX by 95% within 1 month after application and to below detection levels after 3 months. Also, MTBE concentrations were reduced to by 66% 1 month after and 77% 3 months after the application. The target remediation zone was located between approximately 0,6-4m below grade surfaceand the groundwater dissolvedplume/saturated zone covered approximately 410 m2 with portions located insideand outside of the tank berm. Shallow lithologyincluded silty clays with interbedded gravel seams. Real-time monitoring of water quality parameters, including temperature, dissolved oxygen, pH, conductivity and oxidation/reduction potential were performed during the application to determine the effectiveness of the application and allow adjustments of reagents in the field.
Bioventing is an in-situ remediation technology that uses indigenous microorganisms to biodegrade organic constituents adsorbed to soils in the unsaturated zone. Soils in the capillary fringe and the saturated zone are not affected. In bioventing, the activity of the indigenous bacteria is enhanced by inducing air (or oxygen) flow into the unsaturated zone (using extraction or injection wells) and, if necessary, by adding nutrients. All aerobically biodegradable constituents can be treated by bioventing. In particular, bioventing has proven to be very effective in remediating releases of petroleum products including gasoline, jet fuels, kerosene, and diesel fuel. (Fig. 7-6).
Intrinsic permeability, which will determine when environmental engineer consider to apply in situ bioventing system. The rate at which oxygen can be supplied to the subsurface, varies over 13 orders of magnitude (from 10-16 to 10-3 cm2) for the wide range of earth materials, although a more limited range applies for most soil types (10-13 to 10-5 cm2). Intrinsic permeability is best determined from field or laboratory tests, but can be estimated within one or two orders of magnitude from soil boring log data and laboratory tests.
Hydraulic conductivity can be determined from aquifer tests, including slug tests and pumping borehole test. Hydraulic conductivity can be convert to intrinsic permeability using the following equation:
where: k (intrisic permeability cm2),K (hidraulic conductivity cm/sec), µ (water viscosity g/cm sec), ƣ(water density g/cm3), g( acceleration due to gravity cm/sec2) At 20 C˚ : µ/ ƣg= 1,02x10-5
To convert k from cm2 to darcy, multiply 108
Design Radius of Influence (ROI) is an estimate of the maximum distance from a vapor extraction well (or injection well) at which sufficient air flow can be induced to sustain acceptable degradation rates. Establishing the design ROI is not a trivial task because it depends on many factors including intrinsic permeability of the soil, soil chemistry, moisture content, and desired remediation time. The types of soils and their structures will determine their permeabilities. Fluctuations in the groundwater table should also be considered.
When user try to enhance soil microbiological activity in situ or ex situ have to consider more environmental factors. The optimum pH for bacterial growth is approximately 7; the acceptable range for soil pH in bioventing is between 6 and 8.
Bacteria require moist soil conditions for proper growth. Excessive soil moisture, however, reduces the availability of oxygen, which is also necessary for bacterial metabolic processes, by restricting the flow of air through soil pores. The ideal range for soil moisture is between 40 and
85 percent of the water-holding capacity of the soil. Generally, soils saturated with water prohibit air flow and oxygen delivery to bacteria, while dry soils lack the moisture necessary for bacterial growth. Bacterial growth rate is a function of temperature. Soil microbial activity has been shown to decrease significantly at temperatures below 10 C˚. Bacteria require inorganic nutrients such as ammonium and phosphate to support cell growth and sustain biodegradation processes. Using theempirical formulas for cell biomass and other assumptions, the carbon:nitrogen:phosphorus ratios necessary to enhance biodegradation fall in the range of 100:10:l to 100:1:0.5, depending on the constituents and bacteria involved in the biodegradation process. The more complex the molecular structure of the dominant pollution constituent, the more difficult and less rapid is biological treatment. Most low-molecularweight (nine carbon atoms or less) aliphatic and monoaromatic constituents are more easily biodegraded than higher-molecular-weight aliphatic or polyaromatic organic constituents. The presence of very high concentrations of petroleum organics or heavy metals in site soils can be toxic or inhibit the growth and reproduction of bacteria responsible for biodegradation. In addition, very low concentrations of organic material will also result in diminished levels of bacterial activity.
Biopiles, also known as biocells, bioheaps, biomounds, and compost piles, are used to reduce concentrations of petroleum constituents in excavated soils through the use of biodegradation. This technology involves heaping contaminated soils into piles (or “cells”) and stimulating aerobic microbial activity within the soils through the aeration and/or addition of minerals, nutrients, and moisture. The enhanced microbial activity results in degradation of adsorbed petroleum-product constituents through microbial respiration. Biopiles are similar to landfarms in that they are both above-ground, engineered systems that use oxygen, generally from air, to stimulate the growth and reproduction of aerobic bacteria which, in turn, degrade the petroleum constituents adsorbed to soil. While landfarms are aerated by tilling or plowing, biopiles are aerated most often by forcing air to move by injection or extraction through slotted or perforated piping placed throughout thepile(Fig. 7-7, 7-8).
In-situ groundwater bioremediation is a technology that encourages growth and reproduction of indigenous microorganisms to enhance biodegradation of organic constituents in the saturated zone. In-situ groundwater bioremediation can effectively degrade organic constituents, which are dissolved in groundwater and adsorbed onto the aquifer matrix. Bioremediation generally requires a mechanism for stimulating and maintaining the activity of these microorganisms. This mechanism is usually a delivery system for providing one or more of the following: An electron acceptor (oxygen, nitrate); nutrients (nitrogen, phosphorus); and an energy source (carbon). The driving force for the biodegradation of petroleum hydrocarbons is the transfer of electrons from an electron donor (petroleum hydrocarbon) to an electron acceptor. To derive energy for cell maintenance and production from petroleum hydrocarbons, the microorganisms must couple electron donor oxidation with the reduction of an electron acceptor. As each electron acceptor being utilized for biodegradation becomes depleted, the biodegradation process shifts to utilize the electron acceptor that provides the next greatest amount of energy. This is why aerobic respiration occurs first, followed by the characteristic sequence of anaerobic processes: nitrate reduction, manganese-reduction, iron-reduction, sulfate-reduction, and finally methanogenesis. (Figure)
7.11. ábra - Figure. Aerobic respiration, denitrification, iron and manganese reduction, and sulfate reduction result in generation of carbon dioxide. Source: EPA
Generally, electron acceptors and nutrients are the two most critical components of any delivery system. In a typical in-situ bioremediation system, groundwater is extracted using one or more wells and, if necessary, treated to remove residual dissolved constituents. The treated groundwater is then mixed with an electron acceptor and nutrients, and other constituents if required, and re-injected upgradient of or within the contaminant source. This ideal system would continually recirculate the water until cleanup levels had been achieved. If your state does not allow re-injection of extracted groundwater, it may be feasible to mix the electron acceptor and nutrients with fresh water instead. Extracted water that is not re-injected must be discharged, typically to surface water or to publicly owned treatment works.
In-situ bioremediation can be implemented in a number of treatment modes, including: Aerobic (oxygen respiration); anoxic (nitrate respiration); anaerobic (non-oxygen respiration); and co-metabolic. The aerobic mode has been proven most effective in reducing contaminant levels of aliphatic (e.g., hexane) and aromatic petroleum hydrocarbons (e.g., benzene, naphthalene) typically present in gasoline and diesel fuel. In the aerobic treatment mode, groundwater is oxygenated by one of three methods: Direct sparging of air or oxygen through an injection well; saturation of water with air or oxygen prior to re-injection; or addition of hydrogen peroxide directly into an injection well or into reinjected water(Figure).
7.12. ábra - Figure. In situ groundwater bioremediation system using infiltration gallery. Source: EPA
The term “monitored natural attenuation” (MNA) refers to the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. MNA is often dubbed “passive” remediation because natural attenuation processes occur without human intervention to a varying degree at all sites. It should be understood, however, that this does not imply that these processes necessarily will be effective at all sites in meeting remediation objectives within a reasonable time frame. Natural attenuation processes include a variety of physical, chemical, and biological processes that, under favorable conditions, reduce the mass, toxicity,mobility, volume, and/or concentration of contaminants in soil and/orgroundwater. A. Processes that result only in reducing the concentration of acontaminant are termed “nondestructive” and include hydrodynamic dispersion,sorption and volatilization. Other processes, such as biodegradation and abioticdegradation (e.g., hydrolysis), result in an actual reduction in the mass ofcontaminants and are termed “destructive” (Figure).
The emerging phytoremediation technology is cost-effective plant-based approach to remediation takes advantage of the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. Toxic heavy metals and organic pollutants are the major targets for phytoremediation. In recent years, knowledge of the physiological and molecular mechanisms of phytoremediation began to emerge together with biological and engineering strategies designed to optimize and improve phytoremediation. Chelate-assisted phytoextraction has been successfully used to remove lead from contaminated soils using specially selected varieties of Indian mustard (Brassica juncea L.). These varieties combine high shoot biomass with the enhanced ability of roots to adsorb EDTA-chelated lead from soil solution and transport it into the shoots. The transpiration stream is likely to be the main carrier of soluble chelated metal to the shoots, where water is transpired while metal accumulates (Vassil, et al. 1998).The hyperaccumulating plants for example, several Thlaspi species can accumulate Ni and Zn, to 1–5% of its dry biomass. This is an order of magnitude greater than concentrations of these metals in the nonaccumulating plants growing nearby.Unfortunately, most hyperaccumulating species are not suitable for phytoextraction for several reasons: (i) metals that are primarily accumulated (Ni, Zn, and Cu) are not among the most important environmental pollutants; (ii) most have very low biomass and capricious growth habits unsuitable for monoculture; and (iii) agronomic practices and crop protection measures for their cultivation have not been developed. However, many metal-hyperaccumulating species belong to Brassicaceae (mustard) family, and thus are related to B. juncea, the preferred plant for phytoextraction of lead. Unfortunately, B. juncea, while exhibiting a high capacity for metal uptake and translocation, is not very resistant to high levels of lead or other heavy metals in its foliage. Therefore, chelate-assisted phytoextraction is very toxic to B. juncea, requiring harvesting several days after chelate application.Phytoextraction exploits the ability of plant roots to remove unwanted contaminants from their environment.
7.14. ábra - Figure. Phytoremediation of oil field using native poplars and willows Guadal. Source:Guadalupe restoration project USA
Phytoextraction (or phytoaccumulation) uses plants or algae to remove contaminants from soils, sediments or water into harvestable plant biomass (organisms that take larger-than-normal amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been growing rapidly in popularity worldwide for the last twenty years or so. In general, this process has been tried more often for extracting heavy metals than for organics. At the time of disposal, contaminants are typically concentrated in the much smaller volume of the plant matter than in the initially contaminated soil or sediment. 'Mining with plants', or phytomining, is also being experimented with.Phytostabilization creates a vegetative cap for the long-term stabilization and containment of the tailings. The plant canopy serves to reduce eolian dispersion whereas plant roots prevent water erosion, immobilize metals by adsorption or accumulation, and provide a rhizosphere wherein metals precipitate and stabilize. Unlike phytoextraction, or hyperaccumulation of metals into shoot/root tissues phytostabilization primarily focuses on sequestration of the metals within the rhizosphere but not in plant tissues.
Phytostabilization focuses on long-term stabilization and containment of the pollutant. For example, the plant's presence can reduce wind erosion; or the plant's roots can prevent water erosion, immobilize the pollutants by adsorption or accumulation, and provide a zone around the roots where the pollutant can precipitate and stabilize. Unlike phytoextraction, phytostabilization focuses mainly on sequestering pollutants in soil near the roots but not in plant tissues. Pollutants become less bioavailable, and livestock, wildlife, and human exposure is reduced. An example application of this sort is using a vegetative cap to stabilize and contain mine tailings.
Have to calculate nutrition reqirement on TPH polluted site by land farming project:
A conservative approximation of the amount of nitrogen and phosphorus required for optimum degradation of petroleum products can be calculated by assuming that the total mass of hydrocarbon in the soil represents the mass of carbon available for biodegradation. This simplifying assumption is valid because the carbon content of the petroleum hydrocarbons commonly encountered at UST sites is approximately 90 percent carbon by weight.As an example, assume that at a LUST site the volume ofcontaminated soil is 10,000 m3, the average TPH concentration in thecontaminated soil is 1,000 mg/kg, and the soil bulk density is1.75 g/cm3.
The mass of contaminated soil is equal to the product of volume andbulk density: 1,75x 106 T
The mass of the contaminant (and carbon) is equal to the product of the mass of contaminated soil and the average TPH concentration in the contaminated soil:
1,75x 106 kg x 1000mg/kg=1,75 x 103 kg
Using the C:N:P ratio of 100:10:1, the required mass of nitrogen would be 1750 kg, and the required mass of phosphorus would be 175. After converting these masses into concentration units
(mg/kg for nitrogen and mg/kg for phosphorus), they can becompared with the results of the soil analyses to determine if nutrientaddition is necessary. If nitrogen addition is necessary, slow releasesources should be used. Nitrogen additions can lower soil pH, dependingon the amount and type of nitrogen added.