would have to be countered with a makeup water addition. Oxidation need not be provided only by air. Fully saturated systems can be provided with electron acceptors by way of several strategies. Oxygen can be dissolved in groundwater through sparging. It can also be introduced indirectly using hydrogen peroxide dissolved in water, which decomposes to produce oxygen. Alternative electron acceptors such as nitrate may be used in specialized circumstances.
As for the chemical nature of a soil subjected to bioremediation, the pH should be neither excessively high or low. Values between 6 and 8 are generally considered as a desirable operating range. The availability and supply of nutrients also represents an important contributing factor that will affect microbial activity, relative to the availability of key nutrients, including both organic and inorganic (e.g., primarily nitrogen and phosphorus). Ideally, the ratio of available carbon to nitrogen to phosphorus (C/N/P) should fall within the range 100: 10: 1 to 100: 1 : 0.5. As with water availability, these pH and nutrient parameters may also be subject to change during the course of a remediation activity, such that it will need to be checked on a routine basis and corrected if necessary.
Finally, the temperature at which a remediating microbial reaction is maintained will probably have a corresponding impact on the metabolic rate. At depths much below about 1 m the temperature of soil quickly moderates to a fairly stable level of about 12 to 13°C, at which point most bioremediation microbes will experience little negative impact. However, it is possible for temperature to become an important factor, with both ex situ systems and near-surface soils, which are considerably more vulnerable to sizable temperature changes. In both instances, there may be seasonal extremes during which time the rates of microbial activity will be reduced substantially. Frost penetration during winter periods in cold-climate regions could, for example, extend for depths even beyond a meter, such that microbial activity could be commensurately depressed during these periods.
Extending beyond the characteristics and properties of the soils involved, many engineered bioremediation systems also rely on wells to deliver key materials, or air, to the active subsurface site. There are three different types of wells commonly used with bior-emediation operations: those used for ventilating soils (i.e., aerating wells); those used to introduce water into the ground (i.e., dosing wells), possibly mixed with nutrients, and those used for accessing groundwater (i.e., monitoring wells) for sampling or monitoring purposes. The majority of these wells are installed as rigid pipe bodies constructed of inert plastic or metal, the lower reach of which has a series of slits cut into the face of the pipe to act as a screen. The lower screen face of any well serves as the interface between the well and the soil into which it was placed, and the narrowness of the screen openings is sufficiently small to prevent the unwanted migration of all but the smallest soil grains.
Well pipes used for groundwater sampling are commonly installed into a vertically aligned bore hole drilled into the soil to depths of a few meters to 10+ m (sufficiently deep to reach the groundwater table), using fairly narrow diameter (e.g., 2.5 cm) stainless steel or PVC casing. However, in the case of ventilating or dosing wells, there is considerably more variation in the types of wells and their installation, including changes in the alignment of the well, the materials employed, and the means by which they are put into place. The depth of the aerating and dosing wells can vary considerably, from shallow depths extending only a few meters to deep wells that reach 100+ m. To facilitate the desired flow of air through the aerating wells, they are much larger in diameter than dosing or monitoring well lines, with diameters of 7.5 to 10+ cm. Pipes installed into vertically aligned wells are typically installed into drilled wells, although many dosing wells are placed in a horizontal alignment using a predug ditch rather than being placed with drilling. In some cases, particularly with loosely packed loamy or sandy-loam soils, it may also be possible to install shallow-depth vertical wells with a cone penetrometer. However, in this case stainless steel well lines must be used to carry the bearing force during the installation and also to avoid subsequent problems with oxidization of the metal and consequent clogging of the screen.
For aerating wells, the depth of installation is important since air may escape from a shallow screen when dealing with highly permeable soils (i.e., that would offer a path of least resistance back to the soil surface rather than having the incoming air pass through the soil). Diffusers have been suggested, with the idea that they might improve aeration efficiency by creating smaller air bubbles with a higher surface area, but these diffuser bubbles quickly coalesce as the air enters the soil, to the point where there is no overall gain in transfer efficiency.
The type of aeration equipment to be selected depends on the necessary pressure required to push or pull the air through the wells. For remediation systems that require lower pressure [e.g., 10 to 50 kPa (1.5 to 7.5 psi)] they will commonly use rotary air pumps or blowers called positive-displacement (PD) blowers. These types of blowers effectively "push" a relatively constant volume of air drawn into the inlet through to the discharge. The air released is usually oil-free and can then be introduced directly into the soil. Experimental installations have also been constructed to employ passive aeration systems in which the motive power is supplied not by mechanical pumps or blowers but by wind or natural air exchange. The latter strategy depends on barometric-driven changes in airflow in a fashion analogous to that of the air exchange found inside caves.
In the case of more highly compacted and denser soils, high-pressure air compressor systems may be necessary to inject air at higher pressure levels of 50 to 200 kPa (—7.5 to 30 psi), and this airflow often contains a level of oil that needs to be filtered and removed prior to its introduction into soils. Outlet gas temperatures with the latter high-pressure systems may also be considerably higher than would be the case with a lower pressure operation. To some extent, this heat may be helpful in raising the aerated soil temperature and escalating the biokinetic reaction rates, but these higher temperatures may also require a shift in piping materials to alternatives (e.g., to non-HDPE forms such as stainless steel) able to handle these higher temperatures. Aerating wells maintained at high airflow rates and pressures also run the risk of fracturing tightly packed, less permeable soils, at which time problems with short-circuiting might subsequently arise. However, air extraction may also pull water and small soil fines into their well screens, which might then clog their openings and restrict subsequent fluid and air movement.
In general, there are three different configurations for aerating wells on an in situ basis with bioremediation sites, including those that inject air into a contaminated soil zone using pressure (i.e., bioventing), those that use combined air injection and extraction [i.e., soil vapor extraction (SVE)] to mobilize and extra contaminants from soils, and those that inject air into a contaminated groundwater zone using pressure (air sparging).
With bioventing, the incorporated low-level airflow not only oxygenates vadose soil with the goal of promoting enhanced aerobic metabolism of biodegradable contaminants within these soils but may also result in some measure of gas-phase release of the soil's more volatile contaminants. However, the primary goal of bioventing (Figure 16.69) is that of securing metabolic breakdown as opposed to volatilization; on the other hand, these goals are essentially reversed with SVE systems.
Figure 16.69 In situ engineered bioremediation using bioventing.
Figure 16.69 In situ engineered bioremediation using bioventing.
Bioventing has been used successfully to treat a variety of soil contaminants, including petroleum hydrocarbons and nonchlorinated solvents as well as a number of pesticides, wood preservatives, and other organic chemicals. Bioventing may also be effective at removing heavier, semivolatile petroleum hydrocarbons such as diesel, fuel oil, kerosene, and jet fuel. The key technical issues with bioventing all focus on the means of promoting optimal microbial activity. The desired ventilating airflow rate has to be kept sufficiently low to avoid significant losses of the more volatile contaminants, with typical application pressures in the range 10 to 50 kPa.
Moisture and nutrient additions are often provided using a companion dosing well system as replacements for their losses incurred due either to aeration or metabolic uptake. Intermittent pulsed additions of nutrients (NH4Cl, KNO3, NaH2PO4, etc. solutions) and water are often provided, with the intent of spreading these additions into the soil in a fashion that will distribute the zone of microbial activity away from the immediate reach of the well face. Cyclic aeration cycles have also been studied in terms of developing sequential aerobic-anaerobic degradation schemes that couple a far wider, and potentially synergistic, range of metabolic pathways. For example, the anaerobic sequence might enable the reductive dehalogenation of halogenated solvents otherwise unavailable to aerobes, at which point the dehalogenated products could be reoxidized and mineralized during the subsequent aerobic period.
Monitoring provided during the course of a bioventing operation includes measurements of soil moisture content, nutrient presence, and contaminant concentrations. The in situ soil vapor oxygen content can also be used an indicator of metabolic activity, whereby the oxygen uptake rate can be tracked following intermittent shutdowns of the aeration system. Higher respiration rates will, of course, tend to correspond to those areas where the contaminants are still present at the highest concentrations, and with time these rates will drop in parallel with contaminant depletion. Yet another option for tracking the existing metabolic activity is that of the in situ, vapor-phase carbon dioxide content.
The rate and degree to which soil contaminants can be remediated using bioventing depends on a number of factors, including the permeability of the soil and the form and concentration of the contaminants. In some cases, sizable levels of cleanup with easily degradable materials can be experienced in a matter of months, but successful
bioventing operations usually entail operational periods measured in years rather than months. Even then, the long-term prognosis for reaching desired cleanup endpoints will typically extend into a time frame measured in decades. These actual cleanup levels are usually derived on a site-specific basis and developed to achieve desired levels of risk reduction for each of the existing contaminant species.
There is also considerable variation in the size of these operations, ranging from applications with a single aerating well to fields involving dozens of wells. Yet another option is that some bioventing locations opt to add hydrogen peroxide (H2O2) in lieu of delivering oxygen, but this approach may face certain problems. It is possible for this H2O2 chemical to react with the soil and subsequently decompose, and it may also be that catalase-secreting bacteria could evolve at the vicinity of its release that would then lead to rapid decomposition and loss of the desired oxygen.
As the name implies, and as shown in Figure 16.70, soil vapor extraction (SVE) uses both positive and negative pressure to promote in situ bioremediation, essentially melding vacuum extraction to a bioventing system. Here again, both such aeration and vacuum operations are completed within a contaminated soil zone.
The third method of in situ groundwater treatment is that of air sparging (Figure 16.71), by which air is injected into the groundwater to promote the release and removal of contaminants. The key difference in this regard is that of aerating the groundwater zone rather than the overlying soils (as practiced with either bioventing or SVE). Two removal mechanisms will be involved, the first of which involves outright aerobic biodegradation within a reach of the oxygenated saturated soil called the biologically active zone (BAZ). The second mechanism involves volatilization and stripping of contaminants from the saturated to the unsaturated zone (i.e., from the groundwater into the vadose zone). As such, soil vapor extraction is typically used in conjunction with air sparging to ensure that contaminants released from the groundwater will be duly pulled from the overlying soils. Overall, though, sites amenable to air sparging must have soil hydraulic conductivity that is neither so low that it traps the incoming gas, nor too high that it allows short-circuiting of the incoming gas away from (as opposed to being diffusely spread across) a biologically active zone.
In lieu of directly aerating the groundwater, there is yet another engineering approach to in situ groundwater bioremediation, called pump and recycle, which involves the pumping of groundwater back to the soil surface, at which point these waters are then injected and infiltrated back into the overlying soils (see Figure 16.72). This single technology can provide an effective means of remediating contamination found not only in the groundwater but also in the overlying soils. To enhance and promote the metabolic efficiency of the process, an aboveground water-conditioning step may also be incorporated
into this overall process, whereby nutrients and oxygen (using either air or hydrogen peroxide) may be added to the contaminated water prior to its release back to the soil. One limitation with this technology, however, is that subsurface changes in soil layering and density may have a distinct impact on the physical course that this reinjected groundwater prefers to follow, such that these conditioned waters could well fail to reach the targeted areas of contamination.
Rather than attempting to complete a bioremediation effort with groundwaters on an in situ basis, there is also an ex situ bioremediation option in which the applied treatment steps are completed above the soil surface. This concept of pump and treat basically follows the general plan as that of the in situ groundwater bioremediation strategy mentioned previously, but in this case an engineered treatment process (i.e., activated sludge, trickling filter, etc.) is added to the aboveground flow scheme. The treated water is then recycled back to the ground, essentially flushing contaminants from the soil and moving them into the groundwater, where they are then extracted and pumped to the aboveground treatment unit (Figure 16.73). Yet another variation with this approach is that of using soil extraction agents to enhance the effectiveness of this soil washing effort. This process, known as solvent extraction, differs from conventional soil washing in that it uses surfactant-type organic chemicals as the solvent instead of water. The principal application of this process has been with the removal of polychlorinated biphenyls (PCBs), but the technology can be effective for petroleum removal as well. The disadvantages with this process are that these detergents may, at least in some instances, interfere with the desired separation and biotreatment processes.
There are also three other ex situ techniques that can be used to remediate contaminated soils, and to some extent these schemes can be faster, easier to control, and more amenable to a wider range contaminants and soil types than is the case with in
situ techniques. However, the key problem with these ex situ procedures is that they require the initial excavation and relocation of the contaminated soils, and this effort can rapidly become prohibitively expensive. The first such option (Figure 16.74) is that of slurry-phase soil bioremediation, where the extracted contaminated soil is combined with water in an aboveground reactor and then mixed in batch fashion as a suspended slurry to facilitate subsequent microbial degradation.
The microbes involved may not only include a variety of indigenous forms but also specially acclimated organisms seeded into the reactor or recycled from prior treatment batches. Nutrients and oxygen can, of course, be added to the reactor to optimize the environmental conditions for the microorganisms that are degrading the contaminants. After the desired level of contaminant removal is completed, the water is extracted from the solids, and these water and solid residuals are then placed back into the ground. This slurry-phase biological treatment scheme can provide a fairly rapid means of remediating soil contaminants given its ability to maintain a homogeneous soil matrix and optimal microbial contact, particularly when compared to the sort of slow-rate bioremediation processes that might be achieved with in situ treatment of tight clay soils. However, there is clearly a trade-off with this technology in terms of the benefits of accelerated treatment vs. the costs associated with soil extraction.
The second ex situ soil bioremediation scheme (Figure 16.75) is completed as a solidphase process in which the extracted, contaminated soils are placed in carefully engineered, typically enclosed, aboveground mounds known as soil biopiles. These soil piles are built with several meters of soil stacked vertically over an impermeable base constructed with either well-packed clay, plastic liners, or geomembranes, as well as an associated leachate collection system that ensures that the contaminants are not able to leave the site. Air is then drawn through the pile using a vacuum pump tied into an underlying air extraction piping network. Moisture, temperature, nutrients, and oxygen levels can then be controlled within these mounds to enhance their available rates of biodegradation, and in some cases the mounds may also be mixed or turned on an occasional basis to promote homogeneous remediation.
These solid-phase systems are somewhat easier to maintain and operate than is the case with slurry-phase units, but they do require more space, and their metabolic rates are typically lower, due to their reduced effective microbial intensity. There is also a variation in the original makeup of these piles in which the contaminated soils are mixed with various forms of bulking material (i.e., straw, hay, corncobs, wood chips, etc.) to open the soil and to make it more permeable to the passage of water, air, and nutrients. Here again, these piles will often be developed with some form of internal aeration header, or alternatively, mixed on a routine basis with mechanical turning to ensure that the pile contents are kept aerated.
The third option for ex situ soil remediation is that of land farming. This relatively simple process again involves the excavation of the contaminated soils, which are then spread across land areas which are then tilled periodically much like farmlands to entrain air and remix the actively remediating soil matrix. Moisture and nutrients are also monitored and added as needed to promote the desired bioremediation. Given the fact that most of these land-farming operations are maintained on unenclosed lands, there may be instances in which the contaminant removal process could actually be attributable more to volatilization than to biodegradation. If this is expected to be the case, where the rate of contaminants would probably be unacceptably high, this process may have to be shifted to an enclosed operation.
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