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Bioremediation Fundamentals Need to know more? Send us an email or feel free to search our online series with our give away search engine: |
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We can basically distinguish two major approaches, so-called pump and treat and in-situ or treat in place strategies.
Pump and treat systems basically consist of groundwater extraction and treatment at some distance or grade level. Groundwater is pumped out of the aquifer, treated, amended with nutrients and a suitable electron acceptor such as oxygen, and re-injected back. The above ground processes involved may be physical (e.g. air stripping, carbon adsorption) or biological. Re-injected fluid will likely stimulate in-situ biodegradation as well.
Surface treatment may include any of the more popular wwtp configurations, e.g. aeration, activated sludge plants, fixed film. In some cases, seeding with specific cultures capable of degrading contaminants known to be in the groundwater may be conducted. Unfortunately (see Reference 1.), overall, pump and treat have generally "not been successful in remediating groundwater back to drinking standards.".
In situ treatments are characterized in that the bulk of the treatment or biological action occurs in place. Thus, there's the need to deliver oxygen and nutrients subsurface. Oxygen, frequently the rate-limiting factor, can be supplied by way of forced airflow (blowers) or chemically, e.g. hydrogen peroxide.
Pump and treat approaches overcome to a certain extent some of the problems involved with in situ methods as with the latter subsurface condition control may not be that easy. With in situ methods it may not be easy as well to ensure uniform and/or "where needed" oxygen/nutrient supply. It may happen that microorganisms and contaminant constituents either are not in contact or mutually accessible, e.g. isolated and/or in noncontiguous spots. To a certain extent there may be problems in preventing migration or containing further contaminant spread.
As with the case of liquid phase bioremediation, contaminated soils can be either removed and further treated ex situor an in situ approach may be attempted.
In situ remediation approaches treat the soil in the place it's found and no excavation is performed. Ex situ stategies are based on excavation and subsequent treatment in fields, vessels, tanks or lagoons where proper conditions to encourage biological activity are maintained, e.g. temperature, moisture, pH, oxygen, nutrients. Both in situ and ex situ treatments often involve a combination of biological and nonbiological processes and operations.
In situ treatment methods may include soil venting in cases where volatile contaminants are dominant, orbioventing where semivolatile or nonvolatile contaminants are involved. Basically in situ treatment consists on pumping air to the subsurface. If the bulk of the action is phase change (volatilization) as in soil venting, there must be a means of handling or processing the exhaust, as contaminants have simply changed phase. Off-gas handling will be a must. Because nutrient limitations will probably exist, underground biodegradation is considered minimal. In the past, soil venting was aimed at recovering product. However, because of the possibility of biological gas cleaning, it belongs to bioremediation. A soil venting strategy may allocate say 3 to 4 CFM per injection point for enabling as much volatilization as possible. On the other hand, we may want to deliver just enough airflow to maintain or enable biological activity as in bioventing. Were we to use say 0.25 to 2 CFM per injection point we would principally promoting actual biological remediation. This method generally relies on the existing microbial population (soil bacteria) although in some probably very select instances special cultures may be added. In any case, water and nutrients may also be needed. At times, uniform air and water distribution may be a problem even with "good soils." If successful, bioventing is a cost-effective alternative to excavation and surface treatment.
As mentioned earlier, ex situ strategies are based on excavation, removal or transport to some other specially conditioned area or volume. Ex situ processes include slurry-phase approaches employing tanks and/or lagoons, orsolid-phase processes such as landfarming and compost.
Landfarming involved aerating and mixing contaminated soil by tilling, nutrient addition and moisture control by means of periodic irrigation. Land treatment basically follows "principles similar to agricultural landfarming." It has been used for centuries to decompose non hazardous, organic waste spreading the waste, tilling the soil to incorporate the waste into the matrix and providing aeration, e.g. by turning, to sustain biological activity. In most cases, contaminated soils are excavated and treated at a site where contaminant migration can be controlled by way of barriers or liners. In some cases contaminated solids are readily near the surface and there's no need for excavation, being most if not all processes performed onsite. Although there may be a photochemical oxidation component, degradation processes in landfarming are eminently biological. Because of emission level restrictions - landfarming may involve or entail significant contaminant volatilization - this approach is not always admitted.
Compost involves intentionally mixing contaminated soils with organic matter, e.g. manure, reusable wood chips or sawdust for bulking and formed into piles or windrows. Periodical mixing may or may not be carried out. Aeration can be induced by thermal density gradients or by mechanical means; water is regularly added to sustain appropriate moisture levels to sustain microorganism populations. End product is not a full-fledged fertilizer but a reasonable and welcome land conditioner/amendment. Basically (Reference 1), "wet organic solids are oxidized to biologically stable forms such as humus." There are four basic parameters for successful compost, i.e. aeration, temperature, moisture content, and pH. Bulking agents prevent compaction increasing porosity and oxygen availability. There are several compost embodiments or configurations such as windrows arrangements, static piles and closed reactors. Although vessel composters involve significant investment they allow for quite control operation.
Slurry-phase strategies include sophisticated bioreactors. Slurry-phase operations basically dump contaminated sludges or soil chunks in specially conditioned basins or tanks where biological activity is carried out. Operation can be batch or sem-batch. Typical configurations include SBR-type arrangements, ie. fill, react, settle,draw. Very much like a regular wwtp, aeration and mixing equipment is installed to maintain D.O. levels. Because of rather high solids levels,power density levels can be considerable and may need to be maintained for mixing even after process oxygen demand tapers off.
