Bioventing

Organic compounds released into the vadose zone exist in four closely interrelated forms free-phase NAPL, attenuated to surface of soil grains, dissolved in water retained on and between the soil particles, and present as a gaseous phase. The mass distribution of each of these phases is controlled by such factors as concentration gradients, distribution coefficients, and Henry s law constants. 

Consideration of the air permeability of the soil, rate of biological degradation, and rate of volatilization of contaminants is important in the design of a bioventilation system. Reactions that take place in the soil are concentration, temperature, and time 

FIGURE 10.9 Typical distribution pattern for oxygen and carbon dioxide in and around a hydrocarbon plume in porous unsaturated soil. 

Bioremediation as a remediation procedure within the vadose zone can take several forms including bioventing as previously discussed. When conditions of low or inadequate permeability exist, the site may not be suitable for ventilation, and the contaminants may be best degraded by anaerobic reactions. 

In its basic form, bioremediation of the vadose zone involves introduction of nutrients and electron acceptors necessary to stimulate the indigenous bacteria and provide for removal of waste products generated by the reactions. This sometimes takes the form of a series of injections of a soup of nutrients and electron acceptors into the vadose zone through wells, or infiltration galleries. Other sites may require pressure fracturing of the soil before the stimulant blend can be injected. 

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Deeper contamination may be remedied with bioventing, where air is injected through some wells, and extracted through others to both strip volatiles and provide oxygen to indigenous organisms. Fertilizer nutrients may also be added. This is usually only a viable option with lighter refined products. 

R. E. Hiachee, R. M. Miller, and P. C. Johnson, eds.. In situ Meration Air Sparging Bioventing and Belated Kemediation Processes, BatteUe Press, Columbus, Ohio, 1995. 

Ex situ bioremediation may use various biological wastewater treatment processes, soil piles, or land appHcation. With in situ bioremediation, the basic process is the same microbes, soil, and water working together as a bioreactor. Where the in situ techniques differ are in how contaminants and microbes are brought in contact and how oxygen, nutrients, and other chemical supplements ate distributed in the soil—water—air matrix. Typical in situ bioremediation techniques include natural or intrinsic attenuation, air sparging, and bioventing. 

Bioventing. Bioventing is soil venting that enhances biodegradation while extracting volatile compounds from the unsaturated zone. 

The main requirements for the design of a bioventing system are the following9 ... 

An 02 flow must be maintained through the contaminated zone at a level sufficient for the aerobic biodegradation of contaminants. Note that during bioventing the main aim is the maximum utilization of 02 by the microbial cultures. For this reason, air flow rate is usually an order of magnitude lower than that applied in simple SVE systems. A simple empirical rule is that the mean residence time of air in the contaminated soil pore volume should be between 1 and 2 days. 

As previously mentioned, a minimum level of soil moisture is necessary for successful biodegradation. The continuous circulation of air during bioventing results in the evaporation of soil moisture. For this reason, the design of these systems must include an appropriate installation for adding water to the contaminated zone. Care must be taken to avoid the addition of excess water. If soil moisture is significantly increased, e.g., above the limit of 85%, air circulation is no longer effective due to the decrease in free soil porosity. 

A schematic of a bioventing installation, including a system for the addition of water and nutrients, is depicted in Figure 14.4. When the contaminated zone is near the surface and the soil is sufficiently permeable, the addition of water, together with dissolved nutrients, can be carried out using a simple surface irrigation system. When the contamination is located at lower horizons, an underground infiltration system or a network of wells may be more appropriate. 

Mindevalli, O. and Pedram, E.O., Soil bioventing and vapor extraction (in-situ and ex-situ) and ground-water treatment and reinjection at a diesel and gasoline contaminated site, Proc. HAZMACON 94, Bursztynsky, T., Ed., 1994, pp. 386-398. 

Malina, G., Grotenhuis, J.T.C. and Rulkens, W.H., Vapor extraction/bioventing sequential treatment of soil contaminated with volatile and semivolatile hydrocarbon mixtures, Bioremed. J., 6, 159-176, 2002. 

Rathfelder, K.M., Lang, J.R. and Abriola, L.M., A numerical model (MISER) for the simulation of coupled physical, chemical and biological processes in soil vapor extraction and bioventing systems, J. Contam. Hydrol., 43, 239-270, 2000. 

Bioventing. This is a method of treating contaminated soils by drawing air or oxygen through the soil to stimulate microbial activity. 

Bioventing technology was developed by the U.S. EPA Risk Reduction Engineering Laboratory to treat soil contaminated by numerous industrial wastes, which is subjected to aerobic microbial degradation, especially to promote the degradation of polycyclic aromatic hydrocarbons.65 It uses a series of air injection probes, each of which is attached to a low-pressure air pump. The air pump operates at extremely low pressures to allow the inflow of oxygen without volatilization of contaminants. Additional additives such as ozone or nutrients may also be supplied to stimulate microbial growth.77... 

It can be combined with other technologies (e.g., bioventing and SVE) to enhance site remediation. 

Symons, B.D. and Greene, J., Soil vapor extraction, bioventing, and air sparging, in MTBE Remediation Handbook, Moyer, E.E. and Kostecki, P.T., Eds, Amherst Scientific Publishers, Amherst, MA, 2003. 

Despite its humorous name, this technology is a fairly efficient procedure to combine the benefits of vacuum-enhanced recovery and bioventing to promote vapor recovery and in situ biodegradation. Integration of these technologies into a single step results in LNAPL recovery and remediation of residual soil contamination in the vadose zone. 

U. S. Army Corps of Engineers, 1995, Soil Vapor Extraction and Bioventing. Engineers Manual EM 1110-1-4001. 

Wells constructed with perforations and an open unsaturated zone are an efficient procedure to introduce air into the subsurface. A vacuum pump attached to a central well(s) will draw air through the soil to purge the existing soil atmosphere with fresh air. Three commonly used scenarios for bioventing are shown in Figure 10.10. 

T0123 BioTrol, Inc., Biological Aqueous Treatment System T0126 Bioventing—General... 

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