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fuel spill weighing approximatiU3,175 kg (7,000 Ib) would Tat
<br /> 'fable 1 Carrier Fluid Oxygen Suppiy Requirements require 9,525 kg(21,000 lb) of oxygen. This equates to an air
<br /> j Carrier Solution g(lb) Carrier/g (lb) 02 saturated water volume of approximately 8,744,000,000 L
<br /> ure oxygen saturated volume of €
<br /> Water (2,310,000,004 gal), a p i
<br />{ 1,749,000,000 L (462,000,000 gal), or a saturated peroxide
<br /> Air Saturated 110,000
<br /> pure Oxygen Saturated
<br /> 22,000 solution volume of 159,000,000 L(42,000,000 gal)to provide
<br /> 500 mg/L H2O2(100% Utilization) 2,000 the required oxygen for fuel bioremediation. It becomes ap-
<br /> Air{24.9Q10 02} 4.5 parent from these calculations that hydraulic limitations would
<br /> Q be severe for the remediation of a spill even as small as 3,785
<br /> L (1,000 gal) due to massive water volumes required when De.
<br /> temperature in the mesophilic range from 15 to 45'C; and an using saturated phase bioremediation approaches. Ai
<br /> temp Si.
<br /> absence of organic or inorganic toxicants that can inhibit mi-
<br /> crobial activity.
<br /> The most critical limitation to successful bioremediation is BIOVENTING SYSTEMS
<br />' generally the lack of appropriate electron acceptors.A variety
<br /> of electron acceptors can be used by soil microorganisms to Bioventing describes the process in which the air medium is
<br /> carry out the oxidation of organic contaminants.These include utilized to deliver oxygen to the subsurface to stimulate the in C
<br /> It'
<br /> oxygen, nitrate, sulfate, carbon dioxide and organic carbon. Situ biodegradation of organic contaminants. As indicated in
<br /> Of these,oxygen provides the organism with the highest energy 'Fable 1, air is an extremely efficient oxygen transfer medium f
<br /> yield,providing nearly twice that of nitrate, and an order of due to its high oxygen content(20.9 vol%,i.e., 20,900 pPmv) 11
<br /> C
<br /> magnitude higher energy release than sulfate, carbon dioxide and low viscosity as compared to that of saturated water. j
<br /> and organic carbon. Oxygen •metabolism is therefore energet- Bioventing represents a hybrid physical/biological process uti-
<br /> ically selected for, and subsequently, oxygen utilizing micro lizing soil venting systems for oxygen transfer,while focusing
<br /> organisms are ubiquitous in soil environments. Oxygen is also not on contaminant stripping, but rather on in situ aerobic ,
<br /> the pre€erred electron acceptor from an engineering stand- contaminant biodegradation.
<br /> point, as accelerated degradation rates generally occur under Consideration of soil vacuum extraction for oxygen transfer
<br /> aerobic (oxygen rich) conditions as compared to anoxic or to the subsurface was proposed in 1988,by Wilson and Ward
<br /> anaerobic(oxygen deficient) conditions- jg], who noted that systems designed for the removal of vol- 1
<br /> These principles of biodegradation have historically been aides from soil could also be used to transport oxygen. A
<br /> applied to the in situ aerobic bioremediation of contaminated number of other authors have postulated the potential im-
<br /> soils and ground water using,water to carry oxygen to the site provement of in situ,aerobic,subsurface bioremediation using
<br /> of this subsurface contamination. Efforts have been made to SVE for oxygen transfer [10, 11, 1.2, 13, 141 but it has only
<br /> increase the level of oxygen�in this water by saturating the been recently that investigators have collected field data show-
<br /> water with pure oxygen or hydrogen peroxide. These efforts ing the effectiveness of bioventing systems for fuel site re-
<br /> have generally met with limited success, however, because of mediation [15, 16, 171.
<br /> the inability to transfer adequate oxygen to areas of subsurface Bioventing systems are composed of hardware identical to
<br /> contamination due to physical limitations of the transfer of that of conventional soil vacuum extraction (SVE) systems,
<br /> tworks
<br /> the bulk carrier medium through contaminated soils l4, 5 6 with vertical wells and/or literal trenches, Pi ingne
<br /> q, 81. h and a blower or vacuum pump for gas extraction.They differ
<br /> The inherent disadvantage of utilizing water as the carrier significantly from conventional systems,however,in their con-
<br /> graph-is a h- of design and operation. As indi-
<br /> medium for the transfer of olxygen to the s gr P figuration and philosophy
<br /> ically illustrated in Table 1.I�These values represent the mass sated above, the primary purpose of a bioventing system is to
<br /> of fluid required to transferIa unit mass of oxygen under the use moving soil gas to transfer oxygen to the subsurface where
<br /> stated conditions.Due to the�low solubility of oxygen in water, indigenous organisms can utilize it as an electron acceptor to
<br /> rohibitsvei large amounts of oxygen-saturated water are re- out aerobic metabolism of soil contaminants. As such,
<br /> quired even when using ptii•e oxygen or hydrogen peroxide bioventing system extraction wells are not placed in the center
<br /> saturated solutions. This oxygen supply limitation is exacer- of the contamination as in conventional SVE systems (Figure
<br /> bated by the high oxygen demand of hydrocarbon contami- 1), but on the periphery of the site(Figure 2),where low flow
<br /> pants as indicated by the simple stoichiometric reactions for rates [4.6 to 23 actual L/s (10 to 50 acfm)versus 46 to 700+ 3
<br /> hexane oxidation shown below assuming no substrate incor- actual L/s(100 to 1,500+ acfm)for conventional SVE systems]
<br /> poranon into cell material: '' maximize the residence time of vent gas in the soil to enhance -
<br /> in situ biodegradation and minimize contaminant volatilize-
<br /> -- 6 CO,+7 H2O tion.
<br /> ;I 3.5 g 02/g CeHE. Because it is a biological treatment'approacli,however,bio- a
<br /> venting does require the management of environmental con-
<br /> ditions to ensure maintenance of biaactiviry at the site. Man-
<br /> carbon for hydrocarbon mineralization, a 3,785 L (1,
<br /> Assuming an oxygen requirement of only 3 g 02/9 hydro-000 gal) agement of soil moisture and soil nutrient levels to avoid
<br /> inhibition of microbial respiration within the vadose zone can
<br /> €p By-Pass Off-Gas By-Pass Off-Gal
<br /> l Knock-otic Trrarmrnc '� -0uc rrea�ene
<br /> t Drum Blo-erwtu� �°6�30E tkva alaRer
<br /> Weil Well l •`<•"�
<br /> j
<br /> I 1
<br /> €
<br /> QW—
<br /> _ F
<br /> i
<br /> FIGURE 1. Schematic o# typical conventional SVE sys- FIGURE 2. Schematic of recommended bioventing sys-
<br /> tem layout. tem layout.
<br /> 46 February, 1993
<br /> Environmental Progress (Vol. 12, No. 1)
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