|
Table 2 General Design and Application Considerationsppro rime tar Conventional Versus Bioventing
<br /> SVE Systems p
<br /> ��
<br /> Conventional SVE Biovencing
<br /> Parameter Ik Biodegradable
<br /> k Compound Type Volatile @ Room Temperature —
<br /> Vapor Pressure ' >100 mm Hg —
<br /> Hc (dimensionless) I1 >O.DI
<br /> Aqueous Solubility I ¢100 mg/Y' <16/6
<br /> Soil Concentration j� >1 mg/kg
<br /> DeDepth to Ground Water >24 ft
<br /> P 71 x 14-°cm/s
<br /> Air Phase Permeability Little or No Stratification
<br /> Subsurface Conditions Little or None Biodegradable
<br /> NAPL Phase Outside Contamination
<br /> Vent Well Placements Within Contamination
<br /> Maximum Soil Gas Maximum Retention Time
<br /> Operating Mode Exchange Rate &Aerobic Conditions
<br /> ' 46 to 700+ actual L/s 4.6 to 23 actual L/s
<br /> E ; Operating Flow Rates (100 to 1,500�- acfm) (10 to 50 acfm)
<br /> ti 1 to 15 0.1 to 0.5
<br /> Pore Volumes/d j` =7576 Field Capacity
<br /> =25616 Field Capacity
<br /> Optimal Soil Moisture C:N:P=100:14:1
<br />} x Nutrient Requirement — >2 vol%
<br /> Soil Gas O, Levels j� _ Little or None
<br /> Toxicants '
<br /> 4
<br /> be accomplished fairly easily, and have been used to optimize Bioventing System Design
<br /> contaminant biodegradation at field sites when other variables,
<br /> i.e_, toxicity, do noc(limit microbial activity [16, 171. The two major design considerations for bioventing systems
<br /> 1. Oxygen transfer to the subsurface via SVE systems is gen- are first,whether the contaminants of concern are biodegrad-
<br /> erally more rapid than oxygen uptake rates observed under able under prevailing site conditions, i.e., whether inhibition
<br /> field conditions [16; 17]. This results in the oxygenation of or toxicity is evident at the site, and secondly, whether the
<br /> soil gas to near ambient levels if vent system blowers are op- required terminal electron acceptor, i.e., oxygen, can be ef-
<br /> t erased on a can basis. To minimize system operating fectively transported within the soil to encourage aerobic con-
<br /> 1 casts, and more importantly to reduce or even perhaps edm- taminant biodegradation. The first question can be answered
<br /> inate off-gas treatment requirements entirely,cyclic,or"surge" using soil gas composition and in situ respiration measure-
<br /> pumping of vent systems in bioventing operations is recom- meats, while the second question is answered from in situ air
<br /> mended.Surge pumping in a bioventing mode entails operating permeability measurements.
<br /> i the blower system until soil gas oxygen levels reach near am.
<br /> bient conditions throughout the site being remediated_ The
<br /> system would then be shut off for some period of time during Soil Bioactivity Determinations
<br /> which soil gas oxygen concentrations would be routinely mon-
<br /> }
<br /> itored until they reach a level which in aerobic microbial To determine the potential for in situ existing biodegrsoil microadation f
<br /> activity. Once this limiting soil gas concentration is reached, vadose zone contaminants via bioventing,
<br /> �' the vent system would be restarted.and the on cycle would bial activity should be quantified during site assessment in-
<br /> s
<br /> continue once again. Based on a Henry's Law constant for vestigations. This can be readily accomplished through the
<br /> r 4 oxygen, this oxygen limitation would be expected to occur at a=,Aty at analysis of soil gas O, and CO, composition prior to venting
<br /> e site.
<br /> ,and CO,_concentrations can
<br /> a soil gas concentration oenconcentra ionsoately oapproximat ly along with volatile0organics during standard soil gas surveeys
<br /> sponding to soil water oxyg
<br /> tem oxygen/carbon
<br /> t 1 mg/L.An inhibition of soil respiration has been reported at using a variety of measurement techniques. The author- as
<br /> the 2.0 vo1610 soil oxygen level in venting systems treating 3P- successfully used both wet chemical (Fyri PA and
<br /> 4 contaminated soils,[16]and vented thatthis piles contaminated
<br /> �epre entsea electronic Ganalyzer;
<br /> astecB or Model 32 25 charach OX-InstrumeGastecit Inc.,Newark.
<br /> with PCP waste [19] suggest gCA) methods for field soil gas OZ and CO, determinations-
<br /> good operating number for field scale applications. While both uses can be easily measured,-O, concentrations
<br /> oil
<br /> Based on observed field respiration data from various JP-
<br /> are en ']X rlxescenvi-
<br /> 4 jet fuel contaminated sites [20] and bioventing nrates of 0 1 to becausesithere are dered a better
<br /> abiotic sinks for r of respiration
<br /> s ki syst-s
<br /> z taminated soil plies [19] field oxygen uptake
<br /> 0.6 volted s it to lb g O,/ml soil-d a air filled porosity = ronments. Carbon dioxide is producedthrough
<br /> also be affected.
<br /> 40 voi07o) can be expected_These rates can be nearly an order well as aerobic microbial activity [Zl
<br /> lower as remediation progresses to near de min- by assimilation or dissolution of carbonate rock-
<br /> of magnitudeThe key to the evaluation of soil bioacdvitY____
<br /> imus soil hydrocarbon levels {Dupont et al., 1991), allowing theend �Zttwu
<br /> 'a�axyg-__
<br /> typical bioventing systems to be operated an scheduleshon,7 d off and c bon dioxidef 8 h me,hads is the ennricthment in soil gat of 54te_ ...�
<br /> levelsO be
<br /> on, 16 h off at the initation of remediation,to 8 h on, .au_a
<br /> PC- overemphasized that these deterrniX146o,sn
<br /> near the end of the field effort,while still maintaining aerobic to background, uncontaminated_ sa :__
<br /> conditions wichiri�the contaminated soil during nonvenun,p arison to uncontaminated sots pondition.s
<br /> comp 111"e7tGtSS �.
<br /> nods• eracional of O, depletion and CO, enrichmentor-
<br /> Table ? prese tis a summary of general design, op „��•
<br /> and application considerations appropriate for conventional n nialare tbasaltrespirat one d teen int uacoIItainin?t e
<br /> SVE systems versus those utilized in a bioventing F _
<br /> :E site.
<br /> mode. Febru _A7
<br /> �. J
<br />
|