S O I L W A S H I N G AND B I O T R E A T M E N T OF
P E T R O L E U M - C O N T A M I N A T E D S O I L S
By Alok Bhandari, ~ Dennis C. Dove, 2 and John T. Novak, 3 Member, ASCE
ABSTRACT: Soil washing was evaluated in combination with biological treatment as a decontamination technology for petroleum-contaminated soils. The bench- scale soil washing system utilized to clean the soils also fractionated the bulk soil into sand, silt, and clay fractions. With tap water as the carrier, the petroleum removal efficiencies varied from 44% to 55% for three soils. The postwash hydro- carbon levels were in the range of 145-905 mg/kg for sands, 2,000-5,000 mg/kg for silts, and greater than 14,000 mg/kg for clays. Biological degradation was eval- uated as a secondary treatment to reduce the contaminant levels on each fraction to desired levels. Simulated composting lowered hydrocarbon levels on sands to below 50 mg/kg. Slurry treatment of silt and clay fractions reduced hydrocarbon levels to near 100 mg/kg for silt and in the range of 500 to 1,000 mg/kg for clays. It was found that composting and slurry treatment effectively met the suggested target-treatment level of 100 mg/kg for the sand and silt fractions. For the clays, slow desorption of the hydrocarbons and long treatment periods made slurry treat- ment an uneconomical alternative.
INTRODUCTION
Widespread use of petroleum products during the past half century has resulted in many instances of soil contaminat ion by crude oil and oil prod- ucts. Aboveground spills at petrochemical complexes, overfilling of under- ground storage tanks and pipelines, as well as everyday operations at retail outlets have contributed to pollution of soil and ground water. Although some of the leaked product can be recovered by drainage, pumping and extraction, a major portion often remains trapped in the pore spaces of the soil or aquifer material or bound to soil surfaces. Physicochemical t reatments such as soil washing or flushing may be used in combinat ion with biore- mediation to reduce the contaminat ion to desired levels.
Soil washing is a promising technology that can be utilized in the t reatment of pe t ro leum-hydrocarbon-contaminated soils. As defined by Nash et al. (1988), soil washing is the mechanical or chemical dispersal of contaminated soil in order to isolate the contaminant into as little soil as possible. The washing process fractionates the contaminated soil into different particle- size fractions (sands, silts, and clays) and removes contaminants from the soil by mechanical shearing, dispersion, emulsification, dissolution, air strip- ping, froth flotation, or a combinat ion of these. Appropriate postwash treat- ment can then be directed towards each fraction based on the residual contamination and desired t reatment goals.
Field studies of soil washing in Europe have been summarized by Pheiffer
1Grad. Res. Asst., Dept. of Civ. Engrg., Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061.
2Sr. Res. Assoc., Dept. of Civ. Engrg., Virginia Polytech. Inst. and State Univ., Blacksburg, VA.
3prof. of Civ. Engrg., 318 Norris Hall, Virginia Polytech. Inst. and State Univ., Blacksburg, VA.
Note. Discussion open until March 1, 1995. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on October 30, 1992. This paper is part of the Journal of Environmental Engineering, Vol. 120, No. 5, September/October, 1994. �9 ISSN 0733-9372/94/0005-1151/$2.00 + $.25 per page. Paper No. 5034.
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(1990) and Nunno et al. (1988). Experience in Europe has shown that soil washing can be conducted on a large scale and at relatively low costs as long as the clay fraction and level of soil organic matter are low. Studies in the United States have demonstrated that each soil requires a separate evaluation because considerable variations may occur in the effectiveness of the washing process (Nash et al. 1988; Nash 1987; and Esposito et al. 1989). Depending upon target treatment levels, the soil fractions obtained during soil washing may require additional treatment to meet the total petroleum hydrocarbons (TPH) criteria for ultimate disposal. This study focused on bioremediation as a postwash treatment alternative and evalu- ated its effectiveness and limitations in reducing contamination on soil below the suggested cleanup levels.
It is observed that a major portion of the residual contamination after soil washing, soil flushing, or pump-and-treat consists of the higher molec- ular weight hydrocarbons that have physical characteristics that render them unavailable for microbial degradation (Bossert and Barta 1984). These pre- treatment techniques may be incapable of lowering contaminants to levels acceptable for land disposal, but they may reduce the contamination to levels that are not toxic to microorganisms. Therefore, physicochemical pretreatment techniques in combination with bioremediation may offer an economical solution to the problems of decontamination and disposal of petroleum-contaminated soils.
The purpose of this study is to evaluate the effectiveness of soil washing in combination with biological treatment as a remediation strategy for pe- troleum-hydrocarbon-contaminated soils,
MATERIALS AND METHODS
Soils Three different petroleum-contaminated soils were used in this study.
Salem soil was obtained from a contaminated site while New River and Eagle Point soils were obtained clean and then contaminated in the labo- ratory. The laboratory-contamination procedure used 500-g clean soil sam- ples and the contaminant was a petroleum distillate obtained from a fuel depot in San Francisco, Calif. Loss of volatile components was minimized by injecting the contaminant into an inverted glass bottle containing the clean soil. The contaminated soil was mixed in a mechanical shaker for 4 hr and the sealed bottles were then stored at 4~ Salem soil had been contaminated on site as a result of hydrocarbon spills during machinery- washing operations. The likely pollutants were high-molecular-weight, non- volatile hydrocarbons. Soil samples from this site were transferred into glass containers, sealed with teflon tape, and stored at 4~
Petroleum Hydrocarbon Analysis A method for TPH analysis previously used by Falatko (1990) was used
to analyze petroleum hydrocarbons on soil samples. This method is an alternative to the EPA Method 9071 ("Test" 1986) that uses Freon as the extracting solvent and infrared spectroscopy for quantitative analysis. In a comparative study between the two methods (Vogdt 1993), the EPA method 9071 was seen to detect compounds other than petroleum hydrocarbons and, therefore, give erroneously high TPH values. Our method is not sub- jected to such interferences and gives more reliable and repeatable TPH values. Eleven replicates of a contaminated soil were analyzed for TPH
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using this method. The mean was found to be 30,670 ppm with a standard deviation of 150 ppm. When compared with the more elaborate TPH analysis method suggested in the California LUFT Manual ("Leaking" 1987), the method used in this research produced similar results and repeatability. Because of its simplicity and acceptable analytical variability, this method was used for all TPH analyses.
Method for T P H Analysis Samples of 1 g of the contaminated soils were extracted in 20 mL of
methylene chloride (MeCI) for 12 hr. After extraction, 0.5 mL of a 170- ppm 1,4-dichlorobenzene (DCB) solution in MeCI was added to 1.0 mL of the extract. DCB was used as an internal standard to account for changes in TPH levels from loss of solvent during storage. DCB was chosen as the internal standard since it is not a component of the contaminant fluid, it has a lower volatility than the low-molecular-weight component of the con- taminant, and it can be detected efficiently using our analytical method. External standards were prepared from a solution of fuel oil, diesel, ker- osene, petroleum distillate, and crude oil in methylene chloride. A standard curve was constructed and used to estimate TPH values for all samples.
A gas chromatograph with a flame-ionization detector (GC/FID) was utilized for petroleum hydrocarbon analysis. A DB-1 fused silica capillary column with 0.32-mm inner diameter, 30-m length, and 0.25-1xm film thick- ness was used. The initial temperature was kept at 60~ for 4 min, increased at 7~ to 325~ and maintained at that temperature for 5 rain. The injector and detector temperatures were 300~ and 325~ respectively.
Soil-Washing Apparatus The soil-washing apparatus included a high shear mixer, an induced air-
flotation unit, a low shear mixer, an upflow column, and a sedimentation tank (Fig. 1). A 500-g sample of the contaminated'soil was suspended in 0.5 L of water and subjected to high-shear mixing at 4,000 rpm for 45 min. High-shear mixing was able to disintegrate soil clumps and detach fines that adhered to the coarser particles. The slurry was transferred into the fotat ion unit and diluted to 2 L using tap water. Air was bubbled through the slurry and the froth resulting was skimmed off. The flotation unit was then switched to the low-shear mixing mode (200 rpm). Low-shear mixing kept the slurry in suspension during the washing operation.
The low-shear mixer was connected hydraulically to an upflow column and a sedimentation tank. Details of the upflow columns are presented in Fig. 2. The specifications of the columns are given in Table 1 while details of sedimentation tank are listed as follows: particle size of silts = 0.0065 mm; calculated settling velocity -- 0.1998 cm/min; volume of settling basin = 90 cm x 14 cm x 5 cm; flow through basin cross section = 255 mL/ min; horizontal flow velocity = 3.64 cm/min; and detention time = 25 min. The depth of the fluid in the settling basin maintained at 5 cm.
The total slurry volume in the system was increased to 12 L. The slurry was injected into the upflow column through a port situated at the bottom of the column. Peristaltic pumps were used to maintain the required up flow velocities at the column sections A, B, and C. The total upflow velocity of the slurry in the column was maintained at a value lower than the calculated settling velocity of the sand-sized particles (23.75 cm/min), but greater than the settling velocities of silts and clays. The settling velocities for the different particles were calculated by applying Stoke's law for particle sedimentation.
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500 g soil/2 L water
CONTAMINATED SOIL] 500 g dry soil
45 min
I High Shear Mixing I 500 g soil/0.5 L water
~r 45 rain
llnduced Air Flotation} ........... ~ FROTH I
I.ow s.ear M,x,n~ 1500 ~ s0,,/.2 L water 3 hours
Oeotri,ogation [U~,,ow ~,umo Fraotionatlon]
V
......... ISedimentation Tank
FIG. 1. Schematic of Laboratory-Scale Soil-Washing Process
Upflow column Flow #1
C
Sand collection port
FIG. 2. tem
Flow #3
�9 Pump #1
Low-shear mixer
Flow #2
T 0 I "Sedimentati~ tank I
Pump #2
Schematic Illustrating Various Flows and Flow Controls in Washing Sys-
For design purposes, the soil particles were assumed to have a specific gravity of 2.65. A minimum diameter of 0.075 mm was assumed for sand, and 0.0065 mm and 0.002 mm for the silt and clay particles, respectively.
Sand particles settled to the bot tom of the column and were removed through the sand-collection port. The silts and clays remained suspended
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TABLE 1. Cross-Sectional Dimensions, Design Flows, and Upflow Velocities st Sections A, B, and C of Upflow Column
Cross-sectional Column area Flow Upflow velocity sections (cm a) (mL/min) (cm/min)
(1) (2) (3) (4)
A B
10.74 21.74
63.60
255 a 255 a + 255 b
= 510 255" + 255 b q- 510 c
= 1,510
23.74 d 23.46
23.74
Note: Refer to Fig. 2 for column sections, A, B, and C. aFlow recycled from settling basin into upflow column (flow 1). bFlow recycled from top of upflow column to bottom (flow 2). cFlow entering upflow column from low shear mixer (flow 3). dMinimum upflow velocity in column should be less than settling velocity of 23.75 cm/
rain for sand particles.
and their flow was directed into the sedimenation tank. In the sedimentation chamber, the silts settled while the clay particles remained suspended. The effluent from the sedimentation tank was recycled into the system to improve fractionation of the silts and clays. Clay was removed by centrifugation. The supernatant was analyzed for TPH and was referred to as the wash-water fraction.
To evaluate the fractionation efficiency of the washing apparatus, the soil fractions resulting from washing were compared to the particle size frac- tionation achieved by hydraulic screening of the soil. Pressurized water is used in a hydraulic screen to wet-sieve soil and separate it into different particle-size fractions.
Composting Bioreactors The composting study for sand-sized particles was carried out in 50-mL
glass bottles containing 45 g (dry weight) of contaminated sand. The large heterogeneity existing in the sands required replications in the experimental design. A limited supply of the contaminated sand necessitated the small- sized miniature reactors.
Each reactor was dosed with 5 mL of a nutrient solution that consisted of KHzPO4 at 3.8 g/L, K2HPO4 at 12.5 g/L, (NH4)2HPO, at 1 g/L and a micronutrient solution at 1 mL/L. The micronutrient solution was made up of 4 g MgSO4, 0.2 g NaC1, 0.2 g FeSO4.7HzO , 0.2 g MnSO4, and 0.2 g CaC12 in 100 mL of deionized water.
A 5-mL aliquot of active petroleum degraders at a concentration of 6 • 106/mL was added to each reactor. The petroleum degrading bacteria were obtained from Sybron Chemicals Inc. of Salem, Va., and were grown sep- arately in the laboratory on gasoline, diesel, petroleum distillate, and crude oil as sole carbon sources. The bacteria were enumerated by the agar plating technique (Clark 1965). The control reactors did not receive any inoculum or nutrients. The moisture level was maintained at 15% by weight. The reactor contents were mixed daily to provide adequate aeration. Samples were withdrawn from each reactor at intervals of 10 days and analyzed for TPH content.
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Slurry Bioreactors For the silt and clay-sized soil fractions, slurry reactors enriched with
petroleum degraders were utilized. The soil was suspended in water at concentrations of 5 g/L and 3.3 g/L for silts and clays, respectively. Mixing and aeration of the reactor contents was accomplished by a coarse bubble- aeration system. Nutrient doses similar to those for composting bioreactors were applied to the slurry bioreactors. Each reactor was inoculated with a 25-mL aliquot of the mixed petroleum degraders resulting in a concentration of 1.5 • 105/L in the slurry reactors. The hydrocarbon-degrading microbes were obtained from Sybron Chemicals, Inc.
The soils were analyzed for TPH on day zero and thereafter, 40-mL slurry samples were withdrawn, the contents centrifuged, air-dried, and then an- alyzed for petroleum hydrocarbons. The soil slurries were sampled on day 1 and thereafter at intervals throughout the study period. The silt and clay slurry reactors were kept in operation for periods of 60 and 100 days, respectively.
RESULTS
The soil-washing study aimed at assessing the fractionation efficiency of the washing apparatus, determining the postwash hydrocarbon distribution in the system, and evaluating the extent of pretreatment achieved by soil washing. The bioremediation portion of this research investigated the extent of petroleum removal when the soils were inoculated with petroleum-de- grading microorganisms. The rates of TPH degradation were compared for sand, silt, and clay fractions.
Fractionation Efficiency For the soil-washing technique to be effective in removing petroleum
hydrocarbons, it is imperative that a clean fractionation of the sand, silt, and clay-sized particles be obtained. A clean separation is important because hydrocarbon distribution is greatly dependent on particle size.
The soil-washing apparatus utilized an upflow column and a sedimentation tank to produce an accurate fractionation of the bulk soil. The separation resulting during washing was compared to the soil fractionation obtained with a commercial hydraulic screen. As seen in Fig. 3, the separation re- sulting during soil washing was comparable to that obtained by screening. During washing, some clay-sized particles were removed in the froth fraction thus reducing the weight fraction of clay when compared to hydraulic screen- ing.
Generally, a major goal of the soil washing process is to provide a "clean" sand that meets regulatory requirements for handling as a soil or municipal solid waste. Retention of even a small quantity of silt and clay in the sand fraction could, therefore, preclude unregulated disposal. Data from this study show that when flows are adequately controlled and mixing is suffi- cient, the upflow column and sedimentation tank are capable of providing an excellent size separation.
Soil Fractionation and Hydrocarbon Distribution Figs. 4, 5, and 6 illustrate the soil separation and postwash hydrocarbon
distribution for the Salem, New River, and Eagle Point soils. Salem soil had been exposed to the atmosphere at the contaminated site and, therefore, contained much less low-molecular-weight material. Salem soil was pre-
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7 0 1 i i 1
60
5 0 -
o ~ 4 0 -
o ~- 30-
20-
I0-
0
<)<2 <3o , , ( X ) , ( X ) ' ( X ) ' ( X . , " <x_) . .,'C':,,O.
,OK) K X ) K X )
K X )
K X . . , KX . , "
',< X.. .
1,,( x ~' K),(3, r v ~
[ j H y d r a u l i c s c r e e n <x) <x> ~ Soi l w a s h i n g a p p a r a t u s -
SAND SILT CLAY FROTH
FIG. 3. Comparison between Soil Fractionation Obtained by Commercial Hy- draulic Screen and Soil-Washing Apparatus Shows that Washing Produced Clean Fractionation
dominantly sand (61%) and washing resulted in a sand fraction that con- tained only 2.2% of the total initial contamination. Approximately 40% of the original contamination was associated with the silts and clays while nearly half of the TPH was transferred into the wash water as a result of emulsi- fication or dissolution. Volatilization and other losses were calculated by difference and accounted for 4% of the initial TPH.
Sand-sized particles made up 31% and 60.4% of the New River and Eagle Point soils, respectively. In each case, the washed sand contained a small fraction of the initial contamination while the silt and clay fractions possessed larger amounts. Chromatograms confirmed that a considerable amount of the semivolatile contaminants were transferred into the wash water, possibly due to emulsification. Some TPH was removed with the froth, but a major portion of the initial contamination disappeared as a result of volatilization or other losses.
The New River and Eagle Point soils had been contaminated in the laboratory with a petroleum distillate. Low-molecular-weight hydrocarbons, (less than C12), constituted a large fraction of the distillate. Since the lab- oratory-contaminated soils were not exposed to the atmosphere prior to washing, these soils contained a large fraction of volatile petroleum com- ponents in addition to the semi- and nonvolatile hydrocarbons. For these soils, the major reduction in TPH during soil washing was from the loss of
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(A) (5.0%) (14.9%)
(B)
Clay Sill x < ~ ] ~ " %)
( 1 1 . 7 % ~
Washwater
(50.5%)
Froth (4.1%)
Sand (2.2%) Volatile and other losses
(3.6%)
FIG. 4. (a) Different Soil Fractions Obtained while Washing Salem Soil; and (b) Distribution of TPH among Various Soil Fractions, Wash Water, and Volatile and Other Losses
the volatiles to the atmosphere. In a practical situation, most of this removal takes place shortly after a contaminant spill and, as in the case of Salem soil, the highly volatile hydrocarbons are generally not present when the contaminated soil is remediated.
Overall, the mechanical shearing, dispersion, emulsification, dissolution,
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Froth
Sand
(31.0%)
Clay (25.5%)
Silt (41.2%)
(A)
(B)
Volatile end / other losses
(69.2%)
Washwater Silt (10f%) (3.3%)
] 1
i I I
Clay 1 4.2%)
Froth (1.6%) Send (1.1%)
FIG. 5. (a) Different Soil Fractions Obtained while Washing New River Soil; and (b) Distribution of TPH among Various Soil Fractions, Wash Water, and Volatile and Other Losses
air-stripping, and froth-flotation processes during soil washing were capable of producing a considerable amount of contaminant removal in all three soils. This was not only true for the freshly contaminated soils but also for the highly weathered Salem soil. Most of the hydrocarbons remaining were
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Sand
t (60"4%)
(4.8~) (19.9~) Clay (14.9~;)
(B) Washwater (16.7%) Silt
~ ~ 1clay 4/ 3.9~) I Froth (3.0%)
(61.5~) ~,~, Volatile and other losses
1 ~ Sand (0.6~)
FIG. 6. (a) Different Soil Fractions Obtained while Washing Eagle Point Soil; and (b) Distribution of TPH among Various Soil Fractions, Wash Water, and Volatile and Other Losses
distributed between the silt and clay fractions. For each of the soils, clays retained the largest mass of the residual contamination while representing the smallest fraction by weight. Sands made up the largest weight fraction in all soils and contained less than 3% of the original contamination.
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10000
8000
E 6ooo .,._" ~ o
.~ 4000 -1-
2000
FIG. 7. Total Mass Eagle Point Soils
/ / / 1 / / / J
)"//1 . " / / , 4 x x x / / / J X X X " / . f A X X X ," / / J X X X
X X X Axx• ~" / / . , I X X X , - / / J X X X
, - ' / / j X X D K ,- ,,- z .J x x x
x x • I X X X
/ / / '
/ / / / / / / / /
I / / r162 / / / / / /
y/r / / / / / / / / / / / / / / / . / / / / / / / / / / / / / / / / / / / ' / / / / / / / / / / / / / / / / / i i i
I
Before wash
After wash
i i .
i / / / / / / , ' , .A,',. / / / x x x / / / x x x / / / X X X / / - / - X X X ; / / / X>O,(. I / / / - X X') , ( / / / X X X X X X / / / X X X / / / X X X X X X x_x_x X3,O,( / / / X X X X X X ' ~ . X X X X X X XXX / / / / / / X X X / / , ,
X X X X X X X X 3 , ( X X X X X X
i X X X X X N
! X.."Z X X1~2X X X N
SALEM NEW RIVER EAGLE POINT of TPH Removed by Soil Washing for Salem, New River, and
Petroleum Removal Efficiency by Soil Washing The overall hydrocarbon removal by soil washing for the three soil/con-
taminant combinations is shown in Fig. 7. The TPH removal efficiency is defined as the percentage of TPH removed during the washing process by emulsification or dissolution, and by froth flotation. In the case of New River and Eagle Point soils, the initial amount does not include volatile hydrocarbons since this fraction is generally absent from most soils obtained from contaminated sites. Removal efficiencies in the study ranged from nearly 44% for New River soil to 55% for Salem soil. The treatment was less efficient when the contaminated soil contained large amounts of fine particles as in the case of New River soil. Fine particles provide a larger total surface area per unit weight for contaminant adsorption. Fine particles like clays are also known to contain higher amounts of natural organic matter. These factors are responsible for the enhanced surface sorption of hydrophobic contaminants on finer soil particles.
TPH Levels after Soil Washing Fig. 8 shows the TPH concentrations on the different fractions obtained
by soil washing. In the case of Salem soil, prior exposure to the atmosphere was responsible for the lower prewash (initial) hydrocarbon concentration when compared to the New River and Eagle Point soils. The hydrocarbon levels on sands ranged from 145 mg/kg for Eagle Point soil to 950 mg/kg for New River soil. The higher concentration of hydrocarbons on the New
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E o..
13_. I--.
25000
20000
15000
10000 Z /
8000 -
7000 -
6000 -
5000 -
4000 -
3000
2000
1000
0
1
1 1
I 1 Solem [[[1111 New River W Eagle Poin t
/
mL
FROTH CLAY INITIAL SILT SAND
FIG. 8. Initial TPH Levels on Salem, New River and Eagle Point Soils and Postwash Hydrocarbon Concentration on Different Soil Fractions
River sand was thought to be due to the interaction of surface metal oxides with the organic contaminant (Dove et al. 1991). The iron-oxide distribution on the three different sands showed an elevated level of crystalline and amorphous iron oxides on the New River sand particles (Dove and Novak 1992). It is speculated that the iron oxides in New River sand bind hydro- carbons, making them difficult to solubilize in water during the washing operation. Dove and Novak (1992) used scanning-electron-microscope techniques to investigate the surfaces of the contaminated and washed grains of New River sand and found several regions of high carbon content. These regions were also found to possess elevated levels of iron oxides, further suggesting a connection between the iron content and the TPH retention capacity of the New River sand grains. This relationship requires further verification.
Postwash TPH levels for the different silts ranged from approximately 2,000 mg/kg in New River soil to nearly 5,000 mg/kg for Salem soil. The petroleum levels on the clays were as high as 14,000 mg/kg to 20,000 rag/ kg. The fine clays provided larger surface areas for contaminant association and, therefore, hindered easy removal of the petroleum compounds during washing.
The results obtained from the soil washing study suggest that washing of a petroleum-contaminated soil with water without any chemical additives can reduce contamination by as much as 55%. However, the final TPH levels still remained higher than the suggested target cleanup level of 100 mg/kg. A TPH level of 100 mg/kg was chosen as the guidance cleanup level as such a level is
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E O.. n
:E 13.. I.--
E t3.. Q.
::E O_ I---
E Q. Q.
=f Q_ I--
3OO
240
180
120
60
0
1200
900
600
3OO
0
360
300
240
180
120
6O
0
i
I I I I I
q D e t e c t i o n l im i t
I I I I
S A L E M S A N D
I I 1
O
I I I I I I NEW RIVER SAND "
.
o
t �9 w ' O
0
I I 1 1 I t EAGLE
Detection limit I I 1
10 2O 30
1 1 1 POINT SAND
o
- O - - - O - - - - - e - - 0 - - - - -
I I I I I I
4 0 5 0 60 7 0 8 0 9 0 1 0 0
Days
FIG. 9. Removal of TPH from Sand Particles as Seen in Composting Reactors for Salem, New River, and Eagle Point Soils
frequently applied at underground storage-tank-site closures (Kostecki and Calabrese 1991), and is sometimes used as a remediation action level for petroleum-contaminated sites. The postwash TPH levels greater than 100 mg/ kg for all soil fractions, therefore, necessitated additional treatment.
C o m p o s t i n g Composting was evaluated as the primary method of treatment for the
petroleum-contaminated sand fraction. The larger size of the sand particles
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TABLE 2. Zero-Order Rate Constants for TPH Removal in Different Systems
Zero-order Soil type Reactor type degradation rates
(1) (2) (3)
Salem clay Eagle Point clay Salem silt Eagle Point silt New River slit Salem sand Eagle Point sand New River sand
Slurry Slurry Slurry Slurry Slurry Composting Composting Composting
58 mg/kg-d 50 mg/kg-d 58 mg/kg-d 5 mg/kg-d
13 mg/kg-d 4 mg/kg-d
50 mg/kg-d 5 mg/kg-d
precludes their being treated in a slurry system. Moreover, an intensive treatment in a slurry reactor may be unnecessary due to the lower contam- inant levels associated with the sands. Composting was selected to minimize operation costs and at the same time provide adequate mixing and oxygen availability to encourage the microflora to degrade petroleum.
The data for the composting study are presented in Fig. 9, and the zero- order degradation rates for the bioreactors are illustrated in Table 2. In the case of Salem sand, the active petroleum degraders steadily metabolized the TPH to levels below the method detection limit of 50 mg/kg by day 50. The control reactor with no microorganism amendment had a TPH of 220 mg/kg at the end of 80 days. For Eagle Point sand, biotreatment in simulated tempesting bioreactors was able to reduce TPH levels to below 50 mg/kg in 40 days, TPH levels in the controls remained at 300 mg/kg at the end of 80 days.
No considerable reduction in contaminant level was observed in the New River sand. The hydrocarbon level was approximately 600 mg/kg after 15 clays and remained at that level over the remaining length of treatment. The control reactor had a TPH of 830 mg/kg by day 80. The retention of a large nonbiodegradable hydrocarbon fraction for the New River sand could have been due to the binding of hydrocarbons to surface metal oxides on the sand particles, but this relationship has not been verified.
Slurry Biotreatment The postwash biotreatment for silts and clays was conducted in slurry
systems with microorganism amendment. Composting was ruled out in this case as the finer particle size and higher TPH levels on silts and clays were expected to make mechanical mixing and mass transport of oxygen and water in the system inefficient and uneconomical. Slurry systems have the potential to improve removal of organic contaminants from fine particles by enhancing mass transfer from the soil surface to the aqueous phase, thereby increasing the availability of the substrate to microorganisms.
In slurry systems, physical desorption was seen to play a substantial role in lowering the initial TPH from soil particles. Hydrocarbons from the dried soils were released into solution immediately after the contaminated soil was suspended in water. The lower level of TPH on soil particles in seeded systems as compared to controls indicates that microbes played a role in desorption, possibly due to the production of biosurfactants. In evaluating biological treatment of the soil-bound TPH, the focus was kept on the TPH remaining on the soil after day 1.
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4800
E 2000"
1600
:ff 1 2 o o
800
400
0
2400
2300 E 1 2 0 0 / EL
900
600
300
0 3400
E o_ 1200 / EL
900
600
300
0
[ I I I I / ~ TPH following soil washing
/ TPH transferred to aqueous phase O /
L i ofter addition to slurry~
SALEM SILT
I !
> I . Data point I NEW RIVER SILT . / Io Control dotol o l
- 0 Z
I I 1 I ~
0 1 0 20 30 40 50 60
Days
FIG. 10. Removal of TPH from Silt Particles as Seen in Slurry Bioreactors for Salem, New River, and Eagle Point Soils
Silts A substantial removal of TPH was seen for the three silts (Fig. 10). The
TPH levels on Salem silt were reduced to below 100 mg/kg within 30 days. The control with no organism addition showed TPH levels of 1930 mg/kg at the end of 50 days. For the New River and Eagle Point silts, it took 40 and 50 days, respectively, for the hydrocarbon levels to reduce to 100 mg/ kg.
Rates of degradation for the hydrocarbons on silt surfaces were deter-
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15000
14000
13000
6000 /
5000
4000
3000
2000
1000
0
19000
18000
12000" E
10000
Z 8000
6OOO
4000
2000
0
' ' ' ' ' ' ' ' M " l
SALEM C �9 Data point I o Control data
/ /
o
I 1 I I I I I l I
0
EAGLE POINT CLAY J~ Data point J
I Con t ro l data 1
[ I f 1 ! ! [ !
0 10 20 30 40 50 60 70 80
/ / / /
J
Q !
90 100
Days
FIG. 11. Removal of TPH from Clay Particles as Seen in Slurry Bioreactors for Salem and Eagle Point Soils
mined assuming that degradation could be adequately described as zero- order. The rate constants are shown in Table 2. Removal rates were highest for Salem silt. Salem soil was obtained from a site which had been contam- inated for many years and could be expected to contain an indigenous population of petroleum degraders acclimated to the contaminant. A large portion of the contamination that remained on the Salem soil can be ex- pected to include partial-degradation products. Once the environment was made conducive to biodegradation by adding more biomass, providing the
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necessary nutrients for cell growth and encouraging mixing, the petroleum hydrocarbons in this soil degraded fairly rapidly. Biodegration was slower in the controls because these reactors were not stimulated by the addition of growth nutrients.
clays The clay-sized fractions obtained during soil washing were also treated
in slurry bioreactors. The soils studied were from Salem and Eagle Point sites. Slurry treatment resulted in a substantial removal of petroleum con- taminants from both soils. However, the final TPH levels on the clays could not be reduced to the desired level of 100 mg/kg, even after more than 13 weeks of intensive slurry treatment.
It can be seen in Fig. 11 that for Salem clay, the petroleum hydrocarbon concentration was reduced to 400 mg/kg by day 95. The TPH levels on Eagle Point clay dropped to 900 mg/kg in the same duration. At this point, the operation of the slurry batch reactor was terminated due to depletion of the reactor contents caused by sampling. Substantial degradation was ob- served in the control reactor for Salem clay when compared to Eagle Point clay. The removal rate was also seen to be higher in the organism-amended Salem clay bioreactor when compared to Eagle Point clay. Table 2 illustrates these differences in terms of zero-order degradation rates. A possible reason for this difference may be the same as for the silts, that is, the possible presence of acclimated indigenous microorganisms in this soil.
The experiments with slurry reactors indicate that although these systems are capable of producing a considerable reduction in contaminant levels, treatment periods and operational costs may depend on particle size and initial contamination. The study shows that in contrast to sirs, which can be effectively bioremediated to the point where the contamination is lowered to background levels, clays tend to retain a considerable amount of the hydrocarbons even after an extensive biological treatment in slurry systems. If target TPH levels below 100 mg/kg are to be met, then the slurry reactor methodology described here may not be effective. Modified slurry systems that stimulate biosurfactant production or mixed activated sludge systems should be considered.
SUMMARY AND CONCLUSIONS
Soil washing was studied in combination with biological treatment as a remediation strategy for petroleum hydrocarbon contaminated soils. Three soils were washed with tap water and fractionated into sand-silt-, and clay- sized particle fractions. In the absence of any chemical additives, washing could not reduce the TPH levels below the suggested treatment goal of 100 mg/kg for any soil fraction. Bioremediation was evaluated as an additional treatment. Sands were treated in landfarm bioreactors while silts and clays were subjected to slurry biotreatment.
The data obtained from this study suggest that a petroleum hydrocarbon contaminated soil can be effectively treated by utilizing bioremediation in combination with a physicochemical process like soil washing. The sand fraction resulting from the washing operation may be disposed of directly without any further treatment if applicable treatment objectives are met. If not, it can be treated further by land farming. The soil fines can be frac- tionated into silts and clays, and the silts land farmed along with sand or treated in aerobic slurry reactors. For the clay fractions, it is likely that a modified or additional treatment may be required prior to disposal.
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The specific conclusions of this study include:
1. The laboratory-scale soil washing apparatus was able to produce a distinct size separation of the soil into sand, silt, and clay fractions.
2. The residual TPH concentration after washing was found to be a func- tion of particle size; sands were the cleanest while clays contained the highest residual hydrocarbon levels; less than 3% of the original contamination was retained on the sand.
3. With tap water as the carrier, the petroleum removal efficiencies by soil washing varied from 44 to 55% for the three soils.
4. Postwash biotreatment was effective in lowering the petroleum hy- drocarbon levels in all soil fractions; the rate of TPH removal ranged from 4 mg/kg-d to 50 mg/kg-d for sands and from 5 mg/kg-d to 58 mg/kg-d for silts.
5. In the case of clays, slurry treatment was found to be uneconomical because of the long treatment periods required to meet the desired reme- diation level.
ACKNOWLEDGMENTS
The financial support for this research was provided by the Coastal Re- mediation Company of Roanoke, Va., and the Virginia's Center for In- novative Technology.
APPENDIX. REFERENCES
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Clark, F. E. (1965). "Agar-plate method for total microbial count." Methods of soil analysis. Part 2. C. A. Black, ed., American Society of Agronomy, Inc., Madison, Wis.
"Leaking underground fuel tank field manual: guidelines for site assessment, cleanup and underground storage tank closure." (1989). California LUFT manual. State of California Leaking Underground Fuel Tank Task Force, California State Water Resources Control Board, Sacramento, Calif.
Dove, D. C., and Novak, J. T. (1992). "Soil washing and post-washing soil cleaning for permanent disposal." Proc. Federal Envir. Restoration Conf., Hazardous Ma- terials Control Resources Institute, Vienna, Va.
Dove, D. C., Novak, J. T., Bhandari, A., and Falatko, D. (1991). "Evaluation of soil washing for the treatment of petroleum contaminated soils using a laboratory scale system." Proc. Petroleum Hydrocarbon Conf., National Water Well Asso- ciation and American Petroleum Institute, Houston, Tex.
Espositio, P., Hessling, J., Locke, B. B., Taylor, M., Szabo, M., Thumau, R., Rogers, C., Traver, R., and Bartha, E. (1989). "Results of treatment evaluations of a contaminated synthetic soil." J. Pollution Control Association, 39(3), 294- 304.
Falatko, D. M. (1990). "Effects of biologically produced surfactants on the mobility and biodegradation of petroleum hydrocarbons," MSc thesis, Virginia Polytechnic Institute and State University, Blacksburg, Va.
Kostecki, P. T., and Calabrese, E. J., eds. (1991). Hydrocarbon contaminated soil and groundwater. Lewis Publishers, Chelsea, Mich.
Nash, J. H. (1987). "Field studies of in situ soil washing." EPA Document No. EPA/ 600/2-87/110, Office of Solid Waste and Emergency Response, U.S. Envir. Pro- tection Agency, Washington, D.C.
Nash, J., and Traver, R. P. (1988). "Field application of pilot soil washing system."
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Nunno, T. J., Hyman, J. A., and Pheiffer, T. H. (1988). "Assessment of international technologies for Superfund applications." EPA Document No. EPA/540/2-88/O03. Office of Solid Waste and Emergency Response, U.S. Envir. Protection Agency, Washington, D.C.
Pheiffer, T. H. (1990). "EPA's assessment of European contaminated soil treatment techniques.'" Envir. Progress, 49, 582-587.
"Test methods for evaluating solid waste. Physical/chemical methods." (1986). Re- port No. S-846, Office of Solid Waste and Emergency Response, U.S. Envir. Protection Agency, Washington, D.C.
Vogdt, J. (1993). "'Bioremediation of petroleum contaminated soils,'" MSc thesis, Virginia Polytechnic Institute and State University, Blacksburg, Va.
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