Soil Washing and Biotreatment of Petroleum‐Contaminated Soils

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  • 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 adequat...

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