Effect of mixing on the biological degradation of toluene under aerobic conditions.

Author:Ghaly, A.E.
 
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INTRODUCTION

Managing contaminated soils with petroleum hydrocarbons (PHCs) is a common problem. These organic compounds reach soil through spills, leaks from underground tanks and from production, storage and disposal facilities (1). Traditionally, the approach to remediating contaminated soil has been containment or excavation and transport to a landfill (2-5). In most cases, this approach simply does not resolve the problem and the contaminants continue to be present on the site. Soil consists of four phases (Fig. 1) gas phase (15-35 %) water phase (15-35 %), inorganic solids phase (38-45 %) and organic solids phase (5-12 %). PHCs could be distributed among the four phases in relation to their volatility, solubility and ion exchange capacity (6). When released in large quantities at the soil surface, PHCs will penetrate the soil surface and saturate the pores in the soil. The movement of these contaminants through the subsurface layer of the soil is largely governed by the processes of advection, dispersion, sorption and transformation (7). The movement of contaminants within the soil is also affected by the soil characteristics at the site, the concentration of the contaminant and the contaminant characteristics. Eventually, these contaminants will reach surface and ground water causing environmental and health problems. The short term human and environmental health effects of PHCs include acute toxicity to aquatic organisms and skin irritation and itchy eyes at low concentrations to humans. At high concentrations, damage can be done to the liver, lungs, kidneys and nervous system leading to cancer and immunological, reproductive, fetotoxic and genotoxic effects (6,8-9).

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Currently, the objective of remediation is to reduce the volumes of contaminated materials as well as the concentration of the contaminants in these materials[10]. The Atlantic Provinces have established a set of petroleum hydrocarbon (PHC) guidelines outlining their maximum allowable concentrations in soil (Table 1). Contaminated soils should be remediated based on these guidelines. Riser-Roberts (11) reported that new technologies are constantly being developed to treat contaminated soil with various pollutants. Rushton et al. (12) evaluated the current remediation methods for oil contaminated soils. These methods include: physical (soil washing and landfilling), chemical, thermal (incineration, thermal desorption and radio frequency heating) and biological (landfarming, biopiling, composting, bioventing and liquid delivery systems). The study concluded that in order to achieve a complete petroleum hydrocarbon removal, a biological remedial process and a bioreactor should be used.

Table 1: Tier I risk based screening level for soil [12] Compound of Concern in Soil (mg/kg) Receptor Groundwater Soil Ethyl Use Type Benzene Toluene Benzene Xylenes Residential Potable Coarse 0.03 0.38 0.08 11 Fine 0.01 0.08 0.02 2.3 Non-Potable Coarse 0.16 14 58 17 Fine 1.5 120 430 160 Commercial Potable Coarse 0.03 0.38 0.08 11 Fine 0.01 0.08 0.02 2.3 Non-Potable Coarse 1.8 160 430 200 Fine 11 680 430 650 Compound of Concern in Soil (mg/kg) Receptor Modified TPH Receptor Groundwater Soil Use Type Gas Diesel/#2 #6 oil Residential Potable Coarse 39 140 690 Fine 140 220 970 Non-Potable Coarse 39 140 690 Fine 330 4400 8300 Commercial Potable Coarse 450 7400 10000 Fine 520 840 4700 Non-Potable Coarse 450 7400 10000 Fine 10000 7700 10000 Toluene is commonly used as an additive in gasoline and as a solvent in paints, cleaners, adhesives, inks and coatings (9). Of the primary constituents of gasoline (benzene, toluene, ethylbenzene and xylene), toluene is the secondmost mobile in water systems (13). Toluene is a colorless liquid with a sweet odor at room temperature. Its physical-chemical properties are listed in Table 2. Toluene primarily causes central nervous system disorders. In the short term, it can cause fatigue, nausea, weakness and confusion. Long term exposure to toluene can result in spasms, tremors, memory and/or coordination impairment, as well as liver and kidney damage (9). Martinell (14) suggested the use of biological treatment as a tool for the treatment of soil contaminated with toluene.

Table 2: Physical-chemical properties of toluene (11,19) Parameters Value Fluid Density (g/[cm.sup.3]) 0.867 Water Solubility (g/L at 25[degrees]C) 0.530 Vapor Pressure (mm Hg) 28.400 SurfaceTension (dyne/cm at 20[degrees]C) 28.520 Dynamic Viscosity (at 20[degrees]C) 0.631 Molecular weight (g/mol) 92.000 Molecular formula [C.sub.7][H.sub.8] Molecular structure [CH.sub.3] In spite of the widespread establishment of biological remediation methods for various hydrocarbons, there is still the need for better understanding of the process details in order to translate the concept from the laboratory scale to the field scale. The complexity of the soil system (chemical/physical parameters as well as the bioavailability of the contaminants and microbial nutrients present in the soil) is of particular importance (15). Several authors studied the effect of operating parameters on the degradation of toluene. Davis and Madsen (16) reported that the degradation of toluene in soil depends on the moisture content, microbial activity, bioavailability of nutrients and the initial concentration of toluene. Lee et al (17) found that the transfer rate of oxygen is the most important limitation for toluene degradation under aerobic conditions.

The aim of this study was to evaluate the effectiveness of in-vessel bioremediation in reducing the concentration of toluene in contaminated soil under continuous and intermittent mixing conditions.

MATERIALS AND METHODS

Experimental apparatus: The experimental setup shown in Fig. 2 consists of a bioreactor, a mixing unit, an air supply and a temperature measurement and recording unit. The bioreactor was constructed of 6.4 mm thick stainless steel. The sides of the bioreactor measured 340 mm x 280 mm with a radius of 150 mm at the lower end. The lid (340 mm x 800 mm) had 4 hinges on one side and could be clamped down on the opposite side by 4 locking clamps. There was a rubber gasket around the opening to provide a good seal and prevent gas leakages. The bioreactor was insulated with a 25.4 mm thick Styrofoam layer to minimize heat losses. There were three holes at the bottom of the bioreactor used for aeration. The lid had three 60 mm holes which were used as sampling ports and were covered during the operation with rubber stoppers. Inside the reactor, a 6.4 mm diameter stainless steel shaft was mounted on two bearings. There were five stainless steel collars on the shaft in which five 101.6 mm long bolts were mounted to provide mixing. The mixing shaft was rotated by a permanent magnetvariable speed (0-250 rpm), 3/4 ph electric motor (Model No. 2X846, D.C., Dayton Electric manufacturing Company, Chicago, Illinois), which is coupled directly to a gearbox (Model No. 4Z295B, Dayton Electric Manufacturing Company, Chicago, Illinois) of a 30:1 gear reduction ratio. The speed of the motor was controlled by a speed controller (Model No. 60648, Dayton Electric Manufacturing Company, Chicago, Illinois).

[FIGURE 2 OMITTED]

Air was supplied to the bioreactor by a 3/4 hp compressor (Model No. M01 FC75-7.5, Sanborn, Markham, Ontario) with an airflow regulated at 2.5 L/min. The air was passed through a 4.38 L plexiglas canister (35 cm in height X 15 cm in diameter) filled with hydroscopic silica gel (Type 3-8 mesh-ACS Grade, Fisher Scientific, Fair Lawn, New Jersey) which removed the moisture from air. The dry air was then passed through a flow meter (Model No. N103-5G, Cole Parmer, Chicago, Illinois) and then to a manifold which was connected to the holes at the bottom of the bioreactor by 6.4 mm diameter tubing. The condensate from the saturated exhaust gas was collected in a 0.63 L plexiglass water trap. The exhaust gas was then dried in another 4.38 L plexiglas canister (35 cm in height x 15 cm in diameter filled with hydroscopic silica gel (Type 3-8 mesh-ACS Grade, Fisher Scientific, Fair Lawn, New Jersey). Two rubber septums were located on the air inlet and exhaust outlet lines for air/gas sampling.

Temperatures were measured using 10 thermocouples (Model No. LM 35C2, National Semiconductor, Chicago, Illinois). The locations of the thermocouples are shown in Fig. 2. The thermocouples T1-T3 were used to measure the temperature at the center of the reactor. The thermocouples T4-T6 were located on the front wall of the bioreactor while thermoucouples T7-T9 were located on the back wall of the bioreactor. The thermocouple T10 was used to measure the ambient air temperature. All thermocouples were linked to a data logger (Model No. 1000, Omega Tempscan Engineering, Stamford, Connecticut) which was connected to a microcomputer.

The temperature sensors were calibrated using ice and boiling water bathes. The thermocouples including the fittings were immersed into the ice bath and hooked up to the data acquisition system individually. The thermocouple readings were corrected to read zero [degrees]C (offset). They were then immersed into the boiling water bath to correct the upper limit. Before each run, temperature sensors were tested for 24 hours in an empty bioreactor and the temperature readings were always within [+ or -]0.5 [degrees]C of the reactor temperature (measured with thermometer), which is...

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