Environ. Sci. Technol. 2004, 38, 2449-2456
Landfarm Performance under Arid Conditions. 1. Conceptual Framework R A M Z I F . H E J A Z I † A N D T A H I R H U S A I N * ,‡ Environmental Protection Department, Saudi Aramco, Box 5487, Dhahran, Saudi Arabia 31311, and Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5
The primary disposal method for oily sludge in the Kingdom of Saudi Arabia, which is a major oil-exporting country in the world, is landfarming. It is an attractive method of oily sludge disposal in hot arid climatic conditions. Although landfarming technology was introduced to Saudi Arabia in 1982, no scientific studies have been conducted within the Kingdom to support this decision. The results presented in this paper are based on a comprehensive field experiment conducted under Saudi Arabian environmental conditions. Details of experimental setup and conceptual framework of degradation process based on field observations are presented in this paper. The paper also addresses kinetics of oily sludge degradation in landfarm cells under natural and enhanced conditions in the presence of water, nutrients, and tilling. The 12-month field study showed that weathering (evaporation) and not biodegradation is the overall dominant degradation mechanism occurring in landfarms in the study area. The results of this study showed that up to 76% of the oil and grease (O&G) in the sludge has been lost from soil as a result of weathering. However, the results of this study also indicated the primary mechanism for the loss of C17 and C18 alkanes as compared to branched alkanes was due to biodegradation.
Introduction Saudi Arabia produces approximately 8 million barrels of crude oil every day. With seven refineries, 22 bulk plants, several terminals, and several operating tank farms, oily sludge is one of the largest categories of industrial wastes generated in the Kingdom. In a survey conducted by the Saudi Arabian National Oil Company (Saudi Aramco) in 1994, it was reported that the industry generates approximately 30 000 m3 of oily sludge every year (1). The major source of this waste is from tank bottoms. Landfarming technology was introduced to Saudi Arabia in 1982. The decision to use this technology was based on information obtained through a review of technical documents (2, 3). No scientific studies and/or research were conducted to support this decision. The arid environment in Saudi Arabia made landfarming an attractive method. As of 2002, seven landfarms exist in Saudi Arabia with more under construction (4). Landfarming is a managed technology that involves the controlled application of a waste on the soil surface and the * Corresponding author telephone: (709)737 8781; fax: (709)737 4042; e-mail:
[email protected]. † Saudi Aramco. ‡ Memorial University of Newfoundland. 10.1021/es026043s CCC: $27.50 Published on Web 03/16/2004
2004 American Chemical Society
incorporation of the waste into the upper soil zone (5). This technology has been practiced by refineries since 1954 as a disposal method for their oily sludges. During the 1970s when environmental concerns associated with uncontrolled disposal became apparent, landfarming gained popularity, and it became the most common method used by major oil companies in the United States to dispose of their generated oily sludge (6, 7). In 1984, this method lost its popularity when the U.S. EPA issued the Land Disposal Restriction (LDR) as part of the Hazardous and Solid Waste Amendments (HSWA) to the Resource Conservation and Recovery Act (RCRA). In this paper, details of experimental setup and conceptual framework of the degradation process based on field observations are presented. An in-depth investigation on the mechanism of degradation of hydrocarbon constituents under natural and enhanced conditions (i.e., water, nutrients, and tilling) is presented where the individual and combined effect of these conditions were studied (8, 9).
Materials and Methods The experimental site was selected inside the Juaymah Oily Waste Landfarm, operational since 1994, in the Eastern Province of Saudi Arabia. This site is located 20 km northwest of the Ras Tanura Refinery, the largest refinery in Saudi Arabia with a capacity above 320 000 barrels/day. The site is a lowprofile sand dune field over a widespread marine sabkhah. Sediment deposits in the sabkhah include sand and clay. The top 1.2 m of the surface is mainly sand. Localized shallow groundwater has slightly brackish characteristics with total dissolved solids (TDS) ranging from 3500 to 6000 mg/L, making it unsuitable for human consumption. Three monitoring wells (BH-1, BH-2, and BH-3) exist inside the Juaymah landfarm. The shallow water table in these wells is approximately 7 m below ground level. These wells are sampled every 3 months for water characterization. Meteorological data collected near the site between 1964 and 1984 showed that the average annual rainfall in this area is approximately 3.4 in. (85.6 mm) and that the average annual evaporation is approximately 86 in. (2190 mm), which clearly indicates that this area can be classified as an arid region. The objectives of this study include investigating the kinetics of oily sludge degradation in landfarm cells under natural and enhanced conditions (i.e., water, nutrients, and tilling) (8). Since tilling is the most applicable enhancement method in landfarm applications, four cells were assigned to study the individual and all possible combinations of tilling with other enhancements. The remaining two cells were assigned to study the effects of natural attenuation and loading rate. The design of the landfarm cells was based on the specification listed by CONCAWE (3) and the American Petroleum Institute (5). The size of each cell was 2 m × 2 m. The sludge used in this study was fresh sludge obtained from the bottom of a large tank that contained Arab Medium crude, which is the main crude generated in Saudi Arabia. The schematic of the cells is presented in Figure 1, and the functions and experimental work carried out in each of the landfarm cells are described below. Cell LF1 was selected as the control landfarm cell to evaluate the natural attenuation of the organics in sludge. The sludge was applied, and periodical monitoring was conducted without any action to enhance the degradation of the sludge. Tilling was applied in cell LF2 once a week to a depth of 8 in. to provide aeration to the microorganisms inside the zone of incorporation in order to investigate the VOL. 38, NO. 8, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of experimental layout.
TABLE 1. Analytical Protocol
parameter oil & grease metals (As, Ba, Cd, Cr, Cu, Pb, Mn, Hg, Se, Ag, V, and Zn) hydrocarbon analysis n-alkanes, pristanes, and phytanes nutrients (N, P, Na, K, Ca, and Mg) moisture content (%) pH
effect of tilling on the degradation process. Between October and February, tilling and water were added to the landfarm cells once every 2 weeks, but from March to September, tilling and water were added once every week. The main reason for increasing the operating frequency was to keep the moisture content above 6 wt %; however, it was noted that when the landfarm cells were watered, the water evaporated almost immediately. The quantity of water added to each cell was approximately 55 L each time. Cell LF3 was used to investigate the effect of tilling and moisture content on the degradation process. Besides regular tilling, nutrients were also added in cell LF4 to investigate the effect of tilling and nutrients without the addition of water. Cell LF5 was used to investigate the effect of nutrients, tilling, and water on the degradation process. Cell LF6 is similar to LF5, except the oil content was doubled. The loading rate was 300 g of sludge/kg of soil. The goal here was to investigate the effect of hydrocarbon loading on the rate of degradation. The loading rate used for cells 1-5 was 150 g of sludge/kg of soil, which was based on the highest loading rate reported in the literature (5). The N:P:K ratio of the nutrients “Phostrogen” was 84:5.2:5.5. One kilogram of Phostrogen was added to LF4, LF5, and LF6 cells. The C:N ratio used in this study was 87:1. This is in line with the recommended ratio (10, 11). Analytical Methods. On the basis of a comprehensive literature review, a detailed list of parameters to be analyzed was prepared that included microbiological parameters, total hydrocarbon, oil and grease, metals, and nutrients (4, 1115). These parameters are listed in Table 1. This table also lists methods and equipment required for the analysis. A proper quality assurance and quality control protocol was developed for sample collection, use of preservatives, types of containers, and holding time of the samples. This study required extensive laboratory support to perform the required chemical, physical, and biological analyses. The Saudi Aramco laboratories in Dhahran, Saudi Arabia, performed all the analyses. The laboratories used for this work are equipped with the required advanced analytical instrumentations with well-documented quality assurance and quality control (QA/ QC) protocols, using standard procedures on the use of replicates, spikes, blanks, and equipment calibration (16). A sampling protocol was developed to coordinate all of the sampling activities under this research. The sampling activities were divided into background monitoring at initial stage and periodical monitoring on monthly basis. The background monitoring provided baseline data on soil and 2450
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background sludge
background soil
yes yes yes
yes yes yes
yes yes yes
yes yes yes
periodical monitoring in cells yes yes yes yes yes yes yes
sludge. The periodical monitoring helped in collecting information on the activities in the soil zones inside the cells once every month. Samples were collected from the surface to the depth of 6 in. To ensure that the collected samples from the cells were representative, composite samples were prepared by mixing samples from three different locations. The sampling program commenced on September 26, 2000, and was completed on September 4, 2001. A description of the methods used is also listed below. Oil and Grease. EPA 9017A gravimetric method protocol was used for the analysis of O&G, where 10 g of the sludge was Soxhlet-extracted with freon 113 for 4 h. The solvent was removed from the extracts using a Zymark Turbo Vap concentrator, and the O&G were measured gravimetrically. The O&G were determined as follow: 10 g of wet sludge was weighed into a 150-mL beaker, and 10 g of anhydrous sodium sulfate was added to the beaker. The mixture was mixed thoroughly, allowed to stand for 10 min, and then added to the paper extraction thimble. The beaker was rinsed with freon and added to the thimble. It was then extracted in a Soxhlet apparatus for 4 h using 200 mL of freon. Using filter paper (Whatmann No. 2), the extract was filtered into a preweighed Zymark tube, and the flask and filter paper were rinsed with solvent. The solvent was removed by placing the tube in Turbo Vap concentrator for about 90 min. After the Zymark tube was taken out of the concentrator, it was allowed to come to room temperature (about 30 min) and was weighed. The tube was returned to the concentrator for 10 min, and the same steps were repeated. The final weight was taken, and the O&G was calculated as follows:
oil and grease (mg/kg) ) weight of oil (mg) × 1000 (g/kg) weight of wet solid (g) Metals. Trace metal analysis in sludge was determined according to U.S. EPA method 6020 using the 6100 ICP-MS system. The sludge samples were acid digested according to U.S. EPA method 3050B (acid digestion of sediments, sludges, and soils). About 1 g (dry weight) of sample is digested with repeated additions of nitric acid and hydrogen peroxide. The following trace metals were determined in the sludge: Ca, Mg, P, K, As, Ba, Cd, Cr, Cu, Pb, Mn, Se, Ag, Ni, V, and Zn. Hydrocarbon Analysis. Gas chromatograms were obtained using an Agilent 6890 gas chromatograph with a 30 m × 0.53 mm × 0.88 mm HP-1 column, flow control at 3.2 mL/min He, oven programming from 35 to 315 °C at 3 °C/
min, and flame ionization detection. Samples were dissolved in methylene chloride and auto-injected using an injection volume of 0.2 µL, an injector temperature of 300 °C, and a split ratio of 100:1. Analyses were carried out over the course of 3 days with samples arranged in random sequence. n-Alkanes, Pristanes, and Phytanes. Four compounds were used to assess the relative degree of biodegradation: two straight chain alkanes (n-C17 and n-C18) that can be easily biodegraded and two multi-branched cyclic isoprenoids (pristane and phytane) that are relatively more resistant to biodegradation than their normal alkanes counterparts. The onset of the destruction of the isoprenoids (i.e., pristane and phytane) indicates a moderate level of biodegradation, and their complete removal indicates heavy biodegradation. Evaluation of the biodegradation beyond heavy biodegradation requires analysis of other biomarkers referred to as hopanes and steranes. These are detected using GC-MS analysis. The hopanes and steranes are cyclic alkanes known as naphthenes, and they are one of the most resistant hydrocarbons to biodegradation. For each cell, four samples were collected over a period of 1 yr (October 2000, February 2001, May 2001, and September 2001) and were analyzed using GC-FID. The intensity of the GC peaks depends on the concentration of the sample injected into the GC column. Since the concentration of the sample injected into the GC column varies, it is difficult to compare the n-alkanes directly from the GC traces without having a suitable calibration standard. Since no such standards were available at Saudi Aramco labs, the specific concentrations of the carbon compounds could not be determined in this study. As a result, it was decided to determine the degree of biodegradation using the commonly used ratios of n-C17 to pristane (Pr) and n-C18 to phytane (Ph) (16). The chromatographic peak area counts of the two straight chain alkanes (n-C17 and n-C18) and the two multibranched acyclic isoprenoid (pristane and phytane) compounds for each sample along with the computed n-C17/Pr and n-C18/Ph ratios are listed in Table 4. The oily water material was extracted from the soil samples using a pressure flow extraction apparatus. The organic solvent was prepared by mixing methanol, acetone, and chloroform (MAC) in ratios of 15:15:70, respectively. The soluble organic material recovered from the extraction procedure was then submitted for deasphaltening to remove the asphaltene fraction. Excess n-pentane is added to the sample to precipitate asphaltene, which is insoluble in n-pentane. The maltene (asphaltene-free fraction) was then separated into the saturate, aromatic, and resin fractions by HPLC. All fractions were then evaporated to remove the solvent and then weighed to determine the weight percentage of each saturate, aromatic, resin, and asphaltene (SARA) fraction Background Analysis of Soil and Sludge. Baseline data on soil and sludge were collected prior to the degradation study. To assess the degradation process in the soil zones, samples were collected to a depth of 6 in. on a monthly basis for a period of 12 months and composited to analyze for physical, chemical, and microbiological parameters as listed Table 2. Baseline data showed no sign of O&G, with low soil moisture content (0.6%) and high pH (9.6). The general aerobic bacteria (GAB) count was low (9.3E+03) with low TKN (
