Determination of Oil Concentration and Size Distribution in Ship Ballast Waters Method and Representative Results F. E. Witmer' and Arye Gollan2 Hydronautics, Inc., Laurel, Md.
Straightforward techniques to determine the state of dispersion-i.e., quantity and droplet size distributionfor oily ballast waters are outlined. Oil concentrations were determined by using light transmittance through an ultrasonically emulsified, surfactant-stabilized sample, while oil droplet size distributions were directly measured using a microscopic photography cell. When these techniques were applied to actual deballasting operations, it was confirmed that oily ballast clarifiers for crude oil tankage must be able to handle oil fines that are as small as 1 0 in ~ diameter along with very high levels of oil contamination which may exceed 10% on occasion. The physical characterization of oil droplets in ballast water that originates from ship tankage is of significant value in the selection of oil-water separation devices. It is important to determine the oil concentration level and the oil droplet size distribution that must be handled by separation devices. For the meaningful development and laboratory evaluation of' such equipment, field conditions should be simulated. The need for equipment to scavenger oil from contaminated ballast waters has become increasingly urgent in the past several years, with a corresponding increase in the search for shipboard oil separators to handle oily ballast on a practical, cost effective basis. The oil content of ballast water has generally been determined by using extraction techniques which either evaporate the volatile ex tractant to determine the residual by weighing (ASTM, 1970) or by measuring the light transmittance of the extractant and comparing it to known standards (Norris and Bassett, 1967). The extraction step common to both procedures is rather tedious. and the precision of the evaporation technique depends on an appreciable quantity and proportion of heavy oils and residuum being present in the sample. Surprisingly no information concerning the oil droplet size distribution for ballast waters has been published. Shackleton and his coworkers (Shackleton et al., 1960) have studied the effect of actual pumping loops on oilwater mixtures and used a cumbersome droplet rise method to measure oil droplet distributions. Contacts with several U.S. oil companies through the U.S. Maritime Administration (Seelinger, 1969) revealed that none of the companies seem to have adequate data nor do they have an established method for such an evaluation. In evaluating the performance of impingement gravity separators, Lester estimated that at least half the oil is contained by droplets greater than 130p and concluded that nearly all the oil is contained by droplets 511 and larger (Lester. 1970). This paper is concerned with the measurement of oil concentration via light transmittance of emulsified ballast water and the measurement of oil droplet size distribution by direct photography. Membrane Processes Division, Office of Saline Water. U.S. Department ofthe Interior, Washington, D.C. 20240. 2 To whom correspondence should be addressed.
Measuring Procedures Determination of Oil Content. The concentration of crude and Bunker "C" oil (10-5000 ppm) in distilled water samples has been reproducibly measured on a turbidity meter. The instrument consisted of a light source, sample holder cell, an ultrasonic emulsifier, and a photocell. Transmitted light was used to make the measurement. Since output was sensitive to the oil type. calibration curves have been obtained with standards produced from Kuwait, and Venzuela crudes and Bunker "C." The curves are presented in Figures 1 and 2. The sample holder was wetted with a surfactant (5% Triton X-100) to prevent plating out on the walls, and the sample was ultrasonically emulsified (Branson Sonifier, 20KHg, 5.5 mils a t full power) in the sample holder until the transmittance of the sample reduced to a constant value (approximately 2 min). The sample holder (50-ml square sample bottles in this instance) was immersed in a cooling bath during emulsification to preclude heating up and the loss of volatiles. Under such conditions a stable emulsion is produced which does not plate out on the walls of the sample holder. The stability of the emulsion was demonstrated by the fact that prepared samples, which were allowed to settle for a month, had the same transmittance reading (within instrumentation variation) as was observed initially. In the case of field samples, a reference run was performed on oil-free ballast water to account for the natural coloration of the water. The apparent oil content of each sample, as determined by its transmittance and a calibration curve for the specific oil type in question, was cor-
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Figure 1. Turbidity calibration curve for crude oil
Volume 7, Number 10, October 1973
945
rected for the coloration of the carrier water by suhtracting the oil content equivalent associated with its coloration. Samples which contained large proportions of oil (>lo%) were allowed to settle; the oil that separated conveniently served a s a source of supply for preparing the individual calibration curves. Field samples were taken directly in the 50-ml sample bottles prewetted with a surfactant. Emulsification of the samples, prior to analysis, was performed in the lahoratory several days later. The standard sample bottles were used as sample cells and inserted directly into the turhidity meter. The deviation associated with the orientation and the variation of geometry between sample bottles were less than 3%, well within the intended accuracy of the technique (*lo%). Determination of Droplet Size Distribution. The technique developed is based upon microscopic photography and was selected because i t provides a direct and straightforward means of obtaining the desired data. The apparatus consists of a small transparent flow cell y4 in. wide by YS in. mounted on a microscope platform. Photographs are taken of the sample stream while it flows through the cell. The optical system consists of a direct light source (transmitted through the sample cell), microscope barrel and reflex camera. The microscope and light source have been mounted to a shock-insulated platform to facilitate field use. Exposure levels have been reproducibly determined by adapting a conventional spot lightmeter to the microscope barrel and using its reading as a n index to secure good contrast. It h a s been necessary to go to high shutter speeds, 1/1000 see, because of the large displacements associated with flow on a microscale. Fast film (Kodak Tri-X 400 ASA) has been used. In the actual use of this technique, a sufficient number of photographs were taken to develop a statistical basis for the data and the corresponding droplet size distribution. Figure 3 shows representative data: The distance between the fine gradings is 13p, the depth of field is approximately 2 W p , and the nominal oil concentration passing through the cell is 2000 ppm crude.
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Figure 4. Representative influent oil droplet Size distributions of crude oil for constant pressure drop (10 psi) across dispersing orifice
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Figure 2. Turbidity calibration curve for Bunker "C' 946
Environmental Science B Technology
Figure 5. Systems used in obtaining ballast discharge samples in field
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Characterization of Ballast Waters Using Proposed Procedures Laboratory. Ballast water dispersions were simulated in the laboratory by injecting a small stream of oil a t the upstream stem side of the valve seat in a specially modified angle valve which served as a variable orifice. The oil droplet size distribution (both crude and Bunker "C") was controlled by adjusting the pressure drop across the orifice opening. Typical droplet distribution data were measured in the microscopic flow cell and are presented in Figure 4. Field. Field characterization of ballast water was performed on two ships, the Western Sun a t Beaumont, Tex., and the ESSO Houston a t Corpus Christi, Tex. Figure 5 depicts the sampling system used in obtaining the samples. In the case of the Western Sun, samples were taken on the discharge and suction sides of the deballasting
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Figure 8. Fraction of crude oil concentration attributed to all droplets greater than given size (dispersion orifice distribution, Figure 5, serving as basis)
pump immediately a t the pump, while for the ESSO Houston, sampling was performed about 100 yards from the tanker near the point of discharge into a 50,000-bbl settling tank. Figure 6 traces the observed oil content as a function of time during the deballasting operation. Oil concentration levels were measured using the reference emulsificationturbidity determination as outlined above. Figure 7 gives an estimate of the droplet size distribution associated with the discharge of contaminated crude oil ballast through a representative tanker plumbing system. The relationship observed for ballast water oil concentrations vs. time (Figure 6) verified the same levels and trends previously reported by Schackleton et al. (1960) and the Permutit Co., (1966). That is to say that a t the onset of a ballast pumpdown, residuals in the piping system contributed to oil contamination, and as the oil water interface is approached during the dump, oil entrainment increases and rapidly reaches a high level. Our findings that oil fines in the 15-1OOp range can be expected to be prevalent in discharges contaminated with crude oil was confirmed by the field observations of workers from the British Petroleum Co. who investigated the use of impingement gravity-type separators (Lester 1970). It is emphasized that the droplet size distributions discussed were based on the probability of a given size range droplet t o appear in the population. If the fraction of the total oil concentration attributable to droplets below a given size is desired, one must weigh the numerical distribution by the cube of the droplet diameter. For example, a representative droplet distribution obtained for the laboratory runs, that which is shown in Figure 4, has been resolved to this basis in Figure 8. In this example, over 90% of the oil concentration is accountable in droplets whose diameter is greater than 2 5 p . During the deballasting of the Western Sun, microscopic photographs were also taken on the suction side of the pump (see Figure 5 ) . A systematic scan of these pictures revealed no dispersed oil fines. The oil contained in the Volume 7, Number IO, October 1973 947
suction stream was confined to very small slugs which sporadically blanked off the field of vision of the 3/8-in. diameter sight window in the photography cell. The results of the field sampling, while limited, indicated that the influent droplet size distribution used in the laboratory for the evaluation of oil scavenging devices was representative and slightly more severe than one would expect in practice-i.e., a greater fraction of the oil concentration is attributed to smaller size droplets in the laboratory system. It also appears that the oil concentrations (1000-2000 ppm) used in the lab were typical of practice. The laboratory system represented a conservative yet realistic basis of evaluation for candidate ballast water clarifiers. The field results also emphasized that a ballast water clarifier should possess the capability to handle oil fines, if it is to be located on-deck which, of course, will place it a t the discharge side of a pump. The separation process would be measurably facilitated if suction-side clarification could be accommodated. However, such an approach is not practical with existing vessels, the design and oper-
ation of which are generally not amenable to the introduction of additional equipment in the pump wells.
Literature Cited ASTM Procedure ~ 1 1 7 8 .“Determination of Chloroform Extractables,” 1970 Lester, T . E., BP North America Inc.. Drivate communication, 1970. Norris, R. O., Bassett, R. S., “Combustion of Crude Oil in Ships Boilers,” Annual Tanker Conf., Division of Transportation, Am. Petroleum Inst., Absecon, N.J., May 15-17, 1967. Permutit Co., “Research and Development for a Shipboard Oil and Water Separation System,” Report to the U.S. Department of Commerce, Maritime Administration, under Contract Number MA-2722, December 1966. Seelinger, J., U.S. Office of Research and Development, Maritime Administration, Department of Commerce, private communication, 1969. Shacklet‘on, L. R., Douglas, E., Walsh, T., “Pollution of the Sea by Oil,” Inst. Marine Engrgs Trans.. 72, 409-15 (1960).
Received for recieu Februan. 2, 1973. Accepted Jul? 19. 197;1. Work supported by the Maritime Administration, I’.S. Department of Commerce, under Contract N o . (2-0-35467.
NOTES
Solvent Extraction of Sulfur from Marine Sediment and Its Determination by Gas Chromatography Kenneth Y . Chen,’ Mohsen Moussavi, and Amancio Sycip Environmental Engineering Programs, University of Southern California, Los Angeles, Calif. 90007
Among a group of organic solvents studied, toluene and benzene exhibited a good recovery of elemental sulfur from marine sediment. A t a column temperature of 19o”C, the major peaks were produced by Sq,Sg, and SS; with the optimum carrier gas flow rate, the s8 peak predominated. The plot of peak area vs. sulfur injected was linear up to 3 ng in two gas chromatographs equipped with electron capture detectors. The detection limit was a t the picogram level, unmatched by other known methods. Neither the coefficient of variation nor mean values for sulfur were significantly different in comparisons between the gc method and a standard colorimetric method. The level of elemental sulfur in marine sediment is of great interest because sulfur is a reservoir: for the generation of hydrogen sulfide under anaerobic conditions. While the toxicity of hydrogen sulfide is generally known, the presence of elemental sulfur has also been mentioned as the probable intoxicant in fish kills (Vamos, 1964). In addition, sulfur may participate in various biogeochemical interactions important to the transport of pollutants in sediment.
To whom correspondence should be addressed 948
Environmental
Science & Technology
Despite the importance of sulfur to the chemistry of marine sediments, little research has been devoted to its quantification. The purpose of this paper is to examine the feasibility of elemental sulfur extraction and analytical determination by gas chromatography (gc). There are few known methods for the quantitative determination of elemental sulfur in sediment. In the past, determination of the amount of sulfur in soil or sediment was mainly carried out either through reduction of sulfur to sulfide with subsequent applications of spectrophotometric or volumetric titration methods for sulfide determination or oxidation of sulfur to sulfate, then titration with Ba+2 or S r + 2 potentiometrically or polarographically. A number of procedures are available for the measurement of elemental sulfur in a variety of materials (Furman, 1962, Karchmer, 1970) and in soil (Hart, 1961; The Sulfur Institute, 1968). These processes involve extraction and analysis, usually by means of titrimetry, turbidimetry, or colorimetry. The conversion of sulfur to sulfide using metallic copper is also widely used (ASTM, 1971). The copper sulfide is then treated with acid to produce hydrogen sulfide for determination using iodine titration. Kaplan et al. (1963) analyzed the sulfur content in the sediment of Southern California Coastal Waters using a mixture of benzene, methanol, and acetone as extraction solvent, with extensive subsequent treatment and purifi-
