Environ. Sci. Technol. 1990,24, 1069-1079
Xun, L.; Campbell, N. E. R.; Rudd, J. W. M. Can. J . Fish. Aquat. Sci. 1987, 44, 750. Parks, J. W.; Lutz, A.; Sutton, J. A. Can. J. Fish. Aquat. Sci. 1989, 46, 2184. Callister, S . M.; Winfrey, M. R. Water, Air, Soil Pollut. 1986, 29, 453. Bjomberg, A.; Hakanson, L.; Lundbergh, K. Environ. Pollut. 1988, 49, 53. Suns, K.; Jackson, M.; Rees, G. A.; Vickers, R. Ontario Technical Report LTS 79-3 & 80-1, Rexale, Ontario, 1978
Great Lakes Research, Madison WI, 1989; abstract. (54) Finance Report 1988. Western Lake Superior Sanitary District, Duluth, MN, 1989. (55) Mercury. Bureau of Mines Minerals Yearbook; 242377/80037; US.DepL of Interior, US.Govt. Printing Office: Washington, DC, 1988. (56) Mercury, Quarterly. Mineral Industry Surveys. U.S.Dept. of Interior, US.Bureau of Mines-Production and Distribution, Pittsburgh, PA, 1989. (57) Mercury Residue Levels in Minnesota Fish. Minnesota Pollution Control Agency. Data tape from Dan Helwig. St. Paul, MN, 1988. (58) Fish contaminant residues. Data summary. Wisconsin Department of Natural Resources, Madison WI, 1989.
and 1980. List of All Impacted Waterbodies. Minnesota Pollution Control Agency, Prog. Devel. Sec., Div. Water Quality, St. Paul, MN, 1989; p 6 fs. Surface Water Quality for Toxic Substances. Department of Natural Resources, Wisconsin Administrative Code, Chapter NR 105 Register No. 398, Feb 1989, Madison, WI, 1989; Sect. 51, 52. Mercury. National Emission Standards for Hazardous Air Pollutants; 40 CFR 61.52b. Glass, G. E.; Sorensen, J. A.; Schmidt,K. W.; Rapp, G. R., Jr. 32nd Conference of the International Association of
Received for review May 30,1989. Revised manuscript received September 15,1989. Accepted March 10,1990. This work was sponsored by the Minnesota Sea Grant College Program supported by the NOAA Officeof Sea Grant, Department of Commerce, Grant No. NA86AA-D-SG112, the Legislative Commission on Minnesota Resources under MPCA Contract No. 831479, and the US.EPA Great Lakes National Program Office. We thank the US.EPA for salary (G.E.G.),space, and equipment EPA support for this project conducted primarily out of the U.S. Environmental Research Laboratory-Duluth.
Sharpe, M. A,; deFreitas, A. S. W.; McKinnon, A. E. Environ. Biol. Fish. 1977, 2, 177.
Bisogni, J. J.; Lawrence, A. W. J. Water Pollut. Control Fed. 1975, 47, 135.
Ripening in Depth Filtration: Effect of Particle Size on Removal and Head Loss Jeannle L. Darby" and Desmond F. Lawlert
Department of Civil Engineering, University of Texas, Austin, Texas 787 12 Suspensions were filtered in laboratory-scale deep-bed experiments; detailed measurement of particle size distribution at different depths and times and continuous monitoring of pressure at different depths during ripening were performed. Ripening is the improvement in removal efficiency that occurs as previously retained particles begin to serve as additional collectors for suspended particles. Monodisperse, bimodal, and trimodal suspensions of spherical latex particles were filtered under well-controlled physical and chemical conditions. Preferential removal of certain particle sizes was evident throughout ripening, even within the narrow distributions of the monodisperse suspensions. Ripening of the bed significantly affected the relative removal efficiency of particles of different size and surface characteristics;in some cases a decrease in observed initial removal efficiencies was noted. Floc formation and breakoff within the bed changed the particle size distribution of the suspension with time and depth and affected subsequent removal and head loss. The development of head loss in the bed was strongly correlated with the surface area of captured particles.
Introduction In potable water supplies and in wastewaters, many pollutants are particles or are associated with particulate matter. Consequently, the removal of particles is a principal objective of water and wastewater treatment. The need to remove such particles increases as waters of poorer quality are considered for potable water supplies and higher quality is demanded for both drinking water and wastewater discharges. Depth filtration, i.e., filtration in
* Department of Civil Engineering, 211 Walker Hall, University of California, Davis, CA 95616. 0013-936X/90/0924-1069$02.50/0
deep porous beds, is the treatment process most often used for final separation of particles from water. Depth filtration removes particles by attaching them to the media or to previously retained particles. Because of the latter mode of attachment, the removal efficiency of filters improves over time after backwashing; this improvement is known as ripening. Ripening, a consequence of previously retained particles serving as additional collectors for other suspended particles, begins with the relatively low removal efficiency characteristic of clean filters and (when successful) results in a high-quality effluent. Several excellent reviews of depth filtration concepts are available (1-5). Until approximately 20 years ago, understanding of depth filtration was based solely on observations of macroscopic phenomena such as turbidity and head loss. Recently, progress has been made at the microscopic level, particularly in predicting clean bed removal efficiency in terms of fundamental characteristics of the suspension and the bed. Both transport of a suspended particle to a potential collector site and attachment of that particle to the collector are required for effective filtration. Physical mechanisms are often considered to dominate the former step, whereas chemical conditions are frequently considered most influential in the latter step; however, the distinction is not clear. Several investigators have presented detailed theoretical analyses and, in some cases, results of well-controlled experiments of initial removal efficiency in clean beds (6-16). Past work indicated qualitative agreement concerning the role of Brownian motion, sedimentation, and interception in leading to suspended particle transport to the proximity of a single collector. The dominant transport mechanism is related principally to the size of the suspended particle. Brownian motion is predicted to dominate submicron particles, while
0 1990 American Chemlcal Society
Environ. Scl. Technol., Vol. 24,No. 7, 1990 1068
Table I. Characteristics of Suspended Particles" 0.652* 2.02c diameter, fim 5.9c 21.lC 6.3 17.5 coefficient of variation, % 0.736 0.67 1.05 1.027 1.05 1.05 density, g/cm3 styrene vinyltoluene styrene-divinylbenzene monomers charge density: loF3mequiv/g 1.6-7.3 0.8-1.2 not available colloidal silica surface groups surfactant, sulfate emulsion polymerization suspension polymerization preparation a Information provided by manufacturer. * Seragen Diagnostics, Inc., Indianapolis, IN. Duke Scientific Corp., Palo Alto, CA. Estimated from manufacturer information (22).
effects of sedimentation and interception are predicted to dominate larger particles. A minimum in removal efficiency is predicted for particles with diameters of some intermediate size, typically 1-2 pm for conventional filters used in drinking water treatment. Although extensive theoretical and experimental work has been performed on initial removal in clean beds, results of filtration experiments performed with well-controlled physical and chemical conditions throughout ripening are lacking. This lack has hindered the development and testing of theoretical models for ripening. All of the theoretical and the majority of the well-controlled experimental work on ripening and subsequent head loss that is available (16-20) is limited to monodisperse suspensions and provides contradictory evidence concerning the role of particle size in filter performance during ripening. The scarcity of experimental work performed with heterodisperse suspensions has hindered the theoretical investigation of the effect of a particle of one size on a particle of another size and has limited the models to unrealistic conditions. Clearly particle size is an important parameter affecting filtration performance. Yet reliance on turbidity (or suspended solids) measurements has made it impossible to discern the effects of particle size on ripening and pressure development in filtration. Vigneswaran and Aim (21) presented results from filter ripening experiments using suspensions with a bimodal distribution (13 and 26 pm diameter pollen grains). Their results indicated that as the ratio of the concentrations of larger to smaller particles increased, the removal efficiency of the small particles increased. The extent of particle size and concentration combinations reported was limited; the diameter ratio was held constant at 2:l and the number ratio was varied between 1:l and 1:16 (larger to smaller). Measurements of head loss were not reported. The research described herein was directed toward a more fundamental understanding of depth filtration by investigating, experimentally, the interactions between suspended particles and previously retained particles and between suspended particles and the media in a filter bed during ripening. The specific objectives of this research were to determine the change in particle size distribution and the development of head loss in a filter bed as it ripens under well-controlled physical and chemical conditions. Experiments with monodisperse, bimodal, and trimodal distributions of suspended particles were used to investigate systematically the effect of a particle of one size on particles of the same and different sizes. The experimental work was designed such that the results would be optimally useful for development and testing of predictive models for particle removal and head loss during ripening. Experimental Design and Methods
Characterization of Suspension. Four sizes of spherical latex particles were used in this research. Characteristics of the particles are presented in Table I. The particles are referred to herein by their nominal diameters, 0.6,2,6, or 21 pm. The two smaller sized particles 1070
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were prepared by emulsion polymerization, resulting in very uniform size distributions as reflected in their coefficients of variation. The two larger sized particles were prepared by suspension polymerization, resulting in a broader size distribution. The two different polymerization processes produce fundamentally different surface chemistry characteristics. In emulsion polymerization, the resulting negative surface charge and stability of the particles are due to surfactant and initiator sulfate groups. In suspension polymerization, a colloidal suspending agent (e.g., colloidal silica) replaces the surfactant and provides stability to the particles (22). It is not feasible to extend the emulsion polymerization process to particles larger than approximately 3 pm. Solution pH and ionic strength were used to produce relatively favorable conditions for particle removal. A pH of 2.0 and a calcium nitrate concentration of 0.03 M were used throughout the experiments. These conditions were selected on the basis of electrophoresis measurements (Model Mark 11, Rank Brothers, Cambridge, England) and preliminary filtration experiments (23) as well as previous results by others (9, 24). Mean electrophoretic mobility measurements of the particles under the above stated conditions resulted in values of +1.3, -0.4, -0.8, and -1.1 (pm/s)/(V/cm) for the 0.6-, 2-, 6-, and 21-pm particles, respectively. The difference in mobility between the two smaller particle sizes created with emulsion polymerization might be due to the different monomers used in each or the surfactant type (proprietary) and percent used by the respective manufacturers. As proposed by others (24,25), the possible hydration of the colloidal silica coating on the two larger sized poly(styrene-divinylbenzene) (PS-DVB) particles might give rise to an additional repulsive force, resulting in greater stability of the particles than that expected based solely on being at or near the zero point of charge. Apparatus. A laboratory-scale filtration system was designed to allow both detailed measurement of the particle size distribution of a suspension at different depths and times and continuous monitoring of pressure at different depths. A schematic of the system is presented in Figure 1. Wall effects in the filter column were minimized by ensuring that the ratio of column diameter (3.81 cm) to media diameter was greater than 50 (26). Sampling and pressure ports were located at depths of 1.0, 5.5, and 9.0 cm from the top of the media and 1 cm above the media. This arrangement allowed measurement of performance over three discrete segments of the filter (sections 1-2,2-3, and 3-4 with lengths of 1.0,4.5, and 3.5 cm, respectively). The filter media (total depth of 14 cm) consisted of solid glass spheres (500-600 pm in diameter) manufactured from soda lime silica glass. The influent section of the column was designed to distribute the influent evenly across the top of the packed bed and to provide a small chamber for measurement of the influent particle size distribution and pressure. Prior to each experiment, an influent supply reservoir of particle-free water was chemically conditioned [pH 2,
T Transducers and
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Water Reservoir Flgure 1. Schematic of filtration system.
0.03 M Ca(N0,)J. Influent was supplied to the filter by pumping simultaneously from two reservoirs, one containing the particles in distilled/deionized water and the other containing the chemically conditioned water. The influent concentration was monitored frequently throughout each experiment; the average coefficient of variation was 6%. The superficial velocity in all experiments was 0.17 cm/s with a coefficient of variation less than 3 9%. Details of the filtration system are found elsewhere (23). Measurement of Head Loss. Pressure measurements were made via transducers directly linked to a microcomputer. Calibration of the transducers prior to each experiment allowed conversion of the signal response from the transducers (volts) to a relative pressure (centimeters of water). To test the transducers, preliminary experiments with clean media and particle-free water were performed at various flow rates. Pressure measurements made with the transducers were quite similar to those made with manometers and with theoretical predictions based on the Carman-Kozeny model for head loss (23). Particle Size Distribution Measurements. Each sample port was fitted with a pressure seal connector through which a stainless steel needle (19 gauge) was inserted horizontally into the column interior. The deflected point of the needle was inserted with its opening in the center of the bed facing the oncoming water. The flow rate through each needle was controlled with a stopcock. Representative sampling of the particle size distribution and concentration of the suspension in the bed was critical. In theory, it is desirable to have the velocity into the entrance of the needle be the same as that in the surrounding fluid, Le., isokinetic sampling. However, under the operating conditions of the filter, isokinetic sampling was impractical to obtain; the needle opening required would allow the filter media to enter the needle, and covering the opening with mesh was not desirable because of the increased boundary effects on the streamlines. Experiments were conducted which indicated that isokinetic sampling was not necessary with the experimental conditions used in this research (23). Measurements of particle size distributions were obtained frequently throughout each experiment using a system manufactured by Coulter Electron& (Hialeah, FL). The effectiveness of the Coulter counter for such work is described in detail elsewhere (27). Results and Discussion Analysis of a Sample Monodisperse Experiment. (A) Particle Volume Captured. The results of one monodisperse experiment (experiment 11,12.33 mg/L of 2-pm particles) are presented and discussed in detail to provide foundation for the presentation of the results from
120
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Figure 3. Removal efficiency: effect of previously captured 2-pm particles (experiment 11, 12.33 mg/L).
all other experiments. The particle volume concentration at four locations in the filter throughout the experiment is presented in Figure 2. Each point on the figure represents one analysis on the Coulter counter. Both removal and ripening were evident at all three depths monitored in the filter. A horizontal line below the influent concentration would indicate removal but not ripening; the presence of the downward slope indicates that removal efficiency improved with time. This improvement with time provides evidence of the removal of suspended particles by previously retained particles (“ripening”). These results are consistent with expectations and indicate that the experiment was performed well. An alternative analysis of the same results is presented in Figure 3. In the figure, both axes have been normalized in order to compare the results from three sections with different lengths. The fractional removal per unit length within a particular section is plotted against the total volume captured (and accumulated) in a particular section of the bed per unit length. Particle volume captured provides a measure of the previously retained particles (normalized by length of section) that are potentially available to act as additional collectors for incoming particles. This format allows investigation of whether a discernible and consistent relationship between removal efficiency and volume of previously captured particles occurred. In particular, the format quantifies the different performance of the top, middle, and bottom sections of the bed. Several observations can be made from this figure. First, in terms of absolute removal (indicated by values on the abscissa), the greatest particle volume was captured per centimeter in the top section of the bed with lesser amounts captured per centimeter in each subsequent section. As removal occurred, less particle volume was Environ. Sci. Technol., Vol. 24, No. 7, 1990 1071
available to be captured in subsequent sections. Second, ripening in all three sections is evident by the increase in removal efficiency with particle capture. Third, both the top and the bottom sections of the bed illustrated significantly better ripening and removal than the middle section of the bed. To a first approximation, all particles in a true homogeneous, monodisperse suspension have the same probability (according to theory) of being captured in a homogeneous filter bed; however, the latex suspensions used in these experiments actually consisted of a (narrow) distribution of sizes and charge characteristics. In addition, capture of particles changes the geometry of the bed. Therefore, it is reasonable to hypothesize that changes in the suspension distribution can be brought about by selective removal of particular particles (because of size or surface characteristics) or by floc formation and breakoff. Although removal efficiency was less consistent in the top centimeter (section 1-2) than in the other sections, removal improved significantly and surpassed the efficiency of the middle section. A likely explanation for the better removal efficiency in the top section of the bed relative to the middle section is that particles that had the highest probability of being captured (Le., because of size or surface characteristics) were captured in the first centimeter of the bed and subsequently were available to serve as collectors for other suspended particles (accelerating ripening). The experiments were designed to provide such a high probability of capture. Particles that needed more opportunities for contact before capture entered the lower sections of the bed. After a certain volume of particles had been captured, the bottom section of the bed was clearly a more efficient filter than either of the other sections. This unexpected result might be caused by the change in the particle size distribution of the suspension with time and depth. Formation and later breakoff of flocs in the upper sections of the bed would result in larger, and hence more easily captured, particles entering the lower sections of the bed. Detailed evidence of such floc breakoff is presented below. The middle section of the bed, although illustrating obvious ripening, was ultimately the least efficient filter. The particle influent to the middle section had already been preselected by the top section for less likelihood of capture and had not yet had the opportunity to develop larger flocs, which aided removal efficiency in the bottom section. One cause of the scatter in the results for the top section is that the relatively short length of this section (1 cm) resulted in smaller absolute differences between the influent and effluent to the section; extremely accurate measurements of small differences in large numbers are difficult. Another possible cause of the scatter for section 1-2 is that different flow regimes likely prevailed in the top centimeter of the bed and the internal sections. Particle removal and retention in section 1-2 might have been adversely influenced by the transition in flow occurring between the influent measurement chamber (port 1) and the packed bed. (B) Development of Head Loss. The increase in hydraulic gradient as filtration proceeded is presented in Figure 4. The hydraulic gradient is a normalized measure of head loss in the filter bed. The gradients across various sections of the bed were similar in the beginning of the filtration experiment when the bed was clean. As particles were collected in the bed, the hydraulic gradient increased. The increase occurred more rapidly in the upper sections of the bed (particularly the top centimeter), suggesting that more particles per centimeter were captured in this section. A smaller increase occurred in each subsequent section. 1072 Environ. Scl. Technol., Vol. 24,
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Flgure 5. Head loss development as a function of previously captured 2 - ~ mparticles (experiment 11, 12.33 mglL).
The hydraulic gradient across the entire bed depth (section 1-4) fails to distinguish between the wide differences in head loss that developed in the top and bottom sections of the bed. The results of the pressure measurements in the bed (converted into hydraulic gradient) and the particle size distribution measurements of the filtrate (converted into particle surface area captured) are integrated in Figure 5. Section 1-2 is omitted because of the difficulty in obtaining sufficiently accurate measurements of the small differences in pressure and particle count over this short section. Only the additional hydraulic gradient (above that in a clean bed) resulting from accumulation of particles in the bed is utilized. Both axes are normalized by length of section and thus different length sections can be compared. One approach taken to predicting head loss development during filtration is to modify the Carman-Kozeny equation for head loss in clean porous media to include the surface area of captured particles (18). As indicated by the results in Figure 5, the relationship between hydraulic gradient and particle surface area captured in different sections of the bed is almost identical. Such consistency suggests that incremental head loss is caused by the surface area of the captured particles. (C) Change of Particle Size Distribution with Depth and Time. Detailed and extensive particle size data were produced from the particle counter during each experiment; the particle size distribution of a sample is sorted into 100 channels. A useful way of examining this information is to group the data into several size "windows"; each window represents a certain range of particle sizes. Instead of attempting to discern a trend in 100 small intervals, a smaller number of larger windows are examined.
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Figure 6. Selective removal of small particles and floc breakoff (experiment 11, 12.33 mg/L, 2 pm).
In experiment 11, the particle count data were sorted into three windows. Window 1, representing singlets of the 2-pm particles, had boundaries of 1.89 and 2.32 pm. Window 2, representing doublets, had boundaries of 2.38 and 2.80 pm. Window 3, representing triplets and greater sized flocs, had boundaries of 2.85 and 4.23 pm. The number-averageparticle diameter in each of these windows was calculated for each sample at each of four depths in the filter. In the following graphs, only the results from the influent and bottom of the filter are presented for the sake of clarity on the figure; however, the results found in the middle two ports were consistent with the trends noted. Changes with depth and time in the number-average diameter for window 1 are shown in Figure 6A. The numerical differences in this parameter are necessarily small due to the effects of averaging and the small range of sizes in a monodisperse distribution; however, the consistently higher value at a depth of 9 cm in comparison to the influent indicates preferential removal of the smaller particles within this small window. This behavior might reflect the dominance of Brownian motion over sedimentation and interception. Since this window is sized to incorporate mainly singlets, this phenomenon is unlikely to be related to floc development. Results for window 3, presented in Figure 6B, indicate that the number-average diameter of flocs at the bottom of the filter was consistently and strikingly greater than in the influent, and that the difference increased as the experiment proceeded. The influent number-average diameter remained relatively constant throughout the experiment, meaning that some phenomenon in the bed itself caused the trends. Preferential removal of smaller particles might have occurred, as in window 1; this preference accelerated with ripening as particles began to serve as the collector sites. Smaller particles might be easier for the previously retained particles to “hold onto”. A more likely interpretation is the formation and breakoff of flocs in the bed,this phenomenon would be likely to increase with time as is shown in this figure. Particles were captured (probably as singlets), but with time, flocs of captured particles developed, broke off, and appeared in the lower ports. To investiate the second interpretation, the btalnumber concentration in different windows of the influent and at 1 cm into the bed over the course of the experiment is presented in Figure 7. In window 1 (singlets, Figure 7A),
Figure 7. Formation and breakoff of flocs in top centimeter of bed (experiment 11, 12.33 mg/L, 2 pm).
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Figure 8. Effect of ripening on 0.6-pm particles: selective removal of larger particles (experiment 16, 12.6 mg/L, 9-cm depth).
the number concentration of singlets at a depth of 1 cm was less than in the influent, as expected. In window 2 (doublets, not shown), the differences were not as great, but the same trend was present. However, in window 3 (triplets and larger flocs, Figure 7B), the number concentration of flocs was greater at a depth of 1 cm than in the influent. The sensitivity of these results produced through particle size measurements provides strong evidence of formation and breakoff of flocs in the bed. The results of the other monodisperse experiments, analyzed in a similar manner, provide additional evidence of the role of particle size and chemistry on ripening and head loss. These results are discussed next. Other Monodisperse Experiments. (A) Effects of Particle Size. The volume distribution of the effluent at three times in one filtration experiment with 0.6-pm particles (experiment 16, 12.6 mg/L) is shown in Figure 8. These data are typical of the sensitivity of Coulter counter measurements. The area under the curve between any two diameter values represents the volumetric particle concentration in that range. The results portray a “snapshot” view of ripening at different times during the experiment. The peak of the volume distribution curve shifts from a log particle diameter of -0.188 to -0.215 (particle diameter shifts from 0.65 to 0.61 pm) during the first 160 min of filtration, indicating preferential removal of larger particles. Such preferential removal of larger particles was unexpected since the submicron particles were predicted to be influenced principally by Brownian motion. One interpretation of the preference (which accelerated after the bed was ripened) might be that, after Environ. Sci. Technol., Vol. 24, No. 7, 1990 1073
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Figure a. Selective removal of large particles and floc breakoff (experiment 9, 16.2 mg/L, 6 pm).
a critical accumulation of particles occurred, the effect of hydrodynamics became relatively more important than Brownian motion in the selection of which size particles were captured. Interception might have begun to play a larger role, favoring the removal of larger particles. Additional evidence of preferential removal of larger particles in the experiment was seen by analyzing the change in the number-average diameter with time and depth for windows 1 (singlets) and 2 (doublets and flocs),an analysis similar to that shown in Figure 6. The results (not shown) indicate that samples drawn from the lower sections had a smaller number-average diameter than those from the upper sections; this phenomenon increased with time. Experiments with the 6-pm particles also yielded clear evidence of selective removal of certain sizes and floc breakoff. This phenomenon is illustrated by the change in the number-average diameter during experiment 9 (16.2 mg/L), as presented in Figure 9. Preferential removal of larger particles in the singlet window is indicated by the results shown in Figure 9A. Such results are expected when interception dominates transport efficiency. Results from window 2 (mainly doublets and triplets), presented in Figure 9B, indicate that the number-average diameter increased with depth and that the difference between the top and bottom ports increased with time. These results clearly indicate the formation and breakoff of flocs in the bed, which then appeared in the lower sample ports. Similar analyses of results from experiments using 21-pm particles (not shown) showed that, in both the singlet and floc windows, the number-average diameters of the partilces in the lower sections of the bed were greater, indicating preferential removal of smaller particles (singlet window) or floc breakoff (floc window). Preferential removal of the smaller singlets in the relatively broad distribution of the 21-pm particles was unexpected; transport to the proximity of the collector surface was predicted to be dominated by sedimentation and interception, both of which favor collection of larger particles. However, even if larger particles are transported preferentially to the collector surface, they might not be held as well as smaller particles. A comparison of ripening and subsequent head loss, which occurred in monodisperse experiments with the 0.6and the 2-pm particles, is presented in Figure 10. Ripening (Figure 10A) was essentially identical in the two experiments; head loss (Figure lOB), on the other hand, developed significantly more rapidly with the smaller particles. The relationship between head loss development 1074
Environ. Sci. Technol., Vol. 24, No. 7, 1990
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Figure 10. Ripening and head loss: behavior of 0.6- and 2-pm par-
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Figure 11. Head loss development: effect of size of captured partlde (experiment 11, 12.33 mg/L, 2 pm; experiment 16, 12.60 mg/L, 0.6 pm).
and size of particle captured is shown from a different perspective in Figure 11. For identical particle volume captured (Figure l l A ) , the smaller particles created significantly more head loss than the larger particles. However, when the results are normalized for particle surface area captured (Figure 11B), the results of the two experiments are virtually indistinguishable. All monodisperse experiments showed this consistent relationship between development of head loss and particle surface area captured. (B) Role of Particle-Particle versus Particle-Collector Interactions. Rapid ripening in the experiments with the 0.6- and 2-pm particles indicated favorable particle-particle interactions. The larger particles (6 and 21 pm), with different surface chemistry characteristics, behaved otherwise. The volume fraction remaining in the filtrate in experiment 14 with 6-pm particles and experiment 17 with 21-pm particles is presented in Figure 12.
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Flgure 12. Unfavorable partlcle-particle interactions (PS-DVB particles, section 1-4, 9 cm).
Several points about this figure are worth noting. First, clear removal occurred for both particle sizes, as expected. Second, a delay in the onset of ripening with the 6-pm particles is evident. The duration of the delay was not significantly affected by the concentration of the influent; in two other experiments utilizing higher concentrations (10.1 and 16.2 mg/L) of 6-pm particles, clear ripening did not begin until approximately 60 min into the experiments. Given the wide concentration difference between these experiments, it is difficult to discern the cause of this delay. If the phenomenon had been affected by influent concentration, an argument could be made that a certain accumulation in the bed had to occur before ripening developed. If ripening had never occurred, unfavorable particle-particle interactions could be blamed. However, ripening clearly began at about the same time in all the experiments. Some resistance to particle-particle attachment is evident. The electrophoresis results from the present research, as well as work by Tobiason and O’Melia (24), indicate that calcium interacts with the particle surface. Mobility measurements made in the present work also indicate that an equilibrium particle mobility in the solution chemistry is not obtained for approximately 30 min. I t is conceivable that particle-particle interactions were increasingly favored as previously retained particles were exposed to the solution chemistry for a certain time period. Consistent with this interpretation, particle-collector interactions did not show the same resistance because chemically conditioned water was passed through the filter for 2 h prior to an experiment (and thus the glass collectors achieved equilibrium with the calcium ions). In addition, although not explicitly stated in the product literature, the prepared PS-DVB particle suspensions were likely to have contained some added anionic surfactant, which would provide additional stability; this surfactant might wash off with time. However, the manufacturer indicated that the surfactant is not easily removed and that the effect of dilution on particle charge is likely to be small (22). Tobiason and O’Melia (24)have provided a thorough discussion of how changes in (latex particle) surface interaction forces are brought about by specific interaction of ions with particle and collector surfaces and by changes in ionic strength. Third, although removal of the 21-pm particles was obvious, ripening was never observed. In fact, the fraction remaining appeared to have increased slightly with time. Other experiments with 21-pm particles yielded similar results. The slight decrease in removal with time indicates that particleparticle attachments were less favorable than particle to original collector interactions. The electrophoretic mobility measurements indicate that the 21-pm particles had a higher surface charge than the other particles in the same solution chemistry. Tobiason and 0’-
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Flgure 13. Ripening in bimodal experiments (section 1-4, 9 cm).
Melia (24) also found a similar resistance to particleparticle attachment in filtration when using particles of a composition identical with the 6- and 21-pm particles used in this research. Results from Bimodal Experiments. (A) Ripening and Head Loss. The improvement in filtrate quality with time in three bimodal experiments (6-pm particles with each of the other sizes) is shown in Figure 13. Each particle size is treated separately in determining the fraction remaining in the filtrate; this discrimination among different size particles is an advantage of particle-counting techniques. As discussed previously, the chemical composition of the two smaller particles (0.6 and 2 pm) are different from that of the two larger particles (6 and 21 pm). Both the 2- and 6-pm particles in experiment 13 illustrated clear ripening with time (Figure 13A) although the 6-pm particles again exhibited the delay in the onset of ripening. Initially the 6-pm particles were captured slightly better than the 2-pm particles but the smaller particles exhibited ripening quickly and were more efficiently removed than the larger particles for the remainder of the experiment. A slightly greater negative charge and, possibly, repulsive hydration forces are associated with the PS-DVB particles. Although the larger particles are predicted to be transported more efficiently to the collector surface than the smaller particles, the chemical composition of the 2-pm particles favored particle-particle attachment. In the bimodal distribution of 6- and 0.6-pm particles (experiment 20, Figure 13B), ripening of both particle sizes occurred, but again was more pronounced for the small particles. The electrophoresis measurements indicate that the 0.6-pm particles behaved as positively charged particles in the solution chemistry used in this research. Even with the greater ripening of the small particles, the large particles were removed more efficiently throughout the experiment, consistent with predictions of transport efficiency. In the bimodal distribution of 6- and 21-pm particles (experiment 18, Figure 13C), no evidence of ripening of either particle size occurred, although removal was obvious. Environ. Scl. Technol., Vol. 24, No. 7, 1990
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TIME (minutes) Flgrrre 14. Effects of 6- and 2-pm particles on each other (section 1-4, 9 cm).
The larger particles were removed less efficiently with time, as in the monodisperse results. Removal efficiencies were higher for the 21-pm particles than the 6-pm particles, consistent with transport expectations. The similarity of the different sections of the bed in terms of the relationship between particle surface area captured and head loss was analyzed, similarly to that performed for the monodisperse experiments. The results are generally as consistent as those of the monodisperse experiments. More head loss per surface area captured occurred in experiment 13 (6- and 2-pm particles) than in experiment 20 (6- and 0.6-pm particles), perhaps indicating the role of Brownian motion in depositing the smallest particles in shadow regions of the collectors or the previously captured 6-pm particles where head loss would not be increased. (B) Effect of a Particle of One Size and Chemical Composition on a Particle of Another Size. The comparison of the behavior of particles in the bimodal experiments with that of the same particles in the monodisperse experiments allows investigation of the effect of one particle on another. Interactions of 6- and 2-pm particles are considered first. The behavior of 2-pm particles in the bimodal experiment is compared with that in two relevant monodisperse experiments in Figure 14A. Results of experiments 13 and 11 indicate that ripening of the 2-pm particles occurred more rapidly in the monodisperse experiment even though the total influent concentration was smaller. In other words, the presence of 6-pm particles did not increase the removal of the 2-pm particles as much as a similar volume of 2-wm particles did. The effect of surface chemistry might be confounding the effects of particle size in these results. If size differences alone were present, the results would be clear evidence that a particle is more likely to be captured by a particle of similar rather than larger size. A second explanation of these results is that, given identical volumes of previously captured particles, future capture of suspended particles will be helped more by many small particles on the surface of a collector than by a few large particles. A comparison 1076 Environ. Sci. Technol., Vol. 24, No. 7, 1990
of the fraction of 2-pm particles remaining in the monodisperse and bimodal experiments as a function of total particle volume captured (not shown) supports this explanation. A third interpretation is that particle-particle interactions between the 2-pm particles are favored over those between the 2- and 6-pm particles due to the differences in stability between the two particle sizes. A comparison of the behavior observed in experiments 13 and 12 (similar influent concentrations of 2-pm particles), also shown in Figure 14A, indicates that ripening of the 2-pm particles progressed slightly better in the monodisperse experiment. Although the slightly higher concentration of 2-pm particles in experiment 12 might have been a factor in these results, it is likely that the presence of the 6-pm particles hindered the removal of the smaller particles. This effect is discussed further below in conjunction with results from a trimodal experiment. The effect of the 2-pm particles on the 6-rm particles is illustrated in Figure 14B. The lag in ripening, already noted in the monodisperse results, recurred in the bimodal experiment and lasted for approximately the same time. A comparison of the bimodal results with experiment 14 indicates that removal of the 6-pm particles was better in the monodisperse experiments; i.e., the presence of the 2-pm particles apparently hindered the capture of the 6-pm particles. The 2-pm particles, removed preferentially over the 6-pm particles with ripening, served as less efficient collectors of the larger particles than did either the original media or other 6-pm particles. A similar comparison of the effects of the 6- and 0.6-pm particles on each other was performed but is not shown. A comparison of experiments 19 and 20 (identical influent concentrations of 0.6-pm particles) indicated that ripening of the 0.6-pm particles proceeded slightly faster in the bimodal experiment than in the monodisperse experiment. The presence of 6-pm particles helped the removal of the 0.6-pm particles but did not increase removal of the 2-pm particles, as discussed above. This difference might be explained by the apparent negative charge corresponding to both the 2- and 6-pm particles versus the apparent positive charge corresponding to the 0.6-pm particles. A comparison of experiments 16 and 20 (similar total influent volume concentrations) indicated that ripening of the 0.6-pm particles was more far pronounced in the monodisperse experiment than in the bimodal experiment. Therefore, although removal of the 0.6-pm particles was improved slightly by the presence of the 6-pm particles, the improvement was not as great as it would have been in the presence of an equivalent volume of 0.6-pm particles; a similar phenomenon was also noted above in the bimodal experiment with 2- and 6-pm particles. These results might indicate that the opposing charges of the 0.6- and 6-pm particles (compared to the previously discussed 2and 6-pm particles) favored the 6-pm particles serving as collectors of the smallest particles. The charge attraction, however, did not totally negate the size differences, and thus, the smallest particles were more efficiently removed by many small particles on the surface of a collector than by a few large particles. To examine the effect of the 0.6-pm particles on the larger particles, the removal efficiency for the 6-pm particles in experiment 20 (bimodal) was compared with those in experiments 9 and 14 (monodisperse). The results (not shown) indicate that the lag in the onset of ripening, evident in the monodisperse experiments, was missing from the bimodal experiment; the onset of ripening was quicker for the 6-pm particles in the presence of the 0.6-pm particles. However, the ultimate improvement in filtrate
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TIME (minutes) Figure 15. Effects of 6- and 21-pm particles on each other (section 1-4, 9 cm).
quality was not as good as with an equivalent (experiment 9) or lower (experiment 14) concentration of 6-pm particles. This behavior might be explained as follows: the 0.6-pm particles served as better collectors of the 6-pm particles than did the original collectors (and hence improved initial ripening) but served as less efficient collectors than did other 6-pm particles (and hence worse ultimate removal efficiency). The effects of the two larger particles on one another are shown in Figure 15. Similar concentrations of 21-pm particles were used in experiments 17 and 18; the results are compared in Figure 15A. Significant scatter existed in the data for the 21-pm particles due to both the broad size distribution of these particles and the relatively small number concentrations. The 21-pm particles in the bimodal experiment were removed slightly better than in the monodisperse experiment; however, the scatter in the results for the 21-pm particles argues against attributing any significance to this small difference. Similar analysis of the effect of the 21-km particles on the 6-pm particles is illustrated in Figure 15B. No ripening of the 6-pm particles occurred in the bimodal experiment, whereas definite ripening occurred in the monodisperse experiment. Although the influent concentration of 6-pm particles was greater in the monodisperse experiment, the total influent concentration was greater in the bimodal experiment. The 21-pm particles appear to have had a significant detrimental effect on removal of the 6-pm particles. An interpretation of these results is that preferential initial collection of the 21-pm particles occurred, resulting in coverage of the original collectors with sites less favorable for attachment of the 6-pm particles. Interactions between 6- and 21-pm particles were apparently not as favorable as those between 6-pm particles and the original collectors or between two 6-pm particles. Results of Trimodal Experiment. A suspension with a trimodal distribution (11.57 mg/L of 6-pm, 3.98 mg/L of 2-pm, and 5.72 mg-L of 21-pm particles) was used in experiment 15. The fraction of each size remaining in the effluent with time is indicated by the open symbols in
Figure 16. The results indicate that, although the 21-pm particles were removed the best and the 2-pm particles the worst initially, ripening of the smaller particles quickly reversed this trend. The 6-pm particles again showed a delay in ripening (similar to results of monodisperse and bimodal experiments), followed by a very limited improvement in filtrate quality. The 21-pm particles did not ripen at all and, in fact, were removed less efficiently with time, again similar to results of the monodisperse and bimodal experiments. The filtrate quality improved with increasing depth for all three particles sizes (not shown). To examine the effect of the 21-pm particles on capture of other particles, the results of experiment 15 are compared with those in a similar bimodal experiment (experiment 13,11.52 mg/L of 6-pm and 3.11 mg/L of 2-pm particles). The results from experiment 13 are indicated by the filled-in symbols in Figure 16. Comparable results from the relevant monodisperse eperiments are not shown in this figure for the sake of clarity, but are compared to the bimodal results in Figure 14. The effects of the 21-pm particles on 2-pm particles are examined first. Recall that removal of 2-pm particles in the related monodisperse experiment (experiment 12,3.53 mg/L of 2-mm particles) was slightly better than in the bimodal experiment. The results shown in Figure 16 indicate that, although initial capture of the 2-pm particles was better in the trimodal experiment than in the bimodal experiment, this trend reversed in the latter part of the experiments. After ripening, removal was best in the monodisperse experiment and worst in the trimodal experiment. This trend indicates that the 2-pm particles were captured better by other 2-pm particles than by the larger PS-DVB particles and that the larger particles might actually hinder removal of the smaller poly(vinylto1uene) particles. The high rate of capture of the 21-pm particles initially improved removal of the 2-pm particles, perhaps due to the initial increase in surface area sites available; long-term removal was less stable than when the 2-pm particles were captured by other 2-pm particles. The effects of the 21-pm particles on removal of the 6-pm particles are examined next. Recall that the removal of the 6-pm particles observed in monodisperse experiments was hindered by the addition of either 2- or 21-pm particles in bimodal experiments (see Figures 14 and 15). The addition of 21-pm particles to the bimodal distribution used in experiment 13 also significantly hindered capture of the 6-pm particles. Again, as noted in all the experiments, it is clear that the 6-pm particles are removed best by other 6-pm particles. Ramifications Although the experiments in this research were highly Environ. Sci. Technol., Vol. 24, No. 7, 1990
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idealized and designed with theoretical applications in mind, some practical insight is possible. For instance, the observed phenomenon of floc formation, breakoff, and recapture suggests that filter designs incorporating deeper beds, higher velocities, and larger media (even in the bottom sections of the bed) might be reasonable. The observed relationship between particle size and pressure development points out an advantage of controlling the particle size of the filter influent through flocculation without sedimentation; Edzwald et al. (28) made the same suggestion based on their work with humic matter. Monitoring of head loss as a function of bed depth, although not typical in practice, would provide operators with useful information about ripening of the filter and the degree of penetration of solids into the bed. The phenomenon of ripening suggests that, within a relatively short time after backwash, the role of the original filter media is superceded by the role of the particles that are captured first; this initial capture is strongly influenced by particle size. The observed deterioration in effluent quality when large particles with surfaces not favorable for particle-particle attachment (represented by the 21-hm PS-DVB particles) are present suggests that increased focus on the surface chemistry of the larger influent particles might improve bed performance.
Conclusions Ripening is an important aspect of filtration that has been largely ignored in previous theoretical studies. This research was designed to investigate physical aspects of ripening, particularly effects of particle size, in well-controlled experiments with suspensions of latex spheres. The experimental results indicate that the particle size distribution of a suspension changes with both time and depth in a filter bed. Changes occur through both preferential removal of particular particle sizes and floc formation and subsequent breakoff. Retained particles also control the development of head logs. Ripening, for the most part, was found to be strongly correlated with the volume of previously captured particles but was strongly affected by the relative degree of particle-particle interactions versus particle-collector interactions. Head loss was directly related to the particle surface area captured. The major observations are stated below. 1. Removal increased with depth in all the experiments, as expected; ripening with time was observed with all particles except the 21-pm (PS-DVB) particles. Particle-particle interactions for the 21-pm particles were not as favorable as particleoriginal collector interactions. The 6-pm (PS-DVB) particles showed an initial resistance to ripening. 2. Preferential removal of certain particle sizes was evident, even within the narrow distributions of the monodisperse suspensions. Preferential removal of the smaller 2- and larger 6-pm singlets occurred with depth, as expected. However, contrary to expectations, the smaller singlets of the 21-pm size range were removed preferentially to the larger singlets, the larger singlets of the 0.6-pm size range were removed preferentially to the smaller, and in the bimodal experiments, the 2-pm particles were removed preferentially to the 6-pm particles. These results suggest that, in a ripening filter bed, transport is affected by deposit geometry and attachment is affected by suspended particle size and chemistry in ways not accounted for with clean bed models. 3. Substantial evidence of floc formation and breakoff was found. Individual particles were (apparently) captured and then helped to capture other particles; a group of 1078
Environ. Sci. Technol., Vol. 24, No. 7, 1990
captured particles might then break off as a floc. This formation and breakoff of flocs significantly changed the particle size distribution of the suspension with time and depth and affected subsequent removal and head loss. Different sections of the bed were observed to behave differently; lower sections of the bed performed relatively more efficiently than the upper sections, probably as a result of the larger flocs in the influent to the lower sections. 4. Continuous monitoring of pressure in the bed at different depths indicated that the highest increase in hydraulic gradient occurred in the top section of the bed, with progressively smaller increases in each subsequent section. These results are consistent with expectations and experimental results that, in absolute terms, more particles per centimeter are captured in the upper sections of the bed. Head loss was very strongly correlated with particle surface area captured, regardless of the size of the particles. 5. Results from bimodal experiments indicate that the effect of one particle on another is influenced by both the relative size differences and the surface chemistry of each particle. The roles of particle size and particle chemistry were inseparable in these results since latex particles of widely different sizes must be manufactured with different chemical composition. The effect of chemical composition manifests itself through surface charge and specific ion interactions. The effect of size differences is attributable to two factors: (1) a particle of one size apparently prefers to be captured by a particle of a similar size; and (2) for identical particle volume captured, the smaller particles are more numerous and thus provide a greater number of additional collector sites.
Acknowledgments We thank S. Koolik, J. Stewart, F. Hulsey, and C. Paul for their assistance in apparatus setup and R. Cushing for his critical assistance in the experimental studies, figure preparation, and manuscript review.
Literature Cited (1) OMelia, C.R.; Stumm, W. J.-Am. Water Works Assoc. 1967,59,1393-1412. (2) Spielman, L.A. Annu. Rev. Fluid Mech. 1977,9,297-318. (3) Tien, C.;Payatakes, A. C. AZChE J . 1979,25, 737-759. (4) Rajagopalan, R.; Tien, C. In Filtration and Separation; Wakeman, R. J., Ed.; Elsevier: New York, 1979;Vol. 1,pp 179-269. ( 5 ) O’Melia, C. R. J . Enuiron. Eng. 1985,I l l , 874-890. (6) Cookson, J. T.Enuiron. Sci. Technol. 1970,4, 128-134. (7) Yao, K.M.; Habibian, M. T.; O’Melia, C. R. Enuiron. Sci. Technol. 1971,5,1105-1112. (8) Spielman, L.A.; FitzPatrick, J. A. J. Colloid Interface Sci. 1973,42,607-623. (9) FitzPatrick, J. A.; Spielman, L. A. J . Colloid Interface Sci. 1973,43,350-369. (IO) Prieve, D.C.; Ruckenstein, E. AIChE J. 1974,20,1178-1187. (11) Spielman, L.A.; Friedlander, S. K. J . Colloid Interface Sci. 1974,46,22-31. (12) Payatakes, A. C.;Tien, C.; Turian, R. J. AZChE J . 1974, 20,889-905. (13) Ghosh, M. M.; Jordan, T. A.; Porter, R. L. J . Enuiron. Eng. Diu. (Am. SOC.Ciu. Eng.) 1975,101, 71-86. (14) Rajagopalan, R.; Tien, C. AZChE J. 1976, 22,523-533. (15) Rajagopalan, R.; Tien, C. Can. J . Chem. Eng. 1977,55, 246-264. (16) Payatakes, A. C.;Rajagopalan, R.; Tien, C. Can. J. Chem. Eng. 1974,52,722-731. (17) Habibian, M. T.; O’Melia, C. R. J . Enuiron. Eng. Diu. (Am. SOC.Ciu. Eng.) 1975,101, 567-583. (18) O’Melia, C. R.; Ali, W. Prog. Water Technol. 1978, I O , 167-182.
Environ. Sci. Technol. 1990, 2 4 , 1079-1085
Tien, C.; Turian, R. M.; Pendse, H. AIChE J . 1979, 25, 385-395.
Chiang, H. W.; Tien, C. AZChE J. 1985, 31, 1349-1371. Vigneswaran, S.; Aim, R. B. AZChE J . 1985,31,321-324. Bangs,L. B.Uniform Latex Particles; Seragen Diagnostics: Indianapolis, IN, 1984. Darby, J. L. Ph.D. Dissertation, University of TexasAustin, Austin, TX, 1988. Tobiason, J. E.;OMelia, C. R. J.-Am. Water Works Assoc. 1988,80, 54-64. Allen, L. H.; MatijeviE, E. J . Colloid Interface Sei. 1969, 31, 287-295. Rose, H. E. Some Aspects of Fluid Flow; Edward Arnold: London, 1951; pp 136-162.
(27) Lawler, D. F. In Influence of Coagulation on the Selection, Operation, and Performance of Water Treatment Facilities; AWWA No. 20014; AWWA Seminar Proceedings; Kansas City, MO, 1987; pp 19-30. (28) Edzwald, J. K.; Becker, W. C.; Tambini, S. J. J. Environ. Eng. 1987, 113, 167-185.
Received for review June 13,1989. Accepted February 8,1990. Financial support for this research was provided by the US. Geological Survey (Water Resources Research Grants Program) under Grant 14-08-0001 -1502, the American Association of University Women through an American Fellowship to J.L.D., and the National Science Foundation through a Presidential Young Investigator Award to D.F.L.
Hydrocarbon Distributions around a Shallow Water Multiwell Platform James M. Brooks,*Bt Mahlon C. Kennlcutt, Terry L. Wade,+ Alan D. Hart,' Guy J. Denoux,t and Thomas J. McDonaldt
Geochemical and Environmental Research Group, Department of Oceanography, Texas A&M University, Ten South Graham Road, College Station, Texas 77840, and Continental Sheif Associates, Inc., 759 Parkway Street, Jupiter, Florida 33477 Polynuclear aromatic hydrocarbon (PAH) concentrations in nearshore coastal sediments offshore of Matagorda, TX, average 29 f 28 (SD) ppb compared to 96 f 112 ppb for the sediments in adjacent coastal estuaries and bays. PAHs were analyzed in bottom sediments collected in the vicinity of a multiwell platform in -25 m of water, where six wells had been drilled between May 1982 and November 1985. The most elevated PAH concentrations were restricted to within 25 m of the platform discharge point. Mean PAH concentrations for two samplings at 10 and 25 m from the platform were 494 f 251 and 757 f 1820 ppb, respectively. The contaminated platform sites (525 m distant) were dominated by two-ring aromatics while the estuarine/bay and noncontaminated coastal sites were dominated by four- and five-ring aromatic hydrocarbons. Phenanthrene/anthracene ratios suggest a petroleum source for the hydrocarbons at platform stations and pyrolytic sources for the bay/estuarine hydrocarbons. Introduction
Studies of drilling mud and cutting discharges in the marine environment have shown that chemical contamination &e., hydrocarbons, barium, other trace metals) from offshore platforms is generally restricted to the immediate vicinity of the platform (1-5). Most of these studies have been conducted around exploratory wells or in continental shelf environments, where water depths and current regimes are such that drilling mud effluents are unlikely to reach the sediments in significant amount. In contrast to this, development drilling from development platforms discharge less material per well but more total material is generated over the lifetime of the platform. The fate and effects of drilling muds have been reviewed and summarized by several authors (6-8). The study presented here focuses on polynuclear aromatic hydrocarbon (PAH) contamination of benthic sediments generated from multiple development wells in a shallow (
