Electrostatic Enhancement of Fabric Filter Performance Joseph D. McCaln," Wallace G. Klstler, Duane H. Pontlus, and Wallace B. Smith
Southern Research Institute, Birmingham, Alabama 35255-5305
w An experimental study was performed to investigate possible advantages of precharging fly ash before collecting it in fabric filter systems. The tests were performed on a 2500 ft3/min pilot plant at the Electric Power Research Institute Arapahoe Test Facility in Denver, CO. Efficiencies comparable to those of conventional baghouses were measured at substantially higher air/cloth ratios and lower pressure drops when the electrostatic charger was used. Collection efficiencies and pressure drops were measured at air/cloth ratios from 2 to 10 acfm/ft2. Bags of woven glass fiber, felted Nomex, and felted Teflon were tested. Introduction Fabric filters are very effective for the removal of suspended particulate material from gas streams. Collection efficiencies above 99.9% are commonly achieved in welldesigned baghouse systems. The technology is quite versatile, limited principally by the availability of fabrics that can withstand the sometimes hot or corrosive conditions in the gas and by the pressure drop developed across the filter as a result of dust cake buildup on the fabric. The use of fabric filters for collection of fly ash from coal-fired electric utility oilers is comparatively new. Until recently electrostatic precipitators have been used almost exclusively in these applications. The emergence of baghouses in this field may be explained at least partly by the development of more strict pollution control requirements and by the increased use of low-sulfur ,coal for power generation. The fly ash that results from the combustion of low-sulfur coal may, in some circumstances, have an electrical resistivity higher than can be handled efficiently by electrostatic precipitators of conventional design. Baghouse performance, on the other hand, is not intrinsically limited by the resistivity of the ash, although electrostatic effects have been observed to play a part in fabric filtration processes. Electrostatic effects are present to some degree in practically all types of particulate control devices. Even when there is no deliberate charging, the mechanisms by which aerosols are formed and transported tend to produce some distribution of electrical charge on the particles. Recent studies indicate that fabric filter performance can be improved by systematic use of electrostatic effects. Because of the high collection efficiency of a conventional fabric filter, there is little incentive to pursue electrostatic augmentation if the only effect is improved particle capture. But a growing body of evidence indicates that the pressure drop across fiber filters can be substan0013-936X/84/0918-0635$01.50/0
tially reduced under certain controlled electrical conditions. Theoretical background has been developed to describe the effects of electrical charge on particle collection by fibers (1-3) but quantitative explanations of the observed reductions in pressure drop across thick dust cakes are lacking. Two distinctly different approaches have been used for electrical augmentation of fabric filters. One is to charge the particles before admitting them to the filter, and the other is to develop a static electric field in the filter material. The former is accomplished by passage of the aerosol through a corona charger upstream of the filter. The latter method requires an array of electric conductors distributed over the entire surface of the fabric. Voltage sources are applied to the conducting array to create localized electric fields to enhance the collection of charged and uncharged particles. Both of these approaches have been tried under various conditions and found to enhance the performance of fabric filters. In this paper, recent experiments performed by us and by others are briefly reviewed, following which, a summary of the results of experiments performed with a large pilot-scale system which utilized the charged particle approach at a coal-fired power plant is presented. The review includes a brief overview of a number of laboratory-scale pilot-scale experiments, all demonstrating one or more important aspects of the enhancement of filter performance by the application of electrostatics. Background Lamb and Constanza ( 4 , 5 )have performed a number of experiments to examine the behavior of various types of fibers on the collection of uncharged fly ash in the presence of an electric field. Isolation of the effects of cake formation was accomplished by using composite, unwoven filter structures for some of the experiments. In other tests, gold-coated fibers were used to eliminate charge buildup on the fiiter material. It was concluded that lower dust retention and lower pressure drop result when the cake forms close to the upstream surface of the filter, rather than within the filter volume. A number of different filter materials were tested by Ariman and Helfritch (6) for the collection of charged and uncharged particles from a variety of sources. A simple wide-pipe corona discharge device was used to charge the particles. The results varied somewhat for the different types of fibers. The improvement in pressure drop was found greatest for wool felt and orlon felt; fiberglass coated with Teflon B produced the least promising results. Microscopic observations revealed that the dust cakes formed
0 1984 American Chemical Society
Environ. Scl. Technol., Vol. 18, No. 9, 1984
635
DUST IN
I
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LABORATORY SHAKER/ DETROIT EDISON FLY ASH LABORATORY SHAKER/CORONA CHARGED DETROIT EDISON FLY ASH (Hovis et al.)
.9 8
R ’ I TO F A R A D A Y
12.7 cm 15 in.)
I
71
t
t
/ - - I
6
.-e
5
3-
4
3 2
1 1
3
2
TIME, hr
Figure 2. Reduced pressure drop of charged Detroit Edison fly ash (8).
Figure 1. Shaker baghouse inlet modified to accommodate a corona wire (8).
with the charged particles were, in general, more porous and fluffy than those of uncharged particles. These characteristics would explain the improved pressure drop in terms of both reduced flow resistance and better removal of the dust cake upon cleaning the bags. Donovan et al. (7) have described the results of a series of experiments on charged particle collision in a laboratory pulse jet fabric filter unit. In the discussion of their results they attributed much of the measured reduction in pressure drop to thinner dust cakes which were formed as a result of particle collection within the charger. Penney (8)has made extensive laboratory studies with particles charged by impingement on a tungsten carbide surface. It was concluded that nodular or coarse dust layers were formed if the resistivity of the dust cake was sufficiently low t o establish localized electric fields which attracted the charged particles to specific sites. The combination of nonuniform thickness and electric forces resulted in both reduced pressure drop and enhanced collection. The effects of relative humidity on the electrostatic enhancement of filtration have been the subject of several recent studies. Hovis et al. (9) used the laboratory scale shaker baghouse shown in Figure 1to study the performance of a precharger/shaker baghouse combination. The plot of pressure drop vs. time characteristic of the system with and without the charger turned on is shown in Figure 2. It was noted that the improved performance with the corona deteriorated over a period of 8 days at 50% relative humidity until no advantage was seen. When the relative humidity was increased to TO%, the improved performance illustrated in Figure 2 was restored. Iinoya and Mori (10)have made extensive tests to investigate the effects of relative humidity. Their experimental setup is shown in Figure 3. Features of this setup are the settling chamber for collecting large particles and agglomerates and the capability of measuring the particle-size distribution between the charger and filter. As was observed by Penny (8),Iinoya and Mori concluded enhanced performance of the charger/filter system was correlated with the formation of a “nodular” dust cake of uneven thickness (see Figure 4). It was also observed by 636 Environ. Sci. Technol., Voi. 18, No. 9, 1984
PUMP
FILTER
FILTER
HIGH VOLTAGE POWER SUPPLY
Figure 3. Experimental apparatus for electrostatic effects in fabric filtration (3). 70
I N O D U L A R DEPOSIT
{ ‘I
d E
50
- 50-75
40
-0
80
S M O O T H DEPOSIT
-
W I T H O U T CORONA PRECHARGER
00
D U S T L O A D , m(g/m21
Figure 4. Relationship between drags and dust loads on a filter, CaCO, dust (3).
these investigators that the size distribution of the aerosol, measured downstream of the precharger, changed significantly as the relative humidity was varied. When a settling chamber was introduced to remove agglomerates, the differences in particle-size distribution and dependence of the fabric filter performance upon relative humidity both disappeared.
F A B R I C F I L T E R BAGS
Table I. Specifications for the Apitron Pilot Plant charger tube i.d., in. charger tube length, in. no. of tubes length of bags, ft diameter of bags, in. no. of bags operating voltage, full power, kV operating current, mA cleaning interval, min downtown during cleaning, min flow rate at A f C of 4 acfm/ft2, fta/min
7 36 24 12 8
24 30 36 24 6 2400
N
-
Several workers have demonstrated the alternate approach of applying external electric fields to enhance particle collection filters. This approach utilizes two fiely spread conducting grids within or in close proximity to the fabric to provide the requisite fields. This has been done with particles charged positively or negatively and uncharged, all with beneficial effects. This concept has been reviewed by Ariman and Helfritch (6). It may be of less practical value (especially for large installations such as utility boilers) because of the complexity of the electrode/fdter systems and will not be discussed further here. The next section contains a more detailed description of tests performed to evaluate a pilot-scale electrostatic baghouse installed a t the Electric Power Research Institute’s Arapahoe Test Facility.
BAG T U B E SHEET C L E A N I N G PULSE BLOW PIPES ER TUBE SHEET
DISCHARGE ELECTRODE
R/
CHARGER POWER SUPPLY
Evaluation of a Pilot-Scale Electrostatic Fabric Filter System This section presents the results of four tests to characterize the performance of a pilot-scale novel control device, the Apitron electrostatically augmented fabric filter, manufactured by American Precision Industries. Rather than a detailed, quantitative study, the tests were designed to provide a screening evaluation of the charged-particle/baghouseconcept over a wide range of operating conditions. The tests included evaluations of the following: (1) dependence of the particulate mass collection efficiency, fractional efficiency, and operating pressure drop upon the air-to-cloth ratio; (2) dependence of particular mass collection efficiency, fractional efficiency, and operating pressure drop on the level of electrostatic augmentation; (3) dependence of particulate mass collection efficiency, fractional efficiency, and operating pressure drop on the type of filter fabric used (woven glass fiber, felted Nomex, and felted Teflon). The tests were made on an Apitron installed on a slip stream from a pulverized-coal-firedboiler. Measurements made to characterize the unit included the following: (1) inlet and outlet particulate matter concentrations and size distributions using an optical particle counter, an electrical aerosol size analyzer, and University of Washington Mark V impadors; (2) the charge and size of individual particles with a Millikan apparatus; (3) in situ and laboratory measurements of the ash resistivity; (4) characteristic voltage-current curves of the electrostatic charger in the Apitron; ( 5 ) the pressure drop across the filter bags. A cutaway view of the Apitron tested is shown in Figure 5. Pertinent dimensions and typical operating data are given in Table I. During operation of the unit, dust-laden gas enters the electrostatic charger section from below, with the upper portion of the hopper serving as an inlet plenum. The gas then flows upward through a set of tubes with axial charged wires, in which the particulate matter is electrically charged. A large fraction of the particles are collected in the tubes. The walls of the tubes are hollow and designed to be water cooled to control the electrical
U
Figure 5. Cutaway view of the Apltron.
+
10.3
10.4
101
10.2
10.1 100 PARTICLE DIAMETER, lim
101
Figure 6, Average inlet partlcie-sire distributionon a cumulative-mass loading basis for each test period.
resistivity of the precipitated fly ash. The gas then flows upward through the bags where the filtration takes place. Clean gas is exhausted from the unit through a duct located in the side of the bag chamber. Both the bags and the chargers are cleaned by a form of pulse-jet cleaning initiated by an electrical pulse from the control system. The pulse-jet blow pipes are located above the charger tubes between the tube sheeb of the charger and the fabric filter. The cleaning pulses clean the charger tubes by direct blast and the bags by induced flow. The unit is taken off-line for cleaning. Fluctuations in the coal composition and combustion process were minimal over the test period. The average inlet particle-size distributions on a cumulative-massloading basis for the tests are shown in Figure 6, and the corresponding data on a differential basis, dM/d log D vs. Envlron. Sci. Technoi., Voi. 18, No. 9, 1984
637
Table 11. Summary of Test Conditions
test series
filter material
range of A C, acfm/ft
1 2 3 4A 4B
woven glass fiber felted Nomex felted Teflon felted Teflon no filter
4.5-9.1 3.1-10.4 2.7-7.9 1.9-6.8 2.0-9.0"
I
range of electrostatic charger current densities, nA/cm2
range of pressure drop across the apitron, in. W.C.
130-212 53-122 49-114 charger off 62-293
1.4-19.0 0.3-9.7 0.6-7.3 0.5-17.5 b
Values are equivalent air-to-cloth ratios for normal oDeration with filter bags installed for the measured flow rate. *Not applicable. 100.0
I
0
I
I
lo3+
FELTED TEFLON BAGS WITHOUT 0
P, I)
TESTSERIES
+
NO BAGS, ELECTROSTATIC
-I
PRECIPITATOR ONLY
0 1
+
A 2
a
&
1001 10.2
I-
::::3+ts(
0 3 + 4
::::13$e(
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::::4tt+i
100
10'
PARTICLE DIAMETER, Lim
Figure 7. Dlfferentlal Inlet partlcle-slze distribution for each test perbd.
SYMBOL
4
'"1
i X
+ A I.
AIR.TO.CLOTH RATIO, ictmlftZ
FILTER MEDIA NONE WOVEN GLASS FIBER FELTED TEFLON PELTED TEFLON FELTEDNOMEX
4.6. 4.6. 4.7 4.0 4.6. 4.6
CHARGER CURRENT DENSITY nAlcm2 148 183. 172 OFF
4.1
Value is ~ w a v a l m t10 ai~fo.ololhrat10 for normal opcrswn w t h filler baC installed le, the measured flow rate.
*****
I
1
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i
i;
E
it
i 1:
99.0
C
L L
$ Y
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--
x x * x x x x X
X
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+ + + + +
* A 1
i
99.99
I
1
I
1
I
Table 111. Test Series 4: Summary of Apitron Test Results for Operation as a Stand-Alone ESP spec collecting area, ft2/1000 acfm 6 5 7 2 1 3 4
24.5 38.8 48.8 52.8 52.8 108.0 108.0
electrostatic charger kV nA/cm2 24
outlet loading, mg/dncmb
9.0 5.7 4.5 4.2 4.2 2.0 2.0
2129.91 3288.80 1193.37 669.83 1193.37 176.68 293.32
293 130 146 65 138 65 146
28
25 24.8 27.4 26.5 28
Coal pulverizers not working properly.
equivalent A/C, acfm/ft2
collection efficiency,
penetration,
%
%
60.476 61.29 77.162 88.735 77.162 90.177 83.692
39.524 38.71 22.838 11.265 22.838 9.823 16.308
mg/dncm = milligrams per dry normal cubic meter.
1
a
I
A
i4
n'
B W
K
2 4 6 8 10 EQUIVALENT AI R-TO-CLOTH RATIO, acfm/ftZ
3
v)
W
n K
AIR.TOCLOTH RATIO, a c f d f t '
Flgure 11. Pressure drop across the Apltron 15 min after cleaning vs. the air-to-cloth ratio.
CURRENT DENSITY, nA/cm2
Figure 10. Penetrations vs. air-to-cloth (or equivalent flow) and charging current density for the Apitron operating as a Stand-alone electrostatic preclpltatw. Penetratbn is plotted vs. air-to-cloth in Figure 10A and vs. current density in Figure 106. The numbers adjacent to the plotted points denote like condltlons for the two plots.
served in other implementations of electrostatically enhanced fabric filtration. The glass fiber was less efficient, a t least partially due to several small holes found in one of the bags when removed. The results are graphed to illustrate the dependence of the penetration upon A/C in Figure 9. It is seen that the penetration increases with increasing A/C for the felted bags but decreases for the woven glass bags. This probably indicates that diffusion (residence time) plays an important role in the collection mechanism of the felted bags, while impaction is more important for the woven bags. Collection in the Electrostatic Charger. The collection efficiency of the electrostatic charger alone is shown in Figure 10. The penetration varied from 10% to about 40% for operation of the Apitron as a stand-alone electrostatic precipitator (Table 111). Parts A and B of Figure 10 are plots of penetration vs. flow rate, or the equivalent A/C ratio, and charger current density, respectively. In this pair of figures, the data points have been numbered
in order to show the correlation of the effects of increased current density and changes in the air-to-cloth ratio with penetration. These figures show two trends: (1) Data points 4 and 7, and also data points 3 and 2, show that, for an increase in the air-to-cloth ratio at a constant current density, the penetration increases, which is to be expected. (2) Data points 3 and 4, and also points 2 and 1, show that for an increase in current density at a constant air-to-cloth ratio, the penetration increases. This indicates that the charger was operating in back corona. The particulate resistivity was approximately 10l2 cm for all the tests. Effect of the Precharger on Pressure Drop. It was shown in Figures 8 and 9 that that emissions were reduced when the charger was in use. Figure 11 summarizes the data in terms of pressure drop and A/C. The graph is divided into two parts. All of the data taken with the charger off lie above and to the left of the dividing line. All of the data taken with the charger on lie below and to the right. This complete separation of data, for all fabrics, illustrates the dramatic effect that the charger had in reducing the pressure drop. Looking specifically at the data for woven glass and Teflon, it is seen that, like the penetration, the pressure drop was reduced by approximately a factor of 2 when the charger was energized. Partial compensation for the effect of particle collection within the charger on the measured pressure drop reduction can be made by comparing operating pressure drops with and without charging at times within the filter cycle at which like amounts of ash have reached the fabric. Because the bag cleaning took place off-line with a long, no-flow quiescent period after pulsing, one can reasonably Envlron. Sci. Technol., Vol. 18, No. 9, 1984
639
Table IV. Pressure Drop at Equivalent Collected Particulate Points in the Operating Cycle
fabric
AIC, ft/min
felted Teflon felted Teflon woven glass
7 3.5 5
FELTED TEFLON, LOW A/C 0 ‘ FELTED I ITEFLON. I HIGH I IA/C
pressure drop, in. W.C. with without charging charging
6.1 1.4 4.9
Summary Collection efficiencies measured for the test unit were comparable to those of conventional fabric filters, even though the operating conditions were substantially different and clearly not optimized. During these tests at A/C ranging from 1.9 to 10.4 acfm/ft2, efficiencies of 98.64-99.99% were obtained with system pressure drops of 2.3-5.6 in. W.C. (inches water column). Efficiencies of 99.89-99.99% for air-to-cloth ratios (A/C) of 1.1-2.1 acfm/ft2 with system pressure drops of 2.3-5.6 in. W.C. have been reported (10) for woven glass fabric filters used in conventional baghouses. Efficiencies comparable to those of conventional utility baghouses were obtained for operation of the Apitron at higher A/C values with a lower pressure drop when the charger was used. The performance of the pilot unit was enhanced by electrostatic augmentation for all the fabrics tested. However, efficiencies of 60-90% were obtained for the Apitron operating as a stand-alone electrostatic precipitator. Thus, during normal operation of the unit &e., with a filter installed) the particulate loading that reaches the filter is considerably reduced as compared to that which Envlron. Scl. Technol., Vol. 18, No. 9, 1984
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6.8 fpm, CHARGER OFF
13 3.2 7.0
expect that recirculation of material dislodged from the bags during cleaning was minimal with and without charging (although a greater amount of dislodged material will always have been carried back to the bags with the charger off). Under the assumption that fresh fly ash carried by the incoming gas stream dominated in the material arriving at the bags, the data that were obtained permit comparisons to be made of pressure drops after comparable amounts of particulate have been deposited on the bags. In Figure 12,measured pressure drops are plotted vs. time after cleaning for typical tests of performance with Teflon and glass bags. Pressure drop curves are shown for operation at similar air-to-cloth ratios with and without charging. Pressure drops after comparable amounts of material had been deposited on the bags are tabulated for felted Teflon a t two air-to-cloth ratios and woven glass for one air-to-cloth in Table IV. From this we conclude that a substantial portion, about half, of the pressure drop reduction observed with charging resulted from alterations of the cake structure and the remainder from a reduction in the amount of material reaching the bags during each filtration cycle. Alternate Charging System. The system described here has since been modified to employ a “high intensity ionizer” (HII) of the type once marketed by APS, Inc., for particle charging. At the same time, the cleaning method was changed to reverse gas. The use of a single, compact, HI1 charger, located upstream of the pilot plant baghouse inlet, greatly reduces the complexity of the charging system. Preliminary data indicate that the operating presure drop is markedly reduced with charging as compared to without charging in the modified system, with virtually no collection taking place within the charger. Detailed results of this experimental program will be published at a later data upon completion of the work.
840
A
p
O
I 0
I 2
4
I
I 6
I
I
I
I
I
16 18 TIME AFTER CLEANING, min 8
10
12
14
I
I
I
20
22
24
Figure 12. Pressure drop vs. time after cleanlng for typical tests of Teflon and glass bags.
would exist in a conventional fabric filter. The dust layer accumulated in a given time is thinner, which accounts for a portion of the reduction in the pressure drop across the unit. The effects of charged particles on dust cake formation is presumed to account for the remainder of the reduction in pressure drop. In evaluating these test results one must take into account the differences between the test unit and other, more conventional systems. Although the test unit is cleaned by reverse flow induced by a jet, it is an “inside bag” collector, and the cleaning is probably less vigorous than a conventional pulse-jet unit. The bag cleaning method, however, may have been more vigorous than reverse gas and could have contributed to a lower pressure drop as compared to conventional reverse gas units. In any case, the relative differences in performance observed in the various operating configurations (charger off/on, different fabrics, A/C, etc.) are significant and indicate that precharging is a viable method of reducing the pressure drop or, alternately, reducing the required size baghouses while at the same time reducing the emissions.
Literature Cited (1) Natanson, G.Dokl. Akad. Nuuk SSSR 1957,112,696-699. (2) Iinoya, K.;Makino, K. Aerosol Sci. 1974,5, 357-372. (3) Zebel, G.J. Colloid Sci. 1965,20, 522. (4) Lamb, G.;Constanza, P. Chern. Eng. B o g . 1977,73,51-53. (5) Lamb, G.;Constanza, P. “Proceedings, Second Symposium on the Transfer and Utilization of Particulate Control Technology”; E P A Denver, CO, July, 1979;NTIS PB81144800. (6) Ariman, T.; Helfritch, D. J. “Proceedings, Second Symposium on the Transfer and Utilization of Particulate Control Technology”; E P A Denver, CO, July 1979,NTIS PB81144800. (7) Donovan, R.;Hovk, L.; Ramsey, G.; Abbott, J. “Proceedings, Third EPA Symposium on the Transfer and Utilization of Particulate Control Technology”;E P A Orlando, FL, March 1981.
Environ. Sci. Technoi. 1004, 18, 641-647
(8) Penney, G. W. Sept 1978, EPA Report No. EPA-6001778-142. (9) Hovis, L. S.; Abbott, J. H.; Donovan, R. P.; Pareja, C. A. “Proceedings, Third EPA Symposium on the Transfer and Utilization of Particulate Control Technology”; EPA: Orlando, FL, March 1981. (10) Iinoya, K.; Mori, Y. “Proceedings, Second Symposium on the Transfer and Utilization of Particulate Control Technology”;E P A Denver, CO, July 1979; NTIS PB81144800.
(11) Billings, C. E.; Wilder, J. National Air Pollution Control Administration, 1970, Report CPA-22-69-38 (NTIS PB200648). I
Received for review May 12,1983. Revised manuscript received March 1, 1984. Accepted March 6, 1984. This experimental study was supported by the Electric Power Research Institute under Contract RP725-12. Robert C. Carr was the project manager.
Fate of a Tritiated Ekofisk Crude Oil in a Controlled Ecosystem Experiment with North Sea Plankton Morten Laake” Institute of Microbiology and Plant Physiology, university of Bergen, N-5000 Bergen, Norway Kjeli Tjessemt
Institute of Chemistry, University of Bergen, N-5000 Bergen, Norway Knut Rein
Institute of Physics, University of Oslo, Bllndern, Oslo 3, Norway
rn Flexible plastic enclosures were employed with the main intent of determining the fate of an Ekofisk crude oil exposed to North Sea spring conditions. By use of a tritium-labeled Ekofisk crude oil, a dynamic model was developed that allowed calculation of vertical mass fluxes with depth based on actual concentration profiles and measured sedimentation rates. It has been concluded that adsorption and subsequent sedimentation of plankton and organic detritus may cause a rapid sinking of petroleum hydrocarbons. Microbial mineralization seemed to be insignificant on a short-term scale.
Introduction Spilled oil is acted upon by a variety of processes including spreading at the surface, evaporation, emulsification, microbial/ photochemical degradation, and sorption onto particles which carry it at varying rates to reservoirs such as the atmosphere and sediments. The present study intends to give more reliable predictions of the environmental fate of nonviscous crude oils, their rate of transformation and incorporation into the water column. It has utilized a tritium-labeled Ekofisk crude oil in an enclosed planktonic ecosystem, and while preserving nearly natural conditions, they allow for the determination of mass budgets and compartmental residence time. To the authors’ knowledge, an actual mass balance vital to understanding the fate of oil spilled at sea has not previously been obtained. Enclosed planktonic ecosystems lie between the complex and highly variable natural world and the tightly controlled, but less natural, laboratory experiments. Biological and chemical interactions may be characterized more conveniently than in natural systems which pose many logistic problems. Such ecosystems have therefore gained momentum in the studies of trophic interactions, nutrient recycling, and pollutant stress in recent years (1-4). A *Address correspondence to this author at the Environmental Toxicology Laboratory, NVH, P.O. Box 8146, Dep., Oslo 1, Norway. Present address: Statoil A/S, SVK N-4001 Stavanger, Norway. 0013-936X/84/0918-0641$01.50/0
variety of effects upon marine organisms have also been observed in such systems (5).
Materials and Methods Experimental Design. The field experiment was carried out in Rosfjord, southern part of Norway, from March 21 to April 4, 1979. Cylindrical, translucent plastic enclosures were used which consisted of an inner polyethylene film layer (100 pm) supported by an polyamide outer layer (30 pm) (Trikoron S, Alkor-Oerlikon Plastic Gmbh, Munchen). The bags were mounted on floating frames extending the walls 50 cm above sea level (6). The prevailing physical, biological, and chemical conditions in the fjord and the experimental design have been described in detail elsewhere (7). The surface temperature varied within +0.5 to +2.5 “C, while the air temperature varied from -2 to +6 OC. Briefly described, the experiment was performed in three separate enclosures with 1.0-m diameter and 13-m depth. One enclosure served as a control, whereas 4.0 L of Ekofisk crude oil was poured gently onto the surface of the other two bags, giving a fairly large film thickness ( 5 mm) persisting throughout most of the experiment. One of the latter bags was spiked with a tritiated Ekofisk crude oil of specific activity of 2.27 mCi/mg. The tritiation procedure has been described elsewhere (8). The specific activity by weight in the aliphatic fraction of the tritiated Ekofisk crude oil was substantial but as expected somewhat lower (about 40%) than in the aromatic fraction. No major changes take place in the original oil upon tritiation. However, we have at this stage no information as to the extent of tritium exchange in the high molecular weight resin/asphaltene material containing varying proportions of saturated and aromatic moieties. Water samples were systematically collected through PVC tubing at 0.2-, 3.2-, 6.2-, and 10.2-m depth. Sediment traps of 0.04-m2 surface area were used to collect sedimented material a t the termination of the experiment. The plastic bag was sliced horizontally at the surface where visually covered with oil and at each sampling depth (10-cm wall height) and extracted with dichloromethane,
0 1984 American Chemical Society
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Envlron. Scl. Technol., Vol. 18, No. 9, 1984 641
