during periods of low loading and used as an energy source by the microorganisms. When no sludge wasting is practiced, the clinoptilolite can be recycled continuously. (Patent proceedings have been initiated covering various applications of this process.) The complicated regeneration procedure with lime is not necessary in such an application of clinoptilolite. Not considered in the model is the influence of pH upon nitrification. The acid production associated with nitrification may in itself cause a considerable decrease of pH in weakly buffered sewage. The author has also developed a dynamic model for the simulation of p H in activated sludge which can incorporate nitrification (Lijklema, 1972). By combining these two models, a more complete dynamic model can be obtained.
Acknowledgment The author is grateful to Linda W. Little and Charles
R. O’Melia for their stimulating comments. Literature Cited Barnes, R. D., “Invertebrate Zoology,” P. Saunders, Philadelphia, Pa., 1963. Curds, C. R., Cockburn, A,, J. Gen. Microbiol., 54,343-58 (1968). Curds. C. R., Cockburn, A,. ibid., 66,95-108 (1971). Curds. C. R.. Cockburn. A,, Water Res. 4,237-49 (1970). Downing, A. L., Painter, H. A., Knowles, G., J. Inst SeLoage Purification, 2, 130-158 (1964).
Francisco, D. E., Little, Linda W., Lamb, J. C., “Activated sludge modifications for enhancement of trickling filter plant performance,” Department of Environmental Sciences and Engineering, ESE Publication No. 272, University of North Carolina, Chapel Hill, N.C., 1971. Hanson, R. L., Walker, W. C., Brown, J . C., “Variations in Characteristics of Wastewater Influent a t the Mason Farm Wastewater Treatment Plant, Chapel Hill, North Carolina,” Department of Environmental Sciences and Engineering, ESE Publication No. 255, University of North Carolina, N.C., 1970. Haug, R. T., McCarty, P. L., Annual Conference of the Water Pollution Control Federation, San Francisco, Calif., October 1971. Hawkes, H. A., “The ecology of wastewater treatment,” P. Pergamon Press, Ltd., Oxford, England, 1963. International Business Machines Corp., Manual H-20-0367, fourth ed., 1969. Johnson, W. K., Schroepfer, G. T., J. Water Pollut. Contr. Fed., 36, 1015-36 (1964). Lawrence, A. W., McCarty, P. L., J. Sanit. Eng. Diu., Amer. SOC. Civil Eng., 96, (SA3) 757-778 (1970). Lijklema, L., Water Res., 6, 165-82 (1972). Mechalas, B. J . , Allen, P . M., Matyskiela, W. W., Water Pollut. Contr. Res. Ser. 17010 DRD 07/70, Department of the Interior, FWQA, 1970. Murphy, K. L., Timpany, P. L., J. Sanit. Eng. Diu., Amer. SOC. Civil Eng., 93 (SA5), 1-15, (1967). Painter, H. A., Water Res., 4,393-450 (1970). Parker, D. S., Kaufman, W. J., Jenkins, D., J. Sanit. Eng. Diu., Amer. Soc. CiuilEng., 98, (SA1) 79-99 (1972). Straskrabova, Vera, Legner, M., Aduan. Water Pollut. Res., Proceedings of the 4th International Conference, Prague, 1969.
Received for review June 19, 1972. Accepted December 19, 1972.
Ferromagnetic Sorbents for Oil Spill Recovery and Control J. E. Turbeville D e p a r t m e n t of Physics, University of South F l o r i d a , T a m p a , Fla. 33620
Buoyant, ferromagnetic, sorbent particles with an affinity for oil will transform a maritime oil spill into a magnetizable surface film whereby it is possible, with suitable magnetic equipment, to control and recover any oil so treated. Laboratory experiments were conducted to develop ferromagnetic sorbents and study their properties when combined with various test oils. These experiments provide information on the magnetic advantage of such a recovery concept. This advantage is expressed as gain and is shown to increase as the viscosity of the oils under test decreases. A drain rate equation was also obtained from the experiments and is incorporated with results obtained from experimental tests with a model oil recovery unit. Together, they provide a general expression for recovery rates which can be used as a scaling equation for larger units. Prototype floating magnetic grid units were constructed and tested for possible use as “magnetic nets” or “magnetic barriers.” The advantages gained by the addition of magnetic principles to sorbent systems of oil spill recovery could possibly outweigh the disadvantages of sorbents now in general use. Treatment of a maritime oil spill with sorbent particles which are buoyant, ferromagnetic, and water resistant will transform the spill into a magnetizable surface film whereby it is possible, with suitable magnetic fields, to trap and contain the oil or remove it directly from the water surface.
If the oil is a low-grade high-density type fuel oil, the added bouyancy of the ferromagnetic sorbent will tend to support the oil and keep it from sinking, should the water density change from that of seawater to a degree of brackishness where sinking would normally occur. Buoyant ferromagnetic sorbents and the associated magnetic containment and collection devices offer a technique for oil spill recovery and control that is indeed unique. The ideas and methods conceived and tested are outlined below. Ferromagnetic sorbents were developed from commercially available products and are classified as either adsorbent or absorbent. Laboratory studies were made with various oil mixtures to determine the holding characteristics of the permanent magnetic material and the ferromagnetic sorbents which are used in combination to lift oil from the water surface on a n oil recovery test unit. Tests were conducted with the oil recovery unit in a 10-ft tank to establish scale factors and oil recovery rates. Tests were made of prototype floating magnetic grids intended for use in the construction of containment barriers or magnetic nets. Ferromagnetic Sorbents. A commercially available polystyrene bead (Dylite manufactured by Sinclair-Koppers) was used to produce the ferromagnetic adsorbent. The material has a density of 40 lb/ft3 (PCF) in its unexpanded form and, after treatment in a steam expander, Volume 7, Number 5,
May 1973 433
Table I I . Values of k lor Drain Rate Equation Mixture
Value of k
Bunker 'c'
k l = -0.18
90% 'c'/10% diesel 75% 'c'/25% diesel 50% 'C/50% diesel 25% ' Cf 75% diesel
k2
-0.23 = -0.50 ka = -0.59 k 5 = -0.71 k3
Table 111. Gain (Magnetic Advantage) Experienced with Ferromagnetic Absorbents and Test Sample Oils 9O%'C f 10% diesel
75% ' C /
50% ' C /
Bunker ' C
25% diesel
50% diesel
25% 'C/ 75% diesel
Gain 1.95 Visc.y 5056
3.48 1073
3.96 490
8.17 80
9.65 40
a Where viscosity is expressed in CentiStokeS.
Figure 1. Photomicrograph of a single ferromagnetic adsorbent particle. (Particles range in size from 3-mm to 5-mm diameter)
Table I . Oil Sample Data Density at
Mixture
Bunker ' C 90% 'C'/lO% diesel 75% 'C'/25% diesel 50% 'C'/50% diesel 25% 'c'/75% diesel
Diesel
25'C
0.38 gm/cm3 0.96 0.94 0.90 0.87 0.84
Viscosity at
25-C 5056 CS 1073 490 80 43 4.2
has a density of one PCF. The expanded polystyrene bead is then treated with a waterproof adhesive and 40-mesh powdered iron in a forced air tumbler. This allows iron to be coated on to each foam particle and prevents the mixture from sticking together while drying. This ferromagnetic adsorbent has a density of approximately 3.9 PCF and has fairly good breakdown resistance to oils of low terpene content. The heads range in size from 3-mm to 5-mm diameter and have a strong affinity for oil. A (lox) photomicrograph of a single head is shown in Figure 1. Although the data collected with the ferromagnetic adsorbent form the basis of this report, another type of material (Fiberper1 manufactured by Grefco Inc.), which is highly ahsorbent, was also treated with powdered iron and tested for its magnetic recovery properties.
Materials Test Procedures To gain an understanding of the oil retention ability of the permanent magnetic material (magnetic rubher manufactured by B. F. Goodrich), particularly when combined with oil-soaked magnetizable adsorbent, a series of dip and drain experiments was conducted with different mixtures of Bunker 'C' oil and diesel fuel. The results of these tests provide the maximum oil loading capability of the magnets as well as a means by which oil pickup rates may he predicted for the oil recovery unit. The advantage of a magnetic oil recovery system over a system which uses only adhesive bonding of oil for recovery is also obtained from these tests. This advantage, 434
Environmental Science & Technology
which is labeled as gain, can he expressed as a function of viscosity by means of a power law equation. The oil samples, chosen in order to provide a wide range of viscosities, are listed in Table I (Environmental Protection Agency, 1971). The magnetic field strength a t the surface of the rubber magnetic material is approximately 700 gauss as measured with a Model 900 Empire Scientific gaussmeter. Dip and Drain Measurements. These measurements were performed in the several test mixtures of floating oil for three specific reasons: To determine the adhesive holding ability of the ruhber magnets used in the construction of the oil recovery unit; to determine the magnetic and adhesive holding ability of the rubber magnets when the ferromagnetic adsorbent is combined with the particular oil under test; and to determine the holding ability when a nonmagnetic sample of the adsorbent is combined with the oil under test. For each of the dip and drain measurements, maximum possible saturation of oil was attempted during removal of the sample magnet from the oily mixture. The mass of oil and sorbent collected and the allowed drain time were recorded for each of the oil samples listed in Table I. All data collected are presented in graphical form in Figure 2(A-E) and display the drain rates for each of the oil, samples. The numbered curves on each graph represent the following data: Curve 1. Mass of magnetic adsorbent and combined oil as a function of drain time. Curve 2. Mass of oil collected by magnetic adsorbent as a function of drain time (Curve 1 minus average mass of adsorbent). Curve 3. Mass of nonmagnetic sorbent and combined oil as a function of drain time. Curve 4. Mass of oil collected by nonmagnetic adsorbent as a fundtion of drain time (Curve 3 minus average mass of adsorbent and adsorbent loss). Curve 5. Mass of oil collected due to the adhesive properties of the surface of the rubber magnet. Data for the number 2 curves, when plotted on logarithmic paper (Figure 3) and extrapolated to t = 1 sec, provide the maximum loading capability (M,) of the magnets as well as a power law equation for the mass of oil collected per unit length as a function of drain time. The power law equation vias obtained from Figure 3 in the following manner: Maximum load capability .(Mol = 17.1 g14.75 in. (4.75 in. = length of test sample magnet.) Log mass = k log (drain time) + log Mo or M = (3.6 g/in.)tk. M = (3.6g/in.)tA. In general:
M/M,
= tk
Drain Rate Equation
where M , is the mass per unit length at t = 1sec.
(1)
I
I
l5I
15 !
I
0 1 " " " " ' " ' ~ J
5
7
9
1
1
13
3
5
CCl'C'I 1 3 i
!a)
1
'
9
1
1
1
3
D
I ' t " " " " " 1
4
(1)
1~
3
7
SECONDS ( c i 75X'c'I 2%
ZECOhtS
7
5
9
1 1 1 3
3
5
501'c'i 50% 3
1 1 1 3
SECONOS
SECONDS lo1
4
7
Figure 2(A-E). Mass of adsorbent and oil collected by test sample magnet as a function of drain time Curve (1) Mass of ferromagnetic adsorbent and oil, Curve ( 2 ) . Mass of oil from ferromagnetic adsorbent, Curve (3) Mass of nonferromagnetic adsorbent and oil Curve ( 4 ) , Mass of oil from nonferromagnetic adsorbent, Curve (5) Mass of oil due to adhesion only
(El
The h values obtained from the graph (Figure 3 ) depend on viscosity and are listed in Table 11. Gain. The gain which is experienced by the use of magnetic principles on the oil recovery device can also be obtained from the data presented in the graphs of Figure 2(A-E). By assuming the very conservative time of 3 sec as the maximum time required for oil to be moved from the water to the collection hopper, gain can be determined directly from the graphs by comparing Curve 5 with Curve 2. Gain is defined as the ratio of the mass of oil collected by magnetic means to the mass of oil collected by adhesion of oil to the collection device. Table I11 lists the gain at t = 3 sec for the various test oils. A logarithmic plot of these data (Figure 4) provides information for an expression of the gain of this system.
251.'C'/ 751. 0
io3
I
'
'
"""I
'
'
"""I
'
j
Figure 3. Logarithmic plot of oil mass collected with ferromagnetic adsorbent. Curve ( 2 ) , Figure 2(A-E), with extrapolation to t = 1 sec
log gain = K(1og viscosity) + log 34
gain
=
34(vi~cosity)~
(2)
where K = -0.332 and viscosity is expressed in centistokes.
Oil Recouery Unit A rotating magnetic drum concept was developed which employs the buoyant forces on the ferromagnetic sorbent, the magnetic forces produced by the pickup unit, and the viscous forces of the various oils.
VlSaslTT
ICMISTOIISi
Figure 4. Logarithmic plot of gain as a function of viscosity Volume 7, Number 5, May 1973
435
T o simulate horizontal movement of the recovery unit across the surface, a rolling backboard was mounted in the 3-ft wide test tank and used to move the treated oil toward the recovery unit. Deflectors were mounted adjacent to the drum in order to funnel the floating mixture into the working area of the drum (Figure 6). A motor speed control unit attached to the drive mechanism is used to vary the drum speed. Procedures. Recovery rate tests were conducted with the various mixtures of oils and the results provide information for a realistic appraisal of the pickup capabilities of such a system of oil removal. Measured amounts of test oil were poured onto the water and allowed to spread out over the surface as it would in an unconfined situation. No artificially induced thickening was caused hy the tank walls. Measured amounts of the sorbent were then applied to the surface of the oil. A drum speed of 0.3 rps was chosen in order to keep water pickup to a minimum. Because of the small drum diameter, water pickup becomes a large factor at speeds greater than this. With larger drum diameters, water pickup rates would be expected to drop considerably hecause of the greater heights to which it must he lifted. The oil soaked sorbent is herded toward the rotating drum by the backboard a t a rate which will keep drum starvation from occurring. The oil and sorbent are carried over the top of the cylinder, removed by the wiper, and recovered in plastic container bags so that oil and water content may be measured. The test runs were limited to 10 sec because some drum starvation occurs for times greater than this. Results. Tests for all of the mixtures of oils have not been completed, but sufficient data have been collected to form a number of conclusions when comparison of these data is made with the materials test data. In the recovery rate tests, conducted to determine scale factors, two parts (by volume) of the ferromagnetic sorbent are used for each one part (by volume) of the oil under test. On measuring the return for a series of 10-sec runs with the oil recovery test unit, there is approximately a three-to-one (by volume) pickup of sorbent to oil, respectively. This indicates that a three-to-one (by volume) sorbent-to-oil ratio may be a more realistic value to use in actual practice. Because of the fact that this process utilizes adhesion of the oil to the surface of the polystyrene spheres, senaration of the oil and sorbent occurs auite readilv after C:ollection. This is important when separation of oil for sal,age is desirable. A series of 10-sec run5 at 0.3 rps in a 50/50 mixture of >..“I7^.. ‘ 0 , .“A ”. LU,,,,L ” LlllU ,l:n”*l &...I ..:nl,3,.A a , . a L ‘ n a g r -4. C ” , 3 vf oil and water on separation from the recovered sorbent. U l r U r l L U r l JlGL“r;U
-..---I^
“I
“11 C l l l
This is 64.1 cm3/sec or 61 gal/hr. The average water content was less than lO%,and depended on the amount of oil starvation that occurred. Utilizing this information in conjunction with Equation 1 given earlier, we are able to predict with reasonably good, accuracy the volume of liquid which can be recovered when the viscosity of the oil is changed. With the assumption that the initial volume of oil picked up at the interface is independent of the type of oil (based on the data of Figure 3), and using the time ( t ) of a half revolution of the drum, it is possible to predict the recovery rate of oil with a different viscosity, once some initial rate has been established. For example: Using tlie experimental test results of 64.1 cmspec, which we obtained for a 50% ‘C’/50% D mixture of oil while the drum was running a t a speed of 0.3 rps, we can obtain an esti-
Figure 7. Floating magnetic grid unit, intended for use in the construction of "magnetic nets" or "magnetic barriers" mated recovery rate for a 75% 'C'/25% D mixture a t the same speed.
M ( 7 5 / 2 5 ) = (64.1)(1.05) M ( 7 5 / 2 5 ) = 67.3 cm3/sec or 64 gal/hr This agrees well with a series of test 'runs in this type of mixture which yielded an average rate of 66.5 cm3/sec or 63 gal/hr. Floating Magnetic Grids A concept involving small floating units of interconnecting parallel magnetic grids for the capture of treated oil was studied and several prototype magnetic units were constructed for testing (Turbeville, 1970, 1972). The use of small floating units, which join together, is essential because the magnetic fields associated with such a system are relatively short-ranged and the combined units must he capable of following the surface motion of a body of water. The prototype magnetic units which were constructed are capable of moving the treated oil by a process of "raking" n i the" riln he i i a d t.n ''chain off' an a w a to keen
pc'cu,
I 11,.
,up, n'lu
L & ,
LIB.
'"L'6.
L l Y C C 0"C.I
.A'LS..C""
Figure 8. Three magnetic grid units connected in "chain" fashion, used to sweep the tank containing oil soaked adsorbent speeds without significant loss from the grids. Figure 8 displays three of the units, connected in "chain" fashion, which were used to sweep the tank. A concept which envisions large "magnetic nets" utilizing parallel chains of these units for movement of spilled oil away from high risk areas seems quite reasonable. The magnetic nets could be assembled from the basic building block into many different sizes. Small boats could be used to treat the oil with ferromagnetic sorbent, unroll their nets, and proceed to collect the oily mixture. It could then be towed to a convenient area for disposal and dumped by increasing the speed of the boat until the frictional forces of the water exceed the magnetic forces of the grids in the net, A nickun haree could then recover the mixture and the I
a-1
inserted into extruded plastic sleeves 14 in. long and sealed with waterproof cement to aluminum end plates. The plastic covered magnetic bars are suspended on a wooden frame and separated by means of plastic net floats which provide the necessary buoyancy to maintain the assembled units in a floating position. A 14 X 20 in. unit is shown in Figure I . Test Results. Tests were conducted in a 6-ft circular tank to determine the loading ability of the floats and also their ability to retain the oily sorbent when the surface of the water is agitated. All tests were made using Bunker 'C' fuel and a ferromagnetic adsorbent with a density of 2.6 PCF. Sorbent-to-oil ratios of 10-to-1 by volume, respectively, were used for maximum coverage of spilled oil. To capture the oily material between the grids of the floats, it was necessary to sweep the floating oil with the units or agitate the water surface to create a slight wave action, As the oily sorbent is swept into the grids, it remains and continues to pile up until fully loaded. The ability of the units to follow the motion of the surface is important if oil is to remain within the grids. Oil thus captured can he towed about a t moderate
Con FI
__
mu? .. -l...l_ ".._._l-.ll "..~ ...-.-. .l.l -..crit'. the recovery unit tests to formalize a scaled design for a larger recovery unit. Using the maximum load capability of the magnetic bands and the fractional area exposed after bonding to the aluminum drum, we attempt to predict the quantity Of oil which will he recovered as a function of time and then make a comparison with the experimental resul
."..,
magnetic)( band surface circum.
l _ l l .
)
( E ) ( r p s ) (No. of hands)
(oil density)
For a 50150 mixture of Bunker 'c'and die! based on the size of our experimental model, edrate would he, Volume 7, Number 5, May 1973
437
(:)
(3.6$)
(io&)
(0.742) (0.3E) ( 7 ) = 166.2-cm3 sec 0.9o-fF cm3
Based on the experimental average of 64.1 cm3/sec, we see that only about 39% of the maximum load capability is actually collected a t the interface. For a 75/25 mixture of Bunker ‘C’ and diesel fuel our predicted rate would be,
(3.6%)
(i)
(0.776) (0.3z ( 7) ) = 166.4-cm3 g sec 0.94cm3
(40&)
The experimental average for this mixture (66.5 cm3/sec) reveals that about 40% of the maximum load capability is actually collected at the interface. From the results of these two comparisons the assumption is made that 40% of the maximum load capability is collected a t the drum interface with the surface. Equation 4 can now be written in the form of a general expression which will give the recovery rate for a pickup unit of any size, provided the tangential drum speed is low enough for the buoyant, viscous, and magnetic forces to interact. (0.40) (43.2 g/ft)
():
(n diam f t ) (t’) (rps) (7) (width ft)
(oil density g/cm3)
where t = time of a half revolution in seconds. General expression 253.2
(drum diam ft) (width ft) (rps) ( t k ) (oil density, g/cm3)
=
[SI (5)
cm3/sec x 0.951
=
gal/hr
For example: Consider a drum 12 ft wide with a 6-ft diameter running a t 0.2 rps in Bunker ‘C’ oil. Recovery rate
=
(253.2) (6) (12) (0.2) (2.5-O’”) 0.98 3155 cm3/sec = 3000 gal/hr
In actual practice, the drum speed and the forward motion would have to be adjusted to the sea state experimentally in order to find the optimum operating conditions. Variations in the immersion depth would probably not affect the recovery rate to any great extent because of the number of forces involved in the collection process. Mechanisms for the removal of the collected mass of oil and sorbent from the collection hopper to storage tanks
438
Environmental Science & Technology
have not been studied as yet, but because of the consistency of the collected mass it would be reasonable to assume that vacuum hose techniques could be employed for this purpose.
Conclusions The addition of magnetic principles in the development of sorbents and collection equipment for oil spill removal and control offers several advantages over systems which utilize only mechanical principles. Magnetizable adsorbent particles provide for rapid, efficient removal of oil and quick separation upon recovery when salvage is a principal concern. Magnetizable sorbents which are highly absorbent may be used to treat an oil spill when contamination of coast lines and waterfowl are of immediate concern. Separation of oil from the absorbent is more difficult, but the almost total absorption of oil greatly diminishes the possibility of damage to property and wildlife. The test model magnetic pickup drum with parallel rings of flexible permanent magnets bonded to the circumference provides a magnetic holding force which combines with viscous holding to significantly improve recovery rates. Based on the scaling equation which was developed, a recovery rate of approximately 3000 gal/hr would be possible with an oil recovery unit 12 ft wide and 6 ft in diameter running at 0.2 rps. Doubling the rotational speed could conceivably increase this to 6800 gal/hr but this, of course, would depned on sea conditions and the water content of the recovered mixture. The amount of adsorbent necessary for oil recovery by this technique is approximately 3-to-1 by volume of sorbent-to-oil, respectively, or expressed another way, 1.6 lb of sorbent per gallon of oil. The floating magnetic grids tested for the “magnetic net” and “magnetic barrier” concept are important from the standpoint of a total system of oil control. The ultimate objective, however, of any system is total removal of oil from the surface with minimum property damage and maximum oil salvage with minimum cost.
Acknouledgment The author wishes to thank those people and concerns who donated materials for this research and also Paul Schmidt and Ralph Levingston for their very able assistance in the construction of the oil recovery test unit. The author also wishes to thank N. L. Oleson, S. R. Deans, and H. R. Brooker for their helpful comments and criticism.
Literature Cited Environmental Protection Agency, Water Quality Office, Water Pollution Control Research Series-15080 FWN, (July 1971). Turbeville, J . E., Bull. Amer. Phys. Soc., Sec. 11, 15, 1380 (1970). Turbevil1e;J. E., U. S. Patent 3,657,119 (April 18, 1972).
Receiced for recieu. July 10, 1972. Accepted J a n u a c 8, 1973. Project supported in part b ) Cnicersity of South Florida Research Council and College of Natural Science.
