ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - OIL SPILLAGE INTO WATER—TREATMENT pdf

1 PREVENTION OF THE SPILL; 2 CONTAINMENT AND RECOVERY OF THE SPILL; 3 TREATMENT OF THE SURFACE OIL.. 10 Physical Removal of the Contained Oil by Oil Pickup Devices Since oil containm

The general subject of treatment of oil spillage represents

a relatively new area of technology that is unique in that it

encompasses chemical, mechanical, and biological disciplines

There are three major aspects to the problem of oil spills

1) PREVENTION OF THE SPILL;

2) CONTAINMENT AND RECOVERY OF THE

SPILL;

3) TREATMENT OF THE SURFACE OIL

Although this discussion is mainly directed toward the

treat-ment of the spilled oil, the related areas will also be considered

in order to put the overall subject in the correct perspective

PREVENTION OF THE SPILL

The Prevention of Oil Spillages is the Primary

Consideration

It should be emphasized that prevention is the fi rst

consid-eration and, of course, the most complete solution In the

industrial and governmental communities, the major effort

has been directed toward this area There is extensive

ongo-ing research for example, rangongo-ing from operational areas

such as collision avoidance techniques and training to more

novel approaches such as the gellation of crude oil In this

latter approach several chemical systems have been

devel-oped to gel the oil cargo This in-situ solidifi cation thereby

prevents the release of oil from a damaged cargo

compart-ment that may be in danger of failure

Details of this gellation system can be found in US Patent

3,634,050. 1 Other details, such as the effects of mixing, crude oil

type, chemical concentration, and so on, on the strength of the

gel have been outlined by Corino. 2 Gellation is a novel approach

to prevent the release of oil However the fact that there have

been no commercial uses of this method since its conception

twenty fi ve years ago raises questions regarding its practicality

Finally, the removal of the oil cargo from a grounded

tanker is another area where the threat of the release of a fl uid

and mobile oil cargo to the marine environment has been

miti-gated by advances in salvage techniques The offl oading of

the grounded SS General Colocotronis on a reef off Eleuthera

Island in March–April 1969 and the well documented

recov-ery of Bunker C oil from the sunken tanker SS Arrow in

Chedabucto Bay, Nova Scotia during the winter of 1970 3 are

two outstanding examples of this prevention technique

This latter incident represented a singular achievement

in light of the weather conditions encountered during early March in Nova Scotia Over 6000 tons of viscous Bunker C oil were recovered from the sunken wreck The salvage team used a hot tap technique to penetrate the tanker cargo tanks and then used a steam traced pumping system to transfer the oil to a barge at the surface

A more recent and massive removal of oil was the EXXON VALDEZ in March 1989 after its grounding on a reef in Prince William Sound Although approx 250,000 Bbls of North Slope Crude oil was spilled from the grounded vessel, 80 per-cent of its cargo was still in the tanker This offl oading was a signifi cant marine engineering feat since care must be taken to offl oad such a large vessel in the correct sequence since other-wise hull stresses could cause the vessel to break up

CONTAINMENT, RECOVERY OR REMOVAL OF THE SPILLED OIL

If a spill has occurred, it is universally agreed that the rec-ommended procedure is to contain and physically recover

it with or without the use of adsorbents It is obviously the most direct solution to spill incident, if conditions permit its execution This approach may entail three processes:

1) Confinement of the spill by spill booms

2) Recovery of the spill by sorbing agents In this area, more recent advancements have been solidi-fying agents (Solidifiers)

3) Physical removal of the contained oil by oil pickup devices

4) Controlled burning of spilled oil

These aspects of the recovery approach are interrelated as will be appreciated by the following discussion

Confi nement of the Spill by Spill Booms There are many

oil spill booms commercially available today Unfortunately they are signifi cantly limited by the velocity of the surface current and wave height Although there are variations in the materials of construction, strength, geometry, etc., of these various boom designs, as evidenced by the number available and the range of costs, their general forms are quite similar Almost any type of fl oating barrier will hold

conditions Indeed, telephone poles have been employed in

more than one spill instance as a jury rig emergency

mea-sure To improve the capacity of such a fl oating barrier, a

weighted skirt is hung from the fl otating member illustrated

in Figure 1 Design requirements for spill booms have been

published by Lehr and Scherer 4 and Hoult, 5 among others

By a rather cursory inspection of Figure 1, we may now

appreciate some of these requirements such as:

Suffi cient freeboard to prevent overtopping by waves;

Adequate skirt length below water surface to confi ned a

suffi cient quantity of oil;

Adequate fl exibility to permit the boom to bend under

wave action and maintain its retention of the oil spill;

Suffi cient mechanical strength to withstand the forces imposed by the environment

Some of the diffi culties of oil retention against the action

of a steady current are illustrated in Figure 1 A discussion

of draw down phenomena by Hoult 6 outlines that a gradient

in oil thickness, h, is established by the stress imposed by the current fl ow There is, based on the fl uid dynamics of the contained volume of oil in the presence of a water cur-rent, a limiting water velocity above which oil droplets are entrained and fl ow underneath the barrier

The deployment of the above described mechanical booms is also an important consideration In the event of a spill, the speed of response is, of course, most critical Hence,

Flotation Member

Water

Oil Contained Under Quiescent Conditions

Oil Containment Capability Improved

Draw Down Due To Water Current

Flotation Member

Water

Weighted Skirt

Oil

Oil

FIGURE 1 Mechanical boom principle.

easily deployed lightweight booms are desirable However,

these desired properties are not necessarily consistent with

making booms stronger and more capable of withstanding

severe sea conditions

These are the criteria and mechanism of operation of oil

spill booms It is beyond the scope of this chapter to present

the many commercial and changing commercial products

In the world Catalog of Oil Spill Response Products, booms

have been divided into three categories based on maximum

operating signifi cant wave height (Hs) Table 1 shows the

ranges of freeboard and draft corresponding to the expected

maximum waves A boom size can thus be selected based on

the expected environment

In the World Catalog of Oil Spill Response Products,

booms have been divided into three categories based on

maximum operating signifi cant wave height (Hs) Table 1

shows the ranges of freeboard and draft corresponding to the

expected maximum waves A boom size can thus be selected

based on the expected environment

Boom Selection Matrix The selection of a boom depends on

how rapidly it is needed and how readily it can be utilized

Deployment speed and ease relate to the number of people,

the amount of time, and any special equipment (Winches,

etc.—even wrenches) necessary to move the required amount

of boom from storage to the launch site, to deploy it, and to

position it around the spill For example, self-infl atable booms

can be deployed very rapidly either from reels or bundles

Experience has shown, however, that this rapid response boom

should be replaced by a more rugged boom if extended

deploy-ment is required Thus, deploydeploy-ment ease must often be traded

off against ruggedness and durability

The matrix sho

optimum boom for a specifi c application since it indicates:

• Generic types of boom that are most suitable in a

given environment

• Selected booms that have the most needed

perfor-mance characteristics

• Choices with the most desirable convenience

features

Excess or reserve buoyancy is the surplus of fl otation over

boom weight as deployed, and is a measure of resistance

to boom submergence Wave response is a measure of

con-formance to the water surface and is usually improved by

increasing boom water-plane area and fl exibility Other char-acteristics should be evident from the headings

To use the matrix correctly, follow these steps:

1 Identify the most probable environmental condi-tions in which the boom will be used Note those types of booms with an acceptable rating (1 or 2)

2 Identify the most needed performance charac-teristics for the intended application From the booms chosen above, select the ones that have an acceptable rating (1 or 2) in the most important performance characteristics

3 Identify the most desirable convenience features

With booms from steps 1 and 2 above, select the boom with the best rating in the convenience fea-tures of interest

These data (T permission of EXXON from a very informative OIL SPILL RESPONSE FIELD MANUAL by Exxon Production Research Company published in 1992

Recovery of the Spill by Sorbing Agents A most direct

manner of physically removing the spilled oil is by use

of sorbents These materials are buoyant, and preferen-tially wetted by and adsorb oil In essence, they permit this sorbed oil to be physically “picked up” from the water

In addition to making the collection of oil an easier task, the oil is prevented from spreading and remains as a more congealed mass

Materials that have been found useful for this service vary from simple, naturally occurring materials such as straw, saw-dust, and peat to synthetic agents, such as polyurethane foam and polystyrene powder The oil pickup capability varies greatly For example, values of oil pickup, i.e., weight of oil sorbed per weight of adsorption material, have been reported

by Struzeski and Dewling 7 for straw as 3 to 5, although higher values have been reported Polyurethane foam, by comparison,

is capable of oil pick up values of 80 A complete investiga-tion of sorbents for oil spill removal has been published by Schatzberg and Nagy. 8 Of interest is the variation in the oil pickup capability of a given sorbent based on the type of spilled oil For example, in Schatzberg’s controlled tests, oil pickup by straw was 6.4 for heavy crude oil and 2.4 for light crude oil For urea formaldehyde foam, however, oil pickup was 52.4 for heavy crude and 50.3 for light crude Also some

TABLE 1 Boom Classification

Environment

ft meters inches centimeters inches centimeters

wn in Table 2 can be used to select the

able 1 and 2) were extracted and used with the

TABLE 2 Boom Selection Matrix

Legend 1—Good 2—Fair 3—Poor

Type of Boom

Internal Foam Flotation Self-Inflatable Pressure-Inflatable

External Tension

Environmental Offshore

Notes:

* Hs ⫽ Significant Wave Height.

* V ⫽ Velocity of Surface Current.

Not all the booms of a particular generic type have the rating shown in the matrix But at least one or more commercially available booms of the generic

type in question have the rating shown.

* Specially-designed high-current models may be available (river boom)

sorbents are much less effective for oil adsorption if contacted

by water prior to application to the spill

Although highly effective sorbents are available as

noted above, techniques for harvesting (recovering) the oil

soaked sorbent have been limiting For example, there have

been prior instances of oil soaked straw recovery by manual

pickup with pitchforks However, there is development

work underway to mechanize this step as well as the

appli-cation procedure In this regard, some very practical

obser-vations on the use of sorbents have been made by an IMCO

subcommittee on Marine Pollution This guidance manual

outlined that “the use 9 of absorbents involves six basic

oper-ations, the supply, storage, and transportation of the

mate-rial and then the application, harvesting and disposal of the

contaminated absorbent.” The manual further observes that

some of the early applications of sorbents such as the Torrey

Canyon and Santa Barbara suffered because of the lack of

effective and effi cient harvesting techniques

More recently, since the early 1990s a new approach to

oil pickup was conceived by the use of SOLIDIFIERS

Solidifi ers are products which, when mixed with oil,

turn the oil into a coherent mass They are usually available

in dry granular form Unlike sorbents that physically soak

up liquid, solidifi ers bond the liquid into a solid carpet-like

mass with minimal volume increase, and retain the liquid for

easy removal The bonded material also eliminates

dripping-sponge effect by not allowing the material to be squeezed

out, minimizing residue or contamination Some polymers,

in suffi cient quantity or of high molecular weight, can

actu-ally convert the oil to a rubber-like substance

Solidifi ers are most commonly used during very small

oil spills on land or restricted waterways to immobilize

the oil and enhance manual recovery There has been little

documented use of solidifi ers on large spills or open water

However, the possibility that they may reduce the spread of

waterborne oil by solidifying it and increase recovery and

removal rates is a concept with signifi cant potential benefi t

The effectiveness of a solidifi er is based on the amount

of product and time it takes to “fi x” a given volume of oil

The less effective products require larger amounts to solidify

oil Fingas et al (1994) presented results from effectiveness

tests on various solidifi ers and found that generally between

13–44 percent by weight of the product to oil was required to

solidify Alberta Sweet Crude over a 30-minute period

The entire treatment of solidifi ers as an aid to oil spill

response is well covered in an MSRC publication. 10

Physical Removal of the Contained Oil by Oil Pickup

Devices Since oil containment booms have a fi xed capacity

for oil spill containment, it is important to consider means

to physically remove the contained oil from the surface The

use of sorbents has been discussed An alternate approach is

to remove the fl uid oil by means of skimming devices

Oil skimmers have been divided into fi ve categories: 11

brushes, either acting independently, mounted on

a vessel or used in combination with a boom)

• Weirs (simple, self-leveling, vortex assisted, auger assisted, vessel-mounted, and weir/boom systems)

• Vacuum units (portable units and truck-mounted units)

• Hydrodynamic devices (hydrocyclone and water jet types)

• Other methods (including paddle belt and net trawl)

The selection of the optimum skimmer for a particular spill is based on site conditions such as the sea state and characteristics of the spilled oil e.g viscosity and emulsion-forming tendency

There are over 100 commercially available skimmers

on the market that fall within the generic types previously mentioned These are summarized in publications such

as the WORLD CATALOG OF OIL SPILL RESPONSE PRODUCTS

For example, for the principle of oleophilic surfaces, these can comprise either a sorbent belt, an oleophilic rope

or a solid oleophilic disc that rotates through the surface oil

fi lm In heavy sea conditions this type would be more effec-tive than a wier type that is more suited to protected in-shore areas

Controlled Burning of the Spilled Oil Burning represents

a surface treatment of an oil spill that is attractive in that the oil is essentially removed from the water However, some

of the negative aspects of this approach that have hampered its widespread acceptance and use may be summarized as follows:

In many spill instances, there is an obvious concern regarding the combustion of the oil for safety reasons Spills near harbors, tankers, offshore platforms would create an obvious hazard if set afi re

A minimum thickness of oil is required to establish combustion

Air pollution is a concern in some instances There is continuing evaluation and development burning of agents

As reported by Alan Allen, 12 there are fi re retardant booms and ignition methods available to burn the oil under proper conditions e.g oil fi lm thickness and amount of emul-sifi ed water in the oil An effective burn after the EXXON VALDEZ spill on Sat March 25, 1989 was reported by Allen

in this publication

The very encouraging burn rate statistics suggest that only 2% of the original relatively fresh oil remained as residue

In this regard, it is relevant to quote the author of this publication in its entirety because of its concise and suffi -cient analysis of this technique by one well recognized in this method

“It should be recognized that the elimination of spilled oil using in-situ burning must be considered in light of the full range of potential impacts (safety, air quality, etc.) associated with the burning of oil an water The mechanical removal of spilled oil is by far the preferred cleanup technique whenever possible Burning, on the other hand, may provide a safe,

effi cient and logistically simple method for eliminating oil

under certain conditions As a backup for mechanical cleanup

techniques, in-situ burning can provide a useful means of

elim-inating large quantities of oil quickly, while avoiding the need

for recovered oil storage containers Anyone considering the

use of burning should be sure that all regulatory controls have

been satisfi ed, that the ignition and burning operations can be

carried out safety, and that the temporary reductions in local air

quality represent the lower of all other environmental impacts

should the spilled oil not be burned.”

TREATMENT OF THE SURFACE OIL

Chemical Treatment of Surface Oil Should Be

Considered as an Alternate Solution

It is generally agreed, as indicated above, that situations can

arise where the spill cannot be contained and recovered because

sea conditions, weather state, and so on, are beyond the

cur-rent operating capability of containment devices There are

also instances wherein the logistics of containment and

recov-ery equipment, that is, containment boom availability and/or

deployment time and effort, could indicate chemical treatment

as the most practical and expedient handling technique When

physical recovery of the oil pollutant is impractical, there are,

in effect, two courses of action possible In one case, the oil

may be permitted to remain as intact cohesive slick on the

surface of the water and possibly reach shore The alternate

course is to “treat” this surface oil—such treatment essentially

directed toward the removal of the oil from the water surface

and the enhancement of its ultimate removal from the

environ-ment This many be accomplished by chemical dispersion

The Ecological and Economic Damage Caused by an

Untreated Oil Spill Can Be Extensive

The damage resulting from an untreated oil spill is both

visu-ally apparent and extensive It encompasses both biological

as well as property damage The potential damage may be

summarized as follows:

Marine fowl, particularly diving birds, are particularly

vulnerable to an oil spill As reported by Nelson-Smith, 13

sea birds are most obvious victims of an oil spill due to

“mechanical damage.” The oil penetrates and clogs the

plumage which the bird depends upon for waterproofi ng and

heat insulation For example, a duck with oil-impregnated

plumage is under the same stress at a moderate temperature

of ⫹59⬚F as a normal bird would be under a more severe

temperature condition at −4⬚F Some statistics regarding bird

birds, mostly guillemots and razorbills, were killed after the

Torrey Canyon grounding The guillemot casualties equaled

the entire breeding stock between the Isle of Wright and

Cardigan Bay Bird losses in the Santa Barbara spill,

accord-ing to the state Department of Fish and Game, totaled 3500

Shore contamination by beached oil represents

biologi-cal, as well as property damage The tendency of oil to cling

to shore surfaces, such as beach sand, sea walls, and the resul-tant property damage, are well established This is perhaps the most apparent and widely publicized damaging aspect as attested by lawsuits on the part of tourist interests, property owners, etc There is also, in a biological sense, a physical smothering effect on some attached, intertidal organisms such as mussels and barnacles The effects of untreated oil

coming ashore is well illustrated by Blumer et al 15 regarding

a No 2 diesel fuel spill from the barge Florida in Buzzards Bay, Massachusetts in September 1969 Oil was incorpo-rated into the bottom sediment to at least 10 meters of water depth, testifying to the wetting effect of untreated oil in this instance, the oil was physically dispersed by the heavy seas but retained its adhesive characteristics Therefore, it

is deduced that the oil droplets probably came into contact with and wetted and upswept, suspended particulates which later settled again to the bottom Other spill instances depict-ing the importance of this aspect that of the incorporation of oil into the sediment-have been reported by Murphy. 16 In the Buzzards Bay and several other spill incidents of distillate fuels cited by Murphy, there has been a signifi cant kill of all marine life in the area since these highly aromatic products are known to be much more toxic than whole crude oil

Persistent tarry agglomerates are formed as the spilled oil weathers at sea There has been increasing attention directed

to the presence of tar-like globules ranging up to 10 cm in diameter in the open sea As reported by Baker 17 during the voyage of Thor Heyerdahl’s papyrus boat, Ra, during fi ve separate days, they sailed through masses of these agglomer-ates whose age could be substantiated by the growth of goose barnacles adhering to them There have been other inci-dents reported recently by the International Oceanographic

the research craft, R.V Atlantis, as reported by Horn et al 19

In this latter investigation, tarry agglomerates were present

in 75% of over 700 hauls with a surface skimming (neuston) net in the Mediterranean Sea and eastern North Atlantic The amount of tar in some areas was estimated at 0.5 milliliter in volume per square meter of sea surface

The Behavior of Spilled Oil at Sea

Before consideration of the mechanism of dispersing oil and its associated effects, an understanding of the behavior of spilled oil at sea will be useful When a volume of oil is spilled onto the surface of water, the oil has a driving force

to fi lm out or spread-in essence, a spreading pressure usually

expressed as a Spreading Coeffi cient This Spreading S o/w ,

is readily quantifi ed and is determined by a balance of the surface tension forces as follows

So/w⫽gw⫺go/w⫺go,

wherein:

S o/w is the spreading coeffi cient for oil on water ergs/cm 2 or dynes/cm

γ γ γ

o/w

so/w

Water Oil

= Measured Value For Kuwait Crude Oil On Sea Water

= 11 Dynes/cm

FIGURE 2 The spreading behavior of spilled oil.

Film Thickness Inches x 10–6

Appearance

Of Film

Approx

Gals./Sq Mile

Oil Spill

Water Column

FIGURE 3 Oil slick appearance during spreading.

g w is surface tension of water, dynes/cm

g o is surface tension of oil, dynes/cm

g o/w interfacial tension of oil and water, dynes/cm

By an e

it can be seen that if S o/w —the resultant spreading force

is positive, the oil will spread on the water; if negative, it

will not spread but remain a “lens” of Liquid For

exam-ple, spreading coeffi cient values for Kuwait Crude on sea

water, reported by Canevari 20 are positive and confi rm that

for this system the oil readily spreads on the water phase

vari-ous oils on sea water that vary from 25 to 33 dynes/cm

Cochran 22 has also published values that generally agree

for positive spreading coeffi cients, the oil is capable of

fi lming out to very thin fi lms A fi lm thickness of only

3.0 ⫻ 10 −6 inches representing a spill of 50 gallons of oil

distributed over a surface area of one mile will be quite

visible as a “fl at” silver sheen on the surface of the water

However, the initial spreading rate of a large volume

of spilled oil is based on the volume and density of the oil

in essence, sort of static head that overcomes other factors

such as interfacial tension

The Mechanism of Dispersing Surface Oil Slicks by Chemical Dispersants

The dispersion of surface oil fi lms as fi ne oil droplets into the water column is promoted by the use of a chemical dis-persant This oil spill dispersant consists primarily of a surface active agent (surfactant) and a solvent The solvent is added

as a diluent or vehicle for the surfactant It also reduces the viscosity and aids in the uniform distribution of the surfac-tant to the oil fi lm

A surfactant is a compound that actually contains both water compatible (hydrophilic) and oil compatible (lipophilic) groups Due to this amphiphatic nature, a surfactant locates and arranges itself at an oil–water interface as schematically shown

in Figure 4 The surfactant’s molecular structure, e.g ratio of hydrophilic to lipophilic portion, determines the type of disper-sion (oil droplets dispersed in water phase or water droplets dispersed in oil phase), as well as stability of the dispersion In essence, a surfactant that is principally water soluble disperses oil-in-water and established water as the continuous phase; a surfactant that is principally oil soluble, the converse This is Bancroft’s Law, 23 which has been tested and proven empiri-cally true over the years A convenient classifi cation for sur-factants therefore, is based on the ratio or balance of the water

FIGURE 4 Influences of surfactant structure on type of dispersion.

HYDROPHILIC-LIPOPHILIC BALANCE (HLB)

SCHEMATIC OF

SURFACTANT

TYPE OF

EMULSION

FORMED

DISPERSE WATER DROPLETS

DISPERSE

OIL

Increase In Oil Solubility

Increase In Water Solubility

Hydrophilic Group (Water Compatable) Lipophilic Group (Oil Compatable) Oil Soluble Surfactant

Favors Water-In-Oil Dispersion

Water Soluble Surfactant Favors Oil-In-Water Dispersion

xamination of the force balance shown in Figure 2

with these level on sea water As one can see from Figure 3,

compatible portion to the oil compatible portion-sometimes

referred to as HLB (Hydrophilic–Lipophilic Balance). 24

This relationship between the molecular structure of the

the physical concept behind Bancroft’s Law may be

appreci-ated For example, it can be visualized that for a more water

compatible surfactant, the physical location of the larger

hydrophilic group on the outside of the dispersed oil

drop-lets results in a more effective “fender” to parry droplet

colli-sions and prevent droplet coalescence The converse, location

and the larger portion of the surfactant in the dispersed rather

than the continuous phase, would be geometrically awkward

and unstable. 25 The mechanism of oil slick dispersion by the

application of chemical dispersants has been covered in some

detail by Poliakoff 26 and Canevari, 27,28,29 among others From

the above discussion, one can see that the chemical

disper-sant (surfactant) will locate at the oil–water interfaced reduce

interfacial tension This will then act to increase the spreading

tendency of the oil fi lm as shown by Eq (1) More important,

it promotes fi ne droplet formation which can be expressed as:

Wk⫽Ao/wgo/w,

where:

W k mixing energy, ergs

A o/w interfacial area, cm 2

γ o/w interfacial tension, dynes/cm

Thus, for the same amount of mixing energy, a reduction of

γ o/w will result in a corresponding increase in A o/w

It is important to emphasize that, as can be realized from

the above discussion, the chemically dispersed oil does not

sink Rather, the surfactant merely enhances small droplet

formation for a given amount of mixing energy Smaller

diameter oil droplets have a much lower rise velocity per

the familiar Stokes Law Hence, once the oil is chemically

treated, and placed 3 to 5 feet below the surface of the water

by the mixing process, it does not rise to the surface as

There are many surfactants that will aid the formation

of fi ne droplets in the above manner It has already been

noted that the surfactant structure (Hydrophilic–Lipophilic

Balance) infl uences the effi ciency of the emulsifi er

However, a more subtle and less tractable requirement

for an effective dispersant is the prevention of droplet

coalescence once the fi ne oil droplets are formed This is

dispersed by a chemical surfactant and maintained in

sus-pension by gentle bubbling of air After 24 hours, there has

been no coalescence or separation of these fi ne oil

drop-lets In the control sample, with similar volume of oil and

mixing energy, the oil separated almost immediately and

reformed an intact, cohesive fi lm of oil

In essence then, an effective dispersant must parry

drop-let collisions physically For example, dispersed oil may

separate in a sample bottle but even though there may be

a “creaming” effect, i.e oil droplets concentrate near the surface, the droplets should not coalesce to reform an intact slick It is this same “fendering” action that reduces the ten-dency of the droplets to stick to a solid surface

The Physical and Environmental Incentives for Dispersing Oil Slicks

Consideration of the previous summary of the potential damaging aspects of an untreated and unrecoverable oil spill indicates that the removal of the intact, cohesive mass of oil from the surface of the water yields more than a cosmetic effect as is often claimed For this alternate approach when conditions do not permit the recovery of the spilled oil, the removal of oil from the surface by dispersing it into fi ne droplets yields established benefi ts that can be summarized

by the following discussion:

1) Oil properly dispersed with a chemical dispersant will not stick to a solid surface As previously out-lined, the physical fending action of a properly selected surface-active agent prevents the oil drop-lets from coalescing after dispersion This same property also inhibits the oil from wetting out on

a solid surface This has become a controversial point and it has actually been claimed that the con-verse is true For example, in the First Report of the President’s Panel on Oil Spills, 30 it has been stated that such agents cause the oil to “spread into the sand-surfaces which untreated oil would not wet.”

A laboratory experiment was conducted to evaluate this aspect A mixture of 256 cc of sea water, 95

cc of beach sand (New Jersey shore area), and 20

cc Kuwait Crude, were placed in a graduate This represented a vertical cross section of the marine environment after an oil spill The mixture was then agitated to simulate the possible contact of sedi-ment by the oil when turbulent conditions existed

After mixing, the sample was settled to separate the oil–sand water phases In a body of water, either the oil may be driven down into contact with the sandy bottom or the sand may be suspended in the body

of water by wave action, such as deduced from the previously cited Buzzards Bay spill The graduate was then purged with clean water to simulate the return of the environment to a non-contaminated condition

The experiment was then repeated using 20 cc

of Kuwait Crude Oil and 4 cc of a chemical dis-persant (5 parts oil/1 part disdis-persant)

Virtually no “treated” oil impregnated the sand

For the experiment with the untreated crude oil, an analysis of the oil content of the sand bed indicated that 11.20 cc of oil remained of the initial 20 cc

2) Oil removed from surface water prevents bird damage The aforementioned hazard to marine fowl that is presented by the surface oil film is

surfactant and the emulsion type is also shown in Figure 4 and

ily, as illustrated by Figure 5

Oil Droplets

Dispersant Dispersant Prevents Coalescence Of Droplets

FIGURE 6 Dispersant maintains oil droplets in suspension with mild agitation

a) Oil Spill

b) Dispersant Reduces Interfacial Tension

c) Agitation Readily Forms Oil Droplets

Mixing Prop

Water

Fine Oil Droplets

Water Soluble Oil Soluble Water

Surfactant

Water Oil

γ

o

w

FIGURE 5 Dispersant enhances droplet formation.