• Authors
  • School of Biological Sciences, Bangor University, Gwynedd, UK
  • Istituto per l’Ambiente Marino Costiero (IAMC) – CNR, Messina, Italy
  • School of Environment and Natural Resources, Bangor University, Gwynedd, UK
  • Environmental Microbiology Laboratory, HZI-Helmholtz Centre for Infection Research, Braunschweig, Germany
  • Editor
  • Environmental Microbiology Laboratory, Helmholtz Centre for Infection Research, Braunschweig, Germany

Oil Tanker Sludges and Slops


Despite international agreements, economic interests, and increasing environmental protection awareness, oil transportation in the third millennium is a dangerous business causing many cases of pollution. It is a paradigm for the negative effects of globalization, as has been proven by a product tanker in 2006. Following an odyssey, which took her from Europe to Africa, a gasoline tanker entered the port of Abidjian, Ivory Coast on 19 August 2006. Her presence and activities in the port of Abidjian claimed 16 lives and caused more than 40,000 cases of poisoning.

1 Introduction

The voyage of the Panama-registered tanker vessel began from Gibraltar on 26 June 2006. While visiting the port of Amsterdam 1 week later, the tanker was chartered by a Dutch trading company, which attempted to process waste and tanker slops by the port reception facilities. During the inspection of the tanker slops, an immense smell was reported by the waste disposal company's staff. As a result, the port-based company refused to dispose the waste (Bernard et al., 2006; Franoz and Gesret, 2008). An analysis of the tanker slops in the aftermath of the Abidjan catastrophe showed that the gasoline tanker indeed was used as a "swimming refinery" (Persson, 2006). On her last voyage prior to the visit in Amsterdam, the vessel was transporting naphtha, a gasoline precursor containing significant amounts of mercaptans - sulfur-containing hydrocarbons. As these sulfur-containing hydrocarbons should be removed to meet the specifications for gasoline in certain European countries, a chemical process called "Merox" was conducted. By adding specific catalysts, water, and sodium hydroxide to the gasoline, mercaptans are oxidized to disulfides. The products of Merox are upgraded gasoline and sludge containing sulfuric components (Wikipedia, 2006). In the case of the tanker vessel, this chemical process failed due to the addition of too much sodium hydroxide and resulted in a highly toxic sludge containing mercaptans and hydrogen sulfide (Fig. 1 ).
Figure 1 Chemical reactions of the Merox (mercaptan oxidation) process.
After failing to dispose this toxic waste, the charterer attempted to arrange the disposal of the toxic slops using a Dutch waste disposal company, which quoted 500,000€ for this service (Bernard et al., 2006; Persson, 2006). Unwilling to pay this fee, the gasoline tank was ordered to continue its voyage to the Estonian port of Paldiski, where it was bunkering gasoline bound for Lagos, Nigeria. The gasoline was loaded on top of the toxic sludge. After unloading the gasoline in Lagos, the charterer attempted once again to dispose the toxic waste, but decided to keep the slops in the ship because the Nigerian company seemed to intend to refine it and sell it as fuel. As a result, the tanker vessel was called to the port of Abidjan, Ivory Coast. Although Abidjan was a modern port with adequate port reception facilities, it did not provide a slop disposal service. This work was performed by a newly incorporated waste disposal company. This company offered the disposal of the slops at a price of US$ 35 per cubic meter, 20 times less than the price offered in the port of Amsterdam (Bernard et al., 2006). The tanker discharged the slops and continued its voyage back to Paldiski (Fig. 2 ).
Figure 2 Route of the Panama-registered and Dutch-chartered tanker vessel involved in the 2006 Côte d'Ivoire toxic waste spill.

A few weeks later, citizens of Abidjan started to complain about previously unknown symptoms of diseases following heavy rains. Some suffered from nosebleeds, vomiting, headaches, and rashes. Most of them described the water with a smell of rotten eggs or garlic that was running through the streets of Abidjan. More than 40,000 Ivorian citizens showed symptoms described above, 69 were taken to hospitals and 16 died (Bernard et al., 2006).

Although the charterer claimed only to have handed regular tanker slops to the Abidjan-based waste disposal company, analysis showed that indeed the highly toxic sludge also was transferred to a barge and subsequently into tanker trucks (Bernard et al., 2006). The disposal company, obviously overstrained with the disposal of toxic waste, dumped 500,000 l of toxic waste at 14 locations in and around Abidjan, i.e., lakes, rivers, fields, or simply in roadside trenches (Bernard et al., 2006; Franoz and Gesret, 2008). Truck drivers claimed that they were told to "dispose dirty water" by their managers (Bernard et al., 2006; Franoz and Gesret, 2008). Following the heavy rains, the waste was slowly distributed over the whole rural and municipal area of Abidjan, a city with a population of three million.

In the aftermath, many top government officials resigned (Bernard et al., 2006). Two executives of the charter company offered assistance, but were arrested in Ivory Coast upon arrival. The charterer ultimately paid US$ 198 million for compensation and cleanup (Bernard et al., 2006; Franoz and Gesret 2008) (Fig. 3 ).
Figure 3 Sampling on a spill site in Abidjan (right) (From CEDRE).

Although this incident was an extremely severe case of tanker slop dumping, it displays an alarming trend of global trade. Corrupt officials and administrations often are eager or forced into offering inadequate waste disposal for enterprises based in industrialized countries. In 1988, the government of a West African country offered waste disposal including radioactive and chemical waste for US$ 1.6 million down payment and 30 years of economic aid (Knauer et al., 2006). The deal was canceled after a public row and media opposition. In parallel to the illegal disposal of tanker slops in ports as presented above, tanker slops often are disposed directly into the sea.

2 Oil Tanker Sludges and Slops

Tanker slops are result of the need for tank washing when changing the type of cargo after a trip. Oil residues remain on the tank's sides and bottom. Contamination of the refined hydrocarbons particularly occurs when changing from a crude oil to a refined product. In the first 10 decades of tanker shipping, tanks were routinely washed with seawater and detergents, the washings were combined in a tank until a phase separation occurred and the aqueous phase dumped overboard. This measure leads to immense oil pollution and therefore was outlawed (King, 1956). Two other measures - the Load On Top and the Crude Oil Washing (COW) - were implemented and have led to a drastic decrease in oil pollution (Hampton et al., 2006; Yvonnou, 2001). Both measures are presented in Essay 00527. COW reduces the amount of oil spilled during tanker operations to a minimum, since the oil tanks are washed with crude oil that dissolves the residues.

But if COW has reduced this marine oil pollution to zero, why do tanker slops still occur and why do they pose a threat?

As described in Chapter 16, Vol. 1, Part 3, tanker slops are a result of the need to fill some oil-contaminated tanks with seawater as ballast in order to stabilize the ship at sea and prevent capsizing in heavy seas.

The first measure to prevent this was tank washing and subsequent loading of cargo oil on to the washings. The amount of slops was reduced to 300-800 m3 on large tankers transporting more than 320,000 m3 of oil. In most cases, refineries accepted the slops and used them for refining purposes. Refineries, ship owners, and charterer benefited from this arrangement. However, the need for highly refined products finally stopped the use of this method in the 1980s (Hampton et al., 2006; Yvonnou, 2001). COW was implemented and reduced slops from 800 to 250 m3 on large vessels. To reduce tanker slops to zero, modern tanker designs include segregated ballast tanks (SBT). SBT tankers are highly advantageous, because they are considered "ecological" vessels that produce no slops. On the other hand, refineries receive a neat product from these tankers: oil not contaminated with seawater, chemicals, or oil residues. Oil and water cannot come into contact on such SBT tankers, and still the ship's seaworthiness is maintained. Since furthermore the tanks are washed using crude oil, there should, in theory, be no further slops occurring.

However, in the real world, two situations cause slops on all tanker vessels including SBT vessels. The first situation is the compulsory washing of tanks prior to maintenance being carried out on tanks, pipes, or valves either on board or in shipyards. The tank washings in theory should be collected, settled, and kept. In the best case scenario, these slops can be separated via a water or oil separator. Oil is retained and the water can be disposed of. In modern vessels the purity of disposed water furthermore is controlled by detectors fitted to the seawater exit valves. However, these control systems can be bypassed and the slops can be directly dumped into seawater (See Chapter 16, Vol. 1, Part 3).

The second situation that might produce tanker slops is due to a safety measure for heavy sea and stormy weather. The SBT in an SBT tanker's hull are designed to maintain the seaworthiness of a tanker at regular conditions. In a 270,000 dead weight tons (dwt) tanker, the ballast tanks may hold 60,000-70,000 t of seawater. In storm conditions such as hurricanes, the amount of ballast water is increased to 100,000-150,000 t on the same vessel. The tanker has a deeper draught and is less susceptible to sheering forces due to wind or waves. Therefore, the forces affecting the 300-350-m long ship's hull are reduced and thus the risks of damage or, in worst case, sinking. The additional ballast water, however, has to be transferred to some of the oil-polluted cargo tanks. This emergency measure leads back to the initial problem of tanker slops and raises the question what might happen to slop in the cargo tanks when it comes to dispose of the emergency ballast water (Hampton et al., 2006; Yvonnou, 2001).

But even if an SBT tanker, applying COW, is operated in proper manner attempting to minimize pollution, slops might still occur. Firstly, certain oils contain high amounts of paraffin or wax. Like all oils transported in tanker vessels, these oils are heated in order to maintain a low viscosity of the product. Relatively low temperatures of the tank's walls lead to the deposit of waxy residues, which cannot be reheated and remain in the tanks after unloading. Secondly, a tanker vessel needs a good stern trim and properly maintained COW pumps in order to reduce the slops. If the inclination of a tanker to the stern or bow is too severe, COW pumps cannot produce enough suction to perform at optimum efficiency or reach oil residues collected in distant parts of the tank (Hampton et al., 2006; Yvonnou, 2001). Furthermore, technical equipment such as pumps are constantly stressed and endangered by corrosion on the high seas, reducing their performance and leading to the production of tanker slops. As described above, tanker slops are not simply limited to crude oil carriers, but also occur in product tankers such as the vessel involved in the 2006 Côte d'Ivoire toxic waste spill. Considering the chemical reactions potentially conducted inside the ships' cargo tanks during voyages, it can only be assumed that how many other cases of comparable toxic slops are produced. Following the 2006 incident, no further accidents of this kind have been reported. However, catalysts and caustic soda for the Merox process are freely available and can also be applied for upgrading liquid petroleum gas (LPG), which is frequently transported by specialized LPG tankers. Perhaps the 2006 Côte d'Ivoire incident has scared tanker owners from making further attempts of on-board processing of oils. Nevertheless, the opportunity and the economic benefit to perform it remain.

3 Research Needs

Unfortunately, the occurrence of slops in the operations of tankers is inevitable. A number of technical methods to avoid or minimize slops have been invented and are currently employed. For the slops that nevertheless occur, proper disposal is possible in most ports of call. However, as we have presented above, this method of disposal is expensive and the urgent need for it can be abused by waste disposal companies. This economic pressure subsequently leads to the temptation of illegally discharging or dumping slops.

The only possibility of preventing dumping of oil slops is the routine surveillance of tanker routes and prosecution of identified polluters. Many industrialized countries have developed efficient systems of identifying and prosecuting ships suspected of dumping slops together with methods of assigning oil pollution to the individual vessel.

For surveillance, highly specialized airplanes are used for regular reconnaissance flights on frequently used shipping routes. To patrol North and Baltic Sea the German Air Force uses four Fairchild-Dornier DO 228 LM airplanes equipped with side-scanning radar, IR/UV line scanner, LASER-fluoro-scanners, and microwave radiometers. Their equipment is able to identify the type of oil spilt and measures the thickness and extent of oil slicks while in flight. Ships causing pollution, therefore, can be easily identified and countermeasures coordinated. Typically, samples will be taken by coast guard ships and identified by automated gas chromatography. This method enables the authorities to receive a "fingerprint" - a qualitative analysis of the components of the oil spilled (Dahlmann, 2003; Wikipedia, 2008). A ship spotted while dumping slops or discharging contaminated water will be apprehended by the coast guard and forced to call at the nearest port. In case of an oil slick floating on the ocean's surface, sampling and subsequent analysis data enable authorities to compare the pollution to ballast water samples taken from suspicious ships (Fig. 4 ).
Figure 4 A Do 228 LM - reconnaissance airplane (©Bundeswehr/Cherin Hellmich).

Reconnaissance flights are routinely performed over shipping lanes of the North and Baltic Seas.

The HELCOM treaty, signed by states bordering the Baltic Sea, mandates the signatory countries to collaborate together and conduct regular monitoring of the Baltic, and thus gives rise to increased levels of surveillance. This treaty was signed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, and Sweden in order to prevent ship collisions, eutrophication, and oil pollution of the Baltic Sea.

Aerial surveillance of the seas around European countries is a relatively simple task, as shipping routes are short, close to land, and traffic-dense. Furthermore, the industrialized European states have appropriate funds and large air forces/navies to undertake aerial surveillance duties. An equivalent system exists in Japan, the USA, and Canada, though shipping routes surrounding these countries are longer and traffic spread over large distances, aerial reconnaissance is more difficult. States with smaller funds or less strict environmental policies cannot offer this facility or prosecute the polluters. In some cases, corruption can be a major problem in the enforcement of environmental policies.

But even if aerial surveillance is performed, airplanes and their detection systems rely on favorable weather conditions and sunlight. In stormy weather or at night, surveillance flights are difficult and inefficient. In combination with the large number of "black spots" on the map without appropriate surveillance, there are many opportunities for potential polluters to dump tanker slops or to discharge the toxic waste. A solution to the protection of the blue vastness of the ocean lies in the black infinity of space; in the last few decades, a significant number of satellites equipped with specific infrared (IR), microwave RADAR, and other cameras have been launched and are available for aerial reconnaissance. The satellite's sensitive cameras are capable of performing a similar role to reconnaissance aircraft, are cost-effective, and can simultaneously survey much larger areas. Since they can even survey the shipping routes around countries, which cannot perform other measures of detecting oil pollution, satellite-based surveys may be a solution to this problem (Brekke and Solberg, 2004).

However, aerial reconnaissance, oil analysis, and prosecution of illegal dumping only treats the symptoms rather than the cause. In the aftermath of most recent cases of oil pollution due to tanker slops or ballast water, the consequences have been similar. In general, ships were detained in a port until a trial for pollution has started. In many cases, the ship's senior officers were sentenced to imprisonment, faced a financial penalty, or loss of licenses and thus their jobs. The ship's charterers were sentenced to pay fines ranging from US$10,000 up to 500,000 (CEDRE, 2009). However, in an industry under the high pressures of transportation costs and delivery dates, the risk of paying such fines can be acceptable from an economic point of view.

In order to limit oil pollution, the European Union (EU) implemented the directive 2000/59/EC on the basis of a "polluter pays principle": port reception facilities had to be provided by any harbor in the EU. The additional costs for this service were included in the harbor dues or charged as a separate fee. Ships that cause less pollution benefited from this system since reduced fees were charged for lower emissions. As a result, several harbor dues schemes including a free slop disposal service within the fees were set up by a number of port authorities. This free service was readily accepted and significantly reduced the pollution in areas such as the North Sea. However, in recent years, strong competition between the European port companies has occurred. In many cases, slop and waste disposal was performed by subcontractors rather than the port authorities and companies. Shipping companies, being offered several ports in similar locations, such as Antwerp, Esbjerg, Felixstowe, Rotterdam, Amsterdam, Bremerhaven, or Hamburg were able to call at the port charging the lowest harbor dues. The dues therefore had to be reduced to maintain the utilized capacities and thus the free disposal service had to be canceled in many cases. Furthermore, the slop disposal during berthing at a specific port was not mandatory according to the EU directive (Georgakellos, 2007).

However, this example of "free" disposal service shows a potential solution to the problem of tanker slops. Effective prosecution of polluters and draconian penalties for illegal dumping may be good deterrents, but can only act after pollution has already occurred. It is a negative control mechanism, which is considered as an obligation. Moreover, the lack of a mandatory slop disposal at every port of call leaves the decision of disposal to the ship's owner or charterer. Analysis of waste disposal figures for EU port reception facilities for 2000 and 2001 showed approximately 12% of liquid wastes generated in berthed ships that were actually disposed of at their reception facilities. This implies that the current system of waste disposal in the EU is either not strict enough or does not offer satisfactory incentives or deterrents for the ship's charterers and owners in order to dispose slops safely. Illicit discharge or inadequate waste disposal in developing countries therefore cannot be prevented with the current system of harbor dues (Georgakellos, 2007).

A mandatory, free disposal service, which is partly refunded by harbor dues, offers a compromise. Costs for the disposal thus should be shared by charterers, ship owners, and port management together with the consumers of the transported goods. Another important factor is the global implementation of such disposal services. If mandatory disposal fees combined with mandatory disposal at every port are a constant factor in the accounts of all charterers and ship owners, there is no further interest in avoiding disposal or illegal dumping of slops.

Eliminating the mechanism of the free market in environmental issues such as tanker slop disposal could be a very efficient method of significantly improving the situation of maritime oil pollution. It is nevertheless a long and hard path to follow, but considering the consequences of the 2006 Côte d'Ivoire incident, i.e., more than 40,000 victims and a clean-up costs of US$ 200,000,000 in a single case - it might be a path worth traveling on.


  • Bernard F, Follorou J, Stroobants JP (2006) How Abidjan Became a Dump. Guardian Weekly 20 October 2006.
  • Brekke C, Solberg AHS (2004) Oil spill detection by satellite remote sensing. Remote Sens Environ 95: 1-13.
  • CEDRE (2009) Discharge at sea: Illicit discharge. Accessed on 16/1/09 from http://www.cedre.fr/uk/discharge/ill_disch/cas.htm.
  • Dahlmann G (2003) COSI - computerized oil spill identification. Bundesamt fuer Schifffahrt und Hydrographie, June 2003.
  • Franoz B, Gesret S (2008) The Probo Koala. Information days of Cedre, 18 March 2008, INHES, Saint-Denis-La-Plaine (Paris).
  • Georgakellos GA (2007) The use of the deposit-refund framework in port reception facilities charging systems. Marine Pollut Bull 54: 508-520.
  • Hampton S, Kelley PR, Carter HR (2006) Tank vessel operations, seabirds, and chronic oil pollution in California. Mar Ornithol 31: 29-34.
  • King GAB (1956) Tanker practice. The construction, operation and maintenance of tankers. Wokingham: Maritime Press.
  • Knauer S, Thielke T, Traufetter G (2006) Profits for Europe, industrial slop for Africa. Der Spiegel 38/2006.
  • Persson M (2006) Petrochemie met de handboeken dicht. De Volkskrant 2 October 2006.
  • Wikipedia contributors (2006) Côte d'Ivoire toxic waste spill [Internet]. Wikipedia, The Free Encyclopedia 2008 December 27, 21:03 UTC. Accessed on 13/1/09 from http://en.wikipedia.org/w/index.php?title = 2006_C%C3%B4te_d%27Ivoire_toxic_waste_spill&oldid = 260376418.
  • Wikipedia contributors. Dornier Do 228 LM. Wikipedia, The Free Encyclopedia 2008 Jul 14, 10:59 UTC. Accessed on 13/1/09 from http://de.wikipedia.org/w/index.php?title = Dornier_Do_228_LM&oldid = 48367460.
  • Wikipedia contributors. Merox [Internet]. Wikipedia, The Free Encyclopedia; 2008 December 27, 13:01 UTC. Accessed on 13/1/09 from http://en.wikipedia.org/w/index.php?title = Merox&oldid = 260317814.
  • Yvonnou LA (2001) Discharge at Sea - what about slops, Captain? 21 March 2001. Accessed on 16/1/09 from www.cedre.fr.