Contamination of big rivers by edible oils?


* ¿Contaminación de grandes ríos por aceites comestibles?

By now, many people may have heard the following advices: "DO NOT dump cooking oil, poultry fat and grease into the kitchen sink or the toilet bowl. DO NOT use hot water and soap to wash grease down the drain. DO place cooled cooking oil, poultry and meat fats in sealed non-recyclable containers and discard with your regular garbage. DO use paper towels to wipe residual grease or oil off of dishes, pots and pans prior to washing them." Many believe that all this is due to some danger to the natural environment, but does it?

The Environmental Protection Department of the City of New York issued the following explanation: "Liquefied fat, oil, or grease (FOG) that is poured down the kitchen sink drain can cause serious impacts. FOG can cling to the insides of pipes and the sewer system. Over time, it can build up and can eventually block pipes completely. If wastewater can’t move freely through pipes and out into the sewer system, it can back up into your home and can cause unsanitary conditions and damages that can be expensive to repair." ("Grease Disposal Tips To Help the City's Environment", NYC Department of Environmental Protection, The City of New York, 2012). Of course, we have to understand that they have the problem that their winters are very cold and oil solidifies in their pipes easily. In a tropical climate, this issue may not be as severe.


Edible oils are generally biodegradable and non-toxic for the environment. (Zhengkai Li, Brian A. Wrenn and Albert D. Venosa, "Anaerobic biodegradation of vegetable oil and its metabolic intermediates in oil-enriched freshwater sediments.", Biodegradation, Vol. 16, pp. 341-352, Springer, 2005) (Paul Campo, Yuechen Zhao, Makram T. Suidan, Albert D. Venosa, George A. Sorial, "Biodegradation kinetics and toxicity of vegetable oil triacylglycerols under aerobic conditions.", Chemosphere, Vol. 68, pp. 2054-2062, Elsevier, 2007). However, the U. S. Environmental Protection Agency (EPA) determined in 1987 that “a sheen is an appropriate indicator of a discharge of harmful quantities.” ("Sheen Rule", Federal Register, Vol. 52, No. 63, 1987).

As an example, let us see what happens in a large South American river. We can try to calculate how much vegetable oil could go daily, say, to the Paraguay river and try to see if this quantity becomes a "harmful quantity". There are approximately 2 600 000 people in Asunción and the surrounding Central Department of Paraguay. ("Projection of population by department, year 2008", Paraguay Statistical Yearbook 2008, chapter 2: Population and Housing, p. 43, General Directorate of Statistics, Surveys and Census, 2010) (Paraguay: Projection of the National Population by Gender and Age, 2000-2050, p. 24, General Directorate of Statistics, Surveys and Census, 2010). The global vegetable oil production in 2007 reached the mark of 128 million tons ("Atlas of agroenergy and biofuels in the Americas: II Biodiesel", Hemispheric program in agroenergy and biofuels, Inter-American Institute for Cooperation on Agriculture, San José, Costa Rica, 2010). This is 20 kg of oil per person in the world. ("Total Midyear Population for the World: 1950-2050", U.S. Census Bureau, International Data Base, 2010). The daily production would then be 20 kg divided by 365 days, that is, about 0,05 kg per person. With a density for (for example) soybean oil of 900 kg/m3 (Inter-American Institute for Cooperation on Agriculture, cited work), we can round this to 0,05 liters per person per day. If there are 2 600 000 people and each one dumps 0,05 liters per day, we will have 130 000 L of edible oil per day, that is, 130 m3 per day disposed in Asunción and the Central Department.

A concentration of 10 parts per million (ppm) is sufficient to cause an oil sheen on water, which means that 1 liter of oil contaminates 100 000 liters of water, or what is the same, 1 m3 of oil contaminates 100 000 m3 of water. (Steven A. Calanog, Jimmy Y. Chen & Robert F. Toia, "Preliminary evaluation of potential impacts of non-petroleum oils on the aquatic environment.", # 63, 1999 International Oil Spill Conference.) If we have 130 m3 of oil per day and each m3 contaminates 100 000 m3 of water, that means we will have 13 000 000 m3 of potentially contaminated water per day.

The Paraguay River usually has a flow in front of Asunción, according to the season, ranging from about 2500 m3 per second to about 4000 m3 per second. (Stephen K. Hamilton, Suzanne J. Sippel, Deborah F. & John M. Melack Calheiros, "An anoxic event and other biogeochemical effects of the Pantanal wetland on the Paraguay River", Limnology and Oceanography, Vol. 42(2), pp. 257-272, American Society of Limnology and Oceanography, Inc., 1997.) (Guido Duarte, John H. Palmieri, Silvano Frutos, Jose Ortiz Guerrero, "Study of alternatives of water supply for the Central Chaco. Final Consultancy Report", p. 68, Inter-American Development Bank & Paraguay's Technical Secretariat for Planning, 2003.) (Vicente Barros, Lucas Chamorro, Genaro Coronel & Julián Báez, "The major discharge events in the Paraguay River: magnitudes, source regions, and climate forcings", Journal of Hydrometeorology, Vol. 5, pp. 1161-1170, American Meteorological Society, 2004.) (Carlos M. Krepper, Norberto O. García, & Phil D. Jones, "Paraguay River basin response to seasonal rainfall", International Journal of Climatology, Vol. 26, pp. 1267-1278, Royal Meteorological Society, 2006.) ("Study workshop of river dynamics and sedimentology of the Bermejo River and of assessment of the environmental impact in the lower basin from the works planned for the upper basin", National Institute of Hydric Science and Technology, Argentina & Bermejo River Regional Commission, Ezeiza, 1993, cited in "Geo Argentina 2004: Argentina's environment outlook", Chapter 2, p. 120, United Nations Environment Program - Regional Office for Latin America and the Caribbean and Secretariat of Environment and Sustainable Development of the Argentine Republic.) (P. Boulestreau, D. Jouve, V. Piron, "Best practices for flood forecasting and warning systems: the Compagnie Nationale du Rhône’s experience in flood forecasting and the advantages it can provide for the Lower Mekong watershed", Proceedings, 4th Annual Mekong Flood Forum, Siem Reap, Cambodia, 18-19 May 2006, Chapter 2, pp. 106-109.). Now, 2500 m3 per second is 9 000 000 m3 per hour. If the oil is poured evenly over the entire day, except at night, it would be about 12 hours of release per day. During that time, 9 000 000 m3 / h went through the Paraguay river, which when multiplied by those 12 h gives a total of 108 000 000 m3 of water circulating. But those 108 000 000 m3 of water of the Paraguay River is a volume almost ten times higher than the 13 000 000 m3 of water that can potentially be contaminated by the spilled oil, at most. Therefore, in this scenario, the oil would be diluted so much that no sheen would form and there would be no contamination.

However, we could imagine other scenarios. If all the oil is poured at once, say for 1 hour at noon, then the 9 000 000 m3 of water of the river Paraguay that would circulate during this hour would be less than those 13 000 000 m3 of water that all that spilled oil can get to contaminate. In this other scenario, river contamination should occur.


In the real world, attending culinary traditions, this is unlikely to happen. More likely, there would be two major discharges of 1 hour each, one around noon and the other in the evening. In this way we would have 13 000 000 m3 per day of water with oil halved, that is, 6 500 000 m3 of water potentially contaminated in every major oil discharge. I think it would not be far from the truth to consider that the flow of the Paraguay river would remain almost constant during the day. Thus, once again, 9 000 000 m3 of water from the Paraguay river is greater than 6 5000 000 m3 of water with potential of being contaminated, that is, the amount of oil discharged would again be insufficient to cause widespread sheens on that great volume of water.

This amateur model is extremely simplified and suffers from many shortcomings, including: a) the estimated use of cooking oil is an approach in relation to a global average yield; b) it does not discriminate the ratio between the amount of oil used for cooking and the amount used for ingestion, or the amount absorbed and ingested with food; c) does not consider the details of the sewerage system, and especially and firstly, in terms of the total coverage for the population considered (although it is known to be less than what the model took in consideration: in the city of Asunción the real coverage reaches just 75% [all untreated], while in the rest of the Greater Asunción the situation is as follows: Lambaré 16% [untreated], Fernando de la Mora 7% [untreated], San Lorenzo 9% [with a treatment plant but currently collapsed], Luque 7% [untreated], Mariano Roque Alonso 0% and Villa Elisa 0%. "Strategic environmental and social assessment of the Master Plan for sanitary sewer and wastewater treatment of Greater Asunción [sic]", Annex 4 of the Environmental Management Framework of the Modernization Project for the Water and Sanitation Sector, Sanitary Services Company of Paraguay, 2 February 2009, pages 5 and 6) and, secondly, the transit time through the system until reaching the river Paraguay, which could allow the degradation of the oils or the homogenization of the flow throughout the day; d) does not consider periods of droughts with extreme lows in the river water level; e) does not consider local contamination at the discharge points (there are a dozen mouths, half of them underwater, in a 40-km stretch of the river Paraguay between the Botánico and Cateura. [Sanitary Services Company of Paraguay, cited work]); f) does not consider differences among the harmful potentials of certain oils and certain qualities, such as unrefined corn oil; and g) margins of errors have not been calculated, which is especially significant considering that some results fall within the same order of magnitude as of levels considered potentially harmful.

Even so, according to the results obtained with the approaches set out and the main factors involved, namely: a) the river flow, b) the amount of vegetable oil available, c) its non-toxicity and d) its biodegradability, I predict that, most likely, a contamination of the Paraguay River by edible oils would not be happening.

Now it only remains to test this theory to know the truth.

Of course, the intention of this article is not to encourage people to litter (any kind of litter) the river. That must not be done. It would also be good for these urban areas to have someday sewage treatment plants. The authorities certainly will take note of this.

Before closing this essay, we should remember that we are talking of edible oils and not of motor oil, which is not edible nor biodegradable, but on the contrary, is highly toxic and should never be thrown down the drain. The latter should be taken to a service station of those which makes oil changes in order to be processed correctly.

Peace on Earth!

Aldo Loup.

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Photograph: Olive trees ("Olea europaea") in Gethsemane (literally, "oil-press"), at the foot of the Mount of Olives, Jerusalem, Israel. In the book of Exodus, chapter 30, verses 22-33, it is explained how the priests of the Israelites should be anointed ("mshchth", from which the word messiah comes, translated into Greek as "khristós") with a scented liquid based on olive oil. There are other oils, the partially-hydrogenated oils (with trans-unsaturated fatty acids, commonly called "trans"), which can withstand repeated heating without degrading, making them ideal for frying fast foods. However, they should be avoided because they can cause heart disease, stroke, diabetes and other chronic illnesses. (Harvard School of Public Health Nutrition Source, 2011). In this sense, olive oil is a much healthier alternative. Photograph credit: Pat McCarthy / See the Holy,, (original license, of the photograph only, obtained at: With permission from Pat McCarthy /