repsol-ypf.com

The Bens dump, a dramatic example
On 10 September 1996, the Bens dump in A Coruï¿½a collapsed after having been in use since the early 1980s. The spectacular images that showed the unstoppable 100,000 ton mass of rubbish covering the houses of the small port of O Portiï¿½o and rushing on into the sea were seen the world over and were clear evidence of the need to come up with an alternative to the management of urban waste.

The residents ofï¿½A Coruï¿½a learnt from that environmental catastrophe that caused the death of one person, buried by the rubbish. Today, six years after the accident, the dump's 605 hectares are taken up by a park and a modern treatment plant, capable of converting part of the 500 tons of waste received daily into sufficient electricity to power both the plant itself and the town's 22,000 street lights. And without having to rely on the polemical practice of incinerating the rubbish.

The La Coruï¿½a plant is based on the better use of the biogas that is generated during that anaerobic fermentation - that is, free of oxygen - of the rubbish. The process lasts 28 days and generates enough gas to produce 6 megawatts (MW) of electric power. When the organic material cannot generate any more gas, it is converted into compost. The La Coruï¿½a plant is able to produce around 60,000 tons a year of this organic fertiliser.

The controlled generation of biogas using biodigestors such as the Galician plant is the most modern version of biomethanisation. But biogas is produced naturally through bacterial action when organic material decomposes in a humid and oxygen-free environment. As in, for example, the decomposition of rubbish accumulated in dumps, agricultural dumps, water purifying plants or the burning of prickly pear leaves.

Biogas: electric energy generator
The main component of biogas (between 40 and 60% of its volume) is methane. But it also contains CO2 (35%) and small amounts of sulphuric acid, nitrogen, oxygen, water vapour and other substances. The exact proportion of these gases varies depending on factors such as the composition of the waste, the dump's age, the rainfall or the temperature.

The proportion of methane is what determines the calorific power of biogas. The dump's gas is rich in methane and is ideal for the generation of electric energy. Its calorific power, however, is up to 50% less than natural gas, so that to harness its energy you need highly advanced gas turbines and motors.ï¿½ If the levels of biogas are low, it is usually used as fuel for a boiler to obtain thermal energy.

What's more, the harnessing of gases produced by dumps is important not only in terms of energy, but also in ecological terms. Methane has a greenhouse effect that is 21 times higher than CO2. Also, when methane mixes with the air, in proportions of 7 to 15%, it becomes explosive, a combination that is similar to the firedamp effect in mines. Better use of biogas also prevents the unpleasant smells associated with the sulphuric acid it produces.ï¿½ This is why some dumps simply torch the biogas without harnessing its energy in an attempt to minimise its environmental effect.

In Spain there are only around ten plants that harness the biogas they produce: Artigï¿½s (Bizcaya) and La Zoreda (Asturias), since 1992; San Marcos (Guipï¿½zcoa), since 1995; Gï¿½ngora (Navarra) and Meruelo (Cantabria), since 1997; Murcia, since 1999; and Alcalï¿½ de Guadaira (Seville), Can Mata (Barcelona) and Les Valls (Barcelona), since 2001. But the system is gradually being introduced and more and more dumps are being sealed and degased, Vï¿½znar (Granada), Basseta-Blanca (Valencia) and Salto del Negro (Las Palmas de Gran Canaria) among them.

The future of Valdemingï¿½mez
The most ambitious biogas project in Spain will shortly be underway. The largest dump in Spain, Valdemingï¿½mez (Madrid), will soon be degased. Some 21 million tons of waste accumulated over 23 years up to the year 2000 cover an area of 110 hectares.

The first step in order to degas the dump is to seal it, covering it with layers of materials such as polythene and geotextile. Then 280 wells will be dug into the mountain of waste and then the biogas will be piped over to a treatment plant 42.5 km away where the methane will be separated from the oxygen and the sulphuric acid. This will power eight generators that will produce 1,145GW/h from 2002 until 2019. This project will be fully functioning in 2003, producing 140GW/h, Madrid's annual public lighting consumption.

Such production will convert Valdemingï¿½mez into the largest biogas conversion plant in Spain, and one of the largest in the world. Degasing the plant will also prevent huge amounts of methane being launched into the atmosphere and will allow the area to be landscaped once more. According to Joaquï¿½n Fernï¿½ndez Castro, Municipal Director of Madrid's Waste Management and Environmental Quality Service, the project is comparable only to the sealing of Hong Kong's dump. Those wandering around the park that will cover the dump will be aware that the rubbish accumulated under their feet will serve as a source of energy for some 16 years.

- Spaniards generate 18,376,532 tons of solid urban waste a year. Despite the advance of selective collection, 12.82% of all this rubbish (2,356,434 tons) is dumped in an uncontrolled manner and 58.77 % (10,800,200 tons) ends up in one of the 190 controlled dumps found in Spain. Some 48.9 % of Spanish waste is made up of organic material, that can be used to obtain biogas. Source: The State of the Environment 2000, Ministry of the Environment.

- Spain produces over 60% of biogas-derived electric energy of the European Union. The production of biogas in 2000 is comparable to 107,806 tons of petrol and represents 6% of the country's total biomass consumption, excluding domestic applications. Source: Bulletin of Energy Efficiency and Renewable Energy, IDAE, 2001.

- The modern plant of Nostiï¿½n in La Coruï¿½a, began operating in January. There are currently five similar biogas plant construction projects in Spain: in Barcelona, Cadiz, Leon and Zaragoza.