What do sewage sludge, household leftovers and restaurant waste have in common? The answer is that these different types of waste contain high amounts of biological material.

That makes them highly attractive as an alternative energy source. Digesting biological waste under oxygen-free conditions produces biogas consisting of 60 to 70 percent methane. Just like fossil fuels, this gas can be used for heating and electricity generation and to power vehicles. Effective use involves removing excess levels of hydrogen sulfide and carbon dioxide, at which point biogas achieves the same quality as natural gas.

Purifying biogas

The Swedish company Purac, a division of Läckeby Water Group, specializes in technology used in biogas purification. Purac produces vehicle fuel with the Cooab process, which is based on chemical absorption of carbon dioxide at low pressure utilizing an absorbent fluid containing amines mixed with water in a closed system. The company says the process has several advantages over other treatment methods, perhaps most importantly cutting methane emission to 0.1 percent, as compared to traditional processes that release 2 to 4 per cent methane.

The result is higher energy content and less climate impact, since the greenhouse effect from methane is up to 20 times stronger than carbon dioxide. Another environmental gain from Cooab is that excess heat from the process can be captured and put to use.

Thousands of restaurants in China

Purac recently won a major order from China, where restaurant waste in Chongqing City will be used for electricity generation. Energy production is expected to be equivalent to the full-year demand of 1,500 heated Swedish houses. Chongqing City, a megalopolis in the heart of a province of more than 30 million people, is home to thousands of restaurants. In a first stage, 100 tons of restaurant waste will be processed each day, with capacity scheduled to increase to 500 tons per day (180,000 tons per year). This is more than double the capacity of the largest Swedish biogas plant, located in Borås, which is used to turn household waste into vehicle fuel.

The Purac office in Beijing employs 30 Chinese engineers working on development of various types of treatment plants. So far, the company has erected about 70 plants for water purification, wastewater treatment and biogas production in China. The company has sales of about SEK 130 million (USD 19 million) in the country.

World’s largest biogas plant

Concerned about its dependence on Russian natural gas supplies, Germany is investing heavily in a variety of alternative energy sources. Purified to the same quality as natural gas, biogas is projected to constitute an important component of the future German energy mix.

During the autumn of 2009, Purac was commissioned by the German company Weltec Biopower to build the world’s largest plant for biogas purification. This would normally involve multiple processing lines, but the new facility is designed for a single line.

“Purac is unique in the world in its ability to build plants where the flow of gas is purified in a single step,” says Lars-Evert Karlsson, Purac’s sales manager for biogas purification. The German market for biogas purification is estimated at some USD 1.5 billion over the next 10 years.

Biogas from sewage sludge

Sewage sludge from wastewater treatment plants has long been used as an input material for biogas digesters producing heating fuel. Removing carbon dioxide from that gas makes it suitable to be compressed into fuel for vehicles. In the Norwegian capital Oslo, city buses are being converted to run on biogas, and Purac is set to build a biogas production facility for co-location with a municipal sewage treatment plant. Similar projects are underway in Karlstad, Sweden, where the biogas is used for municipal vehicles and private cars.

Article published in December 2009