Precious sewage sludge

From a distance, they look like greenhouses. And even inside them, the ground is covered with a layer of dark brown, about 20 cm deep, which you might think was soil. But what is waiting here to dry on the first sunny days of spring is something quite different: sewage sludge. As the only sewage treatment plant in the country and one of only a few in Europe, the Step Syndicate (with as members the municipalities of Bettembourg, Dudelange, Kayl, Roeser and Rumelange as well as a few French municipalities) has a solar drying plant. But more about that later.

Sewage sludge is a waste product that is produced during the purification of wastewater. As consumers, we don't normally get to see it. Sewage sludge spends its existence mainly in machines, tanks and trucks. When it is turned into compost, its origin is no longer visible. And when it is thermally recycled – i.e. incinerated – it disappears entirely. But even if sewage sludge is not visible to most of us, the issue still concerns all of us.

Why that is, Prof. Dr. Joachim Hansen knows. The engineering scientist has been working on the subject of wastewater treatment for almost 30 years. Building skyscrapers or bridges, on the other hand, never really appealed to him, he confides to the Lëtzebuerger Journal in an interview. "I've always been more interested in environmental issues in civil engineering, " he says. "Whenever we treat wastewater, sewage sludge is produced. And it's in a relatively large quantity." One person produces about 500 liters of sewage sludge a year, he says. "That's roughly equivalent to four full bathtubs."