Can we build buildings on landfill?

Environmental hazard from old landfills : And the garbage ferments forever

A lot has happened in Germany in terms of waste disposal: We collect paper and glass and separate organic waste, packaging and residual waste. Waste from households, public institutions and small businesses has not been allowed to be landfilled without pre-treatment since 2005. A large part is recycled, organic materials are composted and the residual waste is incinerated at high temperatures with low emissions. The mountains of rubbish are only growing slowly. So far, so good, if it weren't for the roughly 4,000 previous landfills for municipal waste. Many of them are still fermenting; odor-intensive and climate-damaging gases are still emerging today, although the landfills have long been closed.

Decomposition not yet complete after 35 years

At an old waste dump in Bochum, researchers now want to try to solve the problem using an example. As in many places in the past, practically everything was disposed of there: from organic materials such as potato peel and leftover food to televisions and refrigerators. For a long time it was believed that organic matter would be almost completely degraded in 15 to 20 years. We now know better: “With the help of test boreholes, we were able to show that, after more than 35 years, a good 15 percent reactive material is still present at the old landfill,” says Frank Otto from the Georg Agricola Technical University in Bochum. Together with the city's environmental agency and the chemist Jürgen Kanitz, he wants to stimulate the breakdown of the organic material again.

Kanitz has observed for years that the decomposition processes in old heaps become slower and slower over time and take much longer than expected. "In a landfill in Switzerland we found yellow hay and even a ski pass from 1968 while drilling a hole at a depth of 15 meters - all undecomposed," he reports. When the lower parts of the landfill have cooled to twelve or 13 degrees Celsius, there is hardly any decomposition.

Gases from the landfill for heating

It takes place in two stages, explains Otto. First, with the help of microorganisms and the oxygen in the air, carbon dioxide and water are generated, and, because the reaction is violent, heat. It is the prerequisite for the second stage of the degradation process to get going: In deeper regions where air can no longer get into, other microorganisms convert biological substances into carbon dioxide and methane. The two compounds make up 90 percent of the landfill gas, the rest are water vapor and traces of other gases. The evaporation is extracted from many larger landfills by means of special wells and can be used for heating and generating electricity.

In many old heaps, however, the decomposition has almost come to a standstill. In order to rekindle them, Otto's scientists want to drill into the Bochum landfill and bring in oxygen in a targeted manner. It is supposed to stimulate the breakdown of organic substances until they are completely implemented. The resulting landfill gas is to be collected in wells so that it can be used, for example, in a combined heat and power plant.

Tree roots do not tolerate the methane gas from the landfills

The creeping outgassing would be over and with it some undesirable side effects. “The landfill we examined is now a recreational area - and there are no large trees growing there. As soon as their roots reach an area with an increased methane content, they die, ”says Otto. In addition, methane is a powerful greenhouse gas. If it is not caught, it will continue to drive global warming.

But as long as no emission limit values ​​are exceeded around the old landfills, most municipalities will not tackle the contaminated sites, says Otto. The quantities of methane that escaped from the old landfills cannot be found in any official emissions statistics. “That is why no politician can improve the climate footprint of his community if he removes the contaminated sites.” The remediation project is therefore intended to examine the extent to which the gas from the old landfill can be used economically in order to recoup at least part of the costs.

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