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Pressure aeration speeds up decomposition

The leakage of organic pollutants and heavy metals along with the emission of methane gas can continue to have a harmful effect on humans and the environment as long as 100 years after a landfill site has been closed down. This is why landfill leachates and gases need to be collected and treated for many years to come. The Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA) at the University of Stuttgart is carrying out a pilot project on a section of a discontinued landfill site in Konstanz with the aim of finding ways to reduce the aftercare period. During the five-year project, waste will be pressure aerated for three years and the effects of aerobic treatment on the decomposition of the organic waste evaluated thereafter. A leachate feedback system was implemented to prevent waste from drying out and a biofilter was installed to treat exhaust gas.

Prof. Dr. Martin Kranert, Chair of the Department of Waste Management and Exhaust Air at Stuttgart University. © ISWA, University of Stuttgart

From June 2005, untreated biologically degradable organic waste from human settlements has no longer been allowed to be stored in landfills. As a result, many landfills, including the Dorfweiher landfill in the Konstanz district had to close down. However, precipitation can cause toxic compounds such as mercury and other heavy metals as well as leachate contaminated with organic compounds to leak from waste up to 100 years after a landfill site has been closed down. This water therefore needs to be collected and cleaned. During the costly aftercare period, ways have to be found to minimise the exhaust gases emitted. “Our pilot project on the Dorfweiher landfill site in the district of Konstanz aims to assess the effects of aeration on a normally anaerobic landfill body. The ultimate goal is to reduce the aftercare period, hence reducing the risk of environmental pollution,” said Prof. Dr. Martin Kranert, chair of the Department of Waste Management and Exhaust Air at the University of Stuttgart.

The project was started in 2009; from the end of 2012, the effects will be evaluated during a two-year observation period. The project involves many scientists from the Institute for Sanitary Engineering, Water Quality and Waste Management (ISWA) in Stuttgart and is financially supported by the district of Konstanz, the Baden-Württemberg Ministry of the Environment, Climate Protection and the Energy Sector and the Freiburg Regional Council. 

Oxygen supply increases microbial activity

The project is focused on monitoring the aerobic and anaerobic degradation of organic constituents of the BA I section of the former Dorfweiher landfill in the district of Konstanz. This area contains 72,000 cubic metres of waste with a high proportion of organic substances originating from domestic waste and composting plants. “Aeration of the waste accelerates the decomposition of organic substances. The supply of oxygen increases microbial activity, and hence the decomposition of waste by bacteria, at the same time as reducing the amount of methane gas that develops,” explains Kranert. 80 so-called injector lances placed at around ten metres from each other guide air into the waste heap. The blowers used for aeration create a pressure of up to 4.5 bar in the lances.

A leachate feedback system and an additional irrigation plant have been implemented in order to counteract the high moisture loss resulting from aeration. “We’ve had relatively high levels of precipitation, so we’ve not yet had to use the second irrigation system. We even had to resort to compressed air shocks to squeeze water from the waste heap,” said Kranert, adding that large amounts of water were discharged during the aerobisation phase when higher microbial activity occurred. The polluted water is collected and treated in the leachate treatment plant.

View on section 4 of the former Dorfweiher landfill in the district of Konstanz. Injector lances accelerate biological degradation of waste. The pilot area is covered by a biofilter consisting of gravel, green cuttings compost and heather.
Injector lances accelerate biological degradation of waste. The pilot area is covered by a biofilter consisting of gravel, green cuttings compost and heather. © ISWA, University of Stuttgart

Methane is converted in the biofilter

In addition to leachate, landfill gas resulting from the bacterial degradation of organic substances is another major problem. The gas consists mainly of methane and carbon dioxide and the high concentration emitted would contribute considerably to global warming. The 1.5-metre-thick biofilter is put on top of the landfill. From bottom to top, it consists of layers of gravel, green cuttings compost and heather. The rising methane is distributed by the gravel and in the next layer, which is 90 centimetres thick, it is oxidised into biomass, water and carbon dioxide. The topmost layer of heather also cleans the gas and retains the moisture in the biofilter. “Under normal circumstances, landfill gas forms up to 30 years after a landfill is closed down,” said Prof. Kranert.

The researchers continuously monitor the changes in the waste heap and in the biofilter using complex measurement equipment and methods, including probe-based gas measurement systems and a remote laser sensor. “We have observed that the temperature increases from top to bottom of the waste heap, which is a clear indication of higher microbial activity,” said Kranert. After around eight months of aeration, the median temperature had already risen from 30 to more than 50°C. As a direct consequence of accelerated aerobic degradation, the concentration of methane has decreased in the waste heap and in the biofilter by more than two thirds. However, the expected improvement of leachate quality has not yet been achieved. “As the waste is decomposed in a much shorter period of time, the contaminants contained therein are also released more quickly,” said Kranert explaining the unexpected situation. The total amount of harmful substances is thus discharged in a relatively short time. 

Innovative surface insulation against leachate

Around 6% of the 72,000 cubic metres of waste have already been decomposed. The height of the waste heap has thus decreased by around 10%. “In addition to the reduction of the aftercare period we hope to achieve, the scientific findings related to the decomposition processes are very valuable. At the end of the monitoring and evaluation phase in about two years’ time, we will have concrete data available on how the aftercare period can be reduced,” said Kranert. The data will also be used to select the final surface insulation for the Dorfweiher landfill. The researchers plan to develop an innovative surface insulation aimed at ensuring that less precipitation leaches into the waste. 

Further information:
Prof. Dr.-Ing. Martin Kranert
University of Stuttgart
Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA)
Bandtäle 2
D-70569 Stuttgart
Tel.: +49 (0)711/ 685 - 65 500
Fax: +49 (0)711 685 - 65 460
E-mail: martin.kranert(at)iswa.uni-stuttgart.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/pressure-aeration-speeds-up-decomposition