Universität für Bodenkultur, Wien, Abteilung Abfallwirtschaft
(University of Agricultural Sciences, Vienna, Dept. of Waste Management);
Nußdorfer Lände 29-31, A-1190 Wien, AUSTRIA
Tel.: ++43/1/3189900-43, Fax: ++43/1/3189900-50
E-Mail: h526di09@edv2.boku.ac.at

Stabilized, mechanically-biologically pretreated wastes (MBP-wastes) still contain up to 30 % of organic substances. Biological tests, however, indicate, that these materials are largely stable and non-reactive. When landfilled can the objective of waste stability be uphold despite the high content of organic? Can stability last forever or will organic matter change during the time of deposition? Do we have to expect release if conditions change?

This paper focuses organic matter of MBP wastes. It is investigated how – apparently stable – solid organic material can be characterized and how it behaves during a 1.5 year period of deposition. Different parameters were used for characterization, looking at easy degradable, long-term degradable and non-degradable organic matter. Special focus was put on the investigation and behavior of humic substances which represent the major part of stable organic matter.

During the deposition of MPB waste a change of organic matter could be observed, depending upon the degree of stabilization. Ignition loss and consequently, the amount of organic substances, above all cellulose, decrease. 60 - 80 % of the – anyway small – gas generation can be ascribed to the reduction of cellulose. A part of the gas generation can be explained by degradation of easy degradable organic substances, i.e. proteins, carbohydrates and to some extent fats. However, chemical analysis of stabilized MBP-waste still determine about 40 % of the total organic substance as easy degradable, although no further biological reactivity could be observed. For explanation parallels to soil science could be helpful. According to soil analysis 1/3 of soil organic matter consists of not yet degraded carbohydrates, lignin, enzymes, proteins and lipids. Their stability can be explained by protection from degradation through covalently binding to other, non degradable substances, sorption and integration or inclusion into organic or inorganic particles. Soil science assumes centuries, when speaking about availability from enclosed organic substances.

A considerable part of the organic substance are humic substances. Depending on the residual reactivity of MBP-waste the content of humic substances remains either constant or even increases during the time of deposition. Increase could be a result of the presence of potentially reactive intermediary products from degradation. Humic substance release in the leachate is negligible, which underlines their stability. It was found that organic substance of MBP-waste contains up to 13 % of humic acids. Based on literature data humic acids represent about one half of the total humic substance (= fulvic acids + humic acids + humine). Consequently, the total amount of humic substances is roughly 1/4 of total organic substance and thus still far away from values that are found in soil (2/3 of organic substance). This may indicate that there is still a potential for degradation. If the amounts of humic substances and plastics are summed up a good estimation for the biological stable fraction could be gained.

Investigations show that about 30 % of the total organic carbon are bound in humic substances and are to a large extent stable. Equally, a high amount of nitrogen could be found in humic substances of MBP-waste. Various further analysis point out that nitrogen is built into humic substances. Therefore it can be concluded that humification has a positive effect on nitrogen release, leading to retention or a " slow release effect ".

Finally, more efforts should be undertaken to characterize composition and behavior of apparently stable organic matter of MBP- and already landfilled waste to understand processes that might happen in the future. For long term - if MBP-waste might approximate soil like characteristics - organic substance must be degraded anyway and thus a strong reduction of by now stable substances must take place. But this might happen over year, decades or even centuries – in periods compatible with natural environmental cycles.