Waste Decomposition: The Main Stages of waste decomposition, otherwise known as the waste degradation process
An understanding of the Main Stages of waste decomposition, or the waste degradation process is essential to understand how a landfill will gradually change over many years.
We are often asked how many years will it take before a particular landfill can be built upon, or will no longer require leachate treatment, and the inability of landfill professionals to provide a quick question often surprises questioners. Unfortunately, there are so many reactions which coincide within a landfill that predictions are hard to come buy. There are many sites on the web that give periods for different materials to rot away with apparent confidence. Our view is that such predictions are too short and so generalised as to be meaningless.
We hope that this article will help provide an understanding of the difficulties in the prediction of landfill degradation rates, let alone the forecasting when all the processes involved will have run their paths and degradation completed.
Furthermore, even after degradation processes have significantly ceased, many landfills will still need further flushing with water, before the leachate produced becomes innocuous and harmless to the environment.
Society’s experience with the present designs of sanitary landfills is very short. Over the past thirty to forty years the increasing size and containment/capping of landfills, which has led to increasingly higher waste densities and lower water contents This has has, no doubt, massively extended the degradation periods of landfills.
At the start of this period most landfills were:
- small and shallow
- would remain uncapped and aerobic throughout,
- did not contain significant plastics, and
- would be quite rapidly degraded and rendered harmless; if not in 10 years, within a few tens of years.
Large current landfills, capped and kept dry by leachate management procedures which seek to minimise landfill leachate disposal costs during the active life of the landfill, and the new generation of mono-disposal hazardous waste landfills,will all take thousands of years to neutralise.
The Aerobic, Anaerobic and Back to Aerobic Sequence
The first point to note about the decomposition of landfills is that the characteristics of a landfill are primarily governed by the presence or otherwise of aerobic or anaerobic conditions.
Clearly, on day one all landfills will be aerobic (there will be a plentiful surplus of air/ oxygen). However, fairly soon all modern landfills become anaerobic in parts of the landfill, as more waste is deposited above and around the waste, and the ability of oxygen to diffuse into the waste-mass is lost within the body of the wastes.
The reactions within the landfill are very much governed by the presence or otherwise of anaerobic condition, so let us consider the effects of this. Logically, all landfills will eventually, at some undefined future date, return to the aerobic state when all significant oxygen-demanding reactions have ceased, allowing the air/ oxygen to flow back-in and not be consumed. Now, it is truly unrealistic to say anything more definitive than that this will take from hundreds of years, to one or more thousands of years.
The diagram below should provide a broad overview of the complexity of the stages of biological decomposition in a landfill.
Printable version for those with Microsoft PowerPoint (Tm).