In humid and temperate countries such as Europe, the evolution of landfills is guided by significant microbiological activity. This, like any microbiological process, is directly dependent on the water content of the landfill. In countries with a drier climate, the humidity rate in the landfill is a very important parameter that will condition the type of evolution of the landfill.
It is essentially defined by climatic conditions and local hydrogeology as in Tallahassee. For example, in Africa, six climatic zones are defined in particular on the basis of the rainfall regime: rainfall and distribution over the year. This may cause some particularly large countries to be globally divided into two or more climatic zones with their own landfill management requirements.
Cities located in desert (II) and Sahelian (III) climatic zones have neither water reserves nor ground humidity. A few cities located far north of the tropical zone with a long dry season (IV) such as Dakar, Ouagadougou and Segou are almost in the same situation. It is expected that no microbiological degradation of waste from landfills located in this way will be observed.
Like in Florida, the humidity brought by the waste evaporates quickly and the rains cannot possibly ensure this type of activity. Only physicochemical degradation could be observed. Such a landfill, far from being a bioreactor, would rather be a place of fossilization of organic waste by desiccation. However, it is essential to determine whether during storms, percolation and hypodermic flow water has time to reach the water table.
On the other hand, the majority of cities located in the humid tropical (V) and equatorial (VI) climatic zone present water balances apparently favorable to a biological evolution of landfills. In addition, the periods of soil humidity are usually long (7 to 10 months). Biomethanation should be easily observed there.
In these same climatic zones, however, there are cities such as Cotonou (zone VI), located in West Africa, which present an intermediate situation similar to several cities in the tropical zone with a long dry season (IV). A soil humidity rate of 50 to 80% is observed there over a short period (2 to 5 months). The type of evolution of landfills subjected to these conditions is not very predictable.
Such an intermediate situation also seems to exist in the Mediterranean zone (I), for example in Tunis. Consequently, each station, in the Mediterranean zone or in a tropical zone with a long dry season, will have to be examined on a case-by-case basis according to soil humidity, climatological data (such as wind speed, drought factor), starting humidity of the waste.
The presence of leachate and biogas must be taken into account in order to classify the landfill objectively. For bringing junk to Tallahassee landfills help reduce the problem. Indeed, the investigations carried out on four landfills in Tunisia and Haiti show that, despite the dryness of the soil and a negative water balance (low or poorly distributed rainfall and lower by a factor of two to five than the potential evapotranspiration data), the interior of the landfill remains sufficiently humid to produce, even during the dry season, low quantities of biogas and little or no leachate.
We are faced with an intermediate discharge that we will call a crust. Rainfall and potential evapotranspiration data are therefore not sufficient to classify a landfill. When the waste arrives at the landfill, with a well-defined humidity, the water activity gradient is relatively high and allows microorganism activity. The water falling on the waste is then only slowly evapotranspired.
Due to the structure of the waste, a certain time is necessary for the evapotranspiration to be complete. During this time, water migrations may have fed the lower layers of waste to activate biodegradation. Similarly, the water that constitutes the waste can play this role. Therefore, only the waste on the surface and in contact with the ground dries out.
We are witnessing the formation of a dry crust around the edge of the mass of waste and a low or even non-existent production of leachate. Subsequently, this crust of dry waste prevents the exchange of water and oxygen between the inside of the landfill and the outside. Thus, the humidity of the waste is maintained within the mass and allows a certain methanogenesis accompanied by a slow but continuous production of biogas.
The management of Tallahassee landfills poses the problem of controlling the biogas that must be evacuated and of evaluating the lifespan of this landfill, which should evolve extremely slowly. In summary, the diversity of climatic and hydrogeological conditions leads us to consider three types of landfills depending on the humidity level present: the dry-fossilized landfill, the wet landfill, and the intermediate landfill.
The search for waste specifies, for Florida, the type of discharge that should be observed according to the climatic zones. However, it is obvious that this classification can easily be extended to other countries around the world. This is particularly the case in Talahassee where most of the existing landfills are either dry-fossilized or intermediate.