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Treatment of leachate from domestic waste landfills
Leachate from domestic waste landfills changes their composition in course of time. High organic loads are measured in seepage waters after 1 to 2 years by the anaerobic process of organic substances in the dump. The BOD reaches values of 4.000 to 40.000 mg/l, the COD values of 6.000 to 60.000 mg/l. Connected with this are a decrease of pH value down to 4,5 to 7,5 and a higher solubility of some inorganic compounds like Fe, Mn, Zn and Mg.
Over the years CH4 and CO2 are produced as final products of metabolism. As a result, the organic load of the leachate decreases. The BOD reaches values of 20 to 550 mg/l, the COD values of 500 to 4.500 mg/l. The pH - value reaches 7,5 to 9.
This does not influence nitrogen load of the leachate. NH4-N reached in both phases 30 to 3000 mg/l and total nitrogen of 50 to 5.000 mg/l. Total phosphorus reaches values of 0,1 to 30 mg/l. High concentrations of AOX and heavy metals are common features.
The removal of nitrogen compounds with simultaneous phosphorus removal is a prior task in all fields of sewage treatment. Thereby is to take into account, that nitrification is a very sensitive process which may have a strong influence also on phosphorus removal.
Complete nitrification is to be expected only in a very narrow range of the pH-value. When the concentrations of NH4+ or NO3- is reach move than 100 mg/l which is the case in leachate of waste landfills, operation of such treatment plants is made more difficult because nitrification itself has a bad influence on the buffering system by the formation of Hydrogen - ions. In contrast to the nitrification, the denitrification leads to an increase of the buffering capacity and of the pH - value by the formation of Hydroxyl - ions.
The arrangement of denitrification section before nitrification section makes it possible to use the organic compounds of sewage water as carbon source and to use the formed nitrate oxygen of the nitrification stage. This guarantees a considerable energy recovery and higher process stability.
In plants with combination of activated sludge and biofilm technology gives specialized groups of bacteria with low growth rates, for example Nitrosomonas and Nitrobacter, a competitive advantage. The concentration of biomass attached to the biofilm and in suspension is much higher than in classical activated sludge plants. And this makes it possible to reach a low sludge loading with nitrogen compounds, even in case of high volumetric loading.
Another substantial influence on nitrification is executed by the ratio of BOD to nitrogen in the inflow. The smaller the ratio BOD to nitrogen in the inflow to the treatment plant is, the greater is the influence of pollution loads variations, degreasing temperatures and changes of the composition of the incoming sewage.
Therefore is it expediently, also the anoxic process stages for denitrification and the anaerobic process stages for phosphorus removal to equip with carrier material for biofilm, to get a high concentration at acetogene bacteria. They decompose the rough sewage compounds, which are retained in these stages, to easily degradable organic compounds and, this way, ensure always sufficient degradable components of the BOD in these stages in time of low inflow. This way, the ratio of the BOD to nitrogen is made more even.
The use of a fluidised bed process possess opposite the tendered solid submerged contact aerators the advantage of a 4 to 5 fold larger surface per volume unit.
