"COD is a measurement of the oxygen required to oxidize soluble and particulate organic matter in water". Therefore i think in the leachate, beside the organic matter, it compose in-organic compounds 

I suggest you consider this: is ammonia organic, or inorganic? is S(-2) and S(-1) organic or inorganic? think of other reduced compounds be organic or inorganic. There is a chemical oxygen requirement to oxidise ammonia and reduced sulphur, and since leachaete is likely to have other reduced compounds, I would hesitate to consider a COD result a measure of oxygen required to oxidise "organic matter". I would call it a measure of oxygen required to oxidise reduced compounds in a sample instead