Leachates from Waste Treatment Centres in Castilla and León region in north-centre Spain have been analyzed and classified attending organic load and nutrients. High content of nitrogen (402 ± 256–3133 ± 1221 mg/L) and potassium (570 ± 151–5120 ± 3421 mg/L) and low concentration of metals make leachate a valuable product for application in agriculture. Biological stabilization in an aerobic laboratory wastewater treatment plant (HRT = 2–10 days) has been studied for three selected leachates, in order to stabilize them before application in agricultural lands. Organic load and biodegradability are strongly influenced by composting fraction in leachate, showing low biodegradability (59.0%) high organic loaded leachates with high composting fraction. Medium and low organic loaded leachates, without composting fraction show high biodegradability (76.0 and 86.6%, respectively). Organic matter has to be differenced from COD, because high concentrations of ammonia, nitrite and chloride present in these liquids mask organic matter value. This work has the intention of being an example in using leachate from solid wastes biologically treated for agricultural disposal. For this purpose, low content of metals and toxics has to be demonstrated to assure soil and groundwater protection in accordance with EU regulations.