Four different heights of lysimeter were prepared to investigate the leachate characteristics produced from waste beds. The solid wastes from Chiang Mai municipal area were filled into the lysimeters with an inplace density of about 0.85 ton/m3. The experiment had been started for 853 days from May, 1990 to October, 1992. The leachates were collected every week to every month depending on the age of the waste and analysed for pH, conductivity, total solids, suspended solids, total dissolved solids, total volatile solids, alkalinity, acidity, total volatile acid, Cl, SO=4, BOD, COD, TOC, TKN and NH3−N. The results showed that conductivity, volatile acid, total solids, total volatile solids, COD, BOD and TOC were very high in the first rainy season or in the first dry season, after that, they decrease abruptly, and then gradually decrease until the end of the experiment. Acidity, alkalinity, suspended solids, TKN and NH3−N were high in the first rainy season and gradually decreased until the end of the experiment. High percentage reduction of acidity, volatile acid, COD, BOD and TOC concentrations in the leachate produced from lysimeters after one year of waste filling showed that the acidogenic phase of the biodegradation process had terminated and that the major portion of carbonaceous organic matter in the waste bed had been utilised within the first year of the experiment. For the deeper waste layer, nitrogenous compounds in the leachate produced after two years of waste filling were still high due to the low activity of nitrification in the waste bed. It was found that the shallower waste layer produced lower concentrations of pollutants in the leachate. Higher amounts of leachate volume and extracted substances per dry weight of wastes were produced from the shallower waste layer.

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