Landfill leachate from municipal solid waste brings serious contamination in downstream waters. Micro-bubble aeration is experimentally conducted in a laboratory tank and nitrification performance is investigated for the purpose of developing an economical water treatment system. Three types of gases, (i) natural air, (ii) oxygen and (iii) ozone, are used for the aeration. A water quality model is developed in order to reproduce water quality behaviors, where balances are considered in respect to dissolved oxygen, bacteria, ammonia, nitrite, nitrate and manganese. Simulated time-dependency of each water quality component is compared with the laboratory data and the model is in good agreement with experiment. The oxygen aeration tank showed most efficient nitrification among the three gasses.