Leachates from a municipal solid waste landfill were treated by anaerobic filter (AF) and two-stage activated sludge (AS) for the removal of ammonia. Nitrification in the first stage AS was efficient, but was unstable with temporary accumulations of nitrite by free ammonia toxicity. With the second stage AS, complete nitrification of ammonia and nitrite was achieved. Denitrification via nitrite occurred with the first stage AS effluent recycle, and the resulting AF effluent NOx concentration was kept lower compared to the case in which denitrification via nitrate only occurred with the second stage AS effluent recycle. This might happen because of low COD/N ratio of the raw leachate, and suggests that denitrification via nitrite may be a valuable option. In AF, denitrifiers out-compete methanogens for COD. Ammonia-removed leachate was treated with Fenton's reagents followed by post-AS for further COD removal. Fenton's treatment reduced COD by 63%. Up to 74 mg/L of COD was removed when 100 mg/L of sucrose was added to post-AS. With the above treatment system, 1400∼1800 mg/L of NH4-N was completely removed, leaving 200 mg/L of nitrate nitrogen. Also, 4000∼7000 mg/L of COD in the raw leachate was reduced to 150∼200 mg/L.

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