Anaerobic treatment of Hong Kong leachate followed by chemical oxidation.

Landfill leachate of Hong Kong was first treated by the upflow anaerobic sludge blanket (UASB) process. At 37 degrees C, pH 7.1-8.5 and a HRT of 5.1-6.6 days, the process removed 66-90% of COD in the leachate for loading rates of 1-2.4 g-COD/l day depending on the strength of landfill leachate. The final effluent contained 1440-1910 mg-COD/l and 70-91 mg-BOD/l. About 92.5% of the total COD removed was converted to methane and the rest was converted to biomass with an average net growth yield of 0.053 g-VSS/g-COD-removed. The granules developed in the UASB reactor were 0.5-1.5 mm in size and exhibited good settleability. The UASB effluent was then further polished by two oxidation processes. The UASB-ozonation process removed 93.0% of the 12900 mg/l of COD from the raw leachate. The UASB-Fenton-ozonation process improved the COD removal efficiency to 99.3%. The final effluent had only 85 mg/l of COD and 10 mg/l of BOD5. Ozonation was most effectively conducted at pH 7-8 with the addition of 300 mg/l of H2O2 and for the duration of 30 min. Ozonation also significantly improved the biodegradability of the organic residues. Nearly 50% of these residues could be used as carbon source in denitrification.