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Anaerobic treatment of Hong Kong leachate followed by chemical oxidation

H.H.P. Fang*, I.W.C. Lau* and P. Wang**

*Centre for Environmental Engineering Research, Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China

**Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China


Abstract Landfill leachate of Hong Kong was first treated by the upflow anaerobic sludge blanket (UASB) process. At 378C, 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 1–2.4g-COD/lday depending on the strength of landfill leachate. The final effluent contained 1440–1910mg-COD/l and 70–91mg-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 0.053g-VSS/g-COD-removed. The granules developed in the UASB reactor were 0.5–1.5mm 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.

Keywords Anaerobic treatment; Fenton reaction; leachate; ozonation


The organic residues in landfill leachate are in general recalcitrant to biodegradation in traditional wastewater treatment facilities, and some are also colloidal in nature. As a result, biological treatment of leachate is often not effective. Hong Kong produces about 9500 tonnes of municipal solids waste daily. They are collected and disposed of at three landfill sites with ages varying from seven to nine years. The leachate is at present treated in oxidation ponds on site with a hydraulic retention time (HRT) of 30 days (Chen, et al., 1997). The process is costly because of its strong energy demand. It is, thus, warranted to develop a more cost-effective alternative.

This project was conducted to study the feasibility of treating Hong Kong leachate with anaerobic process, replacing the existing extended aeration, followed by chemical oxidation. Anaerobic process has become a mature wastewater treatment technology in the past decade. The process not only saves aeration energy but also produces methane, which is a readily useful fuel. In addition, it produces only about 10% of waste sludge as compared to aerobic processes. Thousands of full-scale treatment plants have been installed worldwide (Fang and Liu, 2001). However, due to the recalcitrant nature of the organic residues in the leachate, anaerobic treatment alone was not sufficient. The anaero- bic effluent was further treated in this study with a modified ozonation process.

Ozonation is a widely accepted technology used for the final disinfection of water and

wastewater. In recent years, it has also been increasingly used for the treatment of refrac-

tory and toxic wastewaters from various industries, such as tannery (Jochimsen et al.,

1997), pulp and paper (Zhou and Smith, 1997) and pharmaceutical (Hofl et al., 1997), as 41

Water Science & Technology Vol 52 No 10–11 pp 41–49 Q IWA Publishing 2005

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 Supercritical Fluid Extraction 2005-Anaerobic_treatment_of_Hong_Kong_leachate_followed.pdf Page 001
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