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Low temperature biological treatment of a high ammonia municipal landfill leachate

University of British Columbia

The single-sludge, biological pre-denitrification (i.e., denitrification being carried out be-fore nitrification), completely-mixed activated sludge system, with hydraulic sludge recycle, known as the Modified Ludzack-Ettinger (MLE) system, has proved to be an efficient method for ammonia nitrogen removal. However, because of the sensitivity of microbial growth to temperature changes, this process may be seriously affected at low temperatures. The objective of this project was to study the effects of low operating temperatures on the biotreatment of a high ammonia-N leachate and to optimize the control for the treatment at temperatures from 20°C to 4°C. Two identical bench-scale, single-sludge, pre-denitrification, activated sludge systems, with sludge recycle, were employed during this study. Each system consisted of a 5-literanoxic reactor for denitrification, a 10-liter aerobic reactor for nitrification, and, a 4-liter clarifier for sludge settling. An air diffuser system was installed in the aerobic reactor to ensure that enough dissolved oxygen (more than 1.8 mg/l) was supplied to the nitrifying bacteria. The leachate feed was controlled at 10 liters per day. The settled sludge in the clarifier was returned to the anoxic reactor at a recycle ratio of 6:1 (60 l/d). Methanol was used as an external carbon source for denitrification. Additional phosphorus was added for bacterial growth. Temperatures of 20°C, 12°C and 4°C were studied. Theoretical aerobic SRTs of 20 days and 60 days were operated in system I; theoretical aerobic SRTs of 20 days, 30 days and 40 days were studied in system II. The leachate used in this project was collected from the City of Vancouver Burns Bog landfill in Delta, B.C., Canada. The leachate is characterized by high ammonia-N (average 210 mg/l), low COD (average 400 mg/l) and low BOD5 (average 35 mg/l). This study found that ammonia-N removal of more than 90 %, with effluent ammonia-N of lower than 0.5 mg/l, was achieved at an ambient temperature as low as 12°C, when the theoretical aerobic SRT was set at a minimum of 20 days. Also, an average effluent ammonia-N below 1.9 mg/l was obtained at an ambient temperature of 4°C, when the theoretical aerobic SRT was set at 60 days. However, at a temperature of 4°C, with a theoretical aerobic SRT of only 20 days, the level of ammonia-N removal was observed to be variable and erratic, with average effluent values of 9.2 mg/l. Methanol, as an external carbon source, was found to have a significant effect on the treatment process. When the temperature was suddenly reduced, it was necessary to increase the aerobic SRT and decrease the methanol addition, to protect the nitrifying bacteria against possible competition from heterotrophic bacteria, utilizing the excess carbon in the aerobic basin. After the nitrifying bacteria had been acclimated, methanol addition was increased to support the denitrifying bacterial population in the anoxic chamber. Despite successful denitrification in the anoxic basin, final effluent NOx-N values, at steady state, could still be relatively high, ranging from 20 mgN/l to 50 mgN/l, at various operating temperatures. Optimization of system hydraulic recycle would be necessary to reduce these values even further.

Thesis/Dissertation

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2008-09-05

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http://hdl.handle.net/2429/1643

Civil Engineering

University of British Columbia

UBCV

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