Rate limiting factors of the anaerobic digestion of municipal solid waste in bioreactor landfills

Lai, Takwai Eddy (2001). Rate limiting factors of the anaerobic digestion of municipal solid waste in bioreactor landfills PhD Thesis, School of Engineering, The University of Queensland.

       
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Author Lai, Takwai Eddy
Thesis Title Rate limiting factors of the anaerobic digestion of municipal solid waste in bioreactor landfills
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2001-01-01
Thesis type PhD Thesis
Supervisor Dr Pratap Pullammanappallil
Dr Bill Clarke
Total pages 214
Language eng
Formatted abstract
Anaerobic digestion of municipal solid wastes (MSW) involves the biological conversion of organic matter to methane and carbon dioxide. This conversion process is mediated by several groups of microorganisms, mainly bacteria. It is generally accepted that hydrolysis is the rate limiting step of anaerobic digestion of organic particulate matter. However, when considering the overall conversion of organic particulates of MSW to methane and carbon dioxide in a high solids bioreactor, the hydrolysis process, the availability of appropriate microorganisms, delivery of these microorganisms and the provision of optimal physicochemical conditions for microbial growth could all affect the rate of degradation.

Anaerobic digestion of municipal solid wastes (MSW) involves the biological conversion of organic matter to methane and carbon dioxide. This conversion process is mediated by several groups of microorganisms, mainly bacteria. It is generally accepted that hydrolysis is the rate limiting step of anaerobic digestion of organic particulate matter. However, when considering the overall conversion of organic particulates of MSW to methane and carbon dioxide in a high solids bioreactor, the hydrolysis process, the availability of appropriate microorganisms, delivery of these microorganisms and the provision of optimal physicochemical conditions for microbial growth could all affect the rate of degradation.

Various researchers have shown that the start-up of the degradation of MSW could be enhanced significantly by flushing a fresh bed of MSW with leachate collected from a stabilised MSW bed. This is commonly referred to as a sequenced leach-bed process. It has been widely accepted that this operational strategy provides the fresh bed of waste with inoculum from the stabilised waste bed and thereby overcoming the limitations of the availability of appropriate microorganisms and the delivery of these into the fresh waste bed. However, other research studies have shown that the time scale for degradation was not altered substantially by the usage of micro-filtered leachate from the stabilised MSW bed. This indicated that contrary to the widely accepted opinion, inoculation may not limit the start-up of degradation. Therefore the first aim of this research project was to identify the major limitations to the start-up of degradation during anaerobic digestion of MSW. This was addressed through the following objectives:

• the identification of the biological (inoculum and extra-cellular en2ymes) and the physicochemical (nutrients, vitamins and buffer) factors affecting anaerobic digestion of MSW and

• the development of operational strategies that optimise the degradation of MSW, which are also suitable for implementation in bioreactor landfills.

Eleven sequenced leach-bed experiments were carried out to investigate the effects of inoculation on the start-up of the degradation process. Six reactor studies, which involved the flushing of chemical solutions into the fresh MSW bed, were then carried out to investigate the physico-chemical limitations. The main findings were:

1. inoculation and provision of extra-cellular enzymes are not the major limiting lac ors in starting up the leach-bed process;

2. the provision of moisture is essential for rapid start-up of the leach-bed process;

3. the flushing of buffer solution resulted in rapid start-up;

4. the start-up of the degradation of a fresh MSW bed could not be initiated without the provision of buffer and

5. a fresh bed of MSW was not nutrient deficient.

Although it was shown that the provision of buffering capacity via flushing resulted in rapid start-up, there are other limitations which prevent further improvement of this waste treatment process. Those limitations include the channelling of liquid within the MSW bed and poor mixing within the bed due to the heterogeneous nature of the waste. To overcome these limitations, a fresh MSW bed was flooded instead of flushed with buffer solution. Buffer solution was introduced to the bed from the bottom of the reactor during the flooding procedure. It was shown in three independent reactor studies that this operation significantly shortened the start-up of degradation and sustained methanogenesis was achieved within a shorter time. The results also indicate that the start-up of the degradation of an acidified MSW bed can also be achieved by the provision of a sufficient amount of buffering capacity through the action of flooding.

The results and knowledge generated from the experimental studies were encapsulated in a lumped parameter kinetic model. The kinetic model developed by Nopharatana (2000) to describe the anaerobic digestion of insoluble carbohydrate in a well mixed system was modified. Six modifications were incorporated into the model to improve predictions on the output variables. To improve numerical stability and the efficiency of the model, a novel solution procedure was developed to solve the differential algebraic equations set. Parameter estimation was carried out using the results of one reactor study. Model validation was then carried out using the results of the other two reactor studies. Simulations were carried out on the validated model to demonstrate that the model is capable of satisfactorily predicting the influence of buffer, inoculum and moisture on the start-up of degradation during anaerobic digestion of MSW in flooded conditions.

Keyword Anaerobiosis
Sewage -- Purification -- Anaerobic treatment

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
 
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Created: Mon, 21 Nov 2011, 22:35:45 EST by Ning Jing on behalf of The University of Queensland Library