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Development of a control technique for the automated safe operation of anaerobic digesters at high loading rates

Carnaje, Naomi P. (1995) Development of a control technique for the automated safe operation of anaerobic digesters at high loading rates. Masters by Research thesis, Murdoch University.

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The anaerobic digestion process is of world wide significance for the treatment of highly concentrated wastes such as industrial and rural effluents, and sewage sludge. As its by product, a renewable fuel (CH4) is produced. However, this process is known to be relatively unstable. Especially when loaded with high concentrations of organic material, volatile fatty acids (VFA) are produced rather than methane (CH4) gas resulting in severe ecological, hygienic, and economic problems. This study was aimed to 1) investigate digester behavior under high loading rates; 2) test the usefulness of the stoichiometric conversion efficiency as a digester control parameter and 3) develop a simple yet efficient control technique that could allow the safe operation of anaerobic digesters at high loading rates.

A 1-liter laboratory anaerobic digester was continuously fed with a synthetic wastewater containing glucose as the main substrate. Step changes in the loading rate were applied over a time interval between 12-24 hrs and the volume of the methane produced was monitored on line by an IBM-PC computer. It was observed that, in general, the gas production rate was proportional to the feedrate (loading rate). However, at very high loading rates, the CH4 production rate did not reflect the increase in the feeding rate. Consequently, the VFA accumulated and the H2 partial pressure increased from less than 60 ppm to more than 80 ppm. Two types of overloading were observed: 1) hydraulic overloading which resulted in propionic and acetic acid accumulation and 2) organic overloading which resulted in acetic acid accumulation only.

The proportionality between the loading rate and the gas production rate is stoichiometrically expressed as conversion efficiency. The conversion efficiency was used to control the digester by pre-determined step changes in the loading rate. Using a cut-off conversion efficiency of 75 %, the digester was fed according to its current gas production potential, and constantly pushed to production limit. This control strategy maintained the digester stable and close to its maximum sustainable loading rate. A maximum CH4 production rate of 0.8 L CH4 /L d was observed at a loading rate of 14.5 mmoles glucose/L day and hydraulic retention time of 10 days. However, the conversion efficiency is a useful control parameter only if the feed composition is known.

The relative measure of the ratio of gas production rate to the loading rate was investigated for its potential to be used for digester control in cases where the feed composition is not known. An increase in the loading rate was initiated and the response of the gas production rate to that increase was monitored. As long as the digester operated efficiently, the ratio of the % increase in the CH4 production rate over the % increase in the loading rate (designated as R-factor) was close to 1. By keeping the value of this ratio greater than 0.5, the loading strategy based on the R-factor, was found useful in the control and optimisation of the performance of the digester.

The results indicated that anaerobic digesters could be allowed to operate close to its maximum sustainable loading rate and increase its productivity by using the conversion efficiency and the R-factor as control parameters.

Item Type: Thesis (Masters by Research)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Cord-Ruwisch, Ralf
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