Anaerobic Degradation of Organic Materials - Significance of the Substrate Surface Area

Laurence M. Palmowski, School of Engineering and Technology, Deakin University, Australia

Johannes A. Müller, Institute of Mechanical Process Engineering, Technical University of Braunschweig, Germany


Executive Summary

In anaerobic degradation of organic materials containing principally particulate solids, the rate-limiting step is the biological hydrolysis of the solids. To support the hydrolysis, the particle size of various organic substrates was reduced in these investigations by comminution. Subsequently, the samples were seeded with digested sludge and degraded under anaerobic conditions in batch experiments.

Two positive effects of the comminution could be observed. The first effect mainly occurred using substrates with high fibre content, which are particularly resistant to biodegradation. They have a low degradation degree and low degradation rate. With these substrates, a significant improvement of the degradation degree was obtained for samples whose particle size had been reduced to x < 200 mm by comminution in comparison with the non-comminuted samples. Consequently, the yield of useful digester gas was increased and the amount of residual organic material was reduced. This effect did not occur using substrates with low fibre content. These substrates such as apples are already readily and almost completely degraded (up to 95%) without any mechanical treatment. The second effect of comminution that appeared with all substrates tested, but particularly with substrates rich in fibres, was a significant reduction of the digestion time due to higher degradation rate after comminution.

The first identified cause for both positive effects of comminution on degradation was the increase in the specific surface area of the samples. The present paper discusses and compares several methods for measuring the specific surface area of organic samples, which were particle size analysis, BET-N2-adsorption method and peroxidase adsorption. It was found for hay and rice samples that the outer surface area is much smaller than the pores surface area and that the latter does not change as much as the former (approx. 10 times less). The discrepancy between the surface area determined by BET-measurements and the surface area accessible for enzymes was illustrated in case of a cellulose sample. This pointed out the problems associated with the use of N2-adsorption for the purpose of this study, where the surface area accessible to micro-organisms and exoenzymes is the most important one. Additionally the release of dissolved organic matter was measure to quantify readily degradable matter that becomes available as a result of the comminution process.

To demonstrate the significance of the specific surface area in the anaerobic degradation of particulate substrates, a kinetic model was developed where the rate of hydrolysis was based on the specific surface area of the sample. Good agreements were obtained between model and experimental degradation experiments carried out with samples of various specific surface areas. These results reinforced the significance of the sample surface area in anaerobic degradation processes.

However, it was shown that the surface area is not the only parameter influencing the anaerobic degradation. Other major effects of comminution responsible for the increased degradation degree and degradation rate of the biological process were identified and discussed. These include the increase of dissolved compounds due to the rupture of the sample cells, the exposition of surface areas previously inaccessible for microbial degradation, and the alteration of the sample structure such as the lignin-cellulose arrangements.

Key Words

Specific surface area, biological degradation, anaerobic digestion, particulate organic substrate, mechanical pre-treatment

Origin

L. Palmovski und J. Müller (2002). Anaerobic Degradation of Organic Materials - Significance of the Substrate Surface Area. World Congress of the International Water Association, Melbourne, Australia, April 2002, Conference Proceedings CD-ROM