FACTORS AFFECTING SEDIMENTATION IN THE ANAEROBIC
CONTACT FERMENTATION USING FOOD PROCESSING WASTE
L. van den Berg, Senior Research Officer
C. P. Lentz, Principal Research Officer
Division of Biological Sciences
National Research Council of Canada
Ottawa, Ontario K1AOR6
INTRODUCTION
Stirred anaerobic digestion, the anaerobic filter and the anaerobic contact process are the
main processes available for relatively rapid treatment of organic wastes with concomitant
production of methane [ 1 ]. Stirred or mixed anaerobic digestion is most suitable for relatively concentrated wastes, which allow high loading rates with long solids retention times.
The anaerobic filter was developed for soluble, low strength wastes, and little information is
available on it, especially for other wastes. The anaerobic contact process is suitable for
wastes of low and intermediate strength, particularly those with a relatively high proportion
of suspended solids. The long solids retention times possible even at relatively short hydraulic
retention times are advantageous for the breakdown of particulate matter such as cellulose
and fat. The anerobic contact process would therefore appear to be the method of choice in
the treatment of many moderately strong food processing wastes, as well as other wastes
containing plant biomass material, either for waste treatment or for production of methane.
In the anaerobic contact process the sedimentation and return of the suspended material
(microbial floe and undigested material) largely determines the efficiency of the process.
Yet the suspended material often settles slowly, only in part or, occasionally, not at all
[1-13], Part of the problem may be insufficient flocculation of bacteria, gas hold-up leading
to a buoyant floe, or changes in fermenter (digester) performance such as result from overloading. In many instances, however, no reason is apparent. With some wastes, such as
dilute beet sugar processing waste, settling does not appear to be a problem [5].
Although attempts have been made to get around the sedimentation problem (slow agitation [4], degasification [4,7], flocculating agents [2], and addition of inert material [6],
for example) and these attempts have sometimes been successful, little is known about the
settling characteristics of the suspended material in anaerobic fermenters. The extent to
which sedimentaiton agrees with established theories [14,15] and the factors affecting flocculation [16,17] have not been studied in detail, for example. The effects of factors such as
suspended solids content of fermenter liquid, fermenter loading and conditions, and shape
and volume of the sedimentation vessel on linear and volumetric sedimentation rates have
received relatively little attention. Also, little quantitative information is available on the
extent of (at present largely unexplainable) variability in sedimentaion behavior [4,6,7].
Yet such information is a prerequisite to developing a sedimentation system that will work
consistently as part of the anaerobic contact process.
The results presented and discussed in this paper were obtained mainly with food processing wastes, especially pear waste. Pear waste was used because it contains a large amount of
difficult-to-digest suspended solids (stone cells) and because much information was available
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