COMPARISON BETWEEN UP- AND DOWNFLOW ANAEROBIC FIXED
FILM REACTORS OF VARYING SURFACE-TO-VOLUME RATIOS
FOR THE TREATMENT OF BEAN BLANCHING WASTE
L. Van Den Berg, Senior Research Officer
C. P. Lentz, Principal Research Officer
Division of Biological Sciences
National Research Council of Canada
Ottawa, Canada K1A 0R6
INTRODUCTION
The need for greater efficiency and economy in the treatment of industrial and
municipal liquid wastes and interest in methane production from biomass as a renewable
source of energy has led to renewed interest in anaerobic digestion and the study of new
types of fermenters. These new types have in common the retention of active microbial
biomass in the fermenters, instead of allowing it to escape with the treated effluent.
Included in these types are the anaerobic contact process [1-5], the anaerobic filter
(upflow) [6-9], and upflow [10,11]  and "plug" flow [12]  reactors with indication of
fluidized or expanded bed operation. The anaerobic filter, a tower filled with small stones,
rings, or other types of filter media through which the waste to be treated flowed upward,
was thought to be characterized by an active microbial film attached to the filter medium.
There is now indication that much of the active microbial mass is retained in the void
spaces in the medium in particles, masses, or clumps, perhaps being tumbled or rolled by
the movement of the passing fluid.   This tumbling or rolling action is related to the
fluidized or expanded bed concept [7,9,10-12].   An actual fixed film reactor would also
be a logical development but until now it does not appear to have been studied.
A fixed film reactor has a number of advantages over other types of fermenters
designed to retain the active microbial biomass in the fermenter.   It obviates mixing,
difficult to provide for adequately in large tanks, and settling, a major problem with the
anaerobic contact process.   It can be designed to avoid channelling, a problem inherent
in upflow filters, and possibly in other upflow reactors. Also, with the bacteria adhering
to a surface, the reactor height is not limited as it appears to be with the anaerobic filter
or other upflow reactors.
A fixed film reactor may be conceived using a variety of forms of surface to support
the adhering film, but in this first study of which we report preliminary results vertical
glass tubes or columns were used as fermenters to provide simplicity in predicting active
surface area and volume, as well as pattern of flow. Bean blanching waste supplied by a
commercial canning company (Canadian Canners Ltd., Burlington, Ontario) was used as
substrate. Factors studied during the first year were diameter (hence, surface-to-volume
ratio) and direction of flow (up or down), as well as the relation between performance
and the length of time columns had been in operation. Performance was mainly based on
removal of chemical oxygen demand and of volatile solids and rates of methane production.
Observations on the distribution of microbial activity between surface and bulk of liquid
were also made. Results obtained were compared with those obtained earlier for bean
blanching waste using the anaerobic contact process [3,4].
EXPERIMENTAL
Reactors used and their operation are sketched in Figure 1.   In downflow reactors,
feed (bean blanching waste, 1% total solids, Table I) was pumped in at the top of the
reactor together with liquid recirculated from the bottom of the reactor. Effluent was
removed from the bottom of the reactor through a level control device. Gas was removed
at the top.   In upflow reactors feed was pumped in at the bottom, together with recir-
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