INFLUENCE OF CONTINUOUS VERSUS CHANNELS
ON MIXING CHARACTERISTICS AND PERFORMANCE OF
ANAEROBIC DOWNFLOW STATIONARY FIXED FILM (DSFF)
REACTORS BEFORE AND DURING WASTE TREATMENT
R. Samson, Research Officer
L. Van den Berg, Head
K. J. Kennedy, Environmental Engineer
Division of Biological Sciences
National Research Council Canada
Ottawa, Ontario
Canada KIA OR6
INTRODUCTION
The downflow stationary fixed film (DSFF) reactor developed at the laboratories of Canada's
National Research Council has been shown capable of handling a wide variety of wastes over a large
range of loading rates [1, 2, 3]. Loading rates of up to 20 kg COD/mVd were achieved at hydraulic
retention times of 1 day or less, at temperatures between 10 and 55 C, with methane productivities
as high as 3-4 m3 CH«/mVd, and COD removal efficiencies of 27-88% depending on the type of
wastes and loading rates [1, 4]. The DSFF reactor has also been shown to handle hydraulic overloading (resulting in loading rates of up to 94 kg COD/mVd) as well as intermittent or continuous
loading.
Observations about the performance of the DSFF reactors usually do not take into account the
mixing profiles or differences between the fluid flow patterns from one channel to another. Mixing
studies using a tracer are very useful for determining the effective retention time of reactors, the
presence of dead spaces or shortcircuiting, and the distribution of the liquid at the top of reactors.
Generally, DSFF reactors treating soluble wastes appear to operate well without mechanical mixing.
Biogas production alone is believed to be sufficient to provide a good contact between the substrate
and the bacteria, but little evidence is available. The possibility of channel plugging and channelling
would be greater if mixing is not adequate. Also poor mixing should have a detrimental effect on
the rate of COD removal and on the rate of biogas production.
A recent study by Samson et al [5] has shown that recirculation dramatically improved the mixing
of the DSFF reactors. Some dead space (up to 10%) was observed with a clean-bed reactor (i.e.,
before use). Three months after start-up with a waste, DSFF reactors using vertical continuous clay
channel were shown to have between 20 and 55% of dead space depending on the number of channels
and the design of the flow distributor. It was observed that only a fraction of the total number of
reactor channels was available for the flow. These data indicated that the design of the DSFF reactors
can and should be improved to reduce the occurrence of dead spaces.
The objective of this study was to improve the performance of the DSFF reactors by changing
the configuration of the vertical channels inside the reactor to reduce the volume unavailable for the
biochemical reactions. Differences in performance were analyzed with respect to the mixing profiles
through the reactors.
MATERIALS AND METHODS
Design and Operation of Reactors
The design and operation of fixed film reactors (Figure 1) were reported in detail in a previous
paper [6]. In this experiment, two reactors A and B made of steel, coated with black asphaltic paint,
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