ANAEROBIC WASTE TREATMENT EFFICIENCY COMPARISONS
BETWEEN FIXED FILM REACTORS, CONTACT DIGESTERS
AND FULLY MIXED, CONTINUOUSLY FED DIGESTERS
L. Van den Berg, Research Officer
C. P. Lentz, Research Officer
D. W. Armstrong, Research Officer
Division of Biological Sciences
National Research CouncU of Canada
Ottawa, Canada K1AOR6
Performance comparisons between anaerobic waste treatment processes have seldom
been made, even on a laboratory scale, because of the technical difficulties involved.
Comparisons take a great deal of time and effort to make, but also the practical difficulties
in maintaining steady state conditions in anaerobic waste treatment systems are large.
These difficulties are particularly serious when processes are stressed to their maximum
performance. Also, because the performance of an anaerobic waste treatment process
depends markedly on the type of waste, comparisons between processes should be made
with more than one type of waste, and this compounds the difficulties.
Our interest in advanced technologies of anaerobic waste treatment, especially the
anaerobic contact process 11-6] and anaerobic fixed fdm reactors [7,8] has led us to make
some performance comparisons on a laboratory scale. The processes which were compared
cover completely mixed digesters (equivalent to completely stirred tank reactors), contact
process digesters and fixed film reactors. The last two processes differ in the way microorganisms are retained in the fermenter: the contact process depends on settling and sludge
return, the fixed film reactor depends on attachment and growth of microorganisms on
fixed surfaces. In the present study two widely differing wastes were used: a bean blanching
waste mainly containing soluble starch and protein, and a simulated sewage sludge mainly
containing insoluble organics such as cellulose, fat and protein.
EXPERIMENTAL
The design and operation of the fermenter units used for the anaerobic contact process
have been described in detail earlier [1-5] and have been sketched in a companion paper
[6]. Briefly, the unit consisted of a fermenter (30 - liter volume), a settling flask (4 ft
liters), a refrigerated feed supply, variable speed pumps and stirrers. The units were located
in a room controlled at 35 C (as were all other fermenters). Moderate agitation in the fermenter ensured adequate mixing with as little effect on settleability as possible. A continuous
settling unit was used with bean waste, whde a fdl-and-draw type settling unit was used with
simulated sewage sludge, because of the concentration and settling characteristics of the
sludge [ 5).
Fixed film reactors varied in size from 0.8 to 35 liter. The film support material was
baked potters clay (Miller 50, fired to 1000 C) and the surface-to-volume ratio was 100-140
m2/m3. Small reactors were made from a single cylindrical tube, about 2.8 cm in diameter
[7] and were used only for bean blanching waste. The 35 - liter reactors consisted of a large
glass jar packed with square channels, about 2.8 x 2.8 cm square and 66-68 cm tall (Figure
1). Feed and, where used, recirculated effluent were pumped in at the top of reactors and
effluent was withdrawn at the bottom. Reactors were started by filling them with effluent
from a contact process digester fed simulated sewage sludge or liquid from a municipal
sewage sludge digester.
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