FOOD PROCESSING WASTE TREATMENT BY ANAEROBIC DIGESTION
L. van den Berg, Senior Research Officer
C. P. Lentz, Head
Food Technology Section
Division of Biological Sciences
National Research Council of Canada
Ottawa, Ontario, Canada Kl A 0R6
INTRODUCTION
Previous papers from this laboratory [1,2,3] have dealt with factors affecting treatment efficiencies in the anaerobic digestion of pear peeling waste and rum stillage waste,
as typical but difficult-to-treat food processing wastes, using the anaerobic contact process
at 35 C with 30-liter fermenters.   In this process the active bacterial mass can be retained
in the fermenter for a long time (100 days or more) even with dilute wastes, by recirculating fermenter liquid through a solids separation or settling unit which minimizes loss of
solids (microorganisms) in the effluent.   Factors studied were hydraulic retention time,
volatile solids loading rate, bacteria (sludge or biomass) settling efficiency and nutrient
supply.   This work showed that the process was well suited to the two wastes studied,
with efficiencies in removal of chemical oxygen demand (COD) of up to 90%, depending
on conditions.   Sludge settling efficiency was markedly dependent on fermenter conditions, shape of the sedimentation vessel and degree of agitation in this vessel.   To obtain
a reasonably satisfactory fermentation under most conditions, it was necessary to add an
ammonium salt, phosphate and nutrient factors present in yeast extract or milorganite.
Since the practicality of the anaerobic contact process depends not only on treatment
efficiency, but also on capacity of the fermenter (usually expressed as volatile solids
added per unit volume and time, the volatile solids loading rate, (VSLR) and hence on
the capital investment required, the emphasis in the work reported in this paper was
placed on determining the maximum volatile solids loading rate under a variety of conditions rather than on treatment efficiency.   In addition, the scope of the study was broadened by including bean blanching waste and two types of potato waste as well as pear
peeling waste and rum stillage waste previously studied [1,2,3].   The conditions studied
included hydraulic retention time (HRT, 1-20 days), addition of varying amounts of
ammonium salts, phosphates, sulfide or sulfate and yeast extract, a lower temperature
(29 as well as 35 C), overloading, and long periods (1-7 months) without adding feed
(starvation).   The effect on performance of inoculating unsatisfactory fermenters with
liquid from sewage digesters or from satisfactorily performing digesters using food plant
waste was also determined.   The work presented here, together with that published previously, provides detailed information on treatment of food processing waste by the
anaerobic contact process.   The paper also indicates areas in which insufficient knowledge
limits further improvements in process efficiency and broader application of the process.
The implications of the work in terms of methane production have been discussed in
detail elsewhere [4].
EXPERIMENTAL
Fermenter Design and Operation
Details of design and operation of the fermenter units have been described previously
[2].  Briefly, each fermenter unit consisted of a 30-liter cylindrical fermenter, a 6-liter
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