Anaerobic Digestion of Pear Waste.
Factors Affecting Performance
L. van den BERG, Senior Research Officer
Division of Biology
C. P. LENTZ, Head
Food Technology Section
Division of Biological Sciences
National Research Council of Canada
Ottawa, Canada
INTRODUCTION
This paper presents results from part of a continuing study of the anaerobic digestion
of food plant wastes using the anaerobic contact process. For many food processing plants,
factors such as location, climate, and seasonal operation, as well as high strength and
nutritionally unbalanced wastes may make it difficult or uneconomic to use conventional
methods such as activated sludge treatment, biological filtration, lagooning, or spray
irrigation. The anaerobic contact process, however, with its advantages of relatively simple
equipment, limited space requirements, low accumulation of biological solids, possible high
loading rates (especially with concentrated wastes), and the production of a useful byproduct (methane), would appear to offer a solution for some food plant waste treatment
problems. The anaerobic contact process, developed only recently (1,2,3,4,5,6,7,8) uses
complete mixing of the fermenter contents to overcome the inefficient use of fermenter
volume and low reaction rates characteristic of conventional anaerobic digestion. Long
solids retention times with short liquid retention times are achieved by recirculating
fermenter liquid through a solids separation or concentration unit which minimizes waste
of solids in the effluent from the process (9).
The main objective of the study of this process is to determine the effect of operating
variables such as loading rate, liquid retention time, pH, alkalinity, temperature, and
addition of nutrients, on process efficiency. Study of factors affecting adaptation of the
fermentation to different wastes, solids separation and sludge return, and new methods of
analysis (assessment of anaerobic bacteria populations, for example) are also planned.
Actual rather than synthetic waste was chosen as fermentation substrate because it was felt
that the more direct approach would better serve the objectives of the study. Pear waste was
selected for the initial part of the work because it was typical of many fruit processing wastes
(high carbohydrate content, nutritionally unbalanced), and could be readily obtained and
stored as a frozen concentrate.
Promising results were obtained in the initial phase of the study (10). Treatment
efficiencies, in terms of reduction in chemical oxygen demand, varied between 75 percent
and 95 percent, depending on volatile solids loading rate (0.05 -0.14 lb/ft3/day)and liquid
retention time (10-86 days). Addition of yeast extract, in addition to ammonium and
phosphate salts, was necessary. Separation of solids from the process effluent for return to
the fermenter was a problem, however, and solids return did not seem to be directly related
to return of bacterial floe. Determination of changes in the methane-forming bacterial
population was also a problem.
This paper presents results obtained over a lower range of liquid retention times (0.5 -
30 days), and a higher range of volatile solids loading rates (0.10 - 0.46 lb/ ft3/ day) than were
tested initially, and also includes the results of a study of nutrient requirements. Further
313