63    ANAEROBIC TREATMENT OF APPLE POMACE
AND WASTEWATER
Thomas E. White, Research Associate
David J. Malecki, Research Technician
William J. Jewell, Professor
Agricultural Engineering Department
Cornell University
Ithaca, New York 14853
INTRODUCTION
Apple pomace is the waste solids resulting from the production of apple juice or cider. The
estimated disposal cost of the 1.5 million tons of pomace generated in the U.S. each year exceeds $10
million. Because of the negative impact of these costs, an alternative approach for pomace disposal —
anaerobic digestion — has been proposed by several researchers.1-3 Follow-up studies3'4 have demonstrated that pomace is composed of readily biodegradable materials, easily digested at high efficiencies, and a significant resource for biogas production. An economic feasibility analysis of the
anaerobic process indicated that the payback period for a facility that totally eliminated pomace
would be less than five years.4
In addition to pomace, many processors must discharge large quantities of wastewater generated
during apple processing operations. Effluent limitation guidelines for these streams (point source
discharge) generally require best conventional control technology.5 Currently, most large processors
employ aerobic processes for pre- or final treatment requirements. Because of the high operational
cost of these systems, however, there is considerable interest in anaerobic pretreatment of wastewater
or combined pomace-wastewater digestion. Possible benefits of these alternatives include recovery of
the wastewater energy and reduced cost for effluent polishing.
A major question which remains to be answered with respect to anaerobic digestion of pomace
concerns the potential for use of phase-separated systems. Such systems could potentially exploit the
tendency of pomace to rapidly acidify and thereby improve hydrolysis rates of the pomace solids.6
Furthermore, phase separation, if achievable, greatly increases the options available for apple waste
treatment with regard to reactor design and energy savings. However, little is known regarding the
kinetics of the solubilization process for complex substrates. An endeavor of this study was to expand
the understanding of this area. Specific research objectives were:
• Contrast the kinetics and efficiencies of phase-separated pomace digestion systems with
conventional digestion;
• Investigate the potential of combined pomace-apple wastewater treatment with phase-
separated digesters; and
• Identify options for pomace and apple wastewater treatment which optimize treatment
efficiencies and energy recovery.
WASTE CHARACTERISTICS
Conventional apple juice manufacturing processes lose about 23% of the juice and sugar present in
apples, but recent trends in the industry towards high efficiency presses have resulted in pomace with
less sugar and higher solids concentrations. The physical and chemical properties of apple pomace and
apple processing wastewater are given in Table I. Review of these data indicate that ensiled apple
pomace is generally greater than 20% total solids and approximately 16% carbohydrate on a wet
basis. As shown by the protein fraction and total Kjeldahl nitrogen values, the amount of nitrogen in
pomace is limited to approximately 1% of the dry weight.
The most important factor having an impact on wastewater quality for juice manufacturers is
quality of the raw fruit.7 For other products, the type of peeling operation employed, water use, and
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
551