Treatability of Oily Wastewaters
from Food Processing and Soap Manufacture
P. L. McCARTY, Professor of Environmental Engineering
Stanford University
Stanford, California
D. J. HAHN, Engineer
G. N. McDERMOTT, Senior Engineer
P. J. WEAVER, Project Leader
The Procter & Gamble Company
Cincinnati, Ohio
INTRODUCTION
This paper summarizes and expands upon presentations made at two different
meetings of the American Oil Chemists Society on the subject of animal and vegetable oils
and fats in municipal treatment systems (1,2). McDermott and Polkowski discussed some
of the alleged difficulties associated with fats, oils and other "grease" components in
wastewaters discharged to municipal systems. In general, certain reported treatment
difficulties are attributed to fats, oils, and greases as a group even though there are
considerable differences in the properties of the members. Mineral-type oils are difficult to
degrade and are sometimes toxic, whereas animal and vegetable oils and fats are readily
degradable and non-toxic. In similar ways, there are other differences in the degree of
difficulties that the various forms (flotable and emulsified) and types (hydrocarbons, fatty
acids, glycerides. sterols, etc.) of fats, oils, and greases create in municipal wastewater
treatment systems. At least part of the reason for some of the above misconceptions
regarding fats, oils, and greases stems from a tendency by many to generalize and to lump
similar substances into one category for the sake of simplicity; the more scientific reason,
however, perhaps comes from the lack of suitable analytical techniques to differentiate
between the various forms
This paper will delve into the effect that wastes from processing vegetable and animal
fats and oils have on municipal treatment systems, particularly those involving biological
degradation and sludge filtration. There have been several reports on the biological
treatability of fats and oils and long chain fatty acids resulting from their degradation. The
results of various investigations on aerobic treatability were summarized by Loehr and
Roth (3), who also made some evaluations on the effects of different properties of these
materials on relative rates of degradation. In general, they found that shorter chain length
fatty acids were metabolized more readily than the longer chain acids, unsaturated acids
were more readily degraded than saturated ones, soluble acids could be metabolized faster
than insoluble ones, and the rate of degradation of insoluble fatty acids was a function of the
size of the insoluble particles. The more insoluble and larger fatty acid particles required
greater time for degradation than those with opposite characteristics.
Oils, fats, and fatty acids of animal or vegetable origin are readily degraded in
anaerobic systems and account for the major portion of methane produced from sludge
treatment at municipal sewage treatment plants. O'Rourke (4) conducted an extensive
investigation into the requirements for digestion of this type of grease. He found that over
70 percent of the fatty acids associated with vegetable and animal fats and oils was
converted to methane gas in high rate digestors with detention times often days and at a
temperature of 35 C. Similar observations on the degradation of fatty acids have been noted
by Huekelekian and Mueller (5), and Novak and Carlson (6). Rudolfs (7), in studies of rates
of decomposition of grease in anaerobic digestion, has shown that the greases are rapidly
destroyed producing combustible gases. He noted also that grease skimmings produced
about 40 percent more gas than fresh primary solids.
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