Grease Problems in Municipal Wastewater
Treatment Systems
SHANKHA K. BANERJI, Associate
C. MICHAEL ROBSON, Assistant Chief
BERNARD S. HYATT, JR., Chief
Environmental Division
MCA Engineering Corporation
Baltimore, Maryland 21204
INTRODUCTION
Grease, fat and oil in industrial wastewater repitsent a potential maintenance problem
in a municipal sewerage system. Clogging of sewers and wastewater treatment plant
operational problems may result from heavy concentrations of these substances. To
minimize these problems, standards are frequently established to limit the discharge of
grease, fat and oil.
This paper is based upon the authors' recent experience on several assignments
concerned with problems created by the discharge of industrial wastes with high grease
concentrations into municipal sewers. Two of these assignments will be discussed in more
detail. One assignment was an economic analysis of the relative merits of providing grease
removal facilities prior to discharge to the sewerage system compared with the installation
of removal facilities at the wastewater treatment plant itself. The second assignment was the
process evaluation and design of a pretreatment system for a poultry processing plant from
the basis of the opinions stated.
'This paper will review the following aspects of grease problems in municipal
wastewater treatment systems: 1) Methods of analysis for grease, fats and oils; 2) Problems
caused by the discharge of grease, fats and oils into sewerage systems; and 3) Available
technology for the removal of this material from wastewater, and disposal of the removed
material.
DEFINITION OF GREASE, FAT AND OIL
The Glossary — Water and Wastewater Control Engineering (1) defines grease as
follows:
"In wastewater, a group of substances including fats, waxes, free fatty acids, calcium
and magnesium soaps, mineral acids, and certain other non-fatty materials. The type of
solvent and method used for extraction should be stated for quantification."
The same references defines fats as follows: "Triglyceride esters of fatty acids,
erroneously used as synonymous with grease." Oil is generally defined as a "relatively nonvolatile liquid component that contributes to the formation of oil film and deposits" (2). Oil
is also a glyceride of a fatty acid which is liquid at ordinary temperature. Chemically, it
differs from fat in that it has a predominance of short-chain fatty acids or fatty acids with
unsaturation. The above definition excludes volatile oil of mineral origin, which
presumably is not present in large quantities in municipal sewer systems because of its
volatility at ordinary temperatures. However, for some industrial wastes, such as the wastes
from petroleum refineries or machine shops, the mineral oil component could be
significant.
From the preceeding definitions, it is evident that the terms grease, fat andoilare used
rather loosely in current literature because they do not correspond to a definite compound,
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