FATS, OILS AND GREASES-EFFLUENT REGULATIONS AND
MUNICIPAL TREATMENT PLANT OPERATIONS
David T. Lordi, Coordinator
Technical Studies and Services
Cecil Lue-Hing, Director
Department of Research and Development
The Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois 60611
INTRODUCTION
Greases, oils and fats have historically been considered responsible for causing difficulties in wastewater collection, wastewater treatment systems and in receiving waters.   This
group of compounds is generally defined by the analytical prcoedure used.   The common
method is a soxhlet extraction using hexane as the solvent.   The hexane-extractable materials (HEM) include hydrocarbons, high-molecular-weight fatty acids, sterols and lipids.
The major fractions of grease in domestic sewages have been found to be composed of
glycerides and fatty acids or salts of fatty acids [1-3].
Grease, as a component of sewage, may be derived from several possible sources.   Fecal
material contains approximately 255 mg of grease per gram [3].   Kitchen wastes, garbage
and household wastewaters also contribute to the HEM content of sewage.   Wastes from
meat, poultry and other food-processing plants contain appreciable quantities of grease,
fats and oils.   Refinery wastes and wastes from various manufacturing processes also contribute various forms and quantities of hexane-extractable materials.
In order to protect treatment plants and receiving waters, regulatory agencies and municipal sewage treatment plants usually place a restriction on the quantities of grease, oils
and fats which can be discharged into sewers and subsequently into receiving waters.   The
Illinois Pollution Control Board (IPCB). through its Rules and Regulations, has established
a limit for effluents discharged to the waters of the state at a concentration of 15 mg/1
(as HEM or equivalent) [4].   In addition there can be no discharge of visible oils or
greases.   The Metropolitan Sanitary District of Greater Chicago (MSDGC), under its Sewage and Waste Control Ordinance, limits the concentration of hexane solubles (or HEM)
for wastes discharged to the MSDGC sewers at 100 mg/1 [5].
The hexane-extractable content of domestic and mixed sewages, industrial wastes and
recovered sludges are of considerable significance since it can materially affect the methods
of handling and treatment adopted and the performance of treatment systems.   High concentrations of HEM cause problems in sewers, pumping  stations and treatment plants.
Some fats, oils or greases could solidify in sewers and either ciog sewers or manholes or
interfere with the operation of mechanical equipment.   Where floating HEM is present,
disproportionately large amounts of these materials may be discharged to receiving waters
during high flows in the sewer.   Excessive quantities of fats, oils or greases could overload
skimming devices on primary settling tanks.   Much of the HEM in sewage which is removed through the treatment processes is accumulated in the primary and secondary
sludges which must be ultimately disposed of away from the sewage treatment plant.   Excessive quantities of fats, oils or greases could interfere with anaerobic digestion processes
by causing scum formation; such scum layers may interfere with effective mixing, temperature control and gas separation [6].
In recent years, the question has been raised as to whether the present limits on HEM
should apply to certain types of wastes.   These industrial wastes fall within the broad
classification of food-related industries.   In order to address this question the MSDGC
has undertaken a number of studies concerning greases, oils and fats as defined by the
hexane extraction procedure.   These studies have included evaluation of the MSDGC
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