RESOURCE RECOVERY—OIL FROM SLUDGE
S. P. Gambhir, Senior Chemical Engineer
Gilbert/Commonwealth
Jackson, Michigan 49201
INTRODUCTION
This paper is a review of the oil reclamation techniques used to recover usable cutting, honing, and
grinding oils from oily sludge generated from wastewater treatment processes in metal working industries. The treatment of the wastewaters from these industrial processes is mandated in order to satisfy
regulatory specifications.
Traditionally, the oils used in the metal working industries contain less than ten percent oil. The
life of the oil is extended by filtration techniques as long as possible which reduces the oil and disposal
costs. In many operations, due to spillage, machine changeover, or other reasons, oil is incorporated
as a contaminant in the wastewater. The major portion of the oil is emulsified oil while 20 to 30 percent can be entrained (free) oil.
In a typical oily wastewater plant, the typical unit operations are: skimming, emulsion breaking,
dissolved air flotation, sludge dewatering, centrifuging and/or ultrafiltration. Additional unit operations may be used to improve the effluent quality as dictated by the nature of wastewater and the
effluent standards. Table I lists various unit operations used in the treatment of oily wastewaters. The
separation of free oil from water is dependent on size, specific gravity, viscosity, and the design of the
separation equipment.
The electrically stabilized emulsified oil requires neutralization of the electrical charge before coalescence. The treatment can consist of chemicals to neutralize the charge in case of ionic surfactants or
by coagulation to overcome the effects of surfactants. This is followed by flocculation, commonly
using alum.
Two processes are commonly used: (1) the acid-alum process; and (2) the polymer-alum process.
Flow diagrams of both processes are shown in Figures 1 and 2, respectively. Both acid-alum and polymer-alum processes work well. The differences between acid-alum and polymer-alum processes are
listed in Table II.
Free oil and sludges generated can be further processed to reclaim the oil. Free oil from the CPI's
and API's can be readily extracted by cooking, centrifuging or steam dehydration depending upon the
quality of the skimmings. In the acid-alum process, the oil is treated in cookers either separately or in
combination with flocculated oil. In general, free oil is treated separately. A typical flow diagram of
the oily waste treatment process with oil recovery options is shown in Figure 3.
FREE OIL TREATMENT
The traditional approach in cooking free oil is to acidify the oil to one-half percent or higher in a
heated cooker made of fiberglass or of fiberglass lined steel. The cookers are equipped with steam
coils or live steam injectors. Live steam injection is sometimes preferred for cost and corrosion considerations. The batch is heated up to 180 F. The contents are allowed to settle. Three layers are formed:
(1) a clean oil phase on top; (2) a rag layer in the middle; and (3) a water layer at the bottom.
The three phases are separated. The oil can either be sold to an outside contractor or reused and
burned in the boilers. The rag layer is recycled until it builds up in quantity and is ultimately disposed
once it is further dewatered in a filter press.
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