20    COMPARISON OF THE ANAEROBIC CONTACT AND
PACKED BED PROCESSES FOR THE TREATMENT OF
EDIBLE OIL WASTEWATERS
C. B. Saw, Senior Research Associate
G. K. Anderson, Senior Lecturer
J. A. Sanderson, Research Associate
Department of Civil Engineering
University of Newcastle upon Tyne
Newcastle upon Tyne
United Kingdom NE1 7RU
INTRODUCTION
The process of neutralization, which is carried out during the refining of certain edible oils,
produces a soapstock from which fatty acids are recovered by means of acid splitting. Acid splitting is
often carried out by the addition of sulfuric acid to the soapstock which causes the free fatty acids to
separate from the acidic water phase. Once this separation has occurred, the water phase is drained to
a fat separator where residual free fats are recovered. The final acid water represents the effluent
from the process and contains predominantly glycerol, fat, and sodium sulfate.
Conventional treatment of acid water involves a combination of chemical, physical, and biological
processes with substantial mechanical input required for the dewatering of byproduct sludges. Chemical treatment with lime, followed by sedimentation, has been carried out in order to reduce the
concentration of fatty matter to a level which can be tolerated by conventional aerobic biological
treatment processes, e.g., activated sludge. Since chemical treatment alone removes only 50% of the
COD, there is still a need for a sizable aerobic process to fully treat the effluent. This combination of
treatment is known to be effective but involves high capital and operating costs.
Possible Role of Anaerobic Process in the Treatment of Acid Water
Experience from sewage sludge digestion has shown that anaerobic systems have the ability to
degrade fatty matter. As such, it was thought that anaerobic biological treatment would be preferred
to chemical treatment as a preliminary treatment process for acid water. The benefits sought by
adopting the above process were:
- degradation of fats,
- low chemical requirements,
- low sludge production,
- low power requirements, and
- moderate COD removal efficiency (>70%).
For anaerobic treatment to be economically attractive in terms of minimizing the capital costs, it is
also important that such treatment should be possible in one of the new generation of anaerobic
reactors, such as, a contact digester, packed-bed digester, upflow sludge blanket digester or the
expanded/fluidized bed digester. However, there are no such processes operating in the U.K. on
edible oil refining wastewaters, and therefore it was decided, following preliminary laboratory tests,
that pilot plant studies should be carried out in order to clearly identify the feasibility of treating acid
water by means of an anaerobic process.
178