74    EFFECT OF REACTOR CONFIGURATION ON
OPERATION OF A PHARMACEUTICAL
WASTE TREATMENT SYSTEM
David J. Wolf, Staff Engineer
D. Keith Emerson, Staff Engineer
Merck and Co., Inc.
Elkton, Virginia 22827
INTRODUCTION
A complete mixed nitrifying activated sludge waste treatment system, treating 1.2 MGD
(10,000-15,000 kg BOD/day) of pharmaceutical wastewater from both synthetic and fermentation
chemical processes, has experienced severe filamentous bulking and periods of very poor settling (no
settling at all in a static settling test). The activated sludge contains excessive levels of Type 0092 and
Type 0675 filamentous organisms and high levels of Haliscomenobacter hydrossis and Nostocoida
limicola I. organisms typically associated with low F/M operation. The system operating parameters
are as follows: SRT 5-9 days, MLSS 3000-5000 mg/L, temperature 35°C, HRT 2 days, and F/M
0.2-0.5. A pilot study program was developed to gain an understanding of the bulking problem. A
change in reactor configuration from three aeration basins in parallel flow to series flow proved
successful at the pilot scale in reducing the filamentous population and improving settling characteristics. This configuration has been in operation in the full-scale treatment system for 133 days and, after
several additional modifications, has proved successful at reducing the filamentous organism concentration and improving settling performance.
BACKGROUND
Under ideal conditions, the microbial population found in an activated sludge system consists
primarily of single cell organisms that grow together in dense, compact agglomerations (floes). These
dense floes readily settle out from the treated waste stream. Filamentous bulking occurs when the
activated sludge microbial population is dominated by filamentous microorganisms causing the formation of disperse, poor-settling floes that are interconnected with the long, string-like filamentous
organisms extending from the floes.'
Various nutrient limitations (D.O., phosphorus, nitrogen, or organics) are thought to promote
filamentous bulking; filamentous organisms, which tend to protrude from the floes into the bulk
solution and have a higher surface area to volume ratio than floe forming organisms, may have a
selective advantage in the utilization of substrates present in low concentrations.2-4 The ability to
assimilate limiting substrates provides a growth rate advantage and leads to a dominance of filamentous organisms. At low substrate concentrations, pure cultures of typical filamentous organisms
possess a higher growth rate than pure cultures of typical floe forming organisms, while at high
substrate concentrations, the converse is true.5
Many researchers have shown that reactor configuration plays a significant role in the cause of
filamentous bulking.'•'•6'7 As an activated sludge aeration system approaches a plug flow configuration it is less likely to experience filamentous bulking.* A plug flow reactor will form concentration gradients, allowing the floe forming organisms to proliferate in the elevated temporal substrate concentrations. Conversely, a complete mixed system provides the lowest possible substrate
concentration and is most likely to result in filamentous bulking.
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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