THE EFFECT OF LIQUID RESIDENCE TIME
(INDEPENDENT OF FLOW RATE) ON THE PERFORMANCE
OF A FIXED-FILM BIOLOGICAL REACTOR
Echol E. Cook, Associate Professor
School of Engineering & Technology
Southern Illinois University
Carbondale, Illinois 62901
Stephen M. Katzberger, Engineer
Sargent and Lundy
Riverdale, Illinois 60627
INTRODUCTION
Biological processes for the removal of organics from wastewater were in use long before they were understood.   Of the biological waste treatment processes in use today, the
trickling filter has proven to be both effective and versatile.   Although it is now considered to be less effective than the activated sludge process in the treatment of domestic
wastes, the trickling filter is ideally suited for the treatment of wastes with varying characteristics.   Since the trickling filter has the ability to recover from and handle shock
loads, it has been widely used in the treatment of industrial wastes.
Although many researchers have developed empirical equations to predict the steady-
state performance of trickling filters, they have not agreed upon which variables are most
important.   One of these variables, liquid residence time, has been emphasized by some
investigators.   Liquid residence time in a conventional trickling filter is a function of the
hydraulic loading.
The goal of this study was to examine the effect of liquid residence time on organic
removal efficiency.   In order to clarify this effect, a fixed-film biological reactor was fabricated which allowed one to vary the liquid residence time at a constant hydraulic
loading.
LITERATURE REVIEW
The trickling filter is a complex biochemical system with poorly understood kinetics.
Schulze [1], Eckenfelder [2] and Metcalf and Eddy, Inc. [3], generally agree that the removal of organics in the trickling filter is due to a combination of mechanisms.   The
organic matter in the wastewater is adsorbed onto the biological film, absorbed and utilized as a food source by the microorganisms.   The process is considered to be aerobic,
although some anaerobic activity does occur in the lower layers of the biological film.
Although many empirical equations have been developed to predict filter performance,
there is a lack of agreement as to what the controlling mechanism or mechanisms might
be.   The generally accepted equations use one of three basic approaches.
One approach, taken by Shulze [1] and Eckenfelder [2,4,5], is that hydraulic loading
limits the mass transfer of organics to the biological film.   They have reported experiments in which the hydraulic loading was held constant and the influent organic waste
concentration was changed with no significant change in organic removal efficiency.
Eckenfelder [2,4,5] has also emphasized the use of liquid residence time, which is a function of hydraulic loading in conventional trickling filters, as a variable in empirical equations predicting filter performance.   Craft and Ingols [6], however, observed a filter in
which the organic removal efficiency was independent of the liquid residence time.
A second approach, taken by Galler and Gotaas [7], was that the organic feed concentration is the controlling variable in predicting filter performance.   Galler and Gotaas
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