Kinetics of
Fixed Film Biological Reactors
BILLY H. KORNEGAY, Graduate Student
JOHN F. ANDREWS, Professor
Environmental Systems Engineering
Clemson University
Clemson, South Carolina
INTRODUCTION
In spite of the advances made in the trickling filter process since its inception
in 1889 (1) the search for a more rational design procedure still continues. Progress in achieving such a design technique has apparently been hampered by an
inadequate knowledge of the biological process. Since efficiency of design is contingent upon a fundamental knowledge of the process, more basic studies are required.
In order to gain some insight into processes which depend upon biological
films, a theoretical investigation was made of the kinetics of fixed-film, biological reactors. After several preliminary studies it was found that completely
mixed, annular reactors would permit the greatest degree of control and measurements could be made with ease and accuracy. If the kinetics of fixed biological
films can be described, even for a simple system, then a greater understanding of
biological films will have been gained and a possible foundation established for a
rational method of design. Due to space limitations, only one portion of this investigation is being presented. However, this portion should be sufficient to support the validity of the continuous culture approach. While the basic principles
should be applicable to the trickling filter, it should be understood that the equations presented are not intended to serve as a mathematical approximation ofthe
process.
THEORY
Fixed-film reactors, like other biological processes, depend upon the action
of microorganisms for the removal of soluble organics. Therefore, it should be
possible to describe this removal in terms of biological growth. This approach has
been used in slurry processes through the application of the theory of continuous
culture of microorganisms. It is anticipated that fixed-film, biological processes
are also amenable to description by this theory.
In deriving a mathematical model for a fixed-film reactor, two primary facts
must be recognized. The first is toat toe organisms and substrate do not occupy a
common volume as is the case for slurry reactor systems. Therefore, any relation
between organisms and substrate must be made on a mass basis. The second is
that toe entire mass of attached microorganims is not active in the removal of
soluble organics. This latter point was clearly indicated in the work of Sanders (2)
and investigators at the Water Pollution Research Laboratory in England (3). This
concept is illustrated in Figure 1. It should be noted that the active film thickness d constitutes only a portion of toe total film thickness h.
- 620 -