GROWTHS AND ACTIVITIES OF FIXED-FILMS
IN TREATING SUGAR WASTE
Ju-Chang Huang, Professor
Shoou-Yuh Chang, Assistant Professor
Yow-Chyun Liu, Graduate Research Assistant
Zhang-Peng Jiang, Visiting Scholar
Environmental Research Center
Department of Civil Engineering
University of Missouri—Rolla
Rolla, Missouri 65401
INTRODUCTION
Fixed film biological wastewater treatment processes, such as trickling filters, biological fluidized
beds, and rotating biological contactors (RBC), have gained more attention recently due to their less
energy consumption, ease of operation and low maintenance requirements. However, the dynamic
nature of biofilms and their kinetic characteristics have not been fully understood. Designs of the
fixed film treatment system are mostly empirically oriented, which in some cases could lead to a failure.
Studies conducted by Hoehn [1], Williamson & McCarty [2], Sanders [3], Kornegay & Andrews
[4], Characklis [5], and other investigators [6,7] have described the concepts of film development
through physical, chemical, and biological aspects. These studies have made contributions to our present understandings of the biofilm phenomenon. The study conducted by Hoehn [1] was concerned
with a conceptual description of fixed film development, which was influenced by the molecular diffusion of dissolved oxygen and substrate. These two parameters were also important in controlling the
waste stabilization rate. When the biofilm was thin, oxygen and substrate were not limited to the film
communities, and the film could be described as aerobic. When the film became thicker, diffusion of
either oxygen or substrate could become a limiting factor. In many previous studies [6-9], oxygen had
been assumed to be the rate-limiting factor. The reason for making such an assumption was that the
DO concentration in a treatment system was generally only several mg/l, while organic concentration
could be as high as hundreds or thousands mg/l in terms of the biochemical oxygen demand (BOD).
Williamson & McCarty [2] had suggested that the oxygen to glucose ratio in the bulk solution must be
at least equal to 0.11 (i.e., oxygen:glucose = 1:9) to avoid any oxygen limitation in a fixed film biological system. Matson et al. [10] found a similar ratio, or 0.125, for their suspended biofloc system.
If the oxygen concentration was too low and became a limiting factor, then the stratification of aerobic and anaerobic layers would be formed. In the anaerobe layer, fermentation was the major metabolic mechanism, and the absence of oxygen led sulfate, nitrate, or carbon dioxide to be the electron
acceptors. After the biofilm attained a certain thickness, large portions of microbial mass might suddenly slough off. This could be due to a combination of excessive hydraulic shearings and productions
of some anaerobic metabolites (such as acids) which had weakened the binding strength of microbial
slimes. The maximum film thickness was found to vary from a few hundred microns to several millimeters [1-8] depending on the substrate concentration and shearing stress used in each study.
Generally speaking, aerobic biofilms are more effective in oxidizing organic matters than the
anaerobic counterpart. Comparisons between the oxygenated and regular fixed film systems have also
indicated that the former has a higher oxidative capability and better sludge settleability [11,12]. However, other investigators have shown that an anaerobic system may have similar or sometimes even
better biological activities than an aerobic system in stabilizing organic matter, especially when the
organic strength of the waste is high [13-15].
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