49    EFFECTS OF INITIAL SEED CONCENTRATION
ON THE STARTUP OF THE
ANAEROBIC SEQUENCING BATCH REACTOR
Francis L. De Los Reyes III, Graduate Research Assistant
University of Illinois
Urbana, Illinois 61801
Richard R. Dague, Professor
Department of Civil Engineering
Iowa State University
Ames, Iowa 52203
INTRODUCTION
The anaerobic treatment of wastewater has enjoyed a resurgence of interest in the last decade.
This phenomenon can be traced to two factors: (a) a growing recognition of the advantages of
anaerobic processes, and (b) an increased understanding of the fundamentals of anaerobic bio.
chemistry and microbiology coupled with advances in process engineering. Pioneering efforts in
reactor design have led to the development of new high rate reactors that can operate at higher
loadings than conventional digesters, and systems capable of treating medium to low-strength
wastewaters. Examples of these reactors are the anaerobic filter, fluidized bed reactor, and the
upflow anaerobic sludge blanket reactor (UASB) and the anaerobic sequencing batch reactor
(ASBR, U.S. Patent No. 5,185,079).
One of the most important steps in anaerobic treatment is reactor startup. Economically, it is
important because the reactor is not operating at design load, and therefore the productivity of
the wastewater supplier is limited by the wastewater treatment plant capacity.' The reduction of
startup time is therefore one of the key parameters in increasing the competitiveness of anaerobic
reactors. Unfortunately, despite extensive industrial experience, successful startup is not a routine job.
From a microbiological viewpoint, startup represents a condition of imbalance and stress.
Anaerobic biodegradation requires the coordinated metabolism of different microbial populations, and consists of series and parallel reactions, as illustrated in Figure 1.3 These groups of microorganisms are interdependent, and a dynamic balance must be achieved for efficient treatment. For example, the continuous removal of H2 by methanogens reduces the H2 partial
pressure and allows the acetogenic reactions to proceed. Unless the H, partial pressure is low (<
10 ~* atm), the oxidation of long-chain volatile acids ( propionate and butyrate) to acetate is ther-
modynamically unfavorable. Thus there is a syntrophic relationship between the H,-producing
acetogenic bacteria and the H2~ consuming methanogenic species.4-* If conditions in the system
are such that the methanogens are inhibited, the volatile acids produced in the earlier stages will
not be further metabolized, leading to a drop in pH. This pH drop may further stress the
methanogens and may eventually lead to process failure. The adaptation of the microbial populations to attain dynamic balance occurs during startup.
The duration of startup time depends on many biological, chemical, and physical factors. Wei-
land and Rozzi (1991)' reported that startup is influenced by wastewater composition and
strength, seed biomass volume, activity and adaptation, the environmental conditions, the mode
of operation, and the reactor geometry and size. The most important factors in startup of suspended growth systems are the characteristics of the seed biomass. Improved process startup in
terms of startup time reduction, higher initial loadings and higher COD removal rates have been
reported for systems seeded with granular compared to flocculent biomass.2-6-7 Granular biomass
50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press Inc., Chelsea, Michigan 48118. Printed
in U.S.A.
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