Section Four
WASTE TREATMENT PROCESSES
D. ANAEROBIC BIOLOGICAL
44    FUNDAMENTAL PRINCIPLES OF THE ANAEROBIC
SEQUENCING BATCH REACTOR PROCESS
Shihwu Sung, Graduate Research Assistant
Richard R. Dague, Professor of Environmental Engineering
Department of Civil Engineering
Iowa State University
Ames, Iowa 50011
INTRODUCTION
The first high-rate anaerobic process was developed as a result of studies by Fullen in 1953,'
Schroepfer et al. in 1955,2 Steffen in 1958,} and Schroepfer and Ziemke in 1959.4,s The process was
analogous to the aerobic activated sludge and was called the "anaerobic contact process". Several
other high-rate anaerobic processes have been developed and are being applied throughout the world.
These include the anaerobic filter,6"8 the upflow anaerobic sludge blanket,9"" and the fluidized-bed
process. Speece listed and described these processes in detail.12
In the 1960s, Dague" conducted laboratory studies on methods of increasing the microbial population in anaerobic reactors. The studies involved batch feeding, internal solids separation, and supernatant wasting. The process was called "anaerobic activated sludge."14 The anaerobic activated sludge
process was capable of achieving long solids retention times (SRT) with relatively short hydraulic
retention times (HRT) as a result of bioflocculation and efficient solids separation within the reactor.
Although not recognized at the time (1966), what is now called "granulation" of the biomass was
occurring in the process.13"15
The writers and coworkers at Iowa State University16"18 have been conducting studies on a new
anaerobic reactor called the "Anaerobic Sequencing Batch Reactor" (U.S. Patent Pending). The
system is analogous to the anaerobic contact process except that biomass separation and liquid
clarification occur within the reactor rather than in an external clarifier and no vacuum degasification
is required. As will be discussed later, the ASBR offers attractive advantages over continuous-flow,
suspended-growth anaerobic processes.
The purpose of this paper is to discuss the fundamental principles of the ASBR, present data from
laboratory studies aimed at evaluating the performance of the process and lo present a design model
based on Monod kinetics for use in sizing the ASBR.
PRINCIPLES OF THE ASBR PROCESS
Operating Principles
The operating principles for the ASBR are simple. As illustrated in Figure 1, the reactor sequences
through four steps; settle, decant, feed, and react. During the settle step, mixing is shut off to allow
biomass solids separation. The reactor itself acts as the clarifier. The time required for clarification
will vary depending on biomass settleability, but typically ranges from 10 minutes to one hour. From
an operational standpoint, it is essential that the sludge blanket be below a predetermined decanting
elevation and that the blanket not rise due to the accumulation of biogas within the settled biomass.
As will be discussed later, the concentration of mixed liquor suspended solids (MLSS) in the reactor is
an important variable affecting the settling velocity of the biomass and also the ability to achieve a
clear supernatant for discharge as effluent. An important related variable is the specific process
loading rate (food to microorganism ratio, F/M).
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 4HIIK.
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