53    THE EFFECT OF APPLIED VACUUM ON THE
PERFORMANCE OF THE ANAEROBIC SEQUENCING
BATCH REACTOR
Barbara A. H. Herum, Graduate Research Assistant
Richard R. Dague, Professor of Environmental Engineering
Department of Civil Engineering
Iowa State University
Ames, Iowa 50011
INTRODUCTION
Development of high-rate anaerobic processes has proceeded rapidly during the past forty years.
One characteristic which is common to all high-rate processes is the ability to hold biomass within the
reactor, thereby separating the solids retention time (SRT) and hydraulic retention time (HRT). The
first such process was developed during the 1950s based on work by Fullen.i Schroepfer et al.,2
Steffen and Bedker,3 and Schroepfer and Ziemke.4 This process, called the "anaerobic contact process" operated similarly to aerobic activated sludge and required external clarification and solids
recycle. Later work led to the development of many different systems including the anaerobic filter,
the upflow anaerobic sludge blanket, and fluidized bed processes. A paper by Speece5 describes these
various processes in detail. While the methods differ, each provides an effective means of immobilizing biomass within the reactor.
Research by Dague6 in the 1960s on solids accumulation in anaerobic systems studied the effect of
bioflocculation on sludge settleability. In these studies, reactors were operated in a batch mode, with
solids separation occurring within the reactor prior to decanting supernatant. The process was called
"anaerobic activated sludge,"7 and proved that batch processes were capable of achieving the necessary separation of SRT and HRT.
Research is continuing at the Iowa State University on a newer process called the "Anaerobic
Sequencing Batch Reactor" (U.S. Patent No. 5,185,079) which operates in a manner similar to
Dague's early reactors. Prior studies8"11 have shown that this process is capable of achieving high
COD removal rates over a range of temperatures with a variety of agricultural and industrial waste
streams. Because solids separation occurs within the reactor, it is particularly important for the ASBR
process that the sludge have good settling characteristics. As will be discussed later, settleability is
closely related to the amount of substrate left when settling begins, both because flocculation is
affected by it, and because continued conversion of available waste can cause the biomass to re-
suspend due to internal gassing.
While studying the treatment of swine waste with the ASBR, a co-worker at Iowa State noted that
drawing a slight vacuum prior to the settling step appeared to result in improved settling. This
research was designed to determine whether application of a slight vacuum (6 inches of water head)
during the mixing period just prior to settling actually has such an effect, and if so, what impact this
has on solids accumulation and overall reactor performance.
PRINCIPLES OF ASBR OPERATION
Anaerobic Bioflocculation
Since the ASBR depends on internal clarification it is important that the biosolids settle well. Dague
et al.12 reported in 1966 that anaerobic sludge flocculates like aerobic sludge, and that the specific
process loading rate, also known as the food to microorganism (F/M) ratio, has a large effect on
bioflocculation. When the F/M ratio is low the biomass tends to flocculate and settle well, leading to
low levels of solids in the effluent.
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
529