30    REDUCTIVE DECHLORINATION OF
PERCHLOROETHYLENE (PCE) USING ANAEROBIC
SEQUENCING BATCH BIOFILM REACTORS (AnSBBR)
Patrick J. Hirl, Graduate Research Assistant
Robert L. Irvine, Professor
Department of Civil Engineering and Geological Sciences
University of Notre Dame
Notre Dame, Indiana 46556
INTRODUCTION
The Sequencing Batch Reactor (SBR) is a periodically operated, nonsteady state, biological reactor which has been used successfully for the treatment of municipal, industrial, and hazardous
waste. The SBR is unique from other nonsteady state biological reactors in that it has discrete
fill, react, and decant periods. During the past two years, an Anaerobic Sequencing Batch
Biofilm Reactor (AnSBBR) was used to select for, enrich, and modify the physiological state of
a microbial consortium in order to promote the dechlorination of PCE to ethylene. The AnSBBR,
fed acetate as an electron donor, dechlorinated 6.1 mg (37 pmoles) of PCE to cis- 1,2-
dichloroethylene (cDCE) on a daily basis. When the daily PCE added to the reactor was increased to 80 mg (480 pmoles), the reactor produced vinyl chloride (VC) and ethylene. When the
daily PCE added to the reactor was reduced to 10 mg (63 pmoles), the reactor maintained the
ability to produce ethylene for approximately one month.
BACKGROUND
Periodic Processes for Wastewater Treatment
The SBR is an activated sludge-like, periodic process that has been used effectively for the
treatment of both domestic and industrial wastewaters.1-2 Reactor based periodic systems consist
of one or more identically operated tanks that provide for the time sequencing of two or more
processes or operations (e.g., equalization, biological conversions, and clarification) during a
complete reactor cycle. Each cycle may include up to five periods: FILL, REACT, SETTLE,
DRAW, and IDLE.
During FILL, wastewater enters a partially-full tank containing acclimated biomass. Aerobic
or anaerobic reactions can be initiated during FILL by providing either aeration or mixing. After
the tank reaches its predetermined maximum liquid level, the flow is either stopped or diverted
to another tank by means of a control system. The reactions initiated during FILL are continued
during REACT. The time set aside for REACT must be sufficient to allow the desired effluent requirements to be met. After REACT, the biomass is allowed to SETTLE quiescently for a predetermined period of time by shutting down the mixing and aeration equipment. The treated, clarified effluent is then removed during DRAW. This tank now waits during IDLE for the liquid in
the other tank to reach its maximum level so that it can begin a new cycle.
As can be seen from the above description, the SBR mimics a conventional continuous flow
system's treatment train by providing for piecewise equalization (during FILL), biotreatment,
and sedimentation in the same tank at different times. In fact, the time set aside for each period
in the SBR can be directly related to some corresponding detention time in a conventional continuous flow system (CFS). As a result, it is both appropriate and useful to equate the fraction of
the time dedicated to a particular period with the fraction of the total SBR system volume that
has been set aside for that period. Clearly, then, if the time (i.e., volume) allowed for one period
51st Purdue Industrial Waste Conference Proceedings, 1996, Ann Arbor Press, Inc., Chelsea. Michigan 48118.
Printed in U.S.A.
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