75    ANAEROBIC PRETREATMENT OF HIGH STRENGTH
WASTEWATERS USING A REACTIVE GROWTH
SUPPORT MEDIA
Mervyn C. Goronszy, Executive Technical Manager
TRANSFIELD INC.
Irvine, California 92715
W. Wesley Eckenfelder, Professor
Department of Civil Engineering
VANDERBILT UNIVERSITY
Nashville, Tennessee 37235
INTRODUCTION
Food and similar processing operations typically generate wastewaters containing high concentrations of soluble and organic constituents which are amenable to biological treatment for which there is
a large number of biological processes to choose from. Waste generation patterns from these industries can be highly variable on a day-to-day and a season-to-season basis. There may also be significant periods of time when no wastewaters are generated. It is this feature which poses a number of
problems regarding the design and satisfactory operation and performance reliability of a wastewater
treatment or pretreatment facility. The capacity of a treatment system to handle shock loads relative
to changes in flow, organic loading and shutdown or to no feeding periods is therefore most important.
While food processing wastes are readily degradable by aerobic biological methods, few processes
are able to accommodate wide hydraulic and organic load variations and consistently produce an
effluent of acceptable quality. 12'3.4.5 Many aerobic suspended growth systems have consistently been
troubled by sludge bulking associated problems. It is only in the last decade or so that a more
fundamental understanding has been reached of the many types of filamentous microorganisms and
the related reaction, or growth, conditions which cause their proliferation. With proper design and
operation, aerobic suspended growth systems can be used to provide acceptable and reliable performance although the associated operating costs can be an unacceptable burden if the wastes have a
strength which exceeds 2-3000 mg/L COD. This can be particularly so for the smaller production
facilities where waste volumes are less than about 0.5-1.0 MGD. Most of the high cost is generally
associated with the cost of energy for aeration and related unit operations together with the provision
of relatively long process detention times. This cost can be significantly reduced by the use of
anaerobic digestion technology provided a number of reliability features relative to varying waste
generation patterns can be maintained.
In order to maximize cost effectiveness it is necessary to be able to operate with a high active
biomass concentration and long solids retention time in relation to the mass of substrate that is to be
treated and, in turn, achieve a minimum hydraulic retention time. These desirable principles are often
difficult to apply to smaller continuous flow systems as short reaction retention times can result in
process instability caused by a lack of capacity relative to influent load parameter fluctuations.
There are several anaerobic processes that are capable of achieving good organic removal while
operating at low process retention times. These include the upflow anaerobic sludge blanket reactor,
downflow fixed-film reactor, upflow fluidized bed reactor, upflow packed bed reactor and combined
anaerobic reactor-fixed film reactor. Criteria applicable to three of these systems are summarized in
Table I.
Retention of the biomass as a film on a fixed or a high density media can be used to provide a high
active mass of sludge availability to the substrate and can enable efficient operation at reduced
hydraulic retention times. Process stability problems can occur when the organic loading capacity of
the available biomass is exceeded. This is manifested as a reduction in removal of organics, an
increase in volatile acid concentration (VFA), a reduction of alkalinity buffering, a reduction in
methane content in the offgas and an eventual reduction in pH if the overload condition persists for
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