Error Sources in the Operation of Bench and Pilot
Scale Systems Used to Evaluate the
Activated Sludge Process
RAYMOND F. DRNEVICH, Staff Engineer
J.D. STUCK, Staff Engineer
Union Carbide Corporation
Linde Division
Tonawanda, New York 14150
INTRODUCTION
Laboratory and pilot scale experimentation utilizing continuous flow reactor systems
can be very useful tools for determining the operational characteristics of activated sludge
systems. However, programs run on this size of equipment can also lead to erroneous
conclusions if not conducted properly.
Since laboratory scale testing is limited in size the results obtained are subject to some
of the inherent difficulties encountered in the small scale equipment. Typical equipment
malfunctions such as plugged lines can result in major operational upsets. This can usually
be avoided by preventive maintenance and by screening some of the larger particles from
the feed to the reactor. It is also difficult to maintain the proper balance between mixing
intensity and dissolved oxygen level when operating small systems. It is common to operate
a system at high gas rates, power densities, and DO concentrations because the fine control
required to maintain representative levels is not usually designed into the systems. Mass
balances are especially difficult to perform accurately. Every sample taken from the system
must be analyzed for the components to be balanced and accounted for. A good example of
this is the sludge production measurement. In determining sludge production rates, the
solids removed for suspended solids analysis as well as those removed from the system as
effluent suspended solids and wasting must be taken into account. Further, changes in the
sludge inventory of the system, MLSS and sludge blanket level in the secondary clarifier,
can result in significant errors if not measured. On this scale of equipment fluctuation in
MLSS concentrations cannot be avoided. Changes in the clarifier blanket level can be
prevented by proper selection of clarifier sizes and wasting of the excess sludge that often
builds up in the system.
Activated sludge evaluations on a pilot plant scale results in many advantages over
bench top systems. The larger scale results in a more stable system, less subject to upsets
resulting from plugged lines and the fine control required for the small scale unit. However,
when operating pilot scale equipment, full scale design considerations are sometimes
overlooked. Pilot plant studies are often performed at power densities and dissolved oxygen
levels which would be uneconomical for full scale units. Errors in mass balances can result
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