Environmental Science & Engineering - www.esemag.com - November 2005
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Saving power costs in the activated sludge process
By Mike Rousey
Power consumption is one of
the largest expenses in operating
a wastewater treatment
plant (WWTP). In the activated
sludge process, power consumption
can cost thousands of dollars.
Activated sludge process a
balancing act
Solids present in the primary clarifier
effluent are particles too fine to
“settle out.” Yet, (US) Environmental
Protection Agency regulations require
removal of these organic wastes to prevent
pollution that can cause disease,
fish kills, and algae blooms. The activated
sludge process removes these
suspended particles by converting
them to settleable solids. It uses a
mixed culture of microorganisms
including bacteria, protozoans, and
rotifers to break down organic wastes
into inorganic wastes. However, the
WWTP activated sludge process
speeds up this natural course by controlling
the environment.
Aeration blowers supply oxygen
critical to the survival of the bacteria in
the activated sludge process. However,
dissolved oxygen (DO) levels that are
too high can result in pin flock in clarifiers,
severe sludge bulking in some instances, and large
amounts of wasted electricity.
DO levels that are too
low will not support the
bacteria necessary to
treat incoming wastes.
Inadequate DO concentration,
in effect, can
result in the plant violating
effluent limits
established in its NPDES permit.
Problems with common
control methods
Operators at most US WWTPs use
a handheld DO meter and set activated
sludge process blowers at a constant
speed. This set method of control does
not allow for process adjustments. For
example, when oxygen demand
decreases during winter nights due to
falling temperatures, this method does
not compensate for the decreased need
for aeration and wastes power.
Conversely, a large oxygen demand
during periods of industrial discharge
can deplete DO – and wipe out the
entire activated sludge process – if the
process does not automatically
increase aeration. Operators using
handheld DO meters achieve only hit-or-miss readings that are insufficient
for optimizing plant operation and
energy usage.
Potential power savings with
automation
WWTPs can decrease power consumption
in the activated sludge
process. Energy-efficient motors and
variable frequency drives (VFD) used
with on-line dissolved oxygen analyzers
– installed and maintained properly
– will contain energy costs.
For example, the Hach SC100
Analyzer with an LDO Probe can be
configured to send a 4-20 mA signal to
a VFD or be used in conjunction with
a plant PLC or SCADA. The control
system automatically increases or
decreases aeration blower speed in
response to the measured DO levels in
the aeration basin. A 1998 study by the
Iowa Association of Municipal
Utilities demonstrated the use of this
equipment in the activated sludge
process reduced energy costs as much
as 25%.
In summary, an on-line dissolved
oxygen analyzer when used with the
plant’s energy-efficient motors, variable
frequency drives, and plant SCADA or
PLC systems, adjusts aeration operation
automatically in response to varying
process conditions.
Mike Rousey is with Hach Company
in Colorado.
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