Combined control is a strategy emerging
as a strong alternative to traditional
approaches for maintaining proper
dissolved oxygen (DO) concentrations
in the activated sludge at wastewater
treatment plants.
At its core, this approach sustains
DO concentrations at their setpoints
using a smart design that constantly
examines past and present conditions
within a plant. It ‘learns’ the system and
adjusts the rate of change to calculate
the amount of air required. The system
then sequences multiple blowers and
makes valve adjustments where needed,
to generate the proper process airflow.
Wastewater plants that face more
stringent treatment requirements–
such as those in high-population areas
and with a proximity to more sensitive
waterways–stand to benefit from a
combined control strategy. This type of
precision oversight is also a good fit for
wastewater plant operators who are
concerned with excess oxygen affecting
phosphorus and nitrogen removal or
negatively impacting clarifiers.
A combined control approach is just
one of a variety of methods available to
address biochemical oxygen demand
(BOD), the amount of DO required
to break down organic material in
wastewater.
Where conventional methods
fall short
Proportional-integral-derivative (PID) is
the most common approach to controlling
DO at wastewater plants. It uses a series
of loops operating independently to
perform different functions, such as an
air control valve tethered to a DO probe
reading. PID works well when optimally
tuned but trying to cover a wide range
of flow rates can be problematic. That’s
because PID is a linear control method,
while wastewater isn’t a linear process.
For example, consider a 2-1 swing
in diurnal loading as related to the
throttling capabilities of a butterfly
valve or oxygen exchange in the winter
months compared to summer. Neither
of those items is linear. However, PID
attempts to resolve these DO aspects
with linear solutions. Additionally,
PID too often results in the system
‘hunting’ by increasing or decreasing
blower airflows and overshooting or
undershooting the required DO (think of
a dog chasing its tail) to activate. Even
with a time delay mechanism to start
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Comprehensive approach to dissolved O 2 control
A wastewater treatment plant in Dresden, Saxony.
blowers, PID systems are always hunting
rather than having a true understanding
of where air needs to be applied.
Off-gas analysis, another approach
to DO control, is based on calculating
the oxygen uptake rate. The downside
is that it requires a substantial number
of analytical devices, which need to be
regularly cleaned and calibrated. Any
drift can cause bad data that could work
counter to the DO strategy as well as
increase energy costs.
Model-based systems are also
used for DO control that bypass the
shortcomings of PID loops by addressing
diurnal loading and seasonal changes.
However, the downfall is that model-
based systems require a significant
amount of data to be collected prior to
design. They also fail to adapt to long-
term changing conditions because the
baseline number for the model is static.
So, as the population grows, the models
become irrelevant.
Combined control based on
proof of demand
The combined control strategy provides
dissolved oxygen to the activated sludge
process by calculating the total airflow
required (demand side) as well as the
exact airflow required in each aeration
zone (supply side) to maintain the DO
at the setpoint. It then sequences
the blowers and accurately adjusts
Water Sewage & Effluent May/June 2019
the valves to distribute the airflow as
required. DO is measured in each control
basin, and the valve setting for each
individual aeration zone is adjusted to
achieve the desired DO setpoint.
This flow-based, most-open-valve
method for air distribution allows the
system to operate at the lowest possible
pressure, which reduces energy
consumption. Meanwhile, the proof-of-
demand equation, focused on what the
biological process really needs, occurs
in the background. Beware, though:
some less sophisticated systems will
cycle blowers on and off without a ‘proof
of demand’ verification before turning on
additional blowers.
The potential energy savings of
combined control systems at wastewater
treatment plants will vary based on plant
layout and complexity. Generally, the
system will yield a 20% energy savings
when used in lieu of a typical PID system.
For plants only using a baseline aeration
control – setting airflow based on peak
BOD requirements – the combined
control system can reduce energy usage
as much as 60%.
Precision administration of DO is
key to running an optimal biological
process. For plant operators interested
in improving their processes, effluent
and energy usage, a combined
control strategy for DO is worth
evaluating.
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