As pressure increases to monitor and manage wastewater treatment energy consumption, facility operators are looking at the activated sludge process for energy reduction opportunities.
Compressed air is normally injected into wastewater to mix it with the activated sludge. Controlling the amount of air that is released in the activated sludge process is very important since it controls the growth and the health of the microorganisms. Flow meters are typically installed in the pipes to measure and control the amount of air to run the system properly.
The cost of energy to produce compressed air has increased tremendously due to the high cost of fuel. Regulating and controlling the air injection not only reduces the amount of energy consumed but also optimizes the operation of the plant. Therefore, it is important to understand how flow meter instrumentation can impact wastewater treatment energy consumption.
Thermal Dispersion Flow Measurement
Many technologies can be used to measure the flow rate of air, but thermal dispersion offers unique benefits over more traditional flow meter types. Thermal mass flow meters are inherently capable of directly measuring the mass flow without the need for pressure or temperature correction. This provides a more useful flow measurement and also makes thermal technology highly cost effective. There are other advantages to the use of thermal dispersion gauging, including:
- Excellent low flow sensitivity and turndown
- Low pressure drop
- Ease of installation
Two Types of Thermal Dispersion Flow Technologies
Constant power and constant temperature difference are the two different technologies utilized for thermal mass flow measurement.
- As the name implies, constant power technology supplies a constant power to the heater.
- Constant temperature difference technology maintains a constant temperature difference between the heated sensor and the reference sensor.
Both instrument types permit flow measurement at very high flow rates, providing high turndown capabilities. The primary disadvantage of constant power operation is the slow response time to changes in flow, due to the technology’s dependence on the length of time it takes the heated sensor to reach thermal equilibrium at the new operating conditions.
In contrast, constant temperature difference operation has a faster response time to changes in flow. A PID control circuit varies power to the heater to always maintain a constant temperature difference between the reference RDT and the RTD measuring the temperature of the heater – thus improving response rate.
Magnetrol’s white paper “Energy Saving: Using Thermal Dispersion Air Flow Meters in Wastewater Treatment Plants”, which is available on the Thermal Mass Flow Technology Portal, provides a detailed discussion about the characteristics of thermal mass flow meters, their use in activated sludge process and their benefits in improving overall wastewater treatment energy consumption. Get the white paper by logging in to the Thermal Mass Flow Technology Portal.