By Manish Patel

Wastewater treatment plant automation is getting crucial day by day due to complex processes and uneven wastewater quality due to modern lifestyles having a wide variety of contaminants.

There are many important parameters to be monitored to optimize the purification process. Wastewater treatment mainly depends on loads of wastewater and aeration processes. Various flow measurement techniques are now emerging with respect to fluid and process conditions.

The wastewater treatment process needs accurate and reliable flow measurements for defining treatment conditions. Major areas where Flow measurement in wastewater treatment is required are:

• Influent wastewater (raw wastewater) from drainage or city sewage
• Primary treated wastewater
• Secondary treated wastewater
• Aeration air in the aeration lane for optimizing Dissolved Oxygen (DO)
• Chlorine dosing precise monitoring
• Precise corrosion inhibitor dosing

Flow measurement technologies involved in wastewater treatment plant processes are as below:

• Electromagnetic Flowmeter (full bore or insertion type)
• Non-Contact Ultrasonic Flowmeter
• Insertion Thermal Mass Flowmeter
• Open Channel Ultrasonic Flowmeter
• Coriolis Flowmeter
• Orifice Flowmeter

Above are major flow measurement technologies used for monitoring and controlling wastewater treatment processes.

Let’s describe in detail each flow measurement technique mentioned above – with its advantages and limitations related to its application.

Full Bore Type Electromagnetic Flowmeters

Electromagnetic flowmeters work by using Faraday’s Law of Induction. Inside an electromagnetic flowmeter, there is an electromagnetic coil that generates a magnetic field, and electrodes that capture electromotive force (voltage). Due to this, although it may appear as if there is nothing inside the flow pipe of an electromagnetic flowmeter, flow can be measured.

Under Faraday’s Law of Induction, moving conductive liquids inside of a magnetic field generates an electromotive force (voltage) in which the pipe inner diameter, magnetic field strength, and average flow velocity are all proportional. In other words, the flow velocity of liquid moving in a magnetic field is converted into electricity (E is proportional to V x B x D).

As the flow changes, the electromotive force (voltage) captured by the electrodes changes as followed:

Advantages

• True volumetric liquid flow measurement is independent of temperature, pressure, density, viscosity, and velocity profile.
• Improves reliability with no moving parts with negligible pressure drop.
• Works well even in solid slurries with air bubbles, and with noise suppression circuits.
• Excellent for flow measurement of sewage water and slurries, raw water, effluent water, etc.

Limitations

• Unable to detect liquids with low electrical conductivity (<1µsiemens/cm) such as hydrocarbons and solvent, etc.
• For large pipe diameters, it is more costly than an ultrasonic flowmeter.
• Contact type flow technology has the wearing and fouling of liners and electrodes.

Insertion Electromagnetic Flowmeters

Insertion electromagnetic flowmeters are an economic alternative to full bore flowmeters. They consist of an electromagnetic coil mounted on the end of the sensing probe. It is an alternative for economical installation for large pipes in existing water distribution systems where provision for flow-metering was not originally made. The assembly can be installed in existing pipelines without the need for major excavations or alterations to pipework normally associated with the installation of full electromagnetic flowmeters.

Advantages

• Works well for water distribution piping.
• Excellent and economical for large pipe diameters from 200mm to 2000mm.
• Low cost of installation and easy sensor isolation.

Limitations

• Lower accuracy (2%) than full bore electromagnetic flowmeters.
• Not suitable for pipe diameter less than 200mm.
• Not suitable for liquids with high solid impurities.

Above are some highlights of one of the flow measurement technologies widely used in water and wastewater industries in helping in monitoring and controlling wastewater and raw water, etc.

We will come back with another “important flow technology” in the next edition of this special blog series on Smart Water & Waste World.

Conclusion

The electromagnetic flowmeter is a proven flow technology for liquid flow measurement with long-term reliability and accuracy. There are other alternatives emerging to compete with where existing technology has limitations in performance and the initial cost is a concern.

About the Author

Manish Patel is the Director of Leomi Instruments Pvt. Ltd.

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