Waveguide Solves Digester Level Monitoring Dilemma

March 13, 2003

The city of Orillia, located north of Toronto, Canada, has found a reliable level measurement system for the two anaerobic digesters at its wastewater treatment center.

This facility features five primary and five secondary clarifiers, as well as six aeration basins, servicing a flow of six million gallons a day via 21 pumping stations.

The level in an anaerobic digester is a critical measurement. If the top foam level gets too high, it can enter the gas collection pipeline and lead to costly maintenance.

In extreme cases, pressure can lift the roof off the vessel. Gas can escape, harming staff and the environment. Releasing carbon dioxide and methane gas into the environment requires notification paperwork and has potentially negative effects on a community blessed with clean lakes and rivers.

Potential Problems

The conditions in a digester make level measurement particularly challenging. Bubblers are prone to maintenance problems. Traditional ultrasonic level monitoring systems are adversely affected by sound absorption caused by the combination of carbon dioxide and methane gases above the foamy level surface, a condition aggravated by the steamy environment.

Radar technology is not affected by these conditions; however, if a radar device is simply installed above the valve, excessive signal noise leads to faulty signals and unreliable readings.

This application required a reliable level measurement device to accommodate the existing rotary valve used for instrument isolation. It also needed to be inspected and serviced without discharging digester gas into the environment.

Solution

Eldon Wallis, chief plant operator for the Orillia plant, found an effective solution with the Siemens-Milltronics IQ Radar level measurement device fitted with a built-in sliding waveguide antenna.

The sliding waveguide is a unique feature available with the Siemens-Milltronics radar unit. The waveguide pipe carries the radar signal from the instrument to the horn antenna, bypassing the spool and valve components. The pipe has a clamping seal on the top of the mounting flange which seals the low pressure digester gases after installation.

The system was installed and commissioned fairly quickly. After bolting the flange on the spool section, the valve was opened and the radar instrument was lowered and clamped into place. Set-up was easy, with only six parameter entries required for operation.

Benefits

Previously, the plant needed permission from the Ministry of the Environment to open the tank to install or remove an instrument because the gases in the digester tank would then be released into the environment.

The Siemens-Milltronics IQ Radar instrument now provides reliable level measurement on this difficult challenge. The sliding waveguide configuration eliminates valve signal noise. Using the rotary valve between the radar unit and the digester vessel, the waveguide also has eliminated the related environmental safety issues. The instrument can be raised, inspected or removed at any time without releasing methane gas into the environment.

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For further information, phone 800-964-4114.

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