South East Water (SEW) is committed to ‘healthy water for life’ and looked to upgrade their Mount Martha Treatment Plant to ensure it’s capability would meet projected future population demands.

SEW sought to measure the sludge level in 3 catch-pots installed in a methane reclamation gas pipeline. The pipelines are fed from sewerage digesters which use an anaerobic digestion process involving bacteria to digest the sewerage.

A by-product of this process is methane which is reclaimed to feed site gas process heaters. When the heaters are not required the methane is diverted to a safety gas flare.

The catch pots were installed by SEW upstream of the heaters and flare. The purpose was to separate liquid sludge from the methane gas stream. The separated liquid accumulated in the base of the catch pots and was removed at intervals through an automated drain valve that operated on a timer. This arrangement did not allow reliable control of liquid levels in the catch pots. Consequently, occasional carryover of liquid into the heaters and flare occurred due to inadequate operation of the drain valve and occasional loss of methane into the drainage system occurred due to excessive operation of the drain valve.

SEW engaged SAGE Automation to design a level measurement process and discharge the waste accordingly.

Project Challenges

The solution required a level control system capable of:

• Operating over a range of gas pressures
• Controlling the level of a liquid that could contain up to 3 to 5% solids
• Controlling the liquid level below a variable depth of foam

This requirement was probably the most demanding aspect of the functional requirement. The solution provided by SAGE Automation provided reliable level control despite these difficult requirements.

Another challenge encountered on this project was access to the catch-pots which were located in a pit some 3 meters under ground. The catch-pots are both a confined space and a Zone 1 hazardous area, producing a significant complication during performance of the work.

A specialist dive team was employed to monitor the confined space entry and atmosphere at all times while the wiring and installation of the catch-pot level sensors was undertaken.

A further challenge was the distance between the high and low level probes, which is only 18 mm. The accuracy of level measurement and probe installation was a big factor as the tolerances were very slim. Further, the waste discharge could not break the sludge seal at the bottom of the catch-pot, as it would release methane into the environment.

As a result the control system design was focussed around fast reaction times to frequently changing waste levels.

Technology Utilised 

• SCADA - iFix SCADA 
• PLC - GE

Customer Benefits

• Reduced operation maintenance cost due to less operator interfacing with plant required due to automated waste disposal
• Smaller catch-pot foot is allowable due to an on demand waste evacuation system from the catch-pot
• Intrinsically safe installation
• Reduced methane emissions
• Cleaner downstream gas delivery

Project Highlights 

• Hazardous Area compliant installation 
• High Risk Environment Conditions mitigated by us of professional divers 
• Seamless integration with existing control system 
• Probes selected to meet the switching conditions and able to distinguish between foam and liquid