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Taking the measure of MLSS

Published: 11 January 2012 - Heather Ramsden

Monitoring technology from Partech is being used by MBR plants to measure MLSS - essential for ensuring the removal of pollutants. One of the reasons for using MBR technology to treat wastewater is because units have a relatively compact footprint area; they can be installed where treatment works are located in confined spaces. Where a treatment works is being upgraded, installing an MBR plant means there is no need to use additional land.

MBR technology offers operators further advantages, most notably the non-use of floc to remove the solids by settlement. As a consequence, the biomass can operate at very high levels of mixed liquor suspended solids (MLSS), generally in the order of 12,000-18,000 mg/l, but also capable of going as high as 22,000 mg/l. This high concentration enables a low tank volume and a long sludge age to be utilised, which substantially reduces sludge production.

The process does not require primary or secondary settlement stages or additional tertiary treatment or UV stages to achieve very high disinfection quality: typically better than 5:5:5 BOD: suspended solids: ammonia. What is required, however, is an effective method of measuring suspended solids (MLSS) present during the activated sludge process.

Measuring MLSS

Measuring MLSS is an essential activity in the control of the activated sludge plant as it ensures that the treatment process is actually performing its role in removing pollutants and operating efficiently in terms of aerator and chemical dosing.

Understanding the MLSS trend value enables site operators to change return activated sludge (RAS) and waste or surplus activated sludge (WAS/SAS) rates in response to changing conditions. Typical values are between 2,500 and 5,000mg/l.

Maintaining constant vigilance of MLSS levels in activated sludge tanks can be performed at by employing Partech Turbi-Tech 2000HR probes and 7200 Monitors.

“These monitoring packages provide detailed information on MLSS trend values that are essential in obtaining optimum performance from the solids processing system,” comments Angus Fosten, Partech’s sales & marketing director.

“As a consequence of having our instrumentation systems installed on MBR plants in several UK locations, we have found that treatment works tend to keep the sludge in a state of suspension by the use of blowers and these are normally in constant operation. Blowers do create high levels of turbulence in the tanks, which challenges the robustness of our Turbi-Tech probes, but these are more than capable of maintaining full operation at all times.”

The Partech Turbi-Tech 2000HR operated in conjunction with the Partech 7200 Monitor uses Infrared light that is either scattered or absorbed by the particles in suspension, the amount of received light being proportional to the level of suspended solids. The geometry of the sensor, either light scatter or light attenuation is chosen to suit the suspended solids or turbidity range.

The amount of received light is converted into Suspended Solids by the 7200 Monitor using algorithms that have been developed specifically for these applications. The design of the Turbi-Tech 2000HR minimises the effects of fouling, which is a distinct advantage when it comes to routine maintenance operations.

Integral cleaning mechanism

The integral cleaning mechanism ensures accurate measurement with little routine operator involvement. In normal operating circumstances, the two glass sensor tubes which take readings extend from the head of the instrument and are only retracted when the automatic cleaning regime kicks in.

According to Partech, operators of MBR plants have indicated a liking for the 7200 Monitor because it comes in a robust, all-weather housing and does not deteriorate due to continuous exposure to the elements.

Featuring a large character display of the measured value combined with a scrolling information bar showing output status makes life easy for the works operating team when attending the site for routine inspection and maintenance. Back at central control rooms process scientists are able to keep a real-time check on how the plant is performing as the 7200 Monitor sends readings via a telemetry system. Using this data, all values are logged every 15 minutes.

“The employment of the Turbi-Tech HR at treatment works has demonstrated that this instrument is more than capable of meeting the challenge presented by the high levels of MLSS generated in the MBR activated sludge process”, says Fosten.“Combined with the benefits of the 7200 Monitor, this package has provided this water treatment works with an effective, robust and reliable method of continuously monitoring MLSS.”

Industry Connections: Partech (Electronics) Ltd


 
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