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Posted by Angus Fosten on February 27, 2010
TurbiTech in MBR Plant

Suspended Solids Monitoring Improves Efficiency

Back in late 2004 Southern Water Services replaced the existing wastewater treatment works at Sissinghurst, Kent with a Copa Membrane Bioreactor (MBR) process in order to achieve tightened discharge consent. This was Southern Water’s first full scale submerged membrane reactor process for operational works. This plant has operated continuously since its commissioning in 2005 and has met all its design and operating requirements.

One of the reasons for going down the route of MBR technology at Sissinghurst was that having a relatively compact footprint area, not only it would meet the demanding treatment requirements, but it could be installed without having to use additional land outside of the existing works. MBR technology offers operators several 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 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, more commonly known as Mixed Liquor Suspended Solids (MLSS) present during the activated sludge process. 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. Typically, MLSS values are between 2,500 and 5,000mg/l. However, the MBR plant at Sissinghurst WWTW runs at 12,000 – 15,000mg/l during the winter months and 16,000 – 18,000 during the summer season. The Sissinghurst WWTW is not a large plant, having been designed to handle typical flows between 4 and 10lt/sec. In dry weather, the inlet flow can drop to as low as 2lt/sec.

In order to keep the sludge in a state of suspension, each the tanks is fitted with a blower that are in constant operation. These blowers create high levels of turbulence in the tanks which challenges the robustness of the Turbi-Tech probes. There are no final tanks so the separation of the MLSS and final effluent is done by the filtration membranes. This way the MLSS remains in the tanks..

Maintaining constant vigilance of MLSS levels in the two activated sludge tanks is performed by two 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.

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. 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.

The two Turbi-Tech probes were installed in the activated sludge tanks early in 2009, together with the 7200 Monitors, which replaced the original instruments. The original monitor panels had deteriorated due to continuous exposure to the elements, making them difficult to operate and clean. With Partech’s 7200 Monitors being supplied in robust, all weather resistant housings, this problem has now been overcome. 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 base, Southern Water’s 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 the Sissinghurst WWTW has shown 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”, comments Clive Teobald, field sales engineer at Partech. “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.”

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