Southern Water (SW) has installed eight new Partech Turbi-Tech 2000LA systems at its East Worthing water treatment works (WTW) for measuring mixed-liquor suspended solids (MLSS) in the aeration tanks.

This new installation takes Partech’s Turbi-Tech 2000LA and 7200 Series Monitor a step further in becoming the standard instrument for measuring mixed liquor throughout the whole of Southern Water.

The measurement of MLSS is vital in the control of the activated sludge plant, ensuring that the treatment process is effectively removing pollutants and operating efficiently in terms of aerator usage and chemical dosing. Understanding the trend of the MLSS value enables site operators to change return activated sludge (RAS) and surplus activated sludge (WAS/SAS) rates in response to changing process conditions.

Southern has used Partech’s Turbi-Tech 2000LA at several wastewater treatment works for years. Based on the successful performance of these, the company decided to replace monitors that have proved to be a problem and replace them with the Turbi-Tech 2000LA as part of its Process Compliance Programme.

In older wastewater treatment works, breaking down organic matter is achieved using filter beds. However, aeration is becoming more widely used as technology moves forward. At East Worthing WTW, aeration has been in use for around five years and has proved effective.

David Manley of SW’s Instrumentation Control and Automation (ICA) department, explains: “Handling mixed liquor should not be a problem, but here at East Worthing WTW where we handle some 31,000m3 of wastewater a day, we were finding that the original instruments were losing their calibration very quickly, so we turned to the Turbi-Tech 2000LA as it’s very easy to calibrate and use. Once calibrated, we know that there will be little drift between calibrations.”

To monitor suspended solids and turbidity, it is essential to use instrumentation calibrated in accordance with Reference Method 2540 Total Suspended Solids (Dried).

The Partech Turbi-Tech 2000 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 sensor is designed to cope with the problems of fouling that are inevitably associated with wastewater treatment and also features an integral cleaning mechanism that ensures accurate measurement with little routine operator involvement.

The wastewater entering the WTWs’ aeration lanes comes from the primary settlement tanks and is pumped to the treatment lanes using compressed air. “The objective is to measure the solids in the range 0-5,000mg/l solids and we are trying to keep the solids to an optimum for the site,” says Manley. “The Partech Turbi-Tech 2000LA and 7200 Monitor, which are linked into a SCADA system, are used for collecting information only and armed with this, the works operators can make manual adjustments to the inflow and outflow of the sludge in order to keep it at its optimum level.”

The operators remove the instruments on a weekly basis to check that they have not become fouled with debris. Calibration is undertaken once every six months by Southern’s ICA Sussex team.

SW is moving towards standardising on Partech’s Turbi-Tech 2000LA and 7200 Monitor and the ICA team has an input into the decision-making process when it comes to specifying instruments.

The design of the Turbi-Tech 2000LA minimises the effects of ragging, which is an advantage when it comes to routine maintenance.

In normal operating circumstances, the two glass sensor tubes that take readings extend from the head of the instrument and are only retracted when the automatic cleaning regime kicks in.

But, on those sites where fouling is a particular problem SW has programmed the sensors to remain inside the instrument and only emerge when a reading is required. In this way, the potential for dirt to build up on the surfaces of the sensors or to be affected by fouling is greatly reduced.

Manley says that the correct positioning and installation of MLSS instruments does have a major influence on performance.

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