Pulsar Process Measurement Ltd

Sludge Blanket Level Detection – making new process opportunities possible

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Courtesy of Pulsar Process Measurement Ltd

Much Pulsar's Self-Cleaning Viper Sludge Sensor of the demonstrating the effective maintenance-free solution. discussion around the way we deal with sewage sludge – the way it is moved around the process, the bellmouth valves and decant processes, the downstream treatment of the liquors, important issues around TOTEX and discharge Compliance, plus more recently the green energy benefits of on-site power generation – depends in large part on the accurate and reliable measurement of the strata of the sludge blanket. The technology has moved on massively over the last decade so that now, we are able to monitor sludge blanket processes with as much confidence as we measure open channel flow or control pump station operations, and with very similar ultrasonic technology.

Historically, sludge measurement has been a very hit and miss affair, the usual techniques including a long tube, with an operator standing on the bridge of the clarifier and dipping the tank, using a combination of experience and guesswork to ‘measure’ the level - usually followed by hours of work adjusting penstocks to balance the process between the various tanks. Still in use in probably the majority of smaller sites, it is difficult or impossible to accurately control the process without significant manual intervention. There are clear problems: the safety issues around the need for operators to lean over the rail of the clarifier bridge, especially in icy weather; the need for extended operator intervention to control the process and, when sludge is tankered from a smaller site to final processing, the likelihood of excessive water being shipped, leading to extra tanker movements, wasting time, fuel and money. Possibly more important are issues around Compliance, how we ensure that floc levels are controlled in the final effluent as we become more sophisticated in the handling of the material.

Installation showing a Pulsar Sludge Finder 2 and Viper Sludge Transducer. A dB Level Transducer is also monitoring the liquid level to provide a 'low level' alarm.

There have, of course, been many attempts to automate blanket measurement. These have included mechanical systems of pulleys and ropes, usually repeatedly lowering an optical turbidity or infra-red sensor into the process with obvious issues around probe fouling and maintenance.

Non-contacting ultrasonic technology has become the standard technique throughout the waste water industry for the majority of level measurement and open channel flow applications. Cost-effective, maintenance free, reliable measurement and sophisticated control processes has led to tens of thousands of successful applications world-wide. Applying the same logic to an underwater application is straightforward in theory but in practice has had to wait for the development of very specialised digital signal processing techniques and transducers developed with the very specific output frequency to optimise the response.

All ultrasonic measurement depends on an interface of some kind. In most applications that is the interface between air and water, providing a strong echo. The technique then depends on maintaining a ‘fix’ on the correct signal despite conflicting echoes from chains, pump tops or stanchions. In sludge blanket applications, we use a submerged transducer to discriminate interfaces between densities which may only vary by as little as 0.5%. The measurement is achieved by profiling the echo throughout the range so that the changes in density can be identified. That’s only possible with the latest industrial microprocessor speeds that allow the number of calculations to be completed at the resolution required. In some ways, it is like the development of digital cameras. More processing power and advanced sensors has led to ever improving image resolution.

Obviously, a submerged transducer still has issues around fouling, and at Pulsar, our Viper transducer incorporates a sweeping arm that keeps the face clean. A piezo electric crystal produces a pulse that is reflected throughout the liquor inside the tank, whether that is a primary, secondary, tertiary clarifier or SBR and the controller creates a profile of the spectrum of densities across the depth of the vessel.

Providing a profile has obvious advantages in control terms. Freed from the limitations of a simple high or low level, more complex and detailed control and monitoring is possible. One key benefit is the ability to monitor the floc layer and provide an alarm or automatic control of the process to ensure that floc does not excessively contaminate the final effluent.

Floc level can be difficult to monitor, especially when ‘blooms’ of floc pass close to the transducer face. Additionally, while the main blanket level is being measured, unacceptable levels of floc can creep up the process and overflow into the watercourse, especially in times of high flow or when the process has been disturbed. A new feature introduced by Pulsar is the High Floc Alarm, where the Sludge Finder 2 makes a secondary assessment of the raw data being analysed and looks specifically for an increase of floc rising close to the top of the process but sitting above the level of the main blanket.

The effective control of the floc layer has meant that some sites have now managed to completely automate the final treatment process. At Barnhurst, a Severn Trent site in the West Midlands, fourteen Pulsar Sludge Finder 2 systems are relaying data back to a control cabin via radio telemetry links to adjust bellmouth valves, controlling the process automatically to create a ‘healthy’ sludge level in the clarifiers. Call outs have been significantly reduced, minimising costs, but the key point here is that this critical Compliance control process can now be safely trusted to an automated system, even in final treatment where sludge concentration is very low. Bellmouth valves are one control mechanism, but this approach could just as easily be applied to penstock or decant arm operations.

Good sludge blanket level detection is an important enabler in sludge concentration operations for green energy generation. At Severn Trent’s Stoke Bardolph site, sludge is concentrated from 2% to 6%, before being processed further in on-site digesters and then used to power turbines that help to make the site self-sufficient in energy. This sort of project is very much in line with the ‘spend to save’ approach that supports TOTEX strategies.

As the old saying goes, ‘garbage in, garbage out’. Without accurate, reliable monitoring and measurement, with equipment that is manageable and easy to set up, it would not be possible to rely on the control protocols and, by extension, some of these projects would be difficult, more time consuming and in some cases impossible.

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