Water and Effluent Treatment Magazine
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16 WET NEWS JULY 2015 Why has it proven difficult for critical processes in sludge treatment to be automated? The challenge: Overcoming laborious sludge measurement INSIGHT SeWage & SLUdge treatment Sludge processes have relied on labour-intensive manual measurements being taken, with obvious health and safety implications THE CONCEPT • the Sludge Finder 2 provides continuous or dual channel level control choice • an acoustic signal is generated by a piezo-electric transducer and bounced off an interface level • the control unit has, pre-programmed, the key applications such as primary settlement, final settlement or SBr • transducers operate at very specific frequencies, maximising signal reflection from the interface level NEED TO KNOW 1 there has been a lack of successful r&d to develop the high-end analytical software 2 Sludge blanket detection accuracy has helped with compliance and reduced the number of out-of-hours call-outs at the Stoke Bardolph facility 3 the control unit has, pre-programmed, the key applications such as primary settlement, final settlement or SBr 4 transducers operate at very specific frequencies, maximising signal reflection from the interface level THE VERDICT • Severn trent Water's Stoke Bardolph site uses sludge blanket measurement to help increase sludge concentration from 2% to 6% • It has been able to entrust a key element of site compliance to an automated system R eliable, accurate contin- uous sludge blanket detection has been a long- pursued goal for wastewater treatment plant operators and instrument manufacturers alike. The principles have been well understood, but the execution has been held up by a lack of successful research and development (R&D) to develop the high-end analytical so•ware to allow a manufacturer to provide equipment that users can trust. Even now, most sludge processes rely on laborious, labour-intensive manual measurements being taken, with obvious health and safety implications. Recently, with increased emphasis on totex and compliance throughout the network, it is more important than ever that this particular nut should be cracked, and cracked right across the range of sludge management systems, from primary to final settlement and SBR (sequencing batch reactor). The latest sludge blanket detectors achieve levels of detection accuracy and reliability that, finally, really provide the opportunity to automate these critical processes. So, why has sludge blanket detection proven so difficult? The key issue is the need to discriminate what can be a tiny change in liquor density. Step back That can be done on a point level using infrared, and continuously with ultrasonic measurement, but requires the instrument to detect both tiny changes and also, o•en, a very indistinct boundary layer. Add to that the need to distinguish both a rising and falling level and the challenge becomes clear. Taking a step back, and going back 25 or 30 years, non-contacting ultrasonic measurement was recognised as being an important, developing technology for pumping station control, but also seen as troublesome and difficult to apply repeatedly. Expertise Over the past 20 years, newer players, including Pulsar Process Measurement, have entered the market and brought both a focus to this area and a new expertise digital acoustic engineering. They took ultrasonic measurement to the new level required to make it the standard technology, accepted and implemented without question. The majority of the development in the area has recently focused much more on simplicity of set-up and developing functionality of the latest ultrasonic control systems than the underlying echo processing and discrimination algorithms. The key enabler allowing the development of the technology to the current state has, of course, been increases in microprocessor speed and processing power.