Background
Energy saving possibilities continue to gain importance at sewage treatment plants. A large portion of the cost involved is used to heat the digestion towers. By increasing the suspended solids content of the sludge fed to the digestion tower, these costs can be lowered. A higher suspended solids content in the sludge concentrator means less water in the sludge to be heated in the digestion towers. Optimised prethickening of the sludge contributes directly to energy savings. This requires precise control when filling the thickener. Filling the thickener to a higher level results in the bottom sludge layers in the settling cone being more strongly compressed, thus increasing the suspended solids content at the outlet.
• Optimal filling of the thickener with sludge
• Energy savings in heating the digestion towers by increasing the suspended solids content
• Automation of sludge extraction and filling of digestion towers
Measurement requirements
To avoid overfilling the thickener, it is necessary to measure the sludge blanket. The difficulty here is in reliably measuring the different layers of sludge and swirled solids (fluff zone). Daily visual checks encountered in practice are not sufficient and the fluff zone cannot be measured using typical level measurement methods either. That is why special sludge blanket measuring systems, which use an ultrasonic or optical method to measure, are used for this task. Of the two, the optical method has the
advantage because the sensor runs through all of the layers and by directly measuring the concentration is able to detect all of the phases in the tank more reliably than the indirect measurement offered by ultrasound.
Solution
For this application, KROHNE provides the OPTISYS SLM 2080, a sludge blanket measuring device. It features a suspended solids sensor which is submerged in the medium and travels through all of layers or phases in the tank. The level and the suspended solids content are continuously measured and evaluated. The operator can determine certain concentrations which then characterise the different phases, e. g. 0.5 % suspended solids content for the fluff zone and 5 % for the settled sludge. In addition to the notification of the level in the concentrator, this makes the precise recording of the sludge profile
of the various layers possible. The maintenance for this device is very minimal as both the sensor and the cable are cleaned
via integrated spray cleaning, which automatically eliminates contamination following a measurement.
Customer benefits
Automated measurement of the filling level is considerably more accurate than sight checks. The diagram from the pilot test clearly illustrates that as the level of sludge in the thickener rises, the suspended solids content at the sludge extraction increases and runs more evenly. This enables the different phases to be accurately detected and evaluated, allowing for optimal control when filling the concentrator to the maximum filling height. Thanks to the higher suspended solids content at the outlet,
costs can be saved in the downstream treatment stages such as heating of the digestion towers and the dosage of flocculants. In addition, filling can also be automated via the continuous measurement of the sludge blanket.
Products Used
OPTISYS SLM 2080 optical sludge blanket measuring system
• Direct, continuous measurement of the suspended solids content using optical sensors
• Minimal maintenance thanks to automated flushing of sensor and cable
• IP 55 housing with built-in heating, e. g. for installations outdoors as frost protection
• Digital sensor technology with reliable signal transmission
• Complete system with OPTISENS MAC 080 multi-parameter converter, any type of installation possible
• Automatic sensor detection
