A series of laboratory-based experiments were carried out with the objective of making a comparative analysis of ultrasonic-enhanced irradiation and heater (hot water) treatment of secondary activated sludge. For ultrasound treated sludge, bacterial killing efficiency was gauged with
respect to the transducer radiating surface diameters, time of irradiation, amplitude (%), mode of operation (continuous waves and pulse), reactor system (batch and flow-through) and volume of sludge. In batch experiments, the 34 cm diameter probe resulted in 100% bacterial
killing efficiency in 1 L of sludge at 100% amplitude between 50 and 60 minutes in a continuous wave operation at an average energy consumption of 204 Ws (12.24 Whr). Samples were constantly mixed with a magnetic stirrer, but not water-cooled. Temperature rose from 22°C to about
50°C. With 1 L of sludge and continuously stirred batch reaction, the 55 cm diameter probe resulted in 100% killing efficiency in 600 seconds at 100% amplitude, continuous wave operation and at an average energy consumption of between 240-250 Ws (14.40-15.0 Whr). The temperature
rose from 22°C to ∼70°C within a short time. When the same sample was sonicated and water-cooled, 100% killing efficiency was achieved at 3000 seconds, a factor of 5x time increase compared with sample not water-cooled. Ultrasound was not effective for continuous flow operation.
In batch experiments, hot water treated samples attained 100% bacterial killing efficiency in 9.50 L of sludge between 67°C-75.56°C at an average energy consumption of 0.37-0.5 Whr. Bacterial killing efficiency in hot water treated samples were fluctuated irrespective of the
flow rate and temperature settings. This study showed that bacterial killing efficiency of hot water treated sludge in batch reactor is more efficient than ultrasound enhanced sludge treatment. Neither of the technology can achieve desired killing efficiency, deagglomeration, and cell rupture
to release extracelluar materials can be optimized through the synergetic combination of ultrasound and heater.
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