Coagulation performance and flocs characteristics of recycling pre-sonicated condensate sludge for low-turbidity surface water treatment

• Ultrasound was firstly applied to pre-treat DWTS to enhance recycling process.
• Coagulation performance of recycling pre-sonicated DWTS was assessed.
• Formation, breakage and re-growth properties of re-coagulated flocs were examined.
• Re-growth of broken flocs was irreversible regardless of ultrasound frequencies.
• Morphological analysis was carried out to explore reversibility/irreversibility.

This research innovatively investigated the coagulation performance of recycling pre-sonicated condensate drinking water treatment sludge under different ultrasound (US) conditions (ultrasonic time of 5, 10, 15, 20 and 30 min and US frequencies of 25, 40, 125 and 160 kHz) for low-turbidity surface water treatment. The formation, breakage and re-growth of re-coagulated flocs generated by recycling processes were examined using photometric dispersion analyzer to explore flocs growth properties, flocs strength and re-aggregation potential. The morphological analysis was further conducted to study the structural properties of the broken flocs to elucidate the reversibility/irreversibility of re-growth process. The results indicated that 25 or 40 kHz within 5 min sonication was more favorable for turbidity removal, while 125 or 160 kHz with 10 or 15 min for organics removal. The recoverability of broken flocs was irreversible for the recycling processes regardless of US frequencies. Additionally, the flocs formed preliminarily exposed to 25 or 40 kHz were stronger and more resistant to breakage, and the recoverability was more irreversible. Furthermore, the flocs formed at 25 or 40 kHz were larger and more irregular with more porous and ramose structure. The average size obtained from morphological analysis could also strongly demonstrate the irreversibility of recovery process.

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