Effect of rapid-mixing conditions on the evolution of micro-flocs to final aggregates during two-stage alum addition. Issue 20 (6th September 2021)

• Record Type: Journal Article
• Authors: Du, Peng; Li, Xing; Yang, Yanling; Fan, Xiaoyan; Zhang, Tingting; Wang, Nan; Li, Hang; Ji, Siyang; Zhou, Zhiwei
• Is Part Of: Environmental technology. Volume 42:Issue 20(2021)
• Page Start: 3122
• Page End: 3131
• Publication Date: 2021-09-06
• Subjects: Two-stage coagulant -- micro-floc -- final aggregate -- velocity gradient -- coagulation-flocculation; Environmental engineering -- Periodicals; Environmental protection -- Periodicals; 628.05
• DOI: 10.1080/09593330.2020.1723710
• Languages: English
• ISSNs: 0959-3330

Abstract

ABSTRACT: In a coagulation-flocculation process, optimal separation of the resultant aggregates plays a decisive role on coagulation performance and provides a lower burden for subsequent treatment units. This separation highly depends on the stability of the micro-flocs formed during the initial, rapid stage of coagulation. In this work, a two-stage addition of aluminium sulphate (alum, Al2 (SO4 )3 ) was employed by adding 0.04 and 0.08 mM Al2 (SO4 )3 at the beginning and the end of rapid mixing, respectively. The coagulation performance and floc characteristics were compared to conventional single addition with the same total coagulant dosage, and the effects of variable rapid-mixing speeds (160–850 rpm) and duration time (10–120 s) were investigated. The results showed that the residual turbidity of two-stage coagulant addition was 85.1% lower than single addition when applied at a mixing speed of 580 rpm and a duration time of 120 s. The underlying coagulation mechanism revealed that the two-step addition more effectively neutralized colloids and formed larger aggregates that settled better and could more easily be removed. Moreover, the aggregates were less firm, which was attributed to different interactions among the micro-flocs, the second addition of Al2 (SO4 )3 and destabilized colloids. The present work provides data to broaden the window of rapid-mixing environments for more effective coagulation. GRAPHICAL ABSTRACT: UF0001