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Desalination 204 (2007) 277–295

A comparative study of the flocculation behaviour and final properties of synthetic and activated sludge in wastewater treatment
Tan Phong Nguyena, Nicholas P. Hankinsb*, Nidal Hilala

Centre for Clean Water Technologies, School of Chemical, Environmental and Mining Engineering, The University of Nottingham, Nottingham, NG7 2RD, UK b Department ofEngineering Science, The University of Oxford, Parks Road, Oxford OX1 3PJ, UK Tel.+44 (1865) 273027; Fax +44 (1865) 273010; email:

Received 31 January 2006; accepted 15 February 2006

Abstract In this study, we have investigated in a comparative fashion the flocculation behaviour and final properties of both synthetic and activated sludge. Synthetic sludge was preparedaccording to established procedures; activated sludge was produced in a lab-scale, continuous-flow reactor which was fed with live activated sludge from a wastewater treatment plant. The novelty of our approach lies in the attempt to use the former as a key to characterising the physical and chemical properties of the latter. The effects of calcium ion concentration on flocculation dynamics for bothsludges were measured on-line using the photometric dispersion analyser; the effects of calcium, polyelectrolyte conditioner and shear rate on final sludge properties were determined, including settleability, turbidity of supernatant, sludge volume index and dewatering, sludge conditioning, and floc strength and structure. It was thus possible to link the flocculation dynamics to the final sludgeproperties. The results indicate that calcium ions play an important role in the floc formation and the final floc size for both types of sludge by the construction of calcium bridges, and hence played a significant role in determining the final properties of the sludge. A qualitative link exists between the flocculation dynamics and the final properties for both types of sludge, and between thefloc dynamics and properties of synthetic sludge on the one hand and those of activated sludge on the other. The two types of sludge have very similar settling and dewatering characteristics after cationic polymer conditioning. However, there are quantitative differences in the calcium concentration required for flocculation, the supernatant turbidity, the sludge-volume index and the floc strength.This difference is believed to be due to the absence of filamentous *Corresponding author. Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May2006.
0011-9164/07/$– See front matter © 2006 Elsevier B.V. All rights reserved.


T.P. Nguyen et al. / Desalination 204 (2007) 277–295

material in the synthetic sludge which is present in the activated sludge. The stable and well-defined nature of synthetic sludge makes it useful as a non-complex analogue for the physical and chemical properties ofactivated sludge, but filamentous cellulose should be added to improve quantitative agreement with activated sludge. Keywords: Synthetic sludge; Activated sludge; Wastewater treatment; Flocculation; Floc strength; Sludge properties; Sludge conditioners; Settleability; Dewatering

1. Introduction and review of sludge floc formation Activated sludge is an aerobic, suspended growth process, in whichmicroorganisms are grown in a variety of bioreactor configurations for the purpose of removing soluble organic matter. It is a flexible, reliable process, capable of producing a high quality effluent. Soluble organic matter is reduced to low levels, and a clear effluent low in suspended solids is produced due to the flocculant nature of the biomass [1]. The flocculated microbial aggregates,...