454139 Centrifugal Separation of Flocculated Sludges in Wastewater Treatment

Wednesday, November 16, 2016: 3:40 PM
Mission II & III (Parc 55 San Francisco)
Wallace Woon-Fong Leung, Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong

Decanter centrifuges are commonly used in municipal wastewater treatment to dewater sludge cake to a high consistency. Likewise, the liquid centrate has to be free from solids as it is being re-circulated in the plant as make-up water or being discharged periodically to the environment. Flocculants are used to agglomerate the biological solids in the wastewater otherwise it is practically impossible to separate by centrifugal sedimentation due to the small density difference between the biosolids and liquid. Jar tests are used to identify and screen flocculants to determine the most appropriate flocculant for use with the wastewater under centrifugation. It is also known that the flocculated solids (i.e. flocs) would break up in the centrifuge due to intense shear stress during acceleration of the feed to the pool so that the size of flocs observed from jar tests could be much larger than the ones in the centrifuge. This paper presents a model on separation of flocs in a decanter. The Leung number [1] is used to derive the “cut size” in making separation for the decanter. When the cut size is smaller than the floc size, a high solids recovery can be maintained, and vice versa.

Despite it is not possible to determine the floc size in-situ in the machine, the recovery–size cut curve can be determined for a given wastewater application (i.e. with organic-to-inorganic ratio defined and a selected flocculant), from which the performance of the machine can be inferred and projected for other decanter sizes. 

The model has been applied in a secondary wastewater treatment plant to several decanters with different configurations in separating flocs from raw mixed sludges (48% Primary Sludge, 52% Waste Activated Sludge). The approach also provides a scale-up of decanter centrifuge performance in separation floc factoring in the scale-up, (a) the operating conditions (pool depth, rotation speed, feed rate ) and (b) configuration of the decanter (pool depth, clarifier length, etc.).



[1] Wallace Woon-Fong Leung, “Industrial Centrifugation Technology”, McGraw-Hill, 1998. 



Decanter Centrifuge, Flocculated Solids, Wastewater, Separation 

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