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Frontiers of Environmental Science & Engineering

ISSN 2095-2201

ISSN 2095-221X(Online)

CN 10-1013/X

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2018 Impact Factor: 3.883

Front. Environ. Sci. Eng.    2016, Vol. 10 Issue (2) : 319-326    https://doi.org/10.1007/s11783-014-0758-6
RESEARCH ARTICLE
Simple model of sludge thickening process in secondary settlers
Yuankai ZHANG,Hongchen WANG(),Lu QI,Guohua LIU,Zhijiang HE,Songzhu JIANG
School of Environment & Natural Resource, Renmin University of China, Beijing 100872, China
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Abstract

In wastewater treatment plants (WWTPs), a secondary settler acts as a clarifier, sludge thickener, and sludge storage tank during peak flows and therefore plays an important role in the performance of the activated sludge process. Sludge thickening occurs in the lower portions of secondary clarifiers during their operation. In this study, by detecting the hindered zone from the complete thickening process of activated sludge, a simple model for the sludge thickening velocity, us=aXb(a=0.9925SSVI3.5,b=3.541 ln(SSVI3.5)+12.973), describing the potential and performance of activated sludge thickening in the hindered zone was developed. However, sludge thickening in the compression zone was not studied because sludge in the compression zone showed limited thickening. This empirical model was developed using batch settling data obtained from four WWTPs and validated using measured data from a fifth WWTP to better study sludge thickening. To explore different sludge settling and thickening mechanisms, the curves of sludge thickening and sludge settling were compared. Finally, it was found that several factors including temperature, stirring, initial depth, and polymer conditioning can lead to highly concentrated return sludge and biomass in a biologic reactor.

Keywords wastewater treatment plants      secondary settler      sludge thickening      sludge settling      hindered zone     
Corresponding Author(s): Hongchen WANG   
Online First Date: 24 November 2014    Issue Date: 01 February 2016
 Cite this article:   
Lu QI,Guohua LIU,Zhijiang HE, et al. Simple model of sludge thickening process in secondary settlers[J]. Front. Environ. Sci. Eng., 2016, 10(2): 319-326.
 URL:  
https://academic.hep.com.cn/fese/EN/10.1007/s11783-014-0758-6
https://academic.hep.com.cn/fese/EN/Y2016/V10/I2/319
formula No. formula reference
1 v = v 0 X n [4]
2 v = v 0 n X [5]
3 v = v 0 e n X [6]
4 v = v 0 ( 1 n X 2 / 3 ) [7]
5 v = min ( v 0 [ e γ h ( X f n s X f ) e γ p ( X f n s X f ) ] , v 0 ) [8]
6 v = v 0 e n X / X [9]
Tab.1  Various continuous settling formulae proposed in previous studies
formula No. formula reference
1 v 0 = 5 n = 0.249 e 0.0031 S V I [12]
2 v 0 = 7.8 n = 0.148 + 0.0021 S V I [13]
3 v 0 = 7.6 n = 0.1030 + 0.00255 D S V I [10]
4 v 0 = e 2.605 0.00365 D S V I n = 0.249 + 0.002191 D S V I [14]
5 v 0 = 24.3 e 0.01073 S S V I n = 0.245 + 0.00296 S S V I [15]
6 v 0 = 11.9 0.06 S S V I 3.5 n = 0.16 + 0.0027 S S V I 3.5 [16]
7 v 0 = 15.3 0.0615 S S V I 3.5 n = 0.426 0.00384 S S V I 3.5 + 0.000534 S S V I 3.5 [17]
8 v 0 = 16 0.1 S S V I 3.5 n = 0.16 + 0.003 S S V I 3.5 [18]
9 v 0 = 7.97 n = 0.0583 + 0.00405 S S V I 3.5 [10]
10 v 0 = 161.2046 S S V I 3.5 0.7249 n = 0.22015 e 0.00657 S S V I 3.5 [13]
Tab.2  Summary of relationships between the sludge settleability parameters ( S V I , D S V I , S S V I and S S V I 3.5 ) and the flux theory constants v 0 / n
Fig.1  (a) Thickening curve of activated sludge (the slope of the bold black line is the sludge thickening velocity). A: lag, B: hindered zone, C: transition, and D: compression zone. (b) Batch sludge thickening curves for group A1 (the slope of the bold red lines are the sludge thickening velocities under different initial sludge concentrations)
number S S V I 3.5 /(mL·g−1) a b
WWTP1 102.9 56.420 −3.772
WWTP2 93.9 44.970 −2.900
WWTP3 63.8 22.055 −1.920
WWTP4 59.0 9.283 −1.432
Tab.3   S S V I 3.5 and parameters a and b obtained for the experiments
Fig.2  (a) S S V I 3.5 values, initial concentration X 0 , and sludge thickening velocity from the five WWTPs, (b) sludge thickening and settling curves in WWTP1
Fig.3  Effect of temperature on sludge thickening velocity and sludge viscosity
Fig.4  Sludge thickening velocity under different stirrer speeds (a) and initial heights (b)
Fig.5  (a)–(c) show D 2 P , D 3 , and D B for flocs under 1 mg·L−1 cationic PAM dosage, and (d) and (e) show the sludge velocity and fractal dimension ( D 2 P , D 3 , and D B ) under different cationic PAM dosages, respectively
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