comparative investigation of parameters for determining the dewaterability of activated sludge

8/12/2019 Comparative Investigation of Parameters for Determining the Dewaterability of Activated Sludge

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Comparative Investigation of Parametersfor Determining the Dewaterability ofActivated Sludge

Ge Peng, Fenxia Ye*, Ying Li

ABISTRAC 7: This paper compared four parameters evaluating thedewaterability of sludge-capillary suction time (CST), specific resistanceto filtration (SRI), bound water content, and dry solids content in thecentrifuged sludge cake of the different sludge flocs from seven full-scalewastewater treatment plants. The dry solids content cotrelated with thenormalized CST R2=0.7112, p=0.003,SRF R2=0.6043, p=0.01 I , andthe bound water content R2=0.8106, p=0.001 . The normalized CST wascorrelated significantly with SRF R2=0.9450, p=0.000 and correlatedwith the bound water R2=0.51 10, p=0.0417 . However, SRF correlatedvery weakly with the bound water R2=0.3929, p=0.0448 . It is notnecessary to use both CST and SRF at the same time to estimate the sludgedewaterability. The normalized CST is feasible because of its affordability.simple equipment, and measurement procedure. However, anotherparameter indicating the dewatering extent, such as dry solids content insludge cake, should be applied together to evaluate the dewateringefficiency of tha activated sludge. aterEnviron Res. 83, 667 (2011).KEYWORDS: activated sludge, dewaterability, bound water, capillarysuction time, specific resistance to filtration, dry solids.doi:l 0.2175/10614301OX 12851009156646

IntroductionThe biological treatment of wastewater produces significant

quantities of excess activated sludge, the water content of which isgenerally greater than 95 . Therefore, it is necessary to dewater thesludge to obtain a product that is dry enough to allow a reduction involume, facilitate transportation, and decrease the energy used inthe case of drying or incineration. In many wastewater treatmentfacilities, the bottleneck of th6 sludge handling system is thedewatering operation. Hlowever, it is well-known that the activatedsludge generally is hard to dewater and often exhibits non-traditional filtration behavior (Stickland et al., 2005 .

The water classification is more complex and includes fourcategories, including free water (Lee and Hsu, 1995; Vesilind,1994). The free water can be eliminated by simple thickening orweak mechanical methods. The bound water (BW) is fixed to the

sludge and cannot be removed by mechanical means. This boundwater content is one of the main limiting factors in the waterremoval efficiency (Lee, 1995). Therefore, the bound water

Depaetruent of Chemical Engineering, Ningbo University of Technology,Ningbo, China. 'Department of Chemical Engineering, Ningbo University of Technology,Nitgbo 315016, China; e-mail: [email protected]; [email protected].

content has been chosen as an index for evaluating thedewaterability of the activated sludge (Jin et al., 2004; Vaxelaireand C6zac, 2004).

The most common laboratory procedures for quantifying sludgedewaterability are capillary suction time (CST) and specificresistance to filtration (SRF). However, in full-scale plants,dewatering of the activated sludge typically is carried out usingbelt filter presses or centrifuges. so that sludge dewaterability isexpressed in terms of cake solids content or dry solids or watercontent in the dewatered sludge cake.

In fact, the dewatering efficiency of the activated sludgedepends on not only the rate of dewatering, but also the extent ofdewatering. The rate of dewatering typically is measured in thelaboratory using a CST test or SRF, whereas the extent ofdewatering typically is measured by the dewatered cake solids asthe percent dry solids. Therefore, it is possible that the activatedsludge is easily filterable, but there is a high amount of residualwater in the dewatered sludge (Novak, 2006). Most of theinvestigations focused on the filtration rate measurements,including the CST test and the determination of SRF. However,CST and SRF test are not always able to predict full-scaleperformance (Marinetti et al., 2009). In fact, in our previous study,we found that the SRF value of the activated sludge in a fruitprocessing plant is low, but CST is high (data not shown). Thisimpelled us to investigate the relationships between the dewater-ing parameters. Although bound water content, CST, SRF, and drysolids content have been used for determining the dewaterabilityof the activated sludge, up to now, little attention has been givento achieve an optimum parameter to evaluate the sludgedewaterability and to estimate the correlation between boundwater, CST, SRF, and dry solids.The aim of this study was to determine bound water, CST, SRF,and dry solids values of the activated sludge from differentindustries and urban activated sludge samples and examine theircorrelations. During the investigation of nearly 1 year, theactivated sludge underwent various operation changes understeady and unsteady state. The goal was to obtain an optimumparameter for determining the sludge dewaterability, which ma ybe helpful to estimate the maximum performance of thedewatering process and improve the dewatering efficiency ofthe activated sludge.

MethodologyActivated Sludge Samples. Activated sludge samples weretaken from seven different full-scale activated sludge wastewater


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Table 1 Process descriptions of the different WWTPs.Organic loading rate C:N of

Treatment plant Source of wastewater Biological processa kg COD/kg SS.d) influentbMunicipal North District I (A) Domestic 70 , leachate30 CAS 0.51 + 0.05 4 to 5Municipal North District B) Mainly domestic A2/O 0.37 - 0.05 7 to 9Municipal South District (C) Mainly domestic A/O 0.33 +-0.05 4 to 5Dye plant (D) Printing and dyeing wastewater A/0 0.48 _ 0.05 9 to 11Fruit processing plant (E) Fruit-tinned processing CAS 0.72 + 0.05 13 to 15wastewaterChemicals plant (F) Chemicals wastewater A/0 0.78 + 0.05 14 to 16Petrochemical plant (G) Petroleum refinery wastewater Contacting oxidation-SBR 0.70 + 0.05 13 to 14Pulp-paper plant (H) Paper pulp wastewater A OC 0.81 t 0.05 75 to 85

a CAS = conventional activated sludge, A2/0 = anaerobic-anoxic-oxic, A/O = anaerobic-oxic, A/D/O = anaerobic-oxic-oxic, and SBRsequencing batch bioreactor.b C = COD and N = total nitrogen.

treatment plants (WWTPs) in Ningbo, China. These included twosewage treatment plants and five industrial activated sludgetreatment processes for dyeing, cann ed fruit processing, pulp andpaper, chemicals, and petrochemical industries. Two types ofactivated sludge were collected from the same sewage treatmentplant (North District municipal sewage treatment plant, Ningbo,China), but using different treatment processes. The biologicaltreatment process descriptions of the WWTPs are given inTable 1. The sludge was sampled once per month from theaeration tank of each WWTP and stored in filled plastic containersplaced in an ice cooler during the transportation from the WWTPsite to the laboratory. All of the activated sludges settled for onlyI hour to obtain the concentrated sludge and were not conditionedwith any conditioner. Sample tests started immediately and werecompleted within 20 hours, while being kept refrigerated at 40C.

Sludge Dewaterability Test. The CST was measured by aCST instrument (Model 304M, Triton Electronics Ltd., Essex,England) as detailed in Standard ethods (APHA et al., 1995)with a CST paper purchased from Triton Electronics Ltd. TheCST for distilled water was stable at t I seconds. The CST valueswere normalized by dividing them by the initial total suspendedsolids (TSS) concentration and then expressed in units of secondsper liter per gram TSS. The test was made in triplicate with astandard deviation of 5 .The method of Wisniewski and Grasmick (1998) was modifiedto measure SRF. The SRF test was conducted in a 250-mL stirredcell using a filter with 0.45-jam filter paper. The stirred cell wasfilled with 100 mL of the sludge suspension, and a constantpressure was applied by an air pump. The production of filtrateunder pressure was recorded continuously. The SRF (mlkg) of thesludge was calculated by the following:

SRF = 2000 A2 Apb/tC (1)Where

Ap (35 kPa) = pressure applied;A (0.00502 m2) =filter area;

l = viscosity of the permeate (Pa-s);C = sludge concentration in TSS (kg/in); and

b (s/m6) = time-to-filtration ratio, which is the slope ofthe curve that is obtained by plotting the ratioof the time of filtration to the volume offiltrate (tIV) versus the filtrate volume (V).

The bound water value was estimated by the dilatometricmethod according to Smith and Vesiland (1995). The dry solidcontent was measured by a centrifuge instrument. Approximately50-mL sludge samples were centrifuged at 2215Xg for 10 minutesThe solids content was analyzed following Standard ethod(APHA et al., 1995).

Statistical Analysis. Statistical analysis was carried out withSPSS software version 11.0 for Windows (SPSS, ChicagoIllinois). The Pearson s correlation coefficient (R2) was used toestimate the linear correlation between two parameters. ThePearson's R2 coefficient is always between -l and +1, where -Imeans a perfect negative correlation, +1 a perfect positivecorrelation, and 0 the absence of a relationship. Correlationswere considered statistically significant at confidence intervap< 0 .05 .Results and Discussion

For all of the activated sludge samples studied, the normalizedCST, SRF, bound water content, and dry solids content were in therange 2.8 to 17.9 s.L/g TSS, 3.7 to 14.0 X 108 n/kg, 7.2 to 26.7 gg TSS, and 2. lto 7.0 , respectively. The type and pore size of thefilter paper affect the assessment of filterability of the activatedsludge, such as CST and SRF, which therefore prevents the direccomparison of the results obtained in various studies. Thiindicated a necessity of standardizing filter paper in bench-scalesludge filterability tests (Yigit et al., 2010). Because no pressure iapplied during the measurement of CST, it may not simulate thactual treatment process in many applications. For sample, Wu eal. (1997) found that CST measurement led to excess use oconditioners for sludge. The SRF measurements, on the othehand, are carried out by a Buchner funnel filtration test apparatuat constant vacuum pressure.

In practical WWTPs, dry solids content typically was used athe dewaterability indicator of the activated sludge. Therefore, thcorrelations between dry solids content and the normalized CSTSRF/BW content were investigated in this study. A negativecorrelation between dry solids content and the normalized CSTwas obtained (R2=0.7112, p=0.003 ) (Figure 1.). Similarlynegative correlations were observed between dry solids contenand SRF (R2=0.6043, p=0.01l) (Figure 2) and between drsolids content and the bound water content (R2=0.8106, p=0.001)(Figure 3). For the same dry solids content, the correlation witthe bound water content was strongest, then the normalized CST


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DS content ( )Figure 1-Pearson correlation between normalized CSTand dry solids content from different sludges of sevenWWTPs.and finally the SRF value. The CST had a better correlation withdry solids than SRF, which was consistent with Pan et al. (2003).

Figure 4 shows that the SRF and normalized CST werecorrelated strongly (R2 =0.9450, p=0.000), which may be becausethese two parameters are the indicators of the filterability. Thecorrelation between CST and SRF has been a scientific discussionfor several years (Kavanagh, 1980). The result was consistent withthe findings of Baskerville and Gale (1968). Yu et at. (2010) alsofound that the normalized CST was significantly correlated withSRF (R2=0.95, p


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0 0 0 00 0 0 0

2 0 0 0

C/H

A U. J1 r-.V1+

0 2 4 6 8 10 .12 14 16 18 20CST(s L/gTSS)

Figure 5 Pearson correlation between bound watercontent and CST from different sludges of sevenWWTPs.water do not correspond to a similar indication, in terms ofdewaterability of the activated sludge. This result, with the strongcorrelations between SRF and the normalized CST and betweenthe bound water and dry solids content, revealed that themeasurements of both CST and SRF correspond similarly, interms of dewatering rate of the activated sludge, but the boundwater and dry solids content indicate the dewatering extent of theactivated sludge. On the other hand, even if the water passesthrough the filter cake quickly, the water content inside smallpores and capillaries and water bound inside the floc matrix mayremain high. Therefore, the CST and SRF may be related stronglyto the free water in the activated sludge, whereas the boundwater content may be related to the firmly fixed water in thesludge flocs, then related to the dry solids content in the sludgecake. The results also suggested that the CST or SRF test alone isinadequate to determine the grade of separation of sludge solidsfrom the water.

Historically, a wide range of empirical measurements have beenused for determining dewaterability of the sludge, with one of themost common being CST (Feng et al., 2009; Jarvis et al., 2004;Subramanian et al., 2008; Zhang and Chen, 2009). Although theCST test does not quantify a particular, fundamentally basedphysical parameter of the sludge, there is a strong linearcorrelation between the normalized CST and SRF; the normalizedCST method provides a simple, rapid, and inexpensive method tomeasure the dewaterability potential of the sludge flocs (Houghtonand Stephenson, 2002; Yu et al., 2008), has been accepted andused widely, and has been assigned a specific protocol in theUnited States (APHA et al., 1995). It is obvious that improve-ments in both the rate of dewatering and the extent of dewateringare needed. As a result, the normalized CST measurement andanother parameter indicating the dewatering extent, such as drysolids content in the dewatered cake, should be applied together toevaluate the dewaterability of the activated sludge. Up to thepresent, the most common and successful applicability of thelaboratory studies is the optimum conditioning dose that typicallyis determined using the CST test. By taking into consideration thecost and specific energy requirement, the CST test is the mostinexpensive compared with the other dewatering parameters.

Sludge dewaterability is a complex phenomenon depending onmany factors; however, it is possible to determine which sludgecan be dewatered easily and provide some parameters to evaluate

3025201510

R2=0.39,P=0.04

0 2 4 6 8 10 12 14 16SRF(m/kg, x 108

Figure 6 Pearson correlation between bound watercontent and SRF from different sludges of sevenWWTPs.the sludge. Even so, the relationship between these parametersshould be well-understood. The finding also should be tested andevaluated on a specific plant basis by incorporating cost-benefitanalysis.Conclusions

This paper presented a comparative and correlative study of theparameters for evaluating dewaterability of the activated sludge.The following conclusions may be drawn from this study:(1) The dry solids content correlated with the normalized CST,SRF, and the bound water content. The normalized CST was

significantly corrected with SRF and bound water content.However, SRF correlated very weakly with the bound watercontent.(2) It is not necessary to use the CST and SRF tests simul-taneously to estimate the dewaterability of sludge becausethey are similar indicators of the dewatering rate.

(3) The normalized CST is feasible because of the simpleequipment and measurement procedure. Another parameterindicating the dewatering extent, such as the bound water ordry solids content in sludge cake, should be applied with thenormalized CST measurement to evaluate perfectly thedewaterability of the activated sludge.

CreditsThe authors thank the Zhejiang Provincial Natural ScienceFoundation of China (Hangzhou, China) (Y5090038) and Ningbo

Bureau of Science and Technology Ningbo, China)(2010A610090) for their financial support.

Submitted for publication September 25 2010; revisedmanuscript submitted November 14 2010; acceptedfor publica-tion December 6 2010.ReferencesAmerican Public Health Association; American Water Works Association;

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Peng, Ge; Ye, Fenxia; Li, Ying

Comparative Investigation of Parameters for Determining the Dewaterability

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comparative investigation of parameters for determining the dewaterability of activated sludge

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