Removal of disperse dyes from textile wastewater using bio-sludge

Removal of disperse dyes from textile wastewater using bio-sludge
Suntud Sirianuntapiboon *,Parawee Srisornsak
Department of Environmental Technology,School of Energy and Materials,King Mongkut’s University of Technology Thonburi (KMUTT),
Bangkok 10140,Thailand
Received 17June 2005;received in revised form 1February 2006;accepted 6April 2006
Available online 22June 2006
Abstract
Granular activated carbon (GAC)did not show any significant adsorption ability on the disperse dyes,while resting (living)bio-sludge of a domestic wastewater treatment plant showed high adsorption abilities on both disperse dyes and organic matter.The dye adsorption ability of bio-sludge increased by approximately 30%through acclimatization with disperse dyes,and it decreased by auto-claving.The deteriorated bio-sludge could be reused after being washed with 0.1N NaOH solution.Disperse Red 60was more easily adsorbed onto the bio-sludge than Disperse Blue 60.The Disperse Red 60,COD,and BOD 5adsorption capacities of acclimatized,rest-ing bio-sludge were 40.0±0.1,450±12,and 300±10mg/g of bio-sludge,respectively.The GAC–SBR system could be applied to treat textile wastewater (TWW)containing disperse dyes with high dye,BOD 5,COD,and TKN removal efficiencies of 93.0±1.1%,88.0±3.1%,92.2±2.7%and 51.5±7.0%,respectively without any excess bio-sludge production under an organic loading of 0.18kg BOD 5/m 3-d.Furthermore,the removal efficiencies increased with the addition of glucose into the system.The dye,BOD 5,COD,and TKN removal efficiencies of the GAC–SBR system with TWW containing 0.89g/L glucose were 94.6±0.7%,94.4±0.6%,94.4±0.8%and 59.3±8.5%,respectively,under an SRT of 67±0.4days.Ó2006Elsevier Ltd.All rights reserved.
Keywords:Granular activated carbon (GAC);Sequencing batch reactor (SBR)system;Adsorption;Disperse dye;Disperse Red 60;Disperse Blue 60
1.Introduction
Effluent from industries,such as dyeing,pulp and paper,and textile industries,contain many coloring substances,which are toxic (Benefield et al.,1982;Chu,2001)and need to be removed.Removal techniques for coloring substances
include adsorption,precipitation,coagulation,filtration,and chemical oxidation (Benefield et al.,1982;El-Geundi,1991;Janos et al.,2003;Meshko et al.,2001).Although these physical and chemical treatment processes might be effective for color removal,they use more energy and chem-icals than biological treatment processes.Chemical wastes are also generated from these processes.
The textile industry is one of the most important export industries of Thailand.Textile industries use large amounts of water and chemicals for finishing and dying processes.Dyestuffis a type of refractory organic matter,and thus microorganisms find it difficult to use it as a carbon or energy source.However,many researchers are still inter-ested in biological treatment processes for the treatment of textile wastewater due to the low cost and the absence of chemical waste production.Several types of dyestuff,especially disperse dyes,are used in textile industries due to their chemical properties (Society of Dyes and
0960-8524/$-see front matter Ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.biortech.2006.04.026
Abbreviations:BOD 5,biochemical oxygen demand;COD,chemical oxy-gen demand;F/M,food (BOD 5loading)/microbe (total bio-sludge);GAC,granular activated carbon;GAC–SBR,granular activated carbon-sequencing batch reactor;HRT,hydraulic retention time;MB,moving bio-film;MLSS,mixed liquor suspended solids;SBR,sequencing batch reactor;SDS,sodium dodecyl sulfate;SRT,solid retention time;SS,suspended solids;SVI,sludge volume index;TWW,textile wastewater;STWW,synthetic textile wastewater;TKN,total kjeldahl nitrogen;TP,total phosphorus.*
Corresponding author.Tel.:+6624708656;fax:+6624708660/4279062.
E-mail address:suntud.sir@kmutt.ac.th (S.
Sirianuntapiboon).Bioresource Technology 98(2007)
1057–1066
Colourists,1987).Chemical coagulation was commonly used to treat the wastewater containing disperse dye (Chu,2001),but it still has the many problems mentioned above.
Many researchers have focused on the biological treat-ment process to treat textile wastewater containing disperse dye because it uses little area,is not complicated,and has a low operation cost (Kapdan and Kargi,2002).The biolog-ical removal of color was good with vat and sulfur dyes (Nigam et al.,1995).A previous study (Sirianuntapiboon and Seangow,2004)also reported that vat dye could be adsorbed on the bio-sludge and removed by SBR systems operating under short solid retention operation (SRT).However,little research on the biological removal of dis-perse dyes has been reported.In this study,the disperse dye adsorption capacity of both resting and autoclaved bio-sludge was tested.Both SBR and GAC–SBR systems were applied for treating both synthetic and raw textile wastewaters.The disperse dye removal efficiencies of an active bio-sludge were tested in both SBR and GAC–SBR systems under various organic loadings.2.Methods 2.1.Dyes
The two types of disperse dyes selected for use in this study were Disperse Red 60and Disperse Blue 60.The chem-ical properties of the direct dyes are described in Table 1.2.2.Granular activated carbon (GAC)
The GAC type CGC-11,with a mash size of 8·10mm 2,total surface area of 1050–1150m 2/g and apparent density of 0.46–0.48g/ml was used in the experiment.2.3.Wastewater samples
Two kinds of wastewater samples were used in this study:(1)textile wastewater (TWW)and (2)synthetic tex-tile wastewater (STWW).TWW was collected from an influent sump tank of a central wastewater treatment plant in a textile factory in Samutprakarn province,Thailand.The chemical properties of TWW are provided in Table 2.TWW was supplemented with 0.89g/L glucose (final biochemical oxygen demand (BOD 5)concentration of about 1250±10mg/L)and used as TWW +glucose (Table 2).STWW was prepared according to TWW prop-erties.The BOD 5concentration of TWW was approxi-mately 1100mg/L and the total concentration of dyes (Disperse Red 60and Disperse Blue 60)was 80mg/L.The chemical compositions and properties of the STWW were as follows:composition (glucose 1875mg/L,urea 115mg/L,FeCl 33.5mg/L,NaHCO 3675mg/L,KH 2PO 455mg/L,MgSO 4Æ7H 2O 42.5mg/L and disperse dye 80mg/L);properties (chemical oxygen demand (COD)2008±210mg/L,biochemical oxygen demand (BOD 5)1088±103mg/L,total kjeldahl nitrogen (TKN)52±2.1mg/L and pH 7.8±0.2).
2.4.Preparation of bio-sludge for adsorption test
Three types of bio-sludge were prepared using the bio-sludge from the bio-sludge storage tank of a central domestic treatment plant in Bangkok,Thailand (Sripaya Wastewater Treatment Plant).Bio-sludge type A,bio-sludge type B and bio-sludge type C were prepared by acclimatization of the bio-sludge with STWW containing 80mg/L Disperse Red 60,STWW containing 80mg/L Dis-perse Blue 60,and STWW without dye for 1week,respec-tively.These three types of bio-sludge were used as resting bio-sludge (living bio-sludge)after washing with 0.1M ace-tate buffer pH 6.0,and as autoclaved bio-sludge (dead bio-sludge)after being autoclaved at 110°C for 10min (see Table 3).
2.5.Dye adsorption test
The dye adsorption capacity of both bio-sludges (resting bio-sludge and autoclaved bio-sludge)and GAC were determined by the Jar test system (Rubin,1978)using two kinds of STWW:STWW containing 80mg/L Disperse Red 60dye and STWW containing 80mg/L Disperse Blue 60.The pH of the systems was 7.8±0.2.The dye adsorp-
Table 1
Types and properties of disperse dyes Scientific name Trade name
Type
CI no.Color Wavelength at maximum adsorption (nm)Disperse Red 60Red E-FBL 200%
Disperse dye 60756Red 586Disperse Blue 60Turq.Blue S-GL 200%Disperse dye
61104
Blue
479–
Textile wastewater
Mixed disperse dyes
Red-violet
515
Table 2
Chemical properties of textile wastewater (TWW)and textile wastewater containing 0.89g/L glucose (TWW +glucose)Chemical properties TWW TWW +glucose Range Average ±SD Average ±SD COD,mg/L 1524–36451517±22023±2BOD,mg/L
281–975918±991157±67Suspended solid (SS),mg/L 216–40282±1157±10pH
7.32–8.127.8±0.47.8±0.4Color intensity (A 608),units
0.50–0.70
0.258±0.003
0.258±0.003
±,standard deviation of three replicates.
1058S.Sirianuntapiboon,P.Srisornsak /Bioresource Technology 98(2007)1057–1066
tion capacity of both bio-sludges and GAC were analyzed using Freundlich’s adsorption isotherm equation(Rubin, 1978).
2.6.Elution of dye from bio-sludge
The bio-sludges A,B,and C were suspended in both STWW containing80mg/L Disperse Red60and STWW containing80mg/L Disperse Blue60for12h(until maxi-mal dye adsorption yield).The dye adsorbed bio-sludge was washed in following solutions:0.1M phosphate buffer pH7.0,0.1%Tween80,0.1%Triton X100,0.1%Cholic acid,0.1N H2SO4,0.1N NaOH,and0.1%sodium dodecyl sulfate(SDS)solutions.The reaction mixtures were centri-fuged at6000·g for10min.The eluted dye in the superna-tant was determined under optimal wavelength,as shown in Table1.
2.7.Acclimatization of bio-sludge for use in the SBR and GAC–SBR systems
Bio-sludge from the bio-sludge storage tank at Sripaya Wastewater Treatment Plant in Bangkok,Thailand was
used as the inoculum of the SBR and GAC–SBR systems. The bio-sludge was fed by STWW without disperse dyes in the reactor and acclimatized for1week.
2.8.SBR
Six10-L reactors,made from acrylic plastic(5mm thick)(Fig.1)were used in the experiments.The dimen-sions of each reactor were18cm-diameter and40cm-height,and the working volume was7.5L.A low-speed gear motor(model P630A-387,100V,50/60Hz, 1.7/ 1.3A,Japan Servo Co.Ltd.,Japan)was used for driving the paddle-shaped impeller.The speed of impeller was adjusted to60rpm for complete mixing.One set of an air pump system,model EK-8000,6.0W(President Co.Ltd., Thailand)was used for supplying air for the two sets of reactors(the system had enough oxygen as evidenced by the dissolved oxygen in the system of about2–3mg/L). The excess sludge was drawn during the draw and idle per-iod to control the mixed liquor suspended solids(MLSS)of the system as shown in Table4.2.9.Operation of SBR and GAC–SBR systems
For the SBR system,1.4L of acclimatized sludge(10g/ L)was inoculated in each reactor,and the TWW or STWW was added(final volume of7.5L)within1h.Dur-ing feeding of the wastewater,the system had to be fully and continuously aerated for19h.Aeration was then shut down for3h.After the sludge was fully settled,the super-natant had to be removed(the removed volume of the supernatant was based on the operation program men-tioned in Tables4and5)within0.5h and the system had to be kept under idle conditions for0.5h.The raw waste-water was subsequentlyfilled in the reactor to afinal vol-ume of7.5L and the above operation was repeated.The operation parameters of the SBR system with TWW and STWW are shown in Tables4and5.For the GAC–SBR system,the operation conditions and procedures were sim-ilar to the SBR system,and7500mg of GAC was put in each GAC–SBR reactor.The mixed liquor suspended sol-ids in the bio-sludge of both SBR and GAC–SBR systems was controlled at2500mg/L during operation.
Table3
Properties and compositions of synthetic textile wastewater(STWW)
Chemical composition Chemical properties
Composition Concentration Properties Concentration
Glucose,mg/L1875COD,mg/L2008±210
Urea,mg/L115BOD,mg/L1088±103
FeCl2,mg/L 3.5TKN,mg/L52.0±2.1
NaHCO3,mg/L675pH7.8±0.2
KH2PO4,mg/L55
MgSO4Æ7H2O,mg/L42.5
Disperse dyes,mg/L80
±,standard deviation of three replicates.
Table4
Operation parameters of sequencing batch reactor(SBR)system with
synthetic textile wastewater(STWW)
Parameter Hydraulic retention time:HRT
(days)
357.5
MLSS(mg/L)250025002500
Flow rate(ml/d)250015001000
Hydraulic loading(m3/m3-d)0.330.200.13
F/M ratio(dÀ1)0.1470.0880.059
BOD5loading(g BOD5/d) 2.75 1.65 1.10
Volumetric BOD5loading
(kg BOD5/m3-d)
0.360.220.15
Colorant loading(g colorant/d)0.200.120.08
Volumetric colorant-loading (kg colorant/m3-d)0.0270.0160.011
S.Sirianuntapiboon,P.Srisornsak/Bioresource Technology98(2007)1057–10661059
2.10.Chemical analysis
COD,BOD5,TKN,MLSS,pH of influent and effluent, sludge settled volume test at30min(SV30),and sludge volume index(SVI)of the SBR system were determined using‘‘Standard Methods for the Examination of Water and Wastewater’’(APHA,AWWA,WEF,1998).The color intensity of STWW and TWW was determined as the absorbance at the optimum wavelength(see Table1). The SRT(solid retention time)was determined as the ratio of total MLSS of the system to the amount of excess sludge wasted in a day.
2.11.Statistical analysis method
Each experiment was repeated at least3times.All the data were subjected to two-way analysis of variance (ANOVA)using SAS Windows Version6.12(SAS Insti-tute,1996).Statistical significance was tested using the least significant difference(LSD)at the p<0.05level and the results are shown as the mean±the standard deviation.3.Results
3.1.Adsorption capacity of bio-sludge and GAC
The results are shown in Table6.GAC did not show any significant adsorption capacity for organic matter or disperse dyes in STWW,while bio-sludge from the waste-water treatment plant showed high adsorption capacities for both organic matter and disperse dyes.Adsorption capacities of GAC type CGC-11for Disperse Red60and Disperse Blue60were only3.4±0.3and2.5±0.4mg/g of GAC.However,resting bio-sludge A showed high Dis-perse Red60and COD adsorption abilities of40.0±0.1mg dye/g bio-sludge and450±12mg COD/g bio-sludge,respectively,in STWW containing Disperse Red 60.Resting bio-sludge B showed high Disperse Blue60 and COD adsorption capacities of29.7±2.6mg dye/g bio-sludge and460±13mg COD/g bio-sludge,respec-tively,in STWW containing Disperse Blue60.The dye adsorption capacities of acclimatized bio-sludges(bio-sludge type A and type B)with STWW containing disperse dyes were about30%higher than that of non-acclimatized bio-sludge(type C).The dye adsorption capacity of the resting type of acclimatized bio-sludge decreased by about 5–7%with autoclaving.Also,the organic matter adsorp-tion capacity of resting(living)bio-sludge was about10 times higher than that of autoclaved(dead)bio-sludge. These results were obtained at pH values of7.8±0.2for all of the systems(see Table7).
3.2.Elution of adsorbed dye from bio-sludge
The results are shown in Table8.The disperse dyes were hardly eluted from all types of disperse dye adsorbed bio-sludge(type A,type B and type C)with phosphate buffer and diluted acid solution,but easily eluted by washing with diluted surfactant and inorganic alkali solutions.Adsorbed Disperse Blue60was more easily eluted from the bio-sludge than adsorbed Disperse Red60,possibly due to
Table5
Operation parameter of GAC–SBR and SBR system with textile waste-water(TWW)and textile wastewater containing0.89g/L glucose (TWW+glucose)
Parameter Concentration
TWW TWW+glucose MLSS(mg/L)25002500
Flow rate(ml/d)15001500
Hydraulic loading(m3/m3-d)0.200.20
F/M ratio(dÀ1)0.0330.067
BOD5loading(g BOD5/d) 1.38 1.74 Volumetric BOD5loading
(kg BOD5/m3-d)
0.180.23
Colorant loading(g colorant/d)–a–a
Volumetric colorant-loading
(kg colorant/m3-d)
–a–a
a Cannot be calculated.
Table6
Adsorption capacity of various types of bio-sludge
Type of wastewater Adsorbent Dye adsorption capacity
(mg/g bio-sludge)COD biosorption
(mg/g bio-sludge)
BOD5biosorption
(mg/g bio-sludge)
Source Type
STWW+Disperse Red60Bio-sludge type A Resting40.0±0.1450±12300±10
Autoclaved27.2±0.770±732±3
Bio-sludge type C Resting37.0±0.5453±10295±11
Autoclaved30.2±0.265±630±4
GAC GCG11 3.4±0.3––
STWW+Disperse Blue60Bio-sludge type B Resting29.7±2.6460±13310±11
Autoclaved22.3±1.565±540±2
Bio-sludge type C Resting31.3±3.8455±10300±11
Autoclaved28.7±0.472±639±3
GAC GCG11 2.5±0.4––
*The bio-sludge was acclimatized in various types of STWW for1week before use in adsorption capacity testing,as mention in Table1.±,standard deviation of three replicates.
1060S.Sirianuntapiboon,P.Srisornsak/Bioresource Technology98(2007)1057–1066
the greater molecular weight of Disperse Blue60.For example,115.7±12.3%of adsorbed Disperse Red60and 142.7±19.3%of Disperse Blue60could be eluted from Disperse Red60adsorbed-resting bio-sludge type A and Disperse Blue60adsorbed-resting bio-sludge type B by washing with0.1N NaOH solution,respectively.
3.3.Stability of bio-sludge on dye adsorption capacity
The dye adsorption stability of bio-sludge after washing with0.1N NaOH solution was also determined.The bio-sludge type A,type B and type C could be reused for dis-perse dye adsorption after being washed with0.1N NaOH solution,as shown in Table8,and unexpectedly,the dye adsorption capacity of reused bio-sludge was20–60% higher than that of raw bio-sludge.For example,the Dis-perse Blue60adsorption capacity of bio-sludge type C upon thefirst reuse was55.6%higher than that of raw
bio-sludge type C,and51.1%and48.7%higher adsorption capacities were maintained on the second and third reuses of bio-sludge type C.
3.4.Sequential replacement reaction
The dye adsorption capacities of bio-sludge types A and B in a sequential replacement reaction are shown in Fig.2. Although the adsorption capacity decreased as the replace-ment was repeated,they recovered after the washing of both deteriorated bio-sludges with0.1N NaOH solution.The adsorption capacity of both bio-sludges decreased to about50%of the initial adsorption capacity after three reuses,but it was recovered to about85%of the initial adsorption capacity after washing with0.1N NaOH solution.
3.5.SBR and GAC–SBR systems for treatment of both STWW and TWW
Thefive types of wastewater used were STWW contain-ing80mg/L Disperse Red60,STWW containing80mg/L
Table7
Elution capacity of various types of eluant
Types of wastewater Bio-sludge%of eluted dye from dye adsorbed bio-sludge in various types of eluant
0.1%SDS0.1%Tween800.1%Triton
X-1000.1N NaOH0.1M Phosphate
buffer
0.1%Cholic
acid
0.1N H2SO4
STWW+Disperse Red60Bio-sludge
type A
64.3±1.7250.0±2.1775.1±5.12115.7±12.3017.6±1.4656.3±2.6010.4±0.59
Bio-sludge
type C
32.2±1.0232.2±1.3842.1±3.3442.6±4.639.4±0.9434.6±4.6318.4±1.62
STWW+Disperse Blue60Bio-sludge
type B
127.4±9.04119.0±8.2998.1±4.57142.7±19.2833.8±1.5880.1±1.769.1±2.25 Bio-sludge
type C
136.7±8.0879.9±5.09117.6±4.70164.8±24.1438.5±1.90106.6±2.2126.3±2.50
±,standard deviation of three replicates.
Table8
Adsorption capacity of bio-sludge after re-washing with0.1N NaOH solution
Types of wastewater Bio-sludge%relative value of dye adsorption of bio-sludge after washing with0.1M NaOH solution
Raw
bio-sludge(%)1st reused
bio-sludge(%)
2nd reused
bio-sludge(%)
3rd reused
bio-sludge(%)
STWW+Disperse Red60Bio-sludge type A100118.6±0.0119.1±0.1117.1±1.9 Bio-sludge type C100129.1±0.0124.7±0.2121.3±2.0
STWW+Disperse Blue60Bio-sludge type B100120.1±0.8119.5±2.2118.4±2.1 Bio-sludge type C100155.6±1.0151.1±2.8148.7±2.7
±,standard deviation of three replicates.
S.Sirianuntapiboon,P.Srisornsak/Bioresource Technology98(2007)1057–10661061
Disperse Blue60,STWW without dyes,TWW,and TWW containing0.89g/L glucose(TIWW+glucose).
3.5.1.STWW
The removal efficiencies of the SBR and GAC–SBR sys-tems tended to decrease with the increase of organic load-ing,as shown in Table9.However,the SBR and GAC–SBR systems still showed high COD,BOD5and TKN removal efficiencies of97.3±0.7%and98.6±0.6%, 96.8±1.9%and97.8±1.8%,and90.0±0.6%and92.3±0.4%,respectively with STWW without dyes under the highest organic loading of0.36kg BOD5/m3-d(HRT of3 days).The addition of disperse dyes to the STWW also tended to reduce the removal efficiencies of COD,BOD5 and TKN.Disperse Red60was more easily removed by both SBR and GAC–SBR systems than Disperse Blue60, which might be explained by the lower molecular weight of Disperse Red60in comparison to Disperse Blue60 (Society of Dyes and Colourists,1987).Both the GAC–SBR and SBR systems showed dye removal efficiencies with Disperse Red60that were about5–6%higher than those with Disperse Blue60.
The removal efficiencies of disperse dyes,as well as organic matter,increased through the addition of GAC into the SBR system(GAC–SBR system).The dye removal efficiency of SBR was increased by about6–8%through the addition of1000mg/L of GAC,as shown in Table9.Nei-ther SBR nor GAC–SBR system with STWW,with and without disperse dyes,showed any difference on the SVI value,as shown in Table10.
The SVI of the systems was increased with an increase of hydraulic retention time(HRT)or a decrease of organic loading.However,the SVI of the systems with the addition of disperse dyes into the STWW did not exceed90ml/g, even for the systems operating under the highest organic loading of0.36kg BOD5/m3-d.The SRT of the GAC–SBR and SBR systems with the STWW,with or without disperse dyes,decreased with an increase of the organic loading.For example,the SRT values of the SBR and GAC–SBR systems with STWW containing Disperse Red 60under the highest organic loading of0.36kg BOD5/ m3-d were10±0.9and12±0.8days,respectively.
The total bio-film mass on the GAC of each GAC–SBR system was almost constant at about1.6–1.8g/reactor.The total bio-sludge of the GAC–SBR system was about10% higher than that of the SBR system.The effluent SS of the GAC–SBR system was lower than that of the SBR sys-tem.Effluent SS of the GAC–SBR and SBR systems with STWW containing disperse dyes were greater than 40mg/L,while they were less than30mg/L with STWW without disperse dyes,as shown in Table9.
3.5.2.TWW
The results are shown in Tables9and10.The GAC–SBR system is shown to be more effective than SBR in treating TWW.Dye,COD,BOD5and TKN removal effi-ciencies of GAC–SBR system under the organic loading of0.18kg BOD5/m3-d were93.0±1.1%,92.2±2.7%, 88.0±3.1%and51.5±7.0%,respectively.The effluent SS of the GAC–SBR system was lower than that of the SBR system.Also,the growth rate of bio-sludge of the sys-tem was quite low and no excess sludge was produced.The SVI of the GAC–SBR and SBR system was only38±2 and45±3ml/g,respectively.By supplementing0.89g/L of glucose into the TWW(TWW+glucose),the efficiency of both GAC–SBR and SBR systems increased greatly. The COD,BOD,TKN,and dye removal efficiencies of the GAC–SBR system with TWW+glucose increased by 94.6±0.7%,94.4±0.8%,94.4±0.6%and59.3±8.5%, respectively.The SRT values of GAC–SBR and SBR sys-tems were43±0.3and67±0.4days,respectively.The effluent SS of both systems was less than60mg/L.
4.Discussion
GAC type CGC-11could not be used as the adsorbent for disperse dyes or organic matter of STWW(the main organic matter caused by glucose).This could be explained because the surface of GAC consists mainly of negative charges,while disperse dyes and organic matter(BOD5) that are mainly generated from glucose are non-polar (Metcalf&Eddy Inc.,1991;Cheremisinoffand Morresi, 1978;Perrich,1981).
However,the resting bio-sludge of the domestic waste-water treatment plant could be used as an adsorbent of both organic matter and disperse dyes,and the adsorption capacities decreased by autoclaving.Disperse Red60was more easily adsorbed onto bio-sludge than Disperse Blue 60because the molecular weight of Disperse Red60is less than that of Disperse Blue60(Society of Dyes and Colourists,1987).The adsorption mechanisms of this bio-sludge might be similar to that of the melanoidin adsorption mechanism in Rhizoctonia sp.and Aspergillus oryzae and the vat dye adsorption mechanism of bio-sludge(Sirianuntapiboon et al.,1995;Ohmomo et al., 1988;Sirianuntapiboon and Seangow,2004).The COD and BOD5removal efficiencies of dead bio-sludge(auto-claved bio-sludge)decreased down to only10–15%of the efficiency of resting bio-sludge.This suggests that the biodegradation of organic matter might be the main reaction involved in organic removal rather than pure adsorption(Sirianuntapiboon and Manoonpong,2001; Sirianuntapiboon et al.,1995).Furthermore,the dye might also be degraded through biological oxidation pro-cess(Fu and Viraraghavan,2001;Watanabe et al.,1982). However,the relevant data for this mechanism was not collected in this study.It is therefore recommended that further research regarding the biological oxidation process be conducted to further advance the understanding of dye removal.
Disperse dye adsorption capacity of autoclaved bio-sludge was only5%lower than that of resting bio-sludge because disperse dye can be adsorbed on the surface of both living and dead bio-sludge.This is similar to thefindings
1062S.Sirianuntapiboon,P.Srisornsak/Bioresource Technology98(2007)1057–1066
from our previous work on vat dye and heavy metal adsorp-tion by bio-sludge(Sirianuntapiboon and Seangow,2004; Sirianuntapiboon and Chaiyasing,2000).The dye adsorp-tion capacity of bio-sludge increased through acclimatiza-tion with disperse dye(Bromley-Challenor et al.,2000). This could be explained by the disperse dyes actually induc-ing the adsorption abilities of the bio-sludge,which is similar to thefindings from previous research on the bio-sorption of melanoidin pigment and vat dye,where the adsorption abil-ity of bio-sludge was induced by the adsorbed matter (Ohmomo et al.,1988;Sirianuntapiboon et al.,1995;Siria-nuntapiboon and Seangow,2004).
On successive dye adsorptions,the capacity of bio-sludge gradually decreased with each reuse,but the absorp-tion ability of the deteriorated bio-sludge was recovered upon washing with diluted alkaline or surfactant solutions. This phenomenon shows that the adsorption sites on the bio-sludge for the disperse dyes werefilled with disperse dyes and that the dyes adsorbed on the surface of bio-sludge were released on the alkaline or surfactant-washing (Fu and Viraraghavan,2001).Also,the Disperse Blue60 was more easily released from deteriorated bio-sludge than Disperse Red60,which may be due to the greater molecu-lar weight of Disperse Blue60.However,the dye adsorp-tion ability of deteriorated bio-sludge could not be recovered on washing with diluted acid because the dis-perse dyes could not be eluted by the diluted acid solution (Cheremisinoffand Morresi,1978;Mittal and Gupta,1996; Sirianuntapiboon and Seangow,2004).
With regard to the treatment of STWW by SBR and GAC–SBR systems,the GAC–SBR system showed higher dye removal efficiency than that of SBR system because the GAC–SBR system was operated under higher total bio-sludge concentration due to the bio-film mass(Metcalf& Eddy Inc.,1991;Kapdan et al.,2000;Adam,1973).The SRT of the GAC–SBR system was greater than that of the SBR system,which resulted in an increase in the dye adsorption capacity of bio-sludge(Sirianuntapiboon and Seangow,2004;Sirianuntapiboon and Chaiyasing,2000). This is similar to the results of heavy metal adsorption on the bio-sludge,in which the adsorption ability of bio-sludge was increased with the increase of sludge age(solids retention time:SRT)(Sirianuntapiboon and Chaiyasing, 2000).Furthermore,the bio-sludge of GAC–SBR system (SVI was lower than100ml/g)settled more easily in the secondary clarifier than that of SBR system(Zaoyan et al.,1992;Metcalf&Eddy Inc.,1991).
Another advantage of the GAC–SBR system is that it produced less excess bio-sludge.For the treatment of TWW,both SBR and GAC–SBR systems showed high dye removal efficiencies of87.7±1.1%and89.3±1.1%, respectively.Additionally,the dye removal efficiencies of both SBR and GAC–SBR systems increased by about5–6%when0.89mg/L of glucose was added to the system. This could be due to both the organic matter and disperse dye being rapidly removed with high efficiencies according to the growth of bio-sludge,which is also referred to as the growth association mechanism(Metcalf&Eddy Inc.,1991; Dewalle and Chain,1977;Ohmomo et al.,1988).
5.Conclusion
From the results of this study,it could be suggested that the resting bio-sludge of a domestic wastewater treatment system could be used as an adsorbent of both organic mat-ter and disperse dye due to its high adsorption ability and its ability to be reused after washing with a diluted alkali solution.The biological treatment system,especially the GAC–SBR system,was more suitable for treating raw TWW due to the high removal efficiencies of disperse dyes, as well as organic matter,under low excess sludge produc-tion and low SVI.The advantage of using GAC in the GAC–SBR system is that the GAC acts mainly as the media for bio-film to attach to,but the GAC does not absorb the disperse dyes directly onto its surface.There-fore,the GAC can be used for a long time in the SBR sys-tem because it is the attached bio-film that is largely responsible for dye and organic matter removal(Dewalle and Chain,1977;Perrich,1981).This study also found that dye removal could be improved by increasing the SRT. Moreover,further increases in dye removal could be achieved with the addition of glucose into the systems. Acknowledgements
We wish to express our thanks to the Department of Environmental Technology,School of Energy and Materi-als,King Mongkut’s University of Technology Thonburi for providing the research materials and funding for this project.
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