Journal of Environmental Sciences2011,23(5)875–880
Moisture distribution in sludges bad on different testing methods Wenyi Deng1,∗,Xiaodong Li2,Jianhua Yan2,Fei Wang2,Yong Chi2,Kefa Cen2如何戒烟最科学
1.School of Environmental Science and Engineering,Donghua University,Shanghai201620,China.E-mail:dengwy@dhu.edu
2.State Key Laboratory of Clean Energy Utilization,Institute for Thermal Power Engineering,Zhejiang University,Hangzhou310027,China
Received10May2010;revid03August2010;accepted16August2010
Abstract
Moisture distributions in municipal wage sludge,printing and dyeing sludge and paper mill sludge
were experimentally studied bad on four different ,drying test,thermogravimetric-differential thermal analysis(TG-DTA)test,thermogravimetric-differential scanning calorimetry(TG-DSC)test and water activity test.The results indicated that the moistures in the mechanically dewatered sludges were interstitial water,surface water and bound water.The interstitial water accounted for more than50%wet basis (wb)of the total moisture content.The bond strength of sludge moisture incread with decreasing moisture content,especially when the moisture content was lower than50%wb.Furthermore,the comparison among the four different testing methods was prented. The drying test was advantaged by its ability to quantify free water,interstitial water,surface water and bound water;while TG-DSC test,TG-DTA test and water activity test were capable of determining the bond strength of moisture in sludge.It was found that the results from TG-DSC and TG-DTA test are more persuasive than water activity test.
Key words:moisture distribution;sludge;bond strength
DOI:10.1016/S1001-0742(10)60518-9
Citation:Deng W Y,Li X D,Yan J H,Wang F,Chi Y,Cen K F,2011.Study on moisture distribution in sludges bad on different testing methods.Journal of Environmental Sciences,23(5):875–880
Introduction
Sludge management is an ever-increasing problem in China and many parts of the world.At prent,million tons of dry matter of wage sludge is annually generated in China from municipal and industrial wastewater treatment plants(MIWTP).Most sludge from MIWTPs directly or indirectly is applied to agricultural land or dispod by landfilling(Lin and Zhou,2004).For sludge treatment,us-age,and disposal,low moisture content is usually required. The reprentation of the moisture distribution within the sludge has always been(and it is still)considered to be esntial for the examination of dewatering and drying problems.Quantitative measurement methods describing the distribution of water in wet sludges are propod bad on the obrvation that the physical properties of water adjacent to the particles surface differ from tho of bulk water.The commonly ud testing methods are drying test (Sato et al.,1982;Robinson and Knocke,1982;Tsang and Vesilind,1990;Smollen,1990),DTA or DSC test (Chen et al.,1997;Chu and Lee,1999;Ferras and Lecomte,2004),and water activity test(Vaxelaire et al., 2000;Vaxelaire,2001;Arlabos et al.,2003;Vaxelaire and C´e zac,2004).However,the conclusions of scientific literature in thisfield are often hard to apply becau of *Corresponding author.E-mail:dengwy@dhu.edu controversial data and definitions.Lee et al.(2006)has given a review report on comparison of different testing met
hods.The comparison was bad on results from differ-ent publications,thus the referential value was weakened due to great differences in experimental condition and materials.
In this study,the moisture distribution of three typical ,municipal wage sludge(MSS),printing and dyeing sludge(PDS)and paper mill sludge(PMS),were studied bad on drying test,TG-DSC test,TG-DTA test and water activity test,respectively.The bond strengths of sludge moisture from different testing methods were compared.The advantages and disadvantages of different testing methods were discusd.
1Materials and methods
1.1Materials
Three kinds of mechanically dewatered sludges:MSS, PDS and PMS,which were all produced from aeration basins using anaerobic/oxic(A/O)wastewater treating system,were ud in this study.The moisture content of MSS,PDS and PDS were3.6,2.7and4.3kg mois-ture/kg dry solid(ds),respectively.MSS was sampled from a municipal wage treatment plant in Hangzhou City of Zhejiang Province,PDS from an industrial dyeing
876Journal of Environmental Sciences2011,23(5)875–880/Wenyi Deng et al.V ol.23 wastewater trea
tment plant in Yunchen City of Shandong
Province,and PMS from a paper-making factory in Pinghu
City of Zhejiang Province,China.Sludge samples were
stored at4°C before experiments.The principal character-
istics of the sludges are listed in Table1.
1.2Experimental t-up and procedures
1.2.1Drying test
The apparatus shown schematically in Fig.1was ud
for determining the different types of moisture fractions
in sludge.It consists of a thermostatically controlled
heating oven inside which was placed a digital balance.
The experimental data from the balance was recorded
by a computer automatically.Sludge sample of5g was
introduced to the balance dish.The dish was placed in an
oven and the sample was dried slowly at30°C and at a
不化妆的女人
controlled humidity by sparging the oven with400mL/min
of compresd dry air.The sludge mass was automatically
recorded by the computer at10min intervals,until there
was no change in sludge mass.When the drying process
in the oven wasfinished,the sludge sample was heated at
105°C for12hr to continue drying and thefinal dried mass
was recorded.
八字成语1.2.2TG-DTA and TG-DSC test
By TG-DTA test,a thermal analyzer(Mettler
TGA/SDTA851,Germany)was employed for recording
the thermographs with sludge sample of35mg.Pure N2
was ud as the carrying gas.Cell temperature of the
analyzer was heated from25to80°C with heating rate of
10°C/min,and kept stable at80°C for50min.The TGA
and DTA analysis was conducted simultaneously.
By the TG-DSC test,sludge sample of35mg was
introduced into the thermal analyzer(TA SDT Q600,USA)
Table1Proximate and ultimate analysis of MSS,PDS and PMS(on a
dry basis)
Species(mass fraction)MSS PDS PMS
Ash content(%)54.7564.9049.73
V olatile content(%)37.3832.0242.37
Fixed carbon(FC)content(%) 6.120.12 3.94
Carbon in volatile and FC(%)25.1815.4216.94
Hydrogen in volatile and FC(%) 3.88 3.08 3.64
Nitrogen in volatile and FC(%) 2.560.98 1.07
Sulfur in volatile and FC(%) 1.59 4.75 1.66
Oxygen in volatile and FC(%)9.297.9123.00
MSS:municipal wage sludge;PDS:printing and dyeing sludge;PMS:
paper mill sludge.
Flow meter Compresd dry air Electric resistance Thermocouple Balance
Sludge
30ć
Fig.1Schematic diagram of drying test.with carrying gas of pure N2.Cell temperature was heated from25to90°C with heating rate of10°C/min,and kept stable at90°C for45min.
1.2.3Water activity test
In water activity test,each kind of the dewatered sludges was divided into ven groups which were pre-dried to different moisture contents.Each group of the pre-dried sludge samples was10–15g,and was ground into particles with diameter less than3mm.Thereafter,the pre-treated sludge samples were introduced into a water activity meter(Rotronic HygroPalm AW1,Switzerland)for fast determination of water activity.
2Results and discussion
2.1Drying test
The method of drying test wasfirst propod by Tang and Vesilind(1990).By this method,a typical drying curve (Fig.2)can be divided into a constant-rate period(line AB),first falling-rate period(line BC),cond falling-rate period(line CD),and equilibrium stage.Thefirst critical point B is the transition from the constant-rate period to thefirst falling-rate period,while the cond critical point C is the transition from thefirst falling-rate period to the cond falling-rate period.The moisture content removed during the constant-rate period,thefirst falling-rate period and the cond falling-rate period are regarded as free water,interstitial water,and surface water, respectively.The residual moisture content in equilibrium stage is regarded as bound water.
Figure3shows the drying rate curves of MSS,PDS and PMS.It is obvious that the constant-rate period does not exist in the drying curves,which indicates that the free wa-ter was completely removed during mechanical dewatering process of sludge.As shown in Fig.3,thefirst falling-rate period and the cond falling-rate can be clearly identified. The moisture contents of the cond critical point C for MSS,PDS and PMS are61.0%wet basis(wb),52.5% wb,and53.0%wb,respectively.Therefore,the interstitial water contents were calculated to be2.1kg moisture/kg dry solid(ds)for MSS,1.6kg moisture/kg ds for PDS, and3.1kg moisture/kg ds for PMS.T
he interstitial water accounted for56.9%wb of the total moisture content in the MSS,58.6%wb in the PDS and73.5%wb in the PMS.The D
r
y
i
n
g
r
a
t
e
Moisture content (% wet basis)
First falling rate
Second falling rate
Constant rate Bound Surface water Free water
Interstitical
water
A
B
C
D
Fig.2Typical sludge drying curve.
No.5Moisture distribution in sludges bad on di fferent testing methods 877
0.0005
0.00100.00150.00200.00250.00300.0035Moisture content (% wb)
Moisture content (% wb)
Moisture content (% wb)
D r y i n g r a t e (k g m o i s t u r e /(m i n .k g d s ))
D r y i n g r a t e (k g m o i s t u r e /(m i n .k g d s ))
D r y i n g r a t e (k g m o i s t u r e /(m i n
.k g d s ))
00.0005
0.00100.00150.00200.0025
Fig.3Drying rate curves of MSS,PDS,and PMS.ds:dry solid;wb:web basis.
higher interstitial water content in the PMS was mainly due to its high original moisture content.
Despite of the marked di fference in the interstitial water contents,the surface water contents of the three sludges were more or less the same.The surface water content of MSS was 1.5kg moisture /kg ds,which was only 0.4kg moisture /kg ds higher than that of PDS and PMS.Moreover,the residual bound water content was only 0.1kg moisture /kg ds for the three sludges.
It can be found from Fig.3that the drying rates of the three sludges were di fferent under the same drying conditions.The average drying rate of the interstitial water was 2.8g moisture /(min ·kg ds)for MSS,2.0g moisture /(min ·kg ds)for PDS,and 3.2g moisture /(min ·kg ds)for PMS,which indicated that the interstitial water in PMS was most easily removed during the drying process.The average drying rate of the surface water was 1.2
g moisture /(min ·kg ds)for MSS,0.7g moisture /(min ·kg ds)for PDS,and 1.0g moisture /(min ·kg ds)for PMS.Since the experimental data were all from the same dry-ing conditions,it is reasonable to infer that the drying rate was strongly correlated with the physical properties of the sludges,including thermal conductivity,apparent density,particle diameter,specific heat,etc.(Arlabos and Chitu,2007;D
eng et al.,2009;Yan et al.,2009).It is well known that MSS is a residue resulting from the treatment of wastewater relead from various sources in-cluding homes,industries,medical facilities,street runo ffand business,while PDS and PMS are from treatment of wastewater relead from textile industry and paper manufacturing respectively.The major organic loading of MSS is a complex mixture of fats,proteins,carbohydrates,humic material and fatty acids (Rogers,1996).Di fferently,PDS mainly contains dyestu ff,slurry,dyeing aid,acid or alkali,fiber and inorganic compound,and PMS mainly contains cellulo,varying amounts of highly lignified ma-terials,and molecules of anthropogenic origin depending on the paper manufacturing practices ud (Marche et al.,2003;Zheng and Liu,2006).The high drying rate of PMS may due to its high cellulo content which will produce a capillary force and remove water from inside to outside of sludge.
2.2TG-DTA and TG-DSC test
The TG-DTA method was developed by Chen et al.(1997)for continuous classification of moisture content in activated sludge.The heat flow (Q ,kJ /c)into the sample cell can thereby be estimated by Eq.(1):Q =Ah (T ref −T cell )
(1)
where,A (m 2),h (w /(m 2·K)),T ref (K)and T cell (K)are the e ffective heat transfer area,the average heat transfer coe fficient,the reference temperature and the cell temper-ature,respectively.The group Ah and (T ref –T cell )were determined by pure water test and DTA test,respectively.In the pure water test,the Q in Eq.(1)was the latent heat of pure water,and the (T ref –T cell )can be derived from the DTA test of the pure water,thus the group Ah can be determined through Eq.(1).It is assumed that the group Ah is a constant in both the pure water test and the sludge test.When the heat flow into the sample cell is completely utilized in moisture evaporation,the following Eq.(2)holds:Q =˙mH S
(2)
where,˙m (kg /c)and H S (kJ /kg)are the drying rate and the evaporation heat of moisture in sludge at 80°C,respec-tively,and ˙m was estimated by the TGA data.Therefore,by combining Eqs.(1)with (2),the evaporation heat of sludge water can be determined.The bond strength H B (kJ /kg)between the moisture and the solid pha can thereby be calculated as follows:H B =H S −H W无暇顾及的意思
(3)
where,H W is the evaporation heat of pure water at 80°C.
878Journal of Environmental Sciences 2011,23(5)875–880/Wenyi Deng et al.V ol.23
As shown in Fig.4,the distribution characteristics of the bond strength are similar for the three sludges.When the moisture content was higher than 1.0kg moisture /kg ds (50%wb),there was only a little change in the bond strength which incread from 0to approximately 70kJ /kg with decreasing moisture content.The moisture content higher than 50%wb was mainly compod of interstitial water (Fig.3).Thus it can be inferred that the bond strength of the interstitial water in the three sludges fluctuated in the range of 0–70kJ /kg.The average bond strengths in this moisture range were 38.2,57.8and 68.3kJ /kg for MSS,PDS and PMS,respectively.When the moisture content was lower than 1.0kg moisture /kg,the bond strength
02004006008001000120014000
0.5 1.0 1.5 2.0 2.5 3.0Moisture content (kg moisture/kg dry solid)
Moisture content (kg moisture/kg dry solid)
Moisture content (kg moisture/kg dry solid)
H B (k J /k g )
020040060080010001200140016000
0.5 1.0 1.5 2.0 2.502004006008001000120014001600180020000
0.5
1.0
1.5
2.0
2.5
3.0H B (k J /k g )
H B (k J /k g )MSS
PDS
PMS
Fig.4Bond strength of moisture in MSS,PDS,and PMS through TG-DTA test at 80°C (two replicates).H B is bond strength.
markedly incread from a very low level to more than 1000kJ /kg with decreasing moisture content.It can be found from Fig.3that the moisture content lower than 50%wb was mainly surface water.The average bond strengths in this moisture range were 366.4kJ /kg for MSS,505.6kJ /kg for PDS and 699.6kJ /kg for PMS.It can be found that the average bond strengths in the low moisture range (moisture content lower than 50%wb)was nearly 10times higher than that in the high moisture
range (moisture content higher than 50%wb).It is obvious that the higher the bond strength,the more energy will be consumed during drying process.Thus surface water evaporation will consume much more energy than interstitial water evaporation.
The TG-DSC method ud in this study was propod by Ferras and Lecomte (2004).This method was also developed for continuous classification of moisture content in biological products.In fact,the basic principle of this method is identical with the TG-DTA method,thus it is expected that the bond strength curve from the TG-DSC test is similar with that from the TG-DTA test.Since the heat flow (Q )into the sample cell can be directly measured through the DSC test,the pure water test is not needed in the TG-DSC test.Therefore,the TG-DSC test is more convenient than the TG-DTA test.The evaporation heat of moisture in sludge can be calculated directly through Eq.(2),as well as the bond strength H B in Eq.(3).Figure 5shows the experimental results of MSS and PMS.As expected,the bond strength curves are almost identical with that from the TG-DTA tests.2.3Water activity test
From thermodynamic considerations and under some assumptions (Vaxelaire et al.,2000),the water activity (a w )was defined by the following Eq.(4):a w =
P v P sat (T )
(4)
where,P v (Pa)is the partial pressure of water in the surrounding air,and P sat (T )(Pa)is the saturation pressure of water at the given temperature.It should be noted that Eq.(4)is also the definition of the relative humidity.The water activity test was propod by Herwijn (1996)and Vaxelaire (2001),bad on sorption isotherm data,to
100012001400Moisture content (kg moisture/kg wet basis)
H B (k J /k g )
Fig.5Bond strength of moisture in MSS and PMS through TG-DSC test at 90°C.
No.5Moisture distribution in sludges bad on different testing methods879 evaluate the bond strength between the moisture and the羊肉炒洋葱
solid pha.It is bad on the Clausius-Clapeyron equation
(Zuritz and Singh,1985;Nunes and Rotstein,1991)(Eq.
(5)):电梯是谁发明的
d P v d T =H S
T(V v−V1)
(5)
where,H S(kJ/kg)is the sum of the heat of evaporation of pure water(H W)and the bond strength(H B),and T is the ambient temperature.Neglecting the volume of liquid (V1)(compare to the volume of vapour(V v)),Eq.(5)can be rearranged as:
d[ln(P v)]
d T =H S
RT2
(6)
where,R(kJ/(kg·K))is universal gas constant.For pure water,the heat of evaporation can be expresd as Eq.(7):
d[ln(P sat)]
d T =H W
RT2
(7)
According to Eqs.(4)and(7),the bond strength H B(kJ/kg) can thereby be determined by Eq.(8)(Vaxelaire,2001): H B=−RT ln(a w)(8) The water activity can be determined by the saturated salts method,which is widely adopted as a standard method(Vaxelaire et al.,2000).However,veral days, or even weeks,may be required to obtain the equilibrium value and,at high relative humidity,this delay can easily lead to bacterial growth or even putrefaction,which con-quently invalidates the results.Therefore,the water activity meter,which only takes4–5min for the water activity determination with high precision(±0.015a w),was ud in this study.Table2shows the water activity values of the three sludges at different moisture contents.The results indicate that the water activity decread with the decrea of moisture content,and that there was no obvious change in the water activity values until the moisture content was lower than0.4kg moisture/kg ds.
According to Eq.(8),the bond strength can be deter-mined bad on the water activity data in Table2.Figure 6shows the calculated results of the bond strength in the three sludges.The bond strength curves from the water activity test have similar shapes with that from the TG-DTA or TG-DSC test.It can be found that the bond strength had a marked increa until the moisture content was lower than0.2kg moisture/kg ds.This result was clo with tho reported by Vaxelaire(2001),but was relatively lower than tho from the TG-DTA or TG-DSC test,which was mainly caud by great discrepancies in measuring principles.
2.4Comparison of different methods
Bad on the analysis above,the main advantage of the drying test over the other methods is that it is capable of quantifying the contents of free water,interstitial water, surface water and bound water in sludge.However,due to the low drying temperature,2–3days are often needed until a constant mass of sludge sample is achieved,which may lead to bacterial growth and quality change of sludges. Besides,under drying,the sludge cake will experience volume shrinkage and surface cracking;both cas have a little effect on sludge drying rate.Unfortunately,due to complexity of sludge composition,it is very difficult to pre-cily determine the percentage error of the above effects (i.e.,bacterial growth,quality change,volume shrinkage and surface cracking)on drying rate.
The experimental results from the TG-DTA test,the TG-DSC test and the water activity test are all lead to the formation of the bond strength curves.As stated above, although the TG-DSC test is more convenient than the TG-DTA test by reducing the pure water test,the TG-DTA test is fundamentally the same with the TG-DTA test.Since the bond strength from the water activity test is lower than tho from the TG-DTA or TG-DSC test,it is necessary to ascertain which result is more persuasive.Firstly,it should be noted that there is no bond strength in free water.As can be en from Fig.6,the bond strength was
100
200
300
400
500
加拉帕格斯巨人蜈蚣
600
700
800
Moisture content (kg moisture/kg ds)
H
B
(
k
J
/
k
g
)
Fig.6Bond strength of moisture in MSS,PDS and PMS through water activity test.
薄荷怎么种Table2Water activity of sludges at different moisture contents(ambient temperature of24°C)
MSS PDS PMS
Moisture content Water Moisture content Water Moisture content Water (kg moisture/kg ds)activity(kg moisture/kg ds)activity(kg moisture/kg ds)activity
1.310.98 1.460.96 1.670.99
0.680.970.930.97 1.020.98
0.360.950.470.930.440.97
0.130.790.130.540.140.88
0.070.630.090.380.060.35
0.030.070.070.270.050.23
0.010.040.010.050.020.13
