- Email: [email protected]
Conditioning of oily sludges with municipal solid wastes incinerator fly ash
~
Pergamon
Wal. Sci. Teclo. Vol. 3S. No.8, pp. 23t-238. 1997. C t997 tAWQ. Published by Elsevier Science LId
Prinled in Oreat Brilain.
PU: 50273-1223(97)00172-8
0273-1223197 SINO + 0-00
CONDITIONING OF OILY SLUDGES WITH MUNICIPAL SOLID WASTES INCINERATOR FLY ASH Tay Joo Hwa and S. Jeyaseelan School o/Civil and Structural Engineering. BIle Nl, #lA·37, Nanyang Technological University, Nanyang Avenue. Singapore 639798
ABSTRACT Conditioning of sludges improves dewatering characteristics and reduces the quantity of sludge to be handled. Anaerobic digesled sludge collected from a sewage treatment plant contained 1.8% to 8% oil. TIle increase of specific resistance and capillary suction time (CST) With increasing oil content observed in these samples indicates the inlerference of oil in dewatering. II has been found that addition of municipal solid wastes incinerator ny ash decreases the specific resistances and capillary suction limes of oily sludges rapidly up to 3% dosage. Beyond 3% ny ash. the decrease is less significant and the solids content in the sludge cake increases. This optimum dosage remains the same for sludges with varying oil contents from 1.8% to 12%. TIle total suspended sotids of filtrate decreases with ny ash dosage but the toxic concentrations of heavy metals increases considerably. However at the optimum dosage of 3%. concentrations of heavy metals are within the limits for discharging into the sewers. TIle correlations of CST With the dewatering characteristics such as specific reSistance, filter yield and corrected filter yield are established. These correlations can be used to obtain a quick prediction on dewaterability.
KEYWORDS Sludge dewatering; oily sludges; sludge conditioning; capillary suction time; specific resistance to filtration; dewaterability; municipal solids waste incinerator fly ash.
INTRODUCTION Sludge dewatering is a process whereby water is removed from sludge so as to reduce its volume and alter its physical state from semisolid to damp solid. This physical change reduces the volume of sludge considerably and therefore the cost of disposal. Specific resistance is widely used as a sludge characterization technique for the measurement of filterability since its development in '60s (Coakley, 1956; Christensen and Dick, 1985). Capillary suction time (CST) is a simple method that was developed in 70's (Baskerville and Gale, 1968). CST is a measure of dewaterability analogous to the specific resistance and was experimentally verified (Vesilind, 1988). These sludge dewatering charateristics can be improved by various sludge conditioning methods, which include use of inert filter aids, thermal conditioning, freezing and thawing, elutriation and the addition of chemical conditioners. Freezing and thawing is an efficient, no• odour dewatering operation for plants in cold climates and the cost of dewatering should be less than in drying beds (Martel, 1993). Some of the other factors which may improve dewatering are the method of storing solutions, the type of water used, the time stored, and the method of mixing chemical conditioners 231
232
T. J. HWA and S. JEYASEELAN
(Gale and Baskerville, 1970). In addition to these factors, it was shown that the operations of activated sludge process and anaerobic digestion have significant roles affecting sludge dewatering characteristics (Knocke and Zentkovich, 1986., Lawler et al., 1986). Wastewater carrying oil with it creates treatment problems in the aerobic treatment as well as in the anaerobic digestion. and subsequently in the dewatering processes. Experience in the Bedok sewage treatment plant in Singapore has shown that those oils and grease that enter the treatment plant with the wastewater accumulate in the aeration tank interfering with oxygen transfer, which hinders the aerobic treatment process (Chua, 1989). It has also been shown that accumulation of oil creates difficulties in dewatering. This research work was originated to look into the problems associated with oily sludges and to fmd solutions to improve dewatering. Sludge characteristics such as specific resistance. capillary suction time. filter yield and solid content can be determined in the laboratories. Knowledge of these characteristics with different dosages of sludge conditioners helps in effective dewatering and sludge handling operations. The knowledge of filter yield helps predict the amount of sludge to be handled after dewatering. This paper studies the dewatering characteristics using municipal solid wastes incinerator fly ash as a conditioner. METHODOLOGY Digested sludge samples were collected from a sewage treatment works. Several samples were analysed in the laboratory to determine the characteristics of these digested sludge samples and the results are reported in Table 1. These sludge samples with different oil contents varying from 1.8% to 8.0% by weight have been examined in the laboratory to find out their specific resistance. capillary suction time (CST) and filter yield. Municipal Solids Waste (MSW) incinerator fly ash (or fly ash) was used as a conditioner as it is economical and environmentally friendly. The methods used for determination of the dewatering characteristics and the characteristics ofMSW fly ash are listed in Tables 2 and 3 respectively. The properties of the fly ash samples collected from the Tuas Incineration Plant are reported in Table 3. The fly ash particles were generally spherical and predominantly silt-sized particles. About 86% of fly ash particles are silt-sized and the remaining 14% are sand-sized particles. The loss on ignition of the fly ash is 15% indicating that a significant portion of fine organic matters were not burned during the incineration process. These fine particles were collected by the electroprecipitator from the flue gas after lime neutralization. The fly ash is alkaline in nature with pH values of 11.4. The chemical compositions of the fly ash show that about 80% of the fly ash is composed of silicate. aluminum and calcium. More than two-thirds of the chemical constituents in the fly ash are silicate and calcium. Table 1. Characteristics of raw digested sludge samples
Parameter
Range
Oil content Total solids content Capil1arv Suction Time (seconds) Soecific Gravity of drv solids
3.2% - 4.4% 1.8% - 8.0% 80 - 150
-
Typical Values 3.0 3.8% 140 1.6
Conditioning of oily sludges with municipal solid wastes
233
Table 2. Methods of analysis of parameters Parameter Specific Resistance to Filtration Capillarv Suction Time Filter Yield Solids Content Oil content Particle Size Distribution
Method of Analvsis Buchner Funnel Device CST apparatus Filter Leaf Test Standard method Soxhlet apparatus Marvin Particle Size Analvzer
Table 3. Properties ofMSW fly ash Properties Specific l!ravitv Density (mg/m) Loose stage Comoacted stae:e Particle size distribution Effective size (mm) Uniformity coefficient Coefficient of e:radation Loss on ie:nition % oH value Chemical compositions (%) Silicate Aluminum Ferrus Calcium Potassium Sodium Magnesium Zinc Lead Copper Manganese Chromium Cadmium Nickel Others
Flv Ash 2.45
0.81 1.09 0.01 4.76 1.44 15.0 11.4 35.00 12.50 5.67 32.49 3.80 1.90 1.02 0.48 0.20 0.04 0.08 0.01 0.007 0.008 6.80
Different dosages of fly ash varying from 2% to 12% by weight were used to determine the optimum chemical dosage for varying oil contents. Specific resistance, CST, filter yield. corrected filter yield. total solids contents in the sludge cake and filtrate, and particle size distribution of the sludges after conditioning were measured by the respective methods given in Table 2. The effectiveness of lime and fly ash dosages were also compared by using the same dosages on oily sludges. For the reliability of the experimental results, two or three samples were analyzed to ensure the reproducibility of each parameter. The results of these analyses are presented in Table 4.
234
T. J. HWAand S. JEYASEELAN
Table 4. Dewatering characteristics of oily sludge with MSW fly ash
.. Oil Content
(%)
4%
Fly Ash (%)
(sec)
0 2 3
4
6%
8%
Capillary Suction Time
6 8 10 12 0 2 3 4 6 8 10 12 0 2 3 4 6 8 10 12
323.2 163.5 102.2 68.9 65.4 62.2 59.0 53.4 285.8 231.2 94.9 52.4 56.6 57.8 53.1 52.3 310.7 206.3 93.1 77.4 61.3 50.7 43.8 36.4
Corrected Filter Yield (leg! m'fhr)
Specific Resistance 1 X 10 ' (m/kl!)
Filter Yield
1125.58 696.51 415.70 339.06 318.72 253.56
191~.27
5.62 7.80 8.01 8.33 10.90 15.36 20.90
2290.64 740.03 830.25 645.14 502.05 324.55 260.13
2.60 5.13 9.72 6.21 9.06 8.67 14.87
1.55 2.57 4.38 2.38 2.96 2.41 3.70
1624.78 500.66 521.05 482.13 344.44 319.80 216.31
5.67 7.23 6.50 6.58 8.21 9.77 18.21
3.98 4.50 3.63 3.13 3.41 3.83 6.09
-
-
-
(kg/m'/hr)
-
-
-
-
3.92 3.97 4.04 3.26 3.78 4.55 5.56
-
-
Sludge Solids Content
(%) 3.84 5.51 7.55 7.61 9.80
11.01
12.95 14.43 3.54 5.92 7.05 7.85 9.24 10.83 12.75 14.24 4.99 7.11 8.02 8.93 10.48 12.02 12.74 14.92
Fly ash contains heavy metals. The concentrations of various heavy metals in fly ash are reported in Table 3. The addition of fly ash as a conditioner increases the heavy metals contents in the sludge mix. After dewatering heavy metals expelled from the sludge mix together with the filtrate will be recirculated into the treatment facilities. Heavy metals in the fly ash and the heavy metals carried back into the sewage treatment plant together with the filtrate were examined using ICP. The results are reported in Table 5. DISCUSSION OF RESULTS The variation of specific resistance and capillary suction time with varying fly ash is presented in Table 4 and Figures 1 and 2 respectively. For all the oily sludge samples tested the specific resistance decreases with increasing fly ash dosages. For the sludge with 6% oil content, the specific resistance with 2% fly ash addition is 2290.64 x 10- 12 m1kg. The specific resistance decreases rapidly to 740.03 x 1O- 12 m1kg for 3% fly ash addition. For the same oil content the values of specific resistance at 4% and 12% fly ash dosages are 830.25 x 10- 12 m1kg and 260.13 x 10- 12 m1kg respectively. Similarly for the sludge with 8% oil content, the specific resistance with 2% fly ash addition is 1624.78 x 10- 12 m1kg and decreases rapidly to 500.66 x 10- 12 m1kg for 3% fly ash addition. For the same oil content the values of specific resistance at 4% and 12% fly ash dosages are 521.05 x 10- 12 m1kg and 216.31 x 10- 12 m1kg respectively. The specific resistance decreases with increasing fly ash dosages, but the decreasing rate is very rapid for dosages of less than 3% and become less significant for dosages of more than 4%. This observation is true for CST as well. Therefore it may be concluded that the optimum fly ash dosage to obtain better specific resistance lies within the range of 3% to 4%.
Conditioning of oily sludges with municipal solid wastes
2JS
Table 5. Heavy metals in the filtrates Range of Concentrations (me/L) ND - 0.10 0.42 . 0.48 2038.8 - 2673.6 ND 0.20 • 0.22 0.12 - 1.59 NO 571.6 - 1716.6 0 - 0.16 436.8 - 1069.2 0.03 - 0.06 1.72 - 2.40 0.23 - 1.56
Metallic Element
Ar!.
Al Ca Cd Cr Cu Fe K Ml! Na Pb Si Zn Note: TER NO NM
Typical Values (me/L) ND 0.43 2369.5
NO 0.21 0.70 ND 1158.7 0.08 763.7 0.04 2.15 0.60
TER - Allowable Concentration (ml!/U 5.0 NM NM 1.0 5.0 5.0 50.0 NM NM NM 5.0 NM 10.0
- Trade Effluent Regulations - Not Detectable - Not Mentioned
As shown in Table 4 and Fig 2, for the sludge with 4% oil content, the CST drops from 323.2 seconds to 102.2 seconds for no fly ash addition to 3% fly ash addition. For the sludge with 6% oil content, the CST drops from 285.8 seconds to 94.9 seconds for no fly ash addition to 3% fly ash addition and for the sludge with 8% oil content, the CST drops from 310.7 seconds to 93.7 seconds for no fly ash addition to 3% fly ash addition. With fly ash additions varying from 4% to 12%, the CST decreases from 68.9 to 53.4 seconds for 4% oil content, 57.8 to 52.3 seconds for 6% oil content and 93.1 to 36.4 seconds for 8% oil content in the sludges. Similar to the specific resistance the decrease in CST values is very rapid for fly ash additions of more than 4% for different oil contents in sludge. Therefore the optimum fly ash dosage to obtain favourable CST values lies in the range of 3% to 4%.
2.500..-----------.---------, -4% +6% *8%
• :::' .600 z ;!
~ w
'.000
~
600
II: U
...w
II)
ol.--..l-.---'--......L..--......----'--_.L...----J o 2 4 II 8 10 12 14
FLY ASH DOSAGE f%t
Figure I. Specific resistance vs fly ash dosage (with varying oil conlent).
236
T. J. HWA and S. JEYASEELAN
360r-----------,.------......., -4%
i
!e
+ 6'l(,
)I(
6'l(,
200
Iii 150 u 100 50 0
2
0
..
8
8
12
10
14
FLY ASH DOSAGE 1%1
Figure 2. CST vs fly ash dosage (with varying oil content).
2,500r------..-----------....... ~
12,000 b
-
ol....-__-'-__ o 60
___'___
- - ' ~
100
160
__l._ __ _ J
200
250
CST IS.e) Figure 3. Specific resistance VI CST (with avrying oil content). 25r----------~r__----___,
• 4%
+ e%
lIE 8%
lIE
+0
lIE
0'-----1------'----.1...---......- _ - - 1 o 50 100 150 200 250 CST (Sec) Figure. 4 Filter yield VI CST (with varying oil content).
Table 4 shows the variation of filter yield and corrected filter yield for the different oil contents of the sludges. The filter yield increases with increasing dosage of fly ash. This shows that the increasing dosages of fly ash will increase the volume of sludge to be handled after dewatering. In all the cases the increase of filter yield for dosages between 2% and 8% are not very significant. but for dosages more than 8% the filter yields increase rapidly. The corrected filter yields for all the oily sludge samples in the range of 3% to 4%
Conditioning o( oily sludges with municipal solid wastes
237
fly ash dosages are higher than that of dosages in the range of 4% to 10%. The corrected filter yield is measure of actual solids removed from the original sludge excluding the chemical conditioner. This observation strengthens the conclusion on optimum dosage. Correlation amonK the characteristjcs Measurement of specific resistance is tedious, time consuming and demands skills, whereas measurement of CST is simple, fast and does not require any special skills. The correlations of filter yield and corrected filter yield can also be established. A correlation of these parameters with CST will give a quick indication of the specific resistance, filter yield and corrected filter yield of the samples. The correlations of specific resistance, filter yield and corrected filter yield to CST using the values of the respective parameters for all the sludge samples with varying oil contents tested are presented in Figures 3 and 4. Therefore by measuring CST of a given sample the specific resistance which measures filterability and filter yield can be estimated quickly using the correlations. HeaY)' metals Addition of fly ash introduces heavy metals in the filtrate and as the filtrate is returned to the head of the treatment system, this method of chemical conditioning introduces heavy metals into the treatment system. Fly ash used contains heavy metals such as Ag, AI, Ca, Cr, Cu, K, Mg, Na, Pb, Si and Zn. These heavy metals with concentrations above the treatment threshold limit can create several problems to the treatment processes. Therefore the heavy metal contents in the filtrate were examined and reported in Table 5 together with the allowable concentrations stipulated in the Trade Effluent Regulations established by the Ministry of the Environment, Singapore (TER, 1986). It was found that all the heavy metal concentrations were within the limits prescribed by the Trade Effluent Regulations in force in Singapore. There are some elements that are not mentioned in the Regulations such as AI. Ca, K, Mg, Na, and Si. These elements do not create any problems with the quantities mentioned in Table 5. Therefore this filtrate may be mixed with the influent wastewater to the treatment plant, for treatment. CONCLUSIONS Addition of fly ash decreases the specific resistances and capillary suction times of oily sludges rapidly up to 3% dosage and the decreases of these parameters for fly ash dosages of more than 4% are not significant. Therefore the optimum fly ash dosage as a conditioner to improve dewatering lies between 3% and 4%. The corrected mter yield which reflects the actual solids extracted from the unconditioned dewatered sludge is also relatively higher within this range than the corrected filter yields within the dosages of 4% and 8%. CST is relatively very simple and fast to measure, and even an unskilled person can determine CST with reasonable accuracy. The correlations of specific resistance and filter yield to CST have been established from the experimental results. Therefore by measuring CST of a given sample the specific resistance which measures filterability and mter yield which measures the quantity of sludge to be handled after dewatering can be estimated quickly using the correlations. The addition of fly ash will increase the solids content in the sludge cake. The total suspended solids of mtrate decreases with fly ash dosage but the concentrations of heavy metals increase considerably. However at the optimum dosage of 3%, concentrations of heavy metals are within the limits for mixing with influent wastewater.
REFERENCES Baskerville, R. C. and Gale. R. S. (1968). A Simple Automatic Instrument (or Determining the Filterability of Sewage Sludge. Journal of/he rns/;/u/e ofWaler Pollution Con/rol, 1968. Christrensen. G. L. and Dick. R. I. (I98S). Specific Resistance Measurements: Nonparabolic Data. 10u/7l41 a/Environ. Eng. Div., ASCE. JII. 1985.
238
T. J. HWA and S. JEYASEELAN
Chua, S. E. (1989). The Oil and Grease Problem in Sewage Treatment. Journal of thl! Institution of Enginurs, Singaporl!, 29( I), April 1989. Coakley, P.,and Jones, B. R. S. (1956). Vacuum Sludge Filtration, Sl!Wagl! Industrial Works, 28. Gale, R. S. and Baskerville, R. C. (1970). Polyelectrolytes in the Filtration of Sewage Sludges. Fillm and Sl!parn, 7. 37-44 and 47-52. Knacke, W. R. and Zentkovich, T. L. (1986). Effects of Mean Cell Residence Time and Particle Size Distribution on Activated Sludge Vacuum Dewatering Characteristics, Joumal WPCF, S8( 12), 1118-1123. Lawler, D. F., Yoon, J. C., Shiawjy, H. and Hull, B. A. (1986) Anaerobic Digestion: Effects on Particle Size and Dewaterability, Journal WPCF, 58(12),1107-1117. Martel, C. J.(1993). Fundamentals of Sludge Dewatering in Freezing Beds. War. Sci. TI!Ch., 28(1), 29-35. Novak, J. T. and Knocke, W. R. (1987). Discussion on Specific Resistance Measurements: Nonparabolic Data G. L. Christensen and R. I. Dick., J/Environ. Eng. Div. ASCE 113. TER (1986). Trade Effluent Regulations, Ministry of the Environment, Singapore. Vesilind, P. A. (1988). Capillary Suction Time as a fundamental measure of sludge dewaterability. Journal ofWPCF, 60(2),215• 220.
Pergamon
Wal. Sci. Teclo. Vol. 3S. No.8, pp. 23t-238. 1997. C t997 tAWQ. Published by Elsevier Science LId
Prinled in Oreat Brilain.
PU: 50273-1223(97)00172-8
0273-1223197 SINO + 0-00
CONDITIONING OF OILY SLUDGES WITH MUNICIPAL SOLID WASTES INCINERATOR FLY ASH Tay Joo Hwa and S. Jeyaseelan School o/Civil and Structural Engineering. BIle Nl, #lA·37, Nanyang Technological University, Nanyang Avenue. Singapore 639798
ABSTRACT Conditioning of sludges improves dewatering characteristics and reduces the quantity of sludge to be handled. Anaerobic digesled sludge collected from a sewage treatment plant contained 1.8% to 8% oil. TIle increase of specific resistance and capillary suction time (CST) With increasing oil content observed in these samples indicates the inlerference of oil in dewatering. II has been found that addition of municipal solid wastes incinerator ny ash decreases the specific resistances and capillary suction limes of oily sludges rapidly up to 3% dosage. Beyond 3% ny ash. the decrease is less significant and the solids content in the sludge cake increases. This optimum dosage remains the same for sludges with varying oil contents from 1.8% to 12%. TIle total suspended sotids of filtrate decreases with ny ash dosage but the toxic concentrations of heavy metals increases considerably. However at the optimum dosage of 3%. concentrations of heavy metals are within the limits for discharging into the sewers. TIle correlations of CST With the dewatering characteristics such as specific reSistance, filter yield and corrected filter yield are established. These correlations can be used to obtain a quick prediction on dewaterability.
KEYWORDS Sludge dewatering; oily sludges; sludge conditioning; capillary suction time; specific resistance to filtration; dewaterability; municipal solids waste incinerator fly ash.
INTRODUCTION Sludge dewatering is a process whereby water is removed from sludge so as to reduce its volume and alter its physical state from semisolid to damp solid. This physical change reduces the volume of sludge considerably and therefore the cost of disposal. Specific resistance is widely used as a sludge characterization technique for the measurement of filterability since its development in '60s (Coakley, 1956; Christensen and Dick, 1985). Capillary suction time (CST) is a simple method that was developed in 70's (Baskerville and Gale, 1968). CST is a measure of dewaterability analogous to the specific resistance and was experimentally verified (Vesilind, 1988). These sludge dewatering charateristics can be improved by various sludge conditioning methods, which include use of inert filter aids, thermal conditioning, freezing and thawing, elutriation and the addition of chemical conditioners. Freezing and thawing is an efficient, no• odour dewatering operation for plants in cold climates and the cost of dewatering should be less than in drying beds (Martel, 1993). Some of the other factors which may improve dewatering are the method of storing solutions, the type of water used, the time stored, and the method of mixing chemical conditioners 231
232
T. J. HWA and S. JEYASEELAN
(Gale and Baskerville, 1970). In addition to these factors, it was shown that the operations of activated sludge process and anaerobic digestion have significant roles affecting sludge dewatering characteristics (Knocke and Zentkovich, 1986., Lawler et al., 1986). Wastewater carrying oil with it creates treatment problems in the aerobic treatment as well as in the anaerobic digestion. and subsequently in the dewatering processes. Experience in the Bedok sewage treatment plant in Singapore has shown that those oils and grease that enter the treatment plant with the wastewater accumulate in the aeration tank interfering with oxygen transfer, which hinders the aerobic treatment process (Chua, 1989). It has also been shown that accumulation of oil creates difficulties in dewatering. This research work was originated to look into the problems associated with oily sludges and to fmd solutions to improve dewatering. Sludge characteristics such as specific resistance. capillary suction time. filter yield and solid content can be determined in the laboratories. Knowledge of these characteristics with different dosages of sludge conditioners helps in effective dewatering and sludge handling operations. The knowledge of filter yield helps predict the amount of sludge to be handled after dewatering. This paper studies the dewatering characteristics using municipal solid wastes incinerator fly ash as a conditioner. METHODOLOGY Digested sludge samples were collected from a sewage treatment works. Several samples were analysed in the laboratory to determine the characteristics of these digested sludge samples and the results are reported in Table 1. These sludge samples with different oil contents varying from 1.8% to 8.0% by weight have been examined in the laboratory to find out their specific resistance. capillary suction time (CST) and filter yield. Municipal Solids Waste (MSW) incinerator fly ash (or fly ash) was used as a conditioner as it is economical and environmentally friendly. The methods used for determination of the dewatering characteristics and the characteristics ofMSW fly ash are listed in Tables 2 and 3 respectively. The properties of the fly ash samples collected from the Tuas Incineration Plant are reported in Table 3. The fly ash particles were generally spherical and predominantly silt-sized particles. About 86% of fly ash particles are silt-sized and the remaining 14% are sand-sized particles. The loss on ignition of the fly ash is 15% indicating that a significant portion of fine organic matters were not burned during the incineration process. These fine particles were collected by the electroprecipitator from the flue gas after lime neutralization. The fly ash is alkaline in nature with pH values of 11.4. The chemical compositions of the fly ash show that about 80% of the fly ash is composed of silicate. aluminum and calcium. More than two-thirds of the chemical constituents in the fly ash are silicate and calcium. Table 1. Characteristics of raw digested sludge samples
Parameter
Range
Oil content Total solids content Capil1arv Suction Time (seconds) Soecific Gravity of drv solids
3.2% - 4.4% 1.8% - 8.0% 80 - 150
-
Typical Values 3.0 3.8% 140 1.6
Conditioning of oily sludges with municipal solid wastes
233
Table 2. Methods of analysis of parameters Parameter Specific Resistance to Filtration Capillarv Suction Time Filter Yield Solids Content Oil content Particle Size Distribution
Method of Analvsis Buchner Funnel Device CST apparatus Filter Leaf Test Standard method Soxhlet apparatus Marvin Particle Size Analvzer
Table 3. Properties ofMSW fly ash Properties Specific l!ravitv Density (mg/m) Loose stage Comoacted stae:e Particle size distribution Effective size (mm) Uniformity coefficient Coefficient of e:radation Loss on ie:nition % oH value Chemical compositions (%) Silicate Aluminum Ferrus Calcium Potassium Sodium Magnesium Zinc Lead Copper Manganese Chromium Cadmium Nickel Others
Flv Ash 2.45
0.81 1.09 0.01 4.76 1.44 15.0 11.4 35.00 12.50 5.67 32.49 3.80 1.90 1.02 0.48 0.20 0.04 0.08 0.01 0.007 0.008 6.80
Different dosages of fly ash varying from 2% to 12% by weight were used to determine the optimum chemical dosage for varying oil contents. Specific resistance, CST, filter yield. corrected filter yield. total solids contents in the sludge cake and filtrate, and particle size distribution of the sludges after conditioning were measured by the respective methods given in Table 2. The effectiveness of lime and fly ash dosages were also compared by using the same dosages on oily sludges. For the reliability of the experimental results, two or three samples were analyzed to ensure the reproducibility of each parameter. The results of these analyses are presented in Table 4.
234
T. J. HWAand S. JEYASEELAN
Table 4. Dewatering characteristics of oily sludge with MSW fly ash
.. Oil Content
(%)
4%
Fly Ash (%)
(sec)
0 2 3
4
6%
8%
Capillary Suction Time
6 8 10 12 0 2 3 4 6 8 10 12 0 2 3 4 6 8 10 12
323.2 163.5 102.2 68.9 65.4 62.2 59.0 53.4 285.8 231.2 94.9 52.4 56.6 57.8 53.1 52.3 310.7 206.3 93.1 77.4 61.3 50.7 43.8 36.4
Corrected Filter Yield (leg! m'fhr)
Specific Resistance 1 X 10 ' (m/kl!)
Filter Yield
1125.58 696.51 415.70 339.06 318.72 253.56
191~.27
5.62 7.80 8.01 8.33 10.90 15.36 20.90
2290.64 740.03 830.25 645.14 502.05 324.55 260.13
2.60 5.13 9.72 6.21 9.06 8.67 14.87
1.55 2.57 4.38 2.38 2.96 2.41 3.70
1624.78 500.66 521.05 482.13 344.44 319.80 216.31
5.67 7.23 6.50 6.58 8.21 9.77 18.21
3.98 4.50 3.63 3.13 3.41 3.83 6.09
-
-
-
(kg/m'/hr)
-
-
-
-
3.92 3.97 4.04 3.26 3.78 4.55 5.56
-
-
Sludge Solids Content
(%) 3.84 5.51 7.55 7.61 9.80
11.01
12.95 14.43 3.54 5.92 7.05 7.85 9.24 10.83 12.75 14.24 4.99 7.11 8.02 8.93 10.48 12.02 12.74 14.92
Fly ash contains heavy metals. The concentrations of various heavy metals in fly ash are reported in Table 3. The addition of fly ash as a conditioner increases the heavy metals contents in the sludge mix. After dewatering heavy metals expelled from the sludge mix together with the filtrate will be recirculated into the treatment facilities. Heavy metals in the fly ash and the heavy metals carried back into the sewage treatment plant together with the filtrate were examined using ICP. The results are reported in Table 5. DISCUSSION OF RESULTS The variation of specific resistance and capillary suction time with varying fly ash is presented in Table 4 and Figures 1 and 2 respectively. For all the oily sludge samples tested the specific resistance decreases with increasing fly ash dosages. For the sludge with 6% oil content, the specific resistance with 2% fly ash addition is 2290.64 x 10- 12 m1kg. The specific resistance decreases rapidly to 740.03 x 1O- 12 m1kg for 3% fly ash addition. For the same oil content the values of specific resistance at 4% and 12% fly ash dosages are 830.25 x 10- 12 m1kg and 260.13 x 10- 12 m1kg respectively. Similarly for the sludge with 8% oil content, the specific resistance with 2% fly ash addition is 1624.78 x 10- 12 m1kg and decreases rapidly to 500.66 x 10- 12 m1kg for 3% fly ash addition. For the same oil content the values of specific resistance at 4% and 12% fly ash dosages are 521.05 x 10- 12 m1kg and 216.31 x 10- 12 m1kg respectively. The specific resistance decreases with increasing fly ash dosages, but the decreasing rate is very rapid for dosages of less than 3% and become less significant for dosages of more than 4%. This observation is true for CST as well. Therefore it may be concluded that the optimum fly ash dosage to obtain better specific resistance lies within the range of 3% to 4%.
Conditioning of oily sludges with municipal solid wastes
2JS
Table 5. Heavy metals in the filtrates Range of Concentrations (me/L) ND - 0.10 0.42 . 0.48 2038.8 - 2673.6 ND 0.20 • 0.22 0.12 - 1.59 NO 571.6 - 1716.6 0 - 0.16 436.8 - 1069.2 0.03 - 0.06 1.72 - 2.40 0.23 - 1.56
Metallic Element
Ar!.
Al Ca Cd Cr Cu Fe K Ml! Na Pb Si Zn Note: TER NO NM
Typical Values (me/L) ND 0.43 2369.5
NO 0.21 0.70 ND 1158.7 0.08 763.7 0.04 2.15 0.60
TER - Allowable Concentration (ml!/U 5.0 NM NM 1.0 5.0 5.0 50.0 NM NM NM 5.0 NM 10.0
- Trade Effluent Regulations - Not Detectable - Not Mentioned
As shown in Table 4 and Fig 2, for the sludge with 4% oil content, the CST drops from 323.2 seconds to 102.2 seconds for no fly ash addition to 3% fly ash addition. For the sludge with 6% oil content, the CST drops from 285.8 seconds to 94.9 seconds for no fly ash addition to 3% fly ash addition and for the sludge with 8% oil content, the CST drops from 310.7 seconds to 93.7 seconds for no fly ash addition to 3% fly ash addition. With fly ash additions varying from 4% to 12%, the CST decreases from 68.9 to 53.4 seconds for 4% oil content, 57.8 to 52.3 seconds for 6% oil content and 93.1 to 36.4 seconds for 8% oil content in the sludges. Similar to the specific resistance the decrease in CST values is very rapid for fly ash additions of more than 4% for different oil contents in sludge. Therefore the optimum fly ash dosage to obtain favourable CST values lies in the range of 3% to 4%.
2.500..-----------.---------, -4% +6% *8%
• :::' .600 z ;!
~ w
'.000
~
600
II: U
...w
II)
ol.--..l-.---'--......L..--......----'--_.L...----J o 2 4 II 8 10 12 14
FLY ASH DOSAGE f%t
Figure I. Specific resistance vs fly ash dosage (with varying oil conlent).
236
T. J. HWA and S. JEYASEELAN
360r-----------,.------......., -4%
i
!e
+ 6'l(,
)I(
6'l(,
200
Iii 150 u 100 50 0
2
0
..
8
8
12
10
14
FLY ASH DOSAGE 1%1
Figure 2. CST vs fly ash dosage (with varying oil content).
2,500r------..-----------....... ~
12,000 b
-
ol....-__-'-__ o 60
___'___
- - ' ~
100
160
__l._ __ _ J
200
250
CST IS.e) Figure 3. Specific resistance VI CST (with avrying oil content). 25r----------~r__----___,
• 4%
+ e%
lIE 8%
lIE
+0
lIE
0'-----1------'----.1...---......- _ - - 1 o 50 100 150 200 250 CST (Sec) Figure. 4 Filter yield VI CST (with varying oil content).
Table 4 shows the variation of filter yield and corrected filter yield for the different oil contents of the sludges. The filter yield increases with increasing dosage of fly ash. This shows that the increasing dosages of fly ash will increase the volume of sludge to be handled after dewatering. In all the cases the increase of filter yield for dosages between 2% and 8% are not very significant. but for dosages more than 8% the filter yields increase rapidly. The corrected filter yields for all the oily sludge samples in the range of 3% to 4%
Conditioning o( oily sludges with municipal solid wastes
237
fly ash dosages are higher than that of dosages in the range of 4% to 10%. The corrected filter yield is measure of actual solids removed from the original sludge excluding the chemical conditioner. This observation strengthens the conclusion on optimum dosage. Correlation amonK the characteristjcs Measurement of specific resistance is tedious, time consuming and demands skills, whereas measurement of CST is simple, fast and does not require any special skills. The correlations of filter yield and corrected filter yield can also be established. A correlation of these parameters with CST will give a quick indication of the specific resistance, filter yield and corrected filter yield of the samples. The correlations of specific resistance, filter yield and corrected filter yield to CST using the values of the respective parameters for all the sludge samples with varying oil contents tested are presented in Figures 3 and 4. Therefore by measuring CST of a given sample the specific resistance which measures filterability and filter yield can be estimated quickly using the correlations. HeaY)' metals Addition of fly ash introduces heavy metals in the filtrate and as the filtrate is returned to the head of the treatment system, this method of chemical conditioning introduces heavy metals into the treatment system. Fly ash used contains heavy metals such as Ag, AI, Ca, Cr, Cu, K, Mg, Na, Pb, Si and Zn. These heavy metals with concentrations above the treatment threshold limit can create several problems to the treatment processes. Therefore the heavy metal contents in the filtrate were examined and reported in Table 5 together with the allowable concentrations stipulated in the Trade Effluent Regulations established by the Ministry of the Environment, Singapore (TER, 1986). It was found that all the heavy metal concentrations were within the limits prescribed by the Trade Effluent Regulations in force in Singapore. There are some elements that are not mentioned in the Regulations such as AI. Ca, K, Mg, Na, and Si. These elements do not create any problems with the quantities mentioned in Table 5. Therefore this filtrate may be mixed with the influent wastewater to the treatment plant, for treatment. CONCLUSIONS Addition of fly ash decreases the specific resistances and capillary suction times of oily sludges rapidly up to 3% dosage and the decreases of these parameters for fly ash dosages of more than 4% are not significant. Therefore the optimum fly ash dosage as a conditioner to improve dewatering lies between 3% and 4%. The corrected mter yield which reflects the actual solids extracted from the unconditioned dewatered sludge is also relatively higher within this range than the corrected filter yields within the dosages of 4% and 8%. CST is relatively very simple and fast to measure, and even an unskilled person can determine CST with reasonable accuracy. The correlations of specific resistance and filter yield to CST have been established from the experimental results. Therefore by measuring CST of a given sample the specific resistance which measures filterability and mter yield which measures the quantity of sludge to be handled after dewatering can be estimated quickly using the correlations. The addition of fly ash will increase the solids content in the sludge cake. The total suspended solids of mtrate decreases with fly ash dosage but the concentrations of heavy metals increase considerably. However at the optimum dosage of 3%, concentrations of heavy metals are within the limits for mixing with influent wastewater.
REFERENCES Baskerville, R. C. and Gale. R. S. (1968). A Simple Automatic Instrument (or Determining the Filterability of Sewage Sludge. Journal of/he rns/;/u/e ofWaler Pollution Con/rol, 1968. Christrensen. G. L. and Dick. R. I. (I98S). Specific Resistance Measurements: Nonparabolic Data. 10u/7l41 a/Environ. Eng. Div., ASCE. JII. 1985.
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Chua, S. E. (1989). The Oil and Grease Problem in Sewage Treatment. Journal of thl! Institution of Enginurs, Singaporl!, 29( I), April 1989. Coakley, P.,and Jones, B. R. S. (1956). Vacuum Sludge Filtration, Sl!Wagl! Industrial Works, 28. Gale, R. S. and Baskerville, R. C. (1970). Polyelectrolytes in the Filtration of Sewage Sludges. Fillm and Sl!parn, 7. 37-44 and 47-52. Knacke, W. R. and Zentkovich, T. L. (1986). Effects of Mean Cell Residence Time and Particle Size Distribution on Activated Sludge Vacuum Dewatering Characteristics, Joumal WPCF, S8( 12), 1118-1123. Lawler, D. F., Yoon, J. C., Shiawjy, H. and Hull, B. A. (1986) Anaerobic Digestion: Effects on Particle Size and Dewaterability, Journal WPCF, 58(12),1107-1117. Martel, C. J.(1993). Fundamentals of Sludge Dewatering in Freezing Beds. War. Sci. TI!Ch., 28(1), 29-35. Novak, J. T. and Knocke, W. R. (1987). Discussion on Specific Resistance Measurements: Nonparabolic Data G. L. Christensen and R. I. Dick., J/Environ. Eng. Div. ASCE 113. TER (1986). Trade Effluent Regulations, Ministry of the Environment, Singapore. Vesilind, P. A. (1988). Capillary Suction Time as a fundamental measure of sludge dewaterability. Journal ofWPCF, 60(2),215• 220.