HEAVY METALS REMOVAL FROM MUNICIPAL AND INDUSTRIAL SLUDGES
Mohamed Ragaei Lasheen, Azza Ashmawy and Hanan Ibrahim
(Department of Water Pollution, National Research Center, Dokki, Cairo,
Egypt)
E-mail: ragaei24@intouch.com.
ABSTRACT
The quantity of sludge
produced by Cairo’s WWTP is large (estimated 260,000 m3y -1
and will increase to 610,000 m3y -1 by 2020). Land
application of sewage is widely used in Egypt. However, the principal
environmental concern is due to the inevitable presence of heavy metals. The
main objective of this study is to assess the effective methods for metals
removal from sludge to reduce health risk during land application. The results
showed that acids are suitable for removing heavy metals from sludge. The
optimum treatment efficiencies of metals extraction from sludge are related to
the species of metals in sludge, dosage of extractants, pH, sludge solid
content, and reaction time. The preferred order of solubilization by sulfuric
acid was in the following order:
Fe > Ni > Cr > Cd > Zn > Pb > Cu
Ethylenediamintetracetic
acid (EDTA) showed high removal efficiencies for Fe, Ni and Cd at neutral pH.
Stringent industrial effluent control, coupled with improved industrial
technology should be effective in reducing the heavy metals contents of sludge
in Egypt.
The quantity of sludge produced by the six large wastewater treatment plants (WWTP) in Cairo is currently estimated to be about 155,000 tones of dry solids (tds) per year, and production will increase to about 360,000tds per year by 2020 (ref 1). This large amount of sludge requires effective management and safe disposal.
The basic disposal method for such large quantities of sludge is land application. Sludge from municipal wastewater treatment plants contains a high quantity of nutrients needed for plant growth. However, sludge always contains toxic substance, especially heavy metals. When applied to land, heavy metals can accumulate in the soil and produce harmful effects in animals and vegetation. Therefore, the study of effective methods for removing heavy metals from sludge in order to reduce prospective health risks to the minimum during land application is very important.
The heavy metal species in sludge depend on wastewater characteristics and the treatment processes used. Gould and Genetelli (ref 2) reported that the possible species of heavy metals in sludge can be classified as follows, i) soluble, including metal ions, organic complexes and inorganic complexes, ii) metal oxides, iii) precipitates, iv) adsorbates, including physical, chemosorption and clay lattice, v) Organo metallic complexes, and vi) biological residues. Wu Qi-tang, et al., (ref 3) found that the acid treated sludge showed similar or higher fertilizer value than the untreated sludge and decreased soil heavy metal contamination, but the acidified sludge had to neutralized in order to reduce significantly the plant uptake.
The main objectives of this study are: (1) To assess the variables affecting the efficiency of acidification for the removal of metals from wastewater treatment sludges, and (2) To assess the effect of (EDTA) on metal removal from sludge.
METHODS AND
MATERIALS
Sludge source and metal concentration. Two waste activated sludge samples were obtained from Helwan (WWTP) its capacity is approximately 270,000 m3 /day serves municipal industrialized area south of Cairo, and the new city near Cairo, the 6th of October (WWTP) its capacity 50,000 m3 /day and serves mainly industrial areas. Synthetic industrial sludge was obtained by preparing wastewater contains high concentrations of Cd, Cr (VI), Cu, Pb and Ni and ferrous sulfate was added for Cr (VI) reduction, followed by lime addition for metal precipitation. Table (1) shows the initial metal analysis in the three sludges.
|
Metals |
Helwan |
6th of October |
Synthetic |
|
Cadmium |
8.6 |
3.15 |
72,000 |
|
Chromium |
230 |
450 |
26,175 |
|
Copper |
219 |
650 |
95,025 |
|
Iron |
15234 |
12050 |
116,450 |
|
Lead |
76 |
111.25 |
56,500 |
|
Nickel |
52 |
320 |
76,250 |
|
Zinc |
4483.3 |
2050.8 |
--------------- |
Experimentation and instrumentation A quantity of sludge containing 2g dry solids was diluted to approximately 80ml with distilled water and dosed with either 37% HCL, 89% HNO3, 97% H2SO4 or 0.1M EDTA and diluted to final volume of 100ml. EDTA was added based on the stoichiometric requirement for metals, considering that one mole of metal requires one mole of EDTA. After shaking for a prescribed time the solution was centrifuged and then filtered through Whatman No 42 filter. The percentage of metal removal was calculated according to Lo & Chen (ref 4)
An atomic absorption spectrophotometer was used for metals analysis. Samples were digested and analyzed according to Standard Methods (ref 5)
RESULTS AND DISCUSSION
Effect of acid treatment. Tables (2 and 3) showed that the most difficult metal to be solubilized is copper followed by lead at all acid doses. This observation reflects the nature or form of the copper in the sludge, which suggests that part or all of the copper in the sludge existed as an organic complex. Jenkins, R.L.et al., (ref 6) obtained a similar result. In addition, lead may be existed as sulfate or organic complexes in the sludge and not as hydroxides. The high efficiency for iron and nickel removals might indicate that significant portion of these metals exists as inorganic precipitates or as weakly chelated forms within or/on the sludge solids. For synthetic sludge, Table (4) indicates that lead was the most difficult metal to be solubilized followed by iron.
Table (2) Effect of Different Acids on Metal Removal from Helwan Sludge at pH 2 for two hours.
|
Acid |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
Zinc |
|
H2SO4 |
31.87 |
55.73 |
1.75 |
90.80 |
2.02 |
75.54 |
39.08 |
|
HCl |
29.5 |
20.0 |
1.7 |
90.4 |
0.7 |
58.5 |
18.6 |
|
HNO3 |
22.3 |
21.1 |
1.5 |
57.2 |
0.7 |
50.7 |
17.5 |
Table (3) Effect of Different Acids on Metal Removal from 6th of October Sludge at pH 2 for two hours.
|
Acid |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
Zinc |
|
H2SO4 |
66.9 |
66.8 |
1.4 |
91.0 |
2.1 |
85.2 |
46.8 |
|
HCl |
59.6 |
26.4 |
0.6 |
93.5 |
0.8 |
78.4 |
27.3 |
|
HNO3 |
44.2 |
22.5 |
0.6 |
59.3 |
0.8 |
65.2 |
18.8 |
Table (4) Percentage of Metal Removal from Synthetic Sludge: Extraction with Sulfuric acid for two hours.
|
pH |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
|
4.0 |
58.77 |
3.27 |
22.93 |
1.03 |
0.527 |
31.82 |
|
3.0 |
95.15 |
3.52 |
75.09 |
2.15 |
0.53 |
65.21 |
|
2.0 |
99.04 |
47.76 |
97.61 |
19.67 |
1.35 |
88.72 |
|
1.5 |
99.44 |
99.33 |
99.95 |
60.16 |
1.38 |
98.23 |
Table (5) shows that the efficiencies of metal removal decreased by increasing the sludge solid content. In addition, Table (6) revealed that limitations in metal removal exist regardless of the length of acidification time. Similar results were obtained by using hydrochloric acid, nitric acid or sulfuric acid, as the solubilization of one equivalent of metal requires one equivalent of acid (refs 6,7& 8).
Table (5) Effect of Solid Content on Metal Removal from Helwan Sludge: Extraction
with Sulfuric acid,
pH 2, two hours.
|
Solid Content |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
Zinc |
|
1.67 % |
71.6 |
69.6 |
2.0 |
92.5 |
2.9 |
79.4 |
47.2 |
|
6.4 % |
31.9 |
55.7 |
1.8 |
90.8 |
2.0 |
75.5 |
39.1 |
Table (6) Effect of Extraction Time on Metal Removal from
Helwan Sludge:
Extraction with
Sulfuric acid, pH 2.
|
Hours |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
Zinc |
|
1 |
31.0 |
48.5 |
1.7 |
59.2 |
2.0 |
71.3 |
38.3 |
|
2 |
31.9 |
55.7 |
1.8 |
90.8 |
2.0 |
75.5 |
39.1 |
|
3 |
32.2 |
57.8 |
1.8 |
91.5 |
2.0 |
78.0 |
40.0 |
|
4 |
32.8 |
57.9 |
1.9 |
92.8 |
2.0 |
78.9 |
41.4 |
|
6 |
33.1 |
58.0 |
2.0 |
94.3 |
2.0 |
85.2 |
41.7 |
|
24 |
79.2 |
60.1 |
2.5 |
97.6 |
9.0 |
97.5 |
49.9 |
According to Table (7) the metal removal from wet sludge samples was much higher for all the metals. This may be explained by the fact that in the dry sludge metal was bound to more stabilized forms. Garcia-Delgado, R.A. et al., (ref 9) observed similar behavior. Furthermore, the table shows that metal removal from sludge increased by decreasing the pH.
Effect of EDTA. Table (8) indicates that the percentage of metal removal by EDTA do not corresponds to the published stability constants for the EDTA complexes (ref 10). This may be due to the fact that any heavy metal may be bound by various ligands and/or exist as various insoluble salts. Compared with acid solubilization Pb removal increased while Cr, Ni, Fe, and Zn decreased. Jenkins, R. L. et al., (ref 6) obtained similar results.
It can be concluded that metal removals from sludge via acidification processes were found to be dependent upon the pH, sludge solid content, the specific metal and the length of extraction time. It is recommended to prevent industrial effluent discharged to sewer to reduce the problem of heavy metals and potential risks to the environment are avoided by monitoring sludge quality and control of its use.
Table (7) Percentage of Metal Removal from wet and dry 6th of October Sludge: Extraction with Sulfuric acid, two hours
|
Wet sludge
|
pH |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
Zinc |
|
4 |
56.24 |
12.70 |
0.74 |
53.82 |
0.84 |
54.95 |
13.63 |
|
|
3 |
61.59 |
16.76 |
1.08 |
82.11 |
1.05 |
56.50 |
34.68 |
|
|
2 |
66.95 |
66.84 |
1.37 |
91.01 |
2.11 |
85.24 |
46.78 |
|
|
1.5 |
73.78 |
70.74 |
1.66 |
98.88 |
2.27 |
89.34 |
80.38 |
|
|
Dry sludge dried at105°C |
4 |
52.71 |
2.62 |
1.16 |
33.35 |
1.80 |
6.34 |
11.74 |
|
3 |
55.71 |
4.94 |
1.54 |
46.16 |
2.70 |
11.77 |
26.24 |
|
|
2 |
57.00 |
11.38 |
1.92 |
50.83 |
4.94 |
52.10 |
43.89 |
|
|
1.5 |
71.43 |
22.95 |
2.69 |
67.43 |
14.83 |
57.38 |
72.65 |
Table (8) Percentage of Metal Removal from Helwan Sludge: Extraction with EDTA, two hours .
|
EDTA |
Cadmium |
Chromium |
Copper |
Iron |
Lead |
Nickel |
Zinc |
|
0.5X |
11.32 |
3.30 |
3.39 |
30.06 |
4.04 |
25.34 |
6.38 |
|
1X |
14.30 |
4.01 |
4.51 |
31.58 |
8.76 |
27.29 |
7.51 |
|
2X |
20.85 |
8.49 |
5.38 |
39.01 |
15.50 |
29.05 |
9.76 |
|
4X |
26.21 |
19.69 |
6.20 |
44.66 |
20.90 |
34.70 |
14.07 |
|
6X |
41.70 |
33.21 |
8.28 |
50.88 |
39.77 |
42.59 |
19.23 |
X stoichiometric
requirements
1.
Cairo Sludge Disposal
Study, (1999) European Investment Bank, Cairo Wastewater Organization Cairo,
Egypt.
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Gould M.S. and
Genetelli E.J. (1975), Proc. 30th industrial Waste Conf., Purdue,
Ann Arbor, MI, 689-697.
3.
Wu Q., Nyirandege P.,
Mo C. (1998) J. of Environmental Sciences 10, 122-128.
4.
Lo, K.S.L. and
Chen,Y.H., (1990) The Science of The Total Environment, 90, 99-116.
5.
Standard Methods for
the Examination of Water and Wastewater (1995), 19th Edition
American Public Health Association, Washington D.C.
6.
Jenkins R.L,
Scheybeler B.J, Smith M.L, Baird R., Lo M.P. and Haug R.T., (1981)
J. of WPCF, 53, 25-32.
7.
Tyagi R.D., Couillard
D and Tran F, (1988) Environmental Pollution 50, 295-316.
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Daniel J. W., Jerry
Y.C. H (1982) J. of WPCF, 45 1574-1580.
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Garcia-Delgado,R.A.,
Garcia-Herruzo,F., Gomez-Laho,C. and Rodriguez-Maroto, J.M., (1994) J. Environ.
Sci. Health, A29, 1335-1347.
10.
Principles of Aquatic
Chemistry (1983) Morel, F.M.M., John Wily and Sons. Inc.