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A COMPARISON OF FOUR LEACHABLE EXTRACTION
METHODS ON SEWAGE SLUDGE FOR METAL
DETERMINATION
Graeme Kasselman1 and Heidi G. Snyman2
1
ERWAT Chair in Wastewater Management, Water Utilisation Division, Department of Chemical Engineering,
University of Pretoria, Pretoria, 0002. Tel: (012) 420 3824. Fax: (012) 362 5089.
E-mail: [email protected]
2
Golder Associates Africa (Pty) Ltd, PO Box 6001, Halfway House, 1685.
ABSTRACT
The Addendum to the “Guide, Permissible Utilisation and Disposal of Sewage Sludge” released in
July 2002 introduced leachable extraction analysis on sewage sludge. This was done by specifying
leachable limits for four metals (Cu, Pb, Zn and Cd) for a type D sludge. The analytical method
employed to do the analysis is outlined in the Department of Water Affairs and Forestry, “Minimum
Requirements for the Handling, Classification and Disposal of Hazardous Waste”. It is an
adjustment of the United States of Americas’ (USA) Environmental Protection Agencys’ (EPA)
Toxicity Characteristic Leaching Procedure (TCLP). The applicability of the procedure on sewage
sludge as well as the modification of the procedure for the South African context is being
questioned. Comparative analysis between the TCLP (as prescribed by the USA EPA) with other
internationally known procedures for sludge and sediments was done. These include the
Netherlands 7341 Standard (NEN), the German 38 414 S4 Standard (DIN), and the Australian
4439.3 Standard (AS). The leachable fractions from the extraction methods were analysed by
Inductively Coupled Plasma Mass Spectroscopy. The results showed the German DIN standard to
be the most repeatable procedure. The practicality and duration of the experimental procedure for
the DIN standard and NEN standard questions their suitability for regulatory compliance testing.
BACKGROUND
The Introduction of Leachable Limits and Analysis Methods into the South African
Sludge Guidelines
The present South African Sludge Guidelines are found in two separate publications that are to be
used in conjunction with each other. They are the “Permissible Utilisation and Disposal of Sewage
Sludge. Edition 1” published in August 1997 (1) and the “Addendum to the Guideline on
Permissible Utilisation and Disposal of Sewage Sludge. Edition 1” published in July 2002 (2). The
1997 Guideline was found, after publication, to be too restrictive by the wastewater industry (3). An
evaluation of the data from 77 wastewater treatment plants in South Africa done in 1989 by Smith
and Vasiloudis (4) was done by Snyman et al., (5) and found that none of the plants complied with
the 1997 Guideline (1). After the publication of the 1997 Guidelines, it was found that the limits for
Cd, Cu, Pb and Zn were mathematically calculated based on aquatic toxicity data as leachable
limits and not total limits (3). To clarify the aspects of the 1997 Guideline (1), an addendum was
published (2). The addendum advises wastewater treatment plants to use both total and leachable
metal limit values for Cd, Cu, Pb and Zn and only total values for Co, Cr, Hg, Mo, Ni, As, Se, B and
F.
Since no analytical methods were prescribed or recommended in the 1997 Guideline (1), the
addendum attempted to address this issue not by prescribing analytical methods since “it is an
extremely complex subject and should form part of a separate research study” (2) but by rather
recommending that total extraction be performed by aqua regia and leachable fraction by the
Toxicity Characteristic Leaching Procedure (TCLP) for the interim (2).
Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference
ISBN: 1-920-01728-3
Produced by: Document Transformation Technologies
1346
2 –6 May 2004
Cape Town, South Africa
Organised by Event Dynamics
The TCLP was developed by the United States of America’s (USA) Environmental Protection
Agency (EPA) (6). It is also used in South Africa as an analytical method to determine if hazardous
waste can be co-disposed with general waste on a general waste landfill site (7). For this reason
the TCLP was briefly outlined in the “Minimum Requirements for the Handling, Classification and
Disposal of Hazardous Waste” (7) and that procedure is currently used in South Africa on sewage
sludge.
The Difference Between the USA EPA TCLP, and the TCLP as Practiced in South
Africa
There are two differences between the TCLP outlined in the USA EPA Test Methods for Evaluating
Solid Waste (6) and the abridged South African version in the Minimum Requirements (7). The first
is that of duration of extraction. The USA procedure specifies 18±2 hours while in South Africa the
procedure specifies 20 hours fixed extraction duration. Since the tolerance range of the USA
procedure includes the specified value in the South African procedure, the difference in the
duration of extraction is expected to have no effect.
The second difference is the physical state of the sample. The South African procedure specifies
that that sample should be dry (7). The USA procedure makes compensation for multiphase
samples and specifies that samples with a solid content smaller than 0.5 % solids only requires
filtration. The liquid is then defined as the extract and is analysed for contaminants. If the solid
content is greater than 0.5 % solids, sufficient sample should be collected to yield the equivalent of
100 g dry sample (6). The volume of leachate is also affected since the procedure specifies the
total volume of liquid. This includes the liquid added into the extraction vessel with the sample. It
has been determined that the effect of drying samples will affect the concentration of metal
contaminant that will leach out (8). For this reason no samples were dried in this research.
Selected Leachable Extraction Procedures
The selected leachable extraction procedures were the TCLP as originally specified by the USA
EPA (Method 1311) (6). The Australian Standard 1139.3 (AS) (9) that is a replica of the TCLP but
with site-specific extraction liquids, the Netherlands 7341 Standard (NEN) (10) and the German 38
414 S4 Standard (DIN) (11).
MATERIALS AND METHODS
Apparatus
All apparatus used was initially soaked in HNO3 (1 N) for a minimum period of 24 hours. Thereafter
the apparatus was rinsed three times with distilled water. All reagent water used in this research
was demineralised water irrespective of the procedure. The German Standard (11) suggests
distilled water but to make the protocols comparable it was not used.
Sampling and Sample Preparation
An anaerobically digested sample of 150 L was collected at a wastewater treatment plant on the
East Rand. The sample was stored at 4°C until use. It was initially allowed to settle, and the liquid
decanted off. The solid phase was centrifuged at 4000 rpm (8000 g) for 20 min. The liquid portion
was added to the decanted liquid and the solid phase collected separately.
Moisture Content
Three repeat percent moisture determinations were done on the centrifuged solid phase according
to Standard Methods (12).
For each of the extraction procedures five repeat analyses were done with two blanks.
USA EPA TCLP (6)
A 5 g (dry equivalent) subsample was initially used to determine the correct extraction solution.
96.5 mL of demineralised water was added to the 5 g and it was stirred for 5 min using a magnetic
stirrer. The pH was measured. If the pH was below 5.0 pH units, extraction solution one was to be
used. If the pH was above 5.0 pH units, 3.5 mL HCl (1 N) was added to the sample. It was placed
in a hot water bath at 50°C, allowed to reach 50°C and left at that constant temperature for 10 min.
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The sample was then allowed to cool to ambient temperature and the pH measured. If the pH was
below 5.0 pH units then extraction solution one was to be used. If the pH was above 5.0 pH units,
extraction solution two was to be used.
The extraction solutions were prepared as follows:
• Extraction solution one: 5.708 mL CH3COOH was added to 500 mL demineralised water. 64.3
mL 1.0 N standardised NaOH was added and then diluted to 1 L. The pH was measured and
any solution with pH range outside of 4.93±0.05 pH units was discarded.
• Extraction solution two: 5.708 mL CH3COOH was added to demineralised water and then
made up to a volume of 1 L. The pH was measured and any solution with a pH range outside
of 2.88±0.05 pH units was discarded.
• Using the percent solids determined, the equivalent of 100 g (dry mass) of sewage sludge was
weighed off to an accuracy of 0.01 grams in a 2 L glass bottle with a screw cap. The relevant
TCLP extraction solution was added to make up a total liquid volume of 2 L. The volume of
liquid added into the bottle with the sample is also considered as extraction solution. (Additional
extraction solution added to that originating from the sample should yield a total liquid volume
of 2 L.) The bottle was securely positioned in an end-over-end rotator that rotated the bottles at
40 rpm for 18 hours. Once the samples were removed from the rotator, 250 mL of the mixture
was centrifuged at 12 000 rpm (21 000 g) for 20 min. The liquid portion was filtered through 0.7
µm Gelman glass filter paper. The pH was recorded and 250 mL of leachate was collected,
acidified with 25% (m/m) HNO3 to a pH below 2.00 and stored at 4°C until the metal analysis
was done.
Australian Standard (AS) (9)
This leachable extraction procedure is exactly the same as the TCLP as specified above except
that instead of using a TCLP extraction solution, demineralised water was used.
Netherlands Standard (NEN) (10)
A sample mass of 16.00 g was weighed off into a 1 L Erlenmeyer flask. Eight hundred grams of
demineralised water (ρ = 1.00 kg L-1) was added. The flask was placed on a magnetic stirrer and
magnetic rod inserted. The rotation speed was set so that all the material remained in suspension
(approximately 300 rpm). The instantaneous pH was recorded after one minute of stirring. The
stabilised pH was recorded after 10 min of stirring. If the pH was below 7.00 pH units, no addition
to the sample was made and it was left to stir for 3 hours. If the pH was above 7.00 pH units HNO3
(1 N) was added to maintain the pH at 7.00±0.05 pH units. The addition was checked every 5 min
for the first hour thereafter every 10 min. The total volume of HNO3 was recorded after 3 hours of
stirring.
The entire sample was filtered through a 0.7 µm Gellman glass fibre filter paper. The pH of the
liquid was recorded. The solid phase with the filter paper was again inserted into the original
unwashed Erlenmeyer flask. Eight hundred grams of demineralised water (ρ = 1.00 kg L-1) was
added. (The demineralised water was used to wash the solid material back into the flask). The
solution was again placed on a magnetic stirrer and stirred as previously. The instantaneous pH
was recorded after 1 min and the stabilised pH was recorded after 10 min. The pH was adjusted
with HNO3 (1 N) to a pH of 4.00±0.05 every 5 min for one hour and every 10 min for a further two
hours. The total acid added was recorded.
The sample was again filtered through a 0.7µm Gelman glass fibre filter paper and the liquid
portion collected. The two leachates were added together and acidified with 25% (m/m) HNO3 then
stored at 4°C until metal determination was done.
German Standard (DIN) (11)
A sample of 100 g (dry mass equivalent) was placed in a 2 L glass bottle. One litre of
demineralised water was added to the bottle. The bottle was loaded into an end-over-end rotator
and left to rotate at 40 rpm for 24 hours. The entire contents of the bottle was separated by
centrifugation at 4000 rpm (8000 g) for 20 min. All the solid was collected and returned to the
unwashed glass bottle.
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To the solid portion 1 L of demineralised water was again added and the bottle loaded into the
rotator and set to rotate at 40 rpm for another 24 hours. Extraction after 24 hours was the same as
described above. The two leachates were added together, acidified with 25 % (m/m) HNO3 and
kept at 4°C until further analysis.
Metal Determination
The leachates were analysed on a VG Plasmaquad PQ2 Turbo Plus Inductively Coupled Plasma
Mass Spectrometer (ICP-MS) for B, Cd, Co, Cr, Cu, Fe, Hg, Mo, Ni, Pb, Se and Zn.
RESULTS AND DISCUSSION
Rotation Speed
The speed tolerance of the shaker is reported not to be of such importance. Quevauviller (13)
reported that in an inter-laboratory study the speed tolerance was set to 30 % and that speeds
ranging from 20 to 40 rpm where the procedure stipulated 30 rpm yielded acceptable results. One
laboratory used a shaker with speed of 14 rpm and the results were still comparable. The DIN
standard does not specify a rotation speed (11), while the TCLP (6) and AS (9) specify a rotation
speed of 30±2 rpm.
Results
Table 1 shows the mean values and standard deviations obtained for Cd, Co, Pb and Zn for each
of the extraction procedures used and the limit value specified in the Addendum of the Sludge
Guidelines (2).
Table 1. Table of the leachable limit values and maximum extracted values for
the various extraction procedures.
Metal
Limit Value
TCLP
Cd
Cu
Pb
Zn
15.7
50.5
50.5
353.5
0.0457
1.0221
0.0353
15.8270
AS
mg kg-1
0.0613
0.9910
0.3146
7.6152
DIN
NEN
0.1053
1.2334
0.5752
12.3932
0.0332
0.2369
0.0206
26.8568
The experimental values are lower than the limit values and this was found to be true for a study of
78 different wastewater plants in South Africa as well (3). The test procedure that yielded the
highest values for Cd, Cu, and Pb was the DIN while the NEN yielded the highest value of Zn.
Since South Africa manages all Environmental Affairs based on the Precautionary Principle(7), a
leachate test procedure selection needs to consider which procedure yielded the highest values.
Figures 1(a) to 1(j) and 2(a) and (b) represent the four different test procedures (Toxicity
Characteristic Leaching Procedure, Australian Standard, German Standard and Netherlands
Standard) metal leachate extraction values obtained for B, Cd, Cr, Co, Cu, Fe, Pb, Hg, Mo, Ni, Se
and Zn.
The TCLP yielded the highest values for B, and Se. The DIN yielded the highest values for Cd, Cr,
Cu, Pb, Hg, and Mo. The NEN yielded the highest values for Co, Fe, Ni and Zn.
The TCLP yielded the lowest values for Cr, and Mo. The AS yielded the lowest values for Co, Fe,
Hg, Ni, Se and Zn. The NEN yielded the lowest values for B, Cd, Cu, and Pb.
From this we can deduce that the procedure that yields the highest values of all the metals
analysed for, is the DIN followed by the NEN then the TCLP and the lowest values obtained for the
12 metals analysed for was from the AS.
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210
14
210
0.12
180
12
180
0.10
150
10
150
0.08
120
8
120
0.06
90
0.04
60
30
6
90
4
60
2
30
0.02
0
0.00
0
AS
DIN
0
TCLP
NEN
AS
DIN
Leachate Procedure
Leachate Procedure
Mean leachable values
Mean leachable values
% RSD
50
2.0
40
1.5
30
1.0
20
0.5
10
0.0
2.0
40
1.5
30
1.0
20
0.5
10
0.0
0
DIN
50
0
TCLP
NEN
AS
DIN
Mean leachable values
Mean leachable values
% RSD
1.2
90
1.0
75
0.8
60
0.6
45
0.4
30
0.2
15
0.0
10000
100
1000
75
100
50
10
25
1
0
DIN
0
TCLP
NEN
AS
Mean leachable values
% RSD
240
0.50
200
0.40
160
0.30
120
0.20
80
0.10
40
0.00
Concentration [mg kg-1]
280
0.60
Percent
Concentration [mg kg -1]
0.70
% RSD
0.30
300
0.25
250
0.20
200
0.15
150
0.10
100
0.05
50
0.00
0
DIN
0
TCLP
NEN
AS
DIN
NEN
Leachate Procedure
Leachate Procedure
Mean leachable values
NEN
(f): Fe.
(e): Cu.
AS
DIN
Leachate Procedure
Leachate Procedure
Mean leachable values
Percent
105
Concentration [mg kg -1]
120
1.4
Percent
Concentration [mg kg -1]
1.6
TCLP
% RSD
(d): Co.
(c): Cr.
AS
NEN
Leachate Procedure
Leachate Procedure
TCLP
Percent
60
2.5
Concentration [mg kg ]
3.0
2.5
-1
70
Percent
Concentration [mg kg ]
-1
3.5
AS
% RSD
(b): Cd.
(a): B.
TCLP
NEN
Mean leachable values
% RSD
(h): Hg.
(g): Pb.
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% RSD
Percent
TCLP
Percent
0.14
Concentration [mg kg -1]
240
Percent
Concentration [mg kg -1]
16
90
0.50
75
0.40
60
0.30
45
0.20
30
0.10
15
50
30
45
27
40
24
35
21
30
18
25
15
20
12
15
9
10
6
5
3
0
0.00
0
TCLP
AS
DIN
0
TCLP
AS
NEN
DIN
NEN
Leachate Procedure
Leachate Procedure
Mean leachable values
Mean leachable values
Percent
0.60
Concentration [mg kg -1]
105
Percent
Concentration [mg kg -1]
0.70
% RSD
% RSD
(j): Ni.
(i): Mo.
35
70
0.70
35
30
60
0.60
30
25
50
0.50
25
20
40
0.40
20
15
30
10
20
5
10
0.30
15
0.20
10
0.10
5
0.00
0
0
TCLP
AS
DIN
AS
DIN
NEN
Leachate Procedure
Leachate Procedure
Mean leachable values
0
TCLP
NEN
Percent
40
Concentration [mg kg -1]
0.80
Percent
Concentration [mg kg -1]
Figure 1. Graphs of the mean leachable fractions for B, Cd, Cr, Co, Cu, Fe, Pb, Hg, Mo, and Ni depending on
the extraction procedure.
Mean leachable values
% RSD
% RSD
(b): Zn.
(a): Se.
Figure 2. Graphs of the mean leachable fractions for Se and Zn depending on the extraction procedure.
Accuracy of Repeat Extractions
The repeatability of the different methods was compared by their percentage relative standard
deviations (% RSD) (14). The % RSD is also plotted on figures 1(a) to 1(j) and 2(a) and (b) and is
calculated by the ratio between the standard deviation and the mean value. Low % RSD values
indicate that repeat determinations are close to each other (and the mean) while high % RSD
values indicate that repeat determinations are further away from each other. % RSD values above
100 % show that the standard deviation is larger than the mean. This means that low % RSD
values show a specific procedure to be more repeatable while large % RSD values show a
procedure to be less repeatable.
In comparing the four different methods, for Mo the TCLP was the most repeatable, while for Cd,
Co, Cu, Fe, Pb, Hg, Ni and Zn the DIN standard was the most repeatable. For Cr the NEN was the
most repeatable while B and Se yielded a similar repeatability for the AS and the DIN standard.
For Co, Fe, Ni and Zn the TCLP was the least repeatable, while for Cr, Cu, and Mo the AS was the
least repeatable. For B, Cd, Pb, Hg and Se the NEN was the least repeatable.
From this it can be concluded that the DIN was the most repeatable test procedure when
comparing the % RSD for the 12 metals analysed followed by the NEN, AS and lastly the TCLP.
Experimental Difficulties
The sequential leachate procedures (DIN and NEN) had practical problems with extracting the
liquid from the first step and retaining the entire sample for the second extraction. An assumption
was made that loss of sample between extractions was minimal and should not affect the overall
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extraction results.
The DIN procedure also does not specify how many repeat sequential extractions are to be
performed. Further analysis of the data obtained, shows (not here) that as little as 21 % of the total
metal extracted in two extractions originated from the first extraction (8).
CONCLUSIONS
The DIN yielded the highest metal leachates for three of the four metals regulated by the Sludge
Guidelines.
It was found that the repeatability of the extraction procedure currently used is poor compared to
alternative internationally used procedures. The TCLP is the least comparable across repeat
analyses. This research shows that the German DIN is the most repeatable despite the
experimental difficulties encountered.
This research has further shown that both practical experimental considerations and repeatability
and reproducibility of samples are relevant when selecting a test procedure. The comparison
yielded as the most repeatable an impractical test due to duration and experimental difficulties.
Furthermore, the relationship between test procedure and the limit values needs also to be
considered. It is impractical to do experiments to determine compliance when it is almost
guaranteed.
ACKNOWLEDGEMENTS
The authors would like to thank Mr W Kirsten from the Agricultural Research Council (ARC)
Institute for Soil Climate and Water (ISCW) and Mr AJ Goosen from the University of Pretoria,
Department of Chemical Engineering, Water Utilisation Division for their assistance.
REFERENCES
1. Department of Agriculture, Department of Health, Department of Water Affairs and Forestry, Water
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Department of Health, Sludge Consult, “Addendum No 1 to Edition 1 (1997) of Permissible
Utilisation and Disposal of Sewage Sludge”, pub Water Research Commission, South Africa (2002).
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Sewage Sludge and an Evaluation of Analytical Methods for their Determination” pub Water
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414-S4 pub Deutches Institut für Normung. Berlin. Germany (1984).
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