Snyman, Heidi G. et al - eWISA

METAL CONTENT OF SOUTH AFRICAN SEWAGE SLUDGE
Heidi G. Snyman 1 , J.E. Herselman 2 and G Kasselman 3
1 Corresponding Author: Golder Associates Africa, PO Box 6001, Halfway House, 1685, South Africa.
Tel: (011) 254 4915. Fax: (011) 315 0317. E-mail: hsnyman@golder.co.za.
2 Institute of Soil, Climate and Water, ARC, Private Bag X79, Pretoria 0002, South Africa.
3 ERWAT Chair in Wastewater Management, Water Utilisation Division, Department of Chemical Engineering,
University of Pretoria, Pretoria 0002, South Africa.
ABSTRACT
A survey was done to determine the potentially toxic metals and element (metal) quality of sewage
sludge in South Africa. Sludge samples (78) were collected and analysed for Cd, Co, Cr, Cu, Hg,
Mo, Ni, Pb, Zn, Se, B and F, using three extraction methods. The efficiency of the extraction
methods, aqua regia (total extraction), EPA 3050 (total extraction), and TCLP (leachable) were
compared. The aqua regia extraction method was found to give more reliable results compared to
the EPA 3050 extraction.
The metal content of sludge from the plants receiving industrial effluent was higher or similar to the
metal content of sludge from plants receiving domestic effluent for all the metals with the exception
of B.
The sewage sludge surveyed was evaluated in terms of leachable and total extractable metal
content with respect to compliance to the Sludge Guidelines and related Addendum. None of the
plants surveyed exceeded the TCLP (leachable) extractable concentration limits as specified in the
Addendum of the Sludge Guidelines. The results from the total metal extractions showed a
different picture. The major metals of concern are Ni and Zn. Sixty-one percent (61 %) and 44 % of
the total mass of sludge surveyed exceeded the Ni and Zn concentration limits respectively. A
major fraction (40 % mass basis) of the sludge surveyed did not exceed any of the total metal
limits. Thirty-five percent (35 % mass basis) of the sludge surveyed exceeded on one or two
metals. Only 4% of the sludge surveyed, exceeded the limits for more than seven of the twelve
metals analysed for.
INTRODUCTION
The concentration of potentially toxic metals and elements (hereafter referred to as “metals”) of
South African sewage sludge was determined in 1989 by Smith and Vasiloudis (1) who surveyed
77 wastewater treatment plants. Smith and Vasiloudis (1) used total extraction methods to
determine the metal content of sewage sludge. However, the South African Sludge Guidelines
have been revised since this survey was done. It now includes the requirement that sludge
samples should be extracted using both a total and an available (leachable) extraction method.
This is detailed in the document entitled: “Permissible utilisation and disposal of sewage sludge,
Edition 1” published in 1997 (2) and the related addendum (Addendum No 1 to Edition 1 (1997) of
the Permissible utilisation and disposal of sewage sludge in 2002) (3) that advises use of both the
aqua regia extraction method (total extraction method) and the Toxicity Characteristic Leaching
Procedure (TCLP) (leachable extraction method) to characterise the metals in sewage sludge. The
aqua regia and EPA 3050 extraction methods are commonly referred to as total extraction methods
although they are not an accurate reflection of the total extractable metals in a sample. More
aggressive extraction methods exist for this purpose.
Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 –6 May 2004
ISBN: 1-920-01728-3 Cape Town, South Africa
Produced by: Document Transformation Technologies Organised by Event Dynamics

The TCLP was developed to measure a wastes’ leachability and hence the risk to pollute
groundwater. It is typically used when a waste is to be landfilled in a site that receives a variety of
organic and inorganic wastes. The procedure aims to simulate the dissolving action of the organic
acid leachate formed in a landfill where hazardous waste has been co-disposed with general
waste. It can be used to determine the mobility of organics and inorganics in liquid, solid, and
multiphase wastes. The TCLP has historically not been used by the wastewater industry to assess
the amount of sludge to be safely used in agricultural practices. Most wastewater laboratories have
used the aqua regia (total) extraction, which gives an indication of the acid soluble fraction.
Therefore, limited data is available for applying the TCLP to sludge samples. In addition, the metal
limits in the Sludge Guidelines are still being debated, particularly the analytical methods for
determining metals in sludge. These issues are currently being addressed in the development of
the “Permissible utilisation and disposal of sewage sludge, Edition 2”. In the interim, the
explanatory addendum (3) stipulates that sludge should be extracted with both the TCLP
(leachable) and aqua regia (total) extraction methods. In this study, the extraction procedure
included both the TCLP and aqua regia extraction methods. This data was used to determine the
extent of compliance to the current Sludge Guidelines (2 and 3).
MATERIALS AND METHODS
Two hundred South African wastewater treatment plants (WWTPs) were initially pre-selected and
interviewed telephonically. A sample of 72 plants was selected from the initial 200 plants based on
a questionnaire used to gather background information with emphasis on the treatment methods
and the type of sludge produced. To achieve this some sludge samples did not represent the end
product. For example, in many plants, activated sludge is blended with other sludge types to form
the end product. In such a case, a decision had to be made to either sample the blend or the
activated sludge.
Sludge samples (78 – more than one sample was collected from some plants) were collected at
the selected wastewater treatment plants. Three rounds of sampling was done to include seasonal
variation i.e. winter, summer and autumn. These samples were analysed for microbiological,
nutrient and selected metal concentrations. This paper will focuses on the results of the metal
analysis only.
Two different South African laboratories were selected for the analyses: The East Rand Water
Care Company Laboratory (ERLAB) and the Agricultural Research Councils (ARC) Institute for
Soil, Climate and Water (ISCW) laboratory. No accredited laboratory for the analysis of sludge
could be found at that stage. For this reason, an inter-laboratory extraction and analysis train was
set up. This was done to utilise the expertise of both laboratories to the optimum. Samples were
collected and transported to the laboratories with a maximum delay of 72 hours. A subsample was
used to analyse for pH and moisture content (4). The rest of the sample was dried at 50 0 C for
seven days and crushed to pass through a 1 mm sieve. The samples were extracted using the
following extraction methods: (a) Aqua regia extraction (5); (b) EPA 3050 extraction (6) and (c)
TCLP (DWAF 1998). The F was extracted using the method described by (5). These extracts were
analysed for Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Zn, As, Se and B using a VG Plasmaquad PQ2 Turbo
Plus Inductively Coupled Plasma Spectrometer (ICP-MS).
The analytical data was used to classify the sludge and to determine the impact of the existing
Guidelines on the disposal options.
The metal data was interpreted to reflect the contribution of the plants to metal load of the total
mass of sludge analysed in this study. In other words, as large plants contribute more sludge than
small plants, it was necessary to express the analytical data on a mass basis to obtain a clear
overview of trends in the country. Some of the wastewater treatment plants could not supply the
mass of sludge produced per day. For these plants, the mass of sludge was calculated using the
inflow volume and inflow COD.

RESULTS AND DISCUSSION
Figure 1 gives a geographical map of the plants selected. The plants represented different
treatment methods geographically spread throughout South Africa.
Figure 1. Geographical representation of selected plants categorised into the treatment method.
Anaerobic Digestion
57%
Activated Sludge
20%
Blended sludge
12%
Petro sludge
2%
Oxidation Dams
0.3%
Aerobic digestion
1%
Figure 2. Sludge types produced by the wastewater treatment plants surveyed in South Africa on a mass
percent basis. The blended sludge represents primary and activated or humus sludge blended before or after
digestion.

Figure 2 indicates the types of sludge generated by the wastewater treatment plants surveyed. The
majority of the sludge produced (57% on a mass basis) employs anaerobic digestion of primary
and humus sludge.
Metal Content
The weighted mean values for the metals analysed for, with respect to the extraction method used,
are included in Table 1.
Table 1. Weighted mean* metal concentrations for all the plants surveyed with respect to the
extraction method.
Aqua regia
(mg kg -1 )
EPA 3050
(mg kg -1 )
TCLP
(mg kg -1 )
TCLP
(mg L -1 Metal
)
Total Extraction Leachable Extraction
Cd 10 6 0.03 0.0016
Co 32 21 0.16 0.0079
Cr 414 282 1.02 0.0512
Cu 576 383 0.43 0.0217
Hg 4 3 0.07 0.0035
Mo 9 8 0.03 0.0016
Ni 269 93 0.91 0.0456
Pb 318 190 0.08 0.0042
Zn 3159 1673 7.15 0.3573
Se 11 6 0.07 0.0033
B 57 29 0.84 0.0419
* The values were calculated by determining the total mass of each metal in the sludge for each plant
based on the daily sludge production. These values were then added together and divided by the
total mass of sludge produced by all the plants.
Table 1 shows that the average metal concentration values for the aqua regia extraction method is
larger than the EPA 3050 extraction method. Since the two procedures both aim to determine a
total metal concentration the higher value was chosen as a precautionary principle. Furthermore
the aqua regia extraction data proved to be more reliable compared to the EPA 3050 extraction
method. All further data interpretation was therefore done using the data generated from the aqua
regia extracted samples.
Table 2. Weighted mean* total (aqua regia) metal concentrations in mg kg-1 for the sludge
samples collected in this survey categorised according to the inflow.
Metal
< 10
ML day –1
10 to 25
ML day -1
25 – 80
ML day -1
> 80
ML day -1
Cd 6 6 11 12
Co 14 16 42 37
Cr 375 359 342 504
Cu 469 526 648 583
Hg 6 6 3 3
Mo 7 7 8 11
Ni 190 205 363 261
Pb 190 208 270 446
Zn 2956 2758 3436 3243
Se 10 9 14 11
B 42 55 54 65
F
Mass percentage
157 139 135 101
of total sludge in
each group
11 22 27 40
* The values were calculated by determining the total mass of each metal in the sludge for each plant
based on the daily sludge production. These values were then added together and divided by the total
mass of sludge produced by all the plants in one day.

In the study the TCLP fraction was analysed for different metals to establish compliance with the
Sludge Guidelines (3). The TCLP was designed to predict the concentration of a metal in the
leachate expressed in mg L -1 . However, the maximum leachable concentrations stipulated in the
Sludge Guidelines are expressed in mg kg -1 (3). For this reason both concentrations are included
in Table 1.
Table 2 details the weighted mean metal concentration of the surveyed wastewater treatment
plants’ sludge categorised according to the inflow received. This will allow wastewater plants that
were not included in this study to benchmark their metal values.
Influence of Industrial Effluents on Sludge Metal Quality
The sampling sites were also sorted into two groups; plants receiving mainly domestic effluent (and
industrial effluent that should not affect the metal concentration) and plants receiving industrial
effluent that could affect the metal concentration. For example, if a plant receives effluent from a
candy factory, it was considered a domestic plant. On the other hand, if a plant receives metal
plating effluent, the effluent will impact on the sludge metal quality and was therefore categorised
as an industrial plant. A distinction was only made with respect to metal concentration; we
assumed that a wastewater treatment plant that receives industrial waste should contain more
metals than a domestic plant. Thirty-nine samples were categorised as industrial and thirty-nine as
domestic. The weighted mean metal concentration for the results from this classification is shown
in Figures 3 to 5.
Concentration [mg kg -1 ]
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Zn
Metal
Industrial Total Domestic
Figure 3. The weighted mean Zn concentration for industrial and domestic plants. Total refers to
all the plants (domestic and industrial).
As expected, the Zn concentration (aqua regia; total extraction) for sludge generated from plants
that receive industrial effluent that could affect the metal concentration of the sludge is higher than
plants receiving mainly domestic effluent (Figure 3). The same trend was observed for the other
metals with the exception of B (Figure 5). This indicates that B may not be from industrial origin.

Concentration [mg kg -1 ]
Concentration [mg kg -1 ]
800
700
600
500
400
300
200
100
0
Cr Cu Ni Pb
Metal
Industrial Total Domestic
Figure 4. The weighted mean metal concentrations for industrial and domestic plants for Cr, Cu,
Ni and Pb. Total refers to all the plants (domestic and industrial).
80
70
60
50
40
30
20
10
0
Cd Co Hg Mo Se B
Metal
Industrial Total Domestic
Figure 5. The weighted mean metal concentrations for industrial and domestic plants for Cd, Co, Hg,
Mo, Se and B. Total refers to all the plants (domestic and industrial).
Metal Limits
Table 3 shows the allowable metal limits for a type D sludge according to the Permissible utilisation
and disposal of sewage sludge, Edition 1 (2) and the related addendum (3). The data from the
survey will be compared to the limits detailed in Table 3 in the next two sections.

Table 3. Allowable metal content in a Type D sludge according to the Permissible utilisation
and disposal of sewage sludge, Edition 1 and the related addendum.
Metal
Leachable limit
(mg kg -1 )
Total Limit
(mg kg -1 )
Cd 15.7 20
Co - 100
Cr - 1 750
Cu 50,5 750
Hg - 10
Mo - 25
Ni - 200
Pb 50,5 400
Zn 353,5 2 750
As - 15
Se - 15
B - 80
F - 400
The Compliance of Sewage Sludge to the Leachable Limits
The leachable concentration i.e. TCLP data was converted from mg L -1 to mg kg -1 to compare the
data with the limit values in the Sludge Guidelines (3). Figures 6, 7 and 8 represent the TCLP
(leachable) fraction for the wastewater treatment plants in the study. Each line on the graph refers
to a specific metal. They are cumulative percentage graphs and are interpreted as follows.
Consider Mo at 0.4 mg kg -1 in Figure 6. It reads off at approximately 7 % on the y-axis. This means
that approximately 7 % of the total mass of sludge analysed for has a concentration of 0.4 mg kg -1
or larger.
All the sludge surveyed complied with the TCLP leachable limits (Table 3) for the metals as
stipulated in the Sludge Guidelines (3).
Percent of sludge mass
100
90
80
70
60
50
40
30
20
10
0
0 0.2 0.4 0.6 0.8 1
mg kg
1.2 1.4 1.6 1.8 2
-1
Cadmium Molybdenum Selenium Mercury
Figure 6. Mass percentage distribution of the sludge in terms of the leachable (TCLP) metal
concentration for Cd, Mo, Se and Hg in units of mg kg-1.

Percent of sludge mass
Percent of sludge mass
100
90
80
70
60
50
40
30
20
10
0
0 2 4 6 8
mg kg
10 12 14 16
-1
Copper Cobalt Chromium Lead
Figure 7. Mass percentage distribution of the sludge in terms of the leachable (TCLP) metal
concentration for Cu, Co, Cr and Pb in units of mg kg-1.
100
90
80
70
60
50
40
30
20
10
0
0 5 10 15 20 25
mg kg -1
Zinc Nickel
Figure 8. Mass percentage distribution of the sludge in terms of the leachable (TCLP) metal
concentration for Zn and Ni in units of mg kg-1.
The Compliance of Sewage Sludge to the Total Limits
Figure 9 represents the mass percentage of the surveyed sludge that exceeded the metal
concentration for type D sludge (Table 3) as specified in the Sludge Guidelines (3). The total mass
of sludge was considered (analysed using the aqua regia total extraction method) irrespective of its
classification. In other words, type A, B, C and D sludge was considered in this analysis. Fifty-one
percent (51%) and 44 % of the total mass of sludge surveyed exceeded the Ni and Zn
concentration limits respectively. The survey included the analysis of As, however the analytical
methods used exaggerated the As concentration. This exaggeration was a function of the ICP-MS
and these results was therefore omitted from both the total and leachable results.

Percentage of total sludge analysed
60
50
40
30
20
10
0
9.1
6.0
2.3
23.4
7.6
1.9
Cd Co Cr Cu Hg Mo Ni Pb Zn Se B F
Metal
Figure 9. Mass percentage of the surveyed sludge exceeding the guidelines for a type D sludge based on
the total aqua regia concentrations.
Figure 10 represents the total mass of sludge analysed in this project. The graph specifies the
mass percentage of the total sludge that exceeds a number of metals according to the Sludge
Guideline (2) and Addendum (3). The metals that were considered for the interpretation of this
graph were Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Zn, Se, B, and F. The mass percentage of sludge
surveyed that exceeds with one metal is mainly due to Ni, Zn, B, and Hg. No sludge surveyed
exceeded more than 9 metals. Seven percent (7 %) of the total mass of the sludge analysed
exceeded the limits for more than six metals (3 % exceeded six metals; 2 % exceeded seven
metals; 0 % exceeded exactly eight metals and 2 % exceeded nine metals).
4 metals
4%
3 metals
5%
5 metals
10%
6 metals
3%
7 metals
2%
2 metals
23%
8 metals
0%
50.8
1 metal
12%
9 metals
2%
11.0
0 metals
40%
Figure 10. Mass percentage of the surveyed sludge exceeding the guidelines for a type D sludge based on
the total aqua regia concentrations and detailing the number of metals that exceed.
43.5
10.7
18.8
0.0

CONCLUSIONS
Seventy-two (72) wastewater treatment plants were selected to represent different sludge types
and sludge handling technologies in South Africa. Sludge samples (78) were collected at these
selected wastewater treatment plants. These samples were analysed for Cd, Co, Cr, Cu, Hg, Mo,
Ni, Pb, Zn, Se, B and F, using three extraction methods. The analytical data was used to classify
the sludge and to determine the impact of the existing guidelines on the disposal options.
The efficiency of the extraction methods, aqua regia (total), EPA 3050 (total), and TCLP
(leachable) were compared. The aqua regia extraction method was found to give more reliable
results compared to the EPA 3050 extraction.
The average metal concentration of the sludge from the surveyed plants receiving industrial
effluent (50%) was compared to the plants that do not receive industrial effluent. As expected, the
metal concentration (aqua regia total extraction) of sludge from the plants receiving industrial
effluent was higher or similar to the average metal concentration from sludge plants receiving
domestic effluent for all the metals with the exception of B.
The compliance to the Sludge Guideline and related Addendum of the sewage sludge surveyed in
this study was evaluated in terms of the leachable and total extractable metal content. None of the
plants surveyed exceeded the leachable (TCLP) extractable concentration limits as specified in the
Addendum of the 1997 Sludge Guidelines. The results from the total metal extractions showed a
different picture. The major metals of concern are Ni and Zn. Sixty-one percent (61%) and 44% of
the total mass of sludge surveyed exceeded the Ni and Zn concentration limits respectively. A
major fraction (40%) of the sludge surveyed did not exceed any of the total metal limits. Thirty-five
percent (35% mass basis) of the sludge surveyed exceeded on one or two metals. Only 4 % of the
sludge surveyed, exceeded the limits for more than seven of the twelve metals analysed for.
ACKNOWLEDGEMENTS
The authors wish to acknowledge and thank the Water Research Commission for funding the
research. The authors would also like to thank the East Rand Water Care Company (ERWAT) in
particular, Mr JW Wilken (ERWAT) and the staff at ERLAB for their contributions as well as the
Agricultural Research Commission (ARC) Institute for Soil Climate and Water (ISCW) specifically
Mr A Loock.
REFERENCES
1. R. Smith and H Vasiloudis, Inorganic Chemical Characterisation of South African Municipal Sewage
Sludges. WRC Report 180/1/89 (1989).
2. Permissible Utilisation and Disposal of Sewage Sludge. 1 st Edition. TT85-97, WRC (1997).
3. Addendum No, 1 to Edition 1 (1997) of Permissible Utilisation and Disposal of Sewage Sludge. TT
150/01 WRC (2002).
4. Standard Methods for the Examination of Water and Wastewater, 19 th Edition, pub Water
Environment Federation, USA (ed. M.A.F. Franson, A.D. Eaton, L.S. Clesceri, and A.E. Greenberg)
(1995).
5. Methods of Soil Analysis. Part 3: Chemical Methods, pub Soil Science Society of America and
American Society of Agronomy, USA (ed. J.M. Bigham and J.M. Bartels)(1996).
6. Test Methods for Evaluating Solid Waste, Physical/Chemical Methods (SW-846): Method 3050 Acid
Digestion of Sediments, Sludges and Soils, pub USA EPA, (1986).
7. DWAF, Waste Management Series, Minimum Requirements for the Handling, Classification and
Disposal of Hazardous Waste (1998).