Snyman, Heidi G. et al - eWISA
Snyman, Heidi G. et al - eWISA
Snyman, Heidi G. et al - eWISA
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METAL CONTENT OF SOUTH AFRICAN SEWAGE SLUDGE<br />
<strong>Heidi</strong> G. <strong>Snyman</strong> 1 , J.E. Herselman 2 and G Kasselman 3<br />
1 Corresponding Author: Golder Associates Africa, PO Box 6001, H<strong>al</strong>fway House, 1685, South Africa.<br />
Tel: (011) 254 4915. Fax: (011) 315 0317. E-mail: hsnyman@golder.co.za.<br />
2 Institute of Soil, Climate and Water, ARC, Private Bag X79, Pr<strong>et</strong>oria 0002, South Africa.<br />
3 ERWAT Chair in Wastewater Management, Water Utilisation Division, Department of Chemic<strong>al</strong> Engineering,<br />
University of Pr<strong>et</strong>oria, Pr<strong>et</strong>oria 0002, South Africa.<br />
ABSTRACT<br />
A survey was done to d<strong>et</strong>ermine the potenti<strong>al</strong>ly toxic m<strong>et</strong><strong>al</strong>s and element (m<strong>et</strong><strong>al</strong>) qu<strong>al</strong>ity of sewage<br />
sludge in South Africa. Sludge samples (78) were collected and an<strong>al</strong>ysed for Cd, Co, Cr, Cu, Hg,<br />
Mo, Ni, Pb, Zn, Se, B and F, using three extraction m<strong>et</strong>hods. The efficiency of the extraction<br />
m<strong>et</strong>hods, aqua regia (tot<strong>al</strong> extraction), EPA 3050 (tot<strong>al</strong> extraction), and TCLP (leachable) were<br />
compared. The aqua regia extraction m<strong>et</strong>hod was found to give more reliable results compared to<br />
the EPA 3050 extraction.<br />
The m<strong>et</strong><strong>al</strong> content of sludge from the plants receiving industri<strong>al</strong> effluent was higher or similar to the<br />
m<strong>et</strong><strong>al</strong> content of sludge from plants receiving domestic effluent for <strong>al</strong>l the m<strong>et</strong><strong>al</strong>s with the exception<br />
of B.<br />
The sewage sludge surveyed was ev<strong>al</strong>uated in terms of leachable and tot<strong>al</strong> extractable m<strong>et</strong><strong>al</strong><br />
content with respect to compliance to the Sludge Guidelines and related Addendum. None of the<br />
plants surveyed exceeded the TCLP (leachable) extractable concentration limits as specified in the<br />
Addendum of the Sludge Guidelines. The results from the tot<strong>al</strong> m<strong>et</strong><strong>al</strong> extractions showed a<br />
different picture. The major m<strong>et</strong><strong>al</strong>s of concern are Ni and Zn. Sixty-one percent (61 %) and 44 % of<br />
the tot<strong>al</strong> mass of sludge surveyed exceeded the Ni and Zn concentration limits respectively. A<br />
major fraction (40 % mass basis) of the sludge surveyed did not exceed any of the tot<strong>al</strong> m<strong>et</strong><strong>al</strong><br />
limits. Thirty-five percent (35 % mass basis) of the sludge surveyed exceeded on one or two<br />
m<strong>et</strong><strong>al</strong>s. Only 4% of the sludge surveyed, exceeded the limits for more than seven of the twelve<br />
m<strong>et</strong><strong>al</strong>s an<strong>al</strong>ysed for.<br />
INTRODUCTION<br />
The concentration of potenti<strong>al</strong>ly toxic m<strong>et</strong><strong>al</strong>s and elements (hereafter referred to as “m<strong>et</strong><strong>al</strong>s”) of<br />
South African sewage sludge was d<strong>et</strong>ermined in 1989 by Smith and Vasiloudis (1) who surveyed<br />
77 wastewater treatment plants. Smith and Vasiloudis (1) used tot<strong>al</strong> extraction m<strong>et</strong>hods to<br />
d<strong>et</strong>ermine the m<strong>et</strong><strong>al</strong> content of sewage sludge. However, the South African Sludge Guidelines<br />
have been revised since this survey was done. It now includes the requirement that sludge<br />
samples should be extracted using both a tot<strong>al</strong> and an available (leachable) extraction m<strong>et</strong>hod.<br />
This is d<strong>et</strong>ailed in the document entitled: “Permissible utilisation and dispos<strong>al</strong> of sewage sludge,<br />
Edition 1” published in 1997 (2) and the related addendum (Addendum No 1 to Edition 1 (1997) of<br />
the Permissible utilisation and dispos<strong>al</strong> of sewage sludge in 2002) (3) that advises use of both the<br />
aqua regia extraction m<strong>et</strong>hod (tot<strong>al</strong> extraction m<strong>et</strong>hod) and the Toxicity Characteristic Leaching<br />
Procedure (TCLP) (leachable extraction m<strong>et</strong>hod) to characterise the m<strong>et</strong><strong>al</strong>s in sewage sludge. The<br />
aqua regia and EPA 3050 extraction m<strong>et</strong>hods are commonly referred to as tot<strong>al</strong> extraction m<strong>et</strong>hods<br />
<strong>al</strong>though they are not an accurate reflection of the tot<strong>al</strong> extractable m<strong>et</strong><strong>al</strong>s in a sample. More<br />
aggressive extraction m<strong>et</strong>hods exist for this purpose.<br />
Proceedings of the 2004 Water Institute of Southern Africa (WISA) Bienni<strong>al</strong> Conference 2 –6 May 2004<br />
ISBN: 1-920-01728-3 Cape Town, South Africa<br />
Produced by: Document Transformation Technologies Organised by Event Dynamics
The TCLP was developed to measure a wastes’ leachability and hence the risk to pollute<br />
groundwater. It is typic<strong>al</strong>ly used when a waste is to be landfilled in a site that receives a vari<strong>et</strong>y of<br />
organic and inorganic wastes. The procedure aims to simulate the dissolving action of the organic<br />
acid leachate formed in a landfill where hazardous waste has been co-disposed with gener<strong>al</strong><br />
waste. It can be used to d<strong>et</strong>ermine the mobility of organics and inorganics in liquid, solid, and<br />
multiphase wastes. The TCLP has historic<strong>al</strong>ly not been used by the wastewater industry to assess<br />
the amount of sludge to be safely used in agricultur<strong>al</strong> practices. Most wastewater laboratories have<br />
used the aqua regia (tot<strong>al</strong>) extraction, which gives an indication of the acid soluble fraction.<br />
Therefore, limited data is available for applying the TCLP to sludge samples. In addition, the m<strong>et</strong><strong>al</strong><br />
limits in the Sludge Guidelines are still being debated, particularly the an<strong>al</strong>ytic<strong>al</strong> m<strong>et</strong>hods for<br />
d<strong>et</strong>ermining m<strong>et</strong><strong>al</strong>s in sludge. These issues are currently being addressed in the development of<br />
the “Permissible utilisation and dispos<strong>al</strong> of sewage sludge, Edition 2”. In the interim, the<br />
explanatory addendum (3) stipulates that sludge should be extracted with both the TCLP<br />
(leachable) and aqua regia (tot<strong>al</strong>) extraction m<strong>et</strong>hods. In this study, the extraction procedure<br />
included both the TCLP and aqua regia extraction m<strong>et</strong>hods. This data was used to d<strong>et</strong>ermine the<br />
extent of compliance to the current Sludge Guidelines (2 and 3).<br />
MATERIALS AND METHODS<br />
Two hundred South African wastewater treatment plants (WWTPs) were initi<strong>al</strong>ly pre-selected and<br />
interviewed telephonic<strong>al</strong>ly. A sample of 72 plants was selected from the initi<strong>al</strong> 200 plants based on<br />
a questionnaire used to gather background information with emphasis on the treatment m<strong>et</strong>hods<br />
and the type of sludge produced. To achieve this some sludge samples did not represent the end<br />
product. For example, in many plants, activated sludge is blended with other sludge types to form<br />
the end product. In such a case, a decision had to be made to either sample the blend or the<br />
activated sludge.<br />
Sludge samples (78 – more than one sample was collected from some plants) were collected at<br />
the selected wastewater treatment plants. Three rounds of sampling was done to include season<strong>al</strong><br />
variation i.e. winter, summer and autumn. These samples were an<strong>al</strong>ysed for microbiologic<strong>al</strong>,<br />
nutrient and selected m<strong>et</strong><strong>al</strong> concentrations. This paper will focuses on the results of the m<strong>et</strong><strong>al</strong><br />
an<strong>al</strong>ysis only.<br />
Two different South African laboratories were selected for the an<strong>al</strong>yses: The East Rand Water<br />
Care Company Laboratory (ERLAB) and the Agricultur<strong>al</strong> Research Councils (ARC) Institute for<br />
Soil, Climate and Water (ISCW) laboratory. No accredited laboratory for the an<strong>al</strong>ysis of sludge<br />
could be found at that stage. For this reason, an inter-laboratory extraction and an<strong>al</strong>ysis train was<br />
s<strong>et</strong> up. This was done to utilise the expertise of both laboratories to the optimum. Samples were<br />
collected and transported to the laboratories with a maximum delay of 72 hours. A subsample was<br />
used to an<strong>al</strong>yse for pH and moisture content (4). The rest of the sample was dried at 50 0 C for<br />
seven days and crushed to pass through a 1 mm sieve. The samples were extracted using the<br />
following extraction m<strong>et</strong>hods: (a) Aqua regia extraction (5); (b) EPA 3050 extraction (6) and (c)<br />
TCLP (DWAF 1998). The F was extracted using the m<strong>et</strong>hod described by (5). These extracts were<br />
an<strong>al</strong>ysed for Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Zn, As, Se and B using a VG Plasmaquad PQ2 Turbo<br />
Plus Inductively Coupled Plasma Spectrom<strong>et</strong>er (ICP-MS).<br />
The an<strong>al</strong>ytic<strong>al</strong> data was used to classify the sludge and to d<strong>et</strong>ermine the impact of the existing<br />
Guidelines on the dispos<strong>al</strong> options.<br />
The m<strong>et</strong><strong>al</strong> data was interpr<strong>et</strong>ed to reflect the contribution of the plants to m<strong>et</strong><strong>al</strong> load of the tot<strong>al</strong><br />
mass of sludge an<strong>al</strong>ysed in this study. In other words, as large plants contribute more sludge than<br />
sm<strong>al</strong>l plants, it was necessary to express the an<strong>al</strong>ytic<strong>al</strong> data on a mass basis to obtain a clear<br />
overview of trends in the country. Some of the wastewater treatment plants could not supply the<br />
mass of sludge produced per day. For these plants, the mass of sludge was c<strong>al</strong>culated using the<br />
inflow volume and inflow COD.
RESULTS AND DISCUSSION<br />
Figure 1 gives a geographic<strong>al</strong> map of the plants selected. The plants represented different<br />
treatment m<strong>et</strong>hods geographic<strong>al</strong>ly spread throughout South Africa.<br />
Figure 1. Geographic<strong>al</strong> representation of selected plants categorised into the treatment m<strong>et</strong>hod.<br />
Anaerobic Digestion<br />
57%<br />
Activated Sludge<br />
20%<br />
Blended sludge<br />
12%<br />
P<strong>et</strong>ro sludge<br />
2%<br />
Oxidation Dams<br />
0.3%<br />
Aerobic digestion<br />
1%<br />
Figure 2. Sludge types produced by the wastewater treatment plants surveyed in South Africa on a mass<br />
percent basis. The blended sludge represents primary and activated or humus sludge blended before or after<br />
digestion.
Figure 2 indicates the types of sludge generated by the wastewater treatment plants surveyed. The<br />
majority of the sludge produced (57% on a mass basis) employs anaerobic digestion of primary<br />
and humus sludge.<br />
M<strong>et</strong><strong>al</strong> Content<br />
The weighted mean v<strong>al</strong>ues for the m<strong>et</strong><strong>al</strong>s an<strong>al</strong>ysed for, with respect to the extraction m<strong>et</strong>hod used,<br />
are included in Table 1.<br />
Table 1. Weighted mean* m<strong>et</strong><strong>al</strong> concentrations for <strong>al</strong>l the plants surveyed with respect to the<br />
extraction m<strong>et</strong>hod.<br />
Aqua regia<br />
(mg kg -1 )<br />
EPA 3050<br />
(mg kg -1 )<br />
TCLP<br />
(mg kg -1 )<br />
TCLP<br />
(mg L -1 M<strong>et</strong><strong>al</strong><br />
)<br />
Tot<strong>al</strong> Extraction Leachable Extraction<br />
Cd 10 6 0.03 0.0016<br />
Co 32 21 0.16 0.0079<br />
Cr 414 282 1.02 0.0512<br />
Cu 576 383 0.43 0.0217<br />
Hg 4 3 0.07 0.0035<br />
Mo 9 8 0.03 0.0016<br />
Ni 269 93 0.91 0.0456<br />
Pb 318 190 0.08 0.0042<br />
Zn 3159 1673 7.15 0.3573<br />
Se 11 6 0.07 0.0033<br />
B 57 29 0.84 0.0419<br />
* The v<strong>al</strong>ues were c<strong>al</strong>culated by d<strong>et</strong>ermining the tot<strong>al</strong> mass of each m<strong>et</strong><strong>al</strong> in the sludge for each plant<br />
based on the daily sludge production. These v<strong>al</strong>ues were then added tog<strong>et</strong>her and divided by the<br />
tot<strong>al</strong> mass of sludge produced by <strong>al</strong>l the plants.<br />
Table 1 shows that the average m<strong>et</strong><strong>al</strong> concentration v<strong>al</strong>ues for the aqua regia extraction m<strong>et</strong>hod is<br />
larger than the EPA 3050 extraction m<strong>et</strong>hod. Since the two procedures both aim to d<strong>et</strong>ermine a<br />
tot<strong>al</strong> m<strong>et</strong><strong>al</strong> concentration the higher v<strong>al</strong>ue was chosen as a precautionary principle. Furthermore<br />
the aqua regia extraction data proved to be more reliable compared to the EPA 3050 extraction<br />
m<strong>et</strong>hod. All further data interpr<strong>et</strong>ation was therefore done using the data generated from the aqua<br />
regia extracted samples.<br />
Table 2. Weighted mean* tot<strong>al</strong> (aqua regia) m<strong>et</strong><strong>al</strong> concentrations in mg kg-1 for the sludge<br />
samples collected in this survey categorised according to the inflow.<br />
M<strong>et</strong><strong>al</strong><br />
< 10<br />
ML day –1<br />
10 to 25<br />
ML day -1<br />
25 – 80<br />
ML day -1<br />
> 80<br />
ML day -1<br />
Cd 6 6 11 12<br />
Co 14 16 42 37<br />
Cr 375 359 342 504<br />
Cu 469 526 648 583<br />
Hg 6 6 3 3<br />
Mo 7 7 8 11<br />
Ni 190 205 363 261<br />
Pb 190 208 270 446<br />
Zn 2956 2758 3436 3243<br />
Se 10 9 14 11<br />
B 42 55 54 65<br />
F<br />
Mass percentage<br />
157 139 135 101<br />
of tot<strong>al</strong> sludge in<br />
each group<br />
11 22 27 40<br />
* The v<strong>al</strong>ues were c<strong>al</strong>culated by d<strong>et</strong>ermining the tot<strong>al</strong> mass of each m<strong>et</strong><strong>al</strong> in the sludge for each plant<br />
based on the daily sludge production. These v<strong>al</strong>ues were then added tog<strong>et</strong>her and divided by the tot<strong>al</strong><br />
mass of sludge produced by <strong>al</strong>l the plants in one day.
In the study the TCLP fraction was an<strong>al</strong>ysed for different m<strong>et</strong><strong>al</strong>s to establish compliance with the<br />
Sludge Guidelines (3). The TCLP was designed to predict the concentration of a m<strong>et</strong><strong>al</strong> in the<br />
leachate expressed in mg L -1 . However, the maximum leachable concentrations stipulated in the<br />
Sludge Guidelines are expressed in mg kg -1 (3). For this reason both concentrations are included<br />
in Table 1.<br />
Table 2 d<strong>et</strong>ails the weighted mean m<strong>et</strong><strong>al</strong> concentration of the surveyed wastewater treatment<br />
plants’ sludge categorised according to the inflow received. This will <strong>al</strong>low wastewater plants that<br />
were not included in this study to benchmark their m<strong>et</strong><strong>al</strong> v<strong>al</strong>ues.<br />
Influence of Industri<strong>al</strong> Effluents on Sludge M<strong>et</strong><strong>al</strong> Qu<strong>al</strong>ity<br />
The sampling sites were <strong>al</strong>so sorted into two groups; plants receiving mainly domestic effluent (and<br />
industri<strong>al</strong> effluent that should not affect the m<strong>et</strong><strong>al</strong> concentration) and plants receiving industri<strong>al</strong><br />
effluent that could affect the m<strong>et</strong><strong>al</strong> concentration. For example, if a plant receives effluent from a<br />
candy factory, it was considered a domestic plant. On the other hand, if a plant receives m<strong>et</strong><strong>al</strong><br />
plating effluent, the effluent will impact on the sludge m<strong>et</strong><strong>al</strong> qu<strong>al</strong>ity and was therefore categorised<br />
as an industri<strong>al</strong> plant. A distinction was only made with respect to m<strong>et</strong><strong>al</strong> concentration; we<br />
assumed that a wastewater treatment plant that receives industri<strong>al</strong> waste should contain more<br />
m<strong>et</strong><strong>al</strong>s than a domestic plant. Thirty-nine samples were categorised as industri<strong>al</strong> and thirty-nine as<br />
domestic. The weighted mean m<strong>et</strong><strong>al</strong> concentration for the results from this classification is shown<br />
in Figures 3 to 5.<br />
Concentration [mg kg -1 ]<br />
4500<br />
4000<br />
3500<br />
3000<br />
2500<br />
2000<br />
1500<br />
1000<br />
500<br />
0<br />
Zn<br />
M<strong>et</strong><strong>al</strong><br />
Industri<strong>al</strong> Tot<strong>al</strong> Domestic<br />
Figure 3. The weighted mean Zn concentration for industri<strong>al</strong> and domestic plants. Tot<strong>al</strong> refers to<br />
<strong>al</strong>l the plants (domestic and industri<strong>al</strong>).<br />
As expected, the Zn concentration (aqua regia; tot<strong>al</strong> extraction) for sludge generated from plants<br />
that receive industri<strong>al</strong> effluent that could affect the m<strong>et</strong><strong>al</strong> concentration of the sludge is higher than<br />
plants receiving mainly domestic effluent (Figure 3). The same trend was observed for the other<br />
m<strong>et</strong><strong>al</strong>s with the exception of B (Figure 5). This indicates that B may not be from industri<strong>al</strong> origin.
Concentration [mg kg -1 ]<br />
Concentration [mg kg -1 ]<br />
800<br />
700<br />
600<br />
500<br />
400<br />
300<br />
200<br />
100<br />
0<br />
Cr Cu Ni Pb<br />
M<strong>et</strong><strong>al</strong><br />
Industri<strong>al</strong> Tot<strong>al</strong> Domestic<br />
Figure 4. The weighted mean m<strong>et</strong><strong>al</strong> concentrations for industri<strong>al</strong> and domestic plants for Cr, Cu,<br />
Ni and Pb. Tot<strong>al</strong> refers to <strong>al</strong>l the plants (domestic and industri<strong>al</strong>).<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
Cd Co Hg Mo Se B<br />
M<strong>et</strong><strong>al</strong><br />
Industri<strong>al</strong> Tot<strong>al</strong> Domestic<br />
Figure 5. The weighted mean m<strong>et</strong><strong>al</strong> concentrations for industri<strong>al</strong> and domestic plants for Cd, Co, Hg,<br />
Mo, Se and B. Tot<strong>al</strong> refers to <strong>al</strong>l the plants (domestic and industri<strong>al</strong>).<br />
M<strong>et</strong><strong>al</strong> Limits<br />
Table 3 shows the <strong>al</strong>lowable m<strong>et</strong><strong>al</strong> limits for a type D sludge according to the Permissible utilisation<br />
and dispos<strong>al</strong> of sewage sludge, Edition 1 (2) and the related addendum (3). The data from the<br />
survey will be compared to the limits d<strong>et</strong>ailed in Table 3 in the next two sections.
Table 3. Allowable m<strong>et</strong><strong>al</strong> content in a Type D sludge according to the Permissible utilisation<br />
and dispos<strong>al</strong> of sewage sludge, Edition 1 and the related addendum.<br />
M<strong>et</strong><strong>al</strong><br />
Leachable limit<br />
(mg kg -1 )<br />
Tot<strong>al</strong> Limit<br />
(mg kg -1 )<br />
Cd 15.7 20<br />
Co - 100<br />
Cr - 1 750<br />
Cu 50,5 750<br />
Hg - 10<br />
Mo - 25<br />
Ni - 200<br />
Pb 50,5 400<br />
Zn 353,5 2 750<br />
As - 15<br />
Se - 15<br />
B - 80<br />
F - 400<br />
The Compliance of Sewage Sludge to the Leachable Limits<br />
The leachable concentration i.e. TCLP data was converted from mg L -1 to mg kg -1 to compare the<br />
data with the limit v<strong>al</strong>ues in the Sludge Guidelines (3). Figures 6, 7 and 8 represent the TCLP<br />
(leachable) fraction for the wastewater treatment plants in the study. Each line on the graph refers<br />
to a specific m<strong>et</strong><strong>al</strong>. They are cumulative percentage graphs and are interpr<strong>et</strong>ed as follows.<br />
Consider Mo at 0.4 mg kg -1 in Figure 6. It reads off at approximately 7 % on the y-axis. This means<br />
that approximately 7 % of the tot<strong>al</strong> mass of sludge an<strong>al</strong>ysed for has a concentration of 0.4 mg kg -1<br />
or larger.<br />
All the sludge surveyed complied with the TCLP leachable limits (Table 3) for the m<strong>et</strong><strong>al</strong>s as<br />
stipulated in the Sludge Guidelines (3).<br />
Percent of sludge mass<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 0.2 0.4 0.6 0.8 1<br />
mg kg<br />
1.2 1.4 1.6 1.8 2<br />
-1<br />
Cadmium Molybdenum Selenium Mercury<br />
Figure 6. Mass percentage distribution of the sludge in terms of the leachable (TCLP) m<strong>et</strong><strong>al</strong><br />
concentration for Cd, Mo, Se and Hg in units of mg kg-1.
Percent of sludge mass<br />
Percent of sludge mass<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 2 4 6 8<br />
mg kg<br />
10 12 14 16<br />
-1<br />
Copper Cob<strong>al</strong>t Chromium Lead<br />
Figure 7. Mass percentage distribution of the sludge in terms of the leachable (TCLP) m<strong>et</strong><strong>al</strong><br />
concentration for Cu, Co, Cr and Pb in units of mg kg-1.<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 5 10 15 20 25<br />
mg kg -1<br />
Zinc Nickel<br />
Figure 8. Mass percentage distribution of the sludge in terms of the leachable (TCLP) m<strong>et</strong><strong>al</strong><br />
concentration for Zn and Ni in units of mg kg-1.<br />
The Compliance of Sewage Sludge to the Tot<strong>al</strong> Limits<br />
Figure 9 represents the mass percentage of the surveyed sludge that exceeded the m<strong>et</strong><strong>al</strong><br />
concentration for type D sludge (Table 3) as specified in the Sludge Guidelines (3). The tot<strong>al</strong> mass<br />
of sludge was considered (an<strong>al</strong>ysed using the aqua regia tot<strong>al</strong> extraction m<strong>et</strong>hod) irrespective of its<br />
classification. In other words, type A, B, C and D sludge was considered in this an<strong>al</strong>ysis. Fifty-one<br />
percent (51%) and 44 % of the tot<strong>al</strong> mass of sludge surveyed exceeded the Ni and Zn<br />
concentration limits respectively. The survey included the an<strong>al</strong>ysis of As, however the an<strong>al</strong>ytic<strong>al</strong><br />
m<strong>et</strong>hods used exaggerated the As concentration. This exaggeration was a function of the ICP-MS<br />
and these results was therefore omitted from both the tot<strong>al</strong> and leachable results.
Percentage of tot<strong>al</strong> sludge an<strong>al</strong>ysed<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
9.1<br />
6.0<br />
2.3<br />
23.4<br />
7.6<br />
1.9<br />
Cd Co Cr Cu Hg Mo Ni Pb Zn Se B F<br />
M<strong>et</strong><strong>al</strong><br />
Figure 9. Mass percentage of the surveyed sludge exceeding the guidelines for a type D sludge based on<br />
the tot<strong>al</strong> aqua regia concentrations.<br />
Figure 10 represents the tot<strong>al</strong> mass of sludge an<strong>al</strong>ysed in this project. The graph specifies the<br />
mass percentage of the tot<strong>al</strong> sludge that exceeds a number of m<strong>et</strong><strong>al</strong>s according to the Sludge<br />
Guideline (2) and Addendum (3). The m<strong>et</strong><strong>al</strong>s that were considered for the interpr<strong>et</strong>ation of this<br />
graph were Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Zn, Se, B, and F. The mass percentage of sludge<br />
surveyed that exceeds with one m<strong>et</strong><strong>al</strong> is mainly due to Ni, Zn, B, and Hg. No sludge surveyed<br />
exceeded more than 9 m<strong>et</strong><strong>al</strong>s. Seven percent (7 %) of the tot<strong>al</strong> mass of the sludge an<strong>al</strong>ysed<br />
exceeded the limits for more than six m<strong>et</strong><strong>al</strong>s (3 % exceeded six m<strong>et</strong><strong>al</strong>s; 2 % exceeded seven<br />
m<strong>et</strong><strong>al</strong>s; 0 % exceeded exactly eight m<strong>et</strong><strong>al</strong>s and 2 % exceeded nine m<strong>et</strong><strong>al</strong>s).<br />
4 m<strong>et</strong><strong>al</strong>s<br />
4%<br />
3 m<strong>et</strong><strong>al</strong>s<br />
5%<br />
5 m<strong>et</strong><strong>al</strong>s<br />
10%<br />
6 m<strong>et</strong><strong>al</strong>s<br />
3%<br />
7 m<strong>et</strong><strong>al</strong>s<br />
2%<br />
2 m<strong>et</strong><strong>al</strong>s<br />
23%<br />
8 m<strong>et</strong><strong>al</strong>s<br />
0%<br />
50.8<br />
1 m<strong>et</strong><strong>al</strong><br />
12%<br />
9 m<strong>et</strong><strong>al</strong>s<br />
2%<br />
11.0<br />
0 m<strong>et</strong><strong>al</strong>s<br />
40%<br />
Figure 10. Mass percentage of the surveyed sludge exceeding the guidelines for a type D sludge based on<br />
the tot<strong>al</strong> aqua regia concentrations and d<strong>et</strong>ailing the number of m<strong>et</strong><strong>al</strong>s that exceed.<br />
43.5<br />
10.7<br />
18.8<br />
0.0
CONCLUSIONS<br />
Seventy-two (72) wastewater treatment plants were selected to represent different sludge types<br />
and sludge handling technologies in South Africa. Sludge samples (78) were collected at these<br />
selected wastewater treatment plants. These samples were an<strong>al</strong>ysed for Cd, Co, Cr, Cu, Hg, Mo,<br />
Ni, Pb, Zn, Se, B and F, using three extraction m<strong>et</strong>hods. The an<strong>al</strong>ytic<strong>al</strong> data was used to classify<br />
the sludge and to d<strong>et</strong>ermine the impact of the existing guidelines on the dispos<strong>al</strong> options.<br />
The efficiency of the extraction m<strong>et</strong>hods, aqua regia (tot<strong>al</strong>), EPA 3050 (tot<strong>al</strong>), and TCLP<br />
(leachable) were compared. The aqua regia extraction m<strong>et</strong>hod was found to give more reliable<br />
results compared to the EPA 3050 extraction.<br />
The average m<strong>et</strong><strong>al</strong> concentration of the sludge from the surveyed plants receiving industri<strong>al</strong><br />
effluent (50%) was compared to the plants that do not receive industri<strong>al</strong> effluent. As expected, the<br />
m<strong>et</strong><strong>al</strong> concentration (aqua regia tot<strong>al</strong> extraction) of sludge from the plants receiving industri<strong>al</strong><br />
effluent was higher or similar to the average m<strong>et</strong><strong>al</strong> concentration from sludge plants receiving<br />
domestic effluent for <strong>al</strong>l the m<strong>et</strong><strong>al</strong>s with the exception of B.<br />
The compliance to the Sludge Guideline and related Addendum of the sewage sludge surveyed in<br />
this study was ev<strong>al</strong>uated in terms of the leachable and tot<strong>al</strong> extractable m<strong>et</strong><strong>al</strong> content. None of the<br />
plants surveyed exceeded the leachable (TCLP) extractable concentration limits as specified in the<br />
Addendum of the 1997 Sludge Guidelines. The results from the tot<strong>al</strong> m<strong>et</strong><strong>al</strong> extractions showed a<br />
different picture. The major m<strong>et</strong><strong>al</strong>s of concern are Ni and Zn. Sixty-one percent (61%) and 44% of<br />
the tot<strong>al</strong> mass of sludge surveyed exceeded the Ni and Zn concentration limits respectively. A<br />
major fraction (40%) of the sludge surveyed did not exceed any of the tot<strong>al</strong> m<strong>et</strong><strong>al</strong> limits. Thirty-five<br />
percent (35% mass basis) of the sludge surveyed exceeded on one or two m<strong>et</strong><strong>al</strong>s. Only 4 % of the<br />
sludge surveyed, exceeded the limits for more than seven of the twelve m<strong>et</strong><strong>al</strong>s an<strong>al</strong>ysed for.<br />
ACKNOWLEDGEMENTS<br />
The authors wish to acknowledge and thank the Water Research Commission for funding the<br />
research. The authors would <strong>al</strong>so like to thank the East Rand Water Care Company (ERWAT) in<br />
particular, Mr JW Wilken (ERWAT) and the staff at ERLAB for their contributions as well as the<br />
Agricultur<strong>al</strong> Research Commission (ARC) Institute for Soil Climate and Water (ISCW) specific<strong>al</strong>ly<br />
Mr A Loock.<br />
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