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Sludge is produced as a bypass product of a water treatment process. The sludge is commonly disposed on land, but this result as a health hazards and adverse effects on the ecosystem. All metals above certain concentration have side effects to human being. Among the most hazardous heavy metals are Cardium, Lead, Mercury and hexavalent Chromiun, the first three being toxic at all concentration and have not a known biological function (Moeletsi et al, 2004). When consumed by human, these heavy metals can cause brain and bone damage and as well as kidney and liver disorder (Moeletsi et al, 2004). Although there are metals that are essential to plants and animals under low concentration consumptions(As, Cr, Cu, Co, Fe, I, Ni, Zn, V, Mn and F) (Moeletsi et al, 2004).

Sludge disposal issues

The heavy metals concentration mostly depends on the industrial waste matter contribution to the waste water system. The wastewater from domestic sources has usually low concentration of some of the heavy metals (Richard et al, 2004). High concentration of heavy metals in sludge is of high concern where the sludge is being applied in an agricultural crop, thus being consumed by human body. Another concern is if the soil uptake and accumulates this heavy metals to a phototoxic level and their penetration to ground water (Halday, 2007). A concern is also adhering to the environmental legislation and limitation of heavy metals concentration in sludge when disposed in land and crops

In this study the sludge from two sources, industrial and residential will be analysed to see what is in the sludge and how much of it is in the sludge. Particularly the analysis is for heavy metal such as Manganese, Zinc, Iron, Copper, Lead, Nickle, Chromium, Cadmium , and Mercury. The analysis will be carried through with analytical techniques such as AA and SEM-EDAX.

Atomic absorption spectrophotometer (AA) analysis for metal content and SEM-EDAX images analyse mineral composition, shape and size of the mineral.

Current treatment of the sludge

Sludge is formed from waste water healing process where solids are being separated from liquids. The content of solids in the treatment plant varies for different plants and how the plant is operated. Effective treatment of sludge needs a crucial knowledge of the characteristics of sludge being processed. The sludge from domestic outputs is from the Cape flats wastewater treatment plant and sludge from industrial output is form Athlone wastewater treatment plant. Not much detail is given about the physical procedure that is followed to further treat the sludge at the plants as this information is not disclosed(company property). However the anaerobic digested sludge can not be utilized for farming purpose, for the reason that it contains high concentration of chromium.

Current utilization of the sludge

The sea is the final receiver of all effluents with the near shore waters of False Bay getting about 57% and Table Bay getting 29% of the municipal waste matter discharges. A lot of these localised pollution are nearby to admired entertaining places of which water quality is a most important concern to the health of the people. Industrial influent recycles about 60% of its influents, and it cotains a fear amount of metals in it (Gasson, 2002).

Approximately 27% of wet sludge is reused in municipality uses and composting. The use of land filling being the favored dumping system without doubt puts more interest on the state of the environment. Number of damping sites (about six) exists in the Cape flats area, and this leads to high probability of ground water contamination. As a minimum the Swartklip damp site is closed because of leach ate toxic waste to groundwater. A great deal of the sewage sludge it is not discharged in an environ-mentally accountable way, where this increases the concern with regard to the quality of ground water (Gasson, 2002).

Table 1, waste source output (tons/yr), and total recycling, 1998 from Gasson, 2002







Tons/yr by income group

Tons/yr recycled or reused

Recycling as % of total recycled


895 900 44%

Informal 0,35

102 386

41 000 compost


Low-mid 0,7

300 547

High inc. 1,3

492 967


499 900 24%


293 300

186 700 metals

93 600 paper

13 000 glass


Damp sludge

245 200 12%

65 000



2 050 800

485 000 2


Research significance

There has been a concern in the passed years with regard to increase in heavy metal content in sewage. Particularly Cape Town is facing very stringent laws against waste handling and disposal.

Characterization of the occurrence and mineralization of heavy metals in sewage sludge is an important procedure before the sludge can be exposed and applied in agriculture as a fertilizer, since there is a risk of accumulation of toxic elements by the soil or the plants. However the accumulation of high concentration of heavy metals in the sludge could result in new environmental consideration (Angiliclis and Gibs, 1991; Obrator et al, 1997).

  • When recycled sludge ash could be used as alternative building material (in cement production)

  • When extracted from the sludge the heavy metals can be used for different purposes.

The result of the investigation will be used to can make suitable comments about disposal risks and alternative utilization methods.

Literature review

It has been estimated that each person contributes about 60g of solid waste everyday.

In addition, cities consume about seventy-five percent of the resources on earth and produce about seventy-five of the wastes on the earth, and the magnitude will increase with years motivated by population increase and privileged circumstances (Gasson, 2002). As a result it is of paramount importance to deal with waste matters and the waste handling methods.

South African context

Cape Town has about three million citizens which is rather still a small metropolitan area in comparison with other cities such as Lagos or Cairo, which have citizens in the region of thirteen million, which are known to be among the biggest cities in the world. In the Cape metropolitan area the intension is to increase residential waste recycling by 15 % (Wright-Pierce, 1999). But solid waste is expected to increase by 1,8% annually for next decade (Gasson, 2002). This is of evidence that wastewater treatment plants are on demand, and the waste management has to able to can handle the large expected capacity.

Wastewater works in Metropolitan Area in Cape Town

In Cape Town domestic waste and industrial waste effluents are treated at 23 waste water treatment plant where about 191 million (tons) of the treated effluents are discharged every year (Cape Wastewater Consultants, 1999b).

The Table2 below shows the capacity of water treatment plants in the Cape Town metropolitan. The Cape flats water treatment works is thus revealed to be having the highest capacity for treating 200 ml of wastewater per day. Hence this study investigates the heavy metals concentration of the sewage sludge from the two biggest plants in Cape Town, Cape flats and Athlone, in the Western Cape, with a view of assessing their viability to be used as Agricultural fertilizers in terms of their metal contents.

Table 2. Bulk wastewater treatment works within Cape Town Metropolitan Area




Athlone Wastewater Treatment Works

120,0 Mℓ/day

Potsdam Wastewater Treatment Works

32,0 Mℓ/day

Wesfleur Treatment Works

14,0 Mℓ/day

Melkbosstrand Wastewater Treatment Works

  2,5 Mℓ/day

Llandudno Wastewater Treatment Works

  0,5 Mℓ/day

Dover Wastewater Treatment Works

  0,1 Mℓ/day

Oudekraal Wastewater Treatment Works

   0,03 Mℓ/day


Cape Flats Wastewater Treatment Works

200,0 Mℓ/day

Zandvliet Wastewater Treatment Plant

55,0 Mℓday

Mitchell's Plain Wastewater Treatment Works

 37,5 Mℓ/day

Macassar Wastewater Treatment Plant

34,0 Mℓ/day

Wildevoëlvlei Wastewater Treatment Works

14,0 Mℓ/day

Simon's Town Wastewater Treatment Works

  5,0 Mℓ/day

Miller's Point Wastewater Treatment Works

0,03 Mℓ/day


Bellville Wastewater Treatment Plant

46,0 Mℓ/day

Borcherd's Quarry Wastewater Treatment Works

30,0 Mℓ/day

Kraaifontein Wastewater Treatment Plant

7,0 Mℓ/day

Scottsdene Wastewater Treatment Plant

4,5 Mℓ/day

Gordon's By Wastewater Treatment Plant

3,5 Mℓ/day

Parow Wastewater Treatment Plant

1,2 Mℓ/day

Source: (Gasson, 2002)

The Cape flats area is a big residential area in Cape Town, thus produce a large quantity of waste. This plant is the only plant that produces dry sludge, and dry sludge occupies a smaller volume compared to liquid sludge. Furthermore dry sludge can be incinerated and land filled depending on the concentration of heavy metals content, alternatively the ash is used in the cement manufacturing industry instant of exhausting only one disposal method (Agricultural fertilizer). Athlone is a partly residential and partly industrial area with a lot effluent coming from the industry. Figure1 below show a map of the Cape metropolitan area and show the where the two plants of concern are situated.

It has been adequately discovered by numeral researchers namely Halday, 2007, Moletsi et al, 2004 and Gasson, 2002 that predominantly residential plants contains low level concentration of heavy metals, where else the predominantly industrial area plants shows to have a high concentration of heavy metals. But their discovery still remains incomplete without a detailed knowledge of the source of heavy metals in the domestic sources of the plant.

For many years land has been used as a receptor of waste water, the commonly used being application on land for agricultural purpose. As the population in South Africa increase the land became more difficult to reserve for this purpose, and also the health became more of a concern. Furthermore the world interest on environmental issues in on an increase.

"The Polokwane declaration” In 2001September South Africa committed itself to achieving 50% reduction in the volume of waste generated and 25% reduction in volumes of land-filled waste by 2012 and a zero waste plan by 2022 [12]. With this, one may say is to a certain extent too ambitious considering the increasing population growth.
South Africa is currently formulating its National Waste Bill.  With this initiative we are expecting it to enforce generators to manage their waste according to the hierarchy of waste management in a sustainable way.  That is, industry will have to avoid, minimize, recycle, treat and dispose of their waste as a last resort. Action plans for the implementation of legislation at a local level is currently being discussed. The move is to ensure that generators implement minimization programmes (NEMA 107 of 1998).

Occasionally contamination of the coastal lakes has happened in the Cape Flats and at Noordhoek where high possibility of undesirable outcome for the health of the public and the environmental (Gasson, 2002). Cape flats and Athlone water treatment plants are within the care zone and have a first draft asset management plan that state in accordance customs of putting in action ways of managing the asset ethically (Cape Town water infrastructure, 2004).

In 2003 Richard et al, had done a similar investigation on a all the Cape metro waste treatment plants, and the results showed that Cape flats and Athlone contains high concentration of heavy metals but still below the current DWAF(department of water affairs and forestry) guidelines limits.

Table 3 Heavy metal content of Athlone and Cape flats wastewater plants compared to current guidelines limits, taken from Gasson 2002

Wastewater treatment works

Cu (µg/l)

Pb (µg/l)

Zn (µg/l)

Al (µg/l)

Hg (µg/l)

Data:Jan to Dec 2003











Current DWAF guidelines

















Cape flats











2010 guidelines






European context


The sludge, originating form wastewater treatments are the remains resulting in the primary, secondary and tertiary treatments. A typical metals content of wastewater sludge consist of the following with a median(mg/Kg); Arsenic 10, Cadmium 10, Chromium 500, Cobalt 30, Copper 800, Iron 17000, Lead 500, Manganese 260, Mercury 6, Molybdenum 4, Nickle 80, Selenium 5, Tin 14 and Zinc 1700.

Disposal issues

At present the most commonly used sludge handling option in Europe is agricultural consumption, other new uses and the waste dump places. The options used reflect the political, national, historical, geographic, legal and economical circumstance of that area. Prior 1998 municipal sludge's where mainly dumped at the sea and used on agricultural crops (Odegaard et al, 2002). After 1998, Europe legislation forbidden the damping of sewage influent in the sea, so as to save the marine environment from toxic elements. One may say that world wide the principal disposal method of sewage sludge is agricultural use. In EU thirty seven percent of the sludge formed is being used up in agricultural application, 40% land filled and 12% is used in forestry and the others.

With respect to the values of sustainable development certain limitations have to be forced when the sewage discharge methods are applied.

Table 4 EU Restriction on sludge management (Fytili and Zabaniotou, 2006).

Sludge management method Restrictions

Agricultural use : Sludge components (what is in the sludge)

: Concentration of nutrients and heavy metals in the sludge

: Accepted by the food industry and the public

: Technical restrictions (handling of the sludge)

Land fills : Organic matter in the sludge

: Costs

: Restriction of land

: Permitted new land fills areas

: Recycling necessities

Reclamation : Permits of building an incineration plant

: Costs


Wastewater sludge can be both economical and beneficial application in an agricultural crop, for this reason characterization of sewage sludge before application on land is a vital process. The heavy metal content of a waste stream from a domestic (residential) area is low, and the heavy metal content of a stream from an industrial area is expected to be high. As a result sewage sludge from a predominantly residential area may possibly be used as composed in agricultural crops without endangering plants neither ground water with toxins.

Aim and objectives

As much as 27% of sewage sludge in the Western Cape is recycled for municipal uses, composting, and agriculture (Gasson, 2002). It is well known that all sewage sludge contains varying amounts of heavy metals depending on the feeding area (Richards et al., 2004).

The aim of this study is as follows:

  • The main aim of the study is to determine whether sludge from any wastewater treatment plant in the Western Cape may be used safely for composting purpose, or whether it could lead to build-up of heavy metals in groundwater

  • An alternate aim is to investigate simple treatment techniques with the emphasis of reclaiming heavy metals from the sludge containing high amounts of heavy metals.


Sewage sludge from waste water treatment plants treating wastewater from two different feeding areas in the Western Cape will be collected for study. One area is a predominantly domestic area (Cape flats), the other a predominantly industrial area (Athlone). The sewage sludge will be characterised in terms of heavy metal content and minerals. Also the investigation of the current treatment and utilization of the sewage sludge in the WC region will be discussed.

The analysis section will comprise the techniques such as:

  • Atomic Absorption(AA) and



  • Current treatment process of the sludge from Cape flats and from Athlone area in Western Cape;

  • Current utilization of the sludge;

  • Metal content of the sewage sludge;

  • Is it possible to extract the minerals from the sludge;

  • Make suitable comments about disposal risks and alternative methods.


Two samples namely, raw sludge and high temperature treated sludge(ash) from each sludge sources will be analyzed by using AA and SEM -EDAX

Sample preparation

The sewage sludge will collected from Cape flats water treatment plant in pellet form. The sample will be mixed by riffling the bags received to 1/16 fraction to make sure a representative sample is obtained. A representative sample of the sludge will be put in an oven at temperatures about 500ºC for 12 hours, in order to burn out the organic matter in the sludge. Sludge and sludge ash will be digested in HCl for analysis in AA (method to be further researched).

Mounting Samples for SEM analysis

The following steps are to be followed;

  1. Mix the resin

Samples are mounted for SEM analysis using Epofix

The mix recipe is 7 parts resin to 1 part hardener

This 8ml mix is enough to mount two samples

Use a 20ml syringe for the resin and a 5ml for the hardener.

Draw up the resin into the syringe and inject the resin slowly into the grey mixing cup so as not to form bubbles. Clean the syringe immediately

Inject the hardener drop by drop onto the resin and try to collapse all the formed bubbles in doing so. Clean the syringe immediately

Use the ice cream stick to mix the resin mix carefully. Clean immediatly

  1. Pour thin layer(barely covering the floor of the mounting cup) of resin in each mounting cup

  1. Place the samples in to the mounting cup

  2. Cover the samples completely with resin mix, and clean the grey mixing beaker

  3. Leave the sample to set for at least 15 hours

  4. Remove the mounted samples from the mounting cup by inverting the rubber cup and pushing the sample out.

Sample grinding and polishing for SEM

The following steps are to be followed;

  1. Cut the sample

  2. Clean the sample

  3. Coarse grind the sample to remove the layer of resin that covers the sample surface and to grind flat the meniscus ends on the back of the sample so that it will lie flat in the SEM.

  4. Final grind and polish the sample so as to remove the marks as they may deflect the electron beam and influence the outcome of SEM analysis

It must be noted that the experimental procedures are subjected to changes such as improved methods that are recognized through additional literature reviews and will be updated as this changes are being made.

Resource requirements

Analytical facilities:

  • SEM-EDAX, facility at the Geology department.

  • AA at the process engineering department.

Project plan and resource requirements


  1. A'varez, E.A., Mochon, M.C., Sachez, J.C., Rodriquez, M.T., 2002. Heavy metals extractable form in sludge from waste water treatment plant. Chemospher . pp 765-775

  2. Angeliclis, M., Gibbs, R.J., 1991. Heavy metals in urban sewage sludges chemical forms and possible availability. Treat. Use sewage sludge Liq.Agric. wastes. Pp400-404.

  3. Carter C. A., 1994a. Department of Water Affairs and Forestry (DWAF) Summary Report. Western Cape Systems Analysis. DWAF Report No P G000/00/5293

  4. Carter C. A., de Smidt K., 1994. Department of Water Affairs and Forestry (DWAF) (Main Report. Prepared by Inc. in association with BKS Inc. as part of the Western Cape System Analysis. DWAF Report No PG000/00/5193.

  5. Little P. R., 1998. Department of Water Affairs and Forestry, Feasibility Study of the Skuifraam Dam Supplement Scheme, Main Report.

  6. Fytili, D., Zabaniotou, A., 2006, Utilization of sewage sludge in EU application of old and new methods-A review. Department of chemical engineering, Aristol Universty of Thessalonika., Greece.

  7. Gasson, B., 2000. The Urban Metabolism of Cape Town, South Africa : Planning imperatives in an ecologically unsustainable metropolis. Czech Republic,

  8. Gasson, B., 2002. The ecological footprint of Cape Town: Unsustainable resources use and planning implications. pp 12

  9. Halday, I., 2007, Study of pathways of heavy metals in sewerage system. pp 3, 12-15

  10. Herman, H., Hahn, E., Hoffman, Hllavar, O., 2002. Chemical water and wastewater treatment vii, IWA, Gothenburg. pp 331

  11. http//

  12. Moeletsi, M., Mazema. H., Halday, I., 2004. Strategy for mitigating the impact of heavy metals in wastewater and the economic and environmental implication for the city of Cape Town. pp570-575

  13. Odegaard, H., Paulsrud, B., Karlsson, I., 2002. Wastewater sludge as resource: sludge disposal strategies and corresponding treatment technology aimed at sustainable handling of wastewater sludge. Water Sci Technol. Pp 295-303

  14. Richards, H., Moollan, R.W., Beceri, R., Moche, D., Rus, H., 2004, An overview of the heavy metals concentrations found in municipal wastewaters and wastewater sludges in the city of Cape Town. pp.1515.2-1515.5.

  15. Wright-Pierce, P., 1999. Feasibility Study Towards and Integrated Solid Waste Management Plan for the Cape Metropolitan Area, Cape Metropolitan Council, Cape Town.

  16. Zufiaurre, R., Oliver, A., Chamorro, P., Nerin, C.,Calizo, A.,1998. Speciation of metals in sewage sludge for agricultural uses. Analyst123. pp 255-259