Case Study SA-JW4
SOLAR HEATING AND COMPOSTING OF SLUDGE WITHOUT THE USE OF
BULKING AGENTS
The introduction of the 2006 South Africa Sludge Guidelines heralded a new era in the management,
treatment and disposal of sludge. Coupled with considerable increase in electricity costs from 2009
onward, thought had to be afforded to produce high quality sludge at reduced production cost. This
case study describes the optimisation of mechanical assisted solar sludge drying and composting
without the need for bulking agents and curing, with the sludge sourced from mesophilic digesters.
Sludge Solar Drying and Composting without the aid of Bulking Agents
Description of Process:
Fermented raw sludge and waste activated are stabilised through mesophilic digestion prior to
dewatering on belt filter presses. Dewatered sludge with 20% dry solids content is spread on concrete
beds. These beds were previously used for sludge composting using woodchips as a bulking agent.
The dewatered sludge is turned daily using mechanical sludge turners and the dried sludge is removed
from the beds once the dry solids concentration has reached about 50%. The drying period takes
between 10 - 14 days in summer and 24 - 30 days in winter.
Sludge dried to the required total solids concentration allows for the sludge to be heaped to three
meters high without the heap slumping or forming large clods. It also allows air to freely pass into the
heap, which is essential for the composting process to proceed.
Temperatures in the heaps rise rapidly to 40oC to 50oC and when the average temperature reaches
above 60oC, the heaps are broken down and rebuilt using a mechanical turning. After rebuilding,
temperatures are allowed to rise again and the rebuilding process repeats when temperatures rise >
60oC. Satisfactory stabilisation of the final product is achieved when the volatile solids concentration
has been reduced to below 0.45 kg / kg DS.
Potential Interventions
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Full compliance with the 2006 SA Sludge Guidelines (at all times) is crucial to meet service
delivery performance and the objectives set by the City of Johannesburg.
The low level of technology used ensures that the operation is sustainable and cost effective.
Laboratory analysis of all heaps ensures compliance with the guidelines and protects public
health when used for landscaping etc.
This case present an opportunity to other practisioners that wish to follow a similar process .
Range of potential savings
Sludge treatment and disposal options normally involve high electrical energy requirements (i.e.
incineration, pasteurisation, etc). The composting/solar process described in this case study requires
electrical energy for screening the composted product i.e. 7.5 kW for 100 dry tons per day of sludge
treated. The operational cost reduced from R 560 to R 270/dry ton sludge treated.
Composting using bulking agent
Remote farm disposal
Solar drying/composting and land disposal
R 560/ dry ton sludge treated
R 360/ dry ton sludge treated
R 270/ dry ton sludge treated
Case Study SA-JW4
SOLAR HEATING AND COMPOSTING OF SLUDGE WITHOUT THE USE OF
BULKING AGENTS
Ref
Case Study SA-JW4
Location:
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Sector:
2
Response information, description and remarks
JW are situated in Central and Southern Gauteng Province of South
Africa. The areas are densely populated. Two of the works are
situated in the northern catchment and three in the southern
catchment.
Sewage
sludge
Works Owner or Operator:
The plants are owned by the City of Johannesburg (Water Services
Authority) and operated and maintained by Johannesburg Water
(Water Services Provider).
Size:
Some 3.5 million people are served in the northern catchment and
2.5 million people in the southern catchment. Of the 260 dry tons
per day of sludge produced, 200 dry tons per day are treated by the
solar drying method
Energy Provider:
Power is in form of electricity, provider by the national
electricity agency, ESKOM. A typical plant sized at 450 Ml/day
requires 8.0 MW energy, of which sludge treatment forms the
bulk of the operational budget.
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Process:
Operation consists of front-end loaders, sludge turners and screen.
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Component:
All waste activated and fermented raw sludge
Motivation for the case study:
Most sludge treatment and disposal options available require
enormous amounts of electrical energy to function i.e. incineration,
pasteurisation etc. This process under discussion requires only
electrical energy for screening the composted product i.e. 7.5 kW
for 100 dry tons per day of sludge treated.
Process/Plant changes:
All plant used for composting with a bulking agent was used in the
new operation. The existing inclined face compost turners were
converted to sludge turners at a third of the cost of new machinery.
Only 12 year old tub grinders were scrapped.
Civil/Physical Changes:
No changes were made to the existing civil works or processes were
required. At two of the works, existing concrete beds were
extended.
The only process change was the non-requirement of a bulking agent
and that dewatered sludge was now solar dried prior to composting.
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4
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8
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Operational Changes:
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Risks and Dependencies:
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Implementation:
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The composting process has been in operation in Johannesburg since
1986. The dewatered sludge was dried to the dry solids
concentration as the sludge / bulking agent mixture prior to
composting. Full scale trials were undertaken prior the operation
being implemented at the other works.
The project was procured via a design/build process with a
respectable contractor. Acceptance testing, followed by an O&M
period was included as part of the contract.
Energy Efficiency gains:
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Cost/Benefit analysis:
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With the publication of the 2006 SA Sludge Guidelines, JW were
then not fully compliant and a new operation had to be developed
or the existing high cost composting operation extended.
Consideration was given to other sludge treatment processes but
most required high electrical energy input (heat treatment etc).
The operational cost of sludge treatment and disposal has been
reduced from R 570 per dry ton to R 270 per dry ton. The payback
period of drying bed extensions etc is < 2 years.
Project review:
The project has only been in operation since 2007 and as yet , only
minor changes have been made to the operation.
Confidence grade:
High. Data has been well documented and verified over >2
operational years. The data shows that, to date, only a limited
number of composted batches failed the guideline requirements.
Such batches have either been reformed or mixed with new dried
material. The operation has consistently produced a class A1a final
product which is sort after by maize farmers. At present, 19 farmers
use the product with a waiting list of a further 20 farmers.
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The City of Johannesburg produces 260 dry tons of waste activated and fermented raw sludge per
day and faces many challenges with its treatment and disposal in order to comply with the
“Guidelines for the Utilisation and Disposal of Wastewater Sludge” (Department of Water Affairs,
2006). The revision of a sludge management programme by Johannesburg Water began soon after
the publicationof the guidelines. Company objectives required an operation to be developed that
would be fully compliant with the guidelines in a safe, cost effective and sustainable manner. The
guidelines characterise the quality of the final sludge product prior to disposal by three
criteria:Johannesburg Water chose the classification level
Microbiological class
A, B and C
A1a in order to maximise the available disposal options
Stability class
1, 2 and 3
and ultimate registration of the final product as a sludge
Pollution class
a, b and c
fertilizer.
Description of Process
Johannesburg, with its low humidity, high summer temperatures, no winter rainfall and a high
evaporation rate, has an ideal climate for the solar drying and composting of wastewater sludge.The
short summer thunderstorms are immediately followed by sunshine which rapidly evaporates any
residual moisture.
Waste activated and fermented raw sludge are both stabilised through mesophilic digestion prior to
dewatering on belt filter presses. Dewatered sludge at around 20% dry solids concentration is spread
on concrete beds previously used for sludge composting using woodchips as a bulking agent. The
dewatered sludge is turned daily using mechanical sludge turners and the dried sludge is removed
from the beds once the dry solids concentration has reached about 50%. The drying period takes
between 10 and 14 days in summer and between 24 and 30 days in winter.
Sludge dried to the required total solids concentration allows for the sludge to be heaped to three
meters high without the heap slumping or forming large clods. It also allows air to freely pass into
the heap, which is essential for the composting process to proceed. Above 60% dry solids
concentration, rapid uncontrolled heating of the heap to above 60oC is experienced, which causes
loss of moisture and inhibition of the composting process.
Sludge Solar Drying
% Dry solids - 1
% Dry Solids
% Dry solids - 2
60
60
50
50
40
40
30
30
20
20
10
10
0
Ambient temp
Limit - low d.s.
Limit - high d.s.
0
0
2
4
6
8 10 12 14 16 18 20 22 24 26
June - July
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
October - November
Under the required conditions, temperatures in the heaps rise rapidly to 40oC to 50oC and when the
average temperature reaches above 60oC, the heaps are brokendown and rebuilt using a front end
loader. Any sludge clods that appear around the edge of the compost heap are returned to the
drying beds and broken up using the mechanical turner. After rebuilding, temperatures are observed
to rise to the required average temperature and breaking down and rebuilding of the heaps is
repeated if temperatures rise above 60oC. After the composting period is complete, samples are
taken to the laboratory for analysis. Heaps that comply with Microbiological Class 1 sludge, are
screened through a 15 mm mesh before curing. Satisfactory stabilisation of the final product appears
to be achieved when the volatile solids concentration has been reduced to below 0.45 kg / kg DS.
The control of temperature and moisture content, at between 50% DS (start) and 35% DS (end), is
essential for the success of the composting process. The final curing stage ensures that the final
product does not cause either an odour or vector attraction problem.
The process has been implemented on four of the five large wastewater treatment works in
Johannesburg since 2007.
Potential Interventions

Full compliance with the 2006 Sludge Guidelines at all times is crucial to the service delivery
performance of Johannesburg Water and to the City of Johannesburg.
The low level of technology used ensures that the operation is sustainable and cost effective.
Laboratory analysis of all heaps ensures compliance with the guidelines and protects public
health when used for landscaping etc.
This case present an opportunity to other practisioners that wish to follow a similar process.
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Cost Saving
Johannesburg Water has reduced its sludge treatment
and disposal costs from R 560 / dry ton for the
composting of sludge using a bulking agent down to R
270 / dry tons through solar drying and composting
without the use of a bulking agent.
for Class A1a sludge
Composting using bulking agent
Remote farm disposal
Solar drying/composting and land disposal
R 560/ dry ton sludge treated
R 360/ dry ton sludge treated
R 270/ dry ton sludge treated
The operation does not require the production of woodchips, removal of the woodchip after the
composting process or the disposal of the used woodchip and requires handing of a third of the
product during the composting process.
The 2006 Sludge Guidelines have become a requirement in the licensing of wastewater treatment
plants, thereby becoming a lawful and enforceable standard. With electrical power costs due to
treble over a period of 7 to 10 years, innovations such as described above that do not require high
electrical energy demands may provide meaningful alternatives with higher capital, operational and
energy requirements.
Reference
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Shaun Deacon: Wastewater Engineer, City of Johannesburg