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Waste-landfill sites and the associated environmental
impact to surrounding areas has become a significant concern for local municipalities in recent years as stringent South African legislation requires all
closed landfills to be rehabilitated and capped to prevent environmental degradation and potential pollution of the groundwater bodies.
Despite this
presence of legislation, it is a common reality that, mainly due to the lack of funding, smaller municipalities' are unable to give the necessary
attention to landfill rehabilitation.
Fortunately, this is rarely the case with the larger metropolitans, who tend to carry the pressure of social
responsibility more heavily than their smaller municipality counterparts.
This is evident in that the Ekurhuleni metropolitan municipality, situated to the
east of Johannesburg, has undertaken a proactive approach in preserving the East Rand’s environment by rehabilitating two of three remaining
closed landfill sites, namely Nigel and Sebenza.
The finalisation of these two projects, represents a benchmark in environmental sustainability as both
sites were rehabilitated according to the highest legislative requirements using state-of-the-art geosynthetic materials for the capping of the
waste material.
This method of landfill capping follows an increasing world-wide trend of using geosynthetic materials, which are safer and more
environmentally friendly.
Engineering News spoke to Ekurhuleni and the respective engineering consultants and contractors involved with the Nigel and
Sebenza projects on the innovative approach to landfill capping and the challenges that had to be overcome to effectively rehabilitate these two sites.
Nigel landfill rehabilitation
The Nigel landfill site, situated in the south-eastern area of the Ekurhuleni metropolitan, has been in
operation since the early 1970s and officially ceased operations in the early 1980s.
However, not having an alternative disposal facility the site
continued to be used as a waste disposal site throughout the 1980s and 1990s until final closure in 2004.
A new waste transfer station at Nigel is
currently being constructed to replace the closed landfill.
The location of this particular landfill is quite unusual in that it is situated adjacent to a
natural forming water-pan, which presented some challenging obstacles during the closure and rehabilitation process.
The site was earmarked for
rehabilitation and capping in 2004 by Ekurhuleni Metropolitan Municipality as the water-pan encroaching on the landfill site and the subsequent generation of
leachate was posing a threat to the surrounding environment which could create significant health and environmental concerns for the local area.
Leachate,
a liquid that originates from the waste body, carries dissolved or suspended contaminants and is formed when water and waste come in contact with each other.
Leachate normally forms when rain falls and penetrates uncovered waste material and in this instance also formed due to the water-pan encroaching onto
the waste toeline, which resulted in the pollution of the water.
“Due to the urgency of the situation and the encroaching water on the landfill
toeline, we wanted to rehabilitate the landfill as soon as possible,” explains Ekurhuleni’s project manager Riana Becker.
“As a result,
the design, construction and lining contracts were awarded at the end of last year, and after in-depth consultations with the Department of Water Affairs and
Forestry (DWAF), the rehabilitation design of the site was approved by DWAF and construction commenced in February this year.
“The design contract
was awarded to black economic empowered consultancy firmMasakhe Isizwe Engineers (MIE), the construction contract was awarded to WBHO Construction, who
appointed Aquatan Lining Systems as a specialist sub-consultant for the capping of the site.
“The total value of the project was R11,1-million,
which was funded by Ekurhuleni and by a municipal infrastructure grant (MIG) and was recently completed in June.”
During the design of the capping
system, MIE’s design engineer Pieter de Coning had to take into account the volumes of water adjacent to the landfill as well as the storm water from the
surrounding area and the resultant leachate that was formed.
“One of the problems that we had to tackle was the water flowing down from the north of
the site, partially through or around the landfill and into the closed pan system,” explains de Coning.
“This meant that all the water that
passed through the landfill, including rain falling on top of the landfill, got contaminated and polluted the water in the pan.
“The other problem
was that we could not cap the toeline of the landfill because of the consistent flow of upstream storm water into the water pan resulting in the pan
waterline encroaching onto the waste body.
“So the process that we followed to overcome these challenges was to build a coffer dam at the bottom of
the toeline approximately 20 m into the pan itself.
“Water trapped between the coffer dam and the landfill site was pumped into into the pan.
“This left an area open for the contractors to work and build the waste-water barrier to separate the waste from the water, thereby preventing further
contamination of the water of the formation of leachate.”
The waste-water barrier was constructed of a 1 m thick layer of clay, a geomembrane layer
of 1,5 mm thick high-density polyethylene (HDPE) plastic, and a further two layers of clay to create an impermeable barrier.
A leachate collection drain
was also installed at the bottom of the landfill behind the waste water barrier to collect leachate formed within the waste body.
“The leachate
collected in the two 6 000 litre leachate tanks is taken to a nearby waste water treatment plant,” explains Becker.
“The volume of water that
is being pumped out is not significant at this stage, only amounting to 6 000 litres a month and in time this will decrease as the waste begins to dry
out.”
A HDPE geosynthetic material was used to cap and seal the waste body, which created an impermeable layer that will prevent infiltration of
storm water into the waste body.
The HDPE was used because it was the most cost-effective of the geosynthetic materials at the time of tendering
and displayed characteristics of increased environmental sensitivity required for this particular project.
According to Aquatan’s MD Piet Meyer, 110
000 m2 of smooth HDPE lining meeting the international requirements of GRI GM 13 and installed in accordance with the recently published SABS Sans 10409 code
of practice, was installed on the flat areas on top of the landfill, 30 000m2 of double-textured HDPE lining was installed on the landfill slopes, and 45
000 m2 of geogrid lining was used to cover the lining on the slopes to prevent the topsoil's from sliding down the slopes.
“The smooth HDPE
liner was welded together to prevent any water seepage through the material which may result from ponding on top of the landfill,” explains Meyer.
“The textured HDPE liner together with the geogrid was anchored in an anchor trench separately on the crest of the slope of the waste dump.
“The smooth liner on the flat area on top of the dump overlaps the textured HDPE and geogrid on the slopes so that rain water could run down the slopes
into the water pan without becoming contaminated.
“The textured HDPE and the geogrid was anchored into the anchor trenches all the way around the
slopes to hold the material in position and to prevent any slippage that might occur.”
Six gas wells were also installed on top of the landfill to
help vent the buildup of methane gas within the waste body underneath the HDPE liner.
Gas probes were also installed on site and is monitored on a regular basis.
“The end-use plan for the Nigel site is open space as we cannot allow
access to the public due to the presence of the methane gas,” says Becker.
“The site has been fenced off to prevent public access and will be
hydro-seeded in September in time for the Highveld rainy season.
“This will be done to prevent erosion of the 200 mm thick topsoil together with the
200 mm protection layer covering the HDPE geomembrane.
Sebenza landfill rehabilitation
The Sebenza landfill site, situated on the
outskirts of the suburb of Edenvale, closed its operations in the late 1990’s.
The site was earmarked for rehabilitation by Ekurhuleni in 2002 as
part of its drive to rehabilitate all of its closed landfill sites according to the Minimum Requirements for waste disposal by landfill.
Unlike the Nigel
landfill site, Sebenza is only flanked by a stream, which, as a result, did not create as many challenging obstacles during the rehabilitation process.
However, the closure requirements and the generation of methane gas at the site was of concern to Ekurhuleni, and consequently, the metropolitan put out
tenders for the rehabilitation contract at the end of 2003.
“When awarding the contracts, Ekurhuleni worked on a preferential point system where
the company with the highest points would be awarded the contract,” explains Ekurhuleni’s project manager for the Sebenza landfill Flip Viljoen.
“Points were derived from the company’s BEE stake, its location and the use of local contractors.”
On this point basis, the design
contract was awarded to Kwezi V3 Engineers, who is one of the largest black owned consulting companies in South Africa with a 70% BEE ownership.
The civil
engineering contract was awarded to CML Investments who sub contracted the lining contract to Aquatan Lining Systems.
As the site did not pose as many
obstacles as the Nigel landfill rehabilitation, the total value of the contract was R6,2-million with the MIG funding R5-million and Ekurhuleni funding the
remainder.
“The design portion of the Sebenza project was started in May 2004.
Once the design plans had been approved from DWAF and the Gauteng
Department of Agriculture and Environmental Affairs the construction portion of the rehabilitation started in February 2005,” explains Kwezi
V3 Engineers design engineer Lance Hill.
“The site had to be reshaped as it lay outside the topocadastral boundary.
“This resulted in an excavation of 56 000 m3 of waste, which was reused to shape the site.
“As a precaution to the build up of methane
gas inside the waste body we constructed nine gas wells which vent the gas out into the atmosphere. Subsequently the Metropolitan Municipality
has implemented a project to actively degas the landfill site”
The synthetic material that was used as the membrane over the top of the landfill
was a geosynthetic clay liner (GCL).
“The decision to use a GCL was based on the availability, timeframe for construction and expense of importing
clay to the site,” explains Hill.
65 000 m2 of GCL liner was placed on the site and covered with 12 500 m3 of topsoil, which was then hydro-seeded
to prevent erosion and dust issues.
“Due to the fact that methane gas is constantly produced in the site it would be dangerous to allow access to
the public and so the end use plan of this site is to leave it as an open-space but fenced off from the public,” concludes Hill.
Caption 1:
Nigel coffer dam: the coffer dam was used to drain the water against the toeline of the landfill to enable the installation of the waste-water barrier.
Caption 2: Nigel leachate collection tanks: leachate collection tanks downstream of the landfill site used to collect and store leachate migrating from
the landfill. (The water pan in shown in the background)
Caption 3: Sebenza gas wells: nine gas wells were installed on top of the landfill to vent
the buildup of methane gas in the waste body
Caption 4: Sebenza geomembrane: the GCL liner was covered with a layer of topsoil and hydro-seeded to
prevent damage and erosion
Fact Box 1: Ekurhuleni owns 5 “large” operative landfill sites, which all have DWAF operational permits.
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