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The TSB Malelane and Komati Sugar Mills, situated
in the Lowveld of Mpumalanga, generate molasses as a by product during the sugar cane crushing season, which is typically nine months. Molatek Animal Feeds uses the molasses
generated by the mills as a raw material in the production of animal feeds, and requires molasses throughout the year in order to supply their markets. Storage of molasses during
the crushing season for use during the off-season is therefore required. Both mills are equipped with steel tanks for molasses storage, and a 5 000m3 floating cover molasses storage reservoir was constructed at the Komati Mill in 1999. Insufficient molasses storage existed to provide for the molasses generated during the 2004 crushing season, and a number of alternative storage solutions were investigated, costed and compared by Molatek. The provision of a floating cover reservoir proved the most cost-effective, based on the initial costing exercise.
Jarrod Ball & Associates (JBA) was appointed to design the floating cover reservoir, as well as to provide specifications, contract documentation and drawings, site
supervision and project management. Aquatan were appointed as the main contractor, to undertake the lining required. Following a tender process, TransOos Konstruksie were
appointed to undertake the earthworks for the project, the TSB Malelane Civil Department was appointed to undertake the civil works, and Bensy Engineering was appointed to
undertake the piping.
Given the high cost of storing molasses in road or rail tankers, the time frame during which the reservoir was to be designed and constructed was
extremely tight. JBA were appointed in mid-April 2004, and the dam was commissioned for use in mid-September 2004, just less than five months after the project commenced. Both the
design and construction of the project are worthy of note.
The construction of the reservoir is worthy of mention for the following reasons:
The reservoir was designed to hold 14 800m3, or approximately 20 000tons, at
a specific gravity of 1.35. A 300mm freeboard has been allowed for foam. This is believed to be the largest molasses floating cover reservoir constructed in the southern
hemisphere.
Construction of the dam took a total of 3 months.
The reservoir is lined with a Geocomposite Clay Liner and a 1.5mm thick HDPE
geomembrane, while the cover comprises 1.14mm thick reinforced polypropylene. Construction therefore involved three different geosynthetics, providing the total lining and cover
system for the reservoir. This is believed to be innovative for the storage of molasses.
Four contractors were involved in construction, the earthworks contractor, lining
contractor, civils contractor and piping contractor, which required careful project management and good communication, given the relatively small area of the site.
Two unseasonal rain events caused a total of 9 days delay on the project, and the second
event resulted in damage to the earthworks, which required rectification. The lining contractor made up time to allow for this.
The earthworks were completed to a very high standard, making installation of the lining
easier.
More than 5 000m2 of GCL and 10 000m2 of geomembrane was placed in less than four weeks, to a high standard. This is particularly noteworthy given that the side slopes of the dam are at 1V:1.5H, and the cover material sheets are only 1.9m in width, so that considerable welding was required.
Design changes were made as late as three weeks before completion of construction,
so as to meet programme and cost constraints as well as the client’s requirements, so that the construction team had to be flexible and adaptable.
No spreader pipes were installed during construction. Instead, the herringbone canal
system on the floor was used to spread the molasses uniformly throughout the floor between the HDPE liner and R-fPP cover of the reservoir. Pipes are usually used to spread the
molasses evenly across the floor, limiting stress on the cover. The first filling of the dam went smoothly, with the rate of filling low enough not to result in pulling or
stressing the cover. This is believed to be innovative.
The inlet to the dam was
constructed between the liner and the cover, so that no penetration of the liners was necessary for the inlet, limiting the risk of leakage. This is believed to be innovative.
The dam was constructed with an innovative set of weights and floats, developed taking
the problems experienced with gas collection under the cover at the Komati molasses reservoir. The floats, installed under the cover,run parallel across the width of the
dam, allowing gas the shortest distance to escape. The weights are in place on the centre fold of the dam, so that stormwater will accumulate in this area, which can then be
removed by the stormwater pump in position on the floating cover of the dam and so that gas is forced via the shortest route to the gas removal system around the perimeter of the
dam. Installation of the floats and weights significantly complicated construction, but proceeded well.
The project’s BEE component was significant, with the resident engineer and the
main contractor’s site manager being previously disadvantaged individuals, and the professional engineer and civil technician for the consultants, and the SHEQ
representative and auditor on behalf of the client, being women.
It is also worth noting that the project team worked well together, with the client’s
representatives taking an active role in the project at every stage. Given that the new molasses floating cover reservoir is working well, and the dam was commissioned before the
Mill’s existing storage structures had reached capacity, this project is considered a success.
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