The combined efforts of the Trans-Caledon Tunnel Authority (TCTA), Group Five and all subcontractors must ensure the prevention of acid mine drainage at Central Basin, writes Laura Cornish.
It’s a race against time. The new treatment plant at the Central Basin’s South West Vertical Shaft must be operational before acid mine drainage decants to surface.
The acid mining drainage (AMD) dilemma, not a new concept to the South African public, reached emergency status when gold mining-acidic water started decanting from a defunct shaft in the Western Basin in 2002, polluting nearby spruits and groundwater. Today, the emergency is reaching critical status as government fights to prevent a repeat at the Central and Eastern Basins.
Every day the water level shared between the three Witwatersrand gold basins – Eastern, Central, Western – is rising at a rate of about 300 mm/d, according to the Department of Water Affairs’ (DWA) Inception Report published in March 2012. Pumping stopped in the Western Basin in 1996, the Central Basin in 2008 and the Eastern Basin in 2011.
Fortunately, the crisis on the Western Basin has been curbed thanks to the upgrade of the existing Rand Uranium plant, which has been expanded from 12 Mℓ/d to 36 Mℓ/d. This has stopped and neutralised the uncontrolled decant of water flows.
The severity of the situation has seen government take an active stance. In 2010, the government formed an inter-ministerial committee to evaluate technologies and methodologies to best solve the problem – for the immediate future and the long-term future. TCTA, a special purpose vehicle established by the DWA, was appointed to implement the short-term intervention on its behalf.
The immediate priority: the Central Basin
The Central Basin extends from Durban Roodepoort Deep (DRD) in the west to the East Rand Proprietary Mines (ERPM) in the east. The mine lease areas in this basin extend over about 251 km2.
Considering the amount of water necessary to treat the volume of AMD from the Central Basin is more than double that of the Western Basin, prevention (of AMD reaching the surface) is non-negotiable. Experts are predicting that water levels will reach the environment critical level around September/October this year – only 186 m below the top of the shaft.
“If water rises above this level it could start having an impact on groundwater. The word ‘could’ is used as no one can accurately predict the hydrogeological reaction as the water level in the mine void rises,” explains TCTA business analyst Richard Holden.
Construction of a high-density sludge treatment plant and pump system at the Central Basin’s ERPM South Vertical Shaft near Germiston commenced in February this year and marks another major milestone in government’s fight to prevent AMD entering the Vaal river, which would ultimately cause a shortage of water to consumers in Gauteng and the surrounding provinces. “Without this project, the major impact will be on water security for anyone who draws water directly or indirectly from the Vaal River system,” Holden notes.
During the due diligence, various shafts were evaluated and given the cost to construct a new shaft, an existing defunct shaft was chosen. The plant will be situated at the existing South Vertical Shaft, which intersects the mining void of the central basin. The eastern side of the Central Basin also represents the lowest pumping head.
For TCTA, allocation of sufficient funds to enable the construction contract to be awarded, coupled with obtaining environmental authorisation where legislation has never made provision for this type of project, have been significant challenges Holden explains. Regardless, the urgency of the project has necessitated TCTA bring its strengths to the table: funding and implementing large-scale water projects. “Thanks to much negotiation and compromise with ERPM, Central Rand Gold, the Department of Environmental Affairs, the DWA and all the interested and affected parties who participated in the scoping report, the success of this project will become a reality.”
Group Five Civil Engineering was appointed the full R319 million treatment plant construction contract, including civils, earthworks, building, and mechanical and electrical installation. It will be three times larger than the next biggest similar treatment plant in South Africa (at eMalahleni), with highly complex concrete structures, piping and electrical systems.
Holden says the plant is designed to treat 57 Mℓ/d. “At this stage the final ‘treated’ effluent will still have a high sulphate content (approximately 200 mg/ℓ). This is no different to what was discharged by the mines up until to 2008 when pumping and treatment at this site ceased. The stream bed is already heavily polluted from old tailings and urban stormwater run-off. Although this is not ideal, it is still a lot better than doing nothing until a long-term solution has been completed.”
The biggest impact caused as a result of the project is the increase to the salinity of the water in the Middle Vaal system, which necessitates the release of fresh water from the Vaal Dam to dilute it. “This will result in a deficit in the Upper Vaal system in 2015. What this means is that if there is a drought, water restrictions will need to be imposed.”
Wayne Poulsen, senior site agent for Group Five, explains the basic treatment process: “An extraction pump station, including two huge dewatering pumps, will pump AMD from the existing ERPM shaft. The water will be pumped to two reactors, which combined have an 80 x 80 m concrete footprint and are 5 m deep. This water is then pumped to a lime dosing plant, including silos and eight mixing tanks, each of which are 16 m in diameter and 4 m high. Two thickeners, each 47 m in diameter, will settle out the solids. The treated water will be discharged via pipelines to the nearby Elsburg Spruit and the sludge discharged via pipeline to an existing ERPM tailings facility.” Group Five is further responsible for the construction and delivery of all ancillary tanks and buildings as well as a full-scale laboratory.
Ekurhuleni Metropolitan Municipality will supply the necessary clean water to site and power will be provided via a high-power 4 km overhead line.
“Our construction timeframe is less than a year, meaning not a day can be wasted. Trial commissioning will start in November 2013 , with trial operations running until 18 April 2014,” Howard Wakefield, senior contracts director for Group Five Civil Engineering points out. “We will hand the plant over to TCTA at complete readiness after which it must run permanently, 24/7. Our most critical challenge throughout the project remains time; we have to meet our critical deadlines. But we are confident of success; we started well and are making excellent progress.”
Wakefield believes that even though there were numerous tender bids put forward for the project, “our combination of pricing, technical ability and compliance with TCTA’s stipulated requirements for enterprise development and BEE (including mentorship, procurement, employment, enterprise development and skills training) ranked us highest”.
Current project status
Earthwork construction on site is well under way, says Chris Prinsloo, general manager of Diesel Power’s Civils & Bulk Earthworks Division. Primarily a mining contractor, Diesel Power is focused on growing the civils arm of its business.
The company is responsible for all project bulk earthworks, from excavation through to platform construction and backfilling. Prinsloo admits that their challenge, like Group Five, is time. “It is a tight programme and we need to hand over the platforms so the next let of the project can start. We have also been experiencing a lot of rain. Regardless, our commitment to the project is unwavering, and we will meet our deadlines.”
The company has eight staff members, 20 operators and 16 labourers dedicated to the job. “The reactor platform is already completed, the thickeners area is 70% complete, the dosing area is 60% complete and building platforms are 80% complete,” says Prinsloo. “We have also just started on building the internal roads and the retaining walls.”
Plant on site includes:
- 4 excavators
- 1 dozer
- 1 grader
- 2 rollers
- 2 water trucks
- 22 tip trucks
- 1 tlb
- 1 front end loader.
“This is such an exciting project to be involved with; to be part of a project saving the environment and preserving water resources for future generations is a great motivator,” Prinsloo states.
Pumping – a key critical project element
The scale and complexities of all the project elements are enormous, including the dewatering pumps. The mine shaft is over 1 500 m deep and the pumps, which are 15 m high (four storeys) and weigh 25 t, must be lowered 200 m down the shaft without being dropped.
Ritz Pumps South Africa, with its German joint venture partner Andritz Ritz, has been contracted to supply two heavy-duty mining dewatering pumps to the project. “They originally belonged to Central Rand Gold, which graciously donated them to TCTA for the project. The pumps are currently based at the Andritz facility in Germany and are due to undergo their second factory acceptance test. Following this, they will be shipped to South Africa where we will oversee their installation,” says Chris Munnick, MD of Ritz Pumps SA.
“The beauty of our system is that our pump is assembled above ground and then ‘free hung’ –suspended from surface – which means underground access to get to the pump station, as is generally the case in South Africa, is not needed,” says Munnick. “The pump is simply suspended in the shaft opening with our ‘ZSM’ system, which can withstand in excess of 1 600 t due to an axial non-positive and detachable pipe connection. Our technology enables us to free hang pumps up to 1 250 m with piping from 80 to 600 nominal bore and achieve heads of 1 500 m. We can also insert a pump in large borehole from the surface straight down into the water source, suspend and hang the pump into the water and pump out directly without having to go through the shaft with a complex piping system as is required for the traditional underground high-pressure pump station setup.
“The use of single-suction submersible motor pumps for pumping huge quantities or from great depths is associated with extreme loads on the unit,” Munnick continues. The higher the pump performance, the stronger the axial thrust exerted on the pump, the motor and its thrust bearing. The consequences of this are overloading and untimely shutdown. “The solution is to double up, meaning greater durability. This is what our design is all about, a double-suction pump that provides full compensation for axial thrust. In short, the heavy-duty pump is designed for longer life – between 25 and 30 years.”
While the plant has been designed using historical information, final qualities will only be known when pumping commences and the water level stabilises at and around the environment critical level, Holden points out.
“Projects such as this are extremely complicated and do not naturally fall into the category of national water resource, water board or municipal responsibility. They are a prime example of integrated water resource management and will shape projects to come as South Africa reaches the limit of its allocable water resources. They need time to come together to ensure that they are environmentally and financially sustainable,” Holden reiterates.
Sidebar: Key role players
- Responsible government department Department of Water Affairs
- Implementing agent TCTA
- Design Aecom
- Main contractor Group Five Civil Engineering
- Piping Murray & Dickson Construction
- Earthworks Diesel Power
- Mechanical S.A.M.E
- Electrical Standard Electrical
- Civil foundations M3 Construction
- Building and civil works Enza Construction
The next priority: The Eastern Basin
The severity of the situation in the Eastern Basin is now critical and negotiations are already under way to secure access to land and infrastructure in the Grootvlei mining area. The property (excluding Shaft 4) is in liquidation along with its holding company, Pamodzi Gold.
Sidebar: Critical facts
|Western Basin||Central Basin||Eastern Basin|
|Volume of AMD that needs to be treated||27 Mℓ/d||57 Mℓ/d||82 Mℓ/d|
|Environmental critical level (ECL)||1 550 m AMSL(165 m BCL)||1 467 m AMSL(186 m BCL)||1 280 m AMSL(290 m BCL)|
|Current level||0.88 m BCL||256 m BCL||423 m BCL|
|Breach of ECL if pumping does not commence||Breached already. Objective is to draw the water down to ECL||Sep/Oct 2013||Nov 2014|
|Location of treatment plant||Rand Uranium, Mogale City||South West Vertical Shaft, Germiston||Grootvlei No 3 Shaft, Springs|
AMSL = above mean sea level
BCL = below collar level
Long-term strategy (source: the DWA website)
Addressing AMD in the East, Central and West Rand underground mining basins is taking place through a phased approach and consideration regarding the implementation of a long-term sustainable solution is critical.
A feasibility study is currently under way to determine a sustainable long-term solution. A consortium of consultants, led by Aurecon South Africa, was awarded the tender in December 2011 and the study commenced on 30 January 2012. The consortium is responsible for conducting this study under the strategic guidance of a study management committee, whose purpose is to ensure that the study meets its objectives.
Holden says that the long-term solution, specifically with regards to the Germiston AMD treatment plant, must ensure that the salts do not enter the Vaal River system. One option is desalination – up to potable water standards.
There are five main focus areas that the study is addressing: economics, technical, financial, legal and communication.
The economic analysis will provide a detailed cost benefit analysis (CBA) looking at different scenarios relating to the handling of the AMD problem. The CBA must cover the different options such as the do-nothing scenario as well as an investment scenario. Furthermore, the CBA will examine the benefits and costs of allocating of responsibility for the scenarios to both the public and private sectors. The economic analysis must provide a quantified set of cost and benefits which, wherever possible, can be fed into the financial model.
The technical work is being based on the available reports and is examining technological options that can be used to deal with the problem. One of the questions to be answered is whether a centralised facility or dispersed treatment facilities focusing on the different areas should be deployed.
The technical work also provides input into the financial model by considering the choice of technology, the availability, previous use and sustainability of technologies, as well as the life cycle cost implications. A preliminary design that takes all of the above into account will be prepared to guide government on possible solutions and on deciding on the optimal solution.
It is anticipated that any solution to the problem will require massive investment. The financial analysis must inform the quantum of the investment. A financial model that takes into account the full life cycle costing of all the investments required will be developed.
The financial model must incorporate scenarios of public and private sector investment as well as examining the potential revenue sources, if any, that can be derived from the investment. Where possible, the actual costs and benefits as identified in the CBA will be included in the financial model. The study team will consider strategies for identification, quantification and mitigation of risks that must affect the project and incorporate these in the financial model.
The legal issues associated with the AMD are complex and multifaceted. As part of this study, the legislative issues associated with AMD will be studied and will contribute to the options selection as part of the CBA.
The identification of legal risk, quantification of such risks as well as mitigation of such risks are an important part of the study. The risks that are identified will be translated into a legally sound position to inform any agreements to be negotiated. Legal inputs to any negotiation that may arise from discussions between all the parties will also be provided.
Key stakeholders are engaged throughout the study and measures are in place to effectively communicate study progress. It is expected that the feasibility study will be followed by a formal Environmental Impact Assessment.
Phase 1 (the study initiation phase) is completed, while Phase 2 (the pre-feasibility phase) is in the process of being concluded. Phase 3 (the feasibility phase) is the final phase of this study and has commenced. The complexity of the study necessitated an extension of the study contract. The feasibility study is due for completion on 31 July and will, thus, have been conducted over an 18-month period.