AECOM designed the plant for the Trans Caledon Tunnel Authority (TCTA). The Eastern Basin AMD plant is one of the largest high-density sludge (HDS) plants in the world, with a maximum treatment capacity of 110 Ml/d.
AMD poses a major environmental threat on the Witwatersrand. The depletion of gold reserves in the area has meant a cessation of mining activities and related dewatering operations, which has resulted in the flooding of mining voids.
AMD is generated when ore and other sulphide-containing mining waste is exposed to oxygen and water. The water in the mining voids thus becomes acidic and contaminated with heavy metals.
TCTA was mandated by the Department of Water Affairs to implement the necessary AMD water management and treatment infrastructure. The project encompassed treatment plants in the Randfontein Estates area (Western Basin), the ERPM south-west vertical shaft area (Central Basin) and the Grootvlei mine shaft no. 3 area in Springs (Eastern Basin).
The Eastern Basin plant designed by AECOM (in association with Golder Associates) followed typical industry practice for water or wastewater treatment works, says Claire Hurrell, senior civil engineer:Africa for AECOM. “Some aspects of the detailed design were verified and optimised using state-of-the-art techniques, which not only added value to the engineering, but also ensured cost-savings for the client.”
Hurrell explains that, due to the size of the project, AECOM decided to standardise on tried-and-tested technology, as this has been proven to work best globally. “However, we were able to improve on the standard abstraction method.”
The three 20 m long by 1 m diameter super duplex stainless steel deep-level abstraction pumps selected were installed from ground level down into the 370 m deep shaft, allowing for the abstraction of the AMD without having to re-establish underground workings.
“Construction began in June 2014 and was completed in August 2016. The total construction cost was just under R1 billion. The main contractor was the CMC/PG Mavundla Eastern Basin Joint Venture, with Andritz supplying the deep-level abstraction pumps,” said Hurrell.
Prior to construction, an underwater camera was lowered into the flooded mine shaft at the Eastern Basin to check for any blockages or significant damage to the shaft. A modified underwater sonar system was also deployed to provide a wider field of view in order to reduce the potential risk of damage to the abstraction pumps.
Such preliminary investigation was essential as the mine shaft had been in disuse for several years, and had seen a lack of maintenance, as well as vandalism by illegal miners. Properly equipped and experienced professional mining rescue teams removed debris and cleared any obstacles from the mine shaft.
Additional constraints were posed by a railway on one side and a wetland on the other. Therefore the plant design allowed for the deep excavations for the thickener recycle pump station to be moved as far as possible out of the wetland area for ease of construction.
However, these excavations still posed a considerable construction challenge, as the excavated material was silty clay prone to shear failures. Hence extreme care had to be taken during excavation.
“A decision was taken to use self-drilling anchors, which flush the area with grout while drilling takes place. This allows the hole to remain open while the voids are being grouted and stabilised in the same operation,” Hurrell highlights. “The solution was implemented successfully, allowing for the necessary lateral support to be installed in the large thickener excavations.”
Environmental considerations were also paramount on this project. It was essential to adhere to the programme dates to avoid AMD rising in the mining void, and reaching a point where it threatened to pollute natural aquifers and decant at surface.
The plant, which has been operating successfully since August 2016, is reducing the AMD level in the mine shaft by significantly reducing the iron and manganese levels and increasing the pH to an acceptable level for discharge into the environment.
Hurrell indicates this project signifies AECOM’s commitment to balancing not only South Africa’s, but the world’s, need for safe, reliable water, while protecting this critical natural resource. “Combining our knowledge of local African markets with a global network of creative engineering and technical expertise, our sustainable, multi-disciplinary approach to water engineering produces high-quality, cost-effective, energy-efficient, functional and visionary solutions.”