Eastern basin AMD plant lauded for success achieved since coming on stream in June 2016

3rd March 2017 By: Ilan Solomons - Creamer Media Staff Writer

Eastern basin AMD plant lauded for success achieved since coming on stream in June 2016

EASTERN BASIN TREATMENT PLANT The high-density sludge treatment plant IS running at about 85% of its 108 Mℓ/d capacity, thereby treating around 94 Mℓ/d of acid mine drainage
Photo by: Duane Daws

Water and Sanitation Minister Nomvula Mokonyane has lauded the successes achieved by the Eastern basin acid mine drainage (AMD) treatment plant located in the town of Springs, in Gauteng, since it began operating in June 2016.

Speaking during a visit to the facility last month, the Minister highlighted that the plant formed an integral part of the Department of Water and Sanitation’s (DWS’s) long-term plan to combat the risks posed by AMD. She noted the solutions would cost her department, with support from industry, R10-billion to R12-billion in total by 2020.

The AMD plant services a surface area of about 768 km2 and its infrastructure includes three parallel reactor circuits, three thickeners, a shaft abstraction pump, a high-voltage electrical room, the motor control centre, a lime dosing system, generator stations and a polymer dosing station.

The plant was jointly designed by engineering consultancy Aecom and civil engineering firm Golder Associates. The plant’s main civil infrastructure contractor was the CMC/PG Mavundla Eastern Basin Joint Venture and industrial water treatment company Tecroveer Thanda Manzi supplied the plant with all its mechanical equipment and electrical control and chemical dosing systems.

The R1-billion plant was completed under the instruction of State-owned water management and project implementation authority Trans-Caledon Tunnel Authority (TCTA), as directed by the DWS. The plant is operated by Tecroveer, under the auspices of TCTA.

Mokonyane stated that TCTA had been awarded a second AMD treatment phase. An assessment of the most cost-effective and sustainable method to upgrade the AMD-treated water to a potable standard would be made. She said that the treatment options under consideration included reverse osmosis and electrocoagulation, among several others.

“The implementation of our long-term strategy will provide [environmental] security for the country’s economic hub of Gauteng,” said Mokonyane. She explained that the water treated at the plant would also assist in alleviating water constraints and improve water quality in the Vaal river system for at least the next 30 years.

Plant Capabilities
Eastern Basin AMD treatment plant manager Roelof van Wyk told Mining Weekly during the visit that the high-density sludge treatment plant was running at about 85% of its 108 Mℓ/d capacity, which meant it was treating around 94 Mℓ/d of AMD.

He noted that the plant was designed to run continuously and had three heavy-duty Ritz dewatering pumps, located 150 m below surface, with each pump pushing 1 530 m3/h of water to the surface from the old Grootvlei No 1 shaft, which extends to a depth of 1 km.

“The AMD feed is transferred to a splitter box and, from there, it is separated across three parallel reactor circuits, which consist of preneutralisation, neutralisation and gypsum crystallisation,” Van Wyk explained.

Further, he pointed out that the plant consumed on average 35 t/d of lime and 500 kg/d of polymer, which was required to deliver water of a reasonable quality that was clarified in three 43-m-diameter thickeners before being discharged.

“From the thickeners section of the plant, a clean overflow of water is produced, [as is] a low-quality underflow water. The underflow water is recycled back into the plant for processing purposes, while the excess sludge is disposed down No 1 shaft to a depth of 700 m below surface.”

Van Wyk noted that boreholes were currently being drilled to create a mining void where the plant’s management team intended to deposit the sludge in future.

He said that, currently, there was no need to run the plant at full capacity to maintain the shaft level at 20 m below the environmental critical level (ECL). “We are nearing this limit, owing to recent summer rains. In winter, the plant will likely only run at about 66% capacity,” Van Wyk explained.

He contended that, if the plant’s AMD pumps were not running, the water level in the basin would rise by 600 mm a day. “The way that we are pumping currently ensures that AMD water levels are being reduced by 30 mm a day.” Van Wyk estimated that, if pumping were to be stopped for three weeks, AMD would start decanting in the lower-lying regions of the Eastern basin.

He remarked that the water level at the plant was currently 115 m below surface. However, when the plant started pumping and treating operations in June 2016, the AMD water was at a level of 107 m below surface. Therefore, the AMD was “very close” to breaching the ECL and thus flowing out to the lower lying areas of the Eastern basin into surrounding wetlands.

“We need to get to a level of 120 m below surface to have a safe buffer between having AMD decanting into the surrounding areas of the Eastern basin and ensuring that, instead, it decants into the shafts. This will avoid seriously negative environmental events occurring in the region,” Van Wyk highlighted.

Treated Water Quality
Meanwhile, he said the AMD water had a pH of 6.2 and an iron content of about 115 mg/ℓ, but, after being treated, the water’s pH increased to 8.8. “The salinity of sea water is about 16 000 mg/ℓ, while we are releasing treated water with a salinity of 1 100 mg/ℓ, containing an iron content of 0.1 mg ℓ and manganese levels of 0.01 mg/ℓ. The treated AMD water contains levels of aluminium that are so low that they are not detectable at our laboratory because its equipment can only detect substances with levels of 0.008 and above and it is obviously below this detection threshold,” Van Wyk pointed out.

He added that the uranium levels of the treated AMD water were tested on a biweekly basis at an external laboratory and no traces of the radioactive metal had been detected to date.