JOHANNESBURG (miningweekly.com) – There is a dire need for alternative, sustainable cheap processes to treat mining-impacted water (MIW) and acid mine drainage (AMD), says State-owned research organisation Mintek senior scientist Tamsyn Grewar, who calls for support for irrigation as a preferred reuse activity for the treated MIW that is available in South Africa.
The extent to which MIW could benefit the agriculture industry and also provide potable supplies was highlighted at the yearly Southern African Institute of Mining and Metallurgy (SAIMM) Water 2017 Conference, held from July 10 to 11 at Emperors Palace, east of Johannesburg. The event was hosted by the SAIMM and the Mine Metallurgical Managers Association.
Grewar, who spoke on how South Africa’s AMD could positively impact on the water-energy-food nexus, emphasised the country’s fragile reliance on scarcely available surface water. She said: “We are [among] the top 30 driest countries in the world”.
The drought that has affected vast swathes of the country over the past two years – the worst since 1904 – has led to stringent water restrictions being imposed on consumers in Cape Town.
In light of this water sustainability dilemma, Grewar said water security was critically important, with factors such as pollution, drought and population growth posing significant, growing risks to domestic water resources. “AMD and MIW, particularly from the coal and gold mining industries, are posing a massive threat to the quality of the meagre water resources we have.”
Most of the research and development into water treatment technologies and solutions has been on treatment options, with a basket of technologies and concepts being researched and developed to mitigate the scourge in the short term.
However, Grewar said the way forward was not clear.
Currently, the two most popular options for the treatment of MIW include lime precipitation by means of high-density sludge (HDS) treatment and reverse osmosis (RO), with the former currently the “option of choice” for government in treating AMD in the Witwatersrand area.
However, HDS treatment produces large volumes of sludge and the effluent contains high concentrations of sulphate well above the allowable levels for discharge.
Alternatively, RO offers a suitable solution to treat AMD and MIW and can be used to produce potable water. But this technology has significant energy input requirements and is, therefore, less feasible in terms of operating expenditure (opex). The large volumes of brine produced also result in a “number of issues in terms of disposal”, Grewar stated.
She said HDS treatment and RO had several limitations and would, therefore, not be sustainable in the longer term because the regulations regarding the discharge of sulphates are becoming more stringent worldwide.
An alternative solution to treating MIW and AMD over the long term is biological sulphate reduction (BSR). “It is not new technology . . . and has been around since the mid-1980s,” stated Grewar, adding that previous barriers to entry for BSR technology were costly substrates used for microbial growth; however, she said, Mintek had identified “a number of inexpensive carbon sources” that could be used to reduce operating costs.
Mintek is currently investigating both active and passive options using the BSR method to treat MIW and AMD.
BSR employs an anaerobic process that reduces sulphate and base metal concentrations to very low levels. One of the by-products of the metabolism of the anaerobic biological organisms is alkalinity in the form of bicarbonate, which neutralises the effluent.
According to Grewar, BSR technology produces significantly less solid waste, compared with chemical precipitation methods, making it “highly suitable” for the treatment of MIW and AMD, especially in the coal and gold mining industries.
Mintek has undertaken the testing of its active BSR technology by commissioning a pilot plant at a site in Randfontein, west of Johannesburg, in Gauteng.
The plant used crude glycerol as the carbon source for the microbes. Crude glycerol is a waste product from biodiesel production and is, therefore, “very cheap and readily available”, Grewar pointed out. The plant operating with this substrate managed to achieve efficiencies of greater than 90% for the removal of sulphates, as well as base metals, producing an effluent which meets water quality regulations and may be used in the irrigation of crops.
Meanwhile, Mintek is also researching a passive process using the BSR method. The method employs a more complex substrate – comprising wood chips, wood shavings, cow manure and hay – resulting in longer plant operating times, but reduced operating costs. This longer-term benefit is more suited to applications after mine closure to sustainably treat AMD and any MIW.
Mintek has undertaken laboratory testing using a multistage continuously operated column plant, thereby successfully reducing the sulphate content of the effluent by about 97%. The effluent produced is suitable for irrigation.
TREATED WATER USES
Grewar pointed out that water reuse after treatment was not a priority in South Africa, with the bulk of such water being discharged into water courses. “Although discharge should be the option of last resort, it is often one of the first options.”
In many instances, high-quality water is also used to dilute effluent to within discharge quality limits, and high-quality, potable water is unnecessarily used for applications such as the washing of vehicles and flushing toilets, she said.
Such “wasteful daily practices” could be curbed, with treated MIW and AMD being supplemented for such purposes, she said.
The guidelines and legislation governing the use of water recycling were available, but, she said, in many cases, they were ambiguous and confusing, resulting in a negative impact on any technology development going forward.
However, Grewar proposed some options as highly viable uses for treated AMD and MIW in the agriculture industry.
“We compared the maximum contaminant limits of the more relevant elements for potential reuse activities, including crop irrigation; livestock watering; discharge to sewers for sanitation use; discharge to a water course; and use as drinking water.”
Using such treated water for crop irrigation is highly feasible and would go a long way towards assisting farmers in accessing useable water.
Her enthusiasm in using such water for agricultural purposes is rooted in the fact that agriculture accounts for most of the freshwater withdrawals in South Africa, at well over 60%. “This is not difficult to understand – to produce 1 kg of cereal grain requires 1 000 ℓ of water, and to produce 1 kg of beef requires 43 000 ℓ.”
Further, there is an expected increase in irrigated land in South Africa of about 33% by 2030, which will result in an increased shortage of water, with government predicting a massive countrywide shortfall by 2030.
To mitigate the shortfall, “ . . . govern- ment gives us a hint [at the measures it intends taking] in the National Water Resources Strategy II, [in terms of which] it has committed to exploring the use of new technologies for reusing wastewater and for using treated mine water”, Grewar added.
She pointed out that treated MIW and AMD could be used in crop irrigation, particularly the irrigation of food crops, forage crops (for livestock feed) and energy crops for biofuel production. “This is where MIW fits in the water- energy-food nexus, rapidly gaining global recognition as a strategic area of importance, and it is only going to become more of an issue going forward.”
There are other advantages to using MIW for irrigation: treating MIW and AMD to levels acceptable for irrigation, as opposed to potable quality, results in a “massive capital expenditure reduction, which can be as much as 87%, according to an article in the Journal of the South African Institute of Mining and Metallurgy, titled ‘Collection, Treatment and Re-Use of Mine Water in the Olifants River Catchment’, published in 2001.
The journal also states that the opex for agricultural use results in a reduction of as much as 79% in operating costs.
Moreover, much of the irrigated farmland is often located near mine decant sources, thereby limiting the need and cost for water collection and distribution.
SAFE TO USE
MIW/AMD can be used successfully. Grewar highlights that studies have been investigating the use of such water since 1983.
The Water Research Commission and the University of Pretoria have teamed up over many years to conduct a number of “very successful” research projects; the most recent commercial-scale projects incorporated reused gypsiferous, limed mine water and mine effluent in the production of food and forage crops, she explained.
“They have also done extensive research and modelling regarding the safety of the products and the effects of using treated AMD for irrigation on groundwater quality. Currently, the resulting produce is being sold because it meets safety requirements.”
Grewar concluded that MIW and AMD treatment options needed to become less expensive and more sustainable in the long term. “I believe the most suitable option for reusing treated MIW and AMD is crop irrigation, particularly the irrigation of food, forage and energy crops, as this all fits very neatly [in] the water-energy-food nexus . . . I believe that integrated treatment and reuse plans need to be considered a priority in all cases, with discharge the option of last resort.”