Mining, the great engine of economic development in South Africa over the past 140 years or so, has come with a substantial sting in the tail.
Discharge of acid mine drainage (AMD) wastewaters, which follows closure of gold and coal mines, now threatens the water-scarce and fragile ecosystems of the industrial hub of the country to which mining itself gave rise.
That the implications of AMD discharge following mine closures would be severely problematic, and would ultimately require intensive management, has been highlighted for at least the past 50 years. And, while effective treatment technologies are now available, it is the long-term nature of the problem that threatens to undo a rationally-based and cost-sustainable approach to remediation. Estimates of how long AMD flows can be anticipated range over decades to many centuries. Roman mines in Britain and Bronze Age workings in Spain are still actively generating AMD today. Severe contamination of rivers by coal mines is commonly reported 50-100 years after closure.
South Africa has contributed to a particularly fertile response to this problem in the research and development of technologies for AMD treatment, and an impressive level of innovation has been reported in this area. Notwithstanding this technical excellence, the cost of treatment remains the unresolved issue and substantial financial subventions of the treatment operations are generally needed to allow them to function. Disappearance of mine owners over the past century in some cases, and reluctance of the State to accept financial responsibilities for treatment in others, has resulted in inaction and the problem escalating to current critical levels. The major question remains one of who will pay for treatment over the time frames projected.
One of the indigenous AMD treatment technology developments, the Rhodes BioSURE Process, (sponsored by the South African Water Research Commission), has focused on the specific process development objectives of large volume treatment linked to long-term process sustainability. Following observations by students at Rhodes University some years ago on the enhanced breakdown of complex organic substrates in sulphate-reducing environments it was shown that sewage sludge (and other organic wastes) could be used to fuel a microbially-driven process in which acidity in AMD was neutralized, and sulphate salts and heavy metals could be removed. Apart from the low-cost advantages of biological processing, the opportunity costs in the co-disposal of waste and recovery of treated water as a value-added product offered additional possibilities.
Photograph of the Rhodes BioSURE Process pilot plant under construction at the No.3 Shaft Grootvlei Gold Mine in Springs, Ekurhuleni, Eastern Witwatersrand
The development of the Rhodes BioSURE Process has followed a more or less textbook case of technology R&D, with a lead time of about 15 years from initial observations and fundamental descriptive studies to full-scale process demonstration and implementation. Initial observations of complex organic substrate utilization in fueling sulphate-reducing microbial processes was made in tannery ponds and, in follow-up fundamental enzymology and kinetic studies, it was shown that compared to incomplete methanogenesis, sulphate-reducing organisms enhanced substrate utilization rates. These observations were transferable across a range of complex organic carbon wastes and were shown to be particularly effective where sewage sludges were used.
Following laboratory bench-scale studies in which the various features of the process were characterized, a programme of process development scale-up commenced.
This led to the construction of a 40m3/day pilot plant at Grootvlei Gold Mine, Springs, where AMD was being actively pumped from the mine to the Blesbokspruit Ramsar site wetland. Primary sewage sludge as the carbon source for the process was supplied by ERWAT’s Ancor Sewage Treatment Works close by. This pilot plant was operated over an 18 month period and the operational and design criteria derived provided the basis for a decision to proceed to the construction of a technical-scale plant at the Ancor Works. This required the installation a 2.4 km pipeline to pump AMD from the Grootvlei Mine to the Ancor Works and Dortmund tanks available at the site were converted to function as process reactor vessels. The technical-scale plant was sized to treat 1.6 ML/day AMD and over the study period it was shown that sulphate levels could be reduced to <100 mg/L at hydraulic retention times of around 12 hours.
Photograph of the Dortmund tanks that were converted into reactor vessels in the technical-scale plant constructed at Ancor Works, Springs. (A) reactor under construction (B) completed reactor sealed to prevent escape of sulphide and to maintain anaerobic operating conditions
Photograph of the full-scale Rhodes BioSURE Process plant showing sealed upflow Recycling Sludge Bed Reactor modules, sulphide removal unit operation, mine water feed tanks, clarifiers and minewater balancing tank in the background.
Following detailed process kinetic modeling studies which included the development of a computational process simulation programme in collaboration with engineers at UCT and UKZN, design work commenced on the construction of a full-scale Rhodes BioSURE Process plant to treat 10 ML/day. The process was designed to remove sulphate to levels below 250 mg/L and long-term operation has resulted in the removal of more than 12t/day of sulphate. Sewage sludge was used at a rate of 0.85mg biodegradable COD/mg sulphate reduced.
One of the main outcomes of this development has been the demonstration that the utility company provides a feasible operational environment for dealing with the long-term management of the AMD problem. These organisations have the infrastructure, technology and expertise required to manage large volumes of wastewaters on a daily basis and at as low a cost as possible. Their business investment model focuses over the long-range (50 to >100 years) and the cost recovery potential in waste co-disposal and water recovery present income opportunity incentives. With these factors in place, it is possible that treatment of AMD might well be feasible over the long time-frames that need to be contemplated. However, whether private incentive will work as the prime driver in implementing the technology seems uncertain at present, but it is more likely that decisive input from Government will be required to facilitate the co-operation across various interest sectors necessary to make it happen.
*Peter Rose is Professor of Biotechnology Emeritus, Rhodes University