Constructed wetlands for treating highly alkaline industrial drainage

Lead Research Organisation: Newcastle University
Department Name: Sir Joseph Swan Institute

Abstract

Highly alkaline (pH 9-13) drainage waters can result from the weathering of by-products from globally important industries such as steel manufacture, cement production and electricity generation. The extreme alkalinities of the waters reflect the highly basic nature of the source residues which include lime-rich steel slags and fly ashes. These residues have traditionally been landfilled or stockpiled, historically with little or no control of leachate migration. Drainage waters leaving such disposal sites are of such high pH that they absorb carbon dioxide from the atmosphere and precipitate solids (predominantly calcite: CaCO3) so prolifically that streams and rivers are smothered to the point where little or no aquatic life can be sustained in the waters. In addition, elevated concentrations of metals/metalloids (especially arsenic, chromium, selenium and vanadium) and high sulphate loadings can be of significant environmental concern, and a barrier to compliance with statutory water quality standards such as those set out in the EU Water Framework Directive. Established treatment options for alkaline leachates, such as acid dosing, active aeration and/or recirculation of leachates over stockpiled residues, are very expensive. Given that generation of high pH leachates is now known to continue for many years after the operational life of the associated industrial operations, sustained treatment by these traditional methods is rare, and untreated leachates can produce a legacy of persistent environmental damage. Recent NERC research has highlighted the effectiveness of natural wetlands in lowering the pH and alkalinity of these drainage waters. Microbial respiration in the organic-rich wetland substrate appears to accelerate calcite precipitation, a process which lowers alkalinity. These calcite-rich solids can also serve as a sink for some potentially toxic metals. The work proposed in this study aims to commercially develop constructed treatment wetlands as a low-cost, environmentally sensitive passive option for treating highly alkaline waters. Passive treatment systems are characterised by an initial capital outlay but low running costs for infrequent (albeit regular) maintenance. In addition, constructed wetlands create valuable wetland habitat, provide a useable green public space and integrate well with the wider ecological restoration of post-industrial landscapes. This research will develop some of the technical components of constructed wetlands to ensure effective pollutant treatment with regard statutory environmental quality standards, in liaison with one of the project partners: the Environment Agency. Economic feasibility will also be assessed through quantifying calcite precipitation rates and establishing relationships for sizing and costing treatment systems based on water flows and chemistry. This will be carried out through the construction and monitoring of a pilot wetland system at a site belonging to project partner Corus. The principal focus of the work will however be to engage with potential industrial end-users of the treatment technology and develop opportunities for commercial exploitation. While the technology is unlikely to yield any formal intellectual property rights, the technical know-how for successful design of these treatment systems is a valuable asset. The HERO Group at Newcastle University has a strong track record in commercially-exploiting this know-how for the design of novel treatment systems for other post-industrial pollution sources. End-user engagement will come through various avenues, but will be principally undertaken by project partner the Mineral Industry Research Organisation (MIRO) who count in their membership a range of aggregate and process companies who own problem sites. Workshops and meetings convened with potential end-users will be used to demonstrate the technical capabilities of treatment wetlands and encourage industrial uptake of the technology.

Publications

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Mayes WM (2009) Preliminary evaluation of a constructed wetland for treating extremely alkaline (pH 12) steel slag drainage. in Water science and technology : a journal of the International Association on Water Pollution Research

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Mayes WM (2009) Wetland treatment at extremes of pH: a review. in The Science of the total environment

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Mayes WM (2008) Preliminary evaluation of a constructed wetland for treating extremely alkaline (pH 12) steel slag drainage in Proceedings of the 11th international conference on wetland systems for water pollution control

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Mayes W (2008) Hydrogeochemistry of Alkaline Steel Slag Leachates in the UK in Water, Air, and Soil Pollution

 
Description Some industrial processes produce residues that generate highly alkaline (pH 9 -13) wastewaters (leachates). Such residues include lime works spoil, steelworks slags and coal combustion residues. The leachate arising from these residues is highly damaging to streams into which it discharges. Previous research demonstrated that effective buffering (lowering of pH) of such leachates could be achieved using wetland technology. The aim of this research was to develop commercially exploitable constructed treatment wetlands for remediating alkaline leachates, by engaging end-users of such technology, understanding the rates of the most important geochemical reactions in order to calculate the size of wetland required for individual alkaline discharges, and undertaking a technical and economic feasibility assessment of such wetland technology. The main finding of the research was that wetland technology was a technically and economically viable treatment option for such wastewaters. Consequently there was considerable interest from end-users in exploiting the technology, and the research team was engaged by such end-users to assist with this endeavour beyond the end of the research project.
Exploitation Route This project was funded by the NERC Follow-on Fund, and a key objective of this research was therefore to investigate the potential for end-user exploitation of wetland technology for treatment of alkaline and wastes. There is considerable interest from end-users in exploiting the technology, as it is a cost-effective alternative to chemical treatment of alkaline leachates.
Sectors Chemicals,Construction,Energy,Environment

 
Description The research generated significant interest in wetland technology for treatment of alkaline leachates. There was particular interest from industries that generate such wastewaters, but also from environmental regulators and consulting engineering firms. One industrial partner funded additional work to investigate the feasibility of wetland treatment at a specific site where alkaline leachates are a problem. High pH leachates are a worldwide problem, and the researchers also discussed finding of the work with researchers and industrialists in Europe and Australia. Environmental managers at facilities that generate alkaline wastes are now far better placed to exploit wetland technologies for remediation of these leachates.
First Year Of Impact 2009
Sector Chemicals,Energy,Environment
Impact Types Societal,Economic