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Case Study Wheal Jane: A Clear Success
It was a unique event - one that caught the breath of everyone concerned with the quality of our environment. In January 1992 over 10 million gallons of heavily contaminated water burst from the disused Cornish tin mine workings of Wheal Jane, causing serious pollution.
The highly toxic water, loaded with cadmium, zinc, arsenic and iron, swept into the Carrion River and spectacularly spread its metal-rich contents throughout Restronguet Creek and into Carrick Roads and Falmouth Bay. Action was taken to deal with the pollution and now the Environment Agency has a state of the art treatment works successfully operating. This booklet describes why it is needed and how it works.
A history of minewater treatment
Wheal Jane was one of the last tin mines to be opened and operated in Cornwall. Regular pumping during its productive life depressed the water table by approximately 400 metres. When the mine closed in 1991, the drainage pumps were turned off and the mine began to refill with water.
Due to the geology of the mine, and particularly the presence of iron pyrites in great quantities, the minewater is very acidic (low pH) and carries a variety of dissolved metals that have leached out from residual minerals in the surrounding rocks (in descending order of magnitude, iron, zinc, magnesium, aluminum, potassium, manganese, arsenic, copper, lead, chromium and nickel).
In January 1992 the failure of a plug in the underground Nangiles Adit caused an outburst of heavily polluted water. A highly visible plume of ochreous (iron-rich) and metal-laden water polluted the Carnon River, Restronguet Creek and Falmouth Bay.
One of the Environment Agency's predecessor organisations, the National Rivers Authority, quickly set up a treatment system to prevent further pollution and water has been pumped from the mine and treated ever since. This treatment system was intended as a temporary measure, whilst studies were carried out to determine the best long-term solution to the problem.
The treatment plant used lime to neutralise the water and precipitate dissolved metals. The treated water was then passed to the Clemows Valley tailings dam (CVTD) where the sludge containing solid particles (metallic hydroxides) was allowed to settle. The clarified water decanted from the dam into the Clemows Stream, a tributary of the Cannon River.
The tailings dam was, in effect, both the sludge settlement and sludge storage stage of the process.
The system worked well and prevented many thousands of tonnes of metals entering local watercourses. However, its drawback was its vulnerability to strong winds blowing across the large water surface of the dam and remobilisation of the settled sludge. The effluent was then unacceptable for discharge to the stream. The situation was made worse when ore milling at Wheal Jane ceased in March 1998, and the assisted settling resulting from the input of fine-sized tailings from the milling process stopped. The sand from the mill mixed with the minewater sludge and increased the density of the material deposited so helping to keep the sludge settled.
To ensure polluted water was not discharged to the river the overflow from the dam was shut down and pumping from the mine limited during adverse weather conditions.
The 'UniPure' treatment plant
The Environment Agency employed consultants Knight Piesold Ltd to look into a new treatment plant. An outline design was developed and interest sought from suitable contractors for the plant's design, construction and operation. The contract was awarded to a consortium headed by Hyder Industrial, with Lurgi Invent as process designers and Dean and Dyball as civils design and construct contractor.
The plant proposed by the consortium was based upon the 'UniPure' process - a process based on the same principle of the Ad treatment system - and a small scale plant was set up to optimise the design of the plant.
In the new plant lime is added to raise the pH (make it more alkaline) and cause precipitation of the metal hydroxides, followed by a flocculant to assist in the settlement of solids. Pre-reaction tanks mix the minewater with sludge returned from the clarifier tanks to start the process and help provide a thicker sludge at the end of the process. The reaction tanks are where the lime slurry is added. Following the addition of flocculant sludge settles out in the clarifier tanks. The clear treated water overflows from the top of these tanks, whilst the sludge is continually drawn off at the bottom. A proportion of the sludge is diverted to the sludge-holding tank for discharge to the CVTD, whilst the rest is recirculated through the plant to enhance the settling properties of the sludge.
Lime storage and dosing equipment from the old plant was re-used and the existing mine building houses the new plant control gear.
The clarified water from the plant is discharged direct to the Clemows Stream under a more stringent discharge consent - set by the Environment Agency to protect the natural environment.
The plant creates a far denser sludge than the old plant and this is deposited on the Clemows Valley tailings dam. Only the thickened sludge from the clarifiers is sent to the dam, and this sludge is enclosed in purpose-built 'paddocks'. The paddock idea was developed to enable the supernatant water to be collected and pumped back to the plant for treatment. If the sludge was not confined in this way it would be much more difficult to collect the water and control the distribution of the sludge on the dam. The supernatant water is returned from the dam to the plant for treatment. This system will enable the dam to be gradually dried out, capped and its surface restored to a more natural state.
The plant was completed in October 2000 and is being run by Hyder Industrial Ltd for a 10-year period.
The passive system
The Agency constructed a pilot scale passive treatment system to test the suitability of three passive treatment schemes. The schemes involved the construction of a series of shallow lined lagoons (or cells) within which a combination of aerobic (containing air) and anaerobic (excluding air) environments were created.
When consultants Knight Piesold Ltd carried out their studies into a new treatment plant they concluded that the passive systems worked well, but that too much land was required to extend the passive treatment to all the Wheal Jane minewater which required treatment.
Some 330 litres per second needs to be treated, but the maximum throughput of the pilot passive plant was 6 litres per second. The land area available in the Camon Valley is inadequate to accommodate a full-scale plant.
The passive system is also less controllable and consistent than an active (chemical) treatment plant.
The passive plant is being run as a research project by a consortium of academic institutions, headed by Reading University. There is a leaflet produced by the university that details this project (Tel: 0118 931 6288).
Conservation and recreation
Construction of the pilot schemes in the valley enabled a stretch of bridleway to be improved from Devoran Bridge to above Bissoe Bridge. A footbridge was built at Devoran with a second to be built at Bissoe, which will form part of the Mineral Tramways Project to link North and South Cornwall by bridlepath.
Areas of natural wetland have been created, existing lagoons in the valley have been retained and made safe and extensive landscaping carried out around Jane's Adit.
Principal partners in the Wheal Jane Minewater Project:
- Lead consultant: Knight Piesold Ltd
- WS Atkins Water
- Plymouth Marine Laboratories
- RTZ Consultants Ltd
- Henry Butcher Smith Vincent
- Risk and Policy Analysts Ltd
- Main design consultant: Hyder Consulting
- Main contractor: Hyder Industrial Ltd
- Civil work: Dean and Dyball Ltd
- Process engineering: Lurgi Invent Ltd
- Process licence holder: UniPure UK Ltd
- Electrical contractor: BSA Regal
- Dam operation: Wheal Jane Ltd
- Cost consultants: Capro Franklin AndrewsExternal reviewers:
- Dr P Younger, Newcastle University
- Dr A J Monhemius, Imperial College
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