Hazardous waste salted away in safety

The output of the Winsford rock salt mine in Cheshire has been in the news with the winter snows causing hazardous road conditions nationwide, but the vast underground chambers of the mine itself also offer a convenient facility to deal with another hazard - waste. LAWE reports on the Minosus company which operates safe and secure storage to deal with part of the hazardous waste market which has an estimated value of over £250 million

Minosus is a 50/50 joint venture between Compass Minerals UK, owner of the Winsford rock salt mine and subsidiary of Compass Minerals International Inc, and waste management group, Onyx, part of the global Veolia Environment Group.

The company was formed in 1997 to evaluate and develop waste management options in worked out areas of the mine.

The Minosus mine, operated by Salt Union Ltd within the Compass group, produces rock salt, with 90% of the output from there used for road salt.

Extending some 5km from west to east and 3km from north to south, the void space in the mine now measures some 23 million cubic metres.

The waste storage concept evolved because Salt Union was looking for alternatives to salt production and to find a non-weather dependent income. The company even considered turning part of the mine into a prison.

However, they then came round to looking at document storage and waste storage. Documents have been stored here since 1997.

Recognising that the EU Landfill Directive was likely to be implemented in the UK, spelling the end of co-disposal of hazardous and non-hazardous wastes in landfill sites, Minosus decided to go forward with a project to create an underground storage facility for the permanent disposal of selected hazardous wastes.

The company took the view that the deposition of hazardous wastes in the dry, stable and secure “cocoon” afforded by void space within a 200 million year old bed of salt was a better practicable environmental option than their deposition in hazardous waste only surface landfills.

Onyx estimates that the hazardous waste market has a value of £264million, with 100,000 businesses producing hazardous waste in 2003. The volume of hazardous waste has more than doubled in the past decade.

Initially planning regulations caused a major headache, mainly due to local residents’ concern over increased traffic along the neighbouring roads.

Salt Union MD, David Goadby lives locally, so they had to get it right. He said: “We expected to see pickets at the gates, but when I met a local environmental campaigner and explained what we were doing, he said ‘it seems like a good idea’.”

Work on the surface and underground facilities began in January 2005 and the first consignment of waste was received on the 22 August 2005.

Waste categories

The Minosus IPPC permit, issued by the Environment Agency in August 2004, allows the company to receive 42 individual hazardous waste categories, with a further 24 that can be accepted subject to specific Environment Agency agreement. The waste types are predominately those from the thermal process industries and essentially comprise fly ashes, slags and drosses.

Whilst underground storage facilities are exempt from the need to meet the Waste Acceptance Criteria governing leachate limits – a key issue for surface landfill sites – Minosus has its own criteria to ensure that its activities pose a threat neither to human health nor to the integrity of the mine.

All wastes received must therefore be dry, in solid, granular or powder form and must be non-volatile, non-combustible or flammable and non-reactive. The receipt of radioactive wastes is prohibited by a separate legal agreement.

The first waste type to arrive was Air Pollution Control (APC) residue – the fine powder material that results from the cleaning of the stack emissions at waste to energy plants.

The Minosus planning permission allows the facility to receive up to 100,000 tonnes a year of waste, to be delivered in a maximum of 25 lorries per day. The permitted void space underground occupies two million cubic metres – less than 10% of the whole mine – and is expected to take between 15 and 20 years to fill.

Waste handling

Acceptable waste from the waste producer is transported to Minosus packaged in either flexible intermediate bulk containers (bin bags) or in steel plastic drums on returnable pallets. Waste is sampled and analysed for compliance at an on-site laboratory. Once approved the waste in loaded on to transit capsules – purpose-built steel containers that are use to transport the waste from the surface to the final underground disposal location which is permanently logged.

An automated conveyor system loads full transit capsules into the lift cage at the surface and removes them from the cage on their arrival in the mine.

A tractor unit, capable of pulling 90 tonnes, tows up to six capsules on three self-steer trailers on the 2.5km journey to the disposal area.

The disposal area itself is divided into a number of “rooms” so that different types of waste can be segregated. On arrival the transit capsules, which remain on the trailers, are opened and the waste containers are lifted into their final disposal position.

Once the hazardous waste storage has been filled, concrete bulkheads will be installed that will seal off the disposal area from the remainder of the mine.

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