Nenthead Mine Water Treatment Scheme - How a treatment scheme works

Overview

How would a treatment scheme work?

Mine water would be captured at the Caplecleugh and Rampgill levels before it goes into the River Nent. The mine water would be pumped through a small buried pipeline up to the treatment ponds near the Handsome Mea reservoir. Metals are removed from the mine water by encouraging naturally occurring geochemical, microbiological and ecological processes to take place in a reactive media layer on the base of lined treatment ponds. Sulphates present in the mine water are converted into dissolved sulphides which bind to the metals to form solids that are retained in the reactive media. 

We expect that more than 90% of the metals will be removed in the treatment ponds. The treated water then goes through a wetland which helps put oxygen back into the water before going back down another pipeline and back into the River Nent near the levels. The reactive layer will contain a mixture of woodchip, straw, compost and/or limestone.

The metals are removed from the water by binding with the sulphide that is generated by natural reactions in the reactive layer. It is likely that not all the sulphide will react with the metals and a small amount of hydrogen sulphide (H₂S) may be created which can cause an unpleasant smell if it is not controlled. There will not be any health risk from this hydrogen sulphide.

Odour management

We will install systems to destroy hydrogen sulphide before it can cause an odour nuisance. We expect to add hydrogen peroxide to the effluent in buried tanks next to the treatment ponds since this is the most common method used by industry worldwide. We know that dosing with hydrogen peroxide will work to reduce odours from the site, and that the operational risks, including transport, delivery, storage and use, can be managed safely. This is already done at several Coal Authority mine water treatment schemes.  

We are considering alternative odour management techniques that will achieve the same outcome but are more sustainable. Options include filters that capture the hydrogen sulphide from the air before it causes an odour nuisance. 

Maintenance of treatment beds

We will be frequently monitoring water levels in the treatment ponds along with metal concentrations in the original mine water and the treated water. These data will tell us how the system is performing and particularly whether the permeability or metal removal are decreasing. If performance decreases too much then maintenance will be carried out. Maintenance of the treatment ponds is likely to be required every few years. This will involve turning the material over with a rotovator under water. Completing this under water will prevent the maintenance activity from causing odours. 

Full replacement of the treatment media will likely need to be carried out approximately every 20 years and disposal of waste sediment, which would be similar to industrial effluent sludge, will follow regulations at that time. Under current regulations this would see the waste material go to landfill or another permitted waste site.  

We expect that if planning permission is granted by Cumbria County Council, there would be conditions requiring prior approval of the proposed approach to managing the reactive media. Such a condition was attached to the Nent Haggs scheme.

Reed Beds

The use of reed beds as part of a treatment scheme serves two purposes. Firstly to oxygenate the treated water coming out of the treatment ponds, and secondly to increase biodiversity and carbon capture. Where we install reed beds, appropriate species will be used to support the local habitats and environment.  

We are currently exploring alternative options for aerating the treated water, for example the picture below shows an aeration cascade at the Saltburn scheme which removes the iron from the mine water. More information about the Saltburn scheme can be found here - Saltburn Gill mine water treatment scheme - Case study - GOV.UK (www.gov.uk)

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