Blindwells Mine Water Treatment Scheme

Blindwells Mine Water Treatment Scheme

Location and Background

Situated in East Lothian the Blindwells scheme protects the areas of Longniddry, Musselburgh, Tranent and Cockenzie from flooding as a result of rising mine waters.  The scheme also prevents over 100 tonnes of iron ochre per year entering the Firth of Forth.

Site History

By 1990 both deep and opencast coal operations had finished, this resulted in a rise in the local mine water regime leading to flooding of farmland and property immediately north of the Blindwells Site.  Pumping recommenced at Blindwells to reduce the mine water to a lower level in order to address these problems in 1992.  Pumping also prevented any new uncontrolled, polluting outbreaks of mine water at the coast.  This pumping continues and alleviates the flooding situation.  Until 2010 the pumping resulted in large quantities (circa 300 litres a second) of mine water to polluting the Seton Burn for 1km before discharging across a public beach and into the Firth of Forth.

The treatment scheme maintains the pumping operations at Blindwells whilst removing the polluting iron from the water.  

 

Water Chemistry

Mine water Parameters
Flow rate
300-400l/s
pH
7
Alk/Acy
Net Alkaline
Iron
5mg/l

 

Site Design

A new bypass pipe and associated chambers was constructed to divert the existing flow from existing borehole N°5 to enable the drilling of a new borehole N°5A and the capping of two existing mineshafts.

Borehole N°5 and N°5A will have pumps capable of pumping up to 450 l/s.  Each borehole has new headworks fitted providing air valves, a point for inserting a ‘pig’ and a chamber housing an electromagnetic flow meter.  Within the borehole N°5 compound a control kiosk houses the electrical switch gear and pump controls.  Both borehole compounds are enclosed within a 2.0m high open weld mesh fence.

The two rising mains of 315mm OD SDR17 MDPE pipe, laid with a nominal 0.9m cover.

The treatment area covers an area of 5ha and comprises a 30m long precast concrete cascade, 1.2m deep conditioning zone and 3N° reed beds with associated inlet / outlet structures.  The reed bed levels have been designed to give a gravity flow through the system and also a piped bypass system.

The reed bed is lined with 2mm HDPE membrane with texture finish to both sides.  The cascade and conditioning zone are enclosed by a 2.0m high open weld mesh fence, 2N° double access gates for de-sludging and 1N° personnel gate for regular access.  Both mains discharge into a deep inlet chamber running the full length of the cascade discharging via a weir down the cascade steps and into the conditioning zone.  The cascade steps, conditioning zone and reed bed 1 can be bypassed by opening Penstock Ref P01 in Manhole MH1.

The reed beds are all inter-linked using precast standard concrete units and bypass sumps with penstocks to enable each reed bed to be by-passed individually, to aid maintenance. 

The scheme includes 16,200m2 of reeds, planted at a density of 4/m2.  Topsoil, to a depth of 300mm, will be place over the HDPE membrane in the base of the reed beds as a growing media.  3N° precast headwalls and a 50m length of discharge open ditch, provided with a rectangular flow monitoring weir plate, are present prior to the discharge point at the site boundary into the existing ditch prior to D7 culvert.

 

Access to Blindwells mine water treatment site
Access to Blindwells mine water treatment site
Reed bed at Blindwells shortly after planting
Reed bed at Blindwells shortly after planting

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