MSc Aquatic Ecosystems Management


Water Supply

Reservoirs

Water supplies are best taken from upland reservoirs which have lower levels of nutrients and pollutants, making the treatment process easier and less expensive.

Talla reservoir was built in the late 19th century (completed 1899) to supply Edinburgh with water. Water is gravity fed through tunnels. The construction was done by hard labour, employing many men and powered by steam. The dam is earth-fill, with an impervious core and clossely laid cobblestones protecting the face of the dam. The picture shows the state of the reservoir in mid October 2003, following en extremely dry summer: the level had dropped almost to the bottom of the draw-off tower. The reservoir is supplemented by water from the Fruid reservoir, to the south, which was built in the 1960s. The water can be seen cascading into Talla to the right of the tower. The second picture shows the spillway, which was uncharacteristically dry in October 2003.

   

In October 2004, a wet summer had filled the reservoir to capacity and the spillway was running.

       

This view from the head of the glen shows the upper end of the reservoir. Fruid is behind the forested hill.

Megget reservoir was built in the 1980s (completed 1983) to supplement Edinburgh's water supply, since the older reservoirs (Bonaly, Glencorse, Loganlea, Gladhouse, Rosebery, Edgelaw, Threipmuir, Talla and Fruid) were no longer adequate to meet current and projected demand. It was built by damming a valley at a narrow point. Farms and dwellings in the valley bottom had to be moved, along with a historic ruin of a Scottish Tower house (Cramalt tower). The operators of the reservoir (now Scottish Water) had to provide new housing and have the responsibility of disposing of sewage from the dwellings and sheep-dip used on the farms (mostly devoted to sheep rearing). Farming and forestry activities in the catchment are monitored to ensure that there is no use of chemicals which could damage the water supply.

 

The dam is earthfill but with a steeper slope on the pond side than Talla. It is grouted into the bedrock which has been pressure-grouted to ensure that the fissured rock does not leak. All the control works are built in to the dam itself: It does not have a separate spillway, but the excess water spills down a gap around the draw-off tower, and is discharged into a stilling basin at the foot of the dam.

   

Although the reservoir was 11m below the spilling level in 2003, water was still being discharged through the stilling pond as there is a compensation flow maintained to keep the river in a viable condition, and at the time of our visit this was supplemented by a 'freshet'; an artificial spate to mimic normal flow variation in a river, and specifically to aid the upward migration of spawning salmon.

This is the view from the draw-off tower to the head of the reservoir, and from the head of the loch looking towards the dam. The catchment has an area of 40km-2, and the reservoir a surface area of 2.6km-2 and capacity of 64 million m-3. It is capable of supplying 100,000 m-3 per day.

 

This is a view down the spillway shaft: the draw-off tower is a double cylinder, with the outer annulus forming the spillway and the inner tube containing stairway, lift, draw-off pipes and valves, a laboratory and other monitoring equipment.

   

Water can just be seen (and heard) at the bottom of the shaft: it is being drawn off via the scour pipes at the bottom of the reservoir to form the freshet. These pipes allow water to be run to waste without drawing from the better quality water levels higher up. The tower has 5 levels at which water can be drawn into 2 separate pipes, which means that one side could be shut down for maintenance if necessary. At the time of our 2003 visit, both pipes were drawing water from the second level (from the top).

At the bottom of the tower the vertical draw-off pipes turn to the horizontal and the flow from them into the main supply pipes is controlled by a butterfly valve. The grey cylinder is the control gear for the valve: it can be operated remotely, or electrically by hand, or manually by turning the grey wheel. The round vertical plate is an inspection cover which can be removed to allow inspection of the pipe. The blue valve on top of the pipe is an air-release valve to release trapped air when the valve is opened again. The horizontal pipe runs in parallel with the access tunnel. There is another pipe at the other side, and underneath the tunnel is the spillway channel.

Tthe third image is one of the scour pipes: the grey control gear on these includes a slow release control to prevent the valve being opened or closed too quickly. The upstanding blue part is air release. The pipes are steel lined with concrete.

The tunnel connects the bottom of the draw-off tower with the control gallery; the two draw-off pipes run alongside, with the spillway drain underneath. Inside the control gallery are the distribution pipes. The outer end of the access tunnel can be seen, with alongside them the two draw-off pipes. They are linked so that one side could be taken out of commission without affecting the way the water is distributed

 

The two smaller pipes seen alongside the central one are the compensation pipes: the one on the right is basic compensation, supplying the minimum flow necessary to maintain good conditions in the river downstream. The two outer pipes are the supply pipes which carry the water into the tunnel cut between Megget and the Manor Valley. Once into the Manor side, the water re-enters a pipe for delivery to Edinburgh's water treatment system. Some parts of the pipework are duplicated where it has to pass under roads, rivers, etc. Water is distributed to Gladhouse and Glencorse reservoirs for intermediate storage, or passes directly to water treatment plants at Alnwickhill, Fairmilehead, and Marchbank as well as some smaller works which supply parts of Midlothian.

 

Water Treatment:

Fairmilehead Treatment Works

Water treatment entails some form of filtration, followed by disinfection.

Older treatment works relied on Slow Sand Filtration: large open air tanks containing a bed of gravel overlaid with sand, and a herringbone array of drains beneath to collect the water. The purification is actually carried out by a microbial film called the Schmutzdecke layer which develops on the surface of the sand, and is extremely effective at removing suspended material, including cysts of Cryptosporidium. The water overlying the filter is about 1m deep.

 

Unfortunately the slow sand filter is both land-hungry and labour-intensive to maintain, as every 4 months or so the surface of the filter bed has to be scraped when the microbial film becomes so thick that the water passes through too slowly.

 

 

The dark film is the Schmutzdecke layer. The sand can then be washed and eventually re-used, but every time the bed is scraped it becomes shallower, and eventually the whole bed must be lifted and re-laid.

 

A more modern method is Rapid Gravity Filtration. The water is first mixed with a polymer which causes the organic material in the water to flocculate. It is then run into a filter bed containing a sand/gravel mix and is filtered through the bed. This method removes most of the colour in the water (caused by humic acids, etc.) as required by legislation. This is necessary because these would form trihalomethanes (THMs) during the chlorination process.

The flocculated material is removed but gradually clogs up the bed, so periodically (every few hours) the bed is cleaned by stopping the inflow, allowing the bed to drain down, and blowing air under pressure to scour the sand.

 

Then water is pumped in from beneath to wash the dirt off the bed into a trough, to be carried away to a settling tank. The sludge is tankered away periodically to be disposed of as landfill.

 


Sludge settlement tank

The bed is then allowed to settle again, with the large particles settling first, and then it can be put back into service.

These are the pipes and control gear which manage the whole process of rapid gravity filtration automatically:

The green pipes are the incoming water, the blue are the compressed air.

In works which treat water which may contain high concentrations of phytoplankton or other particulate matter, microstrainers may be used prior to RGF; these are rotating drums of a fine mesh which traps the foreign matter. Water is pumped into the interior of the drum and passes through the holes, while the particulates are carried up the sides of the drum then washed into a trough by a jet of water.

Treated water from both filtration processes is transferred to a chlorine contact tank, which is in an underground tank, the blue rectangle on the right in the plan: the dosing lanes are at the right hand end. Chlorine dosages must be adjusted so that consumers most distant from the works still receive water with residual chlorine to disinfect the pipework. A recent development is the addition of ammonium salts to the water, which allows a chlorine residual to remain longer in the water, thus reducing the initial dose necessary.

Lime is also added to control pH, and Orthophosphoric acid to control lead solubility, since many older buildings in Edinburgh still have some lead pipework.


Lime dosing plant

Treated water then goes to the 7 million gallon storage tank from where it is piped to the consumers.

The plan illustrates very graphically why slow sand filtration is being superseded by RGF: the amount of land needed is almost an order of magnitude greater.

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