The thread about the great untold engineering feat of Edinburgh’s 1970s Interceptor Sewers and the grand scheme to clean up the Firth of Forth

This is Part 4 – the final installment – of my threads on Edinburgh’s sewer story – the great untold engineering feat of the 1970s Interceptor Scheme.

We left off Part 3 of this story at the end of the 19th century, when Edinburgh and Leith had finally managed to clean up the polluted Water of Leith by the construction of two “interceptor” sewers to prevent waste getting into it. However none of the sewage that was diverted by these sewers and prevented from entering the river was simply piped below the low tide level and discharged, raw, into the Firth of Forth where it was hoped for that the tide and currents would move it offshore and out into the estuary. As the city grew in the 20th century, this same basic system was expanded to cope; new sewers were added to keep other rivers and streams such as the Almond, the Jordan / Pow Burn, the Figgate Burn, the Braid Burn etc. clear, but these all just diverted the effluent out into the Forth. hoped for the best that the tides and currents would carry it out to sea

The principal sewers of Edinburgh by the middle of the 20th centurty
The principal sewers of Edinburgh by the middle of the 20th century

In 1951, the Rivers (Prevention of Pollution) (Scotland) Act made provision for the establishment of river purification boards to begin to tackle the issue of discharge of raw waste into tidal waters. This saw the Lothians River Purification Board created. This had the power to make the Forth subject to a “Tidal Waters Order” and in 1960 such an order was made. A follow on act gave the Board the power to make the Edinburgh Corporation, as it was, take action through the licensing of discharges and making that subject to conditions.

River Purification Authorities and Boards in Scotland as set up in 1951
River Purification Authorities and Boards in Scotland as set up in 1951

The clock was ticking now for the Corporation to take action. Initial planning of what should be done started in 1966 and was agreed with the Purification Board in 1967, to be completed by 1976. The fundamental challenge was Edinburgh’s hilly topography that divided the city up into different water catchments – and had caused the sewage network to develop piecemeal following the natural drainage. As such the system was in no way integrated and had 9 separate discharges into the Forth, none of which were treated.

Most of the existing older sewers were “interceptor” sewers, the new system had to “intercept the interceptors”. It then had to consolidate all this waste somewhere for treatment; the natural “centre of gravity” of the city’s drainage is in Leith Docks. It had to run East-West along negligible gradients and yet still be gravity worked, to avoid the expense of pumping. It then had to treat all of the waste and discharge the treated primary effluent somewhere beyond the gently shelfing seabed, where the weak tidal flow made natural dispersion tricky. And lastly it had to do something about the sewage sludge leftover from primary treatment. So there was a huge amount to do.

The 1970s Interceptors (green) intercepted all the existing sewers (brown) to prevent them from discharging directly into the Forth
The 1970s Interceptors (green) intercepted all the existing sewers (brown) to prevent them from discharging directly into the Forth

At the centre of the scheme would be a new primary treatment works near Leith Docks at Seafield and requiring land reclamation. This would dispose of the treated effluent through a single 2.8km long outfall that reached far out underneath the Forth and it would capture the primary sludge to be disposed far off in the other reaches of the Forth by means of a disposal tanker. The works would be fed with the sewage of the city by two vast new interceptor sewers, one each approaching the works from the east and the west, tapping off all of the existing raw discharge outlets into the Forth.

Schematic of the 1970s Sewage Disposal Scheme. Credit ICE

Things moved remarkably quickly. The council approved the scheme in 1970 and got approval from the Scottish Development Department for the necessary financial loans. Shovels began to enter the ground in 1971 for exploratory works and the first construction contract followed within months. The new interceptors were to be 15.3km long in total and of a large diameter, getting wider as they went. The western sewer started at 1.4m diameter and grew to 3.1m. The eastern sewer started at 0.7m and grew to 2.3m. They intercepted the old network at 26 separate points. It was not possible to work entirely under gravity and 6 pumping stations were required to lift the waste uphill via rising mains from parts of the coast that would not naturally drain themselves.

A huge engineering challenge was the sheer range of geological conditions to be dug through; coals, sandstones, mudstones, clays, shales, geological folding, jointing, the Pentland Fault and all sorts of impressively named features like the “Midlothian Syncline“. Most of the route ran under built up areas and therefore 73%, or 11.6km, of the route would have to be tunnelled. The geology required the use of explosives, limited by the presence of the buildings overhead. Much of the excavation was done by compressed air at high pressure, this pressure helped hold back the ground water given how shallow the tunnelling was through fairly wet ground. In some places the ground had to be artificially drained in advance and in others, where there was no other option due to the ground being saturated fine silt and sand, liquid nitrogen was injected into the ground to freeze it, making it stable enough to dig through. As if these weren’t challenges enough, the western sewer also had to cross the Water of Leith at some point. The location was in St. Mark’s Park, which required an inverted syphon on the river bed. This was constructed by building a cofferdam, diverting the river around it, and draining the river within.

The new treatment works was reclaimed out of the East Sands of Leith by extending the existing sea wall of the dock railway yards and then filling in behind it; there was still no shortage of demolition rubble from clearances in the city in the early 1970s, plus an almost endless supply of colliery waste.

Semi transparent overlay of early 1960s OS map over modern aerial photograph. The white line shows the sea wall line prior to the construction of Seafield Sewage Works.
Semi transparent overlay of early 1960s OS map over modern aerial photograph. The white line shows the sea wall line prior to the construction of Seafield Sewage Works.

The sewers and works were calculated to treat the waste of a population of 470,000 within its drainage catchment – 250,000 cubic meters of flow a day. It allowed for an increase to 600,000 people and 336,000 cubic meters a day. While the works has grown since to make use of this built in capacity, the core of it is still the 1970s construction.

Plan of the Seafield Sewage Works as it opened in 1976 overlaid on modern aerial photography. Credit, ICE.

There are a couple of great pictures here of the Seafield works in the later phase of construction from Beqi’s Flickr:

Prepare for the stench
“Prepare for the stench”, embedded form the Flickr of beqi. Looking west towards Leith Docks from the top of one of the lighting towers.
Sewage in the works
“Sewage in the works”, embedded form the Flickr of beqi. Looking southwest towards Leith Links from the top of one of the lighting towers.

In the second picture above, notice the 3 concrete sewage inlets rising out of the ground into the screens, just below the big white shed building. These screens separated insoluble solids and flotsam out of the sewage, which were scraped off the screens, collected in skips and taken to the Corporation incinerator at Powderhall for disposal. Grit was also removed, washed and disposed of and storm tanks collected any excess inflow for later treatment.

Waste scraped from the screens at Seafield, where it is fed into the skips to be taken away for disposal. Embedded form the Flick of Scott.

After screening, the waste was pumped into one of the 8 sedimentation tanks (the large concrete circles) where the solids settled out into a sludge on the base of the tank. The treated liquid effluent was then drained off for disposal at the outfall. The concentrated sludge was collected and pumped to two storage tanks at Leith Docks. The sludge tanks at both Seafield and Leith Docks were fitted with a deodorising system, whereby Ozone was generated in a high voltage discharge, dissolved in water and then sprayed over the stench. I can’t comment if that worked! From the docks, the sludge would be collected by the Corporation’s new, purpose-built sewage tanker, the amusingly named MV Gardyloo.

Gardyloo at Leith, c. 1995. CC-by-SA 2.0, Alljengi
Gardyloo at Leith, c. 1995. CC-by-SA 2.0, Alljengi

Two or three times a week, the Gardyloo would carry it out to the outer reaches of the Forth between the Bell Rock and St. Abb’s Head (depending on the season) and then dump it overboard. In a typical display of 1970s municipal pride, the vessel was painted in the Corporation colours of maroon and white and was licensed to could carry passengers; the city would “treat” school children and pensioners to a 7-hour day trip on the boat, complete with a lunch of fish and chips from the vessel’s cafeteria. The paying public could also take this offer up for a small fee. You can see an amateur cine film of one of these trips at the National Library of Scotland’s Moving Image Archive.

This just left the liquid effluent to get rid of. This was (and still is) by means of a gigantic outfall shaft, 7m in diameter and sunk 52 meters straight down into the ground before turning through 90 degrees, reducing in diameter to 3.7m and heading out to sea for 2.8km.

Sinking the outflow shaft. Credit; ICE.
Sinking the outflow shaft. Credit; ICE.

The outfall tunnel was constructed according to standard colliery practices. This makes perfect sense; the miners and engineers of the Lothians coalfield had been digging roadways under the Forth in this manner for a hundred years or more. During construction, the outfall tunnel had a double track narrow-gauge, battery-powered railway in it to bring the excavated spoil back to the main shaft for disposal. The tunnel was made wider at this point to accommodate a siding of the railway for spoil wagons. The diffuser outlets were drilled down into the end of the outlet tunnel from above the surface by the jack-up rig GEM126 moored overhead in the Forth.

GEM 126 drilling piles for the Immingham oil terminal in 1970
GEM 126 drilling piles for the Immingham oil terminal in 1970

Work on the whole scheme proceeded remarkably quickly and efficiently, slowed down mainly by restrictions on spending. The scheme was transferred from the City of Edinburgh Corporation to Lothian Regional Council during local government reorganisations in 1975. The initially completed part of the scheme started operation in the autumn of 1976, with most of the parts of the wider scheme in place by 1978, when the Gardyloo started operations. The estimated cost in 1975 was £26.04 million, or about £224 million in 2021 money.

Class 26 26007 - Leith.
A coal train for Leith Docks passes the recently opened Seafield Sewage Works in 1981. The concrete building on the left was the technical and scientific block. Embedded form the Flickr of Martyn Hilbert.

The construction of the sewers was relatively incident free, however I have read that a mistake during blasting in one section caused two houses above to become so damaged they had to be demolished. I haven’t yet found out where they may have been.

The success of the scheme can be measured in whether or not it cleaned up the inshore waters of the Forth off of Edinburgh and Leith. Limits on the “amenity” beaches were set at 2,400 E. coli per 100ml water. The raw sewage entering the works at Seafield was 20,000,000/100ml. The scheme reduced the levels in the beach waters to 1,000/100ml in the worst flow conditions; 60% below the limit.

From 1978 to 1998, in more than 2,600 voyages, the MV Gardyloo dumped 8.5 million tonnes of the concentrated sewage sludge of the citizens of Edinburgh and Leith into the North Sea. It was as a direct result of the EC (as it was) Urban Waste Water Treatment Directive that this practice ended and secondary treatment of the sludge was finally commenced. The Gardyloo had been passed to the East of Scotland Water Authority in 1996 during another local government reform, and was sold out of service in 1999. After 5 years service as a general purpose tanker she was bought by a company in Azerbaijan and transferred to the Caspian Sea where, in a strange but somehow apt about turn, she is now the drinking water tanker Shollar.

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These threads © 2017-2023, Andy Arthur

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