The thread about the Interceptor Sewers; how Victorian Edinburgh cleaned up the Water of Leith. Twice.

  1. The first part of Edinburgh’s sewage history was the Thread About the East Foul Burn and the Irrigated Meadows.
  2. The second part of Edinburgh’s sewage history was the Thread About the Sewage Problems of Victorian Edinburgh and How to Fix Them.

At the end of part 2 of this story, Edinburgh and Leith (separate burghs at this time) had finally agreed the terms of an “Interceptor Sewer” for the Water of Leith. What is an Interceptor Sewer? It is a sewer that “intercepts” other sewers and collects their output before it can discharge some place else; in this case into the river of the Water of Leith. This sewer could intercept all the untreated human and animal waste that was currently discharging into the river and carry it safely off somewhere else, out of sight and out of mind. The construction of the interceptor sewer began immediately after approval of the parliamentary bill in 1864, and took 3 years to complete. The engineers were David and Thomas Stevenson.

A tax assessment for the contribution towards the construction of the Interceptor Sewer under the terms of the Edinburgh and Leith Sewerage Act, 1864

The 1864 Interceptor as we shall call it comprises some 5.2 miles (8.4km) of brick-built, egg-shaped sewer. This is the optimum shape for a sewer, ensuring good flow at low volumes, being “self scouring” and requiring a less wide excavation than a circular sewer of the same cross-sectional area.

Egg-shaped (left) vs. oval-shaped (right) sewer cross-sections

The interceptor follows the course of the Water of Leith, from the “irrigated meadow” at Roseburn park – where the Lochrin Burn discharged the waste of the southwest of the city – to the Black Rocks off of Leith. It runs along the river banks in places and along the river bed itself in others. An exception is near Bonnington in Leith where it takes a more direct route, directly under the industries which had been using the river as their sewer. A dogleg branch served the district of Broughton by intercepting the effluent of the Broughton Burn.

Route of the 1864 Interceptor Sewer, overlaid on contemporary Ordnance Survey map.
Route of the 1864 Interceptor Sewer, overlaid on contemporary Ordnance Survey map.

The sewer intercepted some 180 outflows into the river by the method shown below, which meant the old sewers would still overflow into the river during storm events, the internal weir directed most of the waste into the main interceptor. The interceptor got larger as it went downstream to account for the progressively increased volume of waste being added to it.

Cross-section diagram of how the sewer intercepts the old outflows along the river
Sewer overflow of the 1864 Interceptor at the Coalhill in Leith

The brick-built interceptor is around 2.5 feet wide (I couldn’t find a precise measure), with some sections between 2 and 3 feet wide of cast iron pipe, and where it hits the Shore at Leith (picture above) it becomes a 3 foot 6″ pipe, set 20 feet back from the quayside. It is ideally sited to take much of the waste of Leith (which historically either flowed into the river, or was directed towards the shore). The outflow was at a depth 17 feet below the high water spring tides, with the outflow generally being carried away down the Firth of Forth east by the tide. The cost of this scheme was “about £70,000 [£8.9M in current money], entailing a special drainage-rate of 2/6d per £ on the rental within the [catchment area]“.

The pin marked “1864 vent” on the map refers to that curious, square-section “chimney” above the river weir in St. Mark’s Park. It’s not a chimney at all, but a “stink pipe” or sewer vent. Noxious gasses and air compressed by storm events can escape here.

St. Mark's Park 1864 sewer vent, CC-by-SA N. Chadwick via Geograph
St. Mark’s Park 1864 sewer vent, CC-by-SA N. Chadwick via Geograph

The 1864 Interceptor did exactly what it was designed to do, and was very successful at it. It was calculated to carry 600 cubic feet per minute at the outflow, but even by the time it was completed the requirement had more than doubled to 1,340 cubic feet as the population had exploded along its route; “in some cases fivefold“. So the additional waste from the city simply overtopped the collection weirs and continued to find its way into the river.

There was also the continued problem of the waste collecting in the old mill lades at Coltbridge (Roseburn) and Stockbridge, and the same old story of reluctant owners and landlords and fragmented ownership making it very difficult to get anything done about these. In 1873, after nearly 10 years of to-and-fro, the “Edinburgh Improvement Trust” was ready to abandon trying to sort things out. Not even a “forceful petition from Stockbridge residents” having had the required effect. By 1884, a survey by the Trust found that the Coltbridge mill dam had accumulated behind it a 4 foot depth of solid, compacted human excrement and 7.5 feet depth of more liquid matter; the situation was intolerable.

In 1885, the Edinburgh Corporation stepped in and had the Coltbridge mill lades and dam certified as a public health nuisance and obtained an order requiring the owners to clean it up. In the words of the Corporation’s presiding officer:

A deposit of putrescent sewage and other decomposing organic matter [which] is continually evolving noxious gases and germs, of a nature to produce and provoke miasmatic and malarious disease, and to endanger the health of persons who reside in, or who have occasion to frequent the neighbourhood.

Edinburgh’s preferred solution was to fill the mill lades and demolish the weirs below the Coltbridge to cause the river to flow better and scour itself, but leave the lower weir at Leith Mills as a sewage trap. Leith was of course less than pleased by this and the two councils went back to wrangling. The Edinburgh Improvement Act of 1887 gave the Corporation the powers of compulsory purchase along the Stockbridge “Great Lade” (marked brown on the map below). This power they exercised, causing the lade to be progressively bought up and filled in from 1891-1893.

Course of the "Great Lade" through Stockbridge and Canonmills
Course of the “Great Lade” through Stockbridge and Canonmills

A more coordinated approach was required however; in addition to the burghs of Edinburgh and Leith, the Parishes of Colinton and Currie, the County of Midlothian, the Sewerage Commission, the Dock Commission, and the Edinburgh and District Water Trust all had an interest. It was Leith though, that really drove action, after all they were at the bottom of the river so subject to any sewage in the river from their upstream neighbours. They were ill-served by the over-capacity 1864 Interceptor and they were most directly dependent on the water for livelihood and prosperity. Leith Town Council commissioned a series of public health and sewerage reports in 1885 and 1886 to set out the problem and had the whole course of the river surveyed from the uppermost mill to the sea, to detail the causes.

Edinburgh finally got on board with Leith, and together they commissioned a joint report in 1887 that proposed the construction of a duplicate interceptor sewer for the Water of Leith, along a longer course and with a much greater capacity. This sewer would remedy the shortcomings of the 1864 scheme and would serve to once and for all satisfy the requirements of both Edinburgh and Leith regards sewage disposal, and it was to be adequate for the next fifty years of expected growth in demand. Long-term thinking at last!

And so it was that the Water of Leith Purification and Sewerage Act, 1889 went before parliament; opposed by 16 groups of petitioners, mainly mill owners and industries determined to protect their rights to abstract water from the river and discharge effluent into it.

Preamble of the 1889 act
Preamble of the 1889 act

The act passed, and work proceeded on an altogether grander scheme for the whole river, some 22.5 miles long, with a diameter increasing from 4 feet upstream to 9’3″ x 7’2″ downstream. The scheme also included a branch for The Stank, the burn cum drain cum sewer that drained the village of Corstorphine, and the marshy land and farms of the Gyle area into the Water of Leith at Roseburn.

Route of the 1889 Interceptor marked in red, with the 1864 marked in yellow
Route of the 1889 Interceptor marked in red, with the 1864 marked in yellow

The 1889 Interceptor largely follows that of 1864 between Coltbridge and Stockbridge and the two are interconnected in places. At sections it takes a more direct shortcut, allowing the 1864 to continue to intercept the sewers at the river, while channelling the upstream waste by a more direct route to the sea. After Stockbridge the 1889 route heads away from the river to take a more direct route to the sea, running directly under Pilrig Park and Leith Links (where construction was easier), serving the expanding district between Edinburgh and Leith along Leith Walk.

Route of the 1889 Interceptor through Leith
Route of the 1889 Interceptor through Leith

The vent for the 1889 sewer is behind St. Mary’s School on the Links, on the site of the old Roperie, It is another apparently isolated “chimney” and has always proved very hard to get a decent photo of! now that the 15 years and more of being hidden in a stalled building development it is much easier to access. I wonder if the residents of those fancy new townhouses realise what the public “feature” in front of their windows really is!

The 1889 Interceptor vent off of Leith Links, now an architectural feature off Pillans Walk
The 1889 Interceptor vent off of Leith Links, now an architectural feature off Pillans Walk

The two sewers now served a catchment area that was where the city was expanding fastest, shown by the pale orange area in the map below (they also served much of South Leith)

Late 19th century drainage catchment map of Edinburgh, the pale orange area had been served by the Water of Leith until the Interceptor Sewers took over.
Late 19th century drainage catchment map of Edinburgh, the pale orange area had been served by the Water of Leith until the Interceptor Sewers took over.

Long story short, the 1889 scheme was so successful that in 1896 the Edinburgh Corporation found itself needing to secure powers to pass by-laws to protect fishing rights in the river that only a few years before had been nothing but a stinking sewer! The river Purification Act had done exactly that. That said it was only the river which had been purified; the sewage was still entirely untreated and discharged directly into the Firth of Forth off the Black Rocks, where it joined by the effluent from the Old Town and the Southside of the city that flowed through the Irrigated Meadows system to enter the Firth of Forth at Fillyside. But that will be a further thread to cover the final part of this story.

The last part of this story continues at 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.

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These threads © 2017-2023, Andy Arthur


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