Above: Laying the Main Trunk sewer at Ynystawe, to the north of Swansea.

The central element of the scheme was the main trunk sewer which ran over eleven miles through the Borough from Clydach in the north running down the side of the river, around the sea front and ending in a series of large storage tanks under Mumbles Head. Once complete, all the older troublesome outlets into the river could be diverted into the main trunk and thence to the new outfall at Mumbles. Careful planning and engineering ensured a fall of 71 feet from the head of the sewer at Clydach to the outfall pipe at Mumbles. A 40 mile network of subsidiary sewers connected the residential districts to the trunk sewer.

For its time the Main Drainage scheme was very advanced. Although the difficulties of designing effective sea outfalls had yet to be appreciated, the designers of the scheme tried to anticipate many problems. One of the most unsavoury problems with the Tawe outfalls was that floating matter such as faeces, balls of fat and other organic matter could not be broken down. The river was perennially full of such material which also found its way onto the beach. Put simply, whatever was put down toilets flowed directly into the river, or alternatively, it would contribute to various blockages in the sewers. The flow in the new sewer was designed to be fast enough to prevent blockage, and at the Mumbles end of the sewer, a series of screens and settling chambers removed large bodies down to a size of half an inch and also allowed heavy matter such as pebbles to drop out of the flow before they could do any damage to the system.

Once screened the sewage was to be stored in a series of long thin storage tanks under Mumbles Hill. Approximately 7 ½ million gallons of sewage could be stored within the tanks (sufficient capacity to comfortably accommodate the sewage generated by 250,000 people, at least using 1920s water consumption habits!). The stored sewage would be released into the sea at the appropriate state of the tide (i.e. when its going out) through the sea outfall pipes approximately 220 yards offshore from the Mumbles lighthouse.

With the completion of the scheme, the relief on the River Tawe was swift. Although the river still had to cope with massive amounts of industrial pollution (primarily heavy metals and associated toxins), the aesthetic quality of the water was significantly improved. The scheme was widely recognised as a resounding success. It had not only solved the sewerage problem, but it had over many years provided much needed employment for many local men in the greatest years of the Depression.

One of the issues that the plagued the early sewerage schemes was the problem of storm water. Heavy rainfall would flood streets, swamp drains and inundate the town sewers causing unpleasant overflows or even damaging the system. The main drainage scheme tried to take this problem into account but still suffered from overloading during heavy rainfall events which, at times, resulted in the overflow of sewage on to the beach Whilst this was acceptable in the immediate post-war years, such incidents are now considered unacceptable along with the practice of releasing untreated sewage into the bay.

Swansea has been served by the main drainage scheme since its completion in the late 1930s to the present day. Only now, with the completion of Welsh Water’s flagship treatment works on Fabian’s Way on the east side of Swansea Bay, that a radically new approach to Swansea’s sewerage problem has emerged.