Prior to the work, drains in central London were built primarily to cope with rain water, but the growing use of flush toilets frequently meant that they became overloaded, causing sewage and industrial effluent to flow into the River Thames. Bazalgette's London sewerage system project included the construction of intercepting sewers north and south of the Thames. The Southern Outfall Sewer network diverts flows away from the Thames south of the river.
In total, five interceptor sewers were constructed north of the Thames. Three were built by Bazalgette, and two were added 30 years later:
The northernmost (Northern High Level Sewer) begins on Hampstead Hill and is routed past Kentish Town and Stoke Newington and under Victoria Park to the start of the Northern Outfall Sewer at Wick Lane.
Two middle level sewers serve parts of central London and also join the Northern Outfall Sewer at Wick Lane:
One begins close to Kilburn and runs along Edgware Road, Euston Road and past King's Cross, through Islington to Wick Lane.
The flows from the two low-level sewers are raised by some 40 feet (12 m) into the Northern Outfall Sewer at Abbey Mills Pumping Station, to join the flows from the High and Middle Level sewers.
The remaining sections of the NOS carry the sewage from Abbey Mills to the treatment plant at Beckton. The creation of the NOS was a massive undertaking, and involved the construction of huge embankments and several bridges.
Today, the eastern end of the Northern Outfall Sewer, running some 4+1⁄2 miles (7 km) from Wick Lane to Beckton, has been landscaped to form a public footpath/cycleway called The Greenway, with access points along its length.
