---
title: "Where Does London's Waste Water Go? Thames Tideway 2026"
canonical: "https://drainunblockerlondon.co.uk/blog/thames-tideway-london-waste-water-2026/"
pubDate: 2026-08-04
author: John Hanson
description: "From your kitchen sink to the Thames Tideway Tunnel — here's how London's waste water travels through 150 years of Victorian sewers to one of Europe's biggest engineering projects."
tags: [Thames Tideway Tunnel, London sewer system, where does waste water go, London drainage, 2026]
categories: [Drain Advice]
---

Every time you flush a toilet, run a tap, or drain a sink in London, you set in motion a journey through one of the world's oldest urban drainage networks. That water — now wastewater — travels through your property's private drain, into Thames Water's public sewer, through Bazalgette's Victorian interceptor sewers, and ultimately to one of four sewage treatment works before being returned, treated, to the Thames. In 2026, this journey is being transformed by the [Thames Tideway Tunnel](https://www.tideway.london) — Europe's largest infrastructure project of its kind. Here's the full story.

## Bazalgette's System: Still the Backbone

[Sir Joseph Bazalgette](https://www.bbc.co.uk/news/uk-england-london-39406858) built London's main sewer system between 1859 and 1875, following the Great Stink of 1858 when untreated sewage in the Thames created conditions so foul that Parliament considered relocating. His solution was a network of over 1,800 km of street sewers feeding into a series of large interceptor sewers running broadly east–west, parallel to the Thames. These interceptors collected the flow and carried it — by gravity and pumping — to outfall works at Beckton (north) and Crossness (south).

Bazalgette's system was a genuine marvel. It solved the immediate crisis, dramatically reduced cholera and typhoid mortality, and set the template for urban drainage worldwide. It was also designed for a London of 4 million people. Today Greater London has more than 9 million residents, and the system is under chronic pressure.

![Antique-style map detail in black ink on cream paper showing a fictional stylised section of South London with a winding sewer line, vintage engraving aesthetic, no readable text](/images/blog/thames-tideway-london-waste-water-2026/victorian-sewer-architectural-cross-section-brick-arched-tunnel.webp)

## The Combined Sewer Overflow Problem

Bazalgette's interceptor sewers were designed as **combined sewers** — they carry both foul sewage (from toilets and drains) and surface water (from rain) in the same pipe. This was deliberate: Victorian engineers assumed that dilution by rainwater was adequate before discharge to the Thames.

Combined sewers work fine in dry weather. In heavy rain, the volume of water entering the system exceeds the capacity of the interceptors. When this happens, the Victorian system has **combined sewer overflows (CSOs)** — relief valves that discharge directly to the Thames to prevent the sewers from backing up into streets and homes. There are currently 57 CSOs on the tidal Thames, and before the Tideway project they discharged an average of 39 million tonnes of raw or partially treated sewage into the river every year.

This is the problem the Thames Tideway Tunnel was designed to solve.

## The Thames Tideway Tunnel

Construction of the main tunnel began in 2016 and reached substantial completion in 2024. The tunnel is:

- **25 km long**, running from Acton in the west to Beckton in the east
- **7.2 metres in diameter**, bored 40–65 metres below the Thames
- **Designed to capture 94% of the CSO spills** that previously entered the Thames

The tunnel works as a storage and transfer system: during heavy rain, CSOs that would previously have discharged to the river instead drain into the Tideway tunnel. When the rain stops and capacity is available in the treatment works, the stored sewage is pumped to Beckton for treatment.

The project cost approximately £4.5 billion and is funded through an infrastructure charge added to Thames Water bills — known as the Tideway levy — spread across all customer bills over 125 years.

## From Your Drain to the Treatment Works: The Modern Journey

In 2026, the journey of your wastewater from sink to river looks like this:

**Step 1 — Your property drain.** Wastewater leaves your property through the private drain. This section is your responsibility. It connects to the public sewer network at the property boundary (or at the point where your lateral drain was adopted in 2011).

**Step 2 — The street sewer.** Your drain connects to a local street sewer, typically 150–300mm in diameter, which collects flow from all the properties on your street. This is Thames Water's asset.

**Step 3 — The interceptor or combined sewer.** Local sewers feed into larger interceptor sewers or combined sewers. In inner London, many of these pipes are Bazalgette's original brickwork sewers — some over 150 years old, still performing the function they were designed for.

**Step 4 — The Tideway Tunnel (during storms).** When rain pushes flow beyond the interceptors' capacity, flow is diverted into the Tideway Tunnel storage system rather than being discharged to the Thames.

**Step 5 — The treatment works.** Flow reaches one of four treatment works: Beckton (the largest in Europe), Crossness, Mogden, or Long Reach. At each works, the sewage is screened, settled, biologically treated, and the resulting effluent is discharged to the Thames, meeting stringent [Environment Agency](https://www.gov.uk/government/organisations/environment-agency) standards.

**Step 6 — Return to the Thames.** Treated effluent is discharged to the tidal Thames. The quality is strictly regulated — London's treated effluent discharge is one reason the Thames now has a recovering salmon and wildlife population compared to its condition in the mid-20th century.

## What This Means for London Homeowners

The Tideway project doesn't change the responsibilities of homeowners for their private drainage. Your drain from property to the public sewer remains your responsibility; Thames Water maintains the public network.

What it does mean:

**Reduced flooding risk in heavy rain.** With the Tideway tunnel intercepting CSO overflows, the peak pressure on the Victorian combined sewer network during storms is reduced. Properties that previously experienced sewage backup during heavy rain events may see improvement.

**Higher water bills.** The Tideway levy appears on all Thames Water bills and will continue for the project's financing period — approximately 2019 to 2145. It currently adds around £25 per year to a typical household bill.

**Continued maintenance of Bazalgette's sewers.** The main Victorian interceptors are still operating and still require maintenance. Thames Water spends significant capital annually on sewer rehabilitation. Our [drain repair services](/services/drain-repairs/) work in coordination with Thames Water for projects that intersect public and private drainage. For a primer on the boundary between your private drain and the public sewer, see our guide on the [difference between a drain, lateral drain and sewer](/blog/drain-lateral-drain-sewer-difference-london-2026/) and the post on [pipe relining vs drain excavation](/blog/pipe-relining-vs-drain-excavation-london/) for the standard repair options.

## Frequently Asked Questions

### Will the Tideway Tunnel completely stop sewage entering the Thames?

The tunnel captures 94% of the CSO spills that previously entered the river. Some residual overflow will still occur in extreme weather events, but the volume will be dramatically reduced compared to the pre-Tideway baseline.

### How deep is the tunnel under London?

The tunnel varies from 40 metres to 65 metres below the surface, depending on location. At its deepest points it passes beneath multiple Underground lines, foundations, and other utility tunnels.

### Who pays for the Thames Tideway Tunnel?

The project is financed by Tideway, a regulated infrastructure company. Costs are recovered through the Tideway levy on all Thames Water customer bills, spread over approximately 125 years.

### Will my drains be affected by the Tideway Tunnel works?

The tunnel is primarily beneath the Thames or close to it. Shaft sites at various locations across London involved surface disruption during construction. Now that construction is substantially complete, ongoing operational impacts are minimal for most London homeowners.

### Where can I find out more about my local drainage infrastructure?

Thames Water publishes sewer maps and information about the public drainage network. For your private drainage — the pipes from your property to the boundary — a CCTV drain survey gives the most accurate picture. Request a quote online via [our contact form](/contact/) to arrange a survey or visit our [services page](/services/) for more information.