How can we solve the waste water problems of megacities?

The growth of our cities can overburden the existing waste water disposal systems

More than half of the world's population now lives in cities – and the trend is still rising. According to United Nations estimates, 68 % of people will live in urban areas by 2050. The number of megacities with more than ten million inhabitants will grow to over 40 in the next 30 years. The size of these cities poses challenges for utilities and urban planners. How can they deal with the huge volumes of waste water that are produced?

Overflow of a combined sewer system during heavy rain

Is waste water always treated? Not quite

Often, sewer systems cannot keep up with the growth of cities. Take London, for example: many parts of the inner city have so-called combined sewer systems, which drain both stormwater and waste water in a single conduit and date back to Victorian times.

These were designed long ago for a city of four million people. Today, almost nine million people live in London. At the turn of the millennium, the grid was therefore working at 80 per cent capacity, even in dry weather.

However, if there was heavy rain, the combined sewer system overflowed. Untreated waste water then flowed into the Thames via an overflow – about 39 million tonnes per year.

Combined sewer systems with insufficient capacity are now considered a burden on the environment, but are still widespread. In Germany, too, about half of all households are connected to a combined sewer system.

But how can cities like London expand the capacity of their sewers and prevent overflows when their ground is criss-crossed with underground systems, electricity, water and gas pipes?

How deep tunnels solve the waste water problems of big cities

London solved its waste water problem with a "deep tunnel". This is a tunnel system that usually has a diameter of three metres or more. Such systems are constructed with tunnel boring machines deep below cities where there are no more pipelines or underground shafts.

These gigantic pipes collect waste water, rainwater or combined sewage and lead it to underground pumping stations. In doing so, they serve not only as a conduit, but also as a reservoir in the event of heavy rainfall.

A growing number of major cities are turning to deep tunnels to solve their waste water problems – in addition to London, for example, Chicago, New York, Singapore and Hong Kong.

Pumping station shaft of the Lee Tunnel in London

Pumping stations are crucial

Deep tunnels collect waste water deep beneath the city. This usually requires a pumping station that lifts the water back to the surface so that it can be treated there.

Such pumping stations consist of shafts many metres deep and several metres wide into which the water flows from the tunnel. Submersible pumps or dry-installed waste water pumps on the ground then pump the water through pipes to the surface.

And this is where KSB's expertise comes in. After all, hardly anyone has such expertise as KSB does in the manufacture of durable and efficient waste water pumps and the hydrodynamics in pumping stations. We have been able to demonstrate our knowledge in deep tunnel projects in London, Mexico City and St. Petersburg.

A practical example: Auckland Central Interceptor

A current project in Auckland, New Zealand, in which KSB is involved, shows how the construction of such a deep tunnel works. Like London in the past, the city has the problem that waste water and stormwater flow into a combined sewer network. During heavy rainfall, its capacity is insufficient, which means that untreated waste water has to be discharged into the rivers.

That is why the city decided to build a deep tunnel: the Central Interceptor tunnel is to have a diameter of up to 4.5 metres and a length of 14.7 kilometres. It will run from the Grey Lynn district in the north-west under Auckland's inner city and harbour to Māngere in the south-east.

Currently, two German-made tunnel boring machines, named Hiwa-i-te-Rangi and Dominca, are boring the tunnels. When completed, the Central Interceptor will be between 15 and 110 metres below the surface.

Together with two smaller connecting tunnels, it will collect the waste water and convey it to the Māngere pumping station at a gradient of 1:1000. There, KSB pumps transport the water from a depth of 40 metres into a water treatment plant.

Computer simulation of a pumping station shaft

KSB supplies more than just efficient pumps

Our hydraulic know-how ensures perfect functioning

When building pumping stations for deep tunnels, it is crucial that the structure and the pumps are matched to ensure optimal functioning of the equipment. Especially in the dimensioning and design of the inlets to the pumps, no mistakes must be made.

That's why KSB not only supplies pumps, but also the hydraulic know-how for designing the system on request. The experts at KSB use computer models known as CFD (computational fluid dynamics) simulations to test the flow conditions in different designs during the planning phase.

This allows them to detect submerged vortices or points at which solids can form deposits that could block the system. Using experiments with acrylic glass models, they can verify the results from the CFD simulations.

Impellers and motors of our pumps ensure optimum efficiency

For pumping stations, the submersible motor pumps of the Amarex KRT type series and the dry-installed Sewatec or Sewatec SPN waste water pumps are the first choice. In these pumps the innovative impellers play a key role: the design, geometry and free passage of an optimally selected impeller ensure maximum freedom from clogging and energy efficiency.

Different impeller types can be used depending on the respective requirements. In combination with the highly efficient motors, this results in optimum overall efficiency. In addition, KSB offers a range of installation types as well as a large choice of materials for waste water pumps.

In this way, KSB can provide the best solution for each waste water application – for optimum efficiency and operating reliability.

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