There are really three different things being paid for, and the terminology on a water bill can make them look like one service.

Why does taking water away cost more than supplying it?

Because wastewater is much more expensive to deal with than clean water is to supply.

When the company supplies you with 1,000 litres of treated drinking water, it has to:

abstract the raw water;
treat it to drinking-water standards;
pump it through the distribution network;
maintain pipes, reservoirs and treatment works.

But when that 1,000 litres goes down the drain, the company has to:

collect it through the sewer network;
pump it where necessary;
remove solids and other contaminants;
biologically and chemically treat the wastewater;
deal with sludge;
meet environmental discharge standards;
maintain and replace the sewer network.

And wastewater isn't simply the same water going backwards through the system. Sewage is a contaminated product that has to be treated before it can be returned to the environment.

Ofwat says wastewater charges cover collection and treatment of foul sewage, surface-water drainage and highway drainage.

So when you see something like:

Water supply: £X
Sewerage: £Y

the second figure isn't simply a charge for 'taking your water away'. It includes a substantial treatment and environmental infrastructure operation.

What exactly is 'surface water'?

This is the part of your bill that is easy to misunderstand.

Surface water = rainwater falling onto your property.

For example:

Rain -> roof -> gutter -> downpipe -> drain -> public sewer

The water company has to provide and maintain the drainage system capable of carrying that water away.

Ofwat therefore allows companies to charge for surface-water drainage.

It is separate from the water you use in your house.

So, for example:

Your £100 of metered water

You use:

100 cubic metres

The company supplies that water.

Then perhaps most of it goes:

toilet + bath + washing machine + sinks -> foul sewer

You pay for foul sewage collection and treatment.

But simultaneously:

rain -> roof -> gutter -> sewer

You can also pay a surface-water drainage charge.

Why are you paying for rainwater?

Because if the rainwater enters the public sewer, the water company has to provide capacity for it.

Imagine a heavy storm.

Thousands of houses are simultaneously sending rainwater into the drainage system.

The company therefore has to maintain:

large underground sewers;
pumping stations;
storm tanks;
sewage treatment works;
overflow systems;
flood-control infrastructure.

Ofwat specifically says that surface-water drainage is intended to reduce the risk of surface-water and sewer flooding.

But here's something you may want to check

If none of the rainwater from your property enters the public sewer, you may be entitled to a surface-water drainage rebate.

For example, suppose your:

roof -> downpipe -> soakaway -> ground

rather than:

roof -> downpipe -> public sewer

In that situation, Ofwat says you may qualify for a reduction in the sewerage charge.

You normally have to tell the water company because the company doesn't necessarily know how every property's rainwater is disposed of.

And you don't need to pay a third-party company to make the application.

There's another charge: highway drainage

This is slightly different.

Rain -> road -> road drain -> public sewer

That's called highway drainage.

You generally cannot opt out of this charge, even if your own property has a soakaway.

Ofwat explains that companies are permitted to recover the costs of highway drainage from customers regardless of how directly an individual customer benefits from it.

So your sewerage bill can effectively contain:

'We pay water companies to deliver and take away water. The latter costs more, why?'

The answer is that 'take away' actually combines several services.

You're not just paying somebody to transport 100 litres of dirty water through a pipe.

You're paying for:

collection + pumping + sewage treatment + sludge treatment + environmental compliance + sewer infrastructure + rainwater drainage + highway drainage.

That's why the sewerage component can be considerably larger than the water-supply component.

If you give me the name of your water company (for example Yorkshire Water, Severn Trent, Thames Water), I can break down a typical bill into the actual water, foul sewage, surface water and highway drainage charges, and explain exactly what you're paying for.