My water sewerage charge is double my usage charge. Is that normal?

I just got my first water bill in a new property, which is £50. I noticed on the break down that £20 of that is usage, £40 is sewerage (with a £15 credit from government contributions).

Is that normal? I don't really know how they calculate water bills but it says that my water unit is £2 something and my sewerage per unit is £3 something. Is sewerage just charged at a higher rate? I've used the same amount of each so it isn't a leak or anything. The bill was from the start of December to now, a 2 bedroom house with 2 adults. We take showers every morning, I take a bath at the weekend, and we do washing maybe twice a week.

I've had issues with billing in the past (broken meters, malfunctioning water tanks, etc) so I'm really paranoid about it, I'm sure it's probably a totally normal bill but I just want to make sure.