This money is not a total rebate, because they have processed some of your poo into non-poo like they should have done, but it is just for the bit of poo they did not do. Even though they should have, because it wasn't raining, most of the time when they dumped. So the money you get back, just comes from you, in the future. And you're not allowed to do it yourself, because ew-yuck, no, we can't have irresponsible members of the public doing that, the law says we must have proper irresponsible utilities doing that instead. (I should call them sewerage companies, not sewage.