If you are open minded about these things contact your water supplier.
When sewerage goes through the treatment plant there is (as you would expect) a lot of organic waste mixed up with the dirty water

The treatment plants don't just turn the dirty water into clean they also process the solids - filtering and heat treating it so its safe for reuse. As I understand it is freely available. It may well be free for collection or they will charge for delivery. If you have a large works near you and need a lot it may well be worth contacting to see what can be done.
Just be aware tomato seeds aren't destroyed in the heat treatment process
