How to deal with sewage on a map with no water anywhere
Solution 1:
If you are happy to use other than vanilla drainage buildings you can use following.
- Quad's Water Purifer (does not require any road connection) or
- Modular sewage treatment facility
However if you are not happy with those two options you can do the terra-forming and create a lake using following mod in which you can put / place sewage building.
- Extra Landscaping Tools
Hope this helps.