So I know a guy that has his tub separate from his septic system, he runs the grey water out to the back of his property.
He also has a second bathroom that is 90% renovated but the tub is currently connected to the septic and he wants to remove it like the other.
I've often wondered how good of an idea that is just in regards to how it affects the septic system. On one hand you aren't sending a ton of extra water into it, but at the same time your septic winds up filled with only one thing, and isn't diluted it any way like it would be if the tub was hooked up.
The soap, hair, dead skin, etc., that will be in the tub water still should be treated (or digested, like in a septic system) and what he has probably doesn't pass local health codes. if the septic system is sized properly, it should be able to take all of the stuff you send to it. Plus, many people pee in the shower...in fact, there's a public advertising campaign I heard about in London that suggests it to save that extra flush by everyone in the morning to save potable water...it should go into the septic system, regardless.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013