branch to collect bath water for grass

I am thinking about add a branch into the bathtub drain to detour the waste water into the yard for grass. A ball valve will also be added for this purpose.

Please have a look at the attached draft to see if it is practical. What's the potential side effects I may need to deal with?

Thanks,

I'm not a plumber, but I'm thinking that wouldn't meet code

Grey water cannot be connected to sewer at all and it must be treated before being used for irrigation. Also grey water irrigation must be sub-surface only. Your better off looking into rain water collection in my opinion.

It seems ok to use grey water directly: http://extension.umass.edu/landscape/fact-sheets/recycling-gray-water-home-gardens

Not sure that you know this, but since you live in the People's Republic of Massachusetts you are not legally allowed to do any work on your plumbing system unless you are a licensed plumber.

But even if you were licensed, in most areas it is NOT legal to divert grey water for any reason unless it is treated before use. We just passed on a very nice lake-front home here in NH because the owner - oddly enough, a licensed plumber from MA - installed a grey water diversion system when they built the house. That was not legal when the house was built in 1979 and it's not legal today; the ONLY legal use of untreated grey water in NH is to flush toilets. Everything must go into the sewer or the septic system.

What are the side effects? Consider what is IN that water you're diverting to your lawn; not to be too indelicate, but do you wash your backside in the tub?

I would suggest you would be better off with a well engineered irrigation system. Sprinklers that were sized for the area they cover, a timer that would control the time of day and length of time the system operated, and perhaps designing areas that did not require water could make this reasonable.