As jaykay says. I avoid tin and particularly chrome altogether - chrome makes for lovely clear colours, but at what expense to the environment (and the user too)? Definitely not worth it.
If you use chemical acid dyes then the whole lot is taken up by the fibre (quite magically when you see the previously deeply coloured dye solution turn back to clear water
) so any left over water can be tipped onto the soil in an out-of-the-way corner.
If you are going to do the digging a hole option, filling it with fine gravel helps, making a tiny soakaway.
For your alum mordant, try to use the right amount for the weight of wool/fabric, so there is a minimal amount left over. The madder is fine so can be tipped anywhere, as can other dyestuffs from plants.
Don't forget to post us a pic of the finished item. I love dyeing but for one reason and another I haven't done any for a couple of years. It is such fun and so creative