Q: How do I dispose of my indigo vat?
A: Your natural dye kit indigo vat contains these three ingredients : Indigo Pigment (an organic molecule), Ferrous Sulfate (100g) and Hydrated Lime (150g). Both of these inorganic mineral compounds are traditional soil amendments for trees and lawns.
There are two recommended methods for disposal, both start with this step :
When your vat is exhausted (you've pulled all the pigment out that you can) whisk air into the vat (a huge no-no when you're trying to dye). This will introduce Carbonic Acid (dissolved Carbon Dioxide) into the solution, neutralizing the pH and converting most of the Lime to Gypsum.
1) If you are on a septic system : Do not dispose of any liquid down your drain, instead, dilute your vat to 1/4 strength with water and disperse evenly on a lawn or the ground at the base of trees and shrubs. If you have iron deficient soils, this can help green your plants (http://homeguides.sfgate.com/add-iron-garden-soil-95549.html)
2) If you do not have available green space, or are on a municipal sewage system : Allow your whisked vat to settle out. Pour the opaque blue/gray/brown liquid down the toilet being careful not to allow sediment to flow out. Dispose of the remaining chalky sediment in the trash.