Indigo and the Environment: is it that bad?
For this process, sodium dithionite (Na2S2O4) has been considered as a best reducing agent in traditional and industrial dying but the reducing agent releases sodium sulphate, sulphite and thiosulphate ions into the media. Those ions not only cause environmental concerns as major pollutants of the modern textile industry waste waters but they present also no negligible risks of toxicity against living organisms.


It is a very convoluted sentence with so many negations. My simple definition of environmentally dangerous stuff is that anything which is not found in nature is bad for the environment in excessive amounts. Sodium thiosulfate and sulfite are strong reducing agents, in excess amounts, they will slowly deplete the oxygen levels in water. Read the concept of "Chemical Oxygen Demand" in natural waters. These ions will heavily affect the COD, which is a measure of water quality. Sodium sulfate should be fine. You have to check the half-life of these ions (thiosulfate and sulfite) in natural waters.

Untreated textile waste is worse in the sense because it is colored. If large amounts of untreated textile waste, such as those of indigo, are discarded into natural streams they might block the light reaching the depths.