Thanks Ron Reiserer

I have checked but no recommended and/or mandatory limit for dyes are available in the above mentioned document.

As dyes are non biodegradable and unlike heavy metal they have no role in the body.

Methylene blue has been used in the past to treat malaria and urinary tract infections. It's still available as an injectable treatment @ 1% and it is used by the pet industry for treatment of some fish diseases. This means that it will eventually get into the water ways.

Permissible levels will vary by regulatory agency and municipality, but there are some general guidelines available. In the US the EPA has documents like this - https://www.epa.ie/pubs/advice/water/quality/Water_Quality.pdf that contain recommended and/or mandatory limit vales. Check with your local municipality to see what their limits are. Almost all municipalities publish a chemical disposal limit/prohibited list.

Thanks Ron Reiserer

I have checked but no recommended and/or mandatory limit for dyes are available in the above mentioned document.

Look at the bottom of page 66. Looks like a recommendation of 0.2mg/L is given. This doesn't mean that you local regulatory agency uses that number. You should check directly with them. When in doubt, assume it is prohibited.

Okay many thanks