After purge mussels 6 time in salt water 20 mins each, why they still exude much grime? I  buy mussels at Pusateri and Whole Food Market for a year in Toronto. They sell them on ice and dry, not in running water.





10 s this video, Bart teaches add salt to cold water and 
soak for 20 mins. But I repeat 6 times...so 20 mins x 6 times = 120 mins of soaking!!! Mussels exudate less black grime every next time, but even after 6 times, water still has much black grime! See my pic under! Why? This happening for a year! 



Part 2, Step 2 has GIF that shows you what I mean.
If your mussels are really that dirty, scrub them well first, then soak them in fresh water + salt (about 35g per litre) - or sea water - and add a scant handful of ground oats. The oats are supposed to irritate the mussels and they'll purge it together with the sand, while the salt water will keep them alive.

It can take a long time to purge really gritty mussels (according to some sources, from 2 to 12 hours). I'm not sure how clean you expect your mussels to be, just make sure most of the sand is purged.

You should also ask the shop if the mussels have been purged before - it's usually the case.