I often wash my line in laundromats, however I see yellow marks on my white line. 
How to remove this and what is it ?

You mean your sheets etc the stains probably originate from the <deleted> in the water here, happens to us as well, as yet I haven't found a way to remove the stains.

Bleach, buy from Big C

Incidentally i wonder if the yellow stains originate from Ops STI discharge

Bleach removes them, but be careful to not cause bleach stains, speaking from experience.

 

Vanish doesn't help.

Eventually, after searching on internet, I found a way to bring back the whiteness. I ordered Sodium percarbonate on Lazada https://bit.ly/3bDLblC and by soaking more cleaning with detergent, it became 80% white

A troll post has been removed.

Use vinegar with your laundry detergent, restores colours. And a dash of baking powder on the goods before the rinse.

Vanish does work as well, but follow the instructions

And for serious stains on white, Hydroperoxide mixed with baking soda, put the sauce on the stain... it will go!!

On 8/4/2022 at 2:59 PM, Farang two dot zero said:

I often wash my line in laundromats, however I see yellow marks on my white line. 
How to remove this and what is it ?

How does one wash a line? Oh, you mean linen. ????

Dodgy water or rust from drum... 
Keep an eye out for Otteri launderettes. They’re typically cleaner/newer with industrial machines.