Add soapy water or lye to the water at regular intervals and boil the towel for a few minutes. When boiling, the towel should be completely immersed in water to avoid oxidation with air and increase strength and even embrittlement.
In addition, the number of times the towel is used will become greasy and sticky, and the water absorption will be poor, and a smell will be emitted. The more it is washed with soap, the more sticky it is. At this time, you can use a proper amount of salt water to wash, then hot water, and then rinse with clean water, not only can the towel restore the original color, the strange smell can also be eliminated.
After washing the white towel with soapy water or laundry detergent, rinse it off, then re-do the soap again. This time, do not rinse, but directly put it into a clean transparent plastic bag (or fresh-keeping bag) and seal it in the sun. After 2 hours, wash and rinse, it can be as white as new.
If there are more yellow parts, you can turn over the plastic bag in an hour to ensure that every part of the towel is evenly dried. This soap exposure method is sealed in a plastic bag, and it is different from how to prevent the white towel from yellowing, "directly exposed to the sun", one is direct exposure, and the other is sealed exposure.