From the research Ive done, I have not found any examples of damage to kidneys. I have used the SWF (Salt-Water-Flush) 99% of my cleansing experience and have never had a problem using it.
It is important to use UNiodized salt. I like the Himalayan and have only used that.
I also use a measuring spoon so I have a standard of measuring in case I might need to add more or less salt to match the salinity of my blood. I have never had to adjust the amount, although some have.
If you do not feel comfortable with this aspect of the cleanse, you are welcome to use the laxative tea AM & PM.