Since the flow of deionized water is only 1/10 of total water flow... I think the increase in "waste" concentration is also only about 10% even of the filtered

Since the flow of deionized water is only 1/10 of total water flow... I think the increase in "waste" concentration is also only about 10% even of the filtered water is perfectly pure. So if you had 400ppm total dissolved solids it would now be *10/9=444ppm. Not much of a concern.