What makes the dishwasher an imperfect solution for sanitizing a standard kitchen sponge?

The bacterial population can quickly return to high levels if the sponge remains damp after the cycle.

While a dishwasher can be more effective than a microwave at reducing the bacterial load on a sponge through the combination of high-temperature water and detergent, it is not a perfect solution. The primary failure point is the post-wash environment. If the sponge retains water after the cycle finishes, it remains a damp environment. Because the sterilization is rarely 100 percent complete, any surviving bacteria can exploit the moisture to rapidly re-colonize the sponge within days, effectively returning the sponge to a high-density microbial state shortly after it has been cleaned.