Is soil that has been mixed with sewage water and then dried considered pure, given that there is no trace of impurity on it, and is it obligatory to wash everything that touches this soil?

Obsessive doubt has reached a great extent within you, and its cure is to ignore it. There is nothing that warrants anxiety, for the default is that the dust is not impure and has not entered your house. So, abandon these obsessive doubts and do not succumb to them, for they are from Satan to spoil your life, and the matter is easier than what you are doing to yourself.