Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, The Fatwa Center at Islamweb, states:
If it is possible that its color has changed because of other pure things that were mixed with it or because of an impurity, then in principle it is pure.
Therefore, it is permissible for you to pray in clothes stained with this drain water. If you are absolutely certain that this water has changed because of an impurity, then in that case, you must wash the spots on your body or clothes that are stained with this water when you want to pray.
Almighty Allah knows best.