Is the discharge of rainwater with sewage considered a waste of grace and a religious error, and what is the jurisprudential view on this?

The issue is not purely a religious one; rather, it pertains to people's worldly interests, and the reference for it is to the specialists. There are general legal principles that guide such matters, among them the eagerness of the Shari'a for everything beneficial, as the Prophet, peace and blessings be upon him, said: "A strong believer is better and more beloved to Allah than a weak believer, and there is good in both. Be keen on what benefits you, seek help from Allah, and do not lose heart." This address includes the entire Muslim community, which must strive to benefit from rainwater and to learn from the knowledge and experiences of others.