Question

it is possible to reduce river pollution by the use of sewage water for irrigation to collect the accurately

Answer 1

Yes, to some extent. But irrigating sewage water directly is harmful for plants and human because it contains many harmful particels. If you can filter the undesirable elemets(like lead, mercury) from sewage water then you can use it for irrigation. its even rich in urea so there's no need to add urea fertilizers. the sewage acts as a manure.