Mosque- under which a sewage system is running
In regards to your question a similar fatwa can be found in Fatwa Mahmudiyah (p.214 v.6), where Mufti Mahmood Saheb (ra) has written that the wudhu areas and toilets should be made in such a place that the smell does not cause any inconveniences to the musalle’s and nor does the smell stay inside the mosque as this will be against the respect of the mosque. (Fatwa Mahmudiyah p.214 v.6)
If a sewage system is running beneath a mosque, then as long as the smell does not enter the mosque and nor does it cause inconveniences to the musalle’s, then it will be permissible.
Mohammed Tosir Miah
Darul Ifta Birmingham.