Question

why does drain water smell foul

Answer 1

drain smells foul because it is full of dirty stuff where thousands of mosquitoes and flies breed.. also there grow some microorganisms who give out this odour..
odour may also come out if hydrogen sulfide is produced by bacteria..in the drains..
so to fix those stinky drains you need to follow it:
dump a cup of baking soda down the drain. pour two cups of hot vinegar down the drain.(watch out it will fizz).then wait an hour and you can finally flush the drain with water