[graetzer]

That is actually not true, sewage can be treated and processed into 'biosolids'. I remember seeing a documentary about this, where they produced fertilizer from sewage in nyc

http://www.nyc.gov/html/dep/html/wastewater/biohome.shtml

[seiferteric]

Ya, milorganite exists, and has for like 100+ years, and I am sure there are others, but the total impact I am betting, is very minimal.

[TeMPOraL]

So it can; the question is, whether it is, and at what scale.

[anubisresources]

Human waste has been used as fertilizer for ages. Check out night soil