In rural and suburban contexts handling manure and converting it to fertilizer is a solved problem: <i>Eisenia foetida</i> (& co.)<p><a href="https://en.wikipedia.org/wiki/Vermicomposting" rel="nofollow">https://en.wikipedia.org/wiki/Vermicomposting</a><p>and Vermicomposting Flush Toilets:<p>> A simple, proven, low tech and open source approach for ecofriendly human waste disposal<p><a href="http://www.vermicompostingtoilets.net/" rel="nofollow">http://www.vermicompostingtoilets.net/</a>
When I attend VC dog & pony shows, I often find myself reminded of the "New Idea" manure spreader ("pulverizes and spreads a load in three minutes!"):<p><a href="https://www.agriculture.com/thmb/prap3PqgiCmAX8bM2AFUH-kimX0=/1500x0/filters:no_upscale():max_bytes(150000):strip_icc()/NewIdeaSpreaderadvertisement_preview-a0ea13e910f64099a0ce5ea7d757bf55.jpg" rel="nofollow">https://www.agriculture.com/thmb/prap3PqgiCmAX8bM2AFUH-kimX0...</a>
Hm. Not gonna work for the volume of agriculture necessary to support a civilization. Tootle around with such things in your backyard all you like; nobody who's part of feeding billions is going to do more than give lip service to this.