At least in the US, 40% of wastewater solids are already used for agriculture [1]. Another 10% is used for landscaping. The same site estimates that if all US biosolids were applied to agriculture, they would supply 3.2% of applied phosphorus.
You can buy wastewater solids down at the garden store, under the longstanding brand name Milorganite. It's a good lawn fertilizer (6-2-0) if you don't need potassium, which I assume just washes through the waste treatment system. It's also high in iron, which is desirable for deep green grass and moss suppression. And the label says it can be used on vegetables, so there's nothing notably toxic in it.
The primary concern with waste solids would be the presence of heavy metals, I think, but those shouldn't be getting into the waste water stream anyway. Heavy metals are a concern anyway in phosphate fertilizer; many deposits of phosphorites have troublesome amounts of cadmium.
I would have thought biological contamination was an issue. I, perhaps foolishly, presumed wastewater treatment plants would be fairly good at removing heavy metals.
Biological contamination seemed like a bigger issue to me since the solids could be re-colonized post-processing.
Maybe this is a more processed product than I’m thinking of. Surely their not selling literal bags of poop, maybe it no longer resembles human feces enough for bacteria?
It's the stuff that's left after anaerobic, then aerobic, bacteria have chowed down on the sewage. It's basically dead bacteria. Any pathogens would have become food for these decomposers.
I think I'd also worry about residual drugs. Some don't break down quickly.
Maybe we should rethink wastewater streams for selective recycling: stormwater, graywater, blackwater, and yellowwater. Perhaps the latter two should be recycled where possible and then residual wastes close-loop incinerated to prevent leeching any sort of biologically-active molecules or pathogens into the environment.
The quantity of drugs in the waste stream is surprising. For example: I take 1000 mg daily of metformin. Although I have (of course) never measured it, a person produces 100 to 250 g/day of fecal matter. This is 75% water, so reasonably my excreta, on a dry basis, is up to 4% metformin(!). Metformin is largely excreted and breaks down in the environment only slowly, so it would contribute significantly to the nitrogen figure for waste-derived fertilizer.
Wikipedia says this:
"Metformin and its major transformation product guanylurea are present in wastewater treatment plant effluents and regularly detected in surface waters. Guanylurea concentrations above 200 μg/L have been measured in the German river Erpe, which are amongst the highest reported for pharmaceutical transformation products in aquatic environments."
[1] https://www.biosolidsdata.org