[GravitasFailure]

>Can it be used as a manure or is it safe to be dumped underground or in rivers?

Absolutely! A danger of human waste is that it carries a lot of pathogens that might be benign or beneficial in the lower intestine of one person but cause disease in the stomach of another, but the solution to that problem is to let it ferment/decompose for a bit and let all the bacteria kill themselves off through starvation and cooking in their own metabolic heat, and during that process is when you collect the methane. You'll often see large piles of cow manure composting in big piles for similar reasons, though the methane isn't always captured.

The danger to dumping any kind of waste into a river, treated or not, is that it will start killing anything that finds it toxic (most likely your fish and amphibians) and feed anything that finds it nutritious (algae, bacteria), which can cause ecological imbalances and generally screw up your ecosystem. For an adjacent example, look at the Gulf dead zone caused by fertilizer runoff from the American corn belt: Fertilizer feeds algae, which then chokes out other plants fish need to survive and absorb oxygen when the algae dies off, leaving a large swath of ocean without fish or anything that relies on fish.