Non-toxic environmentally friendly degreasers (Simple Green works great for example) are just that, non-toxic. Pour 'em down the drain, water the grass with 'em, drink 'em, whatever, not too bad.

UNTIL you use them.

Now they are contaminated with all the NON-environmentally friendly gunk that was on whatever you were degreasing. At that point you would want to consider disposing of them at an appropriate place.

Even if you use environmentally friendly oil on your chain, after degreasing you're going to have metal deposits, dirt, and road grime mixed in there so I'd still recommend taking it to an appropriate place for disposal.