I ran into this and told them I wouldn't dump it all at once and they were happy with that answer as silly as it sounds. I said i'd just dump a bit here and there over 48hours. 

Post it on Craigs list.  Farmers/gardeners.

Do you have a garden? It's used as fertiliser after all. Otherwise I'd just flush it. Redirect the pump to the shower drain or something.

What is the basis of their request? The salt in the solution, the possible human waste factor, the inclusion of sanitizers, the combination of those things or something else. I agree with Kane, this seems ridiculous and they should specify what their concern is first.

Michael, I wish I had something better to offer than this is a ridiculous position. 800lbs of Epsom salt being flushed down the drain every 6 months or so is insignificant compared to the waste from the entire city everyday, including draino and other such chemicals. It is water soluble an non-toxic. If they are worried about corrosion, it goes down the same drain people shower in so it will be well flushed.

If they are being a stickler about it, I guess you could set up a sceptic disposal and add that to the cost of refills. It will be an insignificant cost when factored over the lifespan of the solution. I think the important thing, in this case, would to not let the sceptic disposal items contaminate your tanks, so you will want to dump to some bins and let them pull from that. Let us know what happens.