Josh and others,
Just some random thoughts ...
Filtering/killing microscopic organisms is something I know very little about.
But...
Would it make sense to fill the 55-gallon drum with diatomaceous earth instead of regular sand? It is what is typically used in swimming pool filters. Could you then set it up to backflush the contaminants on a periodic basis.
Lastly, in relation to the filter, would it make sense to periodically add chlorine or bromine "pucks" to the top of the 55-gallon drum filter, or in a water-tight box at the output, to kill whatever is coming into/through the tank. I believe it might be enough to kill organics, but the concentration would probably be way too low to do any damage in a pond with several million gallons of water. Hopefully, we have some experts who can chime in on this.
Over the last 50 years I've played around with a number of ram pumps. Even though a creek looks like it has very little drop, you might be surprised when you measure it. I know I've sure been surprised whenever I've measure water drop in a stream that looks almost level. An inexpensive
Sight Level can easily give you a pretty good idea of how much drop you have. You can also use a hose or plastic tubing to make a simple
water level. With just a little ingenuity, you can make one for a few dollars -- it just won't be electronic.
I'd recommend a "pressure tank" -- but it too doesn't need to be much more than a sealed can or barrel with a partially inflated tire innertube.
Depending on a lot of factors, figure that you can get about 3-feet of lift for every foot of drop from your system intake to the pump output. People claim that you can get as much as a 10 feet of lift for every foot of drop, but I've not experienced anywhere near that. I doubt I've gotten more than 5 feet of lift per foot of drop, under the best of conditions.
Lastly, I don't remember what the typical input/output efficiency is for ram pumps, but it is fairly low. A lot more water goes through the pump, than it pumps uphill.
The good thing now-a-days is that you can use Schedule 40 pipe for nearly everything, including the check valve. I was mostly using cast iron sewer pipe, sealed with lead, and galvanized pipe, when I last played with these things. When I was doing it, they didn't even have things like inexpensive PVC check valves -- only brass, and they were damned expensive. For the feed pipe, you may even be able to get away with using corrugated HDPE plastic drain pipe, which is available in 100-foot rolls at most places like Lowes, Home Depot, Mennards, etc.
There should be a lot stuff on the Internet about ram pumps. There should also be a lot of old hippies (I consider people like RMEDGAR/Randy and myself to just be young hippies) here on the forum who have a lot more recent experience than we have.
Good luck,
Ken