I have built and used the smaller type with artificial light supplement that are talked about in the previous link I gave, and they work pretty good, just take some tweaking. The basic premise is the same as a bog filter, or any biological filter for that matter, in that you are creating an environment for aerobic (those that thrive on oxygen) bacteria to start the whole ammonia to nitrate conversion cycle. The algae are just another medium for getting the job done. What interests me in the algae scrubbers was more the idea of producing a feed supplement for my aquaponic fish set-up, since they eat pretty much non-stop and the math for food production of fish is basically electricity and fish food needs to be as much less than the value of the finished product as possible. The waterfall style scrubber does work, but its build and maintenance can be a bit tedious if you are like me and have 100 other things to tend to, so I'm always looking for SSL (simple, stupid, lazy) alternatives. This link is where I've found some of the most inspiring ideas, including this latest upflow version.
http://algaescrubber.net/forums/showthread.php?1687-The-new-Upflow-Algae-Scrubber-(UAS)
Ultimately for our ponds what I envision is something that is kind of a morphing of the upflow and waterfall designs. I think I've seen something like this before, maybe in Asia, where the water flow is diverted across shallow tables with very large surface areas exposed to as much sunlight as possible. I haven't tried this yet, but it is definitely on the list for this summer. Maybe take the plastic canvas and iron it into a stretched out fan fold, creating as much surface area as possible, and then placing it on a big table top with sides maybe a few inches high and feeding water through it from a spillway that is as wide as the whole table. Since I wouldn't be using this as the soul filtration I could play around with the flow rates, but I'm hoping that the crinkled up canvas will also create enough water agitation that it will provide its own adequate aeration as well. Another advantage to this approach is that the table with its shallow flow will mimic a shallow rippling stream, where not only is oxygen added, and algae grown, but the minimal depth also allows the sun's rays to better penetrate and work like a free energy U.V. sterilizer, helping to kill potential biological pathogens (something that always concerned me about duck pooh and fish pooh both becoming inputs in my families food supply).
The bog filter is in their catalog I believe.
http://www.marylandaquatic.com/retail/catalog_request.cfm
I have visited there numerous times and can attest to the efficacy of the method.. My thought is that a few ducks could be introduced to a large pond of this sort without issue, but my goal is to provide a low maintenance solution for a higher density of ducks. To do this in a way that does not require a constant dump of water, and near daily maintenance, I believe I'll be using a little bit of everything . So far for an 18'x10' free form pond that slopes from nothing to a 3' depth (approx. 3,200 gallons) that would be :
Large diameter bottom drain with all points of pond bottom sloping toward it to encourage sludge removal.
Large diameter ceramic air stone (or a trio of these if necessary) positioned 6-12" above the main drain to create a strata updraft and aid in moving sludge toward the main drain.
http://www.kensfish.com/moreinfo/ceramic-disk-air-stone-tourmaline-4-3-16inch.html
Large diameter main drain outfall pipe with enough flow and fall to prevent accumulation of sediments and sludge (probably a 3-4" diameter PVC pipe).
Bottom/Main Drain line empties into a conical sediment settlement tank with its own dump valve for emptying.
Outfall from conical separator is gravity flow exiting top and passing through a 50 gallon sized box filter with slide out mechanical filter media pads of increasing fineness and finishing with 10 PPI (pores per inch) reticulated foam before passing to bioreactor.
32" plastic sphere (re-purposed swimming pool sand filter) with bottom center mounted air stone, housing Caldness material (this reactor is essentially what is known as a fluidized bed filter).
Gravity outfall from reactor feeds return pump that send water back uphill and discharges into horizontal algae scrubber/U.V/ filter beds which empty into planted bog filter.
Bog shaped to funnel flow toward waterfall return to main pond. Waterfall provides additional oxygenation as well as minimal U.V. exposure as well.
Surface skimmer, located opposite water fall, is DIY type that stands away from edge of pond by about a foot to allow water to pass in from all sides. This is so that a wreath of wire screen/mesh can hang on the lip but when lifted out, will catch and take all of the feathers that have accumulated with it. A finer mesh lift out basket is fitted to the inside of the skimmer weir which basically functions as a water level limiter.
What I have outlined is basically a H.D. version of a koi pond set up (koi produce copious amounts of pooh and require fairly pristine water to thrive). The risky business with bottom drains begin with the hassles of fixing a leaky installation, so the idea is that you can't be careful enough when installing one. The other is that depending on how you plumb it, there exists the very real possibility that you could have a malfunction that causes the pond to completely drain. I know that this could be controlled by adding solenoid controlled valves connected to a water level float, but the valve sizes and electrical componentry, all made water proof, etc., would be too complicated and pricey to be considered ideal IMO. Still noodling through that one, but a pressure tested watertight assembly is critical to avoid this.