Biological Filtration 201

Chapter 1

Downflow Gravity Biofilters

The Downflow Filter box design that still has many proponents today is a direct successor of the sediment traps of ancient Pompeii, where rainwater passed from one chamber to the next, each chamber filling from above then overflowing into the next, with more and more debris settling out at each successive stage. Named for how water enters and leaves the filter, these “Downflow Gravity Biofilters” have a number of advantages. They are very simple in operation – pump water in at one end and it overflows, clean, back down into the pond at the other. They can be amazingly versatile and effective, because you can create as many stages as needed, filled with just about any medium, to filter out just about anything. Since they are “Gravity Biofilters”, necessarily set higher than the pond level so water can flow by gravity back down into pond, they are usually easy to plumb with bottom drains to siphon out settled debris simply by opening a valve. They easy to make and can be very low maintenance.  Finally, even with all the media clogged, it’s easy to design a simple bypass that lets the water pass back into the pond, unfiltered, yes, but still going its merry way.

So how do they work and what can they do? Imagine a simple box with a series of screens set vertically so water entering at one end has to pass through the screens before exiting at the other end. The first screen will clog rapidly; water, unable to pass through the screen, will be forced to overflow into the second chamber, which will also clog over time. Each successive transition from screened chamber to settling chamber gives the pondkeeper more time before needing to clean the screens, and with a multitude of chambers, fitted with different types of filter media, that time can be lengthened almost indefinitely. Those different types of media can be tailored to enhance biological activity. If you add to the last chamber(s) a medium with lots of surface area that bacteria can readily colonize, the steady flow of clean, oxygenated water will in effect “supercharge” the bacterial conversion of toxic ammonia from fish wastes to relatively benign nitrates – plant food. The “aerobic” bacteria that perform this conversion use up a lot of that oxygen, so you’ll want to return that water to the pond over a waterfall to re-oxygenate it, or you can run some of the deoxygenated water through another chamber where anaerobic bacteria, the ones that work without a lot of O2 present, can directly convert nitrate to gaseous nitrogen, which returns directly to the atmosphere.

OK, if they’re so great why doesn’t everyone use these “Downflow Gravity Biofilters”? There are a couple of inherent disadvantages that have caused them to fall out of mainstream favor. First, they are in effect large boxes above the ground at the edge of the pond, so it takes some creativity to hide them. Two of the better ways to hide them include setting them as the headwater of a stream, well screened by plantings or locating them behind a structure, with large diameter plumbing returning the clean water to the pond. Further complicating matters, the top has to stay accessible for cleaning. Another major issue, the reason why they’ve fallen out of favor, is that the overflow, the bypass that allows water to flow back into the pond when the filters are clogged, can also itself get clogged, with utterly disastrous results. If a leaf follows the overflowing water over the top of the dividers, or falls in from above, and blocks the outflow, then the box itself will overflow. So what, right? Well, if your pump’s down at the bottom of the pond, then that overflow will EMPTY THE POND, killing everything in it and potentially destroying the pump to boot. Not good. The water can only empty out to the depth the pump is located, at which point it starts sucking air, so using a Skimmer will ensure that ONLY the pump dies if the box overflows, but still…. Even though they are technically low maintenance and effective, Downflow Gravity Biofilters really have to be supervised by the homeowner, or the consequences can be dire.