BOG FILTRATION aka WETLANDS and UPFLOW FILTRATION
The purpose of this website is to explain this under-utilized but ancient form of filtration. Ancient, you say? Of course, China, Egypt, Mesopotamia, Rome, the list goes on and on. Well then, if it's so old, how can it be something utilized in the 21st Century? The simple fact is, water filtration has been based on the same principles for eons, and it's still the same today, despite glitzy adds and marketing, high tech shapes and materials used. It boils down to bacteria and media. In other words, Nature has the final say so, and is what makes 21st Century methods the same a 1st Century. Just different equipment to make it work.
If you understand flow rates, with volume of media and bio-load=frequency of cleaning and degree of success, you got it made.
Let's first look at what filtration is: trapping debris or contaminants to enhance the quality of what's being filtered.
Bio-filtration traps organics and bacteria converts them from harmful ammonia to beneficial plant food. Thus the beauty of bog filtration. You're doing it all in one filter, and supplying the plants to eat the food at the same time. This makes this type of filtration very attractive in some circumstances.
Flow rates and the size of the filter are important in bog filtration, as in any type of filter. Furnace, dryer, car and lawn mower air and oil filters, they are all sized to do a job for a finite period of time, based on flow, before necessitating cleaning or replacement. The math will be supplied in a related topics section, but suffice it to say, it boils down to amount of flow per hour/minute per SSA (specific surface area) of the media. We're using different grades (sizes) of smooth gravels. From 6-8" diameter at the bottom level, then 2-4" gravel in the middle, with ¼"- ½ " "pea" gravel at the top. Each layer will be approximately 12" in depth.
What we're building here is an upflow gravel filter. As we have electricity and pumps in the 21st Century, we don't have to rely on gravity and slave labor to do it for us. As a result, we can create more efficient bog filters than the ancients. They function the same, biologically, just different methods of moving the water.
A bog filter's size is a plus. The larger the filter, the longer the intervals between necessary cleaning. Does it need to be periodically cleaned? Only if you want it to keep filtering!!!! Based upon the size and shape of the filter I'm about to describe, it will be, if not a necessity, of huge benefit to clean/backflush these filters every couple of months during your growing season. It beats weekly cleans, and is a lot more attractive than your average "vacuum cleaner canister on steroids" type pressure filter. Do they work? Of course!!!! They are just one of many types of filtration available to today's hobbyist and professional.
At the end of this introduction, I'm going to walk you through sizing and building an upflow bog filter based on a pond's size, and the flow rate of the pump we decide to use based on the pond's capacity of water. NOT THE SURFACE AREA OF THE POND, BUT THE ACTUAL GALLONS CAPACITY OF IT. This is the ONLY way you're going to have a successful application. Some folks out there have arbitrarily decided that volume of a pond doesn't matter in bog filtration. I have already seen their bog filtration systems failing based on this incorrect information and methodology.
TURNOVER: Moving the gallon equivalent of a pond's total volume of water once.
How do you calculate the flow rate and filter size necessary for a pond? Keep in mind, the more often you turnover a pond, the healthier and more successful it's ecosystem will be. On smaller ponds, say 3-400 gallons, we may turn them over 6-7 times per hour. 1000 gallon, 2-3-4 times per hour. 5000-20,000, at least once per hour. Turnover every 2 hours on a 50-60,000 gallon pond would be great. Bigger ponds would benefit from this rate as well, but you have to look for that "economic threshold", of what can be realistically afforded on an electrical cost factor.
Flow rate will be the main determining factor of the final size of your bog filter. We have found, as a "rule of thumb", that using 1 (one) lineal foot of diffuser per 1000 gph (gallons per hour) flow, plus one additional foot as a "fudge factor", has been very successful. This is a MINIMUM size based on flow formula. Bigger is better, to a point. If your bog filter's size is 10 times larger than this minimum, you'll posssibly end up with "dead spots" of stagnant, anerobic water and "crud". I would not recommend more than 3 times minimum, for maximum aerobic (oxygen enriched) circulation. At this size to flow ratio, you're still going to have decent flow rate per SSA.
Well now, after all these pre-qualifiers, let's build one of these things from scratch.
In this case, we're going to take a 10' by 16' pond, with an approximate volume of 1800 gallons, and use a 2800 gph pump. Based on simple math, 3000gph pump=3' of diffuser plus 1' extra= 4' minimum length diffuser. In this case, we used a 5' diffuser, as we had the room, and the client may want to upsize their pump down the road. This upsizing will give them the option to go up to a 4-5000 gph pump. This 5' diffuser, with clean out chimney, will create a bog filter approximately 6' wide by 8' long by 3 ½ ' deep.
Important point: Shaping the bottom profile of your bog filter vault is critical to long term success and ease of cleaning of your upflow bog filter. Your diffuser also becomes your drain during periodic backflush and cleaning. You want to aggressively slope the bottom to the diffuser, 10 degrees or more, and gently slope the diffuser to the chimney vault. About ¼" in 4 feet. This area is trenched lower than the bottom of the vault to put the discharge slots of the diffuser at a level with the bottom of the main vault to facilitate efficient backflush and cleaning.
As to the components of your diffuser and chimney vault. I make my own, and save about 600% over "patented" pieces of plastic offered by some. I'm going to show you photos of my own make, as I've never used any of the "patented" stuff, as I was making and using them before the pre-mades "discovered" bog filtration and "patented" their plastic.
See series of photos and descriptions on how to make your own.
Once everything is shaped and installed, it's really simple to install your gravel medias. Just think, if not shaped and installed properly, this whole gravel pit would need to be exhumed and cleaned and re-installed every 2-3 years to keep it working properly. NOT FUN.
Place the larger media around the diffuser to allow water to flow freely to the farthest reaches of the vault, and to also allow good backflush flow to diffuser. The next layer of 2"-4" media laid in, thicker and higher along the edges and back of the vault, to enhance flow all through-out the vault. Your final level is "pea" gravel. Each layer is approximately 12" deep. The lowest layer, will not be as deep at the edges, due to the slope of the floor of the vault.
The other cool thing about bog filters, is that you can discharge from them wherever you want. Multiple discharges, up to 360 degrees. Great flexibility. All you do is sculpt and shape the dirt to the elevation and areas you want to discharge from.
We sometimes put some larger rocks at random on the surface of a finished bog filter, to make it look more natural. Then we plant it liberally with marginal type aquatic plants, to take up the nutrients from the "filtered" water before it heads back toward the pond.
This completes your introduction to upflow, gravel media bog filters. More information will be added as it comes available from ancient, translated texts, as well as ongoing, modern day use of this wonderful option to the filtration of today's ponds, lakes and water features.
For more information, contact The Pond Professional at (770) 592-2273
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