I'm looking for advice on whether my plan for a bottom drain + skimmer gravity-fed pre-filter to bog to pump to waterfall design will work/be effective (I've attached a photo of the proposed design).
I'm re-lining and slightly adjusting the pond because the previous owners of the house kept the liner below ground/surface level (and there's not enough to bring it above) and the pond is also below ground level in areas which allows for an obscene amount of silt to wash in when it rains.
The pond is roughly 33 inches deep and in the re-build I will likely drop that slightly deeper to accommodate a sloping floor to the pond to facilitate waste removal from the bottom drain. The skimmer is a pool skimmer I've attached a 3 inch PVC pipe to which will run to the top of a prefilter (this is the setup I have in the pond currently). For the bottom drain I'm debating between 3 and 4 inch pipe. I plan to put gate/knife valves on the prefilter inflows to allow for balancing of the skimmer and bottom drain throughput (I'm thinking the skimmer will overpower the bottom drain and may need to be stemmed. I plan to put a filter sock over the prefilter inputs to trap most of the debris, followed by a dam/wall which the water level has to rise to flow over before passing through several layers of sponge filter media before flowing through a 4-6" pipe leading to the bog filter. The bog filter will likely be around 4' x 10' and 3 feet deep. Aquascape has flat floors to the bog (with inset distribution pipe/caterpillar) but I'm wondering if sloping the floors wouldn't increase backwashing effectiveness. I plan to separate the rocks in the bog filter from the bottom with milk cartons or something similar, then have a roughly 8" layer of 4-6" rock followed by a 6-8" layer of 2-3 inch rock, followed by an 8-10 inch layer of 3/4" to 1.5" rock. From what I've seen, although smooth rock should have less surface area, it's advised to use this in bog filters to accommodate flow rates and not get "bogged up". I would then have another dam wall the water has to well up over before entering the pump chamber. I plan to use a 20,000 LPH variable speed pump with 2 inch return lines to a waterfall.
I'm planning to have the feed for the bog be a 6" PVC pipe with slits cut into it and also have a vertical stack for backwashing the bog. I'm debating on putting cleanout/flush valves/pipework at the bottom of the prefilter intake and bottom of the bog filter. In this case I may not need the vertical stack for backwashing the pond (but it may be nice to have a failsafe)?
My main concern is the successive/staged design of the prefilter and bog, and water level drops between each phase with it being gravity-fed. That being said, from a calculator I've found online (https://www.omnicalculator.com/physics/orifice-flow ) it appears that a 2cm water level drop with a 4 inch pipe and 3 inch pipe (bottom drain & skimmer pipes) should provide roughly 4550 GPH (coefficient for liquids is 0.61 i believe) and a 1cm drop is expected for that flow rate with a 6 inch pipe. Does this mean the bog's water level could theoretically be only ~3cm lower than the pond? I have no issue with this if the calculations check out.
I planned to have overflows/dam walls for the 2nd stage (filter media side) of the prefilter and the pump compartment to impact water level drops as little as possible, but if I would only lose ~2cm per side, it may be worth having a 4-6" pipe connect the different comparments as I've been having a tough time figuring out how to make a dual compartment prefilter without plastic welding (I'm afraid of leaks at some point with this method, especially with frost heaving in Southern Ontario, Canada.
Let me know if this seems reasonable, what you think works or won't work, if my water level calculations seem correct (I know it's calculations based on an orifice, but piping will be large with few bends and shouldn't affect flow rate by more than 10-20%). If you have any ideas for how to have a 2 compartment prefilter, etc, I'm all ears!
I just dismantled by bog filter after. its 4m diameter. 3 layers of media up 50cm thick in total in some parts. Took 8 people 6 days to manually remove the media. The layers were choked with fine silt and sediment. Dropping the sump pump or back washing from above did nothing to clean it. So, I am rebuilding it, this time I am going to pre filter (up flow bead fiLTER) the water and reduce the load, secondly I am only using 5cm-8cm of pea gravel. Reducing the flow to 15kl p/hr. So in the event of back washing I can stir the pea gravel up and the pump out from the bottom. 5cm-8cm is much easier to remove. Fingers crossed.
The biggest reason mine bogged up was the load was heavy with solids, and possibly the flow could have been a tad bit too high, however regardless it was a pain to clean out. I should have learnt from my under gravel filter experience.
My bog is really just for aesthetics now. The main filter is going to be RDF to bakki shower plus UVed.
I am about to rip out a pond that was originally built with a bog. What a stinking, putrid mess that was, I would never do that again. And yes, my pond builder "knew how to build them the right way". That is ancient technology. For the money, I would consider installing an RDF (Rotating Drum Filter) to immediately remove solid waste from your water column pumped to a bakki shower filled with ceramic media. I would also recommend you consider a semi-raised design so you can sit and enjoy your koi up close.
If you want a bog for aesthetics, build one next to the pond but not connected to it in any way. Keep it filled with the water from your pond filter flushings and water changes and the run off or overflow from the bog can then feed your garden beds.
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Cheers,
Ci
Do you know if your bog was pre-filtered? One reason I'm heavily leaning towards using a bog filter is for aesthetics and not having any exposed or above-ground filtration devices. Correct me if I'm wrong, but I don't believe I'd be able to use a gravity-fed supply for a drum filter, so I'd have to pump water high and have everything be gravity fed back to the pond, where my current design has the filters gravity fed from the pond and then actively pump back into the pond.
Did you regularly backwash the bog? With a prefilter I'd find it difficult to imagine it clogging...
I put two small plant container bogs on my (also small) pond. It is just finishing up first short season out side.
Similar to PetaPete's suggestion I wanted material that was light, and easy to agitate as part of the cleaning process. I went with the leca (clay) balls. - I bit pricey for clay, about 10x the cost of gravel. I'm thinking I can hook up a hose with a rod at the end (like for digging dandelions) and wash the material if it does get dirty. Also if it does clog I expect it would rise and flow over the top, so I'd know pretty quick.
I'm not saying I know it is a great Idea, but it is what I am trying. Maybe something to consider. FWIW, I expect pea gravel could be cleaned the same way.
I also think with slow flow, and a drainable void at the bottom of the bog, would allow much of the debris to settle out, before it goes into the media. I expect you've seen ozponds.com. I'm not sure that is appropriate for a koi pond though. However if it is largely for ascetics only, the slow flow should work for you, with a tee-off from your main mechanical (and I assume bio) filter so only partial flow through the bog.
I have a (temporary and rather Lame) DIY pre filtered on both my bogs. I expect to be tearing down the pond next month. I'm curious how much dirty it will be after a few months of operation. If I remember I'll flush with a basic flow of water on top, and then (if i find my old dandelion digger t) with the pressurized water injection. I'll also try to remember to post the results.
I'm new to the pond hobby, and this will be my first time, so I don't have a ton of confidence in the above. I'm not going to pretend to have an answer for the other aspects of your original post.
Bogs are kool. But a proper koi pond doesn’t need one and it’s easier to maintain one without a bog. For long-term health of the fish, I would lean toward simplicity and robustness. I remember when I first started keeping koi, the thought of not having natural elements (plants) kid of bummed me. But now many years later…the koi have easily supplanted that yearning.
I would strive for bullet-proof and simple design that is gravity fed. Filter socks will clog and require constant cleaning. Also the slots in your slotted bog pipe will eventually clog. If budget allows maybe get a settlement chamber, sieve or drum filter as the 1st stage.
Last edited by sandthemall; 09-08-2024 at 01:42 PM.
Reason: Grammar and spelling
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