Bog filter advice please

[Sharkologist]

Hello everyone, I'm a new pond owner and this is my first post here. I've picked up a lot of advice from the site already so this seems like a great place to ask for help!

I have a ~25kL wildlife pond on a property we moved to only a few months ago - it was in a bit of a sorry state. I absolutely love the pond so am happy putting time and money (within reason, I'm not rich...) into improving it. I've just had it totally relined as the old liner was poor quality and leaking - this was expensive... There is currently no filtration system at all and I'm looking at building a bog filter myself using whatever knowledge I can find online. I'd like the pond to be clean enough for the kids to paddle/'swim' in (they do this anyway since the reline as the water is pretty clean for now).

I'm posting my proposed bog filter setup here hoping for some input and advice from those more experienced! A sketch of the setup is attached. I'm thinking of using timber sleepers to build the structure and lining this with HDPE. I thought I would attach the liner over the top of the sleepers (with pins/staples) and cap it with wood or stone (see sketch). I will fill with large stones, then pebbles, then pea gravel, and add some plants into the gravel. Cleanout will be via a capped standpipe which I'll use a pond vac on to suck up debris from the bottom of the bog (digging one end deliberately deeper for this purpose). I've added a ball valve to the in-pipe above water level based on advice I've seen that this can help avoid reverse siphoning back into the pond. Water will be pumped directly from the pond - pipe length between pump and filter will probably be ~6m, and head probably ~1.5m. It will flow back out of the filter via a 'spout' into a small streambed (I expect it will only be a trickle, but that's fine) and then into the pond. No plans for fish in the pond (maybe just a few smaller ones down the line). We have some newts, frogs and snails naturally.

Please comment on any part of the setup, but these are a few questions I have, :

• Is the bog big enough? Surface area is limited by space and the standard sleeper sizes. Depth is limited by cost and tree roots. By my calculations the bog capacity will be ~1500L, so ~6% of the pond.
• What pump type/size would I need? From the advice I can gather it seems I need a flowrate ~5-6x the bog capacity, so ~9kLph? I understand the flow in needs to be slow.
• Any better/cheaper way to build the structure? I can't dig deep into the ground due to tree roots so need a mostly raised bog. The sleepers alone will be ~£500+. I couldn't find any decent plastic/metal alternatives at that size.
• I wanted to do cleanout via a bottom drain valve to avoid the need to buy a pond vac, but couldn't think of a way of doing this without needing to pass through the liner with a bulkhead or similar and I don't want to do that - is there a better way?
• I know I need to keep the horizontal pipe on the bottom of the bog raised off the deepest part as well as protected from the stones/gravel above - is there a cheaper way to do this other than using larger rocks? I first thought of using bricks but then realised this would mean lots of brickdust getting into the water (is that a problem?)

Many thanks in advance!

Main sources I used to get to this point are (1) this forum and (2) ozponds.com and his YouTube videos (really helpful)

 

[GBBUDD]

ball valves are great when your standing there when the power goes out but they will not work when the power goes out and it's unattended. ie the siphone breaker or a hole drilled just below the gravel in the bog and when the water drops in the bog it pulls in air and breaks the siphon or the vacuum breaker that works reverse of a check valve .

sorry im a bit lazy to go through converting metric this and that so i can't help there but 6% is way low 10 is minimal but 25 to 30 is the sweet spot

 

[j.w]

and welcome @Sharkologist
Lots here have bogs and can help you. I never made one when I built my pond.
Should have tho!

 

[Sharkologist]

Thanks @GBBUDD. Would you mind explaining a bit more what you mean about the ball valve?

Yes I thought it would be too small, but I'm a bit limited by space and cost at the moment. Maybe I could add a second small bog later and feed it off the same pump. Any advice on pump size/power/flowrate I would need? I realise that's an "it depends" question...

PS: took a look at your links - incredible work!

 

[GBBUDD]

in your drawing you have a ball valve that is before the displacement pipes in the bog . this is to limit or shut down the flow into the bog . a definite must but you also will need something to stop the syphone so when the power goes out that the water doesn't drain out from the bog and empty back into the pond. now if your pond is so big thats that is not a [problem then great with the exception your bog bacteria and creatures will dry out. this is not a good thing. so to prevent this we want a syphone breaker or a check valve to keep the water from back flowing into the pond

 

[Pablo]

Ozponds on youtube is a good resource on building bogs. He also demonstrates adding a siphon break like @GBBUDD mentions

Sorry just noticed that you already have found ozponds. He does have a video on siphon breaks.

 

[Pablo]

As for the size of your bog, the smaller it is then the fewer fish you should have or limit it to just goldfish. If you want more fish then you would probably have to look into mechanical filtering.
Pump size can be tough to calculate. Depends on the amount in the pond and the height going to the bog along with the length of the pipe and any turns, ie calculating the head. The rule of thumb is that you want to turn over the volume of water about every hour.
I would not stress out about the clean out for the bog. There are many people who have never cleaned theirs out, and the ones that do the clean out a lot of times do not find much. Can be variables to that depending on how many fish you have and debris in the pond that you leave in there rather than cleaning, like the leaves.