GPH for Bog Filter..?

[Kaniff]

My bog filter is 1'x1'x12' and drains into my waterfall. The pump on it right now is around 800gph. I plan on building a skimmer with a 950gph pump and I want to hook it into my bog filter... Will 1700ish gph be to fast water for my bog plants? I have cat tails, pickerel rush, lillies in the filter.

 

[Meyer Jordan]

Let's look at it this way. Your "bog", given the stated dimensions, has a capacity of about 80 gallons. If you are using pea gravel as the sub-strate, then you may possibly have space for only maybe 20 gallons of water.
Based on that, It would seem that not only would 1700 gph be entirely too high of a flow rate but also the present 800 gph is too great.
The efficiency of a "bog" filter depends on, among other factors, dwell time or resident time of the water being filtered.

 

[Mucky_Waters]

Let's look at it this way. Your "bog", given the stated dimensions, has a capacity of about 80 gallons. If you are using pea gravel as the sub-strate, then you may possibly have space for only maybe 20 gallons of water.
Based on that, It would seem that not only would 1700 gph be entirely too high of a flow rate but also the present 800 gph is too great.
The efficiency of a "bog" filter depends on, among other factors, dwell time or resident time of the water being filtered.

How does one calculate dwell time in a bog?
800 gph does not seem like too fast a flow for 80 gallons of wet bio-media, but would be quite fast for an 80 gallon solids settlement chamber.
I must profess, I have never quite understood what a bog was suppose to do. Is it a bio-filter? a mechanical filter? a solids settling area? or simply a media for plants to grow roots in and extract nitrates from the water?

 

[Meyer Jordan]

How does one calculate dwell time in a bog?
800 gph does not seem like too fast a flow for 80 gallons of wet bio-media, but would be quite fast for an 80 gallon solids settlement chamber.
I must profess, I have never quite understood what a bog was suppose to do. Is it a bio-filter? a mechanical filter? a solids settling area? or simply a media for plants to grow roots in and extract nitrates from the water?

Wetlands filters are usually considered as a 'water polishing' systems. They are designed for the removal of Nitrate, Phosphous, Heavy Metals and other pollutants from the water through the functions of plants. In some instances, these systems will also reduce the level of pathogenic organisms through natural predation. For this to efficiently occur sufficient dwell or resident time must exist.
In constructed wetlands that are used for 'polishing' waste water, sewage or industrial, the generally accepted optimum flow rate is 21,000 gallon per day/acre.which is an extremely low flow rate.

 

[Meyer Jordan]

How does one calculate dwell time in a bog?

Basically by using the same method for determining turn-over rate for a pond. If the turnover rate for a pond is 2, then the dwell time (resident time) for that particular pond would be 30 minutes.
For the "bog" described by the OP, the dwell time, without any media or sub-strate would be 800 gph/80 or 10 times per hour or 6 minutes dwell time. Assuming that the media will occupy about 80% of the "bog" (using pea gravel), then 800/20 or 40 times per hour or 1.5 minutes dwell time.

 

[addy1]

Basically by using the same method for determining turn-over rate for a pond. If the turnover rate for a pond is 2, then the dwell time (resident time) for that particular pond would be 30 minutes.
For the "bog" described by the OP, the dwell time, without any media or sub-strate would be 800 gph/80 or 10 times per hour or 6 minutes dwell time. Assuming that the media will occupy about 80% of the "bog" (using pea gravel), then 800/20 or 40 times per hour or 1.5 minutes dwell time.

Interesting!
I should, maybe, if feel inspired, figure out our dwell time.
I have no clue, but it works, probably helps that mine is pretty big. i know my pump is around 6800 gph minus pipe head loss, minus some that goes to the small pond. it will be interesting.

 

[Mucky_Waters]

Basically by using the same method for determining turn-over rate for a pond. If the turnover rate for a pond is 2, then the dwell time (resident time) for that particular pond would be 30 minutes.
For the "bog" described by the OP, the dwell time, without any media or sub-strate would be 800 gph/80 or 10 times per hour or 6 minutes dwell time. Assuming that the media will occupy about 80% of the "bog" (using pea gravel), then 800/20 or 40 times per hour or 1.5 minutes dwell time.

Yes the 1.5 minutes dwell time is easy to figure out, but that number alone is meaningless unless you have something to gauge it by. If we use your recommended wetlands dwell time as a gauge, then the OP should be running his water through their 80 gal bog at about .5 drops or so per day or something.

 

[Meyer Jordan]

Yes the 1.5 minutes dwell time is easy to figure out, but that number alone is meaningless unless you have something to gauge it by. If we use your recommended wetlands dwell time as a gauge, then the OP should be running his water through their 80 gal bog at about .5 drops or so per day or something.

True wetlands filters are designed for an extremely low flow rate with the goal of weeks resident time.
Most Garden variety "bogs" are not designed strictly by the same guidelines as an engineered system, but the same mode(s) of filtration is utilized. So a low flow rate would seem indicated if one is looking for similar results.
Granted, the low flow rates of engineered systems can not be practically duplicated on the scale of a Garden Pond obviously because of the very much smaller comparative footprint. But to achieve the "polishing' effect, the flow rate should be as low as is possible and the "bog" in this case would be secondary filtration to a main biofilter.

 

[Mucky_Waters]

I guess I'm not making myself clear.
For comparison, to help gauge the theoretical performance on settling tanks they have a 10% rule. In other words if you were running a flow rate of 800 gph your settling tank should be 10%, 80 gallons (or more) in size to get adequate settling. I believe that works out to a dwell time of about 6 minutes.
I have no idea what the flow rate on a bog full of rocks or pea gravel is suppose to be.

 

[Meyer Jordan]

"Bogs" and settling tanks are not a valid comparison in this case. Each have an entirely different purpose.

 

[Mucky_Waters]

"Bogs" and settling tanks are not a valid comparison in this case. Each have an entirely different purpose.

You have completely missed the point.

 

[Mucky_Waters]

Based on that, It would seem that not only would 1700 gph be entirely too high of a flow rate but also the present 800 gph is too great.

If 1700 gph is too high a flow rate for a bog, what would a good flow rate be? What are you gauging it on?

 

[Meyer Jordan]

If 1700 gph is too high a flow rate for a bog, what would a good flow rate be? What are you gauging it on?

It is already been established what the recommended flow rate for an engineered wetlands filter is. A "bog", by design, is an approximate, but much smaller, version of the larger systems. Since the emphasis of these systems is on the phyto-filtration feature then so should the emphasis be in a "bog" attached to a Garden Pond....phyto-filtration. For this process to be efficient, the flow rate must be as low as is practical. Plants do not process nutrients with the same rate as the bacteria in a biofilter. Bacteria perform at a much higher rate.
If a "bog" in a Garden setting is large enough, it can be utilized as a primary biofilter with a higher flow rate, but the efficiency of the phyto-filtration will be sacrificed and the water will not be 'polished'. The "sparkle" that is the prime feature of polished water will not be achieved.

 

[Mucky_Waters]

flow rate must be as low as is practical.

The question was? If 1700 gph is too high a flow rate that bog, what would a good flow rate be?
I'm looking for a practical answer, some sort of equation for determining what flow rate for a specific sized pond and bog?
I used the settling chamber 10% ratio as an example.
If there is no way to accurately determine these ratios (flow rate vs bog size vs pond size), then we can only guess what they might be.

 

[Meyer Jordan]

Then let me answer it this way.

Flow rate for a "bog" should be calculated to provide the maximum amount of dwell or resident time possible. This will vary based on size (volume) of the "bog" and type of substrate.

I found this guideline:
"Mungasavalli et al. (2006) suggested retention time should be greater than 24 hrs. Shutes et al. (2005) suggested if retention time is greater than 36 hrs, contaminants like TSS, heavy metals, Petroleum Aromatic Hydrocarbon (PAH) and herbicides can be removed."