Slow sand water filtration (biological sand water filters)

[filter_guy]

Hello to all on the pond forum. My name is Dave. I live in the foothills of the Cascade mountains in north western Washington state (USA). There are 3 ponds here fed with biological sand water filters. One of these ponds has been here for 3 years with the filter running 24/7 (with the exception of time during freezing weather). The largest pond is slightly over 1 year old and about 500 gallons. This area is heavily wooded and there are lots of critters accessing the ponds regularly. Without filters, the ponds become horrifically foul. we've already been there and done that.View attachment 60187 I have a website and a blog where the work I have done on slow sand filters, over the past 7 years; and the info on the ponds and the pond filters is recorded. Several thousand hours of research on the website and blog. 8 working filters here, and 1 filter in testing mode.

There were fish in the original pond, but the raccoons ate them. We don't have fish in the ponds right now, the raccoons will just eat them if they can get at them. I am aware that there probably is a limit to the number of fish one can have in a pond and still maintain a clean environment for them. I am wondering if a slow sand filter may work? I am referring to a biological sand filter - no chemicals needed, sand does not need to be replaced - ever. Very low expense. I don't know about fish though. I wonder if anyone has actually tried using a slow sand filter system to maintain pond health with fish involved?

 

[Kristi Kelty]

Wow! I love this. I would like to know if this sand filter method works with fish. ???

 

[filter_guy]

We had 3 large goldfish in the original pond for several months. Before the raccoons got them, it worked quite well. I just don't know how long or how many or how big for the fish? I guess it would depend on the size of the pond, the size of the filter and the size and number of fish in the pond. I do know for sure, that the slow sand filters we have working here with the ponds, take out, and keep out, very nasty stuff that originates from rotting leaves, bird visits, raccoons, dogs, squirrels and anything that goes near water in the summer in the forest when it is super-dry and there is no water for several miles. The ponds stay crystal clear. Just took this pic. I'll need to remove the leaves some time before Thanksgiving I guess.

The filter was started in August of 2010.

Minimal maintenance and no chemicals. Hopefully someone has tried a slow sand water filter on a fish pond with lots of fish - a true biological slow sand filter, that is. I'm sure there are limits??

 

[dieselplower]

Many many posts about sand and gravel filters. Many types of them as well. Yes they work well but eventually they will fill up so you will want a way to backflush.

 

[JohnHuff]

Hi there, fellow Washingtonian.
I built a tiny slow sand filter a long time ago as a project. SSF can be and are used as water filters in third world countries as well as municipal reservoirs for purifying water. It relies on the Schmutzdecke to purify the water. I read a lot about the DIY ones and one thing I remember is that they all say you should not/cannot drink the water it makes!

It is also my understanding that it does need maintenance and the top layer needs to be cleaned out once in a while, like 6 months or so. SSFs work too slowly for fish ponds which need a higher flow rate. I built two horizontal SSFs of my own design with a much faster flow rate, however I tested them with kits and both failed the bacteria test, however they did pretty good with removing nitrites and ammonia.

The short answer is that SSF cannot be used for fish ponds because of the low flow rate. The only way that I can see it working is if a massive one is used.

 

[dieselplower]

Hi there, fellow Washingtonian.
I built a tiny slow sand filter a long time ago as a project. SSF can be and are used as water filters in third world countries as well as municipal reservoirs for purifying water. It relies on the Schmutzdecke to purify the water. I read a lot about the DIY ones and one thing I remember is that they all say you should not/cannot drink the water it makes!

It is also my understanding that it does need maintenance and the top layer needs to be cleaned out once in a while, like 6 months or so. SSFs work too slowly for fish ponds which need a higher flow rate. I built two horizontal SSFs of my own design with a much faster flow rate, however I tested them with kits and both failed the bacteria test, however they did pretty good with removing nitrites and ammonia.

The short answer is that SSF cannot be used for fish ponds because of the low flow rate. The only way that I can see it working is if a massive one is used.

I imagine it would be a very good polishing filter if say half the water from a mechanical filter were ran through it and the other half down a waterfall or something? Slow flow is great for polishing.

 

[JohnHuff]

I imagine it would be a very good polishing filter if say half the water from a mechanical filter were ran through it and the other half down a waterfall or something? Slow flow is great for polishing.

Yes, that is possible. But I would think that the percentage would be much less than 50%, maybe 5-10% depending on size. I'm thinking that some water can be diverted to a SSF with an overflow that the SSF can't handle and drains it back into the pond.

 

[filter_guy]

Interesting. Thanks for the input. I really appreciate it, as I have very little experience with keeping fish in a large pond.

 

[DaveT]

Yes, that is possible. But I would think that the percentage would be much less than 50%, maybe 5-10% depending on size. I'm thinking that some water can be diverted to a SSF with an overflow that the SSF can't handle and drains it back into the pond.

This is Dave (filter guy) with the slow sand filters. It is now 2022. Last post was 8 years ago in 2013. All the filters are still working. The oldest pond (nearly 12 years old) is still crystal clear with no maintenance on the filter. We now have a 200 gallon fish pond with 5 fish in it. The filter is a slow sand filter in a 45 gallon barrel with an overflow that feeds back into the pond. So far (about 7 months) it worked flawlessly until we had an unusual cold weather event here in western Washington state. It got down to 11 degrees F here. Froze everything. Killed 3 of the fish. The slow sand filter has recovered, but is still filtering the mess made by the freeze.
All 4 ponds here have overflows that feed back into the ponds, and the filters run 24/7.

 

[GBBUDD]

Sand filters are a proven OLD method of filtering water for a variety of reasons. They do polish the water but they also strip the water of oxygen. BACTERIA IS BIOLOGICAL FILTRATION. sand filters strip oxygen the less oxygen the less bacteria can thrive. The bogs that we on this site promote are based on Nature giving it the best platform for her to do it's best. We promoted a similar idea to the sand filter but instead of a small pressurized tank we build a pit depending on the size of the pond and place gravel not sand in the pit now a pea stone version is the simplest . but the idea is to allow as much surface area as possible for bacteria and micro organisms to grow . sand is very fine and not the best at allowing much then a few strains to survive. The best part is mother nature does 90% of all the work . there is little need for back flushing though it can be incorporated but after 4 years i have yet to see much come out. but what i have seen is an incredible amount of bugs insects and god knows what they were in the bog growing feeding and doing there job
. Would i use a sand filter YES FOR A POOL. or as a prefilter to a bead filter. OR you can just build a bog and not worry about anything but ripping out plants as they over grow the area. AND a bog is a hell of a lot better looking with iris lillies lotus and a host of other plants than is a round plastic ball with pipes hanging out.

 

[DaveT]

Actually, a slow sand water filter does not remove oxygen; if it is operating properly. If there is no oxygen in the water all the aerobic bacteria, will die, and only anaerobic bacteria will grow. The filter and the water coming out of it will stink horrifically, and the filter will need to have all the sand removed and sterilized. This rarely happens in the real world.

The city of Salem, Oregon, uses a slow sand filter to provide drinking water:

Drinking Water Treatment

www.cityofsalem.net

Here are the results of one series of the tests we've had done on the filters over a time period of 2 and a half years (note the pre-filter and post-filter coliform results):

Also, slow sand water filters are used by thousands in countries other than the U.S.A. :

Biosand Water Filter — Malawi Water Project

www.malawiwaterproject.com

Also read about biological sand water filters on Wikipedia:

Biosand filter - Wikipedia

en.wikipedia.org

Yes, plastic barrels don't look too good. A bamboo fence and bamboo pipes from the filter help:

The filter in the above picture has been running for 8 years. The water is crystal clear, always. The only maintenance is adding some water in the summer. The filter has an overflow which keeps the water oxygenated. The filter does not remove oxygen. Water is always splashing in a slow sand filter. That adds oxygen to the water.