420 Magazine Background

Slow Sand / Rockwool Filter Info

Smokin Moose

Fallen Cannabis Warrior
Where Do I get the Bacteria to Ripen my Slow Sand/Rockwool Filter?
The bacteria needed to "ripen" your slow sand/rockwool filter is in the atmosphere. Just running the filter for one month will give the necessary bacteria time to innoculate the filter.

How Small Can I Make a Slow Sand/Rockwool Filter?
You can make a slow sand/rockwool filter as small as you like, as long as the flow rates are correct for its size and the media depths are right.

Will a Slow Sand/Rockwool Filter Change the PH of My System?
No. A slow sand/rockwool filter will not change the pH of your system if you used an inert media for construction.

What Will Happen if I Construct my own Filter using Maximum Suggested Flow Rates and Minimum Suggested Media Depth?
In a worst case scenario, your filter will not be 99.9% effective against all types of bacteria, viruses and fungi. Typically, you would still expect it to be no less than 94% effective against a range of pathogens.

Will a Slow Sand/Rockwool Filter out my Hydroponic Nutrients?
No. A slow sand/rockwool filter will not filter out any of the nutrients in your system. The bacteria in the filter will use a very small amount of nutrients to grow and multiply, but it is negligible. Slow sand/rockwool filters only filter out bacteria, viruses and fungi.

How Do I Work out the Surface Area of One eEnd of a 4-Inch Storm Water Pipe I am Going to use as my Filter Housing?
Use the formula pi x radius squared. (3.14 x (2"x2")) = 12.56 square inches for a 4" pipe

Can I use Any Other Hydroponic Mediums as my Active System Media instead of Sand or Rockwool?
It is possible there are many other products that can be used as the active media in a slow sand/rockwool filter. All the hard test data I have seen has mainly focused on sand as the active media, as it is the cheapest. Only the Australian tests have trialed rockwool, as well as sand, as far as I know.

Rockwool has superior surface area for the bacteria to live on compared to sand. This is what makes rockwool more efficient. Combined with the fact that you don't get rockwool migrating through the drainage layer makes rockwool easier to use and maintain. There is a company in Holland that has put a lot of money into private research on slow sand filtration and they have developed their own technology. By using a special blend of bacteria to inoculate their filters, they are ready to use immediately. Combined with their own "volcanic" active system medium, they have decreased filter depth and increased flow rates while still maintaining 99.9% efficiency.


New Member
Thankfully I came here with previous knowledge of this type of filter as I have spent some time myself researching and learning about them, so I will try to explain this for you, because honestly they are really that AWESOME!

To start with my friend and I discussed the size of the filter we planned to build, we went on the basis of that if the filter is wider it will allow more water to flow through it faster because you have more surface area for the water to run though. The height of the filter should not be taller than the amount of water in the tank which will be at a constant height. In other words if your water level will never drop below 12-inches, I would make sure my sand level was not higher than 10-inches. But lets get into the reasoning behind this...

The natural bacteria they are talking about being needed to make the sand filter work, (rockwool being added is new to me) is present naturally in the water itself. When I didn't understand this concept myself I was told to think back to when I have gone to a water body, lake, ocean, river, etc... you know that super slimy feeling stuff you can find on rocks, sand, drift wood, etc... that is exactly the natural bacteria they are making reference to here. The "KEY" here is that you can not let the sand dry out for any reason, or for any length of time or you will kill off the bacteria, which needs to remain constantly wet. Again compare this effect to the fine powder on top of sand, rocks, etc... left behind when the water recedes. So now at least you have an idea what the bacteria that they are talking about. Culturing it (getting it and making it grow) is also very simple, keep water exposed to it, that's all it takes.

Which lead me to want to know if I needed rainwater, tap water, etc... in order to culture this bacteria? To my surprise I found out that even some high quality water purification systems for your home years back had one of the filters filled with sand and used this as yet another filter in the filtering process, my friend could not tell me if these systems still exist and work as he is no longer in that field of work. The natural bacteria eats harmful bacteria as the water is passed through the sand and slime, I was told to think of it like a screen filtering out debris from dirty water. I remember seeing these in those heavy duty clear plastic/glass filters hidden underneath of sinks in the dark, if that helps ring any bells for anyone besides me.

The sand is one of natures water filters, any sediment, dirt, etc... will sit on top of the sand where it will become stuck, the sand will settle into place over time and compact itself, and where my friend and I discussed using things like screen at the bottom of the filter we kept having our concerns with it breaking down over time and needing to be replaced. Which is where I am assuming the rockwool comes into play, because one it is fibrous so it keeps the sand from leaving the filter, and two because of what I read above it also allows the bacteria to grow on it really well.

So all in all this sounds awesome to me... Which has be searching around tonight looking for rockwool and going back to research sand again, I know there is different types of sand and there is one type which is sought after for landscaping paver bricks and things like that because of how it interlocks together. I always thought sand was just sand, but I learned last time I researched making this type of filter there are different types or grades of sand. I doubt it matters, but perhaps someone else will come along later and speak about the things I've covered and explain it better than I have.

I just went back to look up some more information on this and they are saying you need to have no less than 12-inches of sand in height, and are recommending 48-inches of sand in height. They also recommend 2-3 layers of rock in the bottom of the sand filter, which also is something my friend and I had previously discussed as well, because of how rock dust is so important to plants for giving them the minerals they need and why garden shops will sell "rock dust".

How effective and important is a sand filter:

Quote "A slow sand filter is a biologically active filter that has been shown to be highly effective against many kinds of fungi, bacteria and some viruses that are common in hydroponic systems, including Phytophthora, Pythium, Cylindrocladium, Verticillium dahliae, Thielaviopsis and Xanthomonas bacteria. Slow sand filtration has been shown to be 99.9% effective against Fusarium spores that are more resistant to heat and UV treatment than other pathogens. The slow sand filter does not affect the pH or electrical conductivity (EC or ppm) of the hydroponic nutrient solution."
Top Bottom