How to Make Your Own Air Filter

This week in the Flog:

  • The Long-Awaited How-To Guide for Air Filtration
  • Use This Simple Technique to Filter Your Air and Kill Smells

A little while back, I wrote an article demonstrating how to make your own fish fertilizer at home. This is a really fun (and kinda gruesome) recipe, one of our more popular ones because it is so easy and such an effective fertilizer.

As I mentioned in that article, you can make your own biofilter very easily at home. This is a great way to keep down the smells associated with some of these recipes. I know many readers live in the city, and want to do things like the fish fertilizer, that make some unpleasant smells during fermentation. Well, read on!

To make your own biofilter, you basically need biochar and a length of pvc pipe (if you don’t have biochar you can use plain old charcoal). The biochar is your charcoal, and the pvc pipe holds it and funnels the air through it, dropping the smells along the way. If you’re wondering what biochar is, we have an article all about it and how to make your own biochar.

Now if you want to enhance your biofilter, inoculate your biochar with microbes, ideally some lactobacillus. You can mix a little sugar+water together, inoculate with lacto, and moisten the biochar with that combo. This is an awesome way to supercharge your biofilter.

The setup involves a length of pvc pipe with two caps. Stinky air from your stink source will travel through a tube, through a little hole in one of the pvc caps, through the pvc pipe full of charcoal, and out a little hole in the other pvc cap at the far end of the pipe. Here’s the whole setup, as pictured in my little fish fermenter:

Carbon Filter in Action

This is the fish fertilizer system, complete with homemade carbon filter that takes care of all the smell coming from the fermenter

Here is the length of pvc pipe next to my bag of carbonized rice hulls. Carbonized rice hulls (CRH) are the hard outer shells of the rice seed, piled up and charred so that they become biochar. You can find details of the process on our page about biochar.

Carbon Filter - First Step

Here is the pvc pipe, and carbonized rice hulls. The hulls are charred to produce a high quality biochar, which I use for many things around the garden.

Meanwhile for the caps, each one gets a little hole that’s just big enough for the tubing to fit in. You can seal this with silicone if you are really worried about air escaping, but if you make it a good tight fit you don’t really need to worry about it.

Carbon Filter - end caps

The end caps go on either end of the pipe after it’s filled with biochar. You don’t need to cap the back end, but I do to help keep it a bit moist inside.

Now the pipe gets packed with your air filter substrate. The CRH, and indeed all biochar, makes an excellent stuffing material, because it’s so porous. No matter how hard you pack it in there, air can pass through no problem.

You want to moisten the substrate before you pack it in, but not make it wet. If it’s too wet, air won’t be able to pass through and you’ll have a clogged filter. Ideally you would moisten it just enough to support a healthy population of microbes. This is where lacto comes in. You can moisten the biochar with some lacto and sugar+water. They will be happy in there and the sealed caps at either end mean it will stay nice and moist inside.

Here’s the pipe packed full of biochar:

Carbon Filter - full of biochar

Here is the pvc pipe full of biochar, which is an excellent filter of bad smells. Inoculating with lacto and sugar makes it even better!

Next the caps go on either end. You can see (if you look closely in the first picture or the picture below) I even have a short bit of tube sticking out the bottom side, you don’t really need this, you just need a little hole in your bottom cap to let the air exit the end of the filter. You technically don’t even need a cap on this end, but I like to use it to help keep the inside moist so the microbes don’t dry out.

Now you can see the filter in action! Even though I packed the pvc with biochar, air passes through nicely as you can see from this one bubbling away as fermentation takes place in the fish fermenter.

Carbon Filter - In Action

Even though I packed the biochar in tight, the air goes through the filter no problem. As the fish ferments, the air goes through the filter, and you can see it here making bubbles in the water bucket.

There you go, pretty simple way to kill the smell when you’re making fish fertilizer! For us living in the city, this is a great little trick to keeping our families, and neighbors, happy!

  • Karry

    X-C-Lent, love this will be doing it for sure. Thanks Patrick.

    • Patrick

      Thanks Karry!

  • Igor

    Bought some PVC pipe (you wouldn’t believe how hard it was to find any caps at all) and some carbon pellets from an aquastore. Great tip about adding lacto!


    • Patrick

      Excellent to hear, that should be great! Yeah the lacto is a good addition..

  • Mick Kruckow

    G’day Patrick, just put down my first brew of fish hydrolysate so looking forward to the end product. Actually put the fish through my mincer & got a nice even product. Probably not in the right blog again, but I was wondering if your filter would suit what I am trying to achieve with my (hydroponic/bioponic/organoponic) grow in the soil system as a air inlet filter?.
    I have got to the stage of a venturi injection system & through trial & error have found out that it is not good to use in the heat of the day, as I end up with jelly in my pipes & filters. I’m going to try early morning injection (4am start) & go to periodic watering during the day & see how that goes. Back to my question, as we’re trying to remain as anaerobic as possible,(in your opinion) would the filter at least let the cleanest possible air in during the injection time.
    Regards, Mick

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