HomeBiogas has 2 filters: the gas filter and fertilizer filter. 

The gas filter is made out of activated carbon. All the biogas produced in HomeBiogas will pass through the filter in order to remove H2S (the rotten egg smell). Thanks to the gas filter, the biogas piped into your kitchen is clean, safe, and smell free!

You will need to change your gas filter once every six months (or whenever there's an unpleasant smell). You can contact HomeBiogas to order a replacement. 

What can you do with your old filter? You can recycle it by burying its contents in your backyard and recycling the container. When you bury the filter it can turn into sulfate, which can be taken in by plants as a nutrient. It is not dangerous for the environment or for your plants :)
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The second filter is the chlorine tablet, which is used to filter out pathogens from the fertilizer. While the anaerobic digestion process that takes place in HomeBiogas does remove pathogens, the chlorine tablet is used as an additional method of pathogen reduction, so that the fertilizer is unequivocally safe and in compliance with local regulations.
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You should replace the chlorine tablet if it has dissolved completely. This will happen every couple of months, so you should check it once a month. You can purchase standard solid pool chlorine tablets in shops near you, online, or from HomeBiogas.