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 dont 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.
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