In the construction of a biofilter, a minimum sand bed depth of 30cm is recommended. This depth is necessary to ensure efficient nutrient assimilation and to provide adequate rooting volume for the plants. The biofilter should be sloped at a rate of 2cm per meter, meaning a 3-meter long bed will have a slope of 6cm. Consequently, the sand depth at the shallow end will be 30cm, and at the deeper end, it will be 36cm.
The optimal depth of the sand bed can vary depending on the plant species being grown. For instance, longer-term crops like banana or fig trees require a more extensive root system and thus a deeper sand bed compared to short-term crops like leaf lettuce or radish. If one wishes to increase the sand bed depth beyond the recommended minimum, the only potential drawbacks would be the additional cost and labor involved. However, whether this additional investment results in a discernible positive impact on productivity is yet to be conclusively established.