**4. Nonconventional methods: biofiltration**

Biofiltration, synonym for biological filtration, is a secondary treatment process for onsite wastewater. Filtration is one of the more common biological treatment processes. Filters are commonly constructed using sand, gravel, peat, or a synthetic material. These synthetic materials, such as foam, fabric, or plastic and natural materials, such as peat, are grouped together under the generic title "biofilter" [34].

Biofiltration is considered an unconventional process that involves the removal of pollutants (such as drugs, fertilizers, dyes, among others) through a physical (adsorption) and biological process simultaneously of a packed material in a filter. The packed material can be a natural one (organic or inorganic). In addition, low-cost adsorbent media can be used (which can even be an agro-industrial waste), such as bentonite, polymeric resins, or peat, which makes this process more eco-friendly and economically competitive compared with physical or chemical processes [32, 35].
