BUPOLSA's biological filter is made of glass fibre reinforced polyester (GRP) and completes the treatment of wastewater from the septic tank, through a filtration and oxidation process. It is used to remove the organic matter found in the wastewater by means of a high-performance porous filtering material, in which the bacteria nest and degrade the organic matter dissolved in the water from the septic tanks. A septic tank removes 35% of BOD5, while a biological filter removes up to 90%.