Clarifier effluents of 2-10 NTU may be improved to <1 NTU by conventional sand filtration. Modern mixed media, AFM and zeolite filters offer effective filtration down to as little as 5 micron.
A mixed media filter refers to a type of filter bed which is graded by size and density. Coarse, less dense particles are at the top of the filter bed, with finer, more dense particles nearer to the bottom. Down-flow filtration allows deep, uniform penetration of particulate matter into the media bed and permits high filtration rates and long service runs. The smaller less dense particles tend to remain at the bottom of the filter even after high-rate back washing. Thus, the different media layers remain intact in the mixed media filter bed.