Slow Sand Filter Basics The sand provides a large surface area for “good” micro-organisms to attach and be retained in the filter (high biomass retention time). Influent dissolved organic matter (DOM) is removed by microbial mineralization (CO2 and H2O) by the good bacteria (reducing color and odor). Influent particles are removed by attachment to (collection on) the large surface area (reducing turbidity). The top Schmutzdecke layer enhances both filtration and mineralization of particles and DOM. Slow sand filters can be operated as continuous flow or batch reactors. The long hydraulic retention time (8 hrs) allows for complete mineralization, and reduces biomass growth that would clog the filter. Jenny-note team that this is all new