Chapter 15 - Biological filtration

Abstract

This chapter describes the biological filtration process. The trickling or percolating filter, also called a bacteria bed, consists of a bed of suitable course porous media on which a biological film or slime grows. The film consists mainly of bacteria upon which higher organisms graze to keep the film active. Settled sewage is periodically distributed over the surface of the medium and, as it flows down through the bed, the fine suspended and dissolved organic matter is adsorbed by the bacteria. Oxygen to sustain aerobic biological oxidation is provided by air, which circulates through the bed via vents at the base of the filter. Trickling filters are well known for their diversity of life-forms participating in the waste-water stabilization. These include procaryotic and eucaryotic organisms as well as higher life forms such as rotifers, nematodes, annelid worms, snails, and various insect larvae. Fungi are also found in trickling filters, which are active in the biofilm and are actively involved in waste stabilization. These organisms tend to dominate at lower pH values, usually associated with industrial waste treatment. Algae are active in the biofilm and produce oxygen during daylight hours. The continuous removal of bacteria by protozoa helps to maintain an active bacterial population, and thus maintains a high decomposition rate. Various factors that affect the efficiency of the aerobic biological filtration process include organic loading rate, volumetric or hydraulic loading rate, depth of media, type, size and shape of media, and the total surface area of media available in the filter.