ABSTRACT
After preliminary and primary treatment wastewater still contains significant amounts of colloidal and dissolved material that needs to be removed before discharge. The problem for the engineer is how to convert the dissolved material, or particles that are too small to settle unaided, into larger particles so that a separation process can remove them. This is achieved by secondary treatment. Chemical treatment using coagulants will deal with a portion of the colloidal solids, but a large portion of the polluting material will be unaffected. Also, the cost of continuous chemical addition and the problem of disposing of large quantities of chemical sludge makes this option normally unattractive (Section 20.2). Alternatively biological treatment can be used. This utilizes naturally occurring micro-organisms to convert the soluble and colloidal material into a dense microbial biomass that can be readily separated from the purified liquid using conventional sedimentation processes. As the micro-organisms are literally using the dissolved and colloidal organic matter as food (substrate), the total volume of sludge will be far less than for chemical coagulation. In practice, therefore, secondary treatment tends to be a biological process with chemical treatment used for toxic wastewater treatment.
