Biological or chemical treatment processes added to a secondary treatment plant including a conventional activated sludge to increase the removal of solids and?BOD. Typical removal rates for advanced secondary plants are on the order of 90 percent removal of solids and BOD. BOD: The amount of oxygen per unit volume of water required to bacterially or chemically oxidize (stabilize) the oxidizable matter in water. Biochemical oxygen demand measurements are usually conducted over specific time intervals (5, 10, 20, 30 days). The term BOD generally refers to a standard 5-day BOD test. Secondary Treatment: The second step in most publicly owned waste treatment systems, in which bacteria consume the organic parts of the waste. It is accomplished by bringing together waste, bacteria, and oxygen in trickling filters or in the activated sludge process. This treatment removes floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids. Disinfection is the final stage of secondary treatment. (See: Primary Treatment, Tertiary Treatment)